History

A 300-kg (660-lb) 22-year-old Shetland Pony cross mare was referred to Littleton Equine Medical Center for evaluation of non–weight-bearing lameness of the left forelimb. Ten days prior to referral, the pony was evaluated by the referring veterinarian for lameness. At that time, the pony had a grade 3/5 left forelimb lameness on the basis of the American Association of Equine Practitioners lameness scale.¹ Results of flexion testing of the left forelimb were positive, and the pony had mild muscle atrophy around the left shoulder joint. The referring veterinarian recommended a treatment course of phenylbutazone (3.3 mg/kg [1.5 mg/lb], PO, q 24 h), which did not improve the lameness. The pony was found the morning of referral with acute non–weight-bearing lameness of the left forelimb, with no evidence of external trauma.

At the time of referral, the pony had a grade 5/5 lameness¹ of the left forelimb with mild atrophy of the infraspinatus and supraspinatus muscles. Palpable heat and swelling was present over the lateral aspect of the triceps muscle, and there was marked instability between the proximal and distal aspects of the humerus when the forelimb was manipulated. Radiographs of the left humerus were obtained (Figure 1).

View Full Size

Craniomedial-caudolateral oblique radiographic view of the left humerus of a 22-year-old Shetland pony cross mare with acute, left forelimb, non–weight-bearing lameness following a 10-day history of milder lameness.

Citation: Journal of the American Veterinary Medical Association 247, 2; 10.2460/javma.247.2.149

• Download Figure
• Download figure as PowerPoint slide

Radiographic Findings and Interpretation

A complete, noncomminuted long oblique fracture of the diaphysis of the left humerus is evident with the distal aspect displaced caudally and proximally. A focal, irregularly margined region of moth-eaten lysis in the cranial aspect of the proximal portion of the humerus is present; the lesion extends from the deltoid tuberosity to the base of the greater tubercle with concurrent irregular periosteal reaction (Figure 2). These findings led to the radiographic diagnosis of a fracture of the left humerus with associated aggressive bone lysis. The primary differential diagnosis is a pathological fracture secondary to primary bone neoplasia, although a clear connection between the fracture line and the aggressive bone lesion is not apparent. Metastatic neoplasia and osteomyelitis are also considered as differential diagnoses for the aggressive bone lysis but considered less likely.

View Full Size

Same radiographic image as in Figure 1. There is a complete, long oblique fracture of the diaphysis of the left humerus with the distal aspect displaced caudally and proximally (arrows). There is also an ill-defined region of moth-eaten lysis with irregular periosteal proliferation at the cranioproximal aspect of the humerus (arrowheads).

Citation: Journal of the American Veterinary Medical Association 247, 2; 10.2460/javma.247.2.149

• Download Figure
• Download figure as PowerPoint slide

Treatment and Outcome

Because of the presence of a complete humeral fracture, the owners elected euthanasia. Postmortem examination of the left humerus confirmed the complete oblique fracture of the left humeral diaphysis with a fracture line extending proximally along the caudal aspect of the metaphysis into the head of the humerus. A firm, osseous mass was present on the lateral aspect of the proximal portion of the humerus, just proximal to the deltoid tuberosity and wrapping cranially and caudally as it coursed proximally up to the greater tubercle and to the head of the humerus. A craniocaudal radiograph of the disembodied humerus shows more clearly the moth-eaten lytic region extending from the deltoid tuberosity proximally and the continuation of the fracture line into the head of the humerus (Figure 3). On cut section, the bone overlying the mass was thin and soft with a necrotic center. The histopathologic diagnosis was undifferentiated sarcoma that, even with special stains, could not be differentiated further as an osteosarcoma or hemangiosarcoma.

View Full Size

Craniocaudal radiographic view of the disembodied left humerus of the pony in Figure 1. The moth-eaten lytic region is more apparent on the postmortem radiograph, compared with the initial radiograph, extending from the deltoid tuberosity proximally toward the lateral aspect of the head of the humerus (arrowheads), and the continuation of the fracture line into the head of the humerus is more clearly seen (arrow).

Citation: Journal of the American Veterinary Medical Association 247, 2; 10.2460/javma.247.2.149

• Download Figure
• Download figure as PowerPoint slide

Comments

Radiographic findings in the pony of the present report not only confirmed the presumptive diagnosis of a humeral fracture made on the basis of physical examination findings, but also provided an explanation for the fracture's occurrence. The radiographic findings of an aggressive lesion, characterized by cortical bone destruction, active periosteal proliferation, and an indistinct margin between radiographically normal and abnormal bone, are consistent with both osseous neoplasia and osteomyelitis.² Osteomyelitis, in this pony, was considered unlikely because it is usually associated with a traumatic wound or surgical implants in adult horses.³ Nevertheless, further diagnostic tests were needed to distinguish between neoplasia and osteomyelitis, given that it is not possible to make this distinction on the basis of radiographic findings alone.

Musculoskeletal neoplasia in horses is considered extremely rare, and the frequency with which it occurs has not been reported. Primary bone tumors, such as osteosarcomas, are more likely to involve the axial skeleton than the appendicular skeleton. Lymphosarcoma and hemangiosarcoma can metastasize to bones, but this rarely occurs in horses and does not usually result in cortical bone destruction.⁴ There are only 2 clinical reports^5,6 in the literature that describe a pathological fracture occurring secondary to a primary bone tumor in horses, highlighting the rarity of these findings in equine patients. Even in dogs, where bone tumors occur more frequently, the incidence of pathological fractures associated with appendicular osteosarcoma is considered low.⁷

In horses, identifying osseous neoplasia in the appendicular skeleton is a diagnostic challenge. In most horses for which a diagnosis of musculoskeletal neoplasia was ultimately made, commonly there was a history of chronic lameness of a few weeks' duration that could not be localized to the distal aspect of the limb.^6,8,a The use of radiography is typically the first step in detecting an osseous neoplasia.⁹ However, obtaining high-quality, diagnostic radiographs of the proximal aspect of the limbs in horses can be difficult because of their large size and anatomy. Although a lateral view of the humerus for the pony of the present report would have helped further characterize and identify the extent of the lysis, it was not deemed necessary because the visible fracture alone had a grave prognosis. Other imaging modalities, such as nuclear scintigraphy and ultrasonography, could have been used to localize and further characterize the suspicious lesion, but ultimately, histologic evaluation was needed to obtain a final diagnosis.