Preliminary development of a hydroxyapatite-backed strain gauge.

Long-term in vivo strain sensing would provide information about deformation changes adjacent to implants during bone remodeling. Biodegradation of the cyanoacrylate adhesive commonly used to attach strain gauges to bone has generally limited in vivo strain sensing to time periods less than one month. Hydroxyapatite (HA) which has been used to attach implants to bone in vivo, was attached to strain gauges using a solvent-thinned polysulfone solution. Three HA-polysulfone surface morphologies were tested in a preliminary bench-top test. The single layer pressed surface morphology, which responded most accurately during bench-top testing, was modified slightly and applied to two gauges which were implanted on the femur of a greyhound. Strain measurements from the HA-backed gauges in place for four months in vivo were compared to strains measured from the contralateral femur. Comparison of the results indicated these gauges were well-bonded and that they were sensing strain accurately. After embedding in PMMA, the femur having the HA-backed gauge and the control femur were sectioned at the level of one of the HA-backed gauges. Microradiographs of these sections indicated no adverse tissue response to the HA-backed gauge on the endosteal or periosteal surface.