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A Study of the Factors Determining Penetration of Polymethyl Methacrylate Bone Cement into Trabecular Bone

Final Activity Report Summary - BONECEM (A study of the factors determining penetration of polymethyl methacrylate bone cement into trabecular bone)

Total joint replacement is a very successful operation where bone cements are primarily used for fixation of artificial joints where it functions as a stress distributor and anchors the prosthesis to bone, relieving pain and disability for approximately 1.5 million people worldwide each year. With changing demographic age distributions in the population, it is predicted that the need for such operations will increase significantly over the coming decades. Most implants use polymethyl methacrylate (PMMA) bone cement, which grouts the prosthesis into bone. A benchmark figure for survival of these implants is now 95% after 10 years. Even so, the number of revision operations is increasing, and the need for improved survival of the implants is great. Problems associated with initial fixation of the implant in bone contribute to later requirements for replacement of the prosthesis. It is therefore important to understand how cement penetrates into cancellous bone (the porous bone beneath a joint), in order to improve the survival of artificial joints.

PMMA bone cement consists of two components: a powder and a liquid. These are mixed at the time of surgery, and the mixtures polymerises, changing from a viscous paste to a solid over a period of around 10 minutes. Two types of bone cement were studied in this project: a low viscosity and high viscosity cement. Detailed measurements of changes of viscosity with time were performed for these cements. Experiments were then performed to assess how these cements penetrated into cancellous bone and also open pore ceramic foam. In these experiments, a silicon oil viscosity standard was used as a control. Viscosity measurements showed that the initially 'low viscosity' cement polymerised more rapidly than the 'high viscosity' cement, and for periods between 3.5 and 8 minutes after the start of mixing, times when a surgeon would be injecting cement into bone, the nominally 'low viscosity' cement actually has a higher viscosity. These findings are confirmed by the penetration studies, in which it was found that it was difficult to get the 'low viscosity cement' to penetrate bone. However, there are indications that factors other than viscosity may affect the ability of cement to penetrate bone, in particular the particle size in the powder, which may be of a similar size to the interconnections between pores in the bone.

Further experiments were performed to investigate a particular clinical problem, that of cementing a prosthesis into a part of the pelvis (acetabulum) in total hip replacement. This is often the part of the prosthesis that fails. A model of the pelvis was constructed using porous ceramic foam. Experiments were performed in which the cement was pressurised in the acetabulum at various times after mixing, and the implant (acetabular cup) was inserted into the acetabulum at various times after cement pressurisation. It was found that early pressurisation of cement and late insertion of the cup resulted in the greatest penetration of cement into the model of the acetabulum. These finding can form the basis of clinical guidelines for surgeons.