A comparison of in vitro and in vivo degradation of two CPC strain gauge coatings.

Calcium phosphate ceramic (CPC) coated strain gauges have been used to measure bone strain in animal models for up to 16 weeks and are being developed to collect measurements in patients for periods of 1 year or more. A published surface roughening and heat treating procedure produced improved dry strength and in vivo stability of CPC-gauge interfaces after 16 weeks. The long term bond strength of two CPC-gauge interfaces prepared using the roughening and heat treating process were evaluated after up to 1 year in vitro and in vivo using a lap shear test. The feasibility of using an in vitro test to predict long term in vivo interface changes was established. A blended tricalcium phosphate + hydroxyapatite had a CPC-gauge interface strength which decreased from 6.07 +/- 2.64 MPa at 16 weeks to 4.71 +/- 1.840 MPa after 1 year in Hanks Balanced Salts (HBS). The same coating had a strength that decreased from 8.51 +/- 2.63 MPa at 16 weeks to 5.35 +/- 1 MPa after 1 year in vivo. A soluble calcium enhanced hydroxyapatite had an interface strength of 4.83 +/- 1.106 MPa after 16 weeks and 4.51+/- 1.100 MPa after 1 year in HBS. The same coating had an interface strength of 8.34 +/- 2.40 MPa after 16 weeks and 5.20 +/- 2.00 MPa after 1 year in vivo. Although interface strengths decreased slightly with time in vivo, after 1 year they were in the same strength range as published CPC-bone interface strengths of 4.8 +/- 2.4 MPa. Comparison of in vitro with in vivo results indicated that in vitro results were a good predictor of strength change in the blended CPC coating, but a poorer predictor of strength changes in the soluble calcium-enhanced coating.