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.

The Achilles tendon is the most commonly ruptured tendon in the human body. Numerous studies have reported incidence of these injuries to be upwards of five times as common in men than women. Therefore, the objective of this study was to investigate the sex- and hormone-specific differences between Achilles tendon and muscle between female, ovariectomized female (ovarian hormone deficient), and male rats. Uninjured tissues were collected from all groups for mechanical, structural, and histological analysis. Our results showed that while cross-sectional area and failure load were increased in male tendons, female tendons exhibited superior tendon material properties and decreased muscle fiber size. Specifically, linear and dynamic moduli were increased while viscoelastic properties (e.g., hysteresis, percent relaxation) were decreased in female tendons, suggesting greater resistance to deformation under load and more efficient energy transfer, respectively. No differences were identified in tendon organization, cell shape, cellularity, or proteoglycan content. Additionally, no differences in muscle fiber type distribution were observed between groups. In conclusion, inferior tendon mechanical properties and increased muscle fiber size may explain the increased susceptibility for Achilles tendon injury observed clinically in men compared to women.

Achilles tendon ruptures are common injuries. Sex differences are present in mechanical properties of uninjured Achilles tendon, but it remains unknown if these differences extend to tendon healing. We hypothesized that ovariectomized females (OVX) and males would exhibit inferior postinjury tendon properties compared with females. Male, female, and OVX Sprague-Dawley rats (n = 32/group) underwent acclimation and treadmill training before blunt transection of the Achilles tendon midsubