Coulomb frictional interfaces in modeling cemented total hip replacements: a more realistic model.

Loosening of cemented femoral hip stems could be initiated by failure of the cement mantle due to high cement stresses. The goals of this study were to determine if realistic stem-cement interface characteristics could result in high cement stresses when compared to a bonded stem-cement interface and to determine if stem design parameters could be chosen to reduce peak cement stresses. Three-dimensional finite-element models of cemented femoral hip components were studied with bonded or realistic Coulomb friction stem-cement interfaces. The results showed that the use of a non-bonded, non-linear Coulomb friction interface resulted in substantially different stress fields in the cement when compared to a bonded stem-cement interface. Tensile stresses in the proximal cement mantel for the Coulomb friction interface case (10.8 MPa) were greater than the fatigue strength of the cement. In contrast, the tensile stresses in the cement mantle were not greater than the fatigue strength for the bonded case (7.5 MPa). Failure of the cement mantle in the proximal femur could therefore be initiated by a lack of a bond at the stem-cement interface. The effect of different cross-sectional stem geometries (medial radii of 3.0, 4.9 and 5.5 mm and antero-posterior widths of 9.8 and 13.7 mm) and different elastic moduli (cobalt chromium alloy and titanium alloy) for the stem material were also evaluated for models with a Coulomb friction interface. Changes in the stem cross-section and elastic modulus had only limited effects on the stress distributions in the cement. Of the parameters evaluated in this study, the characteristics of the stem-cement interface had the largest effect on cement mantle stresses.