For his research, Microtextured Hip Joint to Improve Longevity, assistant professor in mechanical engineering at the University of Utah, Bart Raeymaekers received a NIH (National Institute of Health) RO3 three year grant of $223,500.

More than 280,000 total hip replacement (THR) surgeries are performed in the US each year. This proposal focuses on metal-on-polyethylene (MOP) hip joints, which are the most common type in the US, and consist of a cobalt chromium (CoCr) femoral head that articulates with a polyethylene acetabular liner. The statistical survivorship of MOP hip joints declines dramatically after 15-20 years of use, primarily because polyethylene wear deteriorates the joint, and/or adverse immunological reaction to microscopic wear debris leads to inflammation and osteolysis, which causes mechanical instability of the prosthetic joint. This lack of durability results in risky and costly revision surgery to replace a failed implant, or surgery postponement, which leaves the patient in pain. Hence, longevity of prosthetic hip joints remains an imminent problem.

Research to increase implant longevity can be categorized as improving the mechanical properties of the polyethylene, and/or improving the design of the femoral head. Wear performance has incrementally improved over the past decade by e.g. the introduction of (vitamin E infused) cross-linked polyethylene (XLPE). Changes of the femoral head have focused on new materials and coatings, and creating ultra-smooth surfaces. However, no reports demonstrate a substantial increase of in-vivo longevity as a result of these improvements.

In contrast to the existing paradigm, our approach to reducing polyethylene wear and improving longevity of prosthetic hip joints is to add a patterned microtexture, composed of microsized spherical dimples, to the smooth femoral head. The microtexture enhances the formation of a lubricant film that reduces contact and wear. The objective of this research is to test the hypothesis that microtextured CoCr surfaces articulating with EtO-sterilized XLPE will: (1) create hydrodynamic lubrication at realistic hip joint operating conditions, and (2) substantially reduce XLPE wear to increase longevity and avoid revision.

The patterned microtexture is manufactured using laser surface texturing. We have already implemented microtexture in other applications, yielding significant reductions in friction and wear. Preliminary modeling and experimental data demonstrate feasibility of the concept. In addition, a cost analysis revealed that adding a patterned microtexture to the femoral head costs less than the cost difference between MOP and e.g. ceramic hip joints and other new materials that have been attempted. This is important to obtain insurance reimbursement approval. Also, The microtexture could enable designing patient-specific prosthetic hip joints, where a computer model aids to design the optimal patterned microtexture, based on the patient’s body metrics and activity level. Since the technology base for microtexturing already exists, it can be readily implemented in implant manufacturing. The project team combines expertise in (bio)tribology and microtexturing (PI Raeymaekers), and orthopedics (co-I Bloebaum). In addition, the team’s labs are outfitted to perform the proposed research, with state-of-the art instrumentation and testing equipment, already developed for this project. We propose to accomplish our objective with the following two specific aims.

Aim 1: To design a patterned microtexture to improve longevity of prosthetic hip joints, we will implement an elastohydrodynamic lubrication model of a microtextured femoral head articulating with a deformable polyethylene liner. We will account for non-Newtonian lubricant viscosity and variable loading and sliding speed during gait, with parameters spanning the spectrum of conditions that prosthetic hip joints endure in-vivo. We will use the model to optimize the microtexture geometry and pattern, to minimize friction and wear. We will implement the optimized microtexture designs in prototype MOP joints for in-vitro testing in Aim 2.

Aim 2: To demonstrate that a microtexture on the smooth femoral head improves the longevity of an MOP prosthetic hip joint, we will measure the friction coefficient and wear rate of the optimized microtexture designs of Aim 1, manufactured on the surface of a surrogate femoral head and articulating with a smooth XLPE surrogate acetabular liner. We will test these prototypes under clinically relevant operating conditions and benchmark the results against a conventional smooth surrogate MOP joint. A bench-top hip simulator already built by PI Raeymaekers will create articulation between the joint surfaces, under time-varying speed and load to mimic human gait. In addition, traditional pin-on-disk testing will be performed to allow benchmarking of the results with those in the literature. The lubrication conditions between the articulating surfaces will range from dry to submerged, to test the effectiveness of the microtexture as a function of availability of joint fluid. Subsequently, we will examine the wear-tested articulating surfaces to gain understanding of how microtexture affects wear debris formation and to detect proteins, minerals, and debris depositions that may, over time, reduce the functionality of the microtexture. Additionally, the experimental data will be used to validate the model of Aim 1. This will then enable model-based (predictive) design of custom implant microtextures for e.g. low and high-activity patients.

If successful, this research will shift current paradigms of engineering articulating surfaces of prosthetic hip joints, and produce a new generation of advanced, radically more durable implants. It will demonstrate that a microtextured femoral head surface can dramatically reduce polyethylene wear by creating lubrication more effectively than smooth surfaces.

Learn more about Dr. Raeymaekers and his research at the Utah Tribology and Precision Engineering Laboratory site.