Contains the full lesson along with a supporting toolkit, including teachers’ notes.
Joints Can be Replaced
The joints of the human skeleton are subject to stress forces during activities such as walking, running or lifting. For example, the force acting on the hip joint can be over ten times the body weight. In addition sports injuries and diseases such asosteoarthritis also result in serious damage to the joints. In modern times it is common practice to have damaged joints replaced with artificial parts in a surgical procedure called anarthroplasty. An artificial part is called a prosthesis and the most common of these are replacement hips and knees.
In the case of a total hip replacement both parts of the ball and socket joint are replaced. Hollowed out spaces are made in thefemur and pelvis to accept the new parts. However, this leaves some empty space between the artificial parts and the bone and this must be filled with a suitable material. This material is calledbone cement. In this lesson we look at the chemistry of bone cement production and discuss how it is used by the surgeon. Bone cement is manufactured in the Stryker plant in Co. Limerick.
What is Bone Cement?
The material may be thought of as being similar to cement or mortar. It must be able to absorb the forces acting on the joint during movement and keep the artificial implant in position. It must also be biocompatible, that is it must not cause physical or chemical damage to human tissue. A material that meets these requirements is PMMA (poly-methyl-meth-acrylate).
What is PMMA?
PMMA is a polymer, that is, its molecules are composed of many repeating units of a smaller molecule, or monomer, which in this case is methyl-meth-acrylate (MMA). The process by which monomers join to form a polymer is called polymerisation. This is best understood by considering polythene and its monomer, ethene.