The Role of Scintigraphy in the Lameness Evaluation

Advanced Diagnostic Methods

0749-0739/91 $0.00

+ .20

The Role of Scintigraphy in the Lameness Evaluation

Robert R. Steckel, DVM, MS*

In equine practice, evaluation of patients for lameness consists of a combination of anamnesis, a thorough physical examination, observation of the horse's conformation and gait in hand or during exercise, manipulative tests of suspect areas to help accentuate pain, 5, ,'20, 52 selective regional or local anesthesia, 9, 44 and standard radiographic imaging. 5, 52 Usually, this combination yields a specific lesion or identifies a combination of painful conditions responsible for the altered gait. Most experienced equine practitioners would agree that this described evaluation is most accurate in identifying conditions causing lameness in the equine lower limbs, especially those localized to the carpus or tarsus and below. Many lameness conditions occur, however, when the normal regimen employed by equine practitioners fails to yield a result or when the results of the workup are inconclusive. This is particularly true when an injury or spontaneous disease process is located in the proximal limb region1, 2,11,12,38 above the carpus or tarsus, in the pelvis,21 or in the torso 22, 23 of the horse. In these areas, heavy muscles obscure accurate digital palpation of bony prominences or ligaments to identify points of pain, which so accurately localizes lesions in the lower leg. Similarly, because of the decreased frequency of acute or chronic disease occurring in the upper limb regions and torso, many equine practitioners are unfamiliar with specific manipulative tests to help enhance the pain or with the regional diagnostic anesthesia techniques, either of which might help localize the problem. The heavy muscles over the large bones and joints of the equine upper body, particularly those surrounding the pelvis 22, 23 and axial spine,21 preclude accurate radiographic study of those areas with conventional portable equipment. For these lameness cases, the equine practitioner should .consider alternative diagnostic methods and solicit the help of the nearest referral center at which the necessary equipment and trained

*Diplomate,

American College of Veterinary Surgeons; Associate Professor of Surgery, Department of Clinical and Population Sciences, University of Minnesota College of Veterinary Medicine, St. Paul, Minnesota

Veterinary Clinics of North America: Equine Practice-Vol. 7, No.2, August 1991

207

208

ROBERT R. STECKEL

personnel are available. Specialized diagnostic methods also should be considered for the previously mentioned horses in which more than one disease is identified during the lameness examination. This may be either in the same limb or in different limbs, or the examiner may need to separate the component of soft-tissue injury versus bone disease to arrive at a specific course of treatment and to be ~ble to issue a prognosis for future soundness. Imaging modalities currently used to help differentiate the nature of lesions found during the lameness examination are ultrasonography,15 thermography,55 and scintigraphy. 20, 27, 28 The former two methods are discussed by other authors elsewhere in this symposium. This article deals with the use of nuclear medicine imaging of the musculoskeletal system, or bone scanning, for orthopedic diseases of the horse. The following does not review the physical basis of nuclear medicine. For those interested readers, appropriate references are listed. 4, 7, 10, 14, 18, 27, 28, 36, 37, 43, 48 Rather, the author hopes to show the equine practitioner how scintigraphy can be used to help in diagnosis of lameness. In general, nuclear imaging of skeletal disease is more sensitive than other imaging methods, but it is less specific, and it must always be combined with conventional radiographic imaging. 18, 19 Scintigraphy already occupies a place in the diagnosis of equine lameness,27, 28, 35, 37 and greater familiarity with its benefits and limitations is all that is needed for equine practitioners to request the aid of a bone scan for difficult cases. It is the author's opinion that in the future, increased awareness of the advantages of bone scanning for lameness diagnosis by horse owners will result in their requesting that a scintigraph be done on more horses with obscure lameness.

REVIEW OF NUCLEAR MEDICINE The technique and application of radionuclide bone imaging in human patients was first popularized in the early 1970s to help detect primary or metastatic bone tumors. 18 Since that time, the technology has expanded considerably to include the use of many different isotopes and imaging methods. The positive experience gained has resulted in routine use of scintigraphy for evaluation of many nonneoplastic orthopedic, diseases of human patients. Experience over two decades has taught human nuclear medicine specialists that bone scanning can be used (1) for early detection of suspected bone lesions prior to their appearance on radiographs, specifically occult or incomplete fractures; 16, 18, 19,35,37,42,48 (2) to differentiate the component of bone inflammation versus soft tissue with mixed orthopedic injuries;6, 18, 19, 36, 37 (3) to identify osseous diseases that occur without causing obvious radiographic changes;16, 18,35,36 or (4) to identify acutely damaged muscle. 36, 37, 48,51 In human sports medicine, radionuclide bone imaging now is widely used not only to diagnose acute injuries resulting from competition before their radiographic appearance but also to monitor skeletal change as a response to athletic training. 18, 37 This allows detection of potentially adverse conditions earlier in their course and enables the clinician to

THE ROLE OF SCINTIGRAPHY IN THE LAMENESS EVALUATION

209

recommend corrections in the individual athlete's training regimen to prevent serious injuries from occurring. Application of nuclear medicine for equine diseases was first reported in 1977. 57 Since that time, several reports have appeared describing the extrapolation of knowledge gained in the human medical field to parallel equine conditions. Reports of the use of radionuclide bone imaging in horses have involved diagnosis of chronic lameness conditions,26-28 occult or "stress" fractures, 10, 24, 25, 28, 30, 32, 34 early osteoarthritis,41 osteochondrosis, 4 osteomyelitis, 10, 27, 28 bone-ligament injury (enthesopathy), 10 skull trauma and infected teeth,39 and monitoring the age of growth plate closure in yearlings. 40 It appears that the application of bone scanning technology to the horse is direct and uniformly good results are obtained, identical to those seen in humans.

THE DECISION TO SCAN Inclusion of scintigraphy into the practice of conducting a lameness evaluation does not eliminate the need to conduct: a thorough physical examination. On the contrary, in terms of decreasing radiation exposure to the nuclear medicine personnel, 58 the time and expense of unnecessarily transporting a horse to an imaging facility, and the cost of the scan itself, it behooves the equine practitioner to narrow down the possible sites of musculoskeletal pain as much as possible before considering a scintigraph. More specifically, the results of the anamnesis and the physical and lameness examinations are used to help the nuclear medicine specialist either localize or rule out skeletal lesions in general anatomic areas. An example of a proper request for scintigraphy would be lower forelimbs only, entire forelimbs only, forelimbs plus hindlimbs, or hindlimbs plus the pelvis and vertebral column. The potential use of scintigraphy does change the lameness evaluation in that repetitive observation of the horse on different occasions at different gaits, repetitive selective flexion tests, and repetitive regional anesthetic techniques are not done mindlessly. If a properly conducted lameness evaluation does not lead to a plausible diagnosis and if upon reexamination, the signs of lameness have not changed after an interim period, a bone scan should be recommended to the horse owner. During 8 years of referral lameness practice at Tufts University Large Animal Hospital (TULAH), the author learned to conduct a "modified" lameness evaluation. The order of routine diagnostic tests was changed. 3, 14 Patients referred had already been examined by at least one other veterinarian. In some instances, as many as three experienced equine practitioners examined the horse without locating the cause of lameness. If the results of an abbreviated examination confirmed the previous findings, the horse was subjected to scintigraphy of the suspect area or areas. After the results of the bone scan and appropriate radiographs were obtained, the lameness workup was continued to completion by selective regional anes-thesia to confirm the probable diagnosis based on the combined results of the scintigraphic and radiographic surveys. In the majority of patients examined by the author at TULAH, the

