STRAIN DISTRIBUTION IN THE LIGAMENT USING PHOTOELASTICITY - A DIRECT APPLICATION TO THE HUMAN ACL

Large and highly variable deformations of the anterior cruciate ligame nt (ACL) in the human knee cannot be adequately quantified by one-dime nsional and/or localized measurements. In order to measure strains in the entire area of the ACL, we employed the photoelastic coating metho d to analyze stress on the basis of the strains. A specific kind of po lyurethane possessing optically high fringe-sensitivity was found to b e most suitable for the measurement purposes. Although the photoelasti c method has been successfully applied in various fields for stress an alyses, its use in studying large deformations of biological tissues h as not been reported. Therefore, before proceeding with our main study , we first examined the effects of polyurethane film on the mechanical properties of the ligament. We found that the film had a negligible e ffect on the tissues' properties, and closely reflected the strain beh avior of the tissues. We then applied the method to measure strains on an actual ACL during free flexion-extension of the knee. A specially designed apparatus was used to allow a natural motion of the knee. A p ortion of the femoral bone was removed to expose the ACL to view. Meas urement and analysis gave continuous information about strain distribu tion, including the variations of strain along the principal strain di rections in the ACL. (C) 1998 IPEM. Published by Elsevier Science Ltd.