A Repair-Tension Sweet Zone Maximizes Tendon-to-Bone Healing in a Rat Rotator Cuff Model

Purpose: To define a sweet zone of repair tension that maximizes tendon-to-bone healing while maintaining early functional safety in a rat model of rotator cuff repair. Methods: Forty-eight rats were used to create a graded-tension supraspinatus repair model with four discrete repair tensions: minimal-tension (MinT), low-tension (LowT), medium-tension (MedT), and high-tension (HiT) at two postoperative time points (n = 6 per group per time point). Intraoperative peak tension was measured during supraspinatus tendon reattachment using a digital tension gauge and normalized to tendon cross-sectional area (N/mm²). Functional recovery was assessed by gait analysis at 2 and 4 weeks; magnetic resonance imaging evaluated edema and tendon continuity; biomechanical testing quantified failure load, stiffness, and stress; histology assessed enthesis organization, fibrocartilage formation, and collagen fiber alignment. Results: Measured repair stresses were 0.015, 0.060, 0.110, and 0.305 N/mm² for MinT, LowT, MedT, and HiT groups, respectively. At 4 weeks, MedT showed superior biomechanical healing, achieving the highest LowT ultimate failure load (23.24 ± 2.08 N) versus MinT (14.78 ± 1.40 N, P =.001) and HiT (14.90 ± 0.94 N, P <.001) and the greatest stiffness (11.63 ± 0.46 N/mm) versus LowT (8.44 ± 0.13 N/mm, P <.001), MinT (6.35 ± 1.36 N/mm, P =.003), and HiT (3.05 ± 0.04 N/mm, P <.001). Histologic healing was also best in MedT and worst in HiT (MedT 14.00 ± 1.10 vs. MinT 10.17 ± 0.75, LowT 11.17 ± 0.98, and HiT 8.83 ± 0.75; overall P <.001). Overall, outcomes followed a bell-shaped relationship with repair stress, defining a sweet zone centered at 0.10 to 0.12 N/mm². Conclusions: This study identifies a sweet zone of repair tension (0.10-0.12 N/mm²) that maintains early functional safety and achieves superior late structural and mechanical outcomes compared with under- or over-tensioned repairs. Clinical Relevance: Rotator cuff repairs should aim not only to avoid excessive tension but also to target the sweet zone to optimize enthesis regeneration.