210

ROBERT R. STECKEL

request for scintigraphy was warranted because (1) the signs of lameness were vague, inconsistent, or confusing, or (2) more than one problem had been identified, and specialized imaging was employed to sort out the contribution of each individual disease to the current lameness complaint. Differentiation of bone pathology versus soft-tissue injury through the use of scintigraphy 18, 37, 48 of bone alone or selected isotopes to preferentially image soft tissues 6 allowed more specific treatment recommendations and therefore enhanced the ability to prognosticate accurately. Identification of occult stress fractures in long bones of the upper limbs or degenerative conditions of major joints also allowed not only more accurate diagnoses to be rendered, but better advice to be given regarding the long-term prognosis for future soundness in performance horses. Those years of experience, involving several hundred horses, convinced the author that scintigraphy is an extremely sensitive imaging modality for skeletal diseases of the horse. If the results of a bone scan failed to show occult skeletal disease, the horseowner was informed that the ultimate prognosis was favorable because the current lameness condition most likely resulted from soft-tissue trauma, 37, 48 and the condition should respond to rest from exercise. Equine practitioners who either graduated from veterinary school more than 10 years ago or more recently graduated from a college that did not perform nuclear medicine imaging techniques routinely, may be unfamiliar with this diagnostic tool and skeptical that the time and expense of subjecting a lame horse to a bone scan is justified. Admittedly, compared with other simple diagnostic techniques, a bone scan can be considered an expensive diagnostic procedure depending on the geographic region of the country in which one's practice is located. The cost for scintigraphic imaging is variable and includes the radiopharmaceutical agents, other necessary disposable items, and the use of the gamma camera based on the time required for nuclear medicine technicians to complete the requested study. Prices for a bone scan range from $150 to $550 depending on whether the particular facility is subsidized or private and the amount of skeletal imaging requested by the clinician. Additional costs are incurred because the horse must be hospitalized for 2 or 3 days because of federal regulations regarding the use of radioisotopes in animals. Similarly, institutional quality radiographs may be necessary depending on the results of the bone scan. The total cost of conducting a lameness evaluation incorporating scintigraphy during the last few years that the author was on the staff at TULAH averaged from $350 to $750. Charges at the author's current institution are lower; a complete workup averages $250 to $500. The fee for scintigraphic services at private practice hospitals is, of course, generally higher. The responsible equine veterinarian must temper the cost of arriving at a diagnosis with the total expense of repeated lameness examinations that fail to produce a conclusion and the expense of maintaining a horse that is not serviceably sound for its intended purpose. In some regions of the country, the latter expense on a per month basis is very high. In the author's experience, if a thorough lameness evaluation is conducted without arriving at a diagnosis, the expense of the bone scan is warranted to the

THE ROLE OF SCINTIGRAPHY IN THE LAMENESS EVALUATION

211

horseowner. The continual cost of stabling and repeated inconclusive veterinary examination invariably is greater than the cost of the bone scan.

BONE SCAN PROTOCOL Scintigraphy of the equine skeleton is performed by intravenous administration of a radiopharmaceutical agent and following its distribution through the various body compartments over time by the use of a gamma camera. Bone scans are divided into three phases,7, 10, 18, 27, 28, 37, 43, 48 which overlap and correspond to redistribution of the radioactive compound through different body tissues before its elimination by the renal system. Phase 1 is a so-called nuclear angiogram, which lasts for about 30 seconds while the isotope is contained primarily in major vessels. Phase 2 begins as the material moves out of the vascular compartment into soft tissues. Good quality images of soft-tissue structures are best obtained 10 to 20 minutes after injection. This phase actually lasts until significant accumulation of isotope has occurred in bone and the soft tissues have cleared. Delayed bone images, or Phase 3, begin at least 2 hou,rs after administration of the radioactive material. Most bone scans involve only the delayed phase; for the purpose of clarity, the term "bone scan" refers to Phase 3. Because normal distribution of injected radioactive compounds depends on adequate blood flow, 7, 10, 18 the horse must be well hydrated to obtain reliable results. Horses transported long distances for bone scanning are made to rest overnight to allow rehydration to occur before the procedure is performed. After aseptic placement of a jugular catheter, a standard dose of 100 mCi of 99mTc methylene diphosphonate (99mTc-MDP) is administered for the average 450-kg horse. 10, 27 The time of injection, dose, and route of administration are recorded. If the particular study does not involve all three phases, the horse is allowed to remain in a stall. After 3 or 4 hours have passed, sufficient isotope should have accumulated in bone to permit adequate imaging with the gamma camera. Horses are placed in stocks and administered small doses of sedatives, such as xylazine or detomidine intravenously, to reduce motion during imaging. The head of the gamma camera is then positioned next to the anatomic area or areas to be evaluated, and the scintillation counts are made. Imaging of the limbs always involves lateral views, but anteriorposterior forelimb or posterior-anterior hindlimb views may be included as needed. Imaging of the feet also may be performed from the lateral and anterior-posterior views. By positioning the head of the gamma camera into a pit and then standing the horse over it, a solar image is obtained. Because each individual animal may present a slightly different pattern of isotope distributiori, the contralateral limb or foot image is always recorded for comparison. Imaging of the pelvis, spinal column, or the head typically is performed from both the lateral and dorsal surfaces. The pelvic image may also be recorded posterio-anteriorally. After acceptable images are obtained, the horse is returned to its stall. The scanning procedure is completed on the follOwing morning when a nuclear medicine technician surveys the

212

ROBERT R. STECKEL

patient and the stall for the level of emitted radiation. Because the half-life of technetium is 6 hours, 10, 28 all counts invariably have returned to a safe level to permit completion of the lameness examination or discharge from the hospital on the day after the scan.

