Background:
Several studies have shown that certain cemented total hip replacement
femoral stems have been associated with the complications of early
debonding, loosening, and osteolysis. Some authors have suggested
that these failures may be related to the surface finish of the
stems. We developed an in vitro biomechanical experiment characterized
by simulated stair-climbing to investigate the multiple factors
involved in loosening of cemented femoral stems. In this study,
we measured the effects of stem neck length, body weight, stem size, and
calcar-collar contact on the torsional stability, as reflected by
the strains in the proximal cement mantle, of one design of cemented
femoral stem.
Methods:
Eight Centralign femoral stems (Zimmer, Warsaw, Indiana) were cemented
into eight cadaver femora with use of contemporary cementing techniques.
Prior to insertion, fifteen strain-gauge rosettes were mounted around
the proximal portion of the stem. The stems were loaded on a jig
that simulated static peak loading during stair-climbing. Loading
was repeated for each stem with three different joint reaction forces
and for three different neck lengths. Calcar loading by the collar
was then eliminated by removing a 0.5-mm slice of bone beneath the
collar, and all loadings were then repeated.
Results:
The peak principal tensile strains in the proximal cement increased
linearly with both body weight (r
2
> 0.95) and neck length (r
2
> 0.75). Increasing body weight affected the peak cement strains
far more than did increasing neck length. During simulated stair-climbing,
calcar-collar contact reduced peak strains in the proximal cement
by a factor of 1.5 to two. Peak principal tensile strains in the
proximal cement often exceeded 1000 me when the smaller stems were
used.
Conclusions:
In this stair-climbing test model, the peak proximal cement strains
were increased more by changes in body weight than they were by
changes in neck length. Even during stair-climbing, calcar-collar
contact reduced peak cement strains.
Clinical Relevance:
Many cemented femoral stems that become loose do so by rotating
into retroversion. In this study of one design of a cemented femoral
component during simulated stair-climbing, the peak strain magnitudes
in the proximal cement mantle were increased more by changes in
body weight than by changes in the length of the neck of the stem.
The strong effect of stem size on the cement strains suggests that cemented
femoral stems should not be used in heavy patients with small medullary
canals that require a small cemented stem.