Effect of femoral component alignment on femoral neck remodeling after hip resurfacing - a finite element analysis

L Kohan, M Gillies, R Cordingley

Introduction and aims.

 Varus component alignment and femoral neck notching are associated with fracture.  Notching would be diminished with a more varus alignment of the femoral component. We modeled varus/valgus alignment and looked at bone mineral density change using finite element analysis.

Materials and methods.

CT scans were used to reconstruct the femoral geometry and create a finite element mesh.  The boundary conditions applied were hip loading at 45% through the gait cycle and the muscle forces included.  Two models were created, a preoperative reference one and a postoperative state model, and the differences examined.    Implant orientations analyzed were neutral, 5° of valgus, and 5° of varus.  Even cement distribution and absence of voids was assumed.  The bone mineral density changes at six positions; three along the calcar, and three along the dorsal aspect of the neck were analyzed.

Results

The three alignment positions all produced significant alterations in the bone mineral density model.  The neutral and varus positioning resulted in significant loss of mineralization in the area of the superior neck, just below the edge of the component.  This is the area involved in femoral neck fractures. All three alignment configurations led to increase in bone mineralization at the base of the neck in the region just above the lesser trochanter.  The varus alignment produced significant loss of mineralization along the inferior aspect of the femoral neck, just below the component edge.  This change in bone density occurred within the first nine months after implantation, and from then on, did not change.

These findings are consistent with the clinical experience that if fractures are going to occur spontaneously, they tend to occur early, within the first year.  They also support the biomechanical theory that a valgus alignment is desirable, from a bone remodeling viewpoint