BONE SCAN INTERPRETATION The compounds used to image bone are bound to a phosphate or phosphonate, which exchanges for phosphate in the inorganic matrix. Distribution through osseous tissue is dependent on two physiologic activities: (1) blood flow to bone, and (2) the metabolic activity of bone. Therefore, any process affecting either the blood flow to bone or the rate of bone deposition or reabsorption will be detected as increased uptake of the isotope by more gamma radiation emitted. It is important to remember that bone is a dynamic tissue in a constant state of remodeling. Interpretation of the bone scan image produced therefore is highly dependent on knowledge of the athletic activity of the patient and the results of the lameness examination. As previously mentioned, scintigraphy is more sensitive but less specific than standard radiographic imaging. Increased experience with nuclear imaging has shown that it can be used to characterize a wide variety of changes to bone, and, coincidentally, muscle damage. 3, 36, 37, 50 Over the past decade, application of this technology to evaluating equine skeletal disease has been considerable. There is a strong tendency for the novice to overinterpret the images produced, so the clinician must keep in mind the age of the patient and two physiologic concepts of bone (or muscle) blood flow and the relative metabolic rate of musculoskeletal tissue during scintigraph interpretation. 7, 27, 37, 50 The following general guidelines regarding scan interpretation can be made. As a result of an increased blood supply and higher rate of bone formation, young animals normally retain more radioactive marker than adults, so the image obtained generally appears darker ("hotter):» in all areas of the skeleton. 37 This is particularly true of all the physes and epiphyses of long bones in young growing horses,40 which includes most horses in their first year of racing competition. Even in adult horses, the ends of long bones have more blood supply than the shafts; therefore, all the major joints retain more radioactive marker. 7, 27 Dehydration decreases skeletal blood flow and results in a less radiointense image involving all the bones of the skeleton. 7, 27 Conversely, increased uptake over that expected in certain bones in human patients has been attributed to deranged sympathetic innervation regionally. 18 In addition to these general statements regarding the image produced, abnormal images of increased isotope retention in bone are due to increased adsorption of the carrier compound onto hydroxyapatite of newly formed bone or osteoid. 10, 37, 43 Typical reasons for new bone formation are increased mechanical stress and resultant remodeling37, 48 of either cortical bone or the outer (periosteal) surface or remodeling due to trauma. 37, 43 Fractures identified by scintigraphy, but not radiography, are typically classified as

THE ROLE OF SCINTIGRAPHY IN THE LAMENESS EVALUATION

213

"stress" or incomplete fractures involving only one cortex of the shaft or the end of a long bone. Incomplete fracture of any bone, for example, sagittal fracture of a cuboidal bone of the carpus that has not become distracted, can be detected by a bone scan but usually not by radiography. Incomplete fractures present a focally intense dark image or hot spot on the radiographic film within the confines of the normal skeletal anatomy (Fig. 1). The age of the fracture is a variable that affects the size and intensity of the hot spot, but occult fractures have been detected in human patients within 1 day after their occurrence, and almost all occult fractures are detectable by the third day after injury.37 Focal areas of reduced isotope retention in one portion of a bone, a so-called "cold spot" or photopenic area, have been described in human and equine patients and are believed to be caused by bone infarction that could result in sequestrum formation. 34 ,37 Increased periosteal activity appears as diffuse linear retention of radioactive tracer along the shaft of a bone in horses 25,50 and humans. 42, 48 Enthesopathies, or inflammation of ligament or tendon insertions to bone, have been detected in humans as focally intense areas of increased isotope in areas that correspond to known tendon or ltgament insertions 37 (Fig. 2). Inflammation of individual muscle bellies also has been detected in human athletes 36 , 37 .and horses 27, 50 as diffuse patchy areas of isotope retention overlying or obscuring the normal bone image corresponding to known locations of individual muscles. A narrow window of time exists to detect rabdomyositis, because repeat imaging 48 hours later has failed to identify inflamed muscle in some human subjects. The exact reason for isotope retention is uncertain, but phosphate or phosphonate complexes with increased calcium in inflamed muscle have been proposed. 36, 37, 51 Curiously, bone cysts usually are not detected by scintigraphy unless they are situated near an articular margin and inciting degenerative arthritis, 27, 50 or 1.!nless the subchondral bone surrounding the cyst is inflamed (Fig. 3). Experience with equine scintigraphy of a large number of horses at TULAH indicates that consistent detection of bone cysts is the only limitation of scintigraphic imaging for diagnosis of equine lameness. Otherwise, all changes in the skeleton caused by remodeling or trauma observed in humans are paralleled in the horse.

SPECIFIC SKELETAL IMAGING The Foot The two bones contained within the hoof capsule are frequent sites of osseous inflammation. Scintigraphy can be used to identify occult inflammation or fracture of either the distal phalanx or the distal sesamoid before radiographic changes appear. 10, 27 Typical indications are patients in whom pain has been definitely localized to the foot during the lameness evaluation and regional anesthesia has confirmed the clinical finding, but standard radiographic views are normal. Cases of inapparent osteitis of the pedal bone are difficult to separate from chronic subsolar bruising (Fig. 4). Text continued on page 220

214

ROBERT R. STECKEL

8 Figure 1. A 4-year-old Standardbred gelding was examined for moderate right forelimb lameness of 3 weeks' duration. Routine evaluation failed to localize the source of pain. Bone scanning of both forelimbs was done. Lateral (A) and anterior-posterior (B) views of the right elbow region show intense uptake of isotope in the disto-medial shaft of the right humerus (arrows).

THE ROLE OF SCINTIGRAPHY IN THE LAMENESS EVALUATION

215

Figure 1 (Continued). A lateral radiograph (C) revealed medullary sclerosis and endosteal callus formation in the distal right humeral shaft (arrows) consistent with a chronic stress fracture.

A

L

Figure 2. A 3-year-old Thoroughbred filly was examined for moderate left forelimb lameness of 4 weeks' duration. Routine lameness evaluation failed to localize the site of pain. Bone scanning of both forelimbs was done. Lateral scintigraph of the left elbow region (A) was not symmetric with the opposite limb (B). An abnormal focus of isotope is present in the anterior cortex of the proximal radius (arrow).

216

THE ROLE OF SCINTIGRAPHY IN THE LAMENESS EVALUATION

217

Figure 2 (Continued). A lateral radiograph of the left elbow (C) revealed periosteal new bone formation at the point of insertion of the biceps brachii tendon consistent with enthesopathy (arrows).

218

ROBERT R. STECKEL

Figure 3. An 8-year-old Thoroughbred mare used as a show hunter was presented for intermittent right forelimb lameness of6 months' duration. Routine evaluation failed to localize the source of pain. Scintigraphic imaging of both forelimbs was requested. Delayed bone image of the right elbow region (A) revealed an intense focus of isotope uptake (arrow) compared with the left elbow image (B).

THE ROLE OF SCINTIGRAPHY IN THE LAMENESS EVALUATION

219

Figure 3 (Continued). Anterior-posterior radiograph of the right elbow region (C) revealed a proximal medial radial bone cyst with associated subchondral bone sclerosis (arrows).

220

ROBERT R. STECKEL

Figure 4. An II-year-old Thoroughbred gelding used for 3-day-events was examined for right forelimb lan1eness on three different occasions. On each exan1ination, pain was relieved with paln1ar digital anesthesia. Radiographs of the affected foot on two occasions were normal. Solar scintigraph of both forefeet reveals focally intense activity in the medial wing of the right distal phalanx (arrows). Repeat radiography again was norn1al. A diagnosis of occult osteitis was made. Medial hoof imbalance and a right forefoot angle of 42° were corrected. The lameness resolved in 15 days.

