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© 1997 The Journal of Bone and Joint Surgery, Inc.
Bursal Osteochondromatosis Overlying an Osteochondroma of a Rib. A Case Report*  , HONOLULU, HAWAIIInvestigation performed at Kaiser Permanente, Honolulu
Synovial chondromatosis is a rare, benign, pathological condition characterized by the production and intra-articular extrusion of cartilaginous nodules by metaplastic synovial tissue. These bodies often undergo endochondral ossification—hence the term synovial osteochondromatosis. The disease is idiopathic and appears as a monoarticular arthropathy of a diarthrodial joint, particularly the knee8. Extra-articular lesions have been reported, but they are even rarer than their intra-articular counterparts. In these instances, the chondrometaplasia occurs in tenosynovial or bursal tissue and almost exclusively in the hands and feet9,11,14,16,20,22,26,27. This report describes a case of bursal osteochondromatosis arising within an adventitious bursa over an osteochondroma of a rib. This phenomenon can mimic a malignant tumor and cause clinical confusion.
A twenty-eight-year-old, right-hand-dominant male construction worker had a mass on the posterior portion of the right shoulder. The mass was painless and had gradually increased in size over a two-year period. The medical history was unremarkable with the exception of a remote right clavicular fracture. Physical examination revealed a non-tender fourteen-centimeter-diameter mass in the scapulothoracic region that extended cephalad into the caudad posterior cervical region (Fig. 1). There was no atrophy or neurovascular compromise of the right upper extremity. The range of motion of the glenohumeral joint was normal. Scapular retraction was 25 per cent of normal, and upward scapular rotation lacked the terminal 30 degrees.
Plain radiographs showed diffuse mottled mineral deposition within the lesion (Fig. 2). Computed tomography showed an area of dense calcification at the core of the mass that abutted a cephalad rib and was surrounded by numerous uniformly distributed discrete opacities (Fig. 3). The lesion was not directly contiguous with either the scapula or the clavicle. Magnetic resonance imaging revealed a well circumscribed mass measuring twelve by thirteen by twenty centimeters (Fig. 4). The boundaries of the lesion were the ribs and intercostals anteriorly, the paraspinal muscles medially, the trapezius and rhomboids posteriorly, and the subscapularis posterolaterally. Fat planes were maintained at the interface between the mass and all adjacent muscles. There was no evidence of thoracic invasion. The brachial plexus was displaced anteriorly, but neither it nor the major vessels were involved. The lesion contained a large epicenter of low-intensity T1 signal (adjacent to a cephalad rib) surrounded by diffuse smaller foci of low-intensity signal.
The diagnosis of chondrosarcoma was considered, and an incisional biopsy was performed. The specimen consisted of numerous hard, smooth, white nodules. Some were free-floating, and others were embedded in the segment of excised capsule. Histologically, the lining consisted of synovial-like tissue, and the nodules were composed of benign, hyaline cartilage with varying degrees of endochondral ossification. These findings were consistent with synovial osteochondromatosis. Ambiguity remained regarding what tissue could have given rise to the lesion because the affected region has no synovial tissue under normal circumstances. An arthrogram ruled out occult communication with the glenohumeral joint (Fig. 2). At the time of the definitive procedure, a well defined capsule was encountered and was separated bluntly from surrounding tissue posteriorly. The capsule was firmly affixed anteriorly around the base of a pedunculated mass, which was attached to and appeared to arise from the posterior aspect of the second rib. Complete excision of the lesion necessitated partial resection of the rib (Fig. 5).
Final pathological analysis revealed a bursal sac that contained 395 free cartilaginous masses ranging in size from a few millimeters to three centimeters. The internal surface of the sac was coated with numerous additional nodules that were fixed to the wall within villous synovial projections, yielding a cobblestone-like appearance. Histological analysis confirmed the presence of foci of benign cartilaginous metaplasia within the bursal wall. Ossification was noted within many of the nodules of cartilage. No evidence of malignancy was seen (Figs. 6 and 7).
