Abstract
Introduction: High ion release along with bone resorption at the bone/implant interface is still a problem, leading to pain, poor function and the possibility of bone fracture. Treatment of a loose implant is not easy and can lead to less than satisfactory revision surgery. The reason for ion release, loosening or periprosthetic fracture of an implant is multifactorial. One factor for ion release that has been reported is inclination angle. Another can be the version angle of the implant and subjecting it to an abnormal loading environment. Few studies have been reported in the literature on hip resurfacing performance based on implant orientation. More studies are required into investigating the use of this predictive technique in orthopaedics to investigate the bearing behaviour and potential ion release due to implant surgical positioning. In this study we modeled a number of different version angles and investigated the contact area, stress and wear characteristics using the finite element method.
Methods: CT scans were used to reconstruct the part of the femur and pelvic geometry. A 3D finite element mesh was created using PATRAN (MSC Software, Santa Ana, CA). The femur loading was taken at peak load position of the gait cycle. The loading was applied to the femur and pelvis was fixed. Material properties were applied using the Hounsfield units from the CT file. Two models were generated, a preoperative and a postoperative state model. The post operative model was reconstructed using the Birmingham Hip Replacement (BHR) system (Smith & Nephew Inc, Memphis, TN). The BHR acetabular cup was oriented at different anteversion angles (5°, 30° & 45° to the saggital plane) to investigate the contact mechanics between the head and cup. Serum ion levels were taken from 12 patients and the change in ion levels over the first 12 month period were analysed statistical to investigate the correlation with anteversion angle. Radiographs from the same patients were analysed to determine the cup anteversion angle using image analysis and edge matching techniques.
Results: The contact areas increased with increasing anteversion angle, 137.3, 165.3 and 169.9mm2 respectively. As a consequence, the contact pressure decreased. The change in ion levels for the patients over the first 12 month period correlated significantly (p< .05) with the anteversion angle using Pearson’s r test.
Discussion: Statistical analysis showed a good Pearson’s correlation of anteversion angle to a change in serum ion levels, 0.867 and 0.734 with p values of 0.001and 0.012 respectively. Acetabular version angle appears to be, at the least, important in determining serum metal ion levels and in evaluating causes of metallosis, the influence of anteversion angle needs to be considered when using metal on metal bearing technology when placing the cup in the acetabulum.
Correspondence should be addressed to: EFORT Central Office, Technoparkstrasse 1, CH – 8005 Zürich, Switzerland. Email: office@efort.org
