Studies were performed evaluating the role of Smad3, a transcription factor mediating canonical TGF-β signaling, on scarring and adhesion formation using an established flexor digitorum longus (FDL) tendon repair model. In unoperated animals the metatarsophalangeal (MTP) range of motion (ROM) was similar in Smad3−/− and wild type (WT) mice while the basal tensile strength of Smad3−/− tendons was significantly (39%) lower than in WT controls. At 14 and 21 days following repair Smad3−/− MTP ROM reached approximately 50% of the level of the basal level and was twice that observed in WT tendon repairs, consistent with reduced adhesion formation. Smad3−/− and WT maximal tensile repair strength on post-operative day 14 was similar. However, Smad3−/− tendon repairs maximal tensile strength on day 21 was 42% lower than observed in matched WT mice, mimicking the relative decrease in strength observed in Smad3−/− FDL tendons under basal conditions. Histology showed reduced "healing callus" in Smad3−/− tendons while quantitative PCR, in situ hybridization, and immunohistochemistry showed decreased col3a1 and col1a1 and increased MMP9 gene and protein expression in repaired Smad3−/− tendons. Thus, Smad3−/− mice have reduced collagen and increased MMP9 gene and protein expression and decreased scarring following tendon FDL tendon repair.

The pathogenesis of adhesions following primary tendon repair is poorly understood, but is thought to involve dysregulation of matrix metalloproteinases (Mmps). We have previously demonstrated that Mmp9 gene expression is increased during the inflammatory phase following murine flexor digitorum (FDL) tendon repair in association with increased adhesions. To further investigate the role of Mmp9, the cellular, molecular, and biomechanical features of healing were examined in WT and Mmp9−/− mice using the FDL tendon repair model. Adhesions persisted in WT, but were reduced in Mmp9−/− mice by 21 days without any decrease in strength. Deletion of Mmp9 resulted in accelerated expression of neo-tendon associated genes, Gdf5 and Smad8, and delayed expression of collagen I and collagen III. Furthermore, WT bone marrow cells (GFP+) migrated specifically to the tendon repair site. Transplanting myeloablated Mmp9−/− mice with WT marrow cells resulted in greater adhesions than observed in Mmp9−/− mice and similar to those seen in WT mice. These studies show that Mmp9 is primarily derived from bone marrow cells that migrate to the repair site, and mediates adhesion formation in injured tendons. Mmp9 is a potential target to limit adhesion formation in tendon healing.

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