Development of a low-wearing novel hip joint prosthesis with a longer lifespan

Abstract

The metal-on-polyethylene material combination has been the preferred choice amongst orthopaedic surgeons for artificial hip implants ever since its introduction in the clinical market by Sir John Charnley. In fact, metal-polyethylene hip implants account for the largest share (over one-third) of the global market. Their ubiquitous use has been driven by their ability to restore the biomechanics of the degenerated hip joint. Nonetheless, further improvements in the tribological performance of the polyethylene components are needed, since the average lifespan of these prostheses range between 15 to 25 years. The functionality of the hip joint can be once again restored using revision surgery; however, such procedures tend to be complicated, costly and have lower rates of success. The demand for such medical devices with an improved lifespan is continuously on the rise, due to the ever-increasing global population age and obesity. In order to tackle the prevalent limited lifespan of primary hip joint replacements, research efforts in this work have been primarily focused on developing a new prosthesis design, hereinafter referred to as the MaltaHip, that exploits the molecular reorientation mechanism of the polyethylene material to reduce the extent of osteolysis. By producing uni-directional articulations, using multiple cylindrical joints that are orthogonal to each other, the surface molecules of the polyethylene components become highly aligned due to the molecular orientation hardening mechanisms, effectively producing highly wear resistant articulating surfaces. Further reductions in wear are also aided due to the increased contact area, provided by the cylindrical design of the articulation, which reduce the overall contact stresses. Therefore, the hypothesis that uni-directional cylindrical articulations consisting of polyethylene bearing surfaces provide reduced rates of wear, compared to the ball-and-socket articulation, was tested in this work. A parametric CAD model of the MaltaHip has been developed to optimise the geometrical design of the cylindrical articulations. Finite element simulations were conducted to analyse the stresses that were produced on iterated designs of the prosthesis. The polyethylene components were simulated using an advanced viscoelastic-viscoplastic material model. Furthermore, comparisons of the generated stress were made to the conventional ball-and-socket prosthesis. The results produced from the finite element simulation were also used to predict the theoretical wear rates of the different prostheses designs, using various wear models which are available in literature. Physical prototypes of the MaltaHip implant were fabricated based on the optimised design solution and then subjected to a series of physical tests. A single-station hip joint simulator was designed and built in parallel throughout this work that served as a testbed to validate the functionality of the new mechanism of the MaltaHip implant. Furthermore, the simulator was used to provide initial indications regarding the tribological performance of the new prosthesis design. The test results also indicated that the newly designed prosthesis was able to achieve extreme joint angles with a reduced risk for dislocation. A mock surgery was conducted using the MaltaHip on a Thiel (soft) embalmed cadaver by orthopaedic surgeons, in order to gain insight on the practicality of implanting the new prosthesis. It was observed that the MaltaHip could be implanted with relative ease. Furthermore, the implanted prosthesis demonstrated that it could attain a wide range of motion with a high degree of stability. After completion of the in-house tests, two sets of MaltaHip implants, each consisting of four prototypes, were produced. The first set of implants were produced out of UHMWPE (ultrahigh molecular weight polyethylene) components, whereas the second set of implants were produced out of VEHXPE (Vitamin E-infused highly-crosslinked polyethylene) components. The eight implants were comprehensively wear tested according to ISO 14242-1:2014/Amd 1:2018 at Endolab® Mechanical Engineering GmbH, an accredited implant testing facility in Germany. Gravimetric measurements conducted on the tested specimens demonstrated that the MaltaHip implants produced lower wear rates than conventional ball-and-socket implants that were produced out of the same materials and wear tested under the same conditions. In fact, the MaltaHip implants made from UHMWPE components produced around a quarter of the wear produced by ball-and-socket implants. Furthermore, the MaltaHip implants made from VEHXPE resulted in a negative wear rate, implying that the rate of wear was lower than the rate of fluid lubricant absorption. The results of the study demonstrated that the reductions in the rate of wear were statistically significant, therefore supporting the research hypothesis that was tested in this work. Optical microscopy images demonstrated that the machining marks on the polymeric components were preserved in most cases. This indicated that highly wear resistant surfaces, due to molecular orientation hardening effects, were indeed produced. A particle analysis conducted by Endolab® demonstrated that the produced wear particles ranged between 0.1 – 1.0 μm in size. Wear particles in this size range greatly contribute to the osteolysis effects. Nonetheless, due to the low volumes of wear that were generated by the prosthesis, the net osteolysis effect is postulated to be minimal, as indicated by a numerical method which is used to determine the extent of biological activity of the wear particles, as a function of particle sizes and the volumetric wear rate of the prosthesis. This result indicated that the MaltaHip could significantly reduce the extent of osteolysis, thereby potentially reducing the risk for revision surgery.