Total joint replacement (TJR), as we know it today, came into existence in the 1960’s through the innovative work of Sir John Charnley1. It has relieved the suffering of millions of patients. Over 332,000 Americans undergo total hip arthroplasty, the most frequently performed TJR, each year. In elderly populations, these surgeries provide an effective treatment for the debilitation and pain caused by osteoarthritis and injury. However, medical engineering is yet to present a problem-free prosthesis and, during the past fifty years, investigators have continually sought to improve the success rate of the procedure. This may be related to numerous factors, but here, we will focus on what is considered the most critical consideration: The biomaterial from which a hip implant is made2.
Hip implants may be constructed from ultra high molecular weight polyethylene (UHMWPE), metal or ceramic. The degree to which an implant is tolerated by the body, termed biocompatibility, is determined by the chemistry, mechanical properties and immune response of the material from which it is constructed.
UHMWPE Implants
Unfortunately for medical engineers, in the immune system, size does matter and detrimental host responses to UHMWPE implants often arise not because of the intact implant itself, but from the wear generated over time from bodily motion. These particulates – 0.1 -1 μm in size – are mistaken for pathogens in the body and swiftly attacked. The result of this immune onslaught is a build up of cellular signalling molecules that induce the dissolution of the surrounding bone and consequent loosening of the artificial hip in the patient.3 The issue of wear particle generation has been widely addressed through improved implant sterilization techniques, which can adversely affect the material properties of UHMWPE. UHMWPE prostheses are widely used in TJR but may be considered most ideal for elderly, less active patients.
Metal Implants
Disturbingly, although they do not elicit a significant immune response, particulates generated by metallic implants can cause hypersensitivity, DNA damage and the development of ‘pseudotumors’ – fluid filled masses found near the implant mainly comprised of dead tissue. Whilst any one of these complications can lead to the removal or replacement of a prosthesis, replacements of this type have become more popular in recent times; metallic implants produce 40-100 times less debris and can be used to merely resurface an arthritic joint, minimizing the amount of bone removed during surgery.4 Metal hip replacements are often considered ideal for younger, more physically active individuals.
Ceramic Implants
Ceramics such as alumina and zirconia have been in use in TJR since 1972. They have been coupled with metals and UHMWPE as well as used in pure ceramic prostheses. Early complications caused by their brittle nature were quickly resolved by improvements in design and material quality, and ceramics have performed well in TJR thus far. Hip prostheses made from these smooth, hard-wearing materials generate fewer and less toxic debris in the body, promote good lubrication in the joint and can incorporate minerals that encourage bonding with bone.
Ceramic hip implants sound pretty good, right?
Unfortunately, all biomaterials have drawbacks.
Bone is a dynamic tissue that constantly remodels to satisfy biomechanical needs – bone is produced or resorbed depending on the load experienced. If a joint replacement made from materials far stiffer than natural bone is implanted, it will bear a greater load than the bone in the surrounding area and the bone will start to dissolve, leading to implant loosening. This side effect may be experienced by patients given implants with metal or ceramic prostheses.
Total hip arthroplasty is commonly employed to reduce pain and restore function to damaged or diseased joints and is an effective tool in preventing associated disability. There is no simple answer to the question of which biomaterial is best for manufacturing a hip prosthesis, and like all of medicine, TJR remedies for diseased or damaged hips should be tailored to the unique needs of each patient.
References:
1) Toledo-Pereyra, L. H. John Charnley–father of modern total hip replacement. Journal of investigative surgery : the official journal of the Academy of Surgical Research 17, 299-301, doi:10.1080/08941930490903777 (2004).
2) Goodman, S. B., Gómez Barrena, E., Takagi, M. & Konttinen, Y. T. Biocompatibility of total joint replacements: A review. Journal of biomedical materials research. Part A 90, 603-618, doi:10.1002/jbm.a.32063 (2009).
3) Ingham, E. & Fisher, J. The role of macrophages in osteolysis of total joint replacement. Biomaterials 26, 1271-1286 (2005).
4) Ingham, E. & Fisher, J. Biological reactions to wear debris in total joint replacement. Proceedings of the Institution of Mechanical Engineers, Part H: Journal of Engineering in Medicine 214, 21-37, doi:10.1243/0954411001535219 (2000).
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