Two cases of synovial cysts of bone are reported.

Synovial cysts of bone occur clinically as painful lesions in the small bones of the hands or feet or in the ends of the long bones, while on roentgenogram they appear as expanding lytic lesions with a smooth, dense sclerotic wall. The cysts are adjacent to joint surfaces with which a connection may or may not be evident. The shell of sclerosed bone may serve to distinguish these cysts from the chondromatous and giant-cell tumors. Pathologically, the cysts contain mucinous rather than serous fluid. Histologically, the wall is composed of dense fibrous and loose fibroblastic mucinous tissue and may display, in part, a lining resembling synovial membrane; but there is no giant-cell reaction, new-bone formation, inflammatory response, or pigment and fat deposition. Synovial cysts have none of the features of synovioma or giant-cell tumors of bone and are not true tumors. They do not have the pigment deposition or giant-cell rcsponse seen in unicameral bone cysts to which they may be considered analogous if one chooses to accept Jaffe's concept of an aberration in the development or growth of local osseous tissue as the cause of both lesions. Synovial cysts probably develop from a defect in the joint surface with either simple extrusion of synovial fluid or proliferation of the synovial membrane through the defect. Previous disease or injury may be a factor in the development of such cysts.

The recommended treatment is enucleation of the cyst and packing with bone chips when feasible.