What Is Your Diagnosis?

Rayne D. Ellington-Lawrence 1Department of Clinical Sciences, College of Veterinary Medicine, Cornell University, Ithaca, NY 14853.

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Michelle L. Delco 1Department of Clinical Sciences, College of Veterinary Medicine, Cornell University, Ithaca, NY 14853.

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Laia Reig Codina 1Department of Clinical Sciences, College of Veterinary Medicine, Cornell University, Ithaca, NY 14853.

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Philippa J. Johnson 1Department of Clinical Sciences, College of Veterinary Medicine, Cornell University, Ithaca, NY 14853.

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 BVSc, MSc

History

A 4-year-old 496.8-kg (1,093.0-lb) Thoroughbred racehorse filly was referred for advanced diagnostic imaging because of left forelimb lameness of 3 months’ duration. The referring veterinarian had identified a grade 2 lameness (on a scale1 of 0 to 5) in the left forelimb that improved approximately 80% with an abaxial sesamoid nerve block (ie, local anesthesia of the medial and lateral palmar and palmar metacarpal nerves at the level of the proximal sesamoid bones). The referring veterinarian obtained lateromedial and dorsopalmar radiographs of the left metacarpophalangeal joint (Figure 1).

Figure 1—
Figure 1—

Lateromedial (A) and dorsopalmar (B) radiographic images of the left metacarpophalangeal joint of a 4-year-old 496.8-kg (1,093.0-lb) Thoroughbred filly racehorse with left forelimb lameness of 3 months’ duration. B—Lateral is to the right.

Citation: Journal of the American Veterinary Medical Association 255, 11; 10.2460/javma.255.11.1227

Determine whether additional imaging studies are required, or make your diagnosis from Figure 1—then turn the page →

Diagnostic Imaging Findings and Interpretation

On radiographic images, the distal aspects of the third metacarpal bone (MCIII) and the proximal sesamoid bones had diffuse increased opacity with loss of the normal trabecular bone pattern, consistent with sclerosis of trabecular bone (Figure 2). Within the palmarodistal subchondral bone margin of MCIII, there was also a focal radiolucent concave defect. Osteophyte formation was present on joint margins, most prominently on the bases and apices of the proximal sesamoid bones. Mild flattening of the palmar articular surface of MCIII was evident, as was a smooth, intra-articular osseous fragment near the dorsal proximal margin of the proximal phalanx.

Figure 2—
Figure 2—

Same images as in Figure 1. There is diffuse increased opacity with loss of the normal trabecular bone pattern (brackets; A and B) in the distal aspects of the third metacarpal bone (MCIII) and the proximal sesamoid bones, consistent with sclerosis of trabecular bone. The MCIII also has an area of flattened articular margin (straight line; A) and a poorly defined, focal subchondral defect (half-circle; A). In addition, a basilar osteophyte on the lateral proximal sesamoid bone and an intra-articular osseous fragment (arrow; A) are evident.

Citation: Journal of the American Veterinary Medical Association 255, 11; 10.2460/javma.255.11.1227

To better evaluate the lesions and identify any concurrent soft tissue injury, the horse was anesthetized and MRIa of the left metacarpophalangeal joint was performed (Figure 3). The MRI consisted of 8 sequences and 214 images, including transverse plane (proton density [PD] and short tau inversion recovery [STIR]), sagittal plane (PD, STIR, and T1-weighted 3-D fast frequency echo [3-D FFE]), and dorsal plane (PD, STIR, and 3-D FFE). On MRI, joint effusion and synovial thickening were evident. In the 3-D FFE and PD images, diffuse hypointensity of the signal throughout the subchondral and epiphyseal regions of the distal end of the MCIII and the proximal sesamoid bones was consistent with trabecular bone sclerosis. In a dorsal plane PD image, 2 focal 1.5 × 1.5-cm subchondral defects on the palmarodistal articular surface of the MCIII were evident, and one of these defects, when viewed in a sagittal plane on a T1-weighted 3-D FFE fat-suppressed image, was associated with a thin rim of hyperintense signal in the subchondral region of the MCIII, consistent with a focal bone marrow lesion. In addition, a prominent basilar osteophyte was evident on the lateral proximal sesamoid bone.

Figure 3—
Figure 3—

Representative sagittal plane T1-weighted 3-D fast frequency echo fat-suppressed (A; dorsal to the left) and dorsal plane proton density (B; lateral is to the right) MRI images of the same left metacarpophalangeal joint of the horse in Figures 1 and 2. Joint effusion and synovial thickening are evident. There is diffuse hypointense signal, consistent with sclerosis, in the lateral proximal sesamoid bone (arc; A) and in the epiphyseal trabecular bone of the distal end of the MCIII (bracket; B). There are 2 focal 1.5 × 1.5-cm subchondral defects (arrows; B) on the palmarodistal articular surface of the MCIII, and the subchondral defect on the lateral condyle (arrow; A) is associated with a thin rim of hyperintense signal (arrowhead; A) in the subchondral region of the MCIII, consistent with a focal bone marrow lesion. A basilar osteophyte (asterisk; A) on the lateral proximal sesamoid bone is also present.

Citation: Journal of the American Veterinary Medical Association 255, 11; 10.2460/javma.255.11.1227

Findings on radiography and MRI (severe sclerosis of the distal aspect of the MCIII and proximal sesamoid bones, flattening of the MCIII articular surface, osteophytes on the proximal sesamoid bones, and subchondral bone defects in the MCIII) were consistent with palmar osteochondral disease (POD). The horse's left forelimb lameness was attributed to POD of the left metacarpophalangeal joint.

