- Email: [email protected]
Bone disorders: a radiological approach
BaillieÁre's Clinical Rheumatology Vol. 14, No. 2, pp. 173±199, 2000
doi:10.1053/berh.1999.0061, available online at http://www.idealibrary.com on
1 Bone disorders: a radiological approach Iain Watt
FRCP, FRCR, FFR
Consultant Clinical Radiologist Department of Radiology, Bristol Royal In®rmary, Bristol BS2 8HW, UK
The radiological interpretation of bone and joint disease has emerged as a subspecialty in its own right. This chapter oers simple guidelines for the approach to a plain ®lm examination that has been taken in a patient with a non-in¯ammatory arthritis. It does not discuss what ®lms should be taken or when. Instead, it focuses on what to look for and where. Emphasis is placed on unusual features that may trigger the thought `This doesn't look like ordinary osteoarthritis.' One does not know, of course, that the patient has an unusual cause of arthritis until the radiograph has been taken. Hence the radiographs of all patients deserve to be analysed properly, for only then will the unusual not become the undiagnosed. Key words: radiology; imaging; bone and joint disease; basic principles.
The ®rst port of call after a patient has presented with arthritis or arthralgia is frequently the radiology department in order to obtain a plain radiograph of the presenting joint or joints. The purpose of this chapter is to discuss an approach to the interpretation of such radiographs and to suggest, where relevant, clues for further investigation. Thus, when faced with an X-ray of a problematic joint, what should one look for? It is as important to remember to search a radiograph intelligently as it is to elicit physical signs on the clinical examination of a patient. Naturally, the ®nding of one physical sign leads to the search for other associated ®ndings. With a radiological examination, the same logic should apply. Thus, an examination may start with the soft tissues and move to the texture of the bone, joint capsule and synovium, the interbone distance (or `joint space width') and the subchondral bone itself. It is axiomatic that in order to do this, one must have high-quality radiographs. The radiographs must be of sucient penetration to include bony texture and bony details, while at the same time not `blacking out' soft tissue structures. If one is to proceed to further investigation, a logical progression is necessary. 1. 2. 3. 4.
What is it that the enquiring clinician wishes to know about the patient? How will it in¯uence his or her management decisions? What is the optimum means of ®nding out the information required? Will the therapy that is consequent upon the investigation make a dierence to the patient's outcome? The cost±bene®t relationship here is crucial, remembering that radiation carries a small but de®nite hazard and intravascular contrast medium administration a risk of death.
Finally, it is important to emphasize that one should treat patients rather than radiological images. Notoriously, symptoms and signs do not correlate well with 1521±6942/00/020173+27 $35.00/00
c 2000 Harcourt Publishers Ltd *
174 I. Watt
radiographic evidence and indeed vice versa. Thus, the ®nding of an apparently `degenerate' joint on an X-ray does not mean that the patient has symptoms or even necessarily abnormal physical signs. WHERE TO LOOK ON THE FIRST RADIOGRAPHS The following text will be divided up under a series of headings, but it is important to remember that any joint is a whole organ comprising a number of structures just as a patient is, essentially, a collection of organ systems. Thus, one would start by examining the whole patient (or joint) in order to obtain an overall impression of him or her and their disease. Then, as in clinical examination, one examines the organ systems independently. Thus, the subject headings that follow include: 1. 2. 3. 4. 5. 6.
Soft tissues Bone periosteum Bone (including changes in density) Joint space abnormalities Subchondral bone Synovium.
It is important to emphasize that the distance between two bone ends on a radiograph contains material that is unknown, since hyaline cartilage, joint ¯uid and indeed pannus are non-opaque. To assist in this matter, a number of joints, particularly the knee, bene®t from being examined in weight-bearing, but even then one does not have a full assessment of all the articular surfaces. Other joints, for example the hand and ankle, do not bene®t from weight-bearing ®lms. There is some suggestion that early degrees of cartilage thinning in the hip may be better assessed on weight bearing-®lms.1 Certainly, the dysplastic hip is better evaluated by the false pro®le view.2 FIRST IMPRESSIONS ARE IMPORTANT The clinician should ®rst look for obvious morphological changes. Skeletal dysplasias are extremely rare and dicult to classify, and the reader is referred to a standard textbook for classi®cation.3 Generalized dysplasias are usually obvious, with gross morphological changes (as in achondroplasia or pseudoachondroplasia). However, in some cases, such as minor expressions of multiple epiphyseal dysplasia or spondyloepiphyseal dysplasia, the changes can be subtle. Anteroposterior views of the hips, knees and hands, together with lateral views of the thoracolumbar spine, will usually be sucient to identify and classify a dysplasia. While a single osteochondroma is usually easy to diagnose, multiple osteochondromata in diaphyseal aclasia may show bizarre moulding abnormalities (Figure 1). On the other hand, local changes are relatively common and include, for example, greater or lesser degrees of dysplastic disease of the hip and similar changes in the knee described as part of the spectrum of knee osteoarthritis.4 In the former, the femoral head is, to a greater or lesser extent, uncovered, the lateral opening of the acetabulum is increased and the fovea high and larger than usual. In osteoarthritis, the femoral head may be laterally placed, with subarticular cysts in the labrum, rather than the
Bone disorders: a radiological approach 175
Figure 1. Diaphyseal aclasia. (A) A frontal radiograph of both knees demonstrates multiple metaphyseal abnormalities, all individually classical of an osteochondroma. Note, however, that in spite of considerable metaphyseal abnormality, the joint surfaces are normal. (B) Pelvis. Again, note that there are considerable changes, particularly in the femoral necks and intertrochanteric regions. The hip joints themselves are essentially normal.
acetabular roof (Figure 2). In the latter, the femoral condyles exhibit valgus dysplasia, such that the joint line no longer runs parallel to the ground on weight-bearing ®lms. One may also have a clue about pre-existing disease by looking for evidence of local growth changes, for example the relative enlargement and squaring of former epiphyses that is a characteristic feature of juvenile overgrowth and early metaphyseal fusion (Figure 3). This occurs in a number of conditions, including haemophilia (or other recurrent bleeding disorders), old septic arthritis and juvenile idiopathic arthritis. In addition, bones may be moulded abnormally as a result of a whole number
176 I. Watt
Figure 2. Dysplastic hips. (A) Achondroplasia. Note the considerable shortening and ¯attening of the femoral head and neck, and the secondary remoulding of the acetabulum. The acetabular roof is also abnormally wide. (B) Idiopathic. In this example, the femoral head is partially uncovered and has migrated laterally. Note the loss of cartilage thickness superiorly, the circumferential osteophytosis and the elevation of a fragment of bone from the lateral acetabular margin, the so-called Eggers' cyst.
or conditions, from Paget's disease (Figure 4) in the elderly through to ®brous dysplasia in the younger patient. One should become aware of possible artefacts on the radiographs. These include ®lm faults in processing, gown or clothing folds, and buttons and so on that produce bizarre lines or appearances. A knowledge of radiological anatomy is necessary to avoid misinterpreting overlying shadows such as femoral condyles on somewhat oblique projections. Radio-opaque `foreign' bodies are seldom a problem, although the clinician should distinguish intellectually between a separate osteochondral body, as in osteochondritis dissecans, and a true `foreign' body, for example a bullet. Furthermore, it should be remembered that separate osteochondral bodies may not be loose. They are frequently embedded in the synovium even when they are detached from their site of origin. One may also gauge the degree of bone response in so-called degenerative joint disease. Hypertrophic changes, characterized by abundant osteophytosis, sclerosis and preserved bone density, are characteristic of the bone-forming group of disorders (Figure 5A). These include Forestier's disease (or diuse idiopathic skeletal hyperostosis; DISH) and so-called pyrophosphate arthropathy (hypertrophic osteoarthritis associated with the presence of calcium pyrophosphate dihydrate crystals; CPPD). On the other hand, some variants of degenerative disease are extremely atrophic, with very little evidence of bone response. This is particularly the case with the rapidly destructive variant of osteoarthritis seen in the elderly female, usually in the shoulder5 or hip (Figure 5B), or the more aggressive in¯ammatory erosive osteoarthritis of the hands. The former is, of course, associated with abundant hydroxyapatite crystal formation (BCP) in joint ¯uid. This is, however, no longer considered to be an
Bone disorders: a radiological approach 177
Figure 3. Haemophilia. A lateral view of the knee demonstrates typical overgrowth of the former epiphyses and cartilage thinning. A frontal view would have demonstrated widening of the intertrochanteric notch. The features are typical of juvenile arthritis but are entirely non-speci®c in terms of aetiology.
aetiological factor.5 Neuropathic joints can produce both hypertrophic and atrophic features. A classical cause of atrophic destructive arthritis associated with neuropathy is syringomyelia, the shoulder and elbow being at particular risk. In diabetic osteoarthropathy, a combination of hypertrophic changes with frank bone destruction usually occurs. This is the most common cause of hypertrophic degenerative arthritis with frank destruction, save for superimposed infection in a pre-existing hypertrophic degenerative joint. Degenerative arthritis secondary to another prior arthropathy can be gauged in a number of ways: ®rst, the growth disparities as summarized above, and second, the relative paucity of reparative change as opposed to diuse hyaline cartilage loss (Figure 6). The concept of reparative change is important. Taken at its most simplistic, it is the case that osteophytosis is rare in an in¯ammatory arthritis such as rheumatoid disease, largely because osteophytes are formed by the enchondral ossi®cation of chondrophytes. As rheumatoid disease destroys cartilage, there are few viable chondrocytes. When the rheumatoid disease becomes quiescent, residual cartilage can again hypertrophy and develop chondrophytes, and hence osteophytes. It is thus reasonable to see osteophyte and sclerosis formation in a patient who has had rheumatoid disease as being a reparative rather than a degenerative phenomenon.
178 I. Watt
Figure 4. Paget's disease of the hip. Note the sclerotic, expanded nature of the femoral head and neck, with cortical thickening. There is also, however, a virtually normal joint space width superiorly. Paget's disease almost invariably commences from an articular surface and spreads along the aected bone.
Figure 5. The spectrum of osteoarthritis. (A) Classical hypertrophic osteoarthritis. Note the considerable osteophytosis and hyaline cartilage thinning, yet the absence of obvious bone destruction. (B) Atrophic destructive osteoarthritis. Here there is subchondral sclerosis, obvious destruction of the femoral head and acetabular roof, and superolateral migration of the hip joint accordingly.
Finally, if a scoring system is used to evaluate longitudinal or cross-sectional studies in patients with degenerative arthritis, what system should it be? There is no correct answer since it, as always, depends on what the investigating clinician wishes to know about the individual patient. There are a number of available global scores, but it may be far more relevant to look at individual radiological signs, for example the preservation of interbone distance.
Bone disorders: a radiological approach 179
Figure 6. Old rheumatoid disease with `secondary' osteoarthritis. Note a concentric hyaline cartilage loss in both hips, the relative paucity of sclerosis, osteophytosis, and superomedial migration caused by `soft' bone following an in¯ammatory arthropathy.
