Effects of femoral neck length, stem size, and body weight on strains in the proximal cement mantle.
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.
Harrington
Melvyn A
MA
Orthopaedic Biomechanics and Biomaterials Laboratory and Department of Orthopaedic Surgery, Massachusetts General Hospital, Boston 02114, USA.
O'Connor
Daniel O
DO
Lozynsky
Andrew J
AJ
Kovach
Ian
I
Harris
William H
WH
D001696:Biomechanical Phenomena; D001835:Body Weight; D001843:Bone Cements; D002102:Cadaver; D005269:Femur / Q000033:anatomy & histology*; D006622:Hip Prosthesis; D006801:Humans; D011474:Prosthesis Design; D011475:Prosthesis Failure; D013314:Stress, Mechanical; D016474:Weight-Bearing
D001843:Bone Cements