Validity of photoelastic strain measurement on cadaveric proximal femora

Rosette strain gages indicate shear and principal strains at specific point s, whereas photoelastic coatings provide shear strain information over a br oad area. Information regarding bone loading and load transfer from a prost hetic implant to adjacent bone can be obtained using either strain-measurin g technique on loaded femora. This study compared proximal femoral strains derived from photoelastic coatings to those obtained from rosette strain ga ges applied directly to the bone in order to determine the relationships be tween photoelastic shear strains and rosette shear and principal strains. P hotoelastic shear strains underestimated rosette shear strains and exceeded the larger of the rosette principal strains. Principal strains derived fro m photoelastic coatings augmented with strain separator gages underestimate d their rosette counterparts in most instances. Correlation was strong and nearly linear for all measures, indicating that photoelastic coatings can a ccurately express proportional strain changes despite imperfect agreement i n absolute strain magnitudes. The best agreement between absolute strain ma gnitudes occurred in the proximal medial, or calcar, region. Understanding the relationships between the various measures obtained using the two strai n measurement methods will allow more accurate estimates of actual strains to be made from photoelastic coatings. [S0148-0731(00)01704-0].