Introduction Achilles tendon ruptures affect 15 (women) to 55 (men) per 100,000 people each year, and controversy continues to exist regarding optimal treatment and rehabilitation protocols. The objective of this study was to investigate the temporal effects of surgical repair and immobilization/activity (IM/ACT) on Achilles healing and limb function following complete transection in rodents. Methods Injured tendons (n=128) were repaired or left non-repaired, and animals groups immobilized in plantarflexion for 1, 3, or 6 weeks that later resumed cage and treadmill activity for 5, 3, or 0 weeks (IM1/ACT5, IM3/ACT3, IM6/ACT0). Animals were euthanized after 1- or 6-weeks post-injury. Results At 6-weeks post injury, IM1/ACT5 groups had increased range of motion and decreased ankle joint toe stiffness compared to IM3/ACT3 groups. IM6/ACT0 had decreased tendon cross sectional area, but increased tendon echogenicity and collagen alignment. Surgical treatment dramatically decreased fatigue cycles to failure in repaired tendons from earlier IM1/ACT5 groups. Normalized comparisons between 6- and 1-week post-injury data demonstrated that changes in healing tendon properties (area, alignment, and echogenicity) were maximized by IM1/ACT5 compared to IM6/ACT0. Discussion/Conclusion This study demonstrates how the temporal post-injury healing response of rodent Achilles tendons depends on both surgical treatment and the timing of IM/ACT.

Rotator cuff tears are common musculoskeletal injuries often requiring surgical intervention with high failure rates. Currently, pulsed electromagnetic fields (PEMFs) are used for treatment of long-bone fracture and lumbar and cervical spine fusion surgery. No studies have investigated PEMF in healing soft tissue. Therefore, we investigated the effect of PEMF on rotator cuff healing using a rat rotator cuff repair model. We hypothesized that PEMF exposure following rotator cuff repair would improve tendon mechanical properties, tissue morphology, and alter in vivo joint function. 70 adult male Sprague-Dawley rats were assigned to three groups: bilateral repair with PEMF (n=30), bilateral repair followed by cage activity (n=30), uninjured control with cage activity (n=10). Rats in the surgical groups were sacrificed at 4, 8, and 16 weeks. Control group was sacrificed at 8 weeks. Passive joint mechanics and gait analysis were assessed over time. Biomechanical analysis and µCT was performed on left shoulders; histological analysis on right shoulders. Results indicate no differences in passive joint mechanics and ambulation. At 4 weeks the PEMF group had decreased cross-sectional area and increased modulus and maximum stress. At 8 weeks the PEMF group had increased modulus and more rounded cells in the midsubstance. At 16 weeks the PEMF group had improved bone quality. Therefore, results indicate that PEMF improves early tendon healing and does not alter joint function in a rat rotator cuff repair model. Statement of Clinical Significance PEMF exposure following rotator cuff repair improves early tendon healing.

BACKGROUND Rotator cuff tears affect millions of individuals each year, often requiring surgical intervention. However, repair failure remains common. We have previously shown that pulsed electromagnetic field (PEMF) therapy improved tendon-to-bone healing in a rat rotator cuff model. The purpose of this study was to determine the influence of both PEMF frequency and exposure time on rotator cuff healing. METHODS 210 Sprague Dawley rats underwent acute bilateral supraspinatus injury and repair followed by either Physio-Stim® PEMF or High Frequency PEMF for 1, 3, or 6 hours daily. Control animals did not receive PEMF therapy. Mechanical and histological properties were assessed at 4, 8, and 16 weeks. RESULTS Improvements in different mechanical properties at various endpoints were identified for all treatment modalities when compared to non-treated animals, regardless of PEMF frequency or duration. Of note, one hour of Physio-Stim showed significant improvements in tendon mechanical properties across all time points, including increases in both modulus and stiffness as early as 4 weeks. Collagen organization improved for several of the treatment groups compared to controls. Additionally, improvements in collagen I and fibronectin expression were identified with PEMF treatment. Importantly, no adverse effects were identified in any mechanical or histological property. CONCLUSIONS Overall, results suggest that PEMF has a positive effect on rat rotator cuff healing for each electromagnetic fundamental pulse frequency and treatment duration tested in this study.

Conservative treatment (non-operative) of Achilles tendon ruptures is suggested to produce equivalent capacity for return to function; however, long term results and the role of return to activity (RTA) for this treatment paradigm remain unclear. Therefore, the objective of this study was to evaluate the long term response of conservatively treated Achilles tendons in rodents with varied RTA. Sprague Dawley rats (n=32) received unilateral blunt transection of the Achilles tendon followed by randomization into groups that returned to activity after 1-week (RTA1) or 3-weeks (RTA3) of limb casting in plantarflexion, before being sacrificed at 16-weeks post-injury. Uninjured age-matched control animals were used as a control group (N=10). Limb function, passive joint mechanics, tendon properties (mechanical, histological), and muscle properties (histological, immunohistochemical) were evaluated. Results showed that although hindlimb ground reaction forces and range of motion returned to baseline levels by 16-weeks regardless of RTA, ankle stiffness remained altered. RTA1 and RTA3 groups both exhibited no differences in fatigue properties; however, the secant modulus, hysteresis, and laxity were inferior compared to uninjured age-matched control tendons. Despite these changes, tendons 16-weeks post-injury achieved secant stiffness levels of uninjured tendons. RTA1 and RTA3 groups had no differences in histological properties, but had higher cell numbers compared to control tendons. No changes in gastrocnemius fiber size or type in the superficial or deep regions were detected, except for type 2× fiber fraction. Together, this work highlights RTA-dependent deficits in limb function and tissue-level properties in long-term Achilles tendon and muscle healing.

The Achilles tendon, while the strongest and largest tendon in the body, is frequently injured. Inconclusive evidence exists regarding treatment strategies for both complete tears and partial tears. Well-characterized animal models of tendon injury are important for understanding physiological processes of tendon repair and testing potential therapeutics. Utilizing three distinct models of rat Achilles tendon injury, the objective of this study was to define and compare the effects and relative impact on tendon properties and ankle function of both tear severity (complete tear versus partial tear, both with postoperative immobilization) and immobilization after partial tear (partial tear with versus without immobilization). We hypothesized that a complete tear would cause inferior post-injury properties compared to a partial tear, and that immediate loading after partial tear would improve post-injury properties compared to immobilization. All models were reproducible and had distinct effects on measured parameters. Injury severity drastically influenced tendon healing, with complete tear causing decreased ankle mobility and tendon mechanics compared to partial tears. One week of plantarflexion immobilization had a strong effect on animals receiving a partial tear. Tendons with partial tears and immobilization failed early during fatigue cycling three weeks post-injury. Partial tear without immobilization had no effect on ankle range of motion through dorsiflexion at any time point compared to the pre-surgery value, while partial tear with immobilization demonstrated diminished function at all post-injury time points. All three models of Achilles injury could be useful for tendon healing investigations, chosen based on the prospective applications of a potential therapeutic.