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/mm 2 ). 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/mm 2 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/mm 2 . Conclusions This study identifies a sweet zone of repair tension (0.10‐0.12 N/mm 2 ) 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.