SOFT TISSUES As previously emphasized, adequate quality radiographs of joints require that the soft tissues be visualized. It is easy to look for such phenomena as varicose veins and soft tissue lumps, as in a patient with von Recklinghausen's disease (neuro®bromatosis), if the soft tissues are preserved on the ®lm (Figure 7). It is obviously much easier to see soft tissue lesions using other imaging modalities, such as ultrasound or magnetic resonance imaging, but these are seldom used as the ®rst-line investigation in a possible degenerative joint. Fat-containing masses, simple or malignant lipomata, are often visualized by their relative transradiancy compared with muscle. The fat lines that surround many normal tissue planes are also vital. The ill-de®nition of a normal tendon or swelling around it can diagnose tenosynovitis or tendon trauma. The diagnosis of a joint eusion, and synovitis, can be easy by examining fat planes (see below). Calci®cation Calci®cation in soft tissues occurs in a number of important manifestations including: 1. arterial calci®cation, as occurs in atherosclerosis, diabetes mellitus, when peripheral arterial calci®cation is particularly common, and median necrosis; 2. venous calci®cation, in varicose vein disease, venous thrombosis and arteriovenous malformation; 3. calci®cation in subcutaneous tissue in patients with connective tissue disorders, particularly scleroderma and CREST syndrome; more localized lesions may occur in calcinosis cutis; 4. calci®cation, resulting from BCP, is a classical feature of acute calci®c peri-arthritis, typical sites being the rotator cu, gluteal tendons and ¯exor carpi ulnaris; 5. chondrocalcinosis (see below);
180 I. Watt
Figure 7. Neuro®bromatosis. Note, in the soft tissues, subtle textural changes comprising nodular masses and fatty replacement, with some areas of calci®cation resulting from a plexiform neuro®broma. Note also the gross expansion and thinning of the obturator ring, again caused by neuro®bromatosis, together with moulding abnormalities of the femoral shaft itself.
6. dystrophic soft tissue calci®cation in hyperparathyroidism, renal failure or tumoral calcinosis; 7. the early stages of evolution of a haematoma or, more rarely, a soft tissue sarcoma; 8. parasitic infections, leprosy and thermal injury. Ossi®cation Ossi®cation in soft tissues is, on the other hand, extremely uncommon and may lead to a consideration of either a developmental condition, for example ®brodysplasia (myositis) ossi®cans progressiva, in which fascial planes become ossi®ed (Figure 8), or focal dystrophic ossi®cation, as in a previous haematoma. On very rare occasions, soft tissue ossi®cation may indicate a bone-forming malignancy. BONE PERIOSTEUM Periosteal new bone formation has a wide dierential diagnosis. It is important clearly to distinguish between true periosteal new bone formation and bone formation at speci®c sites, such as osteophytes and enthesophytes.
Bone disorders: a radiological approach 181
Figure 8. Fibrodysplasia ossi®cans progressiva. Classical soft tissue ossi®cation in fascial planes, eectively causing an extra-articular arthrodesis. Note that this is not calci®cation but bone with a trabecular pattern and architecture. Secondary subluxation has occurred at the hip joint. The process here is obviously chronic since the ¯oor of the acetabulum is thick, indicating that the femoral head has subluxed superolaterally for a long time.
Osteophytes Osteophytes develop at the site of enchondral ossi®cation of chondrophytes and are, by de®nition, limited to synovial joints. The sites are classical ± the margins of synovial joints ± or actually on the articular surface, as `stud' or `button' osteophytes. As indicated above, the degree of osteophytosis may re¯ect a more generalized `boneforming' quality of the patient, and abundant osteophytosis may invite a careful search for the presence of crystals in joint ¯uid. Enthesophytes Enthesophytes, on the other hand, classically arise at musculotendinous origins and insertions. They may be subdivided into those in which it is possible to see an obvious erosive component (in which case the possibility of a systemic disorder such as ankylosing spondylitis should be borne in mind) and non-erosive examples, which have a wider dierential diagnosis. Most non-erosive enthesophytes are associated with lowgrade trauma or obesity, but they may be part of a widespread generalized disorder
182 I. Watt
such as Forestier's disease (or DISH). Various metabolic disorders should also be considered, including: 1. 2. 3. 4. 5.
hypoparathyroidism; pseudohypoparathyroidism; sex-linked hypophosphataemic rickets; ¯uorosis; obesity, often in association with hyperuricaemia and non-insulin-dependent diabetes mellitus.
Periosteal new bone formation In the adult skeleton, this presents a wide dierential diagnosis, but for brevity it can be subdivided into local and generalized types. Local new bone formation Local new bone formation may occur in association with trauma and the presence of a local tumour, either primary or secondary. Periosteal new bone formation classically occurs in association with infection, but in this instance, the patient will present with in¯ammatory rather than degenerative features. The more aggressive the cause of the new bone formation, the more ill de®ned and layered will be the appearance. Ill-de®ned new bone formation implies aggressive change, whereas chronic conditions, such as varicose vein disease in the leg, will produce coarse, rather mature-looking bone formation. In the arthritic population, particularly when angular deformity and/or osteopenia are present, the possibility of a stress or insuciency fracture (Figure 9) should be considered. In this, a band of illde®ned sclerosis, crossing the major long bone trabeculae at right angles, may accompany localized periosteal new bone formation. Finally, melorheostosis, in which `dripping candlewax' sclerosis occurs along a bony sclerotome, should be remembered. The importance of this condition lies in the cutaneous manifestations and the associated symptomatology, the so-called linear scleroderma. Generalized periosteal new bone formation Such generalized bone formation by de®nition implies a systemic disorder. In the younger skeleton, one must consider leukaemias and lymphomas, but in the older patient, especially a smoker, one must always consider the possibility of hypertrophic (pulmonary) osteoarthropathy (HOA). Here, the periosteal new bone formation is classically aligned along the diaphyses, sparing the former epiphyses (Figure 10). HOA also has a wide dierential diagnosis, including: 1. Pulmonary: bronchogenic carcinoma, pleural mesothelioma, abscess, bronchiectasis and metastasis; 2. Cardiac and vascular: cyanotic congenital heart disease, bacterial endocarditis or an infected arterial graft; 3. Intestinal and abdominal: portal and biliary cirrhosis, and in¯ammatory bowel disease; 4. Intra-abdominal malignancy: including lymphoma and gynaecological primary tumours.
Bone disorders: a radiological approach 183
Figure 9. Three example of insuciency or stress fracture. (A) Os calcis fracture in a ballerina. Note the band of sclerosis crossing the major trabeculae of the os calcis, extending from the Achilles tendon to the plantar fascia. (B) The distal ®bula. Here a localized zone of periosteal new bone formation is demonstrated laterally. It is just possible to discern a faint band of sclerosis crossing the medullary canal at this level. (C) Involvement of the lateral tibial plateau. Note the ill-de®ned sclerosis, as well as the radiolucency approximately 1.0 cm in from the joint margin, within which there is an area of curvilinear opaci®cation representing the imploded cortex within the insuciency fracture. The appearances are highly reminiscent of avascular necrosis.
The dierential diagnosis of HOA also includes thyroid acropachy, which characteristically involves the medial aspects of the shafts of the ®rst and second metatarsals and metacarpals, and pachydermoperiostitis, in which the coarse periosteal new bone formation is associated with obvious clinical features of subcutaneous thickening. One of the manifestations of the SAPHO syndrome (Synovitis, Acne, Pustulosis, Hyperostosis and Osteitis) is the presence of sclerotic mass lesions in long bones.
184 I. Watt
Figure 10. Hypertrophic (pulmonary) osteoarthropathy. Gross pathology is demonstrated along the radius and ulna (A) and hands (B).
Hence, a search should be made for the appropriate skin rashes and manifestations on the anterior chest wall. BONE Bone sclerosis Sclerosis in bone adjacent to a joint that has been X-rayed may fall into a number of categories.
Bone disorders: a radiological approach 185
Focal lesions that are generalized in the skeleton Multiple bone islands are by far the most common cause of this. These measure some 2±3 mm in diameter, with a discrete zone of radiolucency surrounding them. Innumerable bone islands occur in osteopoikilosis, a condition thought to be unassociated with any signi®cance in terms of joint disease save for the classical clustering of the lesions around the joints (Figure 11). Multiple bone islands should be disregarded in terms of their clinical signi®cance. Larger solitary lesions do occur as osteomata, but these are largely con®ned to the paranasal sinuses. A less common, innocent dysplasia is osteopathia striata, in which streaks of sclerotic medullary bone occur around the joints.
Figure 11. Osteopoikilosis. (A) Hand. (B) An oblique projection of the left hip. Both demonstrate classical multiple bone islands clustered around the joints. The hip lesion also shows some striation in the proximal femur, probably representing additional osteopathia striata.
Focal lesions that are localized to one site The two true focal benign tumours that occur are osteoid osteoma and osteoblastoma. The former characteristically involves individuals in their 20s and 30s, presenting as a small focal zone of radiolucency within which a smaller sclerotic lesion may be seen. Abundant consolidated cortical and periosteal new bone formation occurs with the peripherally situated lesions (Figure 12). When next to a joint, osteoid osteoma may present as an in¯ammatory arthritis. Osteoblastoma, which may for simplicity be regarded as a large variant of osteoid osteoma, classically occurs in the spine and presents with the sclerosis of a posterior element. Focal medullary lesions that resemble bone formation include the so-called cartilage rest or bone infarct, in which serpiginous ill-de®ned calci®cation occurs in the distal diaphysis of a long bone. These too are usually disregarded in terms of signi®cance, although in the older patient the clinician must beware of a growing cartilage lesion in the medulla, which probably will represent a chondrosarcoma (see below). Focal sclerosis occurs also in any old benign ®bro-osseous lesion of bone, for example a previous fracture or osteomyelitis. An appropriate history should be available.
186 I. Watt
Figure 12. Osteoid osteoma. These lesions are usually typical and classical, but on occasions they arise in an unusual area of anatomy. Here, an osteoid osteoma involves the terminal phalanx of a thumb and presented in a patient who complained of joint pain. Note the ill-de®ned radiolucency, within which a central dense opacity is projected on the lateral border of the terminal phalanx.
