Nanomedicine, Volume IIA: Biocompatibility
© 2003 Robert A. Freitas Jr. All Rights Reserved.
Robert A. Freitas Jr., Nanomedicine, Volume IIA: Biocompatibility, Landes Bioscience, Georgetown, TX, 2003
18.104.22.168 Orthopedic Biomaterials
In cases of joint injury or degenerative arthritis, when improvement cannot be gained through physical therapy, nonsurgical treatments, or surgical repairs, orthopedic surgeons often advise joint replacement surgery in which the deteriorated joint is removed and replaced with a man-made device [250-252]. Artificial joints consist of a plastic cup made of ultrahigh molecular weight polyethylene, placed in the joint socket, and a metal (titanium or cobalt chromium alloy) or ceramic (aluminum oxide or zirconium oxide) ball affixed to a metal stem. This type of artificial joint is used to replace hip, knee, shoulder, wrist, finger, or toe joints. Joint replacement surgery is performed on an estimated 300,000 patients per year in the U.S. . In most cases, it brings welcome relief and mobility after years of pain.
Artificial knee joints are used to alleviate pain and restore function in patients who have a diseased joint. Materials and design engineers must consider the physiologic loads to be placed on the implants and must design for sufficient structural integrity. Material choices also must consider implant biocompatibility with surrounding tissues, the environment and corrosion issues, friction and wear of the articulating surfaces, and implant fixation either through osseointegration (the degree to which bone will grow next to or integrate into the implant) or bone cement .
One of the major problems plaguing orthopedic implant devices is purely materials-related: wear and fatigue-induced delamination of the polymer cup in total joint replacements . Any use of the joint, such as walking in the case of knees or hips, results in cyclic articulation of the polymer cup against the metal or ceramic ball. Due to significant localized contact stresses at the ball/socket interface, small regions of polyethylene tend to adhere to the metal or ceramic ball . During the reciprocating motion of normal joint use, fibrils are drawn from the adherent regions on the polymer surface and break off to form submicrometer-sized wear debris . This adhesive wear mechanism, coupled with fatigue-related delamination of the polyethylene (most prevalent in knee joints), results in billions of tiny polymer particles being shed into the surrounding synovial fluid and tissues. The biological interaction with small particles in the body then becomes critical. The body’s immune system attempts, unsuccessfully, to digest the wear particles much as it would a bacterium or virus . Enzymes are released that eventually result in osteolysis, the death of adjacent bone cells . Over time, sufficient bone is resorbed around the implant to cause mechanical loosening, which necessitates a costly and painful implant replacement, or “revision.” Since the loosening is not caused by an associated infection, it is termed “aseptic loosening” . The average life of a total joint replacement is 8-12 years , or even less in more active or younger patients. Because it is necessary to remove some bone surrounding the implant, generally only one revision surgery is possible, thus limiting current orthopedic implant technology to older, less active individuals .
Studies of wear debris extracted from actual tissue samples of patients whose implants failed as a result of aseptic loosening generated significant information regarding wear particle size, shape, and surface morphology . Interestingly, investigators at the Southwest Research Institute used the atomic force microscope (AFM; Section 2.3.3) to produce detailed, high resolution images of polyethylene wear particles measuring a few hundred nanometers in size and sometimes exhibiting a cauliflower-like surface morphology . By combining wear debris and cellular response studies, engineers and biologists are trying to better understand implant failure and to re-engineer implants to avoid future problems [255, 258]. Experiments with diamond-coated hip-replacement implants are in progress .
G.M. Fahy notes that inflammatory cells lack receptors for ultrahigh-density polyethylene or fragments thereof, yet are able to recognize these utterly foreign objects as such and attack them. This might, in part, be accomplished by “recognition” not of specific topological features or chemical groups but instead by “recognition” of a surface with a higher surface energy than the surface energy of the immune cell. The immune cell tries to reduce the free energy of the combined cell-polymer interface by coating the high energy interface – i.e., by adhering to, and if possible engulfing, the particle. This phenomenon would then provide a general guideline as to how to reduce unwanted adhesion: avoid high surface energy interfaces. Surface energy is briefly mentioned elsewhere in connection with diamond (Sections 22.214.171.124 and 126.96.36.199), Teflon (Section 188.8.131.52), and sapphire (Section 184.108.40.206) surfaces, and previously in Sections 9.2.1 and 9.2.3.
Last updated on 30 April 2004