The effect of stem curvature on torsional stability of a generalized cemented joint replacement system

Implant loosening is a common complication that compromises the stability of joint replacement systems. Stem geometry is particularly influential in the stability of cemented implants, both before and after debonding occurs at the stem-cement interface. There are few studies assessing the effect of stem longitudinal curvature as a geometric factor in cemented implant stability. The purpose of this study was to compare the torsional stability of four generalized cemented implant stems (i.e., non-specific to joint), with varying degrees of longitudinal curvatures--zero, two, four, and six degrees.

Twelve specimens of each curvature angle were potted to a depth of 20 mm using bone cement, given 24 hours to cure, and then tested in a materials testing machine. Torque was applied to the stems under monotonic loading at a rate of 2.5 degrees/min, until five degrees of rotation had occurred.

There were no differences in torsional stability among the four stem curvature angles, when the magnitudes of peak torque (P=.72; 1-β = 0.13), rotation of the stem at peak torque (P=0.23; 1-β = 0.38) and work required for five degrees of stem rotation (P=.58; 1-β = 0.07) were compared.

The findings from this study demonstrate that for short stems, stem curvature angles up to six degrees does not improve torsional stability when compared to the straight stem design.