History

A 2-year-old 4.9-kg (10.8-lb) spayed female domestic shorthair cat was referred because of a 4-day history of left hind limb lameness. Conservative treatment with NSAID administration had been initiated by the primary care veterinarian the previous day. The patient was an outdoor cat with no history of recent trauma.

At the time of referral, the patient was quiet and timid but responsive. The cat had severe lameness of the left hind limb, and the lameness was non-weight-bearing when the cat was standing. The left patella was displaced dorsally and the left stifle joint could be hyperextended on assessment of the range of motion. Mild thickening of the periarticular soft tissue was evident, compared with that of the contralateral stifle joint. A cranial drawer sign and tibial thrust were not present. Gentle palpation of the left stifle joint did not elicit signs of pain. The body condition score was 6/9, and findings on the rest of the physical examination were unremarkable.

The PCV and total solids concentration were within reference limits. No abnormalities were detected on serum biochemical analysis, which included albumin, globulins, urea, creatinine, and total protein concentrations and alanine aminotransferase activity. Radiographs of the left stifle joint were obtained (Figure 1).

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Mediolateral (A) and caudocranial (B) radiographic views of the left stifle joint of a 2-year-old 4.9-kg (10.8-lb) spayed female domestic shorthair cat that was referred because of a 4-day history of left hind limb lameness.

Citation: Journal of the American Veterinary Medical Association 252, 4; 10.2460/javma.252.4.399

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

Radiographic Findings and Interpretation

There is marked extra-articular soft tissue swelling in the cranial aspect of the left stifle joint. The infrapatellar fat pad and caudal fascial planes are ill defined. This finding may be due to the presence of slight joint effusion, although thickening and proliferation of the synovium is possible. There is an ill-defined, pinpoint mineral opacity superimposed over the cranial horn of the medial meniscus, and it likely represents a meniscal ossicle or mineralization as a sign of degenerative joint disease of the medial compartment.¹ The medial fabella is not mineralized or is missing, as has been described in large nondomestic cats with meniscal ossicles.² The tibial tuberosity is displaced proximally in the mediolateral view and superimposed over the lateral aspect of the stifle joint and lateral meniscus in the caudocranial view. The fragment has irregular but seemingly blunt margins, is of normal bone density, and is positioned horizontally. Caudal to the larger free fragment there is an additional, small bone fragment adjacent to its surface. The craniodistal aspect of the tibial tubercle has an area of smoothly marginated, heterogeneous mineral opacity with a short zone of transition between affected and unaffected bone. It is interpreted as a benign callus formation. Proximal displacement of the patella is evident.

The final radiographic diagnosis was chronic left tibial tuberosity avulsion fracture with subsequent proximal displacement of the patella and callus formation.

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Same radiographic images as in Figure 1. A—There is marked extra-articular soft tissue swelling in the cranial aspect of the left stifle joint. The infrapatellar fat pad and caudal fascial planes (arrow) are ill defined. Notice the ill-defined, pinpoint mineral opacity just cranial to the intracondylar notch (open arrow). The tibial tuberosity is displaced proximally and has irregular but seemingly blunt margins, is of normal bone density, and is positioned horizontally (asterisk). Notice the small bone fragment caudal to the larger fragment and adjacent to its surface. The craniodistal aspect of the tibial tubercle (arrowheads) has an area of smoothly marginated, heterogeneous mineral opacity with a short zone of transition between affected and unaffected bone. Proximal displacement of the patella is evident. B—The tibial tuberosity is displaced proximally and superimposed over the lateral aspect of the stifle joint and lateral meniscus (arrow). There is a small part of the smoothly marginated, heterogenous mineral opacity evident on the proximomedial aspect of the tibia (arrowheads). Notice the ill-defined pinpoint mineral opacity superimposed over the medial meniscus and that the medial fabella is not evident.

Citation: Journal of the American Veterinary Medical Association 252, 4; 10.2460/javma.252.4.399

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Treatment and Outcome

The fracture was stabilized by use of a standard technique for fixation of the tibial tuberosity with 2 pins and tension band cerclage wire. The new bone was excised, and histologic evaluation of the specimen revealed proliferation of fibrocytes and chondrocytes and the progression of cartilage into woven bone. No neoplastic cells were observed in the specimen, and its histologic features were supportive of the interpretation of callus.

Comments

Avulsion fractures occur at attachment sites of traction elements such as tendons, ligaments, or joint capsules and are caused by excessive forces placed on these structures.³ In a skeletally immature patient, a tibial tuberosity avulsion fracture can be encountered after trauma during which excessive tensile stress is placed on the attachment of the patellar ligament to the tibial tuberosity by the pulling of the quadriceps femoris muscle. However, occurrence after repetitive, chronic stress on the tibial tuberosity with no, or only minor, trauma is also possible.³ Radiographic signs include proximal displacement of the tibial tuberosity with or without rotation.⁴ Radiographic soft tissue changes associated with tibial tuberosity avulsion fracture include various degrees of extra-articular soft tissue swelling that includes the patellar tendon. Chronic avulsion of the tibial tuberosity in the cat of the present report resulted in secondary healing with exuberant callus formation. The presence of meniscal ossicles or mineralizations in the cat of the present report might not be related to the tibial tuberosity avulsion fracture. They are a common condition in domestic cats and seem to indicate degenerative joint disease of the medial compartment of the stifle joint, but may also be indicative of normal skeletal maturation and often are incidental findings in feline patients.^1,2 The suspected joint effusion or thickening of the synovium can be caused by chronic joint instability resulting from the tibial tuberosity avulsion fracture. Additional imaging to support the diagnosis is generally not required. Although the presence of an open physis on orthogonal projections of the contralateral stifle joint could be suggestive of a traumatic tibial tuberosity avulsion fracture, no further imaging was performed in the cat of the present report because of financial restrictions.

Although the callus formation in the cat of the present report was radiographically noticeable and a tibial tuberosity avulsion fracture that was the result of a pathological process was considered,⁵ it was important to adhere to basic radiologic principles to distinguish between aggressive and nonaggressive bone lesions. Characteristics of an aggressive bone lesion include cortical destruction, irregular periosteal reaction, and no transitional zone between affected and unaffected bone.⁶ The callus in the cat of the present report was located in the physis to epiphysis region and thus was surrounded by perichondrium. No periosteal reaction was evident on radiographic evaluation in the cat of the present report.

Although it has been suggested that osteosarcoma can occur in young and adult cats,⁷ findings in a recent study⁸ do not support bimodal age distribution in cats, as is found for osteosarcoma in dogs, with the mean age of detection of 10 years in cats. In contrast to the cat in the present report, primary bone tumors in cats commonly originate in the metaphysis and are likely to have a tubular periosteal reaction. Lastly, sarcoma secondary to fracture is a rare occurrence in cats.⁹

Although the owners of the cat in the present report could not recall any traumatic event initially, they later remembered that the patient had fallen out their first floor window at least 6 months previously. It is possible that the cat sustained the tibial tuberosity avulsion fracture from this fall, although, as discussed, tibial tuberosity avulsion fractures may occur without any trauma. It is also possible that the initial lameness went unnoticed because of spontaneous improvement in hind limb use within 24 to 48 hours, as reported for dogs with tibial tuberosity avulsion fractures.¹⁰