INTRODUCTION: Rotator cuff (RC) tears frequently lead to shoulder pain and dysfunction. In patients who failed nonoperative treatment, surgical RC tendon-to-bone repair is often performed. However, failure of postsurgical repair is not rare. In previous animal studies, it has been demonstrated that FGF-2-soaked gelatin hydrogels (GHs) have the potential to enhance RC tendon-to-bone healing biomechanically and histologically in RC healing rat models, where the supraspinatus tendon insertion site is transected and repaired immediately. In clinical settings, delayed repair of RC tears has become a concern. Whether FGF-2 enhances RC tendon-to-bone healing after surgical repair of chronic tears still remains unclear. We hypothesized that FGF-2 may improve the biomechanical strength and histological status by stimulating the tenogenic healing response in repaired chronic RC tears.
METHODS: Adult male Sprague-Dawley rats (n = 108) underwent unilateral surgery to fix the supraspinatus tendon to its insertion site 3 weeks after detachment. Animals were assigned to either the FGF-2 group or control group. The effects of FGF-2 were assessed via biomechanical testing at 6 and 12 weeks postoperatively, and were examined histologically and immunohistochemically for proliferating cell nuclear antigen (PCNA) and mesenchymal stem cell markers at 2, 6, and 12 weeks postoperatively. The expression of enthesis-related markers, including SRY-box 9 (Sox9), scleraxis (Scx), and tenomodulin (Tnmd), was assessed by real-time reverse transcription polymerase chain reaction, in situ hybridization, and immunohistochemistry. All experimental procedures were approved by the Animal Studies Committee and the Institutional Animal Care and Use Committee of Kumamoto University.
RESULTS: Results of histological evaluation showed hypervascular reparative tissue at the insertion site in both groups at 2 and 6 weeks postoperatively. At 12 weeks, loose fibrovascular tissue was observed at the insertion site in the control group, whereas dense tendon-like tissue was observed in the FGF-2 group. The histological score at 12 weeks was significantly higher in the FGF-2 group than in the control group (FGF-2: 9.3; control: 7.2). With respect to biomechanical testing, significantly higher ultimate load to failure (FGF-2: 18.4, control: 11.8; and FGF-2: 24.0, control: 14.1 [N]), stiffness (FGF-2: 9.0, control: 6.2; and FGF-2: 12.8, control: 7.0 [N/mm]), and ultimate stress to failure (FGF-2: 3.5, control: 2.5; and FGF-2: 5.1, control: 3.0 [N/mm2]) were demonstrated at 6 and 12 weeks, respectively, in the FGF-2 group than in the control group. The cross-sectional area was significantly larger in the FGF-2 group than in the control group at 6 weeks (FGF-2: 5.44, control: 4.62 [mm2]), but no significant differences were observed between the two groups at 12 weeks (FGF-2: 4.76, control: 4.79 [mm2]). At 2 weeks, the percentages of PCNA-positive cells (FGF-2: 36.3, control: 19.8 [%]) and CD73-positive cells (FGF-2: 11.6, control: 7.5 [%]) within the reparative tissue were significantly higher in the FGF-2 group than in the control group. Scx and Tnmd mRNA expressions increased significantly in the FGF-2 group at 12 weeks postoperatively (Scx: FGF-2: 2.0, control: 1.0; Tnmd: FGF-2 group: 1.0, control group: 0.7). Scx and Tnmd mRNAs were mainly detected in the spindle-shaped cells of the tendon mid-substance in both groups, but showed greater expansion into reparative tissues in the FGF-2 group at 12 weeks. Scx mRNA was detected in the reparative tissue around bone surfaces, while Tnmd mRNA was detected in the reparative tissue aligned with collagen fibers.
DISCUSSION: Our findings suggested that FGF-2 enhances the RC tendon-to-bone healing process histologically and biomechanically for surgical repaired chronic RC tears by promoting the increase of undifferentiated mesenchymal cells during the early phase, subsequently facilitating growth of tenogenic cells during later phase of repair. However, the mechanism by which FGF-2 enhances healing response at the reparative tissue remains unclear. Further investigations of the healing responses in chronic RC tears may provide essential clues to help develop clinical strategies to improve healing after surgical repair of RC.

1960 - Fibroblast Growth Factor-2 Promotes Tendon-to-bone Healing By Enhancing Tenogenic Progenitor Expression In A Rat Chronic Rotator Cuff Repair Model