Acetabular loading in rehabilitation

Acetabular fractures are produced by high energy injuries that often cause dislocation of the fragments with gaps and steps (Olson et al., 1997). The goal of operative treatment of such fractures is to restore acetabular anatomy with perfect fragment reduction and stable fixation in order to enable early joint movement (Letournel and Judet, 1993). The fixation of the fragments is not strong enough to allow weight bearing before the bone is healed (Goulet et al., 1994; Olson, 2003) and in some patients even physical therapy with initial passive motion and continued active exercises without weight bearing could lead to dislocation of fragments and early posttraumatic osteoarthritis (Letournel and Judet, 1993). Early physical therapy of patients with acetabular fractures therefore requires careful selection of exercises in order to prevent excessive loading of the injured acetabular region. Current guidelines for nonoperative management of acetabular fractures and postoperative management of surgical procedures in the acetabular region recommend initial bed rest followed by passive motion in the hip joint. Initial active non-weight-bearing exercises commence a few days after surgery and include active flexion, extension and abduction in the hip in the upright position. The same set of exercises in supine or side-lying abduction is usually postponed until 5-14 days postoperatively. Partial weight-bearing with stepwise progression usually starts 6 weeks postoperatively and full weight bearing is eventually allowed at 10 weeks (Maurer et al., 1997). Recently, interesting information was obtained by direct measurements of acetabular contact pressures during rehabilitation exercises in subject with pressure-instrumented partial endoprostheses where it was found that acetabular pressures may not follow the predicted rank order corresponding to the commonly prescribed temporal order of rehabilitation activities (Givens-Heiss et al., 1992; Tackson et al., 1997). It has been found that hip stress magnitudes in some non-weight bearing exercises can exceed hip stress in weight bearing exercises or even gait. Due to technical complexity and invasiveness of direct contact stress distribution measurement, various mathematical models for calculation of the contact stress distribution in the hip joint have been proposed (Brand, 2005; Daniel et al., 2008; Genda et al., 2001; Ipavec et al., 1999; 1995; Legal and Reinicke, 1980; Pedersen et al., 1997). Recently, amathematical model has been developed that enables computation of the contact stress distribution at any given position of