In vitro validation and reliability study of electromagnetic skin sensors for evaluation of end range of motion positions of the hip.

There is growing evidence that femoroacetabular impingement (FAI) is a probable risk factor for the development of early osteoarthritis in the nondysplastic hip. As FAI arises with end range of motion activities, measurement errors related to skin movement might be higher than anticipated when using previously reported methods for kinematic evaluation of the hip. We performed an in vitro validation and reliability study of a noninvasive method to define pelvic and femur positions in end range of motion activities of the hip using an electromagnetic tracking device. Motion data, collected from sensors attached to the bone and skin of 11 cadaver hips, were simultaneously obtained and compared in a global reference frame. Motion data were then transposed in the hip joint local coordinate systems. Observer-related variability in locating the anatomical landmarks required to define the local coordinate system and variability of determining the hip joint center was evaluated. Angular root mean square (RMS) differences between the bony and skin sensors averaged 3.2° (SD 3.5°) and 1.8° (SD 2.3°) in the global reference frame for the femur and pelvic sensors, respectively. Angular RMS differences between the bony and skin sensors in the hip joint local coordinate systems ranged at end range of motion and dependent on the motion under investigation from 1.91 to 5.81°. The presented protocol for evaluation of hip motion seems to be suited for the 3-D description of motion relevant to the experimental and clinical evaluation of femoroacetabular impingement.