Imaging of the pedal bones from the solar surface also allows early detection of occult distal phalanx fractures or osteomyelitis. The author has used scintigraphy of the feet to good advantage to separate true osseous from soft-tissue inflammation in horses diagnosed with navicular syndrome. 54 The degree of accuracy is very high, because the navicular bone does not normally appear on delayed images of the foot. 10, 27, 28 The recognized confusion surrounding the significance of radiographic changes in the diagnosis of navicular syndrome warrants consideration of the use of scintigraphic imaging,47, 54, 56 particularly for those horses in which prepurchase litigation or insurance claims overshadow the disposition of the horse. In the current proposed method of pathologic remodeling of the navicular bone, vascular alteration or flexor cortex remodeling occurs in response to progressive athletic conditioning, similar to other bones in the body. 47 Pool47 suggests the use of scintigraphy as a discriminator to determine in vivo if radiographic lesions are clinically active or benign. Foot scans also can be helpful in early identification of navicular bone osteomyelitis following deep punctures to the frog (Fig. 5), or nondisplaced fractures of the distal sesamoid prior to their radiographic appearance.

Pastern and Fetlock Synovial blood vessels form an anastomotic connection with those of the epiphysis. Increased synovial blood flow due to synovitis therefore results in increased periarticular blood flow and causes increased radionuclide concentration around inflamed joints before degenerative changes

THE ROLE OF SCINTIGRAPHY IN THE LAMENESS EVALUATION

221

Figure 5. A 2-year old Standardbred filly was examined for severe right forelimb lameness of 10 days' duration after sustaining a deep puncture to the right fore frog by a bandage pin. Radiographs failed to identify an abnormality even though the pain was relieved by palmar digital anesthesia on two occasions. On the 12th day, scintigraphy of both forefeet revealed intense focal retention of isotope in the right fore navicular region. Surgical evaluation of the navicular bursa identified focal osteomyelitis of the navicular bone that had not become radiographically apparent. The bone infection healed uneventfully.

occur radiographically. 37 Therefore, bone scans of the pastern and fetlock are helpful in early detection of osteoarthritis (ringbone) of the interphalangeal joints or early degenerative arthritis (osselets) of the metacarpophalangeal joints. In many cases, incomplete proximal phalangeal fractures in the sagittal or frontal plane also are detected long before they are radiographically apparent. In horses of racing breeds, this is a major advantage in preventing severe articular damage with complete fracture or the sometimes catastrophic breakdown injuries associated with complete separation of phalangeal fissure fractures. Identification of incomplete fractures before their distraction permits imaging by high-detail radiography and definitive treatment by lag screw fixation, or, alternatively, conservative management with stall rest. 13, 32, 33 Routine bone scanning of horses presented for poor performance at the TULAH Sports Medicine Center has allowed identification of stressinduced bone sclerosis as a specific entity causing lameness in young horses being trained for racing. The syndrome is well described in young human athletes/ 8 , 37, 48 and, based on a large number of cases, it appears that it is a parallel cause of lameness in young racehorses,25, 50 which can be detected only by scintigraphy and shows no obvious radiographic changes.

222

ROBERT R. STECKEL

These patients typically exhibit lameness due to pain from the middle carpal joints or the metacarpophalangeal or metatarsophalangeal joints, but survey radiographs of the affected areas are normal. Scintigraphy of the limbs usually shows focally intense activity in the medial aspect of the third carpal bone or the condyles of the metacarpal or metatarsal bones (Fig. 6). Intraarticular anesthesia of affected horses' respective joints obliterates the lameness. High-detail radiography of the distal cannon bone region from the lateral view shows increased bone density of the condyle region. Skyline radiographs of affected third carpal bones show changes typical of those that precede osteochondral chip fracture or slab fracture. 8, 45, 59 Sclerosis of other bones, such as the navicular, has been described as a normal response to increased biomechanical stress from athletic activity. 47 In human athletes, the use of scintigraphy can detect stress-induced bone sclerosis while it is only a painful condition, before further deterioration of the bone matrix leading to stress fracture occurs. 18, 37, 48 Early detection of this condition has allowed modification of the training regimen to allow the transition in bone composition to occur without stress fracture ensuing. 37, 48 Similarly, equine sports medicine patients at TULAH have been treated successfully by decreasing the intensity of training for 8 to 12

Figure 6. A 3-year-old Standardbred colt was evaluated for left forelimb lameness that ensued while being reconditioned after 9 months of convalescence for a right forelimb distal phalanx fracture repair. Abaxial sesamoid regional anesthesia of both forelimbs did not affect the lameness. Bilateral forelimb scintigraphy revealed focally intense isotope uptake in the condyles of both third metacarpal bones. The scintigraphic appearance of both distal metacarpii is clearly shown with a left lateral flexed view (arrows). High detail lateral radiographs of both fore fetlocks revealed sclerosis of the distal metacarpal condyles. A diagnosis of stress-induced bone sclerosis was made and the training intensity was reduced for 90 days. The forelimb lameness resolved without further treatment.

THE ROLE OF SCINTIGRAPHY IN THE LAMENESS EVALUATION

223

week periods. Subsequent scintigraphy and lameness reevaluation indicated resolution of the bone inflammation. Metacarpus and Carpus Recently, the value of bone scintigraphy in evaluating the "bucked shin" cortical stress fracture syndrome of racing Thoroughbreds was reported in a study of a large number of cases. 25 Three distinctly different scintigraphic patterns are typical of each respective stage of the disease. This directly parallels the syndrome of tibial stress fractures versus"splints" in human athletes. 35, 37, 42 Thus, the sensitivity of scintigraphy in accurately describing the nature of common clinical entities in two different species is borne out. The previously mentioned stress-induced bone sclerosis was first described in the third carpal bone of racing Thoroughbred horses. 8 , 59 This particular entity is one of the most common causes of lameness in racing Standardbreds exhibiting poor performance that were evaluated at the TULAH Sports Medicine Center. 50 The typical appearance of active third carpal bone sclerosis has been reported. 59 Again, a distinct advantage of scintigraphic evaluation of the carpus is to detect· sagittal fractures of the third carpal or other cuboidal bones, which are essentially unrecognizable on radiographs if the fracture has not displaced or until a full month of healing has occurred and the margins of the fracture become radiolucent. Avulsion fractures of the origin of the suspensory ligament from the posterior metacarpus are difficult to detect by plain radiography because they displace minimally, but they can be readily diagnosed by bone scanning. 10 Recently, incomplete sagittal fractures of the proximal metacarpal cortex were described. 30 Initial detection was by scintigraphy. Radius and Ulna Incomplete fractures of the radial shaft usually result from kick wounds, but they have been described in Thoroughbreds as a result of race training. 31 Although the clinician may suspect that such a fracture has occurred, the delay before detection of a fracture fissure radiographically usually is a full month after the traumatic incident. Even after a lag of 30 days or longer, incomplete radial fractures may be unidentifiable. 31 Bone scanning provides a means to confirm or rule out incomplete radial fractures, enabling measures to be taken to prevent complete diaphyseal disruption in affected patients. As previously mentioned, bone cysts typically do not appear as a hot spot image on bone scans unless the surrounding subchondral bone is remodeling as a result of the presence of the cyst. Figure 3 shows a proximal radial bone cyst in a mature Thoroughbred mare that was detected by scintigraphic evaluation of the forelimbs for obscure chronic lameness. Intraarticular anesthesia of the radiohumeral joint relieved the pain and confirmed the cyst to be the cause of lameness. Humerus and Scapula Spontaneous humeral fractures have occurred in horses during racing. In a recently reported study of young Thoroughbreds, the humerus was

224

ROBERT R. STECKEL

Figure 7. A 2-year-old Standardbred colt was examined for mild right forelimb lameness during race training. Routine lameness evaluation failed to identify the source of pain. A forelimb scintigraph revealed intense radioisotope concentration (arrows) in the distal right scapular region on a lateral image (A) and an anterior-posterior scintigraph (B) of both shoulder joints (arrows). S = the sternal image..

