The use of porcine small intestinal submucosa to enhance the healing of the medial collateral ligament - A functional tissue engineering study in rabbits

Introduction: Small intestinal submucosa (SIS) from porcine has been successfully used as a collagen scaffold for the repair of various tissues, including those of the human vascular, urogenital, and musculoskeletal systems. The objective of this study was to evaluate whether SIS can be used to enhance the healing process of a medial collateral ligament (MCL) with a gap injury in a rabbit model. Methods: A 6 mm wide gap was surgically created in the right MCL of 20 skeletally mature, female New Zealand White rabbits. In 10 rabbits, a strip of SIS was sutured onto the two ends of the MCL, while for the other 10 animals their injured MCL remained untreated and served as a non-treated group. The left MCL of all animals was exposed and undermined serving as the sham-operated side. At 12 weeks post-healing, eight hind limbs from each group were used for mechanical testing. The cross-sectional areas (CSA) of the MCLs were measured. The femur-MCL-tibia complex (FMTC) was tensile tested to failure. The load-elongation curves representing the structural properties of the FMTC and the stress-strain curves representing the mechanical properties of the healing MCL were obtained. The remaining two animals from each group were prepared for histological evaluation. Results: The CSA between the SIS-treated and non-treated groups were not significantly different (p > 0.05). Both treatment groups appeared to increase by nearly 40% compared to the sham-operated side, although statistical significance was not found for the non-treated group (p > 0.05). The stiffness of the FMTC from the SIS-treated group was 56% higher than the non-treated group (45.7 ± 13.3 N/mm vs. 29.2 ± 9.2 N/mm, respectively, p < 0.05) and the ultimate load also nearly doubled (117.434.5 N vs. 66.4 ± 31.4 N, respectively, p < 0.05). These values were lower compared to the sham-operated side (89.7 ± 15.3 N/mm and 332.0 ± 50.8 N, respectively). The tangent modulus of the healing MCL (279.7 ± 132.1 MPa vs. 149.0 ± 76.5 MPa, respectively) and stress at failure (15.7 ± 4.1 MPa vs. 10.2 ± 3.9 MPa, respectively) both increased by more than 50% with SIS treatment (p < 0.05). Yet, each remained lower compared to the sham-operated side (936.3 ± 283.6 MPa and 75.6 ± 14.2 MPa, respectively). Blinded histological comparisons between the SIS-treated MCL and the non-treated control demonstrated qualitatively that the SIS treated group had increased cellularity, greater collagen density, and improved collagen fiber alignment. Conclusion: Healing of a gap MCL injury was significantly enhanced with SIS. The improved mechanical properties and histological appearance of the MCL suggest that SIS treatment improves the quality of tissue and renders the possibility for future studies investigating functional tissue engineering of healing ligaments.