Focal malignant lesions raise the possibility in the younger skeleton of osteosarcoma. Osteosarcoma is relatively easy to diagnose but is frequently overlooked in its earlier stage. The lesion classically occurs in teenagers or individuals in their early 20s, with eccentric bone destruction and bone formation. Periosteal new bone formation is frequently present with the so-called Codman's triangles, associated with a wellde®ned soft tissue mass (Figure 13). In the slightly older subject, paraosteal osteosarcoma presents a slightly more dicult diagnosis. Here, the lesion is characteristically a mushroom-shaped mass of apparently benign bone that is applied to the cortex of a long bone, usually a femur or a humerus. The dierential diagnosis of this includes old trauma. In the older skeleton, solitary sclerotic lesions in medullary bone raise the possibility of metastasis, the most common causes being prostatic carcinoma in the male and breast carcinoma in the female. There are no speci®c features of a sclerotic metastasis to indicate the location of the primary tumour. The dierential diagnosis includes mastocytosis, lymphomas (particularly non-Hodgkin's lymphomas) and the rare sclerotic myeloma. Generalized bone sclerosis This presents a wide dierential diagnosis. Increased density may occur in the cortex, the medulla or both. Medullary sclerosis in the adult skeleton raises three important dierential diagnostic groups: 1. haematological conditions, including the haemoglobinopathies, myelo®brosis and myelodysplasias; 2. metabolic disorders, such as oxalosis, renal osteodystrophy, hypervitaminosis A, hypervitaminosis D and heavy metal poisoning; 3. malignant diseases, including non-Hodgkin's lymphomas, Hodgkin's disease and widespread metastatic disease. Thus, to summarize, generalized medullary sclerosis is always bad news.
Bone disorders: a radiological approach 187
Figure 13. Osteosarcoma. Arising (A) in the proximal humerus and (B) in the medial tibial plateau. The humeral lesion is classical, with a large soft tissue mass, mixed bone formation and bone destruction. There is also tumour bone formation within the mass itself. The second lesion is more subtle. Here, there is undoubtedly destruction under the tibial spines. Notice also the bone formation in the medial plateau itself. Both patients presented with chronic joint pain.
Cortical sclerosis This should suggest, ®rst, a number of dysplasias, the most important being osteopetrosis tarda in the adult. Mixed cortical and medullary sclerosis occurs in a number of dysplasias, including Englemann±Camurati's disease (Figure 14). More commonly, bone expansion and sclerosis raise the possibility of Paget's disease (see Figure 4 above) and haemangioma, particularly when a vertebral body is involved. Bone radiolucency Focal radiolucent lesions Focal radiolucent lesions in long bones adjacent to joints may be dierentiated into those involving the cortex or the medulla, and those subadjacent to the joint. Cortical radiolucencies are common in the younger skeleton, well-de®ned, slightly blister-like lesions being characteristic of so-called ®brous cortical defects. Larger lesions, the non-ossifying ®bromata, often present with fractures, particularly of the tibia. However, both conditions are closely associated with ®brous dysplasia and a number of important variants, including ossifying ®broma. These occur in the older skeleton and may cause diculties in dierential diagnosis. Broadly speaking, however, soap bubble-like lesions involving the cortex and medulla, particularly when associated with the so-called ground glass texture, should raise the possibility of a benign ®bro-osseous lesion of bone. In the older skeleton, frankly malignant lesions such as ®brosarcomata, malignant ®brohistocytomata and metastases are characterized
188 I. Watt
Figure 14. Englemann±Camurati's disease. A radiograph of both femora shows gross bone expansion and marked medullary and cortical sclerosis. Notice, however, that the hips are relatively normal.
by ill-de®ned permeative bone destruction. When bone destruction speci®cally involves the cortex of a long bone, such as the femur, the primary is often carcinoma of the bronchus. Purely medullary lesions suggest either metastasis or myeloma. However, in older patients, a chondroma is common, a chondrosarcoma more rarely being seen. The former are frequent in the hands; a relatively well-de®ned, lytic, expansile lesion is typically shown, with ring-like foci of calci®cation. Chondrosarcoma is notoriously dicult to diagnose, except in some speci®c locations such as when a large calci®ed mass arises from a ¯at bone, or when frank bone destruction has occurred (Figure 15). All cartilage tumours should be treated with suspicion, especially if the patient is locally symptomatic. The focal lesions of myeloma are quite dierent, appearing to be well de®ned, punched out and well circumscribed, without zones of sclerosis. The `raindrop' appearance is typical, with scalloping of the cortex from within. Peri-articular lesions raise a dierential diagnosis and may present, of course, with joint pain. The peri-articular lesions include those listed in Table 1. In the mature skeleton, osteosarcoma is relatively rare, but giant cell tumours are not. These are characteristically placed eccentrically, immediately subarticularly, and are purely radiolucent, with a narrow zone of transmission to normal bone. The knee
Bone disorders: a radiological approach 189
Figure 15. Chondrosarcoma of the hip. Most chondrosarcomata are obvious in so far as they present as large expansile lesions that contain a variable amount of calci®cation. This lesion however is at another classical site and demonstrates more subtle changes. Note the radiolucency involving the ¯oor and roof of the acetabulum, with slight bone expansion.
is a particularly common site (Figure 16), as is the wrist. Histologically, the brown tumour of hyperparathyroidism cannot be distinguished from the idiopathic giant cell tumour (Figure 17). Hence when one is found, metabolic bone disease should be considered as a cause. The joint-related lesions are obviously adjacent to the articular surfaces or bare area and have a sclerotic margin. Thus, large subchondral cysts or geodes (as in rheumatoid disease, when they are characteristically deep to the tibial spines) are obviously benign in terms of their radiological features. These include the ovoid or elliptical outline, the well-de®ned sclerotic margin and the narrow zone of transition. Percutaneous Table 1. Major causes of subarticular lytic lesions. X-ray ®ndings
Cause
Joint-related erosions
Rheumatoid and seronegative diseases
Subarticular cysts
Osteoarthritis, haemochromatosis, Wilson's disease
Subarticular geode
Usually rheumatoid disease, classically of the knee (tibial spines), hip (superior neck of femur) and hand (metacarpal heads)
Synovial tumours
Synovial chondromatosis, pigmented villonodular synovitis
Mass eects
MSUM tophi, amyloid deposits
Tumours
Giant cell tumour, aneurysmal bone cyst, intra-osseous ganglion (usually the ®bula), simple ®bromyxoma (carpus), osteosarcoma
Avascular necrosis
Idiopathic in the knee (sudden onset); also hip and lunate
MSUMmonosodium urate monohydrate.
190 I. Watt
Figure 16. Giant cell tumour of the patella. A classical giant cell tumour is demonstrated, but in an unusual location. Note that it is immediately subarticular, eccentric and lytic in appearance.
injection into one such lesion usually reveals a `synovial' lining and retrograde communication with the joint. Intra-osseous ganglions involve the proximal ®bula or distal ®bula related to the proximal or distal tibio®bular articulations. Cystic lesions secondary to synovial pathology, such as synovial chondromatosis or pigmented villonodular synovitis (PVNS), usually involve those areas of the joint in contact with synovium (primarily the bare area) on both sides of the articulation (Figure 18). Tangential views of these lesions show that they are essentially pressure defects, indenting the bone from the outside. The margins are well de®ned and give the appearance of chronicity. Synovial chondromatosis usually exhibits calci®cation in the synovium or calci®ed and/or enchondrally ossi®ed separate bodies. PVNS is one of the causes of chronic iron deposition in synovium, causing it to be relatively opaque on a plain ®lm. Bone lucency ± generalized It is important to distinguish between bone atrophy, as in prolonged non-use, and osteoporosis from active bone resorption. In the latter, there is evidence of intracortical tunnelling, initial radiolucency being shown in the immediate subchondral areas and at the site of former metaphyses. The distinction between medullary bone and the cortex becomes lost, and in very vigorous hyperparathyroidism, for example,
Bone disorders: a radiological approach 191
Figure 17. Hyperparathyroidism. This 35-year-old lady presented with a history of chronic bone and joint pain. Notice how the skeleton is osteopenic and shows marked intracortical tunnelling. Note also vague areas of ill-de®ned radiolucency in the femoral neck and laterally in the acetabular roof (A). These are caused by brown tumours. (B) A radiograph of the phalanges taken following trauma a year earlier shows classical intracortical tunnelling on the radial aspect of the middle and proximal phalanges, a lack of distinction of the corticomedullary junction and a somewhat simpli®ed trabecular pattern.
periosteal new bone formation becomes obvious. In bone atrophy or osteoporosis, the cortex is simply thinned, the trabecular pattern reduced and the bone radiolucent. Juxta-articular osteopenia in the in¯ammatory arthritides tends to resemble the latter but in a focal zonal position around the involved joints. In algodystrophy, or the various subsets thereof, the features of acute bone resorption are found, but on a
192 I. Watt
Figure 18. Pigmented villonodular synovitis (PVNS). A radiograph of the hip demonstrates cartilage thinning caused by chronic synovitis, but note, however, the multiple well-de®ned subarticular and peri-articular radiolucencies. These suggest a chronic synovial mass or depositional state. The appearances are entirely nonspeci®c but suggest either PVNS or synovial chondromatosis. Were the radiolucencies more predominantly intraosseous, amyloidosis would be an alternative diagnosis.
regional or transitory basis. Furthermore, the radiolucency is, at least initially, localized across former metaphyses and subarticular. If a patient presents with joint pain and evidence of high bone turnover, it is worth looking to see whether chondrocalcinosis or focal mass lesions are present to suggest hyperparathyroidism. As indicated above, the brown tumour of hyperparathyroidism is histologically indistinguishable from an idiopathic giant cell tumour. A loss of bone density will generally also occur in osteomalacia and renal osteodystrophy, when the features of hyperparathyroidism may not be as overt. A coarsened trabecular pattern, evidence of bone softening and the presence of cortical insuciency fractures (Looser's zones) should be sought. Other causes are listed in Table 2.
Table 2. The major causes of osteopenia. Findings
Cause
Generalized osteopenia
Osteoporosis, disuse atrophy, protein de®ciency, homocystinuria, myelomatosis, chronic anaemias, disseminated malignancy, endocrine disorders (adrenal and parathyroid in particular), osteomalacia and rickets Algodystrophy, in¯ammatory arthropathy, tumours, acute lytic Paget's disease, subarticular joint causes
Localized osteopenia
Bone disorders: a radiological approach 193
JOINT SPACE ABNORMALITIES An evaluation of joint space width is dicult, for the reasons already discussed. The interbone distance may comprise hyaline cartilage, joint ¯uid and so on, and such tissues are not visualized on a plain radiograph. Thus, one is in danger of assuming that the interbone distance actually corresponds to the hyaline cartilage thickness. As indicated above, weight-bearing ®lms in some circumstances will help to ensure that the interbone distance really does measure hyaline cartilage thickness. However, granted that the joint surface is in many cases a complex curved structure, even these radiographs will not fully permit a ®ne assessment of cartilage thickness. Other modi®cations have been advocated, including partial knee ¯exion in the posteroanterior view6, with or without other modi®cations such as computer-assisted analysis. For clinical purposes, the interbone distance can be assessed in three ways in order to approach a possible dierential diagnosis of non-in¯ammatory joint disease. The cartilage may appear thick Classically, an increase in the interbone distance occurs in acromegaly (Figure 19). It is one of the earliest and most sensitive radiological features of the disease, being more reliable than looking for thickness changes in other soft tissues (such as the heel pad), terminal tufting or coarsening of the skin thickness. An increased thickness of hyaline cartilage also occurs in hypothyroidism. Cartilage may appear relatively thick in some cases of early joint eusion, when, as indicated above, the increased interbone distance results from joint eusion rather than cartilage. In addition, apparently thick hyaline
Figure 19. Acromegaly. An X-ray of the hand demonstrates classical enlargement of the joint spaces resulting from the thickened hyaline cartilage of these small joints.
