Cyst index: a nonpredictor of simple bone cyst fracture.

Purpose: Treatment is indicated for simple bone cysts (SBC) with high risk of fracture. The cyst index was described as a method to determine if a cyst is at high risk or low risk of fracture. Study Design: The cyst index of 32 femoral and humeral SBCs was determined by 8 reviewers. The risk group cyst index was compared with whether a fracture took place. Sensitivities, specificities, and positive and negative predictive values were calculated across varying cutoff levels. Intraobserver and interobserver reliability testing for 10 cysts was made. Results: Receiver operator curves demonstrate that no cutoff value gave an acceptable level of both sensitivity and specificity. The mean value for the cyst index was significantly different for different observers (P < 0.05). Conclusions: This study does not validate the cyst index to be an accurate predictor of fracture. The test has low intraobserver and interobserver reliability. Clinical Relevance: The cyst index cannot reliably discriminate between the patients that will fracture and those that will not.