Technical note
A comparison of bone strain measurements at anatomically relevant sites using surface gauges versus strain gauged bone staples

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Abstract

Instrumented bone staples were first introduced as an alternative to surface-mounted strain gauges for use in human in vivo bone strain measurements because their fixation to bone is secure and requires not only minimally invasive surgery. Bench-top bone bending models have shown that the output from strain gauged bone staples compares favorably to that of traditional mounted gauges. However their within- and across-subject performance at sites typically instrumented in vivo has never been examined. This study used seven human cadaver lower extremities with an age range of 23–81 years old and a dynamic gait simulator to examine and compare axial strains in the mid tibial diaphysis and on the dorsal surface of the second metatarsal as measured simultaneously with strain gauged bone staples and with traditional surface-mounted gauges. Rosette configurations were used at the tibial site for deriving principal compression and tension, and shear strains. Axial outputs from the two gauge types demonstrated strong linear relationships for the tibia (r2=0.78–0.94) and the second metatarsal (r2=0.96–0.99), but coefficients (slopes) for the relationship were variable (range 7–20), across subjects and across sites. The apparent low reliability of strain gauged staples may be explained by the fact that both strain gauged staples and surface strain gauges are inexact to some degree, do not measure strains from exactly the same areas and strain gauged staples reflect surface strains as well as deformations within the cortex. There were no relationships for the principal tibia compression, tension or shear strain measurements derived from the two rosette gauge types, reflecting the very different anatomical areas measured by each of the constructs in this study. Strain gauged bone staples may be most useful in comparing relative axial intra-subject differences between activities, but inter-subject variability may require larger sample sizes to detect differences between populations.

Introduction

Historically, bone strains have been measured by strain gauges directly bonded to the bone (Perry and Lissner, 1962). When used in human in vivo studies the method is complicated by the relatively invasive surgical technique required for gauge application and by a high percentage of gauge debonding (Lanyon et al., 1975; Burr et al., 1996). To overcome these drawbacks Buttermann et al. (1994) developed the technique of using instrumented staples instead of strain gauges bonded directly to bone, for measuring in vivo bone strain. The staples used by Buttermann et al. were individually fabricated in the laboratory with strain gauges bonded to the staple bridge. This technique was further refined using commercially available 16×15 mm surgical bone staples (Ekenman et al (1998a), Ekenman et al. (1998b)).

In vitro bending models using bones extracted from both large animals (Buttermann et al., 1994; Ekenman et al., 1998a) and small animals (Arndt et al., 1999) have shown good correlations between instrumented staple output and surface-measured strains. Linear coefficients (regression slopes) from these studies have since been generally applied to in vivo studies to enable derivation of surface strain from staple gauge deformation. Human in vivo strain measurements have been reported using these staples aligned axially in the tibia (Ekenman et al., 1998b; Milgrom, 2001) and in the second metatarsal (Milgrom et al., 2002; Arndt et al., 2002). Additionally, the staples have been placed in a 30° rosette pattern in the human tibia to measure principal and shear strains (Milgrom et al., 2000).

Despite the considerable work done in this area, we know of no study that has systematically examined the performance variance of the instrumented staple technique at relevant sites in human bone. Such work is needed to assess the reliability of instrumented staples and a single general calibration coefficient to derive in vivo bone strain values. This study used human cadaver lower extremities and a dynamic gait simulator (Sharkey et al., 1995; Sharkey and Hamel, 1998) to examine the validity of the general calibration approach by evaluating the inter-subject and inter-site variance of the staple-to-surface strain relationship.

Section snippets

Methods

The study used seven unpaired, human frozen lower extremities, each procured at the time of death by mid-shaft tibio-fibular osteotomy. Donors included four men and three women from 23 to 81 years of age (mean age 66 years); four of the lower limbs were left feet and three were right. Once obtained, all specimens were stored frozen at −5°C until the day of testing whereupon each specimen was thawed at room temperature and evaluated. Only anatomically normal feet with no evident pathology were

Results

Complete data sets were collected for five of the seven specimens. In two specimens (ages 74 and 81 years) there was failure of a gauge or of the wire exiting from the gauge. Fig. 2 shows the relationship between the output from the axial aligned element of the rosette surface strain gauge and the output from the axial aligned strain gauged staple multiplied by a factor of 10 for the tibia during a cycle of simulated stance phase of the human DGS for subject seven, a 23 year old male. These

Discussion

The dynamic gait simulator was used in this study to test the validity and reliability of strain gauged staples as a technique for measuring human tibial and second metatarsal strains. Previous in vitro validation studies (Ekenman et al., 1998a; Arndt et al., 1999), did not involve the use of human bone as does the dynamic gait simulator, nor do they allow for simultaneous measurements at two separate anatomical sites, with a known high incidence of stress fracture in both athletes and military

Acknowledgements

This work was supported by Hadassah University Hospital research fund 513.009.

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