Reduced compression time is not sufficient for stability in patellar buttons.

PURPOSE: Patellar resurfacing in total knee arthroplasty requires stable cement fixation of the patellar button. Unlike femoral and tibial components, the patellar button is held under manual compression until cement curing, potentially prolonging operative time. Since the patella experiences limited immediate loading after joint closure, a shorter compression time might suffice for primary fixation. This study investigated the effect of reduced manual compression time on cement morphology, micromotion and load-to-shear failure of cemented all-polyethylene patellar buttons.

METHODS: Twelve human patellae from six donors were implanted with cemented all-polyethylene patellar buttons and assigned to either 30 or 600-s compression group. Micro-computed tomography (µCT) analysis assessed cement volume, penetration depth, expansion and height. Biomechanical testing included cyclic loading to determine micromotion, followed by shear testing to measure maximum failure force. Statistical comparisons and correlation analyses evaluated group differences and associations between cement morphology and biomechanical parameters.

RESULTS: Cement morphology did not differ significantly between groups; volume, penetration depth, expansion and height were comparable. However, micromotion was significantly higher in the 30-s group (0.015 ± 0.009 mm) compared to the 600-s group (0.006 ± 0.003 mm, p = 0.028). The larger standard deviation in the 30-s group indicated greater variability. Maximum shear force did not differ significantly between groups (2450 ± 971 vs. 2383 ± 232 N, p = 0.857). Correlation analyses showed only limited associations between cement morphology and biomechanical outcomes.

CONCLUSIONS: Reduced compression time did not significantly affect cement morphology or ultimate shear strength of cemented patellar buttons. However, shortened compression led to significantly increased micromotion, indicating impaired early interface stability. These findings suggest that micromotion under cyclic loading may be a more sensitive endpoint than maximum shear force for evaluating patellar button cementing strategies. Maintaining compression until full cement curing may optimize primary fixation stability.

LEVEL OF EVIDENCE: Level V, biomechanical study.