THE ROLE OF SCINTIGRAPHY IN THE LAMENESS EVALUATION

225

Figure 7 (Continued). Lateral radiograph of the right shoulder (C) revealed chronic fracture of the supraglenoid tubercle (arrows). Cessation of race training was recommended to the owner.

the most common site of incomplete longbone fracture. 31 Figure 1 shows a distal humeral stress fracture in a racing Standardbred gelding presented for obscure forelimb lameness of 3-weeks duration. The corresponding radiograph demonstrates increased sclerosis of the distal humeral shaft and medullary opacity due to endosteal callus formation. Although the fracture fissure could not be identified, endosteal callus is a typical observation on plain radiography that corresponds to the radioisotope focus. Except for those caused by severe accidents, scapular fractures also are uncommon because of the heavy protection from the shoulder musculature. Radiography to detect scapular fractures often requires general anesthesia. 2, 10 An old fracture of the supraglenoid process in a young Standardbred gelding presented for chronic lameness during race training was detected on a routine bone scan of the horse's forelimbs (Fig. 7). Shoulder osteochondrosis may be detected in some patients by bone scanning if the arthritis has progressed to inflame subchondral bone of the humerus or scapula. Tarsus and Metatarsus Cuboidal bone sagittal fractures are readily detected by bone scanning. An avulsion fracture at the origin of the suspensory ligament similar to that described earlier in the forelimbs was identified in the hindlimb of a young Warmblood gelding being trained for dressage competition (Fig. 8). A typical lesion did develop on radiographic evaluation (Fig. 8), although this is an unusual finding as a cause of hindlimb lameness. In this patient,

226

ROBERT R. STECKEL

Figure 8. A 5-year-old Warmblood gelding competing at dressage previously had been examined scintigraphically for acute right hindlimb lameness. A diagnosis of avulsion fracture of the origin of the suspensory ligament of the affected limb was made. Four months of convalescence was recommended. Resumption of mild exercise re-exacerbated the lameness. A second scintigraph of both tarsii revealed incomplete healing of the avulsion fracture. Lateral scintigraph (A) shows mild focal isotope retention (arrow) in the caudal proximal right metatarsus. Anterior-posterior radiograph (B) of proximal right metatarsus showing central zone of sclerosis (arrows) consistent with healing avulsion fracture. Three months of additional rest were followed by ~ return to soundness.

THE ROLE OF SCINTIGRAPHY IN THE LAMENESS EVALUATION

227

repeat bone scanning was performed after a long convalescent period to ensure that the fracture had healed completely before the horse resumed training for athletic competition. 27, 37 In addition to the previously mentioned stress-induced bone sclerosis of the distal metatarsal condyles, stress remodeling of the anterior metatarsal cortex and of the proximal metaphysis also has been identified by scintigraphy. Tibia Incomplete fractures of the tibial shafts have been described in racing Thoroughbreds. 17, 27, 31 Although a relatively uncommon occurrence, complete tibial fracture during racing usually warrants humane destruction of the horse. Scintigraphic imaging of the tibia allows early detection of these stress-induced fractures (Fig. 9). A bone cyst of the proximal tibial epiphysis was identified by scintigraphic evaluation in three patients evaluated for vague hindlimb or suspected stifle lameness by the author at TULAH during 1990. In all three cases, intraarticular anesthesia of the appropriate stifle joints dramatically improved the horse's gait. Similarly, enthesopathy of the insertion of the cranial cruciate ligament has appeared as a focal area of intense radioactivity in the cranial tibial epiphysis. '

Figure 9. A 2-year-old Thoroughbred colt become acutely lame in the left hindlimb after a race. Routine lameness evaluation failed to identify the source of pain. Two weeks of stall confinement did not alleviate the signs. A lateral scintigraph of the left tibia revealed a focus of intense radioactivity (arrows) in the caudolateral shaft consistent with a cortical stress fracture.

228

ROBERT R. STECKEL

Femur (Stifle) Femoral fractures infrequently occur as a result of athletic activity and usually result in severe disruption of the femoral shaft. Most femoral fractures are a result of severe trauma. Clinical diagnosis of a complete femoral fracture in large animals is not difficult. Incomplete fractures (Fig. 10) or avulsion injuries (Fig. 11), however, preclude standard radiographic assessment in standing patients. Even with the aid of a general anesthetic, their detection can be difficult without specialized radiographic equipment. Scintigraphy affords an accurate means of evaluating the femoral shaft in an adult, standing horse. Despite the heavy thigh musculature, the sensitivity of the technique allows detection of minor injury to bone that cannot be detected by radiography. Moreover, serial scintigraphic evaluation can be used to monitor the healing of injury to large bones such as the femur, in which the sensitivity of standard radiography is limited (Fig. 10). The Pelvis Experienced practitioners realize that the frequency of injury to the equine pelvis resulting from routine athletic activity is small. 22, 29, 49 In most horses in which the pelvic bones are fractured, the history indicates a documented incident of major trauma such as a sudden fall or a collision with an automobile. If comminution occurs and the fracture fragments are displaced, pelvic examination per rectum usually is accurate in arriving at a diagnosis. 22 , 29, 49 However, two recent retrospective studies indicate that

Figure 10. A 10-year-old Selle Francais gelding fell while competing at show jumping. He exited the jump course three-legged lame in the left hindlimb. Survey radiographs of the left stifle region were normal even though effusion of that joint had become marked. The lameness improved somewhat during 3 weeks of stall confinement. Delayed lateral bone image of the left stifle revealed intense uptake of isotope (arrows) in the lateral femoral condyle (posterior-anterior view not shown). Radiographs were still normal, and a diagnosis of acute stress (compression) fracture was made. Two subsequent scintigraphs over a 9-n1onth period showed resolution of the femoral condyle inflammation. The horse returned to hindlimb soundness.

