In vitro measured strains in the loaded femur: Quantification of experimental error

The application of strain gauges to bone surfaces has been extensively employed as a method of determining strain fields in response to implanted devices in orthopaedics. The aim of this study was to determine some of the experimental errors associated with the use of strain gauges in in vitro experimental investigations of the loaded femur. An experimental protocol was devised to obtain strain data at 20 strain gauged locations on the proximal femur. These data were interpolated using a parametric model. The parametric model was then used to estimate the errors associated with mispositioning of the gauges and deviations in their direction of application to the bone. This sensitivity analysis was also supported by a finite element analysis for the purposes of comparison and cross-validation. The results indicated that the nature of the loading normally employed in the literature can contribute to making the readings for some of the gauges (anterior and posterior) unreliable and redundant, even for small positioning errors. The greatest predicted errors for the lateral and medial gauges were due to misalignment of the gauge as opposed to mispositioning. The size of the gauge had a negligible effect on the errors predicted relative to those caused by misalignment.