Development and Verification of a Porous Acetabular Shell Design Manufactured Using Additive Technology.

INTRODUCTION: Porous surface acetabular shells have been successfully used in cementless total hip arthroplasty. Recent advances in additive manufacturing have provided opportunities to optimize the shell designs. The current study describes the design and verification of a new acetabular shell design. MATERIALS AND METHODS: Additive manufacturing technology was used to fabricate acetabular shells using Ti6Al4V powder. A large computed tomography (CT) database was used to verify the screw hole location to ensure the screw trajectories were directed in the safe zone. Benchtop stability tests were conducted to compare the fixation stability of the new shell design to a clinically successful design. RESULTS: Shells were designed with an average pore size of 434 microns, surface porosity of 76%, and a coefficient of friction of 1.2. The CT analysis of various shell orientations demonstrated that at least two useful screws were typically directed toward the acetabular safe zone. The sawbone testing showed that the fixation stability of the new shell was either better or equivalent to the clinically successful design under two different bone preparation conditions. CONCLUSIONS: Using additive manufacturing technology, thin walled acetabular shells were fabricated which allowed for at least two ancillary fixation screws in the safe zone. The thin walls enable the use of a 36mm femoral head with a 48mm diameter shell which may enhance the joint stability in small stature patients. The equivalent or better fixation stability of the new design indicates that good initial fixation may be expected in vivo.

Friction in modern total hip arthroplasty bearings: Effect of material, design, and test methodology.

The purpose of this study was to characterize the effect of a group of variables on frictional torque generated by acetabular components as well as to understand the influence of test model. Three separate test models, which had been previously used in the literature, were used to understand the effect of polyethylene material, bearing design, head size, and material combinations. Each test model differed by the way it simulated rotation of the head, the type of frictional torque value it reported (static vs. dynamic), and the type of motion simulated (oscillating motion vs. continuous motion). It was determined that not only test model may impact product ranking of fictional torque generated but also static frictional torque may be significantly larger than a dynamic frictional torque. In addition to test model differences, it was discovered that the frictional torque values for conventional and highly cross-linked polyethylenes were not statistically significantly different in the more physiologically relevant test models. With respect to bearing design, the frictional torque values for mobile bearing designs were similar to the 28-mm diameter inner bearing rather than the large diameter outer liner. Testing with a more physiologically relevant rotation showed that frictional torque increased with bearing diameter for the metal on polyethylene and ceramic on polyethylene bearings but remained constant for ceramic on ceramic bearings. Finally, ceramic on ceramic bearings produced smaller frictional torque values when compared to metal on polyethylene and ceramic on polyethylene groups.