Читать книгу Materials for Biomedical Engineering - Mohamed N. Rahaman - Страница 34

Major requirements for the femoral head (ball) are that it should articulate in the liner of the acetabular cup with as little friction as possible and should have the requisite strength and stiffness to withstand physiological stresses, as noted for the femoral stem. From a mechanics point of view, the ball is retained within the hemispherical acetabular cup and, thus, is subjected to stresses that are predominantly compressive in nature. When compared to the stem, bending stresses are not as serious a consideration but the biocompatibility requirement is still valid. An added requirement is that the ball should be capable of being polished as smoothly as possible to reduce friction and wear when articulating against the liner of the acetabular cup.

Based on the aforementioned mechanical property requirements, a polymer is unsuitable for use as the ball. From a fabrication standpoint, producing the stem and ball together as a single component and from the same material is an attractive option. A serious problem is that Ti6Al4V is difficult to polish to a very smooth finish, is rather sticky and has poor sliding properties when articulating against itself or other materials. Consequently, Ti6Al4V is not suitable as a femoral head material.

Candidate materials for the femoral head are stainless steel and Co–Cr alloys, which were considered in the materials selection for the femoral stem. Ceramics are now also candidate materials due to the absence of significant bending stresses on the femoral head. Stainless steel generally has properties that are inferior to Co–Cr alloys but it is cheaper to manufacture. It is used when cost is an important factor and when the implant is not required to have an average lifetime of longer than ~10 years. The use of Co–Cr results in a longer average lifetime (~10–15 years) of the implant due to its better mechanical properties. Consequently, Co–Cr is the most commonly used femoral head material.

Al₂O₃, a well‐known ceramic material, has superior strength and stiffness to stainless steel and Co–Cr alloys, higher hardness and the ability to be polished to a very smooth finish. Although it shows a brittle mechanical response, Al₂O₃ created in the last few decades has shown excellent mechanical reliability due to the use of better quality starting materials and improved fabrication techniques. Al₂O₃ has been replaced by a composite composed of Al₂O₃ reinforced with zirconia (ZrO₂) particles, referred to as ZTA, which has a higher resistance to brittle fracture and a higher wear resistance. More recently, ZTA is being replaced by a composite referred to as AMC, which 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.