THE CLASSIFICATION AND RADIOGRAPHIC EVALUATION OF BONE LOSS IN REVISION HIP ARTHROPLASTY

MANAGEMENT OF BONE LOSS DURING REVISION HIP OR KNEE REPLACEMENT

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THE CLASSIFICATION AND RADIOGRAPHIC EVALUATION OF BONE LOSS IN REVISION HIP ARTHROPLASTY Bassam A. Masri, MD, FRCSC, Eric L. Masterson, BSc, MCh, FRCS (Orth), and Clive P. Duncan, MD, FRCSC

Total hip arthroplasty ranks as one of the most successful operations yet devised and has afforded pain relief and improved quality of life for millions of patients worldwide. As a result of this success, there has been a rapid decline in the number of patients who are willing to adopt alternatives to total joint replacement, such as lifestyle modification, osteotomy, or arthrodesis. Joint replacements of limited useful duration are being performed on ever younger patients at a time when average life expectancy is continuing to rise. It can be expected, therefore, that the need for revision hip arthroplasty will continue to rise for the foreseeable future. Most failed hip arthroplasties present.with some degree of associated bone loss, either in the femur, the pelvis, or both. This may range in degree from inconsequential to almost unsalvageable. The causes of bone loss include bone removed at the original arthroplasty to accommodate the index prosthesis, osteolytic response to particulate debris, fretting of the bone from repetitive movement of a loose prosthesis, fracture, and damage to residual bone stock caused during removal of a failed component. As the number of patients with failed hip arthroplasties has continued to rise, so too has the inventiveness and ingenuity of the' orthopedic community in developing solutions to the prob-

lems of diminished bone stock. Other contributors to this volume will outline in detail the relative advantages and disadvantages of the many techniques that are available. Femoral bone loss may be managed by diaphyseal fixation with cemented or cementless prostheses, metaphyseal fixation with modular titanium devices, impaction allografting or proximal femoral replacement with allograft, or a megaprosthesis. On the acetabular side, the options will include cemented or cementless components, reinforcement rings or cages, and morselized or structural allograft. If progress is to be made in our management of this challenging group of patients, it is imperative that there be a commonality of language in describing the nature and extent of bone loss that has been managed with a given technique. This will allow the selection of inclusion criteria for prospective trials and also will permit comparisons to be drawn between reports of different treatment modalities. Ultimately, this should guide the arthroplasty surgeon as to the most appropriate reconstructive technique to use in a given case, based on a rational review of a more uniform literature. Feinstein has coined the term clinimetrics to describe arbitrary ratings, scales, indexes, instruments, or other expressions that have been created as "measurements" for clinical phenomena that

From the Department of Orthopaedics, The University of British Columbia, Vancouver, British Columbia, Canada THE ORTHOPEDIC CLINICS OF NORTH AMERICA

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cannot be measured in the customary dimensions of laboratory data.12 When we need to evaluate therapeutic interventions, clinimetric details serve to identify and define the material that is being studied. He notes that about 220 different indexes have been proposed solely for assessing the function of joints and patients’ responses in arthritis and other locomotor diseases. Clinimetric information may be hard or soft. The classification systems described in this article are derived from varying combinations of ”hard” paraclinical radiographic data and “soft” information derived from the clinician’s intraoperative interpretation of the extent of bone loss in either the acetabulum or femur. As there is no way of independently validating intraoperative findings after the event, there would be obvious attractions in a classification system for bone loss in revision hip arthroplasty that is derived solely from hard reproducible radiographic measurements. A successful classification system for bone loss in the failed hip arthroplasty that is based purely on interpretation of preoperative radiographs should incorporate the following features:

1. It should be based on plain radiography using views that are easily attainable. Computerized tomography and three dimensional reconstruction may occasionally be informative in complex primary arthroplasty or conversion of an excision arthroplasty to a total joint replacement, but is not useful when a prosthesis is in situ. 2. It should have been validated by documented low rates of interobserver and intraobserver error prior to being accepted into routine usage. A number of recent publications have pointed to significant levels of disagreement in well-established but previously untested orthopedic measurement techniques or classification systems?, 13, 20, 26 3. It should have practical application in serving the reconstructive surgeon as a guide to the most appropriate treatment option. A deficiency common to all classification systems is the lack of information on how the radiographic interpretation of the grade of bone loss correlates with the changes that are found intraoperatively. There is also little information regarding the interobserver and intraobserver variability of these classification systems. It is probable that there would be only limited agreement between radiographic and intraoperative grades. Because the latter will be the more accurate, these classification systems for bone loss are probably best confined to describing intraoperative findings. As such, they will always be somewhat interpreter dependent and prone to an understandable tendency by the surgeon to err toward a severer grade of bone loss in marginal cases. The relative benefits of a simple versus a complex classification system are debatable. A simple system is likely to be more easily remembered,

