Introduction
The use of bone cement is become important in many orthopaedic procedures, especially in joint arthroplasty. Polymethyl methacrylate was first used as a cement in dentistry. John Charnley popularised its use in orthopaedics whilst developing his low friction joint arthroplasty. Despite attempts to find alternative means of implant fixation, bone cement still has a common place in orthopaedic surgery. In this article, we review the history of the development of bone cement, discuss the properties of polymethyl methacrylate particularly in relation to the fixation of total hip arthroplasty prostheses and finally we review some potential future developments.
The History of Bone Cement
The history of modern cements began in the early 20th century with Otto Rohm synthesising polymethylmethacrylate [or bone cement] and was first used in dental institutions in the 1930's. In 1936, the Kulzer company discovered that by mixing liquid methylmethacrylate monomer with a polymethylmethacrylate powder and an initiating agent, a mouldable 'dough' was produced. During the Second World War, it was shown that this 'dough' could be polymerized at room temperature and it became useful in cranioplasties [to fill skull defects] and securing the Judet femoral head prosthesis 10.
The late John Charnley popularised the use of bone cement in orthopaedic surgery and by the late 1950's he was searching for ways to fix the femoral component as part of the development of his low friction arthroplasty. He sought a material that was resistant to body fluids, viscous with a low toxicity, could be easily manipulated and would set within a reasonable time. Charnley worked at Manchester University and after experimenting with various materials eventually settled on polymethylmethacrylate: a viscous dough which he formed by mixing the powder with the liquid monomer 20. In 1958, he performed his first case in Manchester. Charnley was the first to realise that it could be easily used to fill the medullary canal and blend with the bone morphology 3. The cement acted to increase the biomechanical stability and decrease the stress on the implant and he settled on the idea of using cement as a 'grout' for the hip implants. Despite all the early scepticism, polymethylmethacrylate is being used as bone cement for implant fixation to this day.
Properties of Bone Cement
Bone cement is supplied as two separate components, a polymer powder and a monomer, which is a colourless and inflammable liquid. The liquid also contains a stabiliser (hydroquinone) to prevent polymerisation, an activator (dimethyl-para-toluidine) and may contain a green dye (chlorophyll). The powder consists of a spherical polymer, an initiator (dibenzoyl peroxide), a radiopacifier (zirconium oxide or Barium sulphate) and often an antibiotic.
As the powdered polymer and liquid monomer are mixed, a viscous dough is formed. The initiator and activator react together to produce an initiation reaction creating free radicals that cause the polymerisation of the monomer molecules. When two polymer molecules meet, they combine to form an unreactive molecule 12. This process is exothermic, with a maximum in vivo temperature of 40