194 I. Watt
cartilage occurs in joints that are eroded by those arthropathies which are largely non-in¯ammatory, such as PVNS, synovial chondromatosis, gout and multicentric reticulohistiocytosis. The cartilage may seem thin A diuse loss of hyaline cartilage thickness occurs in many conditions that essentially `poison' it. Thus, the result of rheumatoid disease (see Figure 6 above), juvenile idiopathic arthritis, septic arthritis and the other in¯ammatory arthritides may all be the same in so far as cartilage will be thinned. The ®nding is, therefore, non-speci®c. However, when diuse hyaline cartilage thinning is found, the possibility of a current or previous in¯ammatory arthritis should be considered. A diuse thinning of hyaline cartilage, particularly in the interphalangeal joints of the hands and knees, is a feature of relapsing polychondritis. The cartilage may be calci®ed The usual cause of hyaline cartilage calci®cation is the deposition of CPPD, although BCP may be found in some joints in the connective tissue disorders. CPPD deposition is an age-related phenomenon and not necessarily associated with any `disease' in the strict meaning of the word. Premature chondrocalcinosis occurs in hyperparathyroidism, haemochromatosis and those families which are genetically susceptible. A shedding of crystals does occur, presenting as the in¯ammatory acute synovitis often called pseudogout. The relationship between chondrocalcinosis and the evolution of the hypertrophic variant of osteoarthritis is strongly debated (see above). The current prevailing view is that a strictly causal relationship does not exist. Chondrocalcinosis may be ¯orid in hyperparathyroidism without the superadded development of a destructive arthritis. Haemochromatosis has a high incidence of CPPD deposition that may manifest prior to any form of speci®c arthropathy.7 Marked chondrocalcinosis, especially in the intervertebral discs and when associated with early and ¯orid disc degeneration, should suggest ochronosis. The crystal found here is BCP.
SUBCHONDRAL BONE Subchondral cysts The relationship between hyaline cartilage and subchondral bone is intimate. There are some clues that may assist the dierential diagnosis in arthritis. A number of conditions are characterized by multiple subchondral `cystic' lesions stretching across the articular surface. There include haemochromatosis, osteoarthritis associated with CPPD and Wilson's disease. Extensive subarticular cystic change can occur also in severe hyperparathyroidism. In haemochromatosis, one of the important ®ndings is multiple subchondral radiolucencies arranged rather like a string of beads across the metacarpal heads and at sites where arthritis is seldom found, for example the ankle joint (Figure 20A). A disease-speci®c form of cartilage pathology occurs in haemochromatotic hips when the zone of provisional calci®cation loses its naturally corrugated form and the cartilage simply peels o at this layer.7 Radiographs show ill-de®ned subchondral radiolucency reminiscent of avascular necrosis (Figure 20B). The latter, however, classically manifests
Bone disorders: a radiological approach 195
Figure 20. Haemochromatosis. (A) Ankle. Note the multiple subchondral cystic lesions stretching across the articular surface. The ankle is rarely involved in osteoarthritis or rheumatoid disease. Such a ®nding should suggest strongly the possibility of haemochromatosis. (B) A radiograph of another 45-year-old male's hip. Notice the ill-de®ned subchondral radiolucencies across the femoral head.
wedged-shaped areas of abnormality and the preservation of the overlying cortex as it receives vitality from the hyaline cartilage. Hypertrophic osteoarthritis associated with CPPD is also characterized by multiple subchondral radiolucencies or cysts. A distinction between the lesions of haemochromatosis and these is subtle8 and not easy to discern in the individual patient. However, the hip features of the latter do seem disease speci®c for haemochromatosis. Subchondral sclerosis Ill-de®ned sclerosis, especially when found adjacent to a `degenerative' joint, suggests trabecular microfractures and callus formation. This implies bone failure and may presage rapid joint failure. Avascular necrosis This chapter can only highlight a few pertinent concepts. The initial ®ndings in avascular necrosis depend on the site of involvement. In the hip, the lesion is initially shown as a subtle change in the architecture of the femoral head trabeculae as a result of callus formation and microfractures. This precedes the development of the typical wedge-shaped defect and classical subchondral lucency or `vacuum' sign. In a metatarsal head, however, prior to collapse and the typical features of Freiberg's infraction, it is usual to see a band of sclerosis 3±4 mm deep to the articular surface, possibly a developing stress fracture. Last, but most important, is the lesion that occurs suddenly in the medial femoral condyle in the middle-aged. The initial sign here is a subtle and enlarging zone of subcortical radiolucency. The cortex subsequently becomes separated, as in the hip, and may implode into the defect beneath it
196 I. Watt
Figure 21. Idiopathic medial femoral condyle necrosis. A classical lesion is demonstrated in the medial femoral condyle. Note the ill-de®ned area of subarticular radiolucency, within which is a linear opacity resulting from the imploded cortex. Chondrocalcinosis is present in the lateral compartment. This is a described association.
(Figure 21). A careful examination of the subchondral plate may make the early diagnosis of these conditions simple, to be con®rmed scintigraphically or by magnetic resonance imaging.
SYNOVIUM An examination of the soft tissues around a joint can be extremely helpful. As already emphasized, it is important to have the soft tissues rendered visible by virtue of the fat planes adjacent to them. What does one look for? Swelling The presence of a joint eusion may frequently be visualized at the metacarpophalangeal, interphalangeal and metatarsophalangeal joints, the wrist, the elbow and the knee. A distension of the suprapatellar pouch indicates the presence of either thick synovium or joint ¯uid. These cannot be told apart, except by using ultrasound or other imaging modalities. In the presence of acute synovitis, however, the outline of the suprapatellar pouch, or other sites, becomes ill de®ned, the degree of ill de®nition paralleling the degree of in¯ammatory change. Soft tissue swelling caused by joint ¯uid or an in¯ammatory synovitis is usually symmetrical. However, an asymmetrical soft tissue swelling should strongly raise the possibility of a depositional disorder, including tophaceous deposits of MSUM, lipid and occasionally a rheumatoid nodule. Asymmetry implies a depositional disorder.
Bone disorders: a radiological approach 197
Opaci®cation Opaci®cation of the synovium is an extremely important sign (Figure 22). Opaque synovium occurs because a high atomic number substance is present in it, the two most common being calcium and iron. The former is characteristic of synovial chondromatosis, in which diuse opaci®cation, often with a ring-like cartilaginous structure, will make the diagnosis straightforward. When enchondral ossi®cation has occurred in some of the separate osteochondral bodies, the diagnosis is easier. Diuse opaci®cation caused by iron is characteristic of the recurrent bleeding disorders, including haemophilia, Christmas disease and PVNS. All of these conditions, when examined on magnetic resonance imaging, show a low signal on T1-weighted sequences, with the degree of `blackness' increasing on T2 weighting. A similar appearance is seen with amyloid deposition.
Figure 22. Radio-opaque synovium. A lateral view of the knee demonstrates densely opaque synovium in the suprapatellar pouch and behind the femoral condyles in a patient with haemophilia.
Radiolucent synovial swelling Radiolucent synovial swelling suggests strongly the presence of fat in the joint itself and that lipoma arborescens may be diagnosed (Figure 23). FURTHER INVESTIGATION The purpose of this chapter was to suggest an approach to the plain radiograph that was often the entry point by which the investigating clinician entered upon a dierential diagnosis of a non-in¯ammatory osteoarticular disorder. Hence, emphasis
198 I. Watt
Figure 23. Lipoma arborescens. A lateral view of the knee with a little intra-articular contrast medium in the suprapatellar pouch outlines a fronded radiolucent tumour arising from the deep surface of the suprapatellar bursa. The lesion was visible on a plain ®lm but is enhanced in this arthrogram.
has been placed on an approach to plain radiographs, their evaluation and the major dierential diagnoses. On occasions, ultrasound scanning may have been the ®rst investigation. Usually, however, ultrasound is used to show joint eusion, tendonitis, swellings and so forth, and is unlikely to be the ®rst port of call in degenerative joint diseases. Similarly scintigraphy and magnetic resonance imaging are powerful investigational tools but should be reserved for asking speci®c management questions, such as which joints are active, how much cartilage loss has occurred or whether this is synovitis responding to therapy. To quote the author's greatest mentor, `the answer to every radiological problem is another radiological examination. The skill lies in de®ning the problem and correctly selecting the next investigation' (J. Roylance, personal communication). CONCLUSION The objective of this chapter was to set an approach to the evaluation of plain radiographs that may have been taken to investigate a patient presenting with a possibly unusual non-in¯ammatory joint lesion. Of course, one does not know that it is unusual until after the radiograph has been taken and the ®lms have been analysed. It is
Bone disorders: a radiological approach 199
upon this analysis that the chapter is focused. The ®lms should be examined in a critical and logical fashion, and such an approach is summarized. First, good-quality ®lms should be obtained and studied in ®rst-class viewing conditions. Second, the ®lm should be approached in an `organ by organ' step-wise progression: soft tissues, bone, joint space and so on. Third, signs should be elicited as for a clinical examination of a patient, a dierential diagnosis ®nally being put together. Further radiological investigation can be undertaken when clear objectives have been set, selecting the best means of coming to the management decision that emerges from that investigation. REFERENCES 1. Conrozier T, Lequesne MG, Tron AM et al. The eects of position on the radiographic joint space in osteoarthritis of the hip. Osteoarthritis and Cartilage 1997; 5: 17±22. 2. Lequesne MG & Laredo JD. The faux pro®l (oblique view) of the hip in the standing position. Contribution to the evaluation of osteoarthritis of the adult hip. Annals of the Rheumatic Diseases 1998; 57: 676±681. 3. Resnick D. Diagnosis of Bone and Joint Disorders, 3rd edn. Philadelphia: WB Saunders, 1996. 4. Cooke TVD. Pathogenic mechanisms in polyarticular osteoarthritis. Clinics in Rheumatic Diseases 1985; 11: 203±210. 5. Campion GV, McCrae F, Alwan W et al. Idiopathic destructive arthritis of the shoulder. Seminars in Arthritis and Rheumatism 1988; 17: 232±245. 6. Rosenberg TD, Paulos LE, Parker RD et al. The 45 degree postero-anterior ¯exion weight-bearing radiograph of the knee. Journal of Bone and Joint Surgery 1988; 70A: 1479±1482. 7. Axford J, Bomford A, Revell P et al. Hip arthropathy in genetic haemochromatosis: radiographic and histologic features. Arthritis and Rheumatism 1991; 34: 357±361. 8. Adamson TC III, Resnik CS, Guerra J Jr et al. Hand and wrist arthropathies of hemochromatosis and calcium pyrophosphate deposition disease: distinct radiographic features. Radiology 1983; 147: 377±381.
doi:10.1053/berh.1999.0061, available online at http://www.idealibrary.com on
1 Bone disorders: a radiological approach Iain Watt
FRCP, FRCR, FFR
Consultant Clinical Radiologist Department of Radiology, Bristol Royal In®rmary, Bristol BS2 8HW, UK
The radiological interpretation of bone and joint disease has emerged as a subspecialty in its own right. This chapter oers simple guidelines for the approach to a plain ®lm examination that has been taken in a patient with a non-in¯ammatory arthritis. It does not discuss what ®lms should be taken or when. Instead, it focuses on what to look for and where. Emphasis is placed on unusual features that may trigger the thought `This doesn't look like ordinary osteoarthritis.' One does not know, of course, that the patient has an unusual cause of arthritis until the radiograph has been taken. Hence the radiographs of all patients deserve to be analysed properly, for only then will the unusual not become the undiagnosed. Key words: radiology; imaging; bone and joint disease; basic principles.