THE ROLE OF SCINTIGRAPHY IN THE LAMENESS EVALUATION

229

Figure 11. A 7-year-old Dutch Warmblood dressage gelding was examined for moderate left hindlimb lameness after falling in an icy pasture 1 month previously. Physical examination failed to localize the source of hindlimb pain. A lateral scintigraph of the left hip revealed abnormal isotope retention in the third trochanter region of the left femur (arrows). The greater trochanter is identified (G). Tangential radiography of the left third trochanter revealed a separated bone fragment that was surgically removed. The horse returned to complete soundness.

a significant portion of pelvic fractures are nondisplaced, and examination per rectum is unreliable in diagnosis. 29, 49 Figure 12 documents spontaneous incomplete fracture of the ileal shaft near the acetabulum 29 in a 4-year-old female polo pony. Other conditions of the pelvis, such as muscle avulsion of the semitendinosus, have been identified by scintigraphy as a diffuse radiointense area overlying the affected ischium. 27 Fracture of other bones of the os coxae also have been identified (Fig. 13). Separation of the sacroiliac joint is a common presumptive diagnosis for vague hindlimb lameness in horses involved in athletic competition that requires jumping. 23 Unfortunately, confirmation of the diagnosis may take weeks to months in some patients as the tuber sacralae becomes more prominent due to subluxation. Radiographic diagnosis is difficult and requires general anesthesia. 22 In the author's experience, scintigraphic imaging of the pelvis from the dorsal view has been extremely accurate in confirming or ruling out suspect cases of sacroiliac dislocation (Fig. 14). The Vertebral Column In addition to diagnosing conditions of the pelvis, bone scanning provides a major advantage for diagnosis of patients suspected of having back pain or disease of the vertebral column. With the horse standing sedated, imaging of the entire column is performed routinely from both the lateral and dorsal views. The most common lesion identified in adult performance horses is impinging dorsal spinous processes,21, 53 or "kissing

230

ROBERT R. STECKEL

Figure 12. A 4-year-old Thoroughbred female polo pony became three-legged lame in the right hindlimb during a match. Physical and lameness examinations failed to identify the source of pain. Three weeks of stall confinement brought about some improvement, but significant lameness persisted. A dorsal pelvic scintigraph (A) revealed abnormal isotope uptake in the right hemipelvis (arrows). Ventrodorsal radiograph (B) under anesthesia identified a nondisplaced right ileal shaft fracture cranial to the acetabulum (open and solid arrows). The mare recovered soundness after long-term rest from polo.

THE ROLE OF SCINTIGRAPHY IN THE LAMENESS EVALUATION

231

A

Figure 13. An 8-year-old Thoroughbred gelding was evaluated for intermittent left hindlimb lameness of 2 weeks' duration; the lameness was obvious only during jumping. Routine examination confirmed the lameness complaint but failed to localize the site of pain. A dorsal pelvic scintigraph (A) revealed focally intense isotope retention in the region of the left tuber ischium (arrow). B = radioisotope accumulation in the urinary bladder. Ventrodorsal radiograph (B) under anesthesia confirmed a left ischial tuberosity fracture (arrows). Longterm stall rest was recommended.

232

ROBERT R. STECKEL

Figure 14. Bone scanning of the pelvis and the hindlimbs was performed after the gelding in Figure 10 had sustained an accident on the jump course. Dorsal scintigraphic image of the pelvis revealed acute inflammation of the left tuber sacrale (arrows). B = radioisotope accumulation in the urinary bladder. Sacroiliac subluxation had occurred simultaneous with the stifle injury. Two followup scintigraphs indicated resolution of inflammation in the left tuber sacralae as the hindlimb lameness resolved completely. The gelding did, however, develop an obvious "hunter's bump.»

spines" (Fig. 15). A typical scintigraphic pattern of increased radioisotope uptake in the tips of the dorsal spines from T 12 through L 2 has .been documented in a large number of cases. 53 Radiographs show combined bone lysis and sclerosis in addition to cyst formation in severely affected vertebral spinous processes that correspond exactly to the area of isotope retention on the scintigraph (Fig. 15). Clinical evaluation of the significance of vertebral column changes has been well described. In the author's experience,53 their contribution to the current lameness complaint is easily confirmed by local anesthesia of suspect areas. Surgical treatment for affected horses has been reported. 46 Occasionally, a scan of the torso reveals an uncommon injury. Figure 16 depicts multiple rib fractures in a horse suspected of having a vertebral column injury after becoming cast in a stall.

SOFT-TISSUE SCINTIGRAPHY The previously mentioned differentiation of soft-tissue inflammation versus bone is another indication for use of scintigraphic evaluation for orthopedic conditions. The nuclear medicine specialist has two options in

THE ROLE OF SCINTIGRAPHY IN THE LAMENESS EVALUATION

233

B Figure 15. A 12-year-old Thoroughbred gelding that had been retired from jumping competition began to exhibit reluctance to be tacked for pleasure riding after having exhibited hindlimb stiffness over the previous 4 months. During the month prior to referral, marked stall behavior changes had occurred. Clinical evaluation indicated the possibility of severe, chronic back pain. A right dorsolateral oblique scintigraph (A) of the spinal column revealed intense uptake of isotope in the tips of the dorsal vertebral spines (arrows)~ of the lower thoracic column. The right kidney (R) still retains radioisotope on the delayed image. Standing lateral radiograph (B) revealed severe crowding and degenerative bony changes of the affected spinous processes of thoracic vertebrae T I2 through T I8 • The gelding was humanely destroyed because of the severe behavioral change.

234

ROBERT R. STECKEL

Figure 16. A 13-year-old Thoroughbred gelding was examined for vague fore and hindlimb lameness after becoming cast in a stall. Routine lameness evaluation failed to identify the source of pain. Bone scanning of the entire skeleton revealed a strained left sacroiliac joint (not shown). Dorsolateral right oblique scintigraph of the caudal thorax revealed five fractured ribs over the right thoracic wall and a fractured spinous process of the ninth thoracic vertebra (arrow). Radiographs of the thorax were not done. Six months of rest from athletic activity were recommended.

evaluating soft-tissue diseases. First, a three-phase scan should be considered. The previously described Phase 2 examines the bone isotope 99mTc_ MDP within 20 minutes of injection 27, 37, 48 as the material moves from the vascular pool (Phase 1) into soft tissues. Sufficient radioactivity to produce a clear image is emitted from soft tissues for only a short time, because the material is quickly redistributed throughout the body. Nonetheless, accurate identification of soft-tissue inflammation can be done. 27 Because some soft-tissue conditions such as suspensory desmitis and concomitant sesamoiditis interface with bone inflammation, the alternative method is to use a separate isotope for soft-tissue imaging that is not as well bound to bone. 6 , 27 A soft-tissue scan using 99mTc-pertechnetate was used as described to separate the component of suspensory desmitis from fetlock arthropathy in a mature Thoroughbred mare used for dressage competition (Fig. 17).

