J . smallAnim. Pract. (1986) 27, 181-187.

Lesions resembling osteochondromatosis in two cats

M A D E L E I N E H U B L E R , K . A . J O H N S O N , R . T . B U R L I N G , D . F . F R A N C I S A N D R . C . C . R A T C L I F F E

Department of Veterinary Clinical Studies, The University of Sydney, NSW 2006, Australia

A B S T R A C T

The clinical and radiographic features of two cats with osseous lesions in the elbow and stifle joints which were suggestive of feline osteo- chondromatosis and synovial osteochondromatosis are reported. Reference is made to previous reports in the veterinary literature and the conditions are compared with similar diseases in man and dogs.

I N T R O D U C T I O N

Osteochondromatosis or multiple cartilaginous exostoses is a skeletal disease described in man (Jaffe, 1964; Aegerter & Kirkpatrick, 1975), horse (Morgan et al., 1962), dog (Pool, 1978 and 1981) and cat (Pool & Harris, 1975; Pool, 1978 and 198 1). The partially ossified cartilage capped exostoses may occur as solitary (osteochondroma) or multiple excrescences (multiple cartilaginous exostoses). The clinical features and pathogenesis of this condition in cats differ in several aspects of other animals (Pool, 198 1). Feline osteochondromatosis appears in mature cats and may involve any bone. The progressive growth of the lesions is characterized by autonomous behaviour and lesions may undergo neoplastic transformation. If clinical signs are present, they are attributable to the encroachment of the bony protuberances on joints, tendons or muscles.

Synovial osteochondromatosis is a rare disease of humans in which cartilaginous masses are formed by hyperplastic metaplasia of synovial cells (Freund, 1937). These cartilaginous masses may undergo calcification and they are predominantly monarticular, affecting most often the knee and elbow. In dogs it occurs usually in the hip and stifles (Schawalder, 1979) while in the cat it has been reported to affect single metacarpal joints (Pool, 198 1).

We report two cases with lesions resembling feline osteochondromatosis and compare them with those described elsewhere (Riddle & Leighton, 1970; Brown et al., 1972; Pool & Carrig, 1972; Pool & Harris, 1975; Pool, 1981).

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C A S E R E P O R T S

Case no. 1 A 6-year-old, neutered, female Burmese cat was presented because of

listlessness. Clinical examination revealed a slight to moderate enlargement of both stifle joints. The swelling was firm but not painful on palpation, and crepitus was elicited by flexion and extension. The other limb joints seemed clinically normal. The cat was not lame and was not impeded by the joint changes.

Under general anaesthesia radiographs of both stifles, elbows and the cervical spine were taken. In the stifles there was bilateral symmetrical calcification of the infrapatellar fat pad and of the medial and lateral collateral ligaments at their attachments (Fig. 1). There was no evidence of secondary degenerative joint

FIG. 1 . (a) Anterior-posterior and (b) mediolateral view of the right stifle of case 1. Calcification of the infra-patellar fat-pad (arrows).

disease. In the elbows there was calcification within the joint capsule, most obvious on the medio-cranial aspect (Fig. 2), but no degenerative joint disease. The cervical spine appeared normal. Although it is an unusual localization for this particular disease in the cat, a diagnosis of osteochondromatosis was made. Unfortunately the owner did not wish for further investigation, and it was not possible to follow the case further.

Case no. 2 An 8-year-old, neutered, male Burmese cat was referred for examination of a

hard, bony enlargement on the right elbow. Six weeks before the owner noticed the

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FIG. 2. (a) Anterior-posterior and (b) mediolateral view of the right elbow of case 1. Intracapsular calcification (arrows) on the medial and dorsal aspect of the joint.

swelling and that the cat had a progressively worsening lameness. The cat was healthy and there was no history of trauma.

At presentation the cat was obviously lame and a pronounced firm swelling was found around the right elbow joint. On the medial aspect it appeared smooth, whereas on the lateral side it felt rather nodular. The range of motion was restricted and pain was elicited by flexion and extension. The other limb joints showed no obvious changes.

Radiographs were taken of both forelegs and the cervical spine, and a biopsy was collected by trephine from the medial aspect of the lesion for histopathology. On the craniomedial aspect of the right elbow, calcification of periarticular soft tissue was present. It was well circumscribed, about 1 - 5 x 3.5 cm in size and originated from the medial humeral condyle. Radiographs of the left elbow revealed similar smaller tumorous lesions. There was no sign of secondary degenerative joint disease. There were no lesions on the cervical vertebrae.

Histologically the biopsy consisted of spicules of bone with islands of cartilage in which there was widespread endochondral ossification and these changes supported a diagnosis of osteochondromatosis.

The cat was re-examined 5 months later and the owner had noticed a marked improvement in the lameness, and it was walking almost normally. On examination the swelling on the right elbow was about the same size and the range of motion was still limited. Around the left elbow joint a slight, firm mass was palpable. A second series of radiographs were taken and by comparison with the first radiographs the lesions on both elbows had only slightly increased in size, but were otherwise unchanged in appearance (Figs 3 and 4).

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FIG. 3. (a) Anterior-posterior and (b) mediolateral view of the right elbow of case 2. Tumorous mineralization of soft tissue mainly on the mediocranial aspect. Small

osteophyte on the lateral aspect (arrows).

FIG. 4. (a) Anterior-posterior and (b) mediolateral view of the left elbow joint of case 2. Periarticular mineralization of soft tissues and perichondral osteophyte formation

(arrows).

D I S C U S S I O N

Feline osteochondromatosis is characterized by an initial appearance of skeletal lesions in mature cats (Pool, 198 1) unlike canine osteochondromatosis, where the

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disease begins in young dogs (Gambardella et al., 1975; Finnie & Sinclair, 1981). Although the lesions may occur in any bone, they have been predominantly found in the scapulae, ribs, vertebrae and pelvis (Pool & Carrig, 1975). The two present cases represent an unusual distribution of the lesions. In none of the previous cases were there exostoses in the stifle joints and elbows, although both radii were involved in one cat (Riddle & Leighton, 1970).

Most cases described elsewhere (Riddle & Leighton, 1970; Brown et al., 1972: Pool & Carrig, 1972; Pool & Harris, 1975; Pool, 1981) were presented because of the rapid onset of growth of one or more exostoses, or they were incidental findings on radiographs taken for other reasons.

The radiographic features reveal osteochondromas as sessile masses with well defined, smoothly contoured borders arising from the surface of the affected bone (Pool & Harris, 1975). They may be pedunculated or have a more cauliflower-like appearance or present a complex bosselated profile (Pool, 1981). The outer part may have an amorphous mottled appearance which occurs during the active growth phase or in case of malignant transformation (Pool, 1978).

In Case 1, calcification of the infrapatellar fat-pads and attachments of the collateral ligaments in both stifles, and the spicule-like calcification within the joint-capsule of both elbows were highly suggestive of synovial osteochon- dromatosis. In man synovial osteochondromatosis has similar features with cartilage formation by the synovial membrane in the joints and later these cartilage nodules undergo endochondral ossification (Aegerter & Kirkpatrick, 1975). Kealy (1979) produced illustrations of similar changes in the stifle of a cat and called it osteochondromatosis, although he admits the calcification within the joint is synovial in origin. The development by metaplasia of cells of the synovial membrane would explain the intra-articular localization of the calcified masses. Exostoses were extra-articular in Case 2 and radiographs and the biopsy supported a diagnosis of osteochondromatosis rather than synovial osteochondromatosis. Histologically, osteochondromas are orderly biphasic growths with an apical margin of hyaline cartilage and a bony base of cancellous bone and intervening marrow spaces. A membrane continuous with the periosteum of the host bone covers the surface of the lesion. In young lesions the cartilage cap is a distinctive structure, while in maturing growths the cap is discontinuous (Pool, 1978 and 198 1). Young, actively-growing osteochondromas mimic a growth plate and produce bone by orderly endochondral ossification. Within the exostoses multiple islands of calcified cartilage scattered through large, rapidly-growing lesions are found. The biopsy of Case 2 had this appearance, except that the growth was not covered by a cartilage cap. This might have been due to an improper collection of the biopsy, the orientation of histological sections of the biopsy, or the osteochondroma may have been a maturing lesion.

Hypervitaminosis A causes exostoses in regions of tendon, ligament or joint capsule attachment, with the first lesions always appearing on the cervical vertebrae in experimentally-induced disease (Clark et al., 19 70). Later lesions were

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found on the scapula, distal humerus and proximal radius and ulna around the joint margins. They consisted of marked periosteal proliferation and formation of irregular masses of woven bone, rather than cartilage. Therefore the present cases of feline osteochondromatosis differed from hypervitaminosis A in the absence of cervical lesions, the intra-articular location of some lesions and in histologic appearance.

The cause of the disturbance that results in feline osteochondromatosis is unknown. A heritable basis for these lesions has been indicated in man (Jaffe, 1964), dogs (Gee 8c Doige, 1970; Chester, 1971) and horses (Pool, 1978). In man several theories have been suggested (Jaffe, 1964; Aegerter & Kirkpatrick, 1975; Gambardella et al., 1975; HUVOS, 1979). Since the canine disease is similar to osteochondromatosis of young humans the same theoretical explanations of the disease have been accepted for the dog (Pool, 1981). The theories fall into two major categories; either there is a chondrodysplasia involving the margins of the growth plates or a perturbation of the periosteum leads to the excrescences. Since feline osteochondromatosis differs from the canine disease in the age of onset, skeletal distribution and growth pattern of the lesions, the hypothesis suggested in man and dog can hardly be accepted for this species. In some cats viral particles isolated from the lesions resembled feline leukaemia (Pool & Carrig, 1972; Pool & Harris, 1975; Pool, 1981). This finding is suggestive of a viral aetiology in the cat. Lesions behave as a neoplasm that progresses from benign to malignant appearance and may undergo transformation to osteosarcoma or possibly chondrosarcoma (Pool, 1981) and affected cats lived less than one year. Surgical intervention provides only transient relief to the cat, for the lesions tend to recur or new exostoses may appear (Riddle & Leighton, 1970).

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