DESCRIPTION AND APPLICATION OF INSTRUMENTED STAPLES FOR MEASURING IN-VIVO BONE STRAIN

In vivo bone strain measurements using strain gages cemented to bony s urfaces with cyanoacrylate polymers are limited in duration due to deb onding of the gages from bone. As an alternative to the bone bonded st rain gages, a technique was developed in which strain gages were first bonded to miniature staples and then the staples embedded into bone. The instrumented staples may be calibrated so that staple strain is di rectly proportional to bone strain. The method was first validated by comparing the staple output with cemented surface strain gages. Compar ison of instrumented staples to cemented strain gages revealed only a 3% deviation from linearity during longitudinal bending; the staples w ere insensitive to transverse loading. The instrumented staples were t hen applied to the in vitro canine lumbar spine to determine L2-3 face t loads. Load testing, repeatibility of facet calibration, and validit y testing of the in vitro instrumented staples were found to be compar able to that of the previous cemented strain gage techniques. In vivo facet joint application of the instrumented staples for periods of gre ater than 5 weeks gave load measurements comparable to our previous sh ort-term in vivo studies obtained with cemented strain gages. The adva ntages of the instrumented staples are a more secure bonding to the bo ne, and less traumatic surgery for fixation.