Focal Defect Resembling a Subchondral Bone Cyst of the Ulnar Trochlear Notch in a Dog

Introduction

Subchondral bone cyst-like lesions are commonly reported in horses, humans, and pigs.^1–3 In the horse, they are most commonly reported in the medial femoral condyle within the stifle joint and may be visible radiographically.³ They may be associated with clinical signs of pain and lameness or reported as an incidental finding. In dogs, benign bone cyst-like lesions are uncommon. The majority of those reports are of unicameral bone cysts that affect the metaphysis or diaphysis of long bones.^2,3 However, there are rare reports of lesions consistent with subchondral bone cysts in dogs.¹ This case report describes a patient in which a lesion consistent with a subchondral bone cyst was identified in the trochlear notch of the ulna during computed tomography (CT) and arthroscopy of the elbow joint.

Case Report

A 13 mo old spayed female golden retriever/standard poodle mixed-breed dog was presented for evaluation of intermittent right thoracic limb weight-bearing lameness, which was exacerbated by activity. Radiography of the elbow joints performed by the referring veterinarian revealed osteophytosis of the right elbow joint, mineral opacities adjacent to the medial epicondyle, and was suspicious for a fragmented medial coronoid process (Figures 1A, B). Conservative treatment with nonsteroidal anti-inflammatory medications and strict exercise restriction did not lead to improvement in clinical signs. The patient was referred to the University of Wisconsin-Madison Orthopedic service for further evaluation.

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On presentation, general physical examination was within normal limits. Orthopedic examination revealed a grade 2 out of 5 right thoracic limb lameness, with marked effusion of the right elbow joint and decreased passive range of motion on the right elbow in flexion. Bilateral medial patellar luxation (grade 2 out of 4 right stifle and grade 1 out of 4 left stifle) was also present. Neurologic examination was normal. No other abnormalities were present on clinical or orthopedic examination. Arthrocentesis of the right elbow joint was performed, and no abnormalities were found on cytological analysis of the synovial fluid.

CT was performed of both elbow joints. There was mild humeral ulnar joint incongruency with a focal hypoattenuating region measuring 0.47 cm × 0.32 cm × 0.5 cm, with a hyperattenuating rim. The hypoattenuating region communicated with the articular margin of the central portion of the ulnar trochlear notch of the right elbow joint (Figure 2). Several small, mineral, attenuating bodies were present adjacent to the medial epicondyle of the humerus, ranging in size from 0.33 to 1.5 cm. The medial coronoid process was considered normal. The left elbow joint was unremarkable. Diagnostic interpretation was subchondral bone cyst communicating with the elbow joint, mineralization of the origin of the antebrachial flexor tendons, and secondary degenerative joint disease of the right elbow joint.

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Bilateral elbow arthroscopy was performed using medial arthroscopic camera and instrument portals. In the right elbow, the central portion of the ulnar trochlear notch contained a large full-thickness cartilage defect covered with vascular ingrowth (Figure 3). Probing of the defect revealed extension into the subchondral bone plate. There was marked vascular infiltration and proliferation of the surrounding articular cartilage and synovial membrane. The medial coronoid process and anconeal process were identified, probed, and considered to be normal.

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The osteochondral defect was debrided using a bone curette, and the joint was lavaged. A biopsy of the cyst-like lesion could not be performed without conversion to an open approach and significant increase in patient morbidity. After arthroscopy, hyaluronic acid^a was injected into the joint. Arthroscopic examination of the left elbow joint was normal.

The patient was discharged with instructions for strict activity restriction, low-impact physical therapy, and treatment with carprofen^b. Recheck examination 6 wk after surgery showed resolution of clinical lameness, though mild effusion and decreased flexion of the right elbow joint persisted.

The patient was re-evaluated 26 mo after the initial presentation. Repeat radiography of the right elbow joint performed by the referring veterinarian showed progressive osteoarthritis (Figure 4). Physical examination remained within normal limits. Gait analysis revealed a grade 1 out of 5 right forelimb lameness that was exacerbated with exercise. Orthopedic examination showed moderate effusion and a mild decrease in passive range of motion of the right elbow joint and mild muscle atrophy of the right forelimb. Bilateral patellar luxation persisted without associated lameness at this time. CT of the right elbow joint was repeated, which showed a static focal area of hypoattenuation communicating with the articular margin of the ulnar trochlear notch of the right elbow joint (Figures 5 and 6A, B). Moderate ulnar subtrochlear sclerosis and osteophytosis of the humeral condyle, and static mineral attenuating bodies adjacent to the medial epicondyle were present. Diagnostic interpretation was a static subchondral bone cyst and flexor enthesopathy with progressive secondary degenerative joint disease of the right elbow joint. Second-look arthroscopy was declined at that time, and conservative medical management of the patient's osteoarthritis was continued.

