T-9 Evaluating the Mechanical Properties of a Tendon Graft, Using Digital Image Correlation (DIC) Technique

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Accurate measurement of the biomechanical properties of tendon grafts such as stiffness, Young's modulus and ultimate strength has important implications in the surgical repair of anterior cruciate ligament injuries. However, the mechanical behaviour of the tendon is complex and difficult to model. The data from the grip-to-grip displacement reported by the standard testing machines may not be an accurate way to determine the mechanical properties of tendon. The goal of this study was to verify the biomechanical property of an anterior cruciate ligament model tendon graft using the experimental data obtained from the standard tensile loading testing machines with a non-invasive digital imaging correlation (DIC) method. In this study the in vitro mechanical properties of the 10 bovine digital flexor tendon constructs was determined. Each construct consisted of a synthetic foam block where the tendon was doubled over an Endo-button loop and fixed with a round head Arthrex ® interference screw. The diameter of the tunnel and interference screw was the same as the diameter of the doubled tendon. The samples were secured and tested in a custom-made apparatus that was mounted in a LOS (Losenhausen, Maschinenbau AG Dusseldorf) uniaxial testing machine. Samples were pre-conditioned and cyclically loaded. A final tensile load to failure was applied to the failure. Experimental measurements were carried out using a DIC system, including two cameras and Vic3D (Limess GmbH, Pforzheim, Germany) software. The technique utilizes two similarly speckled images, which were captured by a solid state video camera, to represent the states of the object before and after deformation. By analyzing any pair of consecutive speckle images with the DIC technique, displacements between the corresponding load levels, incremental displacements and strain of the speckled pattern were obtained. At the beginning of the test, the contour was uniform with same displacement in axial direction, for both limbs of the tendon which verifies that the sample stretched fairly uniformly. By increasing the load the result of displacement in different limbs was slightly different. This meant that there was a relative motion between the limbs and can be concluded that the fixation force is not equal for both strand in the tunnel. At the maximum load, the maximum vertical strain was 0.42 and the shear strain was 0.26. Our results have shown that DIC provides a novel and reliable technique for assessing the biomechanical properties of a tendon graft. This in turn may help clinicians to accurately determine the mechanical properties of tendon grafts used in surgical repair such as a single bundle and a double bundle anterior cruciate ligament repair.