Abstract
Introduction
The Morse taper was adapted into orthopaedics as a connecting junction in total hip replacements. The benefit of modular systems includes the adjustment of leg length, offset and the ability to remove the head for acetabular exposure during revision surgery. The design of the Morse taper facilitates the intimate contact of the conical trunnion of the femoral stem (male component), with the conical bore of the femoral head (female component). Design consideration for tapers involve physical parameters (angle, length, diameter and contact points), and manufacturing finish (surface characteristics). Orthopaedic trunnion tapers are not standardized and vary in length, taper angle and base dimension. Variations in the design and surface characteristics of the trunnion, which directly reflects on the interface at the taper junction can influence the likelihood of subsequent wear, corrosion and ultimately effect longevity of the implant.
Methods and Materials
We studied 11 clean un-used commercial stems from five different companies (DePuy, Stryker, Biomet, Wright Medical, Smith and Nephew). Trunnion surface was scanned with an optical profilometer (Bruker ContourGT-I 3D Optical Microscope (Karlsruhe, Germany), an interference microscopes with the capability to analyse 3 dimensional topographical features of materials. Three measurements of each trunnion were taken on the anterior surface at the proximal, mid-point, and distal segments. Each region scan was conducted under a 20x objective with the scan length of 1000 um and a back scan of 500 um.
Results
Four of the eleven hip stems demonstrated a smooth surface finish and the remaining seven had a distinct threaded surface with the Corail and the SROM surface topography shown in figure 1. All the analysed trunnion surfaces had a Sa value than Sz value (p < 0.05), indicating each trunnion surface consists of repetitive peak heights and valley depth across the scanned surface area shown in figure two. The higher Sa parameter for the threaded trunnions indicates a rougher overall surface due to a recurring threads pattern. The mean dominant thread heights of the trunnions varied from 12.91 μm on the Profemur to 3.24 μm on the SecurFit Max. The Silent, Summit and the Tri-Lock stems demonstrated a variation in thread heights on their respective surfaces. The Summit stem also demonstrated variability in the pitch of its threads which was not seen on the other threaded stems. The Silent had the widest pitch at 0.304 μm whilst the Profemur had the narrowest pitch at 0.13 μm.
Conclusion
Threaded trunnion surfaces for different stems are not uniformly patterned and demonstrate variability in thread height and pitch. These differences could affect micro motion at the taper junction, influencing corrosion and fretting. The study is limited in its capacity as only one stem of each type was analysed, therefore any variation between same stem designs cannot be commented on.
