Читать книгу Materials for Biomedical Engineering - Mohamed N. Rahaman - Страница 35

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.