The ®rst port of call after a patient has presented with arthritis or arthralgia is frequently the radiology department in order to obtain a plain radiograph of the presenting joint or joints. The purpose of this chapter is to discuss an approach to the interpretation of such radiographs and to suggest, where relevant, clues for further investigation. Thus, when faced with an X-ray of a problematic joint, what should one look for? It is as important to remember to search a radiograph intelligently as it is to elicit physical signs on the clinical examination of a patient. Naturally, the ®nding of one physical sign leads to the search for other associated ®ndings. With a radiological examination, the same logic should apply. Thus, an examination may start with the soft tissues and move to the texture of the bone, joint capsule and synovium, the interbone distance (or `joint space width') and the subchondral bone itself. It is axiomatic that in order to do this, one must have high-quality radiographs. The radiographs must be of sucient penetration to include bony texture and bony details, while at the same time not `blacking out' soft tissue structures. If one is to proceed to further investigation, a logical progression is necessary. 1. 2. 3. 4.
What is it that the enquiring clinician wishes to know about the patient? How will it in¯uence his or her management decisions? What is the optimum means of ®nding out the information required? Will the therapy that is consequent upon the investigation make a dierence to the patient's outcome? The cost±bene®t relationship here is crucial, remembering that radiation carries a small but de®nite hazard and intravascular contrast medium administration a risk of death.
Finally, it is important to emphasize that one should treat patients rather than radiological images. Notoriously, symptoms and signs do not correlate well with 1521±6942/00/020173+27 $35.00/00
c 2000 Harcourt Publishers Ltd *
174 I. Watt
radiographic evidence and indeed vice versa. Thus, the ®nding of an apparently `degenerate' joint on an X-ray does not mean that the patient has symptoms or even necessarily abnormal physical signs. WHERE TO LOOK ON THE FIRST RADIOGRAPHS The following text will be divided up under a series of headings, but it is important to remember that any joint is a whole organ comprising a number of structures just as a patient is, essentially, a collection of organ systems. Thus, one would start by examining the whole patient (or joint) in order to obtain an overall impression of him or her and their disease. Then, as in clinical examination, one examines the organ systems independently. Thus, the subject headings that follow include: 1. 2. 3. 4. 5. 6.
Soft tissues Bone periosteum Bone (including changes in density) Joint space abnormalities Subchondral bone Synovium.
It is important to emphasize that the distance between two bone ends on a radiograph contains material that is unknown, since hyaline cartilage, joint ¯uid and indeed pannus are non-opaque. To assist in this matter, a number of joints, particularly the knee, bene®t from being examined in weight-bearing, but even then one does not have a full assessment of all the articular surfaces. Other joints, for example the hand and ankle, do not bene®t from weight-bearing ®lms. There is some suggestion that early degrees of cartilage thinning in the hip may be better assessed on weight bearing-®lms.1 Certainly, the dysplastic hip is better evaluated by the false pro®le view.2 FIRST IMPRESSIONS ARE IMPORTANT The clinician should ®rst look for obvious morphological changes. Skeletal dysplasias are extremely rare and dicult to classify, and the reader is referred to a standard textbook for classi®cation.3 Generalized dysplasias are usually obvious, with gross morphological changes (as in achondroplasia or pseudoachondroplasia). However, in some cases, such as minor expressions of multiple epiphyseal dysplasia or spondyloepiphyseal dysplasia, the changes can be subtle. Anteroposterior views of the hips, knees and hands, together with lateral views of the thoracolumbar spine, will usually be sucient to identify and classify a dysplasia. While a single osteochondroma is usually easy to diagnose, multiple osteochondromata in diaphyseal aclasia may show bizarre moulding abnormalities (Figure 1). On the other hand, local changes are relatively common and include, for example, greater or lesser degrees of dysplastic disease of the hip and similar changes in the knee described as part of the spectrum of knee osteoarthritis.4 In the former, the femoral head is, to a greater or lesser extent, uncovered, the lateral opening of the acetabulum is increased and the fovea high and larger than usual. In osteoarthritis, the femoral head may be laterally placed, with subarticular cysts in the labrum, rather than the
Bone disorders: a radiological approach 175
Figure 1. Diaphyseal aclasia. (A) A frontal radiograph of both knees demonstrates multiple metaphyseal abnormalities, all individually classical of an osteochondroma. Note, however, that in spite of considerable metaphyseal abnormality, the joint surfaces are normal. (B) Pelvis. Again, note that there are considerable changes, particularly in the femoral necks and intertrochanteric regions. The hip joints themselves are essentially normal.
acetabular roof (Figure 2). In the latter, the femoral condyles exhibit valgus dysplasia, such that the joint line no longer runs parallel to the ground on weight-bearing ®lms. One may also have a clue about pre-existing disease by looking for evidence of local growth changes, for example the relative enlargement and squaring of former epiphyses that is a characteristic feature of juvenile overgrowth and early metaphyseal fusion (Figure 3). This occurs in a number of conditions, including haemophilia (or other recurrent bleeding disorders), old septic arthritis and juvenile idiopathic arthritis. In addition, bones may be moulded abnormally as a result of a whole number
176 I. Watt
Figure 2. Dysplastic hips. (A) Achondroplasia. Note the considerable shortening and ¯attening of the femoral head and neck, and the secondary remoulding of the acetabulum. The acetabular roof is also abnormally wide. (B) Idiopathic. In this example, the femoral head is partially uncovered and has migrated laterally. Note the loss of cartilage thickness superiorly, the circumferential osteophytosis and the elevation of a fragment of bone from the lateral acetabular margin, the so-called Eggers' cyst.
or conditions, from Paget's disease (Figure 4) in the elderly through to ®brous dysplasia in the younger patient. One should become aware of possible artefacts on the radiographs. These include ®lm faults in processing, gown or clothing folds, and buttons and so on that produce bizarre lines or appearances. A knowledge of radiological anatomy is necessary to avoid misinterpreting overlying shadows such as femoral condyles on somewhat oblique projections. Radio-opaque `foreign' bodies are seldom a problem, although the clinician should distinguish intellectually between a separate osteochondral body, as in osteochondritis dissecans, and a true `foreign' body, for example a bullet. Furthermore, it should be remembered that separate osteochondral bodies may not be loose. They are frequently embedded in the synovium even when they are detached from their site of origin. One may also gauge the degree of bone response in so-called degenerative joint disease. Hypertrophic changes, characterized by abundant osteophytosis, sclerosis and preserved bone density, are characteristic of the bone-forming group of disorders (Figure 5A). These include Forestier's disease (or diuse idiopathic skeletal hyperostosis; DISH) and so-called pyrophosphate arthropathy (hypertrophic osteoarthritis associated with the presence of calcium pyrophosphate dihydrate crystals; CPPD). On the other hand, some variants of degenerative disease are extremely atrophic, with very little evidence of bone response. This is particularly the case with the rapidly destructive variant of osteoarthritis seen in the elderly female, usually in the shoulder5 or hip (Figure 5B), or the more aggressive in¯ammatory erosive osteoarthritis of the hands. The former is, of course, associated with abundant hydroxyapatite crystal formation (BCP) in joint ¯uid. This is, however, no longer considered to be an
Bone disorders: a radiological approach 177
Figure 3. Haemophilia. A lateral view of the knee demonstrates typical overgrowth of the former epiphyses and cartilage thinning. A frontal view would have demonstrated widening of the intertrochanteric notch. The features are typical of juvenile arthritis but are entirely non-speci®c in terms of aetiology.
aetiological factor.5 Neuropathic joints can produce both hypertrophic and atrophic features. A classical cause of atrophic destructive arthritis associated with neuropathy is syringomyelia, the shoulder and elbow being at particular risk. In diabetic osteoarthropathy, a combination of hypertrophic changes with frank bone destruction usually occurs. This is the most common cause of hypertrophic degenerative arthritis with frank destruction, save for superimposed infection in a pre-existing hypertrophic degenerative joint. Degenerative arthritis secondary to another prior arthropathy can be gauged in a number of ways: ®rst, the growth disparities as summarized above, and second, the relative paucity of reparative change as opposed to diuse hyaline cartilage loss (Figure 6). The concept of reparative change is important. Taken at its most simplistic, it is the case that osteophytosis is rare in an in¯ammatory arthritis such as rheumatoid disease, largely because osteophytes are formed by the enchondral ossi®cation of chondrophytes. As rheumatoid disease destroys cartilage, there are few viable chondrocytes. When the rheumatoid disease becomes quiescent, residual cartilage can again hypertrophy and develop chondrophytes, and hence osteophytes. It is thus reasonable to see osteophyte and sclerosis formation in a patient who has had rheumatoid disease as being a reparative rather than a degenerative phenomenon.
178 I. Watt
Figure 4. Paget's disease of the hip. Note the sclerotic, expanded nature of the femoral head and neck, with cortical thickening. There is also, however, a virtually normal joint space width superiorly. Paget's disease almost invariably commences from an articular surface and spreads along the aected bone.
Figure 5. The spectrum of osteoarthritis. (A) Classical hypertrophic osteoarthritis. Note the considerable osteophytosis and hyaline cartilage thinning, yet the absence of obvious bone destruction. (B) Atrophic destructive osteoarthritis. Here there is subchondral sclerosis, obvious destruction of the femoral head and acetabular roof, and superolateral migration of the hip joint accordingly.
Finally, if a scoring system is used to evaluate longitudinal or cross-sectional studies in patients with degenerative arthritis, what system should it be? There is no correct answer since it, as always, depends on what the investigating clinician wishes to know about the individual patient. There are a number of available global scores, but it may be far more relevant to look at individual radiological signs, for example the preservation of interbone distance.
Bone disorders: a radiological approach 179
Figure 6. Old rheumatoid disease with `secondary' osteoarthritis. Note a concentric hyaline cartilage loss in both hips, the relative paucity of sclerosis, osteophytosis, and superomedial migration caused by `soft' bone following an in¯ammatory arthropathy.
