Polymethylmethacrylate (PMMA) has been used as a bone cement in total hip arthroplasty (THA) since the 1960's and today is still used in a significant percentage of the 1.5 million THA procedures performed worldwide each year. Despite its long clinical history the orthopaedic community acknowledges shortcomings with PMMA including among other things its exotherm, the toxicity of any unreacted monomer, its encapsulation by fibrous tissue and its cement-implant interface strength [1][2]. This proposal will focus on the later point, the cement-implant (or cement-metal) interface strength. Local debonding at this interface initiates a cascade of events;micromotion, particulate debris generation, macrophage activity, bone resorption and finally aseptic loosening of the implant. Attempts have been made to improve the interface strength of PMMA, but gains have been marginal, leading researchers to conclude that it's not an issue of if the interface should fail but when [3]. The use of polyurethane (PUR) based cement provides a completely different approach to addressing this clinical problem. PURs have a proven record of biostability and biocompatibility going back several decades [4]. More recently, in situ curing PUR adhesive formulations have been introduced to European markets and are seeing clinical use as bone void fillers and in stabilizing fractures. These PUR based cements have been shown to provide strong bond to bone and metal. As clinical experience with these materials grows, there is significant interest in development of PUR bone cement for affixation of total joint replacement devices. This research proposes development of a PUR bone cement specifically engineered for the biomechanical demands of THA. Phase 1 of the project will include formulation of new PURs using fundamental structure- property principles of polymer chemistry to provide a reactive two part system that upon mixing achieves full load bearing properties within 24 hour, strong adhesion to bone and metal implants, and long-term biostability. These formulations will then undergo mechanical testing to gain a full understanding of their capabilities. Of specific interest in Phase 1 will be their adhesion to THA devices and bone. The materials will be subjected to both static and dynamic test protocols, reproducing in vivo loads in a laboratory model. In parallel, the new materials will undergo in vitro biostability testing to characterize the risk of in situ oxidative degradation that would limit its clinical viability. Furthermore, level of extractable monomers will be evaluated for developed PURs and compared to PMMA. The overarching objective of these tests is to assess each material's potential as a replacement for PMMA in THA. Later phases of the project would include further refinement of the leading candidate material to improve load bearing capability, adhesion, and biostability, and to complete biocompatibility testing, mechanical assessment under more complex loading, and ultimately human clinical use.
The overall goal of this project is to develop a polyurethane adhesive bone cement for use in total hip replacement surgery. This alternative to polymethylmethacrylate will have superior bond strength between the cement and the metal device, significantly reducing the risk of failure due to loosening at this interface. The technology developed in this study has the potential to improve the lives of over 1.5 million new patients each year who undergo hip replacement surgeries.
