We show that complete loss of Mmp9 during flexor tendon healing results in earlier remodeling of adhesions compared to WT mice, without loss of repaired tendon strength. Accelerated, and increased expression of Mmp2, Gdf5 and Smad8 suggest the remodeling phase of flexor tendon healing occurs earlier in Mmp9−/− mice, resulting in the subsequent decrease in the gliding coefficient. Additionally, the contribution of bone marrow-derived cells to the flexor tendon healing process was demonstrated by presence of GFP-expressing cells in WT mice following bone marrow transplant of GFP+ cells. The functional role of Mmp9 expression in bone marrow-derived cells was demonstrated by the increased adhesions that occurred in myeloablated Mmp9−/− mice transplanted with WT bone marrow cells. The gliding coefficient was similar to that observed in tendon repairs of WT mice. Notably, the strength and stiffness of repairs in Mmp9−/− mice were similar to those observed in WT mice. Thus, lack of Mmp9 reduces tendon adhesions but does not result in a weaker repair tissue.
Based on the causal association between inflammation and adhesion formation 
, previous work has altered the healing course with anti-inflammatory treatment 
. These studies have demonstrated that an attenuated inflammatory response can decrease adhesions, however, decreased adhesions are often accompanied by a decrease in the strength of the repair during the early phases of healing, rendering these tendons unsuitable for aggressive physical therapy, and prone to re-rupture 
. Virchenko et al
have shown that an early decrease in force at failure resulted from Cox-2 inhibition 
, suggesting that altering the normal cascade of inflammatory events leads to a decrease in the force at failure. Furthermore, a greater proportion of untreated specimens had a higher histology score, corresponding a greater amount of mature tissue, suggesting a larger zone of injury in mice treated with a Cox-2 inhibitor. In contrast, our work shows that deletion of Mmp9
results in reduced catabolism of native tendon and a development of a smaller zone of injury compared to WT repairs. However, there is no reduction in the maximum load at failure compared to tendon repairs in WT mice. This is an important distinction because a decrease in the maximum load at failure increases the likelihood of rupture. Our data indicate that targeted inhibition of early Mmp9
expression may improve flexor tendon healing.
The robust remodeling of adhesions and the decreased gliding coefficient by day 21 in Mmp9−/−
mice suggests an accelerated remodeling phase. Prior work in our laboratory has shown that tendon remodeling is associated with elevated expression of Mmp2
. Elevated expression of Mmp2
occurs earlier in Mmp9−/−
mice (14 days) compared to WT mice (21 days). Mmp2
expression is increased immediately before a significant decrease in the gliding coefficient, suggesting an important role for Mmp2
in scar remodeling. The reduced area of initial catabolism of tendon adjacent to the repair site in tendons from Mmp9−/−
mice suggests that while degradation of some native tissue is beneficial, too much catabolism results in a more abundant response with a prolonged period of increased adhesions. While lack of Mmp9
results in a change in the kinetics of tendon remodeling, the overall mechanical strength of the tendon repair, a key component of tissue regeneration, is not reduced in the Mmp9−/−
mice. The return of tendon strength requires a balance between degradation, collagen formation, and tissue remodeling. Alteration in the expression of Mmp9
changes the anabolic phase of healing, suggesting that Mmp9
expression is a key regulator of subsequent tissue responses.
We show that bone marrow cells migrate to the flexor tendon repair site consistent with a prior report showing migration to the patella tendon repair site 
. By day seven post-repair there is a large influx of cells that declines, but persists through day 28. The important role of inflammatory mediators or chemotactic signaling in this “homing” process is demonstrated by the complete absence of GFP+ cells in contralateral control tendons, indicating that BMSCs migrate specifically to the site of injury. Our findings differ somewhat from other models of bone marrow cell migration to the tendon repair site. In a murine partial patella tendon defect model there is a massive influx of GFP+ cells to the repair site within 24 hours 
, while we found very few bone marrow cells at three days post-repair. The rate of healing and the disruption in vascularization in a partial defect model may be different than that of a complete transection. The presence of intact tendon at the repair site may increase the early intrinsic phase of healing, or increase the release of chemotactic signals from the tendon tissue that recruits bone marrow cells to the repair site. Additionally, the synovial sheath surrounding the flexor tendon may be an additional barrier that bone marrow cells must cross before arriving at the intrasynovial repair site.
Expression of Mmp9 specifically in cells derived from the bone marrow was achieved by bone marrow transplantation, while the functional consequences of bone marrow-specific Mmp9 deletion were also shown. Loss of Mmp9 in the bone marrow during flexor tendon healing resulted in earlier remodeling of adhesions, compared to those mice that express normal levels of Mmp9 in the bone marrow, as well as in mice with bone marrow cell-specific Mmp9 expression. Resolution of fibrous adhesions after flexor tendon injury did not occur to the same degree in mice with bone marrow cell-specific loss of Mmp9 compared to healing in Mmp9−/− mice. Based on the low-level expression of Mmp9 transcripts in tendons from WT mice with Mmp9−/− bone marrow, it appears that marrow cells are a major source of Mmp9 during flexor tendon healing. It appears that some WT host cells persisted in the bone marrow and provided a slight contribution of Mmp9 during flexor tendon healing. This would explain why there was not complete remodeling of adhesions to the extent of complete loss of Mmp9, and also why there was low, but detectable expression of Mmp9 in tendons from WT mice with Mmp9−/− bone marrow by real-time PCR.
Bone marrow-specific loss of Mmp9
leads to earlier remodeling of adhesions, without a decrease in the strength of the repair. This study presents a novel mechanism of improved flexor tendon healing, by inhibition of Mmp9
, which in the setting of tendon repair seems to be primarily produced by bone marrow-derived cells. Small molecule Mmp9
inhibitors are currently in clinical trials for Multiple Sclerosis, Chronic Obstructive Pulmonary Disease and Prostate Cancer 
, and may aid the pursuit of Mmp9
inhibition during tendon healing. This work demonstrates that it is possible to regain gliding function of the flexor tendon sooner, without compromising the strength of the repair. One limitation of this study is that we have not directly assessed the in vivo gelatinolytic activity of Mmp9
during flexor tendon healing. Identification of this activity would identify a novel target to improve flexor tendon healing.