MUSCLE INJURIES Scintigraphy using isotopes that are selective for bone has been shown to be extremely accurate in detecting specific muscle damage after exercise

THE ROLE OF SCINTIGRAPHY IN THE LAMENESS EVALUATION

235

A Figure 17. A 12-year-old Thoroughbred mare that had been retired from dressage competition became lame in the right hindlimb after a field hunt. Routine examination revealed severe diffuse soft-tissue swelling and mild joint effusion of the right hind fetlock. Intraarticular anesthesia of the fetlock brought about only partial relief from the painful gait. Delayed bone scintigraphy of both hindlimbs was within normal limits. A separate soft-tissue scan of the distal hindlimbs only was performed. A lateral image of the right hind fetlock (A) revealed diffuse focal uptake of isotope in the region of the suspensory ligament (arrows). Lateral radiograph of the affected fetlock (B) revealed normal bony architecture but also showed displacement of the sesamoid bones distally, indicating chronic breakdown of the suspensory apparatus. The mare was retired for breeding.

in human endurance runners. 36, 37 The areas of most intense isotope uptake correlate with the areas of greatest pain reported in these patients. 36 The mechanism of a technetium-labeled agent binding to injured muscle tissue is incompletely understood, but it appears to be different from hydroxyapatite adsorption in bone. It has been suggested that release of colloidal calcium phosphate from the mitochondria of damaged muscle cells allows formation of hydroxyapatite crystals in the extracellular fluid. 37 In clinical study36 and controlled experimentation, 51 scintigraphy appeared capable of identifying individual muscle injury if the scan was performed within 48 hours. Mter 4 or 5 days, inflamed muscles were not visible by scintigraphy. Scintigraphic imaging of individual muscle injury in equine athletes recently has been reported. 27, 50 Equine practitioners examining patients in race training or other forms of extreme athletic activity should consider the use of scintigraphy to not only confirm the provisional diagnosis of exerciseinduced myositis but also to identify which muscles have been injured so that specific local treatment may be given (Fig. 18).

236

ROBERT R. STECKEL

Figure 18. A 2-year-old Standardbred colt was presented for poor performance and left hindlimb lameness. Pre-exercise serum enzymes for muscle were mildly elevated. A complete bone scan was normal. Treadmill exercise was followed by a marked rise in serum enzymes of muscle origin. During a repeat scan on the day after exercise, posterior-anterior imaging of the upper hindlimb and pelvic region showed intense uptake of Tc-MDP in the left semitendinosus muscle and moderate uptake in the left middle gluteal and right semitendinosus muscles.

SUMMARY

Bone scanning to help diagnose orthopedic disease has been used in human patients for over two decades. The value of this diagnostic tool has been well established in helping to identify a variety of musculoskeletal conditions. It has only recently been used by veterinarians for more accurate characterization of equine musculoskeletal disease. The technique offers the major advantage of increased sensitivity over standard radiographic imaging. The case material illustrated here shows that except for consistent identification of bone cysts, most of the pathologic changes to the horse's musculoskeletal system that might cause lameness are detected on bone scans. Many acute bone diseases can be diagnosed by scintigraphy that

THE ROLE OF SCINTIGRAPHY IN THE LAMENESS EVALUATION

237

cannot be discerned by radiographs until the condition has become chronic: Because of their body size, these conditions may not be diagnosed at all in horses. Scintigraphy in horses offers the other major advantage of affording accurate imaging of the upper limbs, pelvis, and vertebral column without general anesthesia. Therefore, it has a final advantage of increased safety over conventional radiography because it eliminates the need to perform general anesthesia to study these areas. In the author~s experience, if abnormal uptake of isotope in the upper limbs, pelvis or spine is not observed, general anesthesia to radiograph those areas is not warranted. A second major benefit of scintigraphic imaging is to differentiate mixed lameness conditions in which the component of bone disease must be separated from that of soft tissues to arrive at a rational course of treatment or prognosis. Finally, for athletic horses suspected of having lameness due to localized myositis, scintigraphy not only allows confirmation of muscle inflammation but also identifies the muscle bellies injured reasonably accurately so that specific local treatment may be given. Nuclear imaging of equine skeletal disease is an option that should be employed more frequently by equine practitioners for diagnosis of difficult lameness cases. The technique is safe and comparatively inexpensive when one considers the total expense of multiple examinations or radiographic surveys of patients without conclusively diagnosing the source or sources of skeletal pain. This is particularly true when a horse owner becomes dissatisfied and enlists the services of one or more other veterinarians. The equine specialist will maintain better client rapport if he or she suggests referral of the horse to a veterinary medical teaching center or private clinic where scintigraphic imaging can be done rather than having the client become frustrated and seek another opinion elsewhere. The author had the occasion to examine lame horses both in private practice and at a university teaching hospital over a 6-year period, before having the convenience to opt for a bone scan on selected cases. Experience gained over 8 years at TULAH proved the advantage of scintigraphic imaging in helping to thoroughly evaluate equine lameness in many different breeds of horses used for several different types of athletic activity.

REFERENCES 1. Adams SB, Blevins WE: Shoulder lameness in horses: Part I. Compend Contin Educ 11:64, 1989 2. Adams SB, Blevins WE: Shoulders lameness in horses: Part II. Compend Contin Educ 11:192, 1989 3. Allhands RV, Twardock AR, Boero MJ: Uptake of 99mTc-MDP in muscle associated with a peripheral nerve block. Vet Radiol 28: 181, 1987 4. Attenburrow DP~ Portergill MJ, Vennart W: Development of an equine nuclear medicine facility for gamma camera imaging. Equine Vet J 21:86, 1989 5. Beeman GM: The clinical diagnosis of lameness. Compend Contin Educ 10:209, 1988 6. Boerbooms AMT, Buys WCAM: Rapid assessment of 99mTc-Pertechnetate uptake in the knee joint as a parameter of inflammatory activity. Arthritis Rheum 21:348, 1978 7. Charles ND: Skeletal blood flow: Implications for bone scan interpretation. J Nucl Med 21:91, 1980 8. DeHaan CE, O'Brien TR, Koblik PD: A radiographic investigation of third carpal bone injury in 42 racing Thoroughbreds. Vet Radiol 28:88, 1987

