Effect of age on collagen fibril diameter in rabbit patellar tendon repair.

The effect of aging on soft tissue repair is poorly understood. We examined collagen fibril diameter in repairing patellar tendons from young adult and aging rabbits. We hypothesized that repairing tendons from older (geriatric) rabbits would have similar diameter fibrils compared with the younger (young adult) rabbits. Full-length, full-thickness, central-third (2.5 to 3 mm) patellar tendon injuries were made by cutting out the center of the tendon in twelve 1-y-old and thirteen 4- to 5.5 (average, 4.25)-y-old female New Zealand White rabbits. The contralateral tendon served as an unoperated control. The rabbits were euthanized at 6, 12, and 26 wk after surgery. The collagen fibril diameter was examined by electron microscopy at the patellar end, middle, and tibial end of the patellar tendon. There was no significant decline in collagen fibril diameter at any location in the aging rabbit healing patellar tendons compared with those of the 1-y-old rabbits. This study found that collagen fibril diameter was not altered with increasing age in the healing rabbit patellar tendon.

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 x 10(6) and 1 x 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.