SOFT TISSUES As previously emphasized, adequate quality radiographs of joints require that the soft tissues be visualized. It is easy to look for such phenomena as varicose veins and soft tissue lumps, as in a patient with von Recklinghausen's disease (neuro®bromatosis), if the soft tissues are preserved on the ®lm (Figure 7). It is obviously much easier to see soft tissue lesions using other imaging modalities, such as ultrasound or magnetic resonance imaging, but these are seldom used as the ®rst-line investigation in a possible degenerative joint. Fat-containing masses, simple or malignant lipomata, are often visualized by their relative transradiancy compared with muscle. The fat lines that surround many normal tissue planes are also vital. The ill-de®nition of a normal tendon or swelling around it can diagnose tenosynovitis or tendon trauma. The diagnosis of a joint eusion, and synovitis, can be easy by examining fat planes (see below). Calci®cation Calci®cation in soft tissues occurs in a number of important manifestations including: 1. arterial calci®cation, as occurs in atherosclerosis, diabetes mellitus, when peripheral arterial calci®cation is particularly common, and median necrosis; 2. venous calci®cation, in varicose vein disease, venous thrombosis and arteriovenous malformation; 3. calci®cation in subcutaneous tissue in patients with connective tissue disorders, particularly scleroderma and CREST syndrome; more localized lesions may occur in calcinosis cutis; 4. calci®cation, resulting from BCP, is a classical feature of acute calci®c peri-arthritis, typical sites being the rotator cu, gluteal tendons and ¯exor carpi ulnaris; 5. chondrocalcinosis (see below);
180 I. Watt
Figure 7. Neuro®bromatosis. Note, in the soft tissues, subtle textural changes comprising nodular masses and fatty replacement, with some areas of calci®cation resulting from a plexiform neuro®broma. Note also the gross expansion and thinning of the obturator ring, again caused by neuro®bromatosis, together with moulding abnormalities of the femoral shaft itself.
6. dystrophic soft tissue calci®cation in hyperparathyroidism, renal failure or tumoral calcinosis; 7. the early stages of evolution of a haematoma or, more rarely, a soft tissue sarcoma; 8. parasitic infections, leprosy and thermal injury. Ossi®cation Ossi®cation in soft tissues is, on the other hand, extremely uncommon and may lead to a consideration of either a developmental condition, for example ®brodysplasia (myositis) ossi®cans progressiva, in which fascial planes become ossi®ed (Figure 8), or focal dystrophic ossi®cation, as in a previous haematoma. On very rare occasions, soft tissue ossi®cation may indicate a bone-forming malignancy. BONE PERIOSTEUM Periosteal new bone formation has a wide dierential diagnosis. It is important clearly to distinguish between true periosteal new bone formation and bone formation at speci®c sites, such as osteophytes and enthesophytes.
Bone disorders: a radiological approach 181
Figure 8. Fibrodysplasia ossi®cans progressiva. Classical soft tissue ossi®cation in fascial planes, eectively causing an extra-articular arthrodesis. Note that this is not calci®cation but bone with a trabecular pattern and architecture. Secondary subluxation has occurred at the hip joint. The process here is obviously chronic since the ¯oor of the acetabulum is thick, indicating that the femoral head has subluxed superolaterally for a long time.
Osteophytes Osteophytes develop at the site of enchondral ossi®cation of chondrophytes and are, by de®nition, limited to synovial joints. The sites are classical ± the margins of synovial joints ± or actually on the articular surface, as `stud' or `button' osteophytes. As indicated above, the degree of osteophytosis may re¯ect a more generalized `boneforming' quality of the patient, and abundant osteophytosis may invite a careful search for the presence of crystals in joint ¯uid. Enthesophytes Enthesophytes, on the other hand, classically arise at musculotendinous origins and insertions. They may be subdivided into those in which it is possible to see an obvious erosive component (in which case the possibility of a systemic disorder such as ankylosing spondylitis should be borne in mind) and non-erosive examples, which have a wider dierential diagnosis. Most non-erosive enthesophytes are associated with lowgrade trauma or obesity, but they may be part of a widespread generalized disorder
182 I. Watt
such as Forestier's disease (or DISH). Various metabolic disorders should also be considered, including: 1. 2. 3. 4. 5.
hypoparathyroidism; pseudohypoparathyroidism; sex-linked hypophosphataemic rickets; ¯uorosis; obesity, often in association with hyperuricaemia and non-insulin-dependent diabetes mellitus.
Periosteal new bone formation In the adult skeleton, this presents a wide dierential diagnosis, but for brevity it can be subdivided into local and generalized types. Local new bone formation Local new bone formation may occur in association with trauma and the presence of a local tumour, either primary or secondary. Periosteal new bone formation classically occurs in association with infection, but in this instance, the patient will present with in¯ammatory rather than degenerative features. The more aggressive the cause of the new bone formation, the more ill de®ned and layered will be the appearance. Ill-de®ned new bone formation implies aggressive change, whereas chronic conditions, such as varicose vein disease in the leg, will produce coarse, rather mature-looking bone formation. In the arthritic population, particularly when angular deformity and/or osteopenia are present, the possibility of a stress or insuciency fracture (Figure 9) should be considered. In this, a band of illde®ned sclerosis, crossing the major long bone trabeculae at right angles, may accompany localized periosteal new bone formation. Finally, melorheostosis, in which `dripping candlewax' sclerosis occurs along a bony sclerotome, should be remembered. The importance of this condition lies in the cutaneous manifestations and the associated symptomatology, the so-called linear scleroderma. Generalized periosteal new bone formation Such generalized bone formation by de®nition implies a systemic disorder. In the younger skeleton, one must consider leukaemias and lymphomas, but in the older patient, especially a smoker, one must always consider the possibility of hypertrophic (pulmonary) osteoarthropathy (HOA). Here, the periosteal new bone formation is classically aligned along the diaphyses, sparing the former epiphyses (Figure 10). HOA also has a wide dierential diagnosis, including: 1. Pulmonary: bronchogenic carcinoma, pleural mesothelioma, abscess, bronchiectasis and metastasis; 2. Cardiac and vascular: cyanotic congenital heart disease, bacterial endocarditis or an infected arterial graft; 3. Intestinal and abdominal: portal and biliary cirrhosis, and in¯ammatory bowel disease; 4. Intra-abdominal malignancy: including lymphoma and gynaecological primary tumours.
Bone disorders: a radiological approach 183
Figure 9. Three example of insuciency or stress fracture. (A) Os calcis fracture in a ballerina. Note the band of sclerosis crossing the major trabeculae of the os calcis, extending from the Achilles tendon to the plantar fascia. (B) The distal ®bula. Here a localized zone of periosteal new bone formation is demonstrated laterally. It is just possible to discern a faint band of sclerosis crossing the medullary canal at this level. (C) Involvement of the lateral tibial plateau. Note the ill-de®ned sclerosis, as well as the radiolucency approximately 1.0 cm in from the joint margin, within which there is an area of curvilinear opaci®cation representing the imploded cortex within the insuciency fracture. The appearances are highly reminiscent of avascular necrosis.
The dierential diagnosis of HOA also includes thyroid acropachy, which characteristically involves the medial aspects of the shafts of the ®rst and second metatarsals and metacarpals, and pachydermoperiostitis, in which the coarse periosteal new bone formation is associated with obvious clinical features of subcutaneous thickening. One of the manifestations of the SAPHO syndrome (Synovitis, Acne, Pustulosis, Hyperostosis and Osteitis) is the presence of sclerotic mass lesions in long bones.
184 I. Watt
Figure 10. Hypertrophic (pulmonary) osteoarthropathy. Gross pathology is demonstrated along the radius and ulna (A) and hands (B).
Hence, a search should be made for the appropriate skin rashes and manifestations on the anterior chest wall. BONE Bone sclerosis Sclerosis in bone adjacent to a joint that has been X-rayed may fall into a number of categories.
Bone disorders: a radiological approach 185
Focal lesions that are generalized in the skeleton Multiple bone islands are by far the most common cause of this. These measure some 2±3 mm in diameter, with a discrete zone of radiolucency surrounding them. Innumerable bone islands occur in osteopoikilosis, a condition thought to be unassociated with any signi®cance in terms of joint disease save for the classical clustering of the lesions around the joints (Figure 11). Multiple bone islands should be disregarded in terms of their clinical signi®cance. Larger solitary lesions do occur as osteomata, but these are largely con®ned to the paranasal sinuses. A less common, innocent dysplasia is osteopathia striata, in which streaks of sclerotic medullary bone occur around the joints.
Figure 11. Osteopoikilosis. (A) Hand. (B) An oblique projection of the left hip. Both demonstrate classical multiple bone islands clustered around the joints. The hip lesion also shows some striation in the proximal femur, probably representing additional osteopathia striata.
Focal lesions that are localized to one site The two true focal benign tumours that occur are osteoid osteoma and osteoblastoma. The former characteristically involves individuals in their 20s and 30s, presenting as a small focal zone of radiolucency within which a smaller sclerotic lesion may be seen. Abundant consolidated cortical and periosteal new bone formation occurs with the peripherally situated lesions (Figure 12). When next to a joint, osteoid osteoma may present as an in¯ammatory arthritis. Osteoblastoma, which may for simplicity be regarded as a large variant of osteoid osteoma, classically occurs in the spine and presents with the sclerosis of a posterior element. Focal medullary lesions that resemble bone formation include the so-called cartilage rest or bone infarct, in which serpiginous ill-de®ned calci®cation occurs in the distal diaphysis of a long bone. These too are usually disregarded in terms of signi®cance, although in the older patient the clinician must beware of a growing cartilage lesion in the medulla, which probably will represent a chondrosarcoma (see below). Focal sclerosis occurs also in any old benign ®bro-osseous lesion of bone, for example a previous fracture or osteomyelitis. An appropriate history should be available.
186 I. Watt
Figure 12. Osteoid osteoma. These lesions are usually typical and classical, but on occasions they arise in an unusual area of anatomy. Here, an osteoid osteoma involves the terminal phalanx of a thumb and presented in a patient who complained of joint pain. Note the ill-de®ned radiolucency, within which a central dense opacity is projected on the lateral border of the terminal phalanx.
