Dynamic radiostereometric analysis for evaluation of hip joint pathomechanics

Background Dynamic RSA (dRSA) enables non‐invasive 3D motion‐tracking of bones and may be used to evaluate in‐vivo hip joint kinematics including hip pathomechanics such as femoroacetabular impingement (FAI) and the biomechanical effects of arthroscopic cheilectomy and ‐rim trimming (ACH). The study aim was to evaluate the kinematic changes in the hip joint after ACH. Methods Seven non‐FAI affected human cadaveric hips were CT‐scanned and CT‐bone models were created. dRSA recordings of the hip joints were acquired at five frames/s during passive flexion, adduction to stop, and internal rotation to stop (FADIR). ACH was performed and dRSA was repeated. dRSA images were analyzed using model‐based RSA. Hip joint kinematics before and after ACH were compared pairwise. The volume of removed bone was quantified and compared to the postoperative range of motion (ROM). Results Mean hip internal rotation increased from 19.1 to 21.9° ( p  = 0.04, Δ2.8°, SD 2.7) after ACH surgery. Mean adduction of 3.9° before and 2.7° after ACH surgery was unchanged ( p  = 0.48, Δ‐1.2°; SD 4.3). Mean flexion angles during dRSA tests were 82.4° before and 80.8° after ACH surgery, which were similar ( p  = 0.18, Δ‐1.6°, SD = 2.7). No correlation between volume of removed bone and ROM was observed. Conclusions A small increase in internal rotation, but not in adduction, was observed after arthroscopic cheilectomy and ‐rim trimming in cadaver hips. The hip flexion angle of the FADIR test was reproducible. dRSA kinematic analysis is a new and clinically applicable method with good potential to evaluate hip joint kinematics and to test FAI pathomechanics and other surgical corrections of the hip.