Repeatability of a novel technique for in vivo measurement of three-dimensional patellar tracking using magnetic resonance imaging

PURPOSE: To determine the repeatability of a novel noninvasive MRI-based technique for measuring patellofemoral kinematics in vivo. MATERIALS AND METHODS: The patellar kinematics measurement method relies on registering bone models (with associated coordinate systems) developed from a high resolution MRI scan to loaded bone positions derived from fast, low resolution MRI scans. The intrasubject variability, high resolution to low resolution registration error, and interexperimenter repeatability were quantified in experiments on three healthy subjects. RESULTS: The intrasubject variability and registration error were within range of the accuracy of our procedure; specifically, less than or equal to 1.40 degrees for orientation and 0.81 mm for translation. The interexperimenter repeatability was less than or equal to 1.28 degrees for orientation, with the exception of patellar spin, and 0.68 mm for translation. CONCLUSION: Our novel measurement technique can measure three-dimensional patellar tracking noninvasively during loaded flexion in a repeatable manner. Our results compare well to another noninvasive tracking protocol, fast phase-contrast MRI, which has a reported subject interexam variability of 2.4 degrees or less for patellar orientation. A particular strength of our method is that axes and high-resolution bone models need only be determined once for intrasubject comparisons. The method is sufficiently accurate and repeatable to detect clinically significant changes in patellofemoral kinematics.