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Queen Mary University of LondonQueen Mary University of London
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School of Engineering and Materials Science
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PhD Thesis: Development of PEMA based bone cements and assessment of their short and longer term tensile properties

Author: ZNAOR, Neven

Year: 2001

Supervisor(s): Liz Tanner

Conventional poly(methyl methacrylate) (PMMA) bone cements possess a number of well-documented mechanical and biological disadvantages. An experimental material based upon poly(ethyl methacrylate) (PEMA) and n-butyl methacrylate (nBMA) has demonstrated potential as an orthopaedic bone cement due to a low polymerisation exotherm, high ductility and good biocompatibility. A clinical trial undertaken a the London Hospital Medical College revealed higher femoral stem subsidence for implants implanted with PEMA/nBMA, compared to those fixed with PMMA after only 2 years.

A range of PEMA formulations have been studied in order to reduce the creep whilst retaining the benefits of PEMA/nBMA. PEMA was polymerised with different acrylic monomers; nBMA, tetrahydrofurfuryl methacrylate (THFMA) and a mix containing nBMA/THFMA (50/50 wt.%). The cements were tested in quasi-static tension, isochronous creep and tensile creep at 8 MPa for one week. The highest strength was obtained for PEMA/nBMA/THFMA, followed by PEMA/THFMA and PEMA/nBMA. The highest modulus and lowest ductility were obtained for PEMA/THFMA, followed by PEMA/nBMA/THFMA and PEMA/nBMA.

The influence of cross-linking on PEMA/nBMA was investigated by the addition of ethyl glycol dimethacrylate (EGDMA) or triethyl glycol dimethacrylate (TEGDMA). These cements were tested in tension and isochronous creep. The strongest and most creep resistant cement was PEMA/EGDMA, followed by PEMA/TEGDMA and PEMA/nBMA.

Hydroxyapatite (HA) was added to reinforce the cement and potentially offer an increase in bioactivity. Thirty wt.% (approximately 9.5 vol.%)(w.r.t. polymer) of three different kinds of HA were incorporated into PEMA/nBMA. In addition, the HA particles were treated with a silane coupling agent to introduce a chemical bond between the HA and the PEMA/nBMA matrix. The HA was studied using particle size analysis, FTIR and SEM. The inclusion of HA generally increased the modulus, but lowered the strength and ductility. The strength was improved with the silane treatment.