2Background

Trauma or diseases such as rheumatoid arthritis can cause the movement of a joint to become painful. When physical therapy and medication are found to be ineffective in restoring the function of the joint, orthopedic surgery is often required to repair the damaged joint tissues. When the damage to the joint tissues is severe, portions of the joint can be surgically replaced with an orthopedic implant. Orthopedic implants are fabricated from various metals, polymers, and ceramics that are known to be well tolerated by the body. The decision regarding which combination of specific materials that should be used to construct the implant depends on the joint that the implant is being designed to replace. Finger joints have been successfully replaced with a simple polydimethylsiloxane ('silicone') hinge. 'Ball and socket' joints such as the hip, on the other hand, are much more complex in terms of their stress states and ranges of motion. Many different ceramic, polymeric, and metallic materials have been combined to construct a device that mimics the complex function of the original, natural ball and socket joint.

Early attempts at developing materials for permanent placement within the body were rarely successful; the recipient either died soon from the effects of the operation or later from the toxicity of the implanted material.

As biomaterials researchers began to consider all factors in the design process for each material that was to be implanted, implants became less dangerous and more effective. By the time of World War II, materials specifically developed to exhibit degradation resistance (e.g., high purity stainless steel, nickel superalloys, tantalum, titanium alloys, silicone rubber, polymethylmethacrylate) were used in humans after extensive testing in animals. Biomaterials intended for implantation have been synthesized from commercially available materials found in the lab, such as polymethylmethacrylate (PMMA), or are simply purified from naturally occurring compounds (such as coral from the ocean). Now, there are very few types of body tissue that cannot be replaced by a wide variety of biomaterials and combinations of biomaterials. Table 1 provides a summary of the polymeric, ceramic, metallic and composite materials that are commonly used as biomaterials.

All types of the more common metals have been used for prosthetic applications in the body. Gold, silver, copper, lead, zinc, cadmium, tin, iron, nickel, aluminum, magnesium, vanadium, vanadium steel, bronze, brass, tantalum-niobium alloys, plated steels, cobalt-chromium-molybdenum alloys, titanium, zirconium, palladium, and 302, 304, 316L, and 317 stainless steels have been employed with the goal of aiding bone healing and function. Eventually it was discovered that relatively few of the above listed metals and alloys (mainly gold, titanium, zirconium, tantalum-niobium, palladium, 316L, and CoCrMo alloys) could be tolerated by the body quite well (although stainless steels were found to affect bone growth and high amounts of ionic Co, Cr, and Mo have been linked to health problems). Detailed discussions regarding the microstructure-property correlations and appropriate manufacturing techniques of these metals currently used for implant construction have been provided by a number of authors.

Over the past thirty years, many new designs and material combinations for artificial hip joints have been proposed. Now that people are living longer, more active lives, more people are developing arthritis and other disorders of the joints. The exact number of artificial joints that have been implanted to date is a difficult figure to determine. However, most sources estimate that there are now nearly 1,000,000 artificial joints in over 800,000 recipients around the world. They are currently being implanted at an accelerating rate of over 200,000 per year. Ideally, these artificial joints would perform exactly as the natural ones they replace. Unfortunately, not all of these joint replacement systems have provided trouble-free service. A 4% long term failure rate for consumer products may be acceptable for some applications, but when one considers that a failure rate of 4% after 5 years in service for hip prostheses translates to 40,000 patients who need surgery to alleviate pain or immobilization, the term 'acceptable failure rate' changes significantly. Collier et al. reported a mean implant duration time of 40 months before the patient exhibited symptoms indicating that the artificial joint was in need of repair (revision) surgery. Some of these symptoms are obvious to the patient (e.g., severe pain with each step) whereas other symptoms may only be ascertained by the surgeon from radiographs (x-rays) of the hip region. In the study performed by Collier et al., the shortest service life before the implant had to be removed was 2 months, whereas the longest duration time found was over 13 years with trouble-free operation. Other studies of the performance of different implant systems indicate that the 40 month average service lives reported by Collier et al. are rather short (see Clarke et al.). However, no implant system to date has exhibited completely 'failure-free' performance. In the case of hip joint replacement systems, 'failure-free' can be taken to mean the following:

• The patient's body's immune system would not react adversely to the implanted materials
• For younger patients the implants should function properly for at least twenty years, or for the remainder of an elderly patient's life
• No particulate or corrosion debris would be generated, and no mechanical failure (such as fracture or loosening of the implant components) would occur.

It is important to note that violation of this last constraint can lead to violation of the previous two. In other words, the prevention of debris particle generation is crucial for the prevention of pain and/or implant malfunction. However, before the major debris generation modes can be eliminated, their corresponding debris products and their sources must be identified, thus the impetus for this research. Once the mechanisms of debris generation are determined, appropriate modification of the implant's structure and content can be recommended.

It is generally believed that the introduction of a hip joint prosthesis into the body will rehabilitate the patient without any side-effects, and this is generally the case. However, long term failure rates ranging from 3 to 11% (depending on the study and types of implants considered) have caused concern among surgeons, hospitals, patients, lawyers, insurance companies, and biomaterials researchers alike. A detrimental aspect of this concern over implant failure is that well functioning implants are rarely examined at autopsy, thus depriving the scientific community of the important material properties of systems that functioned satisfactorily. Occasionally, patients or hospitals will insist that they have a right to keep the implant without allowing it to be subjected to a materiallographic examination.

All sources that address the generation of titanium debris from orthopedic implants emphasize that the titanium alloy present in the tissues was generated in particulate form, with constituent elemental concentrations usually proportional to alloy composition (e.g., Ti:Al:V = 90:6:4). Buchhorn et al. report that the experimentally generated debris of their research retained its original crystal structure and the resulting passivation layers were found to be thin oxides similar to those generated in air. It is important to note that these debris particles were generated in ethanol under controlled amounts of motion in sealed containers of the same material as the debris. Conditions within the human body vary considerably from this ideal and may generate particles of a much different nature.

Most claim that micromotion between bone tissue or the CoCrMo head and passive titanium layer (i.e., fretting) as well as Ti/UHMWPE interfacial interaction in the presence of chemical attack are the primary causes for debris generation. Interestingly, the sources that emphasize wear mechanisms also implicate initial and continual chemical or mechanical attack as having a pronounced weakening effect upon the oxide layer that normally protects the bulk material.

The results of research recently presented at the 1993 International Symposium for Biomaterials (Birmingham, AL) indicate that the non-articulating surfaces of nearly all titanium and CoCrMo alloy joint replacement components have been glass bead or grit blasted to provide the implant with a favorable surface state (with the surface in compression) for corrosion resistance and biocompatibility. Ricci et al. indicated that approximately 20% of the implant surface was in fact covered by either Al2O3, SiO2, or other non-intrinsic ceramic or metallic components. Electron Dispersive X-ray spectra were identical for the embedded grit particles and for glass beads and alumina-zirconia grit particles supplied by various manufacturers. When the implants were subjected to a highly corrosive environment, particles of grit were discovered that were not initially detectable with secondary electron imaging of the original uncorroded surface. That is, the grit was found embedded several microns below the implant surface. The grit was buried deeper and more extensively for titanium alloys than those of CoCrMo. This phenomenon is explained by titanium's higher ductility and lower hardness. It does not require a great stretch of logic to conclude that these grit particles can and will contribute to three body wear upon their release into the implant's surroundings.

The next section will discuss how a biomaterials engineer would approach the design of a total hip replacement so that the above requirements can be met. Details for the specific design requirements of a Ti6Al4V-CoCrMo implant will then be provided.

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