Instability in total hip replacement - can magnets help?

Aims: Dislocation after primary total hip arthroplasty (THA) is a common and devastating complication and one of the main indications for revision procedures. Risk factors include implant choice, patient and technical factors. Efforts to improve postoperative stability following revision surgery may impact the incidence of recurrent dislocations. This study investigated an innovative device design to address this problem using rare earth magnets. Methods: Computer simulations using design and magnetic finite element analysis software were used to quantify forces between hip implants with embedded magnets. Multiple Neodymium-Iron-Boron magnets were modeled within a hip system's acetabular shell and femoral head. The model included a 13mm x 5mm cylindrical magnet located at the shell's dome, 16mm x 16mm cylindrical magnet within the femoral head, and a 4-magnet array (10mm x 5mm) located in the transacetabular screw holes. A 16mm spherical magnet within the femoral head, which allowed for magnetic field realignment relative to opposing cup magnets and rotation of the sphere, was also simulated. Biomechanical testing was performed using implant models embedded with magnets to verify simulation retention forces at various joint positions. Results: The magnetic field simulations generated retentive forces between the acetabular cup and femoral head magnets ranging from 5.9N to 12.0N without generating significant off-axis forces that could compromise retention. The use of a spherical magnet within the femoral head improved the retentive force between the head and acetabular cup for a range of motion. Mechanical testing validated loads obtained in the simulations but indicated that some simulation configurations overestimated the effect of the femoral head magnet. Greater forces may be obtained with specially designed femoral head bores and acetabular shells that incorporate additional or larger magnets. Conclusion: Rare earth magnets provide exceptional attractive strength and may prove useful in imparting stability to prevent dislocation in revision THA.