Quantitative evaluation of both histological and mechanical recovery in injured tendons using Fourier-transform second-harmonic-generation microscopy

Quantitative assessment of healing after tendon injury is important for orthopedic practice. Although histological and mechanical recovery are assessed by histological staining and tensile testing respectively, both tests are destructive and invasive, making it difficult to be applied to single sample. Here, as a non-destructive and less invasive approach for quantitative assessment of tendon healing, we applied Fourier-transform second-harmonic-generation (FT-SHG) microscopy to an animal model of tendon healing. FT-SHG microscopy enables orientation analysis of collagen fibers independently of the SHG image brightness. The extracted FT-SHG parameter showed a significant difference between the healing sample and the control sample, indicating the potential for quantitative parameter of histological recovery. More importantly, the FT-SHG parameter indicated good correlation (coefficient of determination, R2 = 0.62) with the Young's modulus obtained by tensile testing of the same sample. These results indicate the potential of the proposed method for quantitative evaluation of both histological and mechanical recovery after tendon injury.