The Truth Behind Subchondral Cysts in Osteoarthritis of the Knee

BACKGROUND Subchondral cysts have always been taught to be one of the cardinal radiological features of knee osteoarthritis but are not well understood. We aimed to evaluate the radiological prevalence and epidemiology of subchondral cysts in patients with knee osteoarthritis to determine if they are truly a cardinal radiological feature. METHODS All patients of a single surgeon with symptoms of knee osteoarthritis were selected for this study. All patients had failed a trial of conservative therapy and were planned for total knee arthroplasty. Patients with symptoms of and documentary evidence of inflammatory arthritis, other neurological and orthopaedic problems causing functional deficits were excluded from this study. A total of 806 plain radiographs were analyzed with the aid of an atlas for the presence of narrowed joint space, osteophytes, subchondral sclerosis and subchondral cysts. The radiological prevalence of each feature was then calculated. Demographics and pre-operative measurements were compared between patients with and without radiological evidence of subchondral cysts. RESULTS Subchondral cysts were only present in 30.6% of the study population. Narrowed joint space was present in 99.5%, osteophytes in 98.1% and subchondral sclerosis in 88.3% of all radiographs. The differences in prevalence were statistically significant. There was a higher proportion of females in patients with radiological evidence of subchondral cysts. These patients also had a greater varus deformity preoperatively. CONCLUSION With a radiological prevalence of 30.6%, subchondral cysts should not be considered a cardinal radiological feature of osteoarthritis. Subchondral cysts may be associated with the female gender and genu varum.