Usefulness of the photoacoustic measurement method for monitoring the regenerative process of full-thickness defects in articular cartilage using tissue-engineering technology

Abstract

We demonstrated the capability of photoacoustic measurement for viscoelastic characterization. Since tissue viscoelasticity affects the propagation and attenuation of photoacoustic waves generated in the tissue, the relaxation times of the photoacoustic waves give the viscosity-elasticity ratio of the tissue. The relaxation times of photoacoustic waves of articular cartilage tissues engineered under various culture conditions were closely correlated with intrinsic viscosity-elasticity ratios measured by using a conventional viscoelastic analyzer (R > 0.98). In order to apply the photoacoustic measurement method to evaluation of the regeneration of articular cartilage as a method to validate the surgery, the method should enable not only evaluation of engineered tissue during cultivation in vitro but also evaluation after transplantation of engineered tissue in vivo. The aim of this study was to verify the usefulness of the photoacoustic method for repeated measurement of viscoelastic properties in order to evaluate the process of regeneration of a full-thickness defect in rabbit articular cartilage using allografted tissue-engineered cartilage. Photoacoustic waves were induced by 266- and 355-nm, 5-7 ns, light pulses delivered through an optical silica fiber from an Q-switched Nd:YAG laser and were detected by a piezoelectric transducer, which we had designed. About a 40% difference between the viscosity-elasticity ratio of allografted cartilage that of tissue surrounding the defect was shown just after surgery. The difference was significantly reduced at 4 and 12 postoperative weeks. Therefore, since the photoacoustic measurement method enables assessment of the progress of restoration of the viscoelasticity of articular cartilage, its main function, this method would be useful as an evaluation method in regenerative medicine.