238

ROBERT R. STECKEL

9. Derksen FJ: Diagnostic local anesthesia of the equine front limb. Equine Pract 2:41, 1980 10. Devous MD, Twardock AR: Techniques and applications of nuclear medicine in the diagnosis of equine lameness. J Am Vet Med Assoc 184:318, 1984 11. Dyson S: Diagnostic technique in the investigation of shoulder lameness. Equine Vet J 18:25, 1986 12. Dyson S: Shoulder lameness in horses: An analysis of 58 suspected cases. Equine Vet J 18:29, 1986 13. Ellis DR, Simpson DJ, Greenwood RES, et al: Observations and management of fractures of the proximal phalanx in young Thoroughbreds. Equine Vet J 19:43, 1987 14. Gaughan EM, Wallace RJ, Kallfelz FA: Local anesthetics and nuclear medical bone images of the equine forelimb. Vet Surg 19:131, 1990 15. Genovese RL, Rantanen NW, Simpson BS: The use of ultrasonography in the diagnosis and management of injuries to the equine limb. Compend Contin Educ 9:945, 1987 16. Hajek MR, Noble HB: Stress fractures of the femoral neck in joggers: Case reports and review of the literature. Am J Sports Med 10:112, 1982 17. Haynes PF, Walters JW, McClure JR, et al: Incomplete tibial fractures in three horses. J Am Vet Med Assoc 177:1143, 1980 18. Holder LE: Current concepts review: Radionuclide bone-imaging in the evaluation of bone pain. J Bone Joint Surg 64A:1391, 1982 19. Holder LE, Michael RH: The specific scintigraphic pattern of "shin splints in the lower leg": Concise communication. J Nucl Med 25:865, 1984 20. Hopes R: Lameness: Diagnosing the site of pain. Equine Vet J 18:89, 1986 21. Jeffcott LB: Diagnosis of back problems in the horse. Compend Contin Educ 3:S134, 1981 22. Jeffcott LB: Pelvic lameness in the horse. Equine Pract 4:21, 1982 23. Jeffcott LB: Sacroiliac lesions as a cause of chronic poor performance in competitive horses. Equine Vet J 17:111, 1985 24. Johnson PJ, Allhands RV, Baker GJ, et al: Incomplete linear tibial fractures in two horses. J Am Vet Med Assoc 192:522, 1988 25. Koblik PD, HornofWJ, Seeheman HJ: Scintigraphic appearance of stress-induced traun1a of the dorsal cortex of the third metacarpal bone in racing Thoroughbred horses: 121 cases (1978-1986). J Am Vet Med Assoc 192:390, 1988 26. Lamb CR, Morris EA: Coxofemoral arthrosis in an aged mare. Equine Vet J 19:350, 1987 27. Lamb CR, Koblik PD: Scintigraphic evaluation of skeletal disease and its application to the horse. Vet Radiol 29:16, 1988 28. Lamb CR, Koblik PD, O'Callahan MW, et al: Comparison of bone scintigraphy and radiography as aids in the evaluation of equine lameness: Retrospective analysis of 275 cases. Proc Am Assoc Equine Pract 34:359, 1988 29. Little C, Hilbert B: Pelvic fractures in horses: 19 cases (1974-1984). J Am Vet Med Assoc 190: 1203, 1987 30. Lloyd KCK, Koblik PD, Ragle C, et al: Incomplete palmar fracture of the proximal extremity of the third metacarpal bone in horses: Ten cases (1981-1986). J Am Vet Med Assoc 192:798, 1988 31. Mackey VS, Trout DR, Meagher DM, et al: Stress fractures of the humerus, radius, and tibia in horses: Clinical features and radiographic and/or scintigraphic appearance. Vet RadioI28:26, 1987 32. Markel MD, Martin BB, Richardson DW: Dorsal frontal fractures of the first phalanx in the horse. Vet Surg 14:36, 1985 33. Markel MD, Richardson DW: Noncomminuted fractures of the proximal phalanx in 69 horses. J Am Vet Med Assoc 186:573, 1985 34. Markel MD, Snyder JR, Hornof WJ, et al: Nuclear scintigraphic evaluation of third metacarpal and metatarsal bone fractures in three horses. J Am Vet Med Assoc 191:75, 1987 35. Matheson GO, Clement DB, McKenzie DC, et al: Stress fractures in athletes. Am J Sports Med 15:46, 1987 36. Matin P, Lang G, Caretta R, et al: Scintigraphic evaluation of muscle damage following extreme exercise: Concise communication. J Nucl Med 24:308, 1983 37. Matin P: Basic principles of nuclear medicine techniques for detection and evaluation of trauma and sports medicine injuries. Semin Nucl Med 18:90, 1988 38. May SA, Wyn-Jones G: Identification of hindleg lameness. Equine Vet J 19:185, 1987

THE ROLE OF SCINTIGRAPHY IN THE LAMENESS EVALUATION

239

39. Metcalf MR, Tate LP, Sellett LC: Clinical use of 99mTc-MDP scintigraphy in the equine mandible and maxilla. Vet Radiol 30:80, 1989 40. Metcalf MR, Sellett LC, Smallwood JE, et al: Scintigraphic characterization of the equine foredigit and metacarpophalangeal region from birth to six months of age. Vet Radiol 30:111, 1989 41. Metcalf MR, Tate LP, Sellett LC, et al: Radiographic and scintigraphic imaging of a proximal radial physeal injury in a young horse induced by olecranon fracture repair. Equine Vet J 22:56, 1990 42. Mubark SJ, Gould RN, Lee YF, et al: The medial tibial stress syndrome: A cause of shin splints. Am J Sports Med 10:201, 1982 43. Nutton RW, Fitzgerald RH, Kelly PJ: Early dynamic bone-imaging as an indicator of osseous blood flow and factors affecting the uptake of 99mTc hydroxymethylene diphosphate in healing bone. J Bone Joint Surg 67A:763, 1985 44. Nyrop KA, Coffman JR, DeBowes RM, et al: The role of diagnostic nerve blocks in the equine lameness examination. Compend Contin Educ 5:S669, 1983 45. Pool RR, Meagher DM: Pathologic findings and pathogenesis of racetrack injuries. Vet Clin North Am Equine Pract 6:1, 1990 46. Roberts EJ: Resection of thoracic or lumbar spinous processes for the relief of pain responsible for lameness and some other locomotor disorders of horses. Proc Am Assoc Equine Pract 14:13, 1968 47. Pool RR, Meagher DM, Stover SM: Pathophysiology of navicular syndrome. Vet Clin North Am Equine Pract 5:109, 1989 48. Rupani HD, Holder LE, Espinola EA, et al: Three-phase radionuclide imaging in sports medicine. Radiology 156:187, 1985 . 49. Rutikowski JA, Richardson DW: A retrospective study of 100 pelvic fractures in horses. Equine Vet J 21:256, 1989 50. Seeheman HJ, Morris E, O'Callaghan MW: The use of sports medicine techniques in evaluating the problem athlete. Vet Clin North Am Equine Pract 6:239, 1990 51. Siegel BA, Engel WK, Derrer EC: 99mTc-diphosphonate uptake in skeletal muscle: A quantitative index of acute damage. Neurology 25:1055, 1975 52. Stashak TS: Diagnosis of Lameness. In Adam's Lameness in Horses. Philadelphia, Lea & Febiger, 1987, p 100 53. Steckel RR, Mitchell S: Clinical aspects of thoraco-Iumbar spinal disease in horses: A review of 52 cases. In preparation. 54. Trout DR, Hornof WJ, O'Brien TR: Soft-tissue and bone-phase scintigraphy for diagnosis of navicular disease in horses. J Am Vet Med Assoc 198:73, 1991 55. Turner TA, Purohit RC, Fessler JF: Thermography: A review in equine medicine. Con1pend Contin Educ 8:855, 1986 56. Turner TA, Kneller SK, Badertscher RR, et al: Radiographic changes in the navicular bones of normal horses. Proc Am Assoc Equine Pract 32:309, 1986 57. Ueltshi G: Bone and joint imaging with 99mTc-Iabeled phosphates as a new diagnostic aid in veterinary orthopedics. J Am Vet Radiol Soc 18:80, 1977 58. Widmer WR, Shaw SM, Hurd CD, et al: Radiation biology and radiation safety. Compend Cantin Educ 11:1237, 1989 59. Young A, O'Brien TR, Pool RR: Exercise-related sclerosis in the third carpal bone of the racing Thoroughbred. Proc Am Assac Equine Pract 34:339, 1988

Address reprint requests to Robert R. Steckel, DVM, MS Department of Clinical and Population Sciences University of Minnesota College of Veterinary Medicine 1365 Gortner Avenue St. Paul, MN 55108