Mechanical, histological, and functional properties remain inferior in conservatively treated Achilles tendons in rodents: Long term evaluation

Abstract

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.

Effects of immobilization angle on tendon healing after achilles rupture in a rat model

“…These splints were padded with Webril (Hanna Pharmaceutical; Wilmington, DE), and secured to the skin with silk tape stirrups (3M; St. Paul, MN), and Webril padded CoFlex (Andover Healthcare; Salisbury, MA) straps. This construct was then overwrapped with an additional layer of Webril and CoFlex from the knee to the toes before being coated with poly(methyl‐methacrylate) (PMMA) (Patterson Dental; St. Paul, MN) to resist chewing damage …”

“…Achilles tendons were harvested at the time of animal sacrifice (n = 6/group), and were processed by standard paraffin procedures and previously described assessments . Cell density and nuclear shape on H&E stained images were independently graded using a scale of one to three for cellularity (1, low; 2, moderate; 3, high) and one to four for nuclei shape (1, spindle shaped; 2, mixed/spindle; 3, mixed/rounded; 4, rounded).…”

“…Early rehabilitation strategies have been tested in our rat model of Achilles tendon rupture, which also support the use of nonsurgical treatment and early return to activity . The objective of this study was to use this well‐described rat model to investigate the effects of ankle flexion and immobilization duration, assessing ankle joint function and Achilles tendon mechanical, structural, and histological properties 6 weeks post‐injury.…”

Tendon Healing: Mechanical Loading, Microdamage and Gene Expression

“…Palmes et al showed in a mice model that the full loading group had reached to non-injured levels of mechanical properties after 16 weeks, whereas the immobilized group had not (43). This effect could not be seen in a rat model (21). However, an immobilized group was not evaluated in this study so the full loading tendons might still be better compared to immobilization.…”

“…The improvement by mechanical loading is usually evaluated in the early phases of Achilles tendon healing, 1-3 weeks after injury (3,9,16,18,22,27), but later evaluation has also been made (21,43). Most animal studies evaluating the effect of full loading during the early phase of Achilles tendon healing have shown beneficial effects (3,9,16,18,19,22).…”