HYDROXYAPATITE-COATED STRAIN-GAUGES FOR LONG-TERM INVIVO BONE STRAIN-MEASUREMENTS

The aim of this study was to examine the bonding process between hydro xyapatite-coated strain gauges and bone in order to continue developme nt of a long term in vivo strain measurement device. Two types of comm ercially available hydroxyapatite (HA) particles were applied to the s ensing surface of uniaxial strain gauges using a polysulfone solution as an adhesive. Characterization by scanning electron microscopy and x -ray diffraction (XRD) was used to determine materials property differ ences between the two powders. Interfacial strengths between the HA co atings and the strain gauges were tested and found comparable to inter facial strength obtained by a plasma sprayed HA coating on the surface of a titanium implant. Gauges were surgically placed on the periostea l surface of greyhound femora. Three groups of dogs were implanted wit h gauges for periods of 3, 6, and 12 weeks using cyanoacrylate, resorb able sutures, and cable ties to initially hold the gauge against the s urface of the bone. Following euthanasia, the femora of the dogs were explanted and subjected to cantilever loading. Response of the implant ed RA-coated gauges were compared to a control set that had been fresh ly glued onto the contralateral femur. Full response, that is, 100% of the strain measurement with respect to the control, was obtained afte r 12 weeks in vivo. Attachment of HA-coated gauges with a circumferent ial suture showed bonding, while HA-coated gauges attached with cyanoa crylate did not bond to bone. After mechanical testing, femora were em bedded in polymethylmethacrylate, cut, ground, and polished. Sections were stained using mineralized bone stain (MIBS) and optical microscop y was performed using transmitted and fluorescent light to allow analy sis of remodeling occurring in the region of the strain gauges. Bone f ormation occurred at the HA surface of sutured gauges, and a fibrous t issue layer developed between the bone and HA coating when the tissue adhesive was used to initially bond the gauge. Fluorescence microscopy indicated an increase in the number of areas of bone remodeling adjac ent to the gauge but a normal rate of remodeling of 0.93 +/- 0.07 mum/ day was observed. No gross bone remodeling due to strain gauge placeme nt was observed. Backscattered electron imaging (BSE) indicated new bo ne apposition at all time periods.