Effects of Cell-to-Collagen Ratio in Mesenchymal Stem Cell-Seeded Implants on Tendon Repair Biomechanics and Histology

Autogenous tissue-engineered constructs were fabricated at four cell-to-collagen ratios (0.08, 0.04, 0.8, and 0.4 M/mg) by seeding mesenchymal stem cells (MSCs) from 16 adult rabbits at one of two seeding densities (0.1 × 10 6 and 1 × 10 6 cells/mL) in one of two collagen concentrations (1.3 and 2.6 mg/mL). The highest two ratios (0.4 and 0.8 M/mg) were damaged by excessive cell contraction and could not be used in subsequent in vivo studies. The remaining two sets of constructs were implanted into bilateral full-thickness, full-length defects created in the central third of the patellar tendon (PT). At 12 weeks after surgery, repair tissues were assigned for biomechanical ( n = 13) and histological ( n = 3) analyses. A second group of rabbits ( n = 6) received bilateral acellular implants with the same two collagen concentrations. At 12 weeks, repair tissues were also assigned for biomechanical ( n = 4) and histological ( n = 2) analyses. No significant differences were observed in any structural or material properties or in histological appearance among the two cell-seeded and two acellular repair groups. Average maximum force and maximum stress of the repairs were approximately 30% of corresponding values for the central one-third of normal PT and higher than peak in vivo forces measured in rabbit PT from one of our previous publications. However, average repair stiffness and modulus were only 30 and 20% of normal PT values, respectively. Current repairs achieved higher maximum forces than in previous studies and without ectopic bone, but will need to achieve sufficient stiffness as well to be effective in the in vivo range of loading.