The bursa was attached to the resected segment of the second rib at the base of the pedunculated osseous mass. Ossifying cartilage covered the head of this osseous projection. The gross and histological appearances of this protuberance were consistent with a solitary osteochondroma. This osteochondroma corresponded to the large mineralized focus that had been demonstrated at the epicenter of the bursa by computed tomography and magnetic resonance imaging. We believe that the adventitious bursa over the osteochondroma provided the tissue of origin for this unique occurrence of osteochondromatosis in an area that is normally devoid of synovial tissue.
Although osteochondromatosis can develop in any synovial-tissue-lined space, extra-articular lesions have been rarely described. Bursal osteochondromatosis is exceptionally uncommon. Isolated cases in the popliteal bursa, in the iliac bursa, and in shoulder bursae have been reported1,3,7,12,18,22,24,28. Friction between an osteochondroma and overlying soft tissue can give rise to the formation of an adventitious bursa2,5,8,15,21,23. A review of the literature revealed descriptions of free osteocartilaginous bodies within bursal cavities overlying osteochondromas4,5,8,13. However, the presence of intrabursal osteochondral nodules does not in itself confirm the diagnosis of bursal osteochondromatosis. Histological documentation of cartilaginous metaplasia and formation of nodules of cartilage within the synovial-like tissue lining the bursa is necessary for the diagnosis6,8. Such proof was not found in the reports just mentioned. The lesions in those reports may represent a quiescent stage in which no active intrasynovial lesion persists (Milgram17 stage 3). Without confirmation of active metaplasia, the diagnosis of bursal chondromatosis remains uncertain. There are alternative potential sources for the intrabursal osteochondral bodies. They could stem from repetitive shearing off of fragments from the osteochondroma itself. In addition, trauma to the bursal wall could produce synovitis and subsequent fibrin coagula within the bursal cavity, which, in turn, could undergo chondrification6,8. The chondrometaplasia within the synovial-like tissue of the bursa in the specimen from our patient rules out such alternative etiologies and establishes the diagnosis of bursal chondromatosis (Milgram17 stage 2). To our knowledge, only three previous case reports have documented bursal chondromatosis associated with osteochondromas. Two of the lesions were over the femur, and the third occurred at the scapula2,19. In all three reports, there was histological evidence of chondrometaplasia within the bursal lining. We know of no previous case of bursal chondromatosis associated with an osteochondroma of a rib. Many authors have acknowledged the potential for synovial osteochondromatosis to radiographically resemble malignant lesions with stippled calcification, such as chondrosarcoma, synovial sarcoma, and periosteal sarcoma22,27. Histological similarity to chondrosarcoma has also been noted because benign synovial chondromatosis commonly contains cytological atypia or multinucleation, or both2,4,10,25. These radiographic and histological parallels with malignant tumors must be emphasized. If these features are not recognized as being consistent with synovial chondromatosis, they could lead to an erroneous diagnosis of a malignant tumor and an inappropriate radical operation. Osteochondromatosis within a bursa over an osteochondroma may resemble chondrosarcoma clinically and radiographically2,5. Cardinal warning signs for sarcomatous transformation of an osteochondroma include pain, an apparent increase in size, and new adjacent calcifications. Malignancy must be definitively ruled out in the presence of such signs. However, these characteristics may also stem from the development of benign osteochondromatosis within an associated adventitious bursa. Note should be made of an isolated case report of chondrosarcomatous transformation of a solitary osteochondroma, which subsequently shed malignant nodules of cartilage into an associated adventitious bursa and necessitated a disarticulation of the hip10. It is important to recognize the potential anatomical distribution of bursal chondromatosis. This condition not only should be considered in the differential diagnosis of lesions occurring at known bursal locations, but also should be considered in regions that normally do not have bursal tissue because adventitious bursae can form in unusual locations.
*No benefits in any form have been received or will be received from a commercial party related directly or indirectly to the subject of this article. No funds were received in support of the study.
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