more quickly completed, and hence more widely used in the busy clinical environment in which most orthopedic surgeons work. However, it may not adequately define the exact nature of a given defect. As a result, randomized trials that are based on simple classification systems, although more practical, will inevitably be open to criticism. THE ACETABULUM Acetabular Anatomy

The normal bony acetabulum is a socket formed by the confluence of the pubis, ischium, and ilium that completely covers all but the inferior and anteroinferior articular surface of the femoral head in the neutral position. The articular cartilage of the normal acetabulum is replaced by nonarticulating fibroadipose tissue medially and inferiorly, marking the site of attachment of the foveal ligament. The bony floor of this medial deficiency is formed by the quadrilateral plate. In the context of hip arthroplasty, the bone stock ideally should be such that it will permit the acetabular component to be completely covered by intact bone superiorly, medially, and posteriorly, almost completely covered anteriorly, and lying such that the center of rotation of the reconstruction is in a normal anatomic relationship to the quadrilateral plate, thus avoiding a high center of rotation of the hip reconstruction.6 For the purposes of description, the acetabulum is most conveniently divided into anterior and posterior columns, a medial wall, and a roof. The concept of anterior and posterior columns first appeared in the literature pertaining to acetabular fracture^,'^ but is usefully applied in describing bone stock in arthroplasty surgery. The anterior column is formed by the confluence of the superior pubic ramus and the anterior ilium, whereas the posterior column is made up of the ischium and posterior ilium. The floor or medial wall of the acetabulum is formed by the quadrilateral plate that represents the site of closure of the triradiate cartilage. The obturator internus muscle lies on the medial surface of the quadrilateral plate, forming a thin muscular layer between the floor of the acetabulum and the pelvic viscera. The roof of the acetabulum i.s formed by the ilium. This is widest near the joint and becomes progressively thinner more proximally. It, therefore, becomes increasingly deficient as an acetabular component migrates more proximally. Radiographic Evaluation of the Acetabul urn

The most commonly available radiographs are an anteroposterior (AP) view of the pelvis and a shoot-through lateral radiograph of the affected hip joint. The AP view provides useful information

THE CLASSIFICATION AND RADIOGRAPHIC EVALUATION IN REVISION HIP ARTHROPLASTY

about the condition of the roof and floor of the acetabulum. A high center of rotation of the failed hip relative to the quadrilateral plate is indicative of a deficiency in the acetabular roof or superior rim. Penetration of the implant (including any associated cement) more medially than Kohler 's line suggests that the floor of the acetabulum is deficient. Loss of the integrity of the teardrop indicates that the medial wall (i.e., the quadrilateral plate) is damaged.3The extent of a protrusio deformity can be measured in an axial direction by inward movement of the acetabulum beyond Kohler's line that is drawn from the medial border of the ilium to the medial border of the ischiurn.I7Protrusio can be measured also in terms of proximal migration beyond the level of the acetabular subchondral bone.24 The AP view provides only limited information regarding the integrity of the anterior and posterior columns because these structures are superimposed and partly obscured by the implants. Indirect evidence of anterior column deficiency sometimes may be suggested by excessive medialization of the failed cup as this is commonly the result of combined floor and anterior column bone loss. The presence of ischial lysis suggests that there is significant posterior column bone The columns are partially viewed on the shootthrough lateral view of the hip joint, though they are not seen in profile, and the quality of the images are often poor because of poor x-ray penetration and an inability of the patient to co-operate with the radiography technician. The best views of the columns are provided by oblique views of the pelvis as described by Judet et al.19The obturator view (Fig. 1A) is obtained by rotating the affected side 45" toward the x-ray tube. As the name suggests, this view demonstrates the obturator foramen in profile and gives an excellent view of the anterior column and the iliopectineal line. The iliac view (Fig. 1B) requires that the affected hip be turned 45" away from the x-ray tube and affords a clear view of the posterior column, ilioischial line, and sciatic notch. Angiography of the iliac vessels is occasionally indicated when there is concern that the external iliac vessels might be damaged during extraction of a failed intrapelvic acetabular component. If the angiograms confirm that the vessels are sitting between the bony pelvis and the prosthesis, it may be wise to consider a retroperitoneal approach to extract the prosthesis.', Cross-sectional imaging modalities are of little value in assessing bone stock in the failed acetabular component of a hip arthroplasty. Metal produces a dramatic artifact effect on computerized tomograms and this prevents useful interpretation when a prosthesis is in situ. It occasionally may be useful in assessing bone stock prior to revision of an excision arthroplasty to a total hip arthroplasty. Planning can be further facilitated by the production of three-dimensional images or CT-generated

