Biomechanical influences after implantation of a total hip endoprosthesis on the periprosthetic bone density around the cup

Aim: The aim of this investigation was to prove how changes of the biomechanical joint load affect the distribution of the bone density around a cementless socket implant as the consequence of a total hip endoprosthesis (THEP). Method: 43 patients with different diagnoses were treated with Duraloc cup implants. Using a known biomechanical model, the biomechanical situation, the changes of the joint load and their influence on the local bone density were determined. The periprosthetic bone was divided into three zones. By means of DEXA measurement on the 10th postoperative day as well as at 3 and 12 months after the implantation the change of periprosthetic bone density was analysed. Results: Following THEP implantation the biomechanical parameters normalised. The mean biomechanical score (with a maximum of 12 points) increased significantly from 6.9 preoperatively to 9.7 postoperatively. Patients with dysplasia coxarthrosis showed the most significant improvement from 2.1 to 8.6 points. The periprosthetic bone density was reduced on an average of 14.4%, with highest values after 3 months (-9.5%). The zone- and the diagnosis-related analysis of bone density in combination with analysis of the biomechanical load showed normalised biomechanical parameters and harmonised values of bone density. A remarkable loss of bone was seen in regions with primary high values of bone density. These zones are the lateral cranial edge of the socket in patients with dysplasia coxarthrosis and patients with coxa valga as well as the zones from the lower socket entrance to the socket pole in patients with protrusio acetabuli. Conclusion: In order to ensure a long life-time of the artificial joint, treatment with a THEP of patients suffering from coxarthrosis should lead to a biomechanically optimal reconstruction of the hip joint. Preoperatively existing pathological deformities which, due to mechanical fail loading, lead to an unequal distribution of the bone density must be corrected in order to avoid higher loading of the implant and increased polyethylene abrasion of the socket layer.