Changes in strain distribution of loaded proximal femora caused by different types of cementless femoral stems.

BACKGROUND: A number of clinically used total hip femoral implants are claiming a more or less physiologic load transfer, mostly without providing experimental data. To compare three clinically cementless total hip stems of fundamentally different design, the strain distribution before and after insertion in human cadaver femora was measured in vitro.
METHODS: A conventional straight stem based on a distal anchorage concept, a so-called "anatomic" stem designed to have a proximal force transmission and a "stemless" femoral neck prosthesis were evaluated under similar loading conditions. Strain distribution was measured with tri-axial strain-gauge rosettes before and after implantation of the stems. The same bending moment was used in all femora tested to compare magnitude and direction of the resulting strains.
FINDINGS: The straight and the "anatomic" stem both led to a decrease of the longitudinal strains in the proximal femur, while the femoral neck implant mainly led to an increase of measured strains on the lateral side of the greater trochanter. The observed medial strains were closer to physiological values in the "stemless" prosthesis than those of the two full-stem prosthesis.
INTERPRETATION: The decrease in strains seen in the proximal region of the femora with implanted conventional hip prosthesis corresponds well to the decrease of bone density in this region noted in clinical follow-up studies. The more physiological strain at the inferior base of the neck seen in "stemless implant" may induce a remodelling process that better retains bone stock in that area. However, the increase of strains noted after implantation of this prosthesis require further investigation to assess the risk they may pose to bone failure.