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foam models. This technology also can be used to generate customized pros these^.^^ Metal also produces significant susceptibility artifact on MR imaging because of distortion of magnetic fields, especially on gradient echo sequences. It is possible to confine this artifact to the immediate vicinity of the metal implant by the use of fast spin-echo sequences that produce minimal distortion. However, even with optimal sequencing, interpretation of bone loss on MR images is often difficult. This imaging modality will only be indicated in the rare situations where the adjacent soft tissues need to be assessed to determine the exact location of nerves, vessels, or other organs. It also may find indications in orthopedic oncology to rule out local tumor recurrence.

Classification of Acetabular Bone Stock

Numerous classification systems have been described to assess bone stock in hip arthroplasty. Some of these encompass a wide range of defects encountered in both primary and revision arthroplasty, although others are designed specifically to address those deficiencies encountered in revision of the failed hip arthroplasty. The following classification systems have all been published within the last 10 years. The American Academy of Orthopedic Surgeons Classification

This classification system was devised by the American Academy of Orthopedic Surgeons Committee on the Hip and was reported by DAntonio et a1 in 1989.6In addition to providing a standard nomenclature for classifying acetabular bone defects, the group provided details of preoperative planning, suggested inventories, and surgical techniques. Although they provided information regarding the radiographic determination of the extent and nature of bone loss, the authors pointed out that ultimately the bone loss is determined intraoperatively. The classification system was designed to encompass both primary and revision cases and has two basic categories: segmental (Type I) and cavitary (Type 11). A segmental deficiency was described as any complete loss of bone in the supporting hemisphere of the acetabulum (including the medial wall). Cavitary defects were defined as a volumetric loss in bone substance of the acetabular cavity with retention of an intact acetabular rim. Both segmental and cavitary deficiencies are subdivided into peripheral (superior, posterior, or anterior) and central (medial) categories. Both segmental and cavitary deficiencies may coexist, especially in revision arthroplasty for the failed hip replacement, and these combined defects are referred to as Type I11 bone loss. For the purpose of completeness, Type IV and Type V are

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Figure 1. A, Obturator oblique view of the left hip. B, Iliac oblique view of the left hip.

added to refer to pelvic discontinuity and hip arthrodesis, respectively. Paprosky

This system is only designed to address bone loss found in association with the failed hip arthropla~ty.~~ It centers around the development of four specific radiographic criteria that, the authors claim, permit accurate preoperative evaluation of the failed acetabulum and determination of the defect that will be encountered intraoperatively. The authors suggest that the system can be used to choose the most appropriate reconstruction in a given case. The four radiographic criteria used in this system are 1. Superior migration of the cup less or more than 2 cm. 2. The presence or absence of ischial lysis (which provides evidence of posterior column bone loss). 3. The presence or absence of an intact Kohler’s line. This determines the ability of the anteromedial portion of the acetabulum to support a prosthesis. 4. The presence or absence of teardrop lysis, indicating whether or not host bone remains in the inferomedial acetabulum, thus allow-

ing ingrowth into the surface of a porouscoated prosthesis. These four radiographic criteria are used to divide the acetabular defects into Types 1 to 3. Type 1 defects have an intact acetabular rim and no socket migration. The areas of bone loss within the acetabulum are relatively minor and are compatible with the use of a porous bio-ingrowth prosthesis. Type 2 defects have superior migration of the socket of up to 2 cm, but with minimal ischial lysis and an intact Kohler’s line. Type 2 defects are subdivided into A to C subtypes. In Type 2A, the acetabular component migrates superomedially, producing a cavitary defect in the roof of the acetabulum. The acetabular rim remains intact. The defect is said to be a 2B when the direction of socket migration is superolateral, producing both segmental and cavitary bone loss in the acetabular roof. Type 2C defects have additional severe teardrop lysis, inferring loss of the medial wall of the acetabulum. Type 3 defects under the Paprosky classification system have more than 2 cm of superior acetabular migration, severe posterior column bone loss (indicated by severe ischial lysis), and severe medial wall damage (indicated by severe teardrop lysis). There are two subtypes: Type 3A defects have an intact anteromedial wall (indicated by an intact