Focal malignant lesions raise the possibility in the younger skeleton of osteosarcoma. Osteosarcoma is relatively easy to diagnose but is frequently overlooked in its earlier stage. The lesion classically occurs in teenagers or individuals in their early 20s, with eccentric bone destruction and bone formation. Periosteal new bone formation is frequently present with the so-called Codman's triangles, associated with a wellde®ned soft tissue mass (Figure 13). In the slightly older subject, paraosteal osteosarcoma presents a slightly more dicult diagnosis. Here, the lesion is characteristically a mushroom-shaped mass of apparently benign bone that is applied to the cortex of a long bone, usually a femur or a humerus. The dierential diagnosis of this includes old trauma. In the older skeleton, solitary sclerotic lesions in medullary bone raise the possibility of metastasis, the most common causes being prostatic carcinoma in the male and breast carcinoma in the female. There are no speci®c features of a sclerotic metastasis to indicate the location of the primary tumour. The dierential diagnosis includes mastocytosis, lymphomas (particularly non-Hodgkin's lymphomas) and the rare sclerotic myeloma. Generalized bone sclerosis This presents a wide dierential diagnosis. Increased density may occur in the cortex, the medulla or both. Medullary sclerosis in the adult skeleton raises three important dierential diagnostic groups: 1. haematological conditions, including the haemoglobinopathies, myelo®brosis and myelodysplasias; 2. metabolic disorders, such as oxalosis, renal osteodystrophy, hypervitaminosis A, hypervitaminosis D and heavy metal poisoning; 3. malignant diseases, including non-Hodgkin's lymphomas, Hodgkin's disease and widespread metastatic disease. Thus, to summarize, generalized medullary sclerosis is always bad news.
Bone disorders: a radiological approach 187
Figure 13. Osteosarcoma. Arising (A) in the proximal humerus and (B) in the medial tibial plateau. The humeral lesion is classical, with a large soft tissue mass, mixed bone formation and bone destruction. There is also tumour bone formation within the mass itself. The second lesion is more subtle. Here, there is undoubtedly destruction under the tibial spines. Notice also the bone formation in the medial plateau itself. Both patients presented with chronic joint pain.
Cortical sclerosis This should suggest, ®rst, a number of dysplasias, the most important being osteopetrosis tarda in the adult. Mixed cortical and medullary sclerosis occurs in a number of dysplasias, including Englemann±Camurati's disease (Figure 14). More commonly, bone expansion and sclerosis raise the possibility of Paget's disease (see Figure 4 above) and haemangioma, particularly when a vertebral body is involved. Bone radiolucency Focal radiolucent lesions Focal radiolucent lesions in long bones adjacent to joints may be dierentiated into those involving the cortex or the medulla, and those subadjacent to the joint. Cortical radiolucencies are common in the younger skeleton, well-de®ned, slightly blister-like lesions being characteristic of so-called ®brous cortical defects. Larger lesions, the non-ossifying ®bromata, often present with fractures, particularly of the tibia. However, both conditions are closely associated with ®brous dysplasia and a number of important variants, including ossifying ®broma. These occur in the older skeleton and may cause diculties in dierential diagnosis. Broadly speaking, however, soap bubble-like lesions involving the cortex and medulla, particularly when associated with the so-called ground glass texture, should raise the possibility of a benign ®bro-osseous lesion of bone. In the older skeleton, frankly malignant lesions such as ®brosarcomata, malignant ®brohistocytomata and metastases are characterized
188 I. Watt
Figure 14. Englemann±Camurati's disease. A radiograph of both femora shows gross bone expansion and marked medullary and cortical sclerosis. Notice, however, that the hips are relatively normal.
by ill-de®ned permeative bone destruction. When bone destruction speci®cally involves the cortex of a long bone, such as the femur, the primary is often carcinoma of the bronchus. Purely medullary lesions suggest either metastasis or myeloma. However, in older patients, a chondroma is common, a chondrosarcoma more rarely being seen. The former are frequent in the hands; a relatively well-de®ned, lytic, expansile lesion is typically shown, with ring-like foci of calci®cation. Chondrosarcoma is notoriously dicult to diagnose, except in some speci®c locations such as when a large calci®ed mass arises from a ¯at bone, or when frank bone destruction has occurred (Figure 15). All cartilage tumours should be treated with suspicion, especially if the patient is locally symptomatic. The focal lesions of myeloma are quite dierent, appearing to be well de®ned, punched out and well circumscribed, without zones of sclerosis. The `raindrop' appearance is typical, with scalloping of the cortex from within. Peri-articular lesions raise a dierential diagnosis and may present, of course, with joint pain. The peri-articular lesions include those listed in Table 1. In the mature skeleton, osteosarcoma is relatively rare, but giant cell tumours are not. These are characteristically placed eccentrically, immediately subarticularly, and are purely radiolucent, with a narrow zone of transmission to normal bone. The knee
Bone disorders: a radiological approach 189
Figure 15. Chondrosarcoma of the hip. Most chondrosarcomata are obvious in so far as they present as large expansile lesions that contain a variable amount of calci®cation. This lesion however is at another classical site and demonstrates more subtle changes. Note the radiolucency involving the ¯oor and roof of the acetabulum, with slight bone expansion.
is a particularly common site (Figure 16), as is the wrist. Histologically, the brown tumour of hyperparathyroidism cannot be distinguished from the idiopathic giant cell tumour (Figure 17). Hence when one is found, metabolic bone disease should be considered as a cause. The joint-related lesions are obviously adjacent to the articular surfaces or bare area and have a sclerotic margin. Thus, large subchondral cysts or geodes (as in rheumatoid disease, when they are characteristically deep to the tibial spines) are obviously benign in terms of their radiological features. These include the ovoid or elliptical outline, the well-de®ned sclerotic margin and the narrow zone of transition. Percutaneous Table 1. Major causes of subarticular lytic lesions. X-ray ®ndings
Cause
Joint-related erosions
Rheumatoid and seronegative diseases
Subarticular cysts
Osteoarthritis, haemochromatosis, Wilson's disease
Subarticular geode
Usually rheumatoid disease, classically of the knee (tibial spines), hip (superior neck of femur) and hand (metacarpal heads)
Synovial tumours
Synovial chondromatosis, pigmented villonodular synovitis
Mass eects
MSUM tophi, amyloid deposits
Tumours
Giant cell tumour, aneurysmal bone cyst, intra-osseous ganglion (usually the ®bula), simple ®bromyxoma (carpus), osteosarcoma
Avascular necrosis
Idiopathic in the knee (sudden onset); also hip and lunate
MSUMmonosodium urate monohydrate.
190 I. Watt
Figure 16. Giant cell tumour of the patella. A classical giant cell tumour is demonstrated, but in an unusual location. Note that it is immediately subarticular, eccentric and lytic in appearance.
injection into one such lesion usually reveals a `synovial' lining and retrograde communication with the joint. Intra-osseous ganglions involve the proximal ®bula or distal ®bula related to the proximal or distal tibio®bular articulations. Cystic lesions secondary to synovial pathology, such as synovial chondromatosis or pigmented villonodular synovitis (PVNS), usually involve those areas of the joint in contact with synovium (primarily the bare area) on both sides of the articulation (Figure 18). Tangential views of these lesions show that they are essentially pressure defects, indenting the bone from the outside. The margins are well de®ned and give the appearance of chronicity. Synovial chondromatosis usually exhibits calci®cation in the synovium or calci®ed and/or enchondrally ossi®ed separate bodies. PVNS is one of the causes of chronic iron deposition in synovium, causing it to be relatively opaque on a plain ®lm. Bone lucency ± generalized It is important to distinguish between bone atrophy, as in prolonged non-use, and osteoporosis from active bone resorption. In the latter, there is evidence of intracortical tunnelling, initial radiolucency being shown in the immediate subchondral areas and at the site of former metaphyses. The distinction between medullary bone and the cortex becomes lost, and in very vigorous hyperparathyroidism, for example,
Bone disorders: a radiological approach 191
Figure 17. Hyperparathyroidism. This 35-year-old lady presented with a history of chronic bone and joint pain. Notice how the skeleton is osteopenic and shows marked intracortical tunnelling. Note also vague areas of ill-de®ned radiolucency in the femoral neck and laterally in the acetabular roof (A). These are caused by brown tumours. (B) A radiograph of the phalanges taken following trauma a year earlier shows classical intracortical tunnelling on the radial aspect of the middle and proximal phalanges, a lack of distinction of the corticomedullary junction and a somewhat simpli®ed trabecular pattern.
periosteal new bone formation becomes obvious. In bone atrophy or osteoporosis, the cortex is simply thinned, the trabecular pattern reduced and the bone radiolucent. Juxta-articular osteopenia in the in¯ammatory arthritides tends to resemble the latter but in a focal zonal position around the involved joints. In algodystrophy, or the various subsets thereof, the features of acute bone resorption are found, but on a
192 I. Watt
Figure 18. Pigmented villonodular synovitis (PVNS). A radiograph of the hip demonstrates cartilage thinning caused by chronic synovitis, but note, however, the multiple well-de®ned subarticular and peri-articular radiolucencies. These suggest a chronic synovial mass or depositional state. The appearances are entirely nonspeci®c but suggest either PVNS or synovial chondromatosis. Were the radiolucencies more predominantly intraosseous, amyloidosis would be an alternative diagnosis.
regional or transitory basis. Furthermore, the radiolucency is, at least initially, localized across former metaphyses and subarticular. If a patient presents with joint pain and evidence of high bone turnover, it is worth looking to see whether chondrocalcinosis or focal mass lesions are present to suggest hyperparathyroidism. As indicated above, the brown tumour of hyperparathyroidism is histologically indistinguishable from an idiopathic giant cell tumour. A loss of bone density will generally also occur in osteomalacia and renal osteodystrophy, when the features of hyperparathyroidism may not be as overt. A coarsened trabecular pattern, evidence of bone softening and the presence of cortical insuciency fractures (Looser's zones) should be sought. Other causes are listed in Table 2.
Table 2. The major causes of osteopenia. Findings
Cause
Generalized osteopenia
Osteoporosis, disuse atrophy, protein de®ciency, homocystinuria, myelomatosis, chronic anaemias, disseminated malignancy, endocrine disorders (adrenal and parathyroid in particular), osteomalacia and rickets Algodystrophy, in¯ammatory arthropathy, tumours, acute lytic Paget's disease, subarticular joint causes
Localized osteopenia
Bone disorders: a radiological approach 193
JOINT SPACE ABNORMALITIES An evaluation of joint space width is dicult, for the reasons already discussed. The interbone distance may comprise hyaline cartilage, joint ¯uid and so on, and such tissues are not visualized on a plain radiograph. Thus, one is in danger of assuming that the interbone distance actually corresponds to the hyaline cartilage thickness. As indicated above, weight-bearing ®lms in some circumstances will help to ensure that the interbone distance really does measure hyaline cartilage thickness. However, granted that the joint surface is in many cases a complex curved structure, even these radiographs will not fully permit a ®ne assessment of cartilage thickness. Other modi®cations have been advocated, including partial knee ¯exion in the posteroanterior view6, with or without other modi®cations such as computer-assisted analysis. For clinical purposes, the interbone distance can be assessed in three ways in order to approach a possible dierential diagnosis of non-in¯ammatory joint disease. The cartilage may appear thick Classically, an increase in the interbone distance occurs in acromegaly (Figure 19). It is one of the earliest and most sensitive radiological features of the disease, being more reliable than looking for thickness changes in other soft tissues (such as the heel pad), terminal tufting or coarsening of the skin thickness. An increased thickness of hyaline cartilage also occurs in hypothyroidism. Cartilage may appear relatively thick in some cases of early joint eusion, when, as indicated above, the increased interbone distance results from joint eusion rather than cartilage. In addition, apparently thick hyaline
Figure 19. Acromegaly. An X-ray of the hand demonstrates classical enlargement of the joint spaces resulting from the thickened hyaline cartilage of these small joints.
