The finite helical axis of the knee joint (a non-invasive in vivo study using fast-PC MRI).

An understanding of the in vivo knee joint kinematics is critical for the further improvement and validation of knee joint models and for the development of better surgical and rehabilitative protocols. Unfortunately, most studies exploring the finite helical axis (FHA) tend to produce excellent qualitative results, but quantitative results are often lacking. Thus, the purpose of this study was to non-invasively and in vivo quantify the tibiofemoral FHA in a relatively large normal population during volitional knee extension using fast-PC MRI, to report the data relative to consistent coordinate systems (making it available for modeling input, experimental comparison and for device design), to determine the variability of the FHA, to investigate the screw home mechanism and to test the hypothesis that knee joint kinematics are independent of gender. Intra- and inter-subject repeatability was excellent. The intra- (inter-) subject repeatability of the FHA orientation in the frontal and axial planes was 1.8% (3.3%) and 3.7% (6.0%) of the average value, respectively. At the beginning of extension, the FHA was directed laterally and slightly superiorly and at the end of extension, it was directed in the lateral-inferior direction, indicative of the screw-home mechanism. The FHA location was not fixed during extension. There was small, but significant differences in all FHA parameters between genders and normalizing positional data relative to epicondylar width helped to reduce this difference. The data obtained in the current study forms an excellent base for future knee joint modeling and clinical studies.