THE CLASSIFICATION AND RADIOGRAPHIC EVALUATION IN REVISION HIP ARTHROPLASTY

Kohler's line) and bone loss confined to the 10- to 2-o'clock positions. Type 3B defects have bone loss from the 9- to 5-o'clock positions and disruption of Kohler's line, indicating that little host bone remains on which to seat a prosthesis. Paprosky et a1 suggest that in Type 3A defects there is generally 50% to 70% of host bone available for bone ingrowth into an porous-coated acetabular component. The superior defect is filled with a carefully fashioned distal femoral allograft. In Type 3B defects there is less than 50% available host acetabular bone stock for fixation of a porous-coated prosthesis. For these defects, the authors suggest the use of a whole acetabular transplant and an allpolyethylene cemented acetabular component. Gross

Like the Paprosky system, the Gross classification of acetabular bone loss is specifically designed to describe defects that are found in association with a failed arthroplasty and to indicate the type of reconstruction that will be ne~essary.'~ Three types of pelvic defects are described; protmsio defects, minor column (shelf) defects, and major column defects. Protrusio is a contained cavitary defect in which the acetabular walls and columns are intact. It is usually managed by filling the cavitary defect with morselized bone. Minor column, or shelf, defects are characterized by the loss of part of the acetabular rim with the corresponding area of acetabular wall, but involving less than 50% of the acetabulum. This is managed by a structural graft in which less than half of the acetabulum is replaced. Major column defects describe the loss of one or both columns with its corresponding acetabular wall involving over 50% of the acetabulum. These are managed by structural grafts involving over 50% of the acetabulum. The authors of this classification system suggest that the bone defects usually can be classified with plain radiography. Engh et a1

Like the Gross classification, the system described by Engh et al consists of just three types of acetabular bone loss.1oThe system was developed to facilitate analysis of comparable patient populations undergoing revision hip artliroplasty. Type I (mild) is characterized by minimal bone loss with an intact acetabular cavity and rim. Type I1 bone loss exists when the rim is deficient but the cavity is intact. In Type I11 bone loss, both the rim and the cavity of the acetabulum are deficient. Protrusio and pelvic discontinuity are classified as subcategories of Type 111.

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ments using a titanium bio-ingrowth stem.lhThey included in their results a classification system of femoral and acetabular bone loss found in association with the failed hip prosthesis. Both femoral and acetabular bone loss were divided into four grades, and the preoperative and postoperative Harris hip scores were found to correlate inversely with the severity of bone loss, that is, the least satisfactory scores were found in those patients with the worst bone loss. On the acetabular side, Type I bone loss was said to exist when there was minimal enlargement of the acetabular wall with a loose prosthesis. Type I1 bone loss was the most common type encountered and consisted of marked enlargement and thinning of the acetabular wall, but without any wall defect. In Type I11 bone loss, a local wall defect was present either superiorly, anteriorly, posteriorly, or centrally. Type IV bone loss existed when there was massive and global collapse or defect involving two or more acetabular walls. Chandler and Penenberg

In 1989 Chandler and Penenberg published a classification of acetabular and femoral bone loss found in association with total hip replacement? Like the American Academy classification system published in the same year, it is based on the division of the acetabulum into a roof, a floor, and anterior and posterior columns. A further similarity was the division of defects into rim and intraacetabular types, corresponding to segmental and cavitary defects in the American Academy system. Medial wall deficiencies were classified somewhat differently with protrusio acetabuli and medial wall perforation both designated as separate categories. Finally, combined defects were described in six different patterns:

1. Protrusio and perforation of the medial wall 2. Combined superior wall and superior intraacetabular defects 3. Combined superior wall and superior intraacetabular defects and medial wall perforation 4. Superior intraacetabular defect and perforation of the medial wall 5. Global deficiency involving the superior, anterior, and posterior rims and their associated intra-acetabular areas 6. Column deficiency. This is the most severe form of acetabular bone loss. The extensive bone loss involves the anterior or posterior columns and the medial wall to the point that there is discontinuity between the anterior or posterior column and the adjacent portion of the ilium.

