Understanding the mechanical environment of the hip

The hip functions as a ball and socket joint, with cartilage layers that cover the joint surfaces on both sides protecting it from impacts and permitting smooth movements. When the cartilage is impaired by mechanical, infectious or inflammatory causes, the joint might eventually wear down - a disabling condition known as osteoarthritis. Recent literature indicates that up to 80\% of all osteoarthritis cases are potentially caused by subtle hip variations: the round shape of the ball (femur head) that is disturbed by a bump or/and the socket (acetabulum) that overcovers the femur head. These abnormal variants can give rise to conflicts and altered load distribution in the hip joint. When the load on the joint is no longer evenly distributed, peak stresses can arise in certain areas of the hip posing a risk of developing focal cartilage damage. Since the apparent prevalence of these morphological hip abnormalities is reported to be much higher than the number of actual patients, the question remains how to differentiate potential patients from incidental findings.The aim of this thesis was to describe and explore the impact of shape variation in the hip joint and by doing so improve the understanding of the mechanical environment of the hip joint. First differences in hip anatomy between white and Chinese subjects were mapped using a cross sectional design. Pelvic computed tomography scans of 201 subjects (99 white Belgians and 102 Chinese; 105 men and 96 women; 18-40 years old) were assessed. Ten radiographic parameters predisposing to femoroacetabular impingement were evaluated. The white subjects had a less spherical femoral head than the Chinese subjects. The Chinese subjects had less lateral acetabular coverage than the white subjects. A shallower acetabular configuration was predominantly present in Chinese women. Static and dynamic variation in hip joint reaction forces was evaluated using an experimental computational modeling design. We therefore calculated the hip joint reaction force and hip flexion angle in a virtual representative male Caucasian population by means of musculoskeletal modeling of three distinct sitting configurations: a simple chair, a car seat and a kneeling chair configuration. The observed median hip joint reaction force in relation to body weight and hip flexion angle, respectively, was 22.3$\%$ body weight and 63° for the simple chair, 22.5$\%$ body weight and 79° for the car seat and 8.7$\%$ body weight and 50° for the kneeling chair. The kneeling chair appears to hold the greatest potential as an ergonomic sitting configuration for the hip joint since it requires the lowest hip flexion angle and hip joint reaction force of these 3 distinct sitting configurations. Dynamic mapping of deep squat hip kinetics was performed in young, athletic adults using a personalized numerical model solution based on inverse dynamics. Thirty-five healthy subjects underwent deep squat motion capture acquisitions and MRI scans of the lower extremities. Musculoskeletal models were personalized using each subject’s lower limb anatomy. The average peak hip joint reaction force was found to be 274$\%$ body weight. Average peak hip and knee flexion angles were 107° and 112° respectively. Deep squatting kinetics in the younger population differ substantially from the previously reported in vivo data in older subjects. In order to map variation in cartilage stress, a numerical discrete element analysis algorithm was developed. A validation study with hip joint contact stress data from 10 healthy subjects calculated by means of subject-specific finite element analysis was performed. Furthermore an efficient cartilage anatomy prediction tool was defined that does not require manual cartilage image segmentation. We showed that this novel population-averaged cartilage anatomy prediction method, integrated with the discrete element analysis algorithm could provide an efficient platform to predict cartilage contact stresses in large populations compared to subject specific finite element analysis. The mechanical effect of arthroscopic cam resection in femoroacetabular impingement was explored with a case-control study design. For this purpose, patient-specific discrete element models from 10 cam type femoroacetabular patients (all male, aged 18-40 years old) were defined based on preoperative CT and postoperative MRI scans. Complete cam resection postoperatively on MRI was confirmed with alpha angles $<$ 55°. The preoperative and postoperative peak contact stress findings during impingement testing were compared against a matched virtual control group. Peak contact stress was significantly elevated in patients with cam type femoroacetabular impingement during impingement testing with increasing amount of internal rotation. This effect was however normalized following arthroscopic cam resection and loading patterns matched those of the control group. Using multidimensional statistics and personalized load and stress predictions, we were able to demonstrate that the important population variation in shape and joint mechanics adds to differences in the onset and progression of cartilage lesions of the hip joint. Further, our work contributes to an improved identification and classification of patients who are truly at risk for developing cartilage damage. The final step of this thesis was to gradually transfer these findings into practice at the operating theater. We demonstrated that an accurate surgical treatment of cam lesions has the potential to effectively restore the normal mechanical environment of the hip