Hip replacement: ceramic head destroys titan acetabular cup

Hip replacement: ceramic head destroys titan acetabular cupThe prosthetic implant used in hip replacement consist of different parts, the acetabular cup, the femoral component and the articular interface.

One piece (monobloc) shells are either polyethylene or metal. A monobloc polyethylene cup is cemented in place while a metal cup is held in place by a metal coating on the outside of the cup. Modular cups consist of two pieces, a shell and liner. The shell is made of metal, the outside has a porous coating while the inside contains a locking mechanism designed to accept a liner. Additional fixation is achieved as bone grows onto or into the porous coating. Polyethylene liners are placed into the shell and connected by a rim locking mechanism, ceramic and metal liners are attached with a Morse taper.

The femoral component consists of the femoral stem with attached prosthetic femoral head (ball).Stems are made of multiple materials (titanium, cobalt chromium and stainless steel) and they can be monolithic or modular. There are two types of fixation: cemented and uncemented. Femoral heads are made of metal or ceramic material. Metal heads, made of cobalt chromium for hardness, are machined to size and then polished to reduce wear of the socket liner. Ceramic heads are more smooth than polished metal heads, have a lower coefficient of friction than a cobalt chrome head, and in theory will wear down the socket liner more slowly. As of early 2011, follow up studies in patients have not demonstrated significant reductions in wear rates between the various types of femoral heads on the market. Ceramic implants are more brittle and may break after being implanted.

The articular interface of the hip is a simple ball and socket joint. Different combinations of materials have different physical properties which can be coupled to reduce the amount of wear debris generated by friction. Typical pairings of materials include metal on polyethylene (MOP), metal on crosslinked polyethylene (MOXP), ceramic on ceramic (COC), ceramic on crosslinked polyethylene (COXP) and metal on metal (MOM). Each combination has different advantages and disadvantages.