Gustilo and Pasternak

Johnston et a1

In 1988 Gustilo and Pasternak published a minimum 2-year follow-up on 57 revision hip replace-

In 1990, Johnston et a1 published a standard system of terminolgy for reporting results of total

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hip arthroplasty.'* This represented a consensus between The Hip Society, the Commission on Documentation and Evaluation of the Societe Intemationale de Chirurgie Orthopedique et de Traumatologie (SICOT), and the Task Force on Outcome Studies of The American Academy of Orthopedic Surgeons. The purpose of the report was to provide nomenclature in which each term, whether applying to a functional or a radiographic parameter, was specifically defined so as to have a constant meaning. It was suggested that investigators should use the nomenclature provided in the report so that meaningful comparisons of the results of different investigators could be carried out. This report did not recommend a distinct classification system as to the severity of acetabular bone loss. It did, however, recommend means of describing acetabular component migration. Acetabular migration and the center of rotation of the hip should be measured in millimeters in both superior and medial directions, and these measurements should be related to the teardrop. THE FEMUR

As on the acetabular side, numerous classification systems for bone loss in association with the failed hip arthroplasty have been described. The emphasis in the different classification systems varies with the object that each was designed to achieve. Gross has described a simple classification system in which the emphasis is on determining the necessity for using bone allograft in the recon~truction.'~ Engh et a1 and Paprosky have both produced classification systems that revolve around the ability of the femoral diaphysis to support a cementless prosthesis.*0* The Endo-Klinik classification places more emphasis on the ability of the femur to support a cemented prosthesis? The emphasis in the American Academy classification is on being comprehensive and including all femoral abnormalities in both primary and revision arthr~plasty.~ A comprehensive classification system that is applicable to all types of reconstructions has definite advantages over those classification systems that only cater to one type of reconstruction, because these systems should be able to accommodate comparisons between different reconstructive techniques. Radiographic Evaluation

As in the acetabulum, preoperative evaluation of femoral bone stock in the context of a failed hip arthroplasty is based almost exclusively on high quality plain radiography. Occasionally, a computerized tomogram of the proximal femur with or without three-dimensional reconstruction will provide useful information in patients without a prosthesis currently in situ. The femur should always be viewed in at least two orthogonal planes. Ante-

roposterior and lateral radiographs are normal; nevertheless, variable information may, on occasion, be ascertained from oblique projections obtained when Judet views are obtained for the evaluation of bone loss in the acetabulum. The anteroposterior radiograph should include the entire femur from hip to knee joints. The lateral radiograph is performed often as a shoot-through and is frequently very disappointing in terms of clarity because the bone details are obscured by the soft tissue shadows. A much superior lateral view of the proximal femur is obtained using the Lauenstein and Hickey methods. The pelvis is rotated 45" toward the affected side. The hip is then abducted and externally rotated so that it comes to lie on the plate that is below the patient. The beam is directed vertically and the resultant image is a high quality lateral view of the proximal femur with an iliac oblique view of the acetabulum.2As in the acetabulum, cross-sectional imaging modalities have little to offer in the assessment of femoral bone stock in the presence of a failed prosthesis, though computerized tomography may be useful in evaluating bone stock prior to revision of an excision arthroplasty. One specific indication for the use of MR imaging that is occasionally useful is in the assessment of the extent of radiolucent cement distal to a failed prosthesis in arthroplasties that were performed prior to the routine addition of radio-opaque markers to bone cement.ll American Academy of orthopedic Surgeons

This collaborative effort of the American Academy of Orthopedic Surgeons Committee on the Hip was published in 1993 with the aim of facilitating preoperative planning and surgical treatment of femoral bone deficiencies in both primary As in the acetabular and revision hip arthr~plasty.~ classification by the same group, the basic categories of segmental, cavitary, and combined deficiencies were used. In addition, the group introduced categories for femoral malalignment, stenosis, and discontinuity in order to provide a comprehensive classification system for the full range of bone abnormalities found in hip arthroplasty. A segmental defect is defined as any loss of bone in the outer supporting cortical shell of the femur. These are subdivided into partial or complete. A partial defect that involves the greater trochanter is listed separately because of the additional reconstructive difficulties presented by extensive trochanteric bone loss. Cavitary defects are classified according to severity. Cancellous cavitary defects exist when the normal cortical bone is retained. Most revision arthroplasties will involve cancellous cavitary deficiency to a greater or lesser extent. When there is additional endosteal erosion of the cortical bone, the defect is described as a cortical cavitary deficiency. Finally, in long-standing implant failure the femur becomes expanded in association with severe cortical thinning and

