Fabellar prevalence, parameters and association with medial meniscal tear: causality or coincidence?

Abstract Background: Fabella is a sesamoid bone of knee that has potential biomechanical function. The correlation between medial meniscal (MM) tear and fabella has not been rigorously examined. The aim of this study is to examine the fabellar prevalence and parameters in Chinese population and test the hypothesis that fabellar presence and morphology are associated with MM tear.Methods: A total of 1011 knee magnetic resonance imaging scans were analyzed. The fabellar presence was documented and parameters were measured. Further ratios were calculated to present fabellar morphology. Differences among subgroups were compared and correlation of fabellar presence and parameters with advancing age was assessed by Spearman correlation analysis. Odds ratios (ORs) and 95% confidence intervals (CIs) were calculated to investigate whether factors have relationship with MM tear. Diagnostic performance of risk factors was assessed and cut values were determined.Results: The overall prevalence of fabellae was 39.8% and increased with the increasing age (r = .237, P < .001). The fabellar were larger in male and ones with an articulating groove (all p < .001). The length/thickness ratio and width/thickness ratio were found significantly different between gender, age and articulating groove condition (all P < .05) and showed a moderate correlation with age (r = .463, p < .001; r = .303, p < .001), respectively. Fabella presented more often in knees with MM tears with a multivariate OR of 2.960 (95% CI, 1.853-3.903). Age, fabellar length, width, length/thickness ratio and width/thickness ratio yielded an area under the ROC curve (AUC) of 0.604-0.766 to predict an MM tear. In combination with age, fabellar width and length/thickness ratio, the diagnostic performance was improved to an AUC of 0.791 (95% CI, 0.744-0.837), a sensitivity of 73.0% and a specificity of 74.6% (OR, 7.939; 95% CI, 4.094-12.852). Conclusion: The presence of fabellae, increased fabellar length and width as well as flatter fabellar morphology, are significantly associated with an increased risk for MM tear. These findings might aid clinicians in identifying patients at risk for a MM tear and informing them.