A new analysis method for hip joint rotation angle validation in a three‐dimensional motion analysis

<p>【Background】</p><p>　The integration of the Plug‐in Gait model (PIGM) into the VICON Nexus three‐dimensional motion analysis system has limitations in measuring hip joint rotation angles. Consequently, a new analysis method using the anterior‐posterior axis of the femur within a motion analysis software is proposed. This method's validity and reliability are compared to those of the PIGM using a rotation angle on a two‐dimensional image as a reference.</p><p>【Methods】</p><p>　The study involved five healthy male subjects (mean age: 21 years). Measurements were synchronized between VICON Nexus (OMG Corporation) and a digital camera using a marker set of the PIGM. The motion tasks included internal and external rotations of 30° from the neutral hip flexion/extension position (0°). Each task was measured five times. For each limb position, the hip joint rotation angles were evaluated using: (1) the angle between the femur shaft and the sagittal horizontal axis of the horizontal plane image, analyzed using an image analysis software (ImageJ) (two‐dimensional image angle); (2) angle calculated using the PIGM (three‐dimensional analysis angle); and (3) angle between the femur anterior‐posterior axis and the sagittal horizontal axis of the PIGM (femur anterior‐posterior axis angle). Correlation coefficients between the parameters were computed, and the intraclass correlation coefficients (ICCs), standard error of measurement (SEM), and minimum detectable change at 95% confidence level (MDC95) were calculated for the femur anterior‐posterior axis angle.</p><p>【Results】</p><p>　No significant correlation was observed between the hip joint rotation angles obtained from two‐ and three‐dimensional analyses. In contrast, a strong positive correlation was observed between the femur anterior‐posterior axis angle and the two‐dimensional image angle. ICCs for intra‐ and inter‐rater reliability were high for both internal and external rotation positions, with SEM<0.04 and MDC95<0.12.</p><p>【Conclusion】</p><p>　The devised femur anterior‐posterior axis angle‐based method can effectively assess hip joint rotation motion.</p>