Surface Strain Gage Testing of Whole Bones and Implants

Strain is the relative change per unit length of an object's physical dimensions caused by an applied mechanical stress. High strains in whole bones, implants, and whole bone–implant constructs may cause mechanical failure, but low strains in whole bone may cause “stress shielding,” which leads to bone atrophy, bone resorption, and implant loosening. Consequently, a number of strain analysis techniques are commonly used in orthopaedic biomechanics applications, such as extensometers, fiber optic sensors, pressure sensitive film, thermography, and ultrasound. However, strain gages are the “gold standard” tool for assessing whole bones and prostheses, as well as for validating other experimental and computational methods. Strain gages are a sensitive, precise, inexpensive, widely used, and highly tested technology, although they can be physically fragile, inconsistent, and prone to drift. Strain gages are composed of a metal alloy wire or foil laid out in a grid-like pattern, whose electrical resistance changes due to mechanical strain. Therefore, this chapter explains how to perform strain gage testing for whole bones, fracture fixation devices, total joint replacements, and biomaterials, as well as how to analyze, present, and interpret results.