The intention of the article 1 was to assess the likely impact of the environment on medical-grade silicone, and what effect this may have on the material properties. Two medical-grade silicones were chosen for their similarity to those materials used previously as small joint replacements. The difference in mechanical strength of the two silicones before testing was not important to the authors; what was important was how environmental effects may change these values.
Hutchinson et al. 2 used Q7-4565 (67A), the closest available material to HP100 at that time. I requested samples of Q7-4765 (65A) and Q7-4780 (77A) from Dow Corning, which have similar mechanical properties to Q7-4565, shown by the Shore A hardness measurements in brackets. From this request Dow Corning advised by email, in January 2005, for us to use materials C6-165 and C6-180 as ''the C6 materials are equivalent materials, with only different testing qualification.'' In addition, we were asked to sign an agreement in accordance with this. Indeed, more than one of these agreements was signed, stating in each that the materials supplied would be used for the following applications: ''I am a research student at Birmingham University studying crack growth rate in elastomers used for biomedical applications. I would like to utilise the elastomers in both mechanical and physical testing including tensile, compressive and hardness tests. I would also like to use the samples for examination of certain imaging techniques, such as MRI scanning and SEM examination.'' Dow Corning also requested a short report on the findings of the tests which was provided by email on January 23, 2006. Dow Corning responded by email and subsequently supplied further materials on occasions both before (January 2005) and after (August 2006) the report was submitted. In response to the point raised on curing temperatures, samples were indeed cured at 116 and not 1108C. This was a typing error from the authors. It is still however possible that cross linking was occurring, though possibly as a competing process.
The authors did not intend to imply that data from these materials be used to assess HP100 finger joints in particular. Instead we used medical-grade silicones with a similar-ity to those used in finger joint implants, not necessarily Dow Corning's, as an example of silicones used in small joint replacements and to assess their response to environmental ageing.
We take issue with the criticism of using a reference by Naidu 3 which you state ''has many shortcomings including the use of unsuitable equipment, and so forth.'' Firstly, the criticism is in a non-peer reviewed letter from two of the authors (Curtis et al.). 4 Secondly, this letter was published (December 2007) after our article had been submitted for publication (June 2007). In addition, it should be noted that in his reply to the letter, Dr Naidu states that Curtis et al. 4 ''have seriously misunderstood the article.'' 5 The debate and future research raised from academic publications is always anticipated and welcomed as it can only add to the greater knowledge of the scientific community.