Effect of length of the engineered tendon construct on its structure-function relationships in culture

Constructs containing autogenous mesenchymal stem cells (MSCs) seeded in collagen gels have been used by our group to repair rabbit central patellar tendon defect injuries. Although these cell-gel composites exhibit improved repair biomechanics compared to natural healing, they can be difficult to handle at surgery and lack the necessary stiffness to resist peak in vivo forces early thereafter. MSCs are typically suspended in collagen gels around two posts in the base of a well in a specially designed silicone dish. The distance between posts is approximately the length of the tendon wound site. MSCs contract the gel around the posts prior to removal of the construct for implantation at surgery. We hypothesized that in vitro construct alignment and stiffness might be enhanced in the midregion of the longer construct where the end effects of the posts on the bulk material (St. Venant effects) could be minimized. Rabbit MSCs were seeded in purified bovine collagen gel at 0.04 M cells/mg collagen. The cell-gel mixture was pipetted into silicone dishes having two post-to-post lengths (short: 11 mm and long: 51 mm) but equivalent well widths and depths and post diameters. After 14 days of incubation, tensile stiffness and modulus of the constructs were measured using equivalent grip-to-grip lengths. Collagen fiber orientation index or OI (which measures angular dispersion of fibers) was quantified using small angle light scattering (SALS). Long constructs showed significantly lower angular dispersion vs. short constructs (OI of 41.24 degrees +/-1.57 degrees vs. 48.43 degrees +/-1.27 degrees , mean+/-SEM, p<0.001) with significantly higher linear modulus (0.064+/-0.009 MPa vs. 0.024+/-0.004 MPa, p=0.0022) and linear stiffness (0.031+/-0.005 MPa vs. 0.018+/-0.004 N/mm, mean+/-SEM, respectively, p=0.0404). We now plan to use principles of functional tissue engineering to determine if repairs containing central regions of longer MSC-collagen constructs improve defect repair biomechanics after implantation at surgery.