Materials for Biomedical Engineering. Mohamed N. Rahaman
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has similar composition to ZTA but slightly improved mechanical properties due to a modification of the ZTA microstructure. While the potential for brittle failure of AMC (or ZTA) in vivo should be recognized, the actual incidence of failure under the physiological stresses of the hip is now very low.
Another ceramic material that has been used as an articulating bearing material in hip implants is yttria (Y2O3)‐stabilized zirconia (abbreviated YSZ). While YSZ can have comparable or somewhat better mechanical properties than AMC, its manufacturing conditions have to be carefully controlled to achieve these properties. Some compositions of YSZ can also be chemically unstable in an aqueous environment, which can lead to a degradation in mechanical properties and wear resistance in vivo. Based on these limitations, YSZ is not currently approved by the US Food and Drug Administration for use as femoral head devices in vivo.
When articulating against the liner of the acetabular cup, AMC femoral heads generate a lower amount of wear particles than Co–Cr. Consequently, hip implants produced with AMC femoral heads can have a longer lifetime in vivo when compared to similar implants with Co–Cr femoral heads. This improvement in implant lifetime can be a significant factor in the selection of hip implants, particularly for younger patients. However, despite this advantage, femoral heads composed of AMC are used far less frequently than Co–Cr due to their higher manufacturing cost and lingering concerns about their brittle mechanical response.
Acetabular Cup
The femoral head articulates against the liner of the acetabular cup whereas the shell provides mechanical support for the liner and stability of the acetabular cup within the pelvic bone. In addition to a consideration of biocompatibility phenomena, friction, and wear are properties of primary importance for the liner, whereas strength and stiffness are important properties for the shell. Articulation of the femoral head against the liner leads to the production of wear debris, which has been shown to cause adverse responses by tissues surrounding the implant, such as an inflammatory response (Chapter 24). This adverse response, in turn, can lead to loosening of the stem within the femur, necessitating replacement of the implant (revision surgery).
Sir John Charnley selected polytetrafluoroethylene (PTFE) as the liner material in his early implants because it was available off the shelf and was known to have the capacity to be machined and polished to a smooth surface finish with low friction. However, the use of PTFE resulted in a high incidence of implant failure and Sir John Charnley tried another available polymer, PE, which proved to be more successful. Advances in materials science in the last several decades have resulted in the production of PE having better mechanical properties and higher wear resistance. PE with high molecular weight, referred to as ultrahigh molecular weight polyethylene (UHMWPE) or a highly cross‐linked UHMWPE is now the most commonly used liner material.
Other candidate liner materials include the metals and ceramics discussed for the femoral head material, which can be polished to a smooth low‐friction surface and have a much higher hardness than UHMWPE. Based on their mechanical properties, Co–Cr alloys are worthy of selection and they have been used as the liner in hip implants. However, articulation of Co–Cr femoral heads against Co–Cr liners (sometimes referred to as metal‐on‐metal bearing couples) have been associated with adverse responses in vivo. These bearing couples are prone to scratching and sticking, which reduces the ease of articulation. Of greater importance is the release of metal particles and ions, which has been shown to cause adverse immune responses in patients, such as metal ion sensitivity (Chapter 19). Based on these adverse effects, Co–Cr liners are no longer approved for in vivo use by the US Food and Drug Administration.
Ceramic liners composed AMC have been used for articulating against AMC femoral heads over the last few decades. Implants composed of these ceramic‐on‐ceramic articulating couples produce a much smaller amount of wear particles in vivo when compared to the commonly used couple consisting of Co–Cr and UHMWPE. AMC bearing couples show the lowest amount of wear of all the articulating couples currently used in hip implants, and a longer average lifetime when compared to the commonly used Co–Cr on UHMWPE couples. On the other hand, ceramic liners suffer from the same disadvantages described earlier for ceramic femoral heads. They are more difficult to manufacture and more costly than UHMWPE liners, there is a lingering concern about their brittle mechanical response, and they are sometimes prone to developing a squeaking noise when articulating against a ceramic femoral head.
Selection criteria for the shell of the acetabular cup, such as mechanical properties and biocompatibility, are, in general, similar to those already described for the stem. Consequently, Ti6Al4V is the commonly used shell material. The shell can be stabilized in the pelvic bone using Ti6Al4V metal screws or bone cement. Press fitting the shell (with its liner) into the pelvic bone is now also a commonly used procedure. Bone ingrowth into the roughened surface of the shell, as described for the stem, serves to stabilize the acetabular cup in place.
Modern Hip Implants
Based on the foregoing discussion, we can summarize the materials currently used in implants for total joint surgery (Figure 1.8). Ti6Al4V is used almost exclusively for the femoral stem and the shell of the acetabular cup, while Co–Cr and UHMWPE are the most commonly used combination for the femoral head (ball) and liner of the acetabular cup, respectively. Stainless steel balls are used far less often than Co–Cr. The ball is impacted on to a tapered end of the stem to hold it firmly in place and to allow its removal, if necessary. AMC finds use for both the ball and the liner but far less frequently when compared to the Co–Cr on UHMWPE couple. While AMC bearing couples show the lowest wear rates and the longest average lifetime when compared to all articulating couples used in hip implants, they are expensive to manufacture, their brittleness can be a concern, and a small fraction can suffer from a squeaking noise in vivo. AMC balls articulating against UHMWPE liners are also used. They show lower wear than Co–Cr on UHMWPE bearing couples but higher wear than AMC on AMC bearing couples.
Figure 1.8 Various combinations of materials currently used in implants for total hip replacement.
1.6 Brief History of the Evolution of Biomaterials
Materials have been used to heal the human body since the early civilizations, although they were not described by the term biomaterials until more recently. Biomaterials have evolved enormously over time, particularly over the last several decades, and they are continuing to evolve with developments in materials science and engineering and a greater emphasis on incorporating biological principles and advances into their design (Section 1.3). While various systems have been used to describe this evolutional history of biomaterials, they all reflect the enormous increase in the sophistication, performance, and application of biomaterials over time. In order to highlight some of the major advances, we provide a brief description of the evolution of biomaterials over time, divided into a few historical periods (Ratner 2013). These periods cover the years prior to World War II, a few decades after World War II, and what can be described as a more contemporary period, covering approximately the last 30–40 years (Figure 1.9).