194 I. Watt
cartilage occurs in joints that are eroded by those arthropathies which are largely non-in¯ammatory, such as PVNS, synovial chondromatosis, gout and multicentric reticulohistiocytosis. The cartilage may seem thin A diuse loss of hyaline cartilage thickness occurs in many conditions that essentially `poison' it. Thus, the result of rheumatoid disease (see Figure 6 above), juvenile idiopathic arthritis, septic arthritis and the other in¯ammatory arthritides may all be the same in so far as cartilage will be thinned. The ®nding is, therefore, non-speci®c. However, when diuse hyaline cartilage thinning is found, the possibility of a current or previous in¯ammatory arthritis should be considered. A diuse thinning of hyaline cartilage, particularly in the interphalangeal joints of the hands and knees, is a feature of relapsing polychondritis. The cartilage may be calci®ed The usual cause of hyaline cartilage calci®cation is the deposition of CPPD, although BCP may be found in some joints in the connective tissue disorders. CPPD deposition is an age-related phenomenon and not necessarily associated with any `disease' in the strict meaning of the word. Premature chondrocalcinosis occurs in hyperparathyroidism, haemochromatosis and those families which are genetically susceptible. A shedding of crystals does occur, presenting as the in¯ammatory acute synovitis often called pseudogout. The relationship between chondrocalcinosis and the evolution of the hypertrophic variant of osteoarthritis is strongly debated (see above). The current prevailing view is that a strictly causal relationship does not exist. Chondrocalcinosis may be ¯orid in hyperparathyroidism without the superadded development of a destructive arthritis. Haemochromatosis has a high incidence of CPPD deposition that may manifest prior to any form of speci®c arthropathy.7 Marked chondrocalcinosis, especially in the intervertebral discs and when associated with early and ¯orid disc degeneration, should suggest ochronosis. The crystal found here is BCP.
SUBCHONDRAL BONE Subchondral cysts The relationship between hyaline cartilage and subchondral bone is intimate. There are some clues that may assist the dierential diagnosis in arthritis. A number of conditions are characterized by multiple subchondral `cystic' lesions stretching across the articular surface. There include haemochromatosis, osteoarthritis associated with CPPD and Wilson's disease. Extensive subarticular cystic change can occur also in severe hyperparathyroidism. In haemochromatosis, one of the important ®ndings is multiple subchondral radiolucencies arranged rather like a string of beads across the metacarpal heads and at sites where arthritis is seldom found, for example the ankle joint (Figure 20A). A disease-speci®c form of cartilage pathology occurs in haemochromatotic hips when the zone of provisional calci®cation loses its naturally corrugated form and the cartilage simply peels o at this layer.7 Radiographs show ill-de®ned subchondral radiolucency reminiscent of avascular necrosis (Figure 20B). The latter, however, classically manifests
Bone disorders: a radiological approach 195
Figure 20. Haemochromatosis. (A) Ankle. Note the multiple subchondral cystic lesions stretching across the articular surface. The ankle is rarely involved in osteoarthritis or rheumatoid disease. Such a ®nding should suggest strongly the possibility of haemochromatosis. (B) A radiograph of another 45-year-old male's hip. Notice the ill-de®ned subchondral radiolucencies across the femoral head.
wedged-shaped areas of abnormality and the preservation of the overlying cortex as it receives vitality from the hyaline cartilage. Hypertrophic osteoarthritis associated with CPPD is also characterized by multiple subchondral radiolucencies or cysts. A distinction between the lesions of haemochromatosis and these is subtle8 and not easy to discern in the individual patient. However, the hip features of the latter do seem disease speci®c for haemochromatosis. Subchondral sclerosis Ill-de®ned sclerosis, especially when found adjacent to a `degenerative' joint, suggests trabecular microfractures and callus formation. This implies bone failure and may presage rapid joint failure. Avascular necrosis This chapter can only highlight a few pertinent concepts. The initial ®ndings in avascular necrosis depend on the site of involvement. In the hip, the lesion is initially shown as a subtle change in the architecture of the femoral head trabeculae as a result of callus formation and microfractures. This precedes the development of the typical wedge-shaped defect and classical subchondral lucency or `vacuum' sign. In a metatarsal head, however, prior to collapse and the typical features of Freiberg's infraction, it is usual to see a band of sclerosis 3±4 mm deep to the articular surface, possibly a developing stress fracture. Last, but most important, is the lesion that occurs suddenly in the medial femoral condyle in the middle-aged. The initial sign here is a subtle and enlarging zone of subcortical radiolucency. The cortex subsequently becomes separated, as in the hip, and may implode into the defect beneath it
196 I. Watt
Figure 21. Idiopathic medial femoral condyle necrosis. A classical lesion is demonstrated in the medial femoral condyle. Note the ill-de®ned area of subarticular radiolucency, within which is a linear opacity resulting from the imploded cortex. Chondrocalcinosis is present in the lateral compartment. This is a described association.
(Figure 21). A careful examination of the subchondral plate may make the early diagnosis of these conditions simple, to be con®rmed scintigraphically or by magnetic resonance imaging.
SYNOVIUM An examination of the soft tissues around a joint can be extremely helpful. As already emphasized, it is important to have the soft tissues rendered visible by virtue of the fat planes adjacent to them. What does one look for? Swelling The presence of a joint eusion may frequently be visualized at the metacarpophalangeal, interphalangeal and metatarsophalangeal joints, the wrist, the elbow and the knee. A distension of the suprapatellar pouch indicates the presence of either thick synovium or joint ¯uid. These cannot be told apart, except by using ultrasound or other imaging modalities. In the presence of acute synovitis, however, the outline of the suprapatellar pouch, or other sites, becomes ill de®ned, the degree of ill de®nition paralleling the degree of in¯ammatory change. Soft tissue swelling caused by joint ¯uid or an in¯ammatory synovitis is usually symmetrical. However, an asymmetrical soft tissue swelling should strongly raise the possibility of a depositional disorder, including tophaceous deposits of MSUM, lipid and occasionally a rheumatoid nodule. Asymmetry implies a depositional disorder.
Bone disorders: a radiological approach 197
Opaci®cation Opaci®cation of the synovium is an extremely important sign (Figure 22). Opaque synovium occurs because a high atomic number substance is present in it, the two most common being calcium and iron. The former is characteristic of synovial chondromatosis, in which diuse opaci®cation, often with a ring-like cartilaginous structure, will make the diagnosis straightforward. When enchondral ossi®cation has occurred in some of the separate osteochondral bodies, the diagnosis is easier. Diuse opaci®cation caused by iron is characteristic of the recurrent bleeding disorders, including haemophilia, Christmas disease and PVNS. All of these conditions, when examined on magnetic resonance imaging, show a low signal on T1-weighted sequences, with the degree of `blackness' increasing on T2 weighting. A similar appearance is seen with amyloid deposition.
Figure 22. Radio-opaque synovium. A lateral view of the knee demonstrates densely opaque synovium in the suprapatellar pouch and behind the femoral condyles in a patient with haemophilia.
Radiolucent synovial swelling Radiolucent synovial swelling suggests strongly the presence of fat in the joint itself and that lipoma arborescens may be diagnosed (Figure 23). FURTHER INVESTIGATION The purpose of this chapter was to suggest an approach to the plain radiograph that was often the entry point by which the investigating clinician entered upon a dierential diagnosis of a non-in¯ammatory osteoarticular disorder. Hence, emphasis
198 I. Watt
Figure 23. Lipoma arborescens. A lateral view of the knee with a little intra-articular contrast medium in the suprapatellar pouch outlines a fronded radiolucent tumour arising from the deep surface of the suprapatellar bursa. The lesion was visible on a plain ®lm but is enhanced in this arthrogram.
has been placed on an approach to plain radiographs, their evaluation and the major dierential diagnoses. On occasions, ultrasound scanning may have been the ®rst investigation. Usually, however, ultrasound is used to show joint eusion, tendonitis, swellings and so forth, and is unlikely to be the ®rst port of call in degenerative joint diseases. Similarly scintigraphy and magnetic resonance imaging are powerful investigational tools but should be reserved for asking speci®c management questions, such as which joints are active, how much cartilage loss has occurred or whether this is synovitis responding to therapy. To quote the author's greatest mentor, `the answer to every radiological problem is another radiological examination. The skill lies in de®ning the problem and correctly selecting the next investigation' (J. Roylance, personal communication). CONCLUSION The objective of this chapter was to set an approach to the evaluation of plain radiographs that may have been taken to investigate a patient presenting with a possibly unusual non-in¯ammatory joint lesion. Of course, one does not know that it is unusual until after the radiograph has been taken and the ®lms have been analysed. It is
Bone disorders: a radiological approach 199
upon this analysis that the chapter is focused. The ®lms should be examined in a critical and logical fashion, and such an approach is summarized. First, good-quality ®lms should be obtained and studied in ®rst-class viewing conditions. Second, the ®lm should be approached in an `organ by organ' step-wise progression: soft tissues, bone, joint space and so on. Third, signs should be elicited as for a clinical examination of a patient, a dierential diagnosis ®nally being put together. Further radiological investigation can be undertaken when clear objectives have been set, selecting the best means of coming to the management decision that emerges from that investigation. REFERENCES 1. Conrozier T, Lequesne MG, Tron AM et al. The eects of position on the radiographic joint space in osteoarthritis of the hip. Osteoarthritis and Cartilage 1997; 5: 17±22. 2. Lequesne MG & Laredo JD. The faux pro®l (oblique view) of the hip in the standing position. Contribution to the evaluation of osteoarthritis of the adult hip. Annals of the Rheumatic Diseases 1998; 57: 676±681. 3. Resnick D. Diagnosis of Bone and Joint Disorders, 3rd edn. Philadelphia: WB Saunders, 1996. 4. Cooke TVD. Pathogenic mechanisms in polyarticular osteoarthritis. Clinics in Rheumatic Diseases 1985; 11: 203±210. 5. Campion GV, McCrae F, Alwan W et al. Idiopathic destructive arthritis of the shoulder. Seminars in Arthritis and Rheumatism 1988; 17: 232±245. 6. Rosenberg TD, Paulos LE, Parker RD et al. The 45 degree postero-anterior ¯exion weight-bearing radiograph of the knee. Journal of Bone and Joint Surgery 1988; 70A: 1479±1482. 7. Axford J, Bomford A, Revell P et al. Hip arthropathy in genetic haemochromatosis: radiographic and histologic features. Arthritis and Rheumatism 1991; 34: 357±361. 8. Adamson TC III, Resnik CS, Guerra J Jr et al. Hand and wrist arthropathies of hemochromatosis and calcium pyrophosphate deposition disease: distinct radiographic features. Radiology 1983; 147: 377±381.