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Aerospace Materials and Orthopaedic Applications: Opportunities and Challenges

Introduction

The evolution of biomaterials adopted for orthopaedic use might well be described as slow and incremental. There are a number of contributing reasons, but the conservative nature of orthopaedic surgeons and the effects of the 510(k) approval process on the development of novel materials and PMAs that require clinical trials remain major factors. It could be argued that advances in hip and knee implants have been minor since the days of John Charnley, since both stainless steel and ultra high molecular weight polyethylene are still widely employed in orthopaedic implants 50 years after their introduction. Of course, the highly successful clinical outcomes of hip and knee arthroplasty attests to excellence in the original Charnley design, but it must be recognized that current orthopaedic materials are sub-optimal from several standpoints. In this article, we will address how aerospace materials, particularly composites, might have direct applications to orthopaedic practice due to advanced material properties.

Major Drawbacks to Current Orthopaedic Materials

Although complications in arthroplasty such as infection and operative error have fallen below five percent for most procedures,[1] three areas of adverse outcomes are well recognized in orthopaedic science:

Wear debris-induced osteolysis. Joint arthroplasty requires the replacement of an articulating joint with a prosthesis that enables movement through a load-bearing surface. All current bearing surfaces are subject to the generation of wear during use, and the current materials in debris form have proven provocative of macrophage recruitment and attendant chronic inflammation. This outcome ultimately drives osteoclast-mediated bone resorption that leads to loss of fixation of the prosthesis commonly termed aseptic loosening.[2] We believe that low friction aerospace polymers and composites such as PEEK (polyetheretherketone) may provide superior bearing surfaces that improve the performance and life expectancy of joint prostheses. 

Stress shielding. The quality of bone and its structural stability are dependent upon force applied to the skeleton during normal activities (Wolffe’s Law). The implantation of metal femoral and tibial components during arthroplasty results in abnormal distribution of forces to the bone tissues, due to the stiffness of the prosthesis components. This attenuation of the normal bone loading during motion results in reduced osteoblast activity and subsequent regional bone loss in tissues adjacent to the prosthetic components. Aerospace engineering suggests that composites can be matched to the modulus of bone, allowing natural force transmission from the implant to the bone resulting in the alleviation of bone loss due to stress shielding.

Material biocompatibility. Recent reports of adverse reactions to metal-on-metal (MoM) hip implants have alerted patients and surgeons to the phenomenon of metal hypersensitivity. The release of metallic ions from orthopaedic implants (due to both bearing wear and corrosive action on metals) has been reported to result in hypersensitive responses to nickel, cobalt and chrome in 30% of patients with well functioning MoM implants (vs. 15% in normal controls), and hypersensitivity can reach as high as 66% in patients with failing prostheses.[3] The mechanism for the development of metal hypersensitivity is well-recognized by clinical immunologists, and this form of allergy is inevitable in a large subset of orthopaedic patients. The minimal biological response to many aerospace materials indicates a high level of biocompatibility appropriate for use in orthopaedic procedures.


Several studies have illustrated the potential for composite materials to provide successful adaptation to orthopaedic implantation, but some difficulties have arisen in clinical acceptance of these designs.[4,5] Aerospace technology may provide the key to the advancement of composite materials, since several decades have been devoted to material performance in harsh environments and extreme mechanical loading. It is also recognized that composites can provide advantages over metals in instruments and equipment used over the vast range of delivery of orthopaedic care, and this philosophy is illustrated in projects under development at the National Center of Innovation for Biomaterials in Orthopaedic Research (nCiBOR). The particular advantages of an improved strength to weight ratio and radiolucency during imaging are major motivating factors for innovation using composite based materials. Challenges arise due to the limited knowledge of long-term performance of these materials. We describe three applications of aerospace materials under development of orthopaedic applications.

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