History
A 1-year-old 350-kg (770-lb) Thoroughbred filly was referred for examination because of moderate focal swelling overlying the dorsodistal aspect of the left metacarpal region and lameness in the left forelimb. On physical examination, no lameness was detected; however, palpation of the left forelimb revealed moderate effusion around the common digital extensor tendon sheath and a hard, well-delineated mass within the sheath proximal to the metacarpophalangeal (fetlock) joint. No signs of pain were elicited with palpation, and other findings on physical examination were unremarkable. Radiography was performed (Figure 1).
Lateromedial (A) and dorsomedial-palmarolateral oblique (B) radiographic images of the left metacarpophalangeal (fetlock) joint of a 1-year-old 350-kg (770-lb) Thoroughbred filly referred because of moderate focal swelling overlying the dorsodistal aspect of the left metacarpal region and lameness in the left forelimb.
Citation: Journal of the American Veterinary Medical Association 257, 6; 10.2460/javma.257.6.595
Lateromedial (A) and dorsomedial-palmarolateral oblique (B) radiographic images of the left metacarpophalangeal (fetlock) joint of a 1-year-old 350-kg (770-lb) Thoroughbred filly referred because of moderate focal swelling overlying the dorsodistal aspect of the left metacarpal region and lameness in the left forelimb.
Citation: Journal of the American Veterinary Medical Association 257, 6; 10.2460/javma.257.6.595
Lateromedial (A) and dorsomedial-palmarolateral oblique (B) radiographic images of the left metacarpophalangeal (fetlock) joint of a 1-year-old 350-kg (770-lb) Thoroughbred filly referred because of moderate focal swelling overlying the dorsodistal aspect of the left metacarpal region and lameness in the left forelimb.
Citation: Journal of the American Veterinary Medical Association 257, 6; 10.2460/javma.257.6.595
Formulate differential diagnoses and treatment strategies from the history, clinical findings, and Figure 1—then turn the page →
Radiographic Findings and Interpretation
Radiography revealed a well-defined broad-based mineral opacity projecting from the dorsomedial aspect of the distal portion of the third metacarpal bone and moderate focal thickening of the soft tissues in the area of the common digital extensor tendon and sheath (Figure 2). This focal thickening of the soft tissues paralleled the underlying bone proliferation. Furthermore, evidence of soft tissue swelling adjacent to the area of the common digital extensor tendon sheath was suggestive of tenosynovitis. On the basis of physical and radiographic findings, our primary differential diagnosis was osteochondroma or periosteal exostosis in response to external trauma, and surgical removal was recommended.
Same radiographic images as in Figure 1. There is a well-defined broad-based mineral opacity (long arrows) projecting from the dorsomedial aspect of the distal portion of the third metacarpal bone. In the area of the common digital extensor tendon and sheath, there is moderate focal thickening of the soft tissues (short arrows), and the thickening parallels the underlying bone proliferation.
Citation: Journal of the American Veterinary Medical Association 257, 6; 10.2460/javma.257.6.595
Same radiographic images as in Figure 1. There is a well-defined broad-based mineral opacity (long arrows) projecting from the dorsomedial aspect of the distal portion of the third metacarpal bone. In the area of the common digital extensor tendon and sheath, there is moderate focal thickening of the soft tissues (short arrows), and the thickening parallels the underlying bone proliferation.
Citation: Journal of the American Veterinary Medical Association 257, 6; 10.2460/javma.257.6.595
Same radiographic images as in Figure 1. There is a well-defined broad-based mineral opacity (long arrows) projecting from the dorsomedial aspect of the distal portion of the third metacarpal bone. In the area of the common digital extensor tendon and sheath, there is moderate focal thickening of the soft tissues (short arrows), and the thickening parallels the underlying bone proliferation.
