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Description: The characterization of biological tissues depends on accurate measurements of deformation and strain, but less attention has been given to the role of out-of-plane deformation in ligament strain. The objective of this study was to investigate the influence of out-of-plane deformation on surface strain measurements in healthy and damaged ligaments. Tensile tests on five porcine posterior cruciate ligaments (PCL) were performed before and after damage using the femur – PCL – tibia construct. Damage was simulated by loading the ligament to its maximum force capacity. Digitized surface dots were tracked using an optical motion capture system. The transverse strain (ε_xx), longitudinal strain (ε_yy), and shear strain (γ_xy) distributions on the ligament surface were obtained for the control and damaged states using two-dimensional (2d) strain and three-dimensional (3d) strain measurements. There was no significant difference between the 2d and 3d strains in the control state for all three strains. However, the value and location of the peak strain values (tensile and compressive) in ligament surfaces did change. The 2d peak tensile strain was both over and under-estimated, compared to 3d strain, when out of plane deformation was included for ε_xx and ε_yy; but consistently overestimated for positive γ_xy. The percentage of damaged regions, quantified as a loss in tensile strength, after damage was overpredicted by 2d strain for ε_yy. Care should be taken when using 2d surface strain as peak values and local damage is sensitive to out-of-plane deformation.