Femoral Collar Effect

Femoral Collar Effect on bone strains after cemented hip replacement

William A. Allen, BS Gary S. Beaupré; PhD Dennis R. Carter, PhD; Virginia Giddings, MS; Stuart B. Goodman, MD, Ph.D; David J. Schurman, MD; Marjolein C. H. van der Meulen, PhD

The effect of a collar in cemented hip replacement has been long debated and remains a controversial issue. Proponents of collars believe that the collar will transfer hip loads directly to the bone at the medial calcar, thereby reducing proximal stress shielding and bone resorption. 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.

In this study we evaluated the effect of a femoral collar using two implant designs; one having a conical shaped collar and the other having no collar. 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. The strain value recorded for each implant was significantly lower than the strain value recorded for the natural femur (Fig. 1). 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.

Figure 1. 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.

Republished from the 1996 Rehabilitation R&D Center Progress Report. For current information about this project, contact: William A. Allen.