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Discussion

Bone cysts are uncommon findings in veterinary patients. Bone cysts and cyst-like lesions can be either benign or malignant and are classified based on their location and content.² Differentiation between true cysts and cyst-like lesions is based on the presence of an epithelial lining, which occurs in cysts.² Subchondral bone cysts are round, cyst-like lesions of the subchondral bone located between the epiphyseal plate and the articular cartilage.² If visible on plain radiographs, they appear as round, radiolucent defects in the subchondral bone. They are commonly reported in horses and humans and may be found either incidentally or reported as a cause of lameness. In humans, subchondral bone cysts appear to have a synovial cell lining.³ Cyst development is thought to arise because of synovial membrane proliferation through a traumatic cartilage defect.³ In contrast, subchondral bone cysts in horses often lack a synovial lining and are thought to occur secondary to focal retention of cartilage at a weight-bearing site leading to pressure-induced infolding of cartilage.³ A biopsy sample was not taken from the defect in the dog described in this report, so it is not known whether this suspected subchondral bone cyst contained a synovial lining.

Elbow dysplasia is a common cause of lameness in the juvenile dog, and includes fragmentation of the medial coronoid process (FMCP), ununited anconeal process, osteochondritis dissecans (OCD) of the humeral condyle, and joint incongruity of the elbow joint. Flexor enthesopathy of the elbow has also recently been described as either a primary or secondary condition in the elbow dysplasia complex.^4,5 Those conditions can occur either alone or in combination. Techniques that aid in the diagnosis of elbow dysplasia include physical and orthopaedic examination findings, conventional radiography, CT, joint fluid analysis, and arthroscopy. Radiography is often the initial diagnostic test performed for elbow dysplasia, but some lesions can be difficult to definitively diagnose using radiography. An ununited anconeal process is best visualized on a flexed mediolateral projection and is seen as a radiolucent gap between the anconeal process and the ulna.⁶ Elbow OCD lesions, best identified on the craniocaudal radiographic view, are typically located on the distal medial aspect of the humeral condyle.⁶ Elbow OCD lesions are typically seen as a flattening of the subchondral bone of the condyle, often with surrounding sclerosis.⁶ Elbow incongruity and FMCP are typically more difficult to definitively diagnose using radiography. Due to the complex three-dimensional anatomy of the elbow joint, CT is often beneficial in diagnosing elbow incongruity and FMCP. Whereas bone cysts in other locations have been identified using radiography, it is suspected that this lesion was not noted due to its location in the elbow joint and superimposition of surrounding structures.

This is a rare report of a lesion consistent in appearance with a subchondral bone cyst in the ulnar trochlear notch of a dog. The patient was undergoing a diagnostic workup for elbow dysplasia due to intermittent thoracic limb lameness. Mineralization of the origin of the flexor tendons was noted as an incidental finding that remained static on repeat imaging. The pathophysiology of that lesion and its contribution to clinically relevant lameness is still unclear. CT of the affected elbow was particularly helpful in the patient described herein because the lesion was not visible on conventional radiographs. The subchondral bone cyst was identified on arthroscopic examination and debrided arthroscopically. In horses, success rates of simple debridement of a subchondral bone cyst range from 35 to 64%, with success decreasing with advancing age.⁷ Another described technique is arthroscopic injection of corticosteroids into the cyst lining, with success rates reaching 67%.⁸ Success rates with both simple debridement and corticosteroid injection decreased with presence of pre-existing osteoarthritis at the time of treatment.^7,8 Other methods of surgical treatment described for bone cysts include bone or chondrocyte grafting, injection with bone cement, or excision. A recent article evaluating growth factor-enhanced chondrocyte grafts in horses with subchondral bone cysts showed success rates of 74% regardless of age, size of cyst, or presence of pre-existing osteoarthritis.⁹ Due to the rare occurrence of the described lesion in canine patients, limited information is available regarding treatment options and success rates in dogs. In the patient described herein, simple debridement of the cyst was performed, followed by injection of hyaluronic acid. Temporary resolution of the lameness resulted; however, objective gait analysis using a force plate is not routine in the study authors' clinical practice and was not performed in the patient to document changes in clinical function. At the time of long-term follow-up examination, lameness, elbow joint effusion, and progressive osteoarthritis were present and the CT appearance of the bone cyst was unchanged.

As many cases of suspected elbow dysplasia are treated presumptively using conservative therapy and other cases have only radiographs and synovial fluid analysis performed, it is possible that lesions such as the described herein are currently underdiagnosed. The use of CT and arthroscopy allow for greater sensitivity of subtle lesions in the elbow joint of dogs. This case was particularly challenging considering the diagnostic dilemma to distinguish between multiple potential sources of lameness and limited information available to guide therapeutic intervention.

Conclusion

Subchondral bone cyst-like lesions should be considered as a differential diagnosis for dogs with clinical signs resembling elbow dysplasia. Further information regarding treatment options is needed in canine patients.