Bradford Royal Infirmary, Duckworth Lane, Bradford BD9 6RJ, UK
SIR, Although femoral head subchondral pseudocysts of up to 25 mm in diameter are recognized in osteoarthritis [13], they rarely occur in the femoral neck, and giant lesions are uncommon. The radiological appearances of such lesions, especially when associated with rheumatoid arthritis, can present diagnostic difficulty, in some cases simulating malignancy [46].
A 59-yr-old man was referred to the orthopaedic clinic because of increasing right hip pain. Rheumatoid factor antibody assay was <20 IU/ml, and the uric acid level was 0.39 mmol/l, within the normal range. Radiographs indicated moderate degenerative change in both hips, and a lucency measuring 40 x 25 mm in diameter in the right femoral neck (Fig. 1). Repeat views at 2 months failed to show any progression in size. This, together with a sclerotic margin suggested a benign process, and subsequent total hip replacement was performed at which a well-defined cyst was identified containing amorphous grey soft tissue. Other than a dense sclerotic bony margin, there appeared to be no capsule, and no communicating channel. In addition, a complete pathological fracture of the femoral neck was discovered through the cyst. The cyst was curretted back to macroscopically normal cancellous bone and bone grafted, prior to insertion of an uncemented total hip replacement.
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Subchondral cavities in the presence of degenerative joint disease are not associated with an epithelial lining and as such are not typically cystic. The term geode, from the geological term for hollows within rocks, has been adopted as a more appropriate descriptive term for such lesions [2]. Two theories consider the pathogenesis of geode formation in osteoarthritis. The theory of synovial fluid intrusion [3, 7] suggests that elevation of intra-articular pressure forces synovial fluid through focal areas of damaged articular cartilage with secondary resorption of bony trabeculae. This may be associated with joint communication, the finding of fragments of articular cartilage within the cyst, and the similarity of the fluid in the cyst with synovial fluid. In contrast, the theory of bony contusion [8, 9] suggests a focus of bone necrosis secondary to violent impact between apposing bone surfaces unprotected by cartilage. In support of this theory, the lack of communication between the cyst and the articular cavity, the presence of metaplastic cartilage as opposed to articular cartilage in the wall of the geode, and osteoclast activity and granulation tissue resembling fracture callus at the site of the geode were noted. In addition, it has been shown that lesions with a similar pattern and shape to subchondral geodes can be created experimentally by overloading photoelasticity models to produce a specific zone of overloading (and presumably cancellous bone death) at some depth below the bone surface. This may result in secondary synovial intrusion during healing in which the communicating channel is created from within the bone, rather than by the pressure of synovial fluid from the joint [1].
The association of cysts of the femur with degenerative joint disease has been known for many years [3, 7, 8], and usually involves joints under the greatest pressure [1, 3]. As we have shown, these lesions can contain soft tissue including myxoid and adipose elements [2] which can present diagnostic difficulty. In those giant geodes previously described in association with rheumatoid arthritis, pathological fracture has been noted [6, 10]. Treatment of such lesions, when associated with a pathological fracture, is by prosthetic replacement of the femoral head [10]. The treatment of these lesions in the absence of fracture or gross evidence of joint disease is, however, less clear cut, although bone grafting and internal fixation to prevent pathological fracture have been suggested [6, 10].
This case demonstrates the difficulty which can be encountered in distinguishing giant geode formation from malignancy. Indeed, the patient in this case commenced a course of radiotherapy before confirmation of the benign nature of the lesion. In addition, we have shown that a giant geode complicated by pathological fracture can occur in osteoarthritis, and may therefore hasten the need for referral for prosthetic replacement. Geode formation in this case was some way distant from the articular surface, with no macroscopic communication with the joint cavity, lending support to the bony contusion theory of geode formation.
Notes
Correspondence to: A. P. Cohen.
References