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 st udy were to determine if realistic stem-cement interface characteristi cs 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-elemen t models of cemented femoral hip components were studied with bonded o r realistic Coulomb friction stem-cement interfaces. The results showe d that the use of a non-bonded, non-linear Coulomb friction interface resulted in substantially different stress fields in the cement when c ompared to a bonded stem-cement interface. Tensile stresses in the pro ximal cement mantel for the Coulomb friction interface case (10.8 MPa) were greater than the fatigue strength of the cement. In contrast, th e tensile stresses in the cement mantle were not greater than the fati gue strength for the bonded case (7.5 MPa). Failure of the cement mant le in the proximal femur could therefore be initiated by a lack of a b ond at the stem-cement interface. The effect of different cross-sectio nal stem geometries (medial radii of 3.0, 4.9 and 5.5 mm and antero-po sterior widths of 9.8 and 13.7 mm) and different elastic moduli (cobal t chromium alloy and titanium alloy) for the stem material were also e valuated 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 th is study, the characteristics of the stem-cement interface had the lar gest effect on cement mantle stresses.