I

THE CLASSIFICATION AND RADIOGRAPHIC EVALUATION IN REVISION HIP ARTHROPLASTY

complete loss of cancellous bone (femoral ectasia). Combined defects are said to exist when segmental and cavitary defects coexist. Malalignment is classified as either angular or rotational. Femoral stenosis is a further category referring to relative narrowing or obliteration of the femoral medullary canal. It is to be encountered more likely in certain primary arthroplasties. The final category of femoral discontinuity refers to the existence of a fracture of the femur with or without an implant present. For all the defect categories, the level of bone loss is indicated by the assignment of one of three levels. Level I is all bone proximal to the inferior border of the lesser trochanter. Level I1 is the 10 cm distal to Level I. Level I11 is all bone distal to Level 11, and deficiencies at this level are usually only seen following failure of long-stem prostheses. Chandler and Penenberg

This classification system for femoral bone loss includes six main categories, each of which may exist in isolation or in conjunction with another category5 The six basic categories are calcar deficiency, trochanteric deficiency, cortical thinning, cortical perforation, femoral fracture, and circumferential deficiency. Calcar deficiency may be intramedullary or complete. Intramedullary loss exists when there remains a thin shell of cortical bone in the region. Trochanteric deficiency is seen commonly in combination with bone loss elsewhere in the femur and may influence the surgical approach. Similarly, cortical thinning may occur in isolation but will be more commonly seen in combination with other deficiencies. Cortical perforation may occur as a result of osteolysis or migration of a loose stem (most commonly into a varus alignment), or may be produced intraoperatively during attempts to remove a failed prosthesis from a damaged femur. Femoral fracture may occur both in the host femur and in a prior proximal femoral allograft reconstruction. The final category is circumferential deficiency of the metaphysis and proximal diaphysis. This is subdivided into those cases where a thin rim of diaphyseal cortical bone remains and cases where the proximal femur is completely absent.

Gustilo and Pasternak Four categories of femoral bone loss are described in this classification system used to describe bone stock loss in revision arthroplasties.I6 In Type I there is minimal endosteal or inner cortical bone loss. In Type I1 there is proximal canal enlargement with cortical thinning of 50% or more and sometimes a lateral wall defect with an intact circumferential wall. Type 111bone loss exists when there is a posteromedial wall defect involving the lesser trochanter, indicating instability. Finally, in Type IV there is total circumferential bone loss in

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varying distances below the lesser trochanter. The bulk of revisions in this series were for Type I1 and I11 bone loss. The authors were able to demonstrate that the postoperative Harris hip scores were highest for those patients with Type I bone loss preoperatively and worst for those with Type IV. Engh et a1

This classification system is simple and is intended to provide a guide to implant selection in revision arthroplasty.loThe three categories of bone loss described are minimal, moderate, and severe. In minimal bone loss, the patient is treated in a manner similar to a primary arthroplasty. Moderate damage exists when bone loss is confined to femoral neck and intertrochanteric regions. This accounts for the bulk of revision arthroplasties and precludes the use of a standard proximally coated cementless implant. Although a standard length prosthesis is adequate, it should, in the authors’ opinion, be fully porous-coated to ensure adequate implant stability and diaphyseal fixation. Severe bone loss exists when there is damage to both the proximal femur and the femoral shaft, thus precluding immediate mechanical stability with a standard-length prosthesis. Stability can be achieved only with a long-stem prosthesis that extends into healthy bone more distally. Using this classification, the authors were able to achieve optimal femoral implant fixation in 86.8% of cases with moderate bone loss and 71.4% of cases with severe bone loss. Johnson et a1

In recommending a uniform nomenclature for describing the results of total hip replacement, this report suggested standardized terms for radiographic evaluation of the femoral component.18As part of a more extensive reporting system, the authors recommended that calcar resorption be described in terms of millimeters of height and thickness lost. It was suggested that shaft resorption, loss of bone density, and endosteal cavitation should be recorded on a zonal bas’is using the seven zones described by Gruen et a1 on the AP view,15 and an additional seven zones on the lateral view. They suggested reporting the extent of endosteal cavitation in terms of its length and width (in millimeters). Paprosky

