Wear of ultra-high molecular weight polyethylene manufactured with laser powder bed fusion

Abstract

More than 750,000 total knee replacement (TKR) surgeries are performed annually in the United States (US) to treat degenerative joint diseases such as osteoarthritis. A prosthetic knee implant typically comprises a cobalt chromium (CoCr) femoral component and a polyethylene tibial insert that is constrained in a tibial tray. The articulation between the femoral component and tibial insert replaces the function of the natural knee joint. Advances in design, manufacturing, and materials have increased the statistical survivorship of prosthetic knee implants. However, despite extending the longevity of prosthetic knee implants, approximately 20% to 40% of patients are unsatisfied with their TKR. The advent of image processing and additive manufacturing (AM) technologies has raised interest in personalized prosthetic (knee) implants. Such patient-specific implants could potentially improve patient outcomes and satisfaction by improving knee kinematics compared to a standard implant, which improves functionality, and by enabling an almost perfect anatomical fit and improved positioning of the implant, which reduces stress-shielding and promotes bone ingrowth, thus reducing potential complications. However, AM of the polyethylene tibial insert has not yet been achieved, yet it is crucial to implement patient-specific implants because it defines the knee kinematics. The low melting point of ultra-high molecular weight polyethylene (UHMWPE) and the entanglement of long polymer molecules prevents use of extrusion-based AM techniques such as fused filament fabrication (FFF). Hence, in this work, we use laser-powder bed fusion (L-PBF) to manufacture UHMWPE specimens. We print and post-process cylindrical pin specimens and perform pin-on-disc wear testing against stainless steel countersurfaces. We measure wear as a function of sliding distance for 3D-printed UHMWPE specimens of different density and compare the results to those of conventionally extruded and machined UHMWPE specimens. The results demonstrate that the wear rate of 3D-printed UHMWPE specimens decreases with increasing density and ultimately approaches that of extruded UHMWPE specimens. The results of this work are relevant in the context of 3D printing patient-specific knee implants to improve clinical outcomes for patients.