Citation: Journal of the American Veterinary Medical Association 257, 6; 10.2460/javma.257.6.595
Treatment and Outcome
Four months later, the filly was returned for surgical removal of the exostosis on its left third metacarpal bone. During surgery, the common digital extensor tendon sheath was confirmed to have been thickened and the structure's synovial fluid had low viscosity. The removed exostosis was submitted for evaluation. Perioperative medications included cephalothin sodium (20 mg/kg [9.1 mg/lb], IV) and phenylbutazone (4.4 mg/kg [2.0 mg/lb], IV). Postoperative care included a compressive bandage that was changed every 2 days for 10 days, barn rest for 14 days, and phenylbutazone (2.2 mg/kg [1.0 mg/lb], IV, q 12 h) for 3 days. Histologic examination of the surgically removed exostosis revealed a thin layer interpreted as periosteum and a layer of cartilaginous tissue, suggestive of osteochondroma.
When the filly was reevaluated 3 months after surgery, no lameness, soft tissue swelling, or common digital extensor tendon sheath effusion was observed. Recheck radiography was performed (Figure 3), and no abnormalities were detected at the site from which the osteochondroma had been removed.
Lateromedial (A) and dorsomedial-palmarolateral oblique (B) radiographic images of the left front fetlock of the filly in Figures 1 and 2 obtained 3 months after surgery. There is no residual bony protrusion at the site from which the osteochondroma had been removed (brackets), and the soft tissue swelling has resolved.
Citation: Journal of the American Veterinary Medical Association 257, 6; 10.2460/javma.257.6.595
Lateromedial (A) and dorsomedial-palmarolateral oblique (B) radiographic images of the left front fetlock of the filly in Figures 1 and 2 obtained 3 months after surgery. There is no residual bony protrusion at the site from which the osteochondroma had been removed (brackets), and the soft tissue swelling has resolved.
Citation: Journal of the American Veterinary Medical Association 257, 6; 10.2460/javma.257.6.595
Lateromedial (A) and dorsomedial-palmarolateral oblique (B) radiographic images of the left front fetlock of the filly in Figures 1 and 2 obtained 3 months after surgery. There is no residual bony protrusion at the site from which the osteochondroma had been removed (brackets), and the soft tissue swelling has resolved.
Citation: Journal of the American Veterinary Medical Association 257, 6; 10.2460/javma.257.6.595
Comments
Osteochondromas, characterized by anomalous bone formation on otherwise clinically normal bone and covered by a cartilaginous cap, are common in humans1 and are also described in horses,2 mainly in an active growth phase. In horses, solitary osteochondromas have been reported involving the distal aspect of the radius (seemingly most common),2–5 tibia,6 calcaneus,7,8 or nasal bone.9 To our knowledge, osteochondroma has not previously been described in the distal aspect of the third metacarpal bone in a horse, as was the case with the filly of the present report; however, osteochondroma has been reported in a similar location in a white rhinoceros.10
Radiographic findings in the filly of the present report greatly facilitated diagnosis and treatment. We recognized that ultrasonographic examination of the affected limb, particularly the common digital extensor tendon and sheath, may have also been performed to help evaluate focal soft tissue swelling; however, on the basis of the surgeon's experience combined with the treatment plan to remove the exostosis, intraoperative assessment of the common digital extensor tendon and sheath was selected.
Similar to the findings in the filly of the present report, horses with exostoses (from other various causes) on the palmar or plantar surface of a third metacarpal or metatarsal bone have shown inconsistent lameness.11 Surgical removal of the osteochondroma from the filly in the present report successfully reduced tendon sheath inflammation and improved the appearance of the affected limb. In addition, arthroscopic- and tendoscopic-assisted removal of osteochondromas from other anatomic locations in horses has been successful, and prognosis after surgery, regardless of the technique or anatomic location, is good for return to athletic activity.3–6 Given the soft tissue impingement by the physical protrusion of the osteochondroma in the filly of the present report and resolution of clinical and radiographic abnormalities after surgery, we believed, with reasonable confidence, that physical presence of the osteochondroma induced the filly's tenosynovitis.
Findings in the filly of the present report underscored that osteochondroma should be included on the differential diagnosis list for radiographic evidence of exostosis on the distal aspect of the third metacarpal bone in horses and that tenosynovitis may occur because of physical impingement. Physical, radiographic, and histologic examinations are crucial in diagnosing osteochondroma.
References
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