An in vitro comparison of bone deformation measured with surface and staple mounted strain gauges

Chicken tibiae were chosen as a model for human second metatarsals. Local surface bone deformation in a 4-point bending configuration was measured in vitro by both strain gauge instrumented staples and strain gauges bonded to the bone's cortical surface. A series of staple bridge dimensions (0.5, 0.6, 0.8 and 1.0 mm) was compared to test for staple influence on bone characteristics and greatest measurement validity and reliability. Thicker staple inhibition of bone deformation was the greatest but differences to thinner staples were not statistically significant (p>0.05). All staples except 0.5 mm had maximum deviations from linearity less than 1%. The 1.0 mm staple had an R² value of 0.992±0.006 plotted against the 4-point bending input force and 0.994±0.002 plotted against the surface strain gauge signal. The mean intraclass correlation coefficients (ICC) calculated with four input forces (30, 60, 90 and 120 N) and for loading and unloading conditions for the 0.5, 0.6, 0.8 and 1.0 mm staples were 0.75, 0.83, 0.87 and 0.92, respectively. Finally, the differences in slope of the staple strain gauge signal plotted against surface strain gauge signal between input force loading and unloading conditions (0.32), and between input compression and tension conditions (0.79) was least for the 1.0 mm staple which also resulted in the lowest standard deviations. These results suggested the appropriateness of the 1.0 mm staple for in vivo application.