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

Abstract

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.

Introduction

Human second metatarsals are a common site for stress fractures in military recruits (Milgrom et al., 1985; Finestone et al., 1992). Local bone deformation is regarded as an indicator of stress fracture risk and this in vitro study presents the development of a methodology suitable for future in vivo application in ascertaining local bone deformation of human second metatarsals.

There are disadvantages involved in the in vivo application of strain gauges directly onto bone surfaces. Direct adhesion is quite invasive as the periosteum must be removed and the bone prepared to obtain an optimal bonding surface. The insertion of instrumented staples involves less traumatic surgery and is thus more appropriate for human in vivo application. Strain on the human tibia has previously been measured in vivo with rosette strain gauges (Lanyon et al., 1975; Burr et al., 1996), transcutaneous extensometers (Fyhrie et al., 1998) and inserted staples (Ekenman et al., 1998a). No such investigation has, however, been performed on metatarsals or bones of corresponding size and previous values for human metatarsal loading have been provided by theoretical biomechanical models (Stokes et al., 1979; Gross and Bunch, 1989) or investigations of human cadaver specimens (Lease and Evans, 1959; Sharkey et al., 1995; Courtney et al., 1997). This study tested the viability of the staple technique (Ekenman et al., 1998b) when applied to bones representing the human second metatarsal. Chicken tibiae were chosen as a metatarsal model because of ethical difficulties in obtaining human bone specimens. The aim was to assess the in vitro validity and reliability of this methodology and to determine the interactive effects between staple and bone during induced bending deformation.