FACTORS AFFECTING CEMENT STRAINS NEAR THE TIP OF A CEMENTED FEMORAL COMPONENT

A generic three-dimensional finite-element model of the upper half of the femur containing a cemented femoral stem of a total hip arthroplas ty was developed to study those factors influencing cement strains nea r the tip of a cemented femoral component. This generic model was veri fied through another three-dimensional finite-element model that had b een created based on the precise geometry of a cadaver femur implanted with a contemporary cemented femoral component. This cadaveric femora l reconstruction had been created with strain gauges embedded in the c ement mantle and was then loaded under conditions simulating single le g stance and stairclimbing. By use of the cement strains measured expe rimentally in the cadaver femur, and comparison of them with those obt ained from the finite-element model of that cadaver femur, it was poss ible to establish proper material properties, boundary conditions, and loading conditions for the generic model. The generic model was then modified parametrically to determine those factors that influence the strains occurring within the cement mantle near the tip of a cemented femoral component. These models suggest that the single factor that mo st adversely influenced peak strains at or near the tip of the prosthe sis was a thin cement mantle. This effect was present both when the ce ment mantle was reduced in thickness and when a similar effect occurre d by virtue of a varus or valgus placement of the stem. Factors that d ecreased the peak cement strains near the tip of the femoral stem incl uded a more flexible stem and thicker cement mantles. This effect of a more flexible stem could be obtained by changing the modulus of the m etal implant, by uniformly reducing the thickness of the stem, or by t apering the stem within the same bone geometry. Thicker cement mantles reduced both the axial and the shear strains occurring at the tip of the prosthesis. The presence or absence of a hole in the tip of the pr osthesis per se, as for a centralizer had no significant effect on the peak cement strains seen around the tip of the prosthesis; however, t runcating the tip of the prosthesis from a hemisphere to a flat profil e, which resulted in a sharp corner at the tip of the prosthesis, prod uced a 35% increase in cement strains at the tip as a result of a stre ss concentration effect. Thus, the common way of modifying the rip to have a hole for a centralizer, which involved truncating the tip, incr eased the cement strains occurring near the tip of the prosthesis.