Perspectives on cell and collagen composites for tendon repair.

Cell and matrix composites recently have been used to repair tendons and ligaments. In the current study the authors briefly review prior studies in this area, and the contributions these findings have made toward solving this clinical problem. The authors then provide a perspective on the necessary characteristics that the cells and delivery vehicles of these composites must possess, separately and in combination, to function successfully after surgery. The authors conclude by applying these functional tissue engineering principles to two tendon injury models in which mesenchymal stem cells have been suspended in Type I collagen gel to form composites for patellar and Achilles tendon repair. In the first study, mesenchymal stem cells were suspended in gel (5 million cells/mL) with no attempt to align the cells during incubation. The resulting composites were implanted in window defects in one patellar tendon in the rabbit knee, with gel alone in a matching defect on the contralateral side. Biomechanical evaluation at 4 weeks showed that the material properties of the mesenchy-mal stem cell based repairs were 18% to 33% greater than results for contralateral controls. In the second study, mesenchymal stem cells were suspended in gel (4 million cells/mL), contracted on a tensioned suture during incubation, placed in an Achilles gap defect, and compared with repairs of contralateral gap injuries containing suture alone (controls). By 4 weeks, the repairs treated with mesenchymal stem cells had achieved twice the structural properties of the contralateral controls and 50% to 60% of the stiffness and strength of normal tendons that were not surgically treated. In addition, the material properties of the repairs treated with mesenchymal stem cells had increased with time to 37% of normal by 12 weeks after surgery. Both studies reveal the benefits of using pluripotential cells in a collagen gel matrix, and suggest additional research that might enhance the repair quality of healing tendons.