Treatment and Outcome

The horse's left metacarpophalangeal joint was injected with triamcinolone (6 mg total), hyaluronic acid (250 mg total), and amikacin (250 mg total), and the horse was discharged from the hospital with a poor prognosis for return to racing. The horse had persistent lameness and was retired for use as a broodmare within 2 months after resuming race training (4 months after receiving the intra-articular treatment).

Comments

Palmar and plantar osteochondral disease, alone or in combination, are common, progressive biomechanical disorders in the metacarpophalangeal and metatarsophalangeal joints, respectively, of working racehorses.2 Intense training regimens of racehorses produce cyclic high-impact loading that results in focal subchondral bone and articular cartilage injury on the palmar aspect of the third metacarpal or metatarsal condyles.2 Excessive loading results in reactive bone turnover, progressive sclerosis of trabecular bone, and accumulation of microfractures.3 Early cartilage damage may contribute to increased loading of subchondral bone, and repeated injury leads to further degradation of adjacent articular surfaces.3 End-stage POD may culminate in severe osteoarthritis and catastrophic fracture, alone or in combination.2

Flattening and ulceration of articular cartilage and bruising of subchondral bone are used to definitively diagnose POD on postmortem examination.2 However, early detection of POD through diagnostic imaging allows for alteration of exercise regimens to potentially mitigate progressive joint degeneration. Radiography is usually the first step in diagnosing POD but has poor sensitivity and specificity (mean sensitivity and specificity of 0.37 and 0.75, respectively, in a recent study4) because lesions are subtle. Therefore, practitioners should use caution when ruling out POD in horses with compatible clinical signs but no evident radiographic lesions. Awareness of common POD lesions may increase the ability of practitioners to detect POD with radiography.4 Primary POD lesions include trabecular bone sclerosis, radiolucent subchondral bone defects in the distal end of the MCIII, and roughening of the palmar articular margin of the MCIII. Secondary lesions associated with joint degeneration, such as proximal sesamoid bone osteophytes, flattening of the palmar condylar surface, and cavitation of the dorsodistal surface of the MCIII, are all highly correlated with POD.4 Of particular importance, if sclerosis of the condyles, flattening of the palmar aspects of the condyles, and proximal sesamoid osteophytes are present, as in the horse of the present report, then POD is highly probable.4 Radiographic abnormalities are more likely to be found with moderate to severe end-stage POD2; therefore, the use of advanced diagnostic imaging to diagnose early stages of POD and monitor progression of the disease has several advantages over conventional radiolography.5

For instance, MRI can be used to evaluate the 3-D distribution and severity of structural changes in the trabecular and subchondral bone and adjacent soft tissue structures.6 High-field MRI provides high-resolution images with reduced motion artifact, allows for detailed evaluation of soft tissue and skeletal structures, and is the preferred imaging modality for assessing small soft tissue lesions and subtle osseous changes.6 In the horse of the present report, the high resolution of MRI allowed for maximum visualization of a bone marrow lesion deep to the subchondral bone. Alternatively, standing low-field MRI produces lower resolution images with increased motion artifact; however, images can be obtained under standing sedation, eliminating risks of general anesthesia.5 Low-field MRI is sensitive enough to detect POD lesions and may be preferred if lesions can be localized with other modalities, such as nuclear scintigraphy.5 In contrast, CT provides excellent visualization of osseous changes and is useful in evaluating for trabecular sclerosis and subchondral thickening but has low contrast for soft tissues structures or bone marrow lesions.6 With the increasing use of standing low-field MRI and CT, earlier diagnosis and monitoring of POD with advanced imaging are likely to increase. However, current standing diagnostic imaging modalities cannot detect cartilage damage as early in the process as can high-field MRI, which if used may maximize potential for early diagnosis of POD in horses.

In the horse of the present report, high-field MRI was elected to evaluate soft tissue structures and further explore the radiographic abnormalities evident. Although the radiographic abnormalities were highly suggestive of POD, the radiographic images could not provide a 3-D assessment of the abnormalities evident or the details of articular cartilage and other soft tissue structures. In contrast, MRI provided us with detail so that we could accurately determine the distribution of bone sclerosis, evaluate the depth of subchondral bone defects, and measure the collapse of articular cartilage. For example, in the dorsopalmar and lateral radiographic images, subchondral defects were either not visible or poorly defined, whereas in the MRI dorsal plane PD image, 2 large condylar defects and complete collapse of the condylar articular surface were present. Lesions of this nature underscored the advantage of advanced diagnostic imaging in assessing and diagnosing lesions in the distal portions of the limbs of horses. Further, being able to obtain 3-D images with high sensitivity for potential lesions of the distal portion of the limb makes it possible to diagnose diseases like POD earlier and easier.

The goal of POD treatment is to prevent further structural damage and facilitate remodeling of sclerotic bone. Treatment often consists of long periods of rest followed by slowly increasing levels of exercise. Although corticosteroids are often used in POD treatment, such treatment is considered palliative, and subsequent clinical improvement is often shortlived. The use of biologic products, such as platelet-rich plasma and autologous stem cells, may be elected; however, there is little evidence to support these treatments for POD.7 Arthroscopic debridement of diseased articular cartilage is also ineffective because of the irreversible nature of structural changes once present.3 For these reasons, it is essential that equine practitioners know the important radiographic abnormalities attributed to POD so that advanced diagnostic imaging and changes to exercise regimens can be recommended potentially in time to prevent more severe and possibly catastrophic developments. The prognosis for horses with POD is poor for returning to racing; however, with proper rest for affected horses and realistic expectations of owners, horses with POD can have otherwise clinically normal lives and serve in other capacities, including as athletes in less demanding disciplines and as breeding stock.

Footnotes

a.

Vantage Atlas 1.5 T MRI, Canon Medical Systems USA Inc, Tustin, Calif.

References

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