Effect of head size and rotation on taper corrosion in a hip simulator

Aims This study investigates head-neck taper corrosion with varying head size in a novel hip simulator instrumented to measure corrosion related electrical activity under torsional loads. Methods In all, six 28 mm and six 36 mm titanium stem-cobalt chrome head pairs with polyethylene sockets were tested in a novel instrumented hip simulator. Samples were tested using simulated gait data with incremental increasing loads to determine corrosion onset load and electrochemical activity. Half of each head size group were then cycled with simulated gait and the other half with gait compression only. Damage was measured by area and maximum linear wear depth. Results Overall, 36 mm heads had lower corrosion onset load (p = 0.009) and change in open circuit potential (OCP) during simulated gait with (p = 0.006) and without joint movement (p = 0.004). Discontinuing gait’s joint movement decreased corrosion currents (p = 0.042); however, wear testing showed no significant effect of joint movement on taper damage. In addition, 36 mm heads had greater corrosion area (p = 0.050), but no significant difference was found for maximum linear wear depth (p = 0.155). Conclusion Larger heads are more susceptible to taper corrosion; however, not due to frictional torque as hypothesized. An alternative hypothesis of taper flexural rigidity differential is proposed. Further studies are necessary to investigate the clinical significance and underlying mechanism of this finding

Image registration demonstrates the growth plate has a variable affect on vertebral strain

Characterizing the biomechanical behavior of the vertebrae is important in understanding the impact of structural and material changes on spinal growth and fracture risk. The growth plate is critical for the normal development of the skeleton, with abnormalities leading to uneven maturation. Little is known about how growth plates affect the stress and strain experienced by the surrounding bone. Concentrated strain within the growth plate may influence mechanical cell signaling during development, lead to increased fracture risk at this site and may influence average bone strain measures. It is hypothesized that the growth plates and adjacent bony areas will take up a large amount of the strain within rat-tail vertebrae under axial compressive loading, leading to increased average bone strain measures. The sixth caudal vertebrae of 8 rnu/rnu rats were muCT scanned in both loaded (20-32 N axial compression) and unloaded configurations. Image registration was used to calculate strain in the bone due to the applied load by finding a spatial mapping between the two scans. In seven of the eight rats, the majority of the strain measured within their vertebrae was concentrated in the growth plates. Five of the specimens had growth plates that demonstrated rigid behavior in contrast to compliant growth plate behavior seen in the other three rats. The presence of a compliant growth plate led to higher average (-0.03 vs. -0.01) and maximum (-0.13 vs. -0.02) strains. The strain within the growth plate is important to consider when interpreting apparent tissue level biomechanical data commonly reported in the literature as this study suggests strains are not uniformly distributed with high concentrations in and around the growth plate. This strain distribution may provide insight into the mechanical signals that cells experience during the formation of new bone, with the higher strains near the growth plate signaling cells to lay down more bone, while also leading to increased risk of fracture in this region