Elevated contact stress after acetabular fracture correlates with development of radiographic OA

Abstract

Purpose: Elevated contact stress following intra-articular fracture has been linked to subsequent development of post-traumatic osteoarthritis (PTOA). This relationship has been well established in the ankle, and elevated contact stresses in the hip have been tied to osteoarthritic changes. However, changes in hip contact stress due to articular fracture have not been well studied. Additionally, though elevated hip contact stresses due to residual incongruity have been documented in in vitro studies, to our knowledge, they have never been studied in a series of clinical fracture cases. Therefore, the present study utilizes discrete element analysis (DEA) to investigate changes in contact stress that may occur in patients following surgical reduction of their intra-articular acetabular fractures and the relationship between elevated hip contact stress and radiographic OA outcome in these patients. Methods: Under IRB approval, post-operative CT scans from 10 patients with surgically reduced acetabular fractures were obtained. Five of the cases were considered to not have progressed to OA (KL grade < 2, No OA group), and the remaining five were considered to have developed OA (KL ≥ 2, OA group). Femoral and pelvic anatomy for each patient was segmented from CT using a semi-automated program developed in MATLAB (Mathworks, Natick, MA). Each patient model was smoothed, aligned to an appropriate coordinate system, and moved through a series of loadings drawn from gait data collected using an instrumented total hip. Forces applied to each model were scaled based on the patient’s body weight. DEA was used to compute contact stress during the stance phase of gait. For each patient, the maximum contact stress that developed at any time during the stance phase of gait was compared to their KL grade. The contact stress patterns for these two different groups of acetabular fracture patients were also compared over the full stance phase of gait to DEA-computed contact stresses in 5 trauma patients without hip pain or injury. Results: The maximum contact stress for the OA group was 10.4 MPa, which was significantly (P < 0.001) higher than that for the No OA group (7.2 MPa) or for the patients with normal hips (8.9 MPa, Figure 1). While there was no significant difference in maximum contact stress between the No OA group and the patients with normal hips, the No OA group did have lower contact stress near heel-strike and higher contact stress near toe-off (Figure 1). There was a positive correlation between the maximum contact stress and KL grade (R2 = 0.546, Figure 2).View Large Image Figure ViewerDownload Hi-res image Download (PPT) Conclusions: The acetabular fracture patients who developed OA had significantly higher contact stress than the fracture patients who did not and the patients with normal hips. This suggests that exposure to abnormally high contact stress in the hip leads to the development of OA. The lack of a significant difference in contact stress between the fracture patients who did not develop OA and the patients with normal hips indicates that the contact stress levels in both groups were less than that needed to initiate OA development. However, the shift in maximum contact stress to later in the gait cycle for these fracture patients indicates that the surgical reductions have not returned the hip joints to a fully normal mechanical state. DEA-calculated contact stress holds potential as an early predictor of patient specific radiographic outcomes.