Abstract
Introduction
Implant stability immediately after implantation is vital for long term success and improvement in patient outcomes for total hip arthroplasty (THA). Of the many factors that influence stability and resistance to migration for press-fit metaphaseal-fixed stems, this work attempted to evaluate the effect of distal stem taper angle on initial stem axial stability. Two THA stem designs having a distal taper angle of 3.5° and 5° were evaluated with cadaveric specimens with a simulated walking gait while stem motion was measured with respect to the periprosthetic bone.
Methods
Specimen Preparation
Three fresh-frozen matched pairs with no sign of orthopedic deformities were procured and evenly distributed between the two stem implant groups. The stems were manufactured without a neck region to eliminate ancillary bone-implant contact from confounding the interactions between taper angle and implant motion and migration. A board-certified orthopedic surgeon prepared each specimen and implanted the stems before each specimen was rigidly constrained within a custom test fixture. The test fixture oriented each femur vertically (±1°).
Simulated Gait & Motion Measurement
The simulated gait loading and motion measurements were performed separately, with bone-implant relative motion measured immediately after the loading challenge had been completed. Loading was applied in three stages of incremental load increase of 1000 cycles. The magnitudes included 0.5× body weight (BW), 1×BW, and 2.5xBW with the final magnitude corresponding to loads measured in vivo with an instrumented femoral stem during a normal walking gait. Force was applied sinusoidally at 2Hz with a minimum force of 100N consistent between all loading magnitudes. Values for BW were taken from the specimen data sheets provided for each matched pair.
Stem motion was measured at the apex and trough of the loading waveform for the last 10 cycles of the last loading magnitude with a high resolution (±0.001mm) digital displacement indicator (Fowler High Precision; Newton, MA). Measurements were made with the displacement indicator superiorly fixed to the femur in order to limit bone flexion from influencing the results. The trough measurements were subtracted from the apex to calculate axial displacement of the stem (see Figure 1) with respect to the superior periprosthetic bone.
Results
The ten sets of measurements recorded for each specimen were averaged and plotted for Figure 2. Bone-implant motion ranged from 9 – 29µm, with an overall measurement uncertainty of 1.9µm. The magnitude of displacement was found to be independent of BW (Pearson correlation = 0.027), and the difference between stem designs was not found to be statistically significant (p≤0.803).
Conclusion
The paired cadaveric study demonstrated that both the 3.5° and 5° THA stem taper angles had high initial stability, with bone-implant motion less than 30µm during a simulated gait. The difference between stem designs was not found to be statistically significant, and future work aims to determine spatial differences in bone-implant contact, and possible influence on the susceptibility for periprosthetic fracture.
