A new process to improve the mechanical performance of crosslinked UHMWPE injoint replacement prostheses

Polymerix Llc, Allston MA

Investigators

Abstract

While total joint replacement prostheses have been a great success for over 80 years in providing mobility to patients with osteoarthritis, post-surgery complications like aseptic loosening, dislocation and infections remain as problems, often leading to costly and complicated revision surgery. The objective of this proposal is to address implant dislocation, the second most common complication for total hip replacement surgeries. One solution to reduce the incidence of dislocation is to increase the diameter of the femoral head and accordingly decrease the thickness of the ultra-high molecular weight polyethylene component. The consequences of these design changes are that the larger articular surface area could lead to higher volumetric wear and the thinner polyethylene components are subjected to higher stresses with a greater likelihood for fracture. The highly wear resistant crosslinked polyethylenes introduced in the late 90’s have addressed the issue of wear but these crosslinked materials also have lower mechanical toughness, limiting the use of thinner polyethylene components. To solve this problem, we have developed a patent-protected rapid-pressurization process which has the potential to greatly increase the ductility of crosslinked polyethylene without compromising its strength. We hypothesize that pressure-quenching of antioxidant containing crosslinked polyethylene will exceed the toughness, strength and resistance to fatigue crack propagation over uncrosslinked polyethylene without compromising wear or oxidation resistance. Our preliminary data on pressure quenching and past research on thermal quenching show that a pseudo-hydrostatic process in which the melted, crosslinked polyethylene is rapidly pressurized and crystallized can be performed on large cylinders using a simple hydraulic press, which is easily translatable into a manufacturing process, providing bulk crosslinked polyethylene from which implants can be machined. The specific aims of the proposal are: Aim 1 will optimize the pressure-quenching process of Vitamin E containing crosslinked polyethylene and verify its superior mechanical properties using tensile, fracture toughness and impact tests. Aim 2 will use accelerating aging to demonstrate that the pressure-quenched Vitamin E containing crosslinked polyethylene is oxidation-resistant. Aim 3 will include long-term wear and fatigue crack propagation tests to verify that pressure-quenched, Vitamin E containing crosslinked polyethylene preserves wear resistance and has a higher resistance to fatigue crack propagation. This proposal has the potential to guide in the fabrication of total hip replacement components which are less likely to dislocate, without compromising either wear resistance or risk for mechanical damage. Furthermore, knee and shoulder joint components, which are subjected to higher stresses could be better protected from mechanical damage. The long-term plan is to leverage this materials-formulation study to investigate implant designs in a larger project that can reduce the incidence of dislocation without compromising either wear or mechanical damage. Longer lasting joint replacements would greatly benefit the elderly with osteoarthritis who require these implants for mobility.