This classification system was designed to determine the ability of an extensively coated cementless prosthesis to achieve diaphyseal anchorage in the femur?* It consists of three types. In Type 1 the femoral metaphysis and diaphysis are intact and bone loss is minimal. Type 2 is characterized by complete loss of the calcar and significant anteroposterior bone loss. The diaphysis remains intact. There are three subtypes: in

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Type 2A, bone loss is confined to above the lesser trochanter; in Type 2B the lateral metaphysis is deficient; and in Type 2C the medial subtrochanteric metaphyseal bone is deficient. In Type 3 lesions the entire proximal femur is deficient (both metaphysis and diaphysis). Mallory

As part of a comprehensive report on technical aspects of preparation of the proximal femur in cementless revision arthroplasty, Mallory described a simple classification system for femoral bone loss?' A Type I defect was said to exist when the femur remained essentially unchanged, and both the medullary canal and the femoral cortex remained intact following removal of a previous cemented implant. This type of defect can be managed by a cemented or cementless prosthesis of standard length. Type I1 exists when the medullary cancellous bone is absent but the cortical tube remains intact. For this, the author recommends the use of a long-stem prosthesis. In Type 111, both the medullary contents and the cortical tube are destroyed. This may be proximal to the lesser trochanter (Category A), between the lesser trochanter and the femoral isthmus (Category B), or distal to the femoral isthmus (Category C). The author suggested the use of fresh frozen proximal femoral allografts for Type 111, Categories B and C. Endo-Klinik

This classification system subdivides bone loss in association with a failed cemented femoral component into four grade^.^ In Grade 1 the prosthesis is clinically loose and radiolucent lines are present along the proximal half of the cement mantle. In Grade 2 the medullary cavity of the proximal fe.mur is expanded by endosteal erosion and radiolucent lines are present circumferentially. Grade 3 bone loss exists when the proximal femur is expanded with resultant widening of the medullary cavity. Finally, Grade 4 bone loss is characterized by gross destruction of the proximal third of the femur that precludes even the use of a long-stem prosthesis.

Gross Gross has described a classification of bone deficits associated with loose hip implants, which is based on the type of bone graft that is used in the recon~truction.~~ Femoral defects are either intraluminal or cortical. Intraluminal defects exist when the medullary canal is widened but the cortex is still intact and thought to be strong enough to support an implant. Cortical defects may be noncircumferential (and require only strut grafts), or circumferential (requiring a segmental graft). The circumferential cortical defects are further subdivided into calcar defects (less than 3 cm in

length) and proximal femur (large fragment) defects (greater than 3 cm in length). DISCUSSION

Many authors have produced classification systems for bone loss about the hip in order to quantify the severity of bone loss in the revision cases they are undertaking, to explain the indications for a particular revision arthroplasty technique, and to assess the results of these interventions. Many of the classification systems are similar. For example, the concept of contained versus uncontained acetabular defects is common to most systems. On the femoral side, most systems emphasize the importance of the three levels at which the bone loss exists in determining the type of reconstruction that will be necessary. Ultimately, the orthopedic community needs to know how the different techniques compare to each other. This becomes exceedingly difficult in the face of a plethora of classification systems. A single comprehensive classificationsystem that can adequately describe all types of bone loss associated with hip arthroplasty should become a standard for reporting purposes. The system recommended by the American Academy probably comes closest to achieving this objective, though even this system remains untested in its reliability, reproducibility, and validity, and some of the terms would benefit from more rigid definition. Objective radiographic measurements form an important part of at least one bone loss classification systemz3and further work in determining the reliability of such measurements may represent a logical next step in the evolution of the ideal system. SY NOPSlS

Many classification systems have been described over the past 10 years fur bone loss that is found in association with the failed hip arthroplasty. Most are based on assessments of bone stock that are made intraoperatively. Good-quality plain radiography is the most useful preoperative investigation and provides important information regarding the residual bone stock. There is a need for critical appraisal of the validity of classification systems currently in use and the development of a consensus system that will permit comparison between the published results of different techniques.

References 1. Al-Salman A, Taylor DC,Beauchamp CP, et al: Prevention of vascular injuries in revision total hip replacement. Can J Surg 35:261-264, 1992 2. Ballinger PW Merrill's Atlas of Radiographic Posi-

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