In vivo and noninvasive six degrees of freedom patellar tracking during voluntary knee movement

Objective. The purpose of this study was to investigate in vivo and noninvasively patellar tracking in six degrees of freedom during voluntary knee extension and flexion. Design. Patellar tracking was evaluated in vivo and noninvasively with corroboration using in vivo fluoroscopy and in vitro cadaver measurements. Background. Patellofemoral pain is closely related to abnormal patellar tracking and malalignment. However, there is a lack of quantitative and convenient methods to evaluate six degrees of freedom in vivo patellar tracking, partly due to difficulty in evaluating 3-D patellar tracking noninvasively. Methods. Six degrees of freedom patellar tracking was measured in vivo and noninvasively using a small clamp mounted onto the patella and an optoelectronic motion capture system in 18 knees of 12 healthy subjects during voluntary knee extension and flexion. Results. The patella tracked systematically following a certain pattern during knee extension and flexion. Patellar tracking patterns during knee extension and flexion were not significantly different in the 18 knees tested. When the knee was voluntarily extended from 15° flexion to full extension, the patella was extended 8°, laterally tilted 2°, and shifted 3 mm laterally and 10 mm proximally. The results were consistent with previous in vitro and in vivo studies. Conclusions. Six degrees of freedom patellar tracking can be evaluated in vivo and noninvasively within the range of 20° flexion to full knee extension.