Pre-use intraoperative testing of autologous tissue for valvular surgery: A proof of concept study

Background and aim of the study: Autologous tissue is being used more commonly for valvular repair and reconstruction, but methods for testing the mechanical properties of the tissue intraoperatively have not been available. Before committing to its use, the surgeon should know its thickness, strength, and if it is isotropic or anisotropic. We have developed an intraoperative tissue tester (ITT), based on optical and mechanical methods that allows a surgeon to assess these parameters rapidly. Methods: Tissue thickness was measured with a micrometer. Specimens of tissue were incrementally pressurized to 200 mmHg, resulting in a dome-shaped distortion. The dome is a section of a spheroid whose dimensions are determined by the diameter of the circle and the height of the dome. The LaPlace equation for a sphere was used to calculate tissue stress. Moire fringe patterns are visualized as concentric circles or ellipses produced by directing a beam of collimated light at 45 degrees through a grid with Ronchi rulings onto the surface of the dome. Circles indicate isotropy and ellipses indicate anisotropy. Uniaxial testing was performed on strips of tissue taken from orthogonal and intermediate axes to validate Moire fringe pattern analysis. Specimens of human pericardium (n = 8), lightly tanned by a 10-min immersion in buffered 0.625% glutaraldehyde, were examined. Results: Mean thickness of eight specimens was 0.61 mm (range: 0.41-0.99 mm). During inflation, the lightly tanned human pericardium showed non-linear deformation characteristic of pericardium. Mean peak stress for the dome was 136 kPa. There was good correlation between dome height in the inflation tester and the uniaxial Young's modulus. Moire fringe pattern analysis showed minimal anisotropy to 200 mmHg pressure, confirmed by uniaxial testing. Conclusions: The basic mechanical properties of autologous tissue to be used for valvular repair or reconstruction should be determined to avoid use of tissue that is outside an expected range of values for strength, anisotropy/isotropy and thickness. A self-contained ITT, based on optical and mechanical principles, has been developed to measure those parameters.