4-year-old, castrated male, Bullmastiff dog (Canis lupus familiars).Dog had a lump on the lateral aspect of L elbow for an unknown period of time. Punch biopsy sent to a diagnostic laboratory resulted in a suspected diagnosis of chondroma. Dog continued to be lame and the owners elected amputation.
Received an elbow joint, which had an ulcerated firm mass, measuring approximately 5 x 5 cm, over the lateral aspect.Â The ulceration was from the site of the previous punch biopsy.Â On section, the mass was cavitated; continuous with the elbow joint space; filled with variably-sized, smooth, white, hard masses.Â Sections of the mass were placed into decalcification to allow sectioning.Â Fluid injected into the joint space opposite the mass drained out through the section area of the mass suggesting the mass communicated with the joint space.
The mass consists of numerous nodules consisting of well-differentiated hyaline cartilage arising from the synovial membrane and occasionally found within it.Â Residing within the lacunae are mature and well-differentiated chondrocytes.Â The nuclei are round and eccentric and consist of homogeneous dark purple chromatin.Â There is a moderate amount of eosinophilic cytoplasm, often containing small basophilic granules.Â The cells have irregular, almost spiculated cellular margins.Â Often these nodules show evidence of endochondral ossification.Â The synovium is markedly thickened and infiltrated with lymphocytes and plasma cells.Â The overlying fibrous capsule is also thickened by numerous reactive fibroblasts.Â
1.Â Synovial osteochondromatosis.
2.Â Synovial hyperplasia with lymphocytic plasmacytic synovitis.
Synovial osteochondromatosis (SOC) is a condition described infrequently in dogs.(1,2,3,4,5,9) and rarely in cats(7), horses(8), pigs(13), great horned owls(6,10) and a red tailed hawk.(10) It is characterized by the formation of small intra- or periarticular cartilaginous or osseous nodules.Â Histologically, the cartilaginous lobules are hypercellular and can show features of nuclear and cellular atypia which can complicate the diagnosis, particularly if only small samples are taken.Â Compared to the condition in humans, relatively little is known about the pathogenesis of the disease in dogs.Â The majority of the descriptions in veterinary medicine are based on case reports of individual dogs.(1,2,3,4,5,9)
In humans, SOC is considered to be a metaplastic condition rather than a neoplastic disease.(11) There are however, at least two case reports of malignant transformation of synovial osteochondromatosis to chondrosarcoma in dogs.(1,2)
Based on the description of the condition in humans numerous subcategories have been described.Â These include chondromatosis where the nodules are strictly cartilaginous in nature and osteochondromatosis where some of the nodules have undergone endochondral ossification as in this case.Â They are further subdivided based upon the suspected origin as either primary or secondary.Â SOC is defined as being secondary when there is a preexisting history of joint trauma or chronic mechanical irritation, while primary SOC is an idiopathic condition.12 In this case, there was no previous history of joint injury or disease and careful examination of the articular surfaces of the joints did not reveal any sign of disease suggesting that this is a primary SOC. There are limited numbers of cases describing the treatment of this condition; those that do exist suggest that removal of the nodules and partial synovectomy may improve the clinical signs, although radiographic reoccurrence of nodules was noted in some of these cases.(4)
Synovium: Cartilaginous nodules, multiple, with marked proliferative and lymphocytic synovitis.
The contributor provides an excellent summary of this uncommon condition.Â Conference participants considered both primary and secondary synovial chondromatosis as differential diagnosis for this case, and discussed differences between the two.Â The moderator provides the following table for comparison:
|Primary synovial chondromatosis||Secondary synovial chondromatosis|
|Degenerative joint disease||Mild||Severe|
|Number of nodules||More||Fewer|
Participants also considered chondroma and chondrosarcoma as potential diagnoses; however, most agreed that a chondroma would more likely present as a single, discrete mass rather than as multiple nodules, and chondrosarcoma would likely not appear so well-demarcated or demonstrate such a repeatable and orderly appearance within each nodule.Â
The classification of primary synovial chondromatosis is controversial, as there is evidence of both a reactive process and a metaplastic/proliferative process.(1) Recent cytogenic studies demonstrate that in human primary osteochondromatosis, a subpopulation of mesenchymal stem cells undergo clonal proliferation, suggesting a benign neoplastic process.(1)
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