Critics argue that the beneficial effects of the collar are never realized in practice since the ability of a collar to transfer load relies on intimate contact between the underside of the collar and the resected bone surface. Additionally, critics contend that a collar will neither increase the proximal loading to the degree necessary to maintain bone nor load the bone in a truly physiological manner. By recording the surface strains in femora before and after hip replacement we were able to assess the performance of the two implant designs and compare the results to the natural femur. At a location 5 mm below the collar, the strains were not different between the two implant designs. At a location 25 mm below the collar the strain for the conical collar implant was closer to normal, but still significantly less than normal.
Surface strains in the proximal femur before and after hip replacement.
While we did find a difference in the strain levels with and without a collar, the biological significance of this strain difference in terms of adverse bone remodeling is unknown. The levels of bone strain that are necessary for bone maintenance is an important question and the focus of much research. Based upon the present study, we believe that the conical collared implant will perform better in terms of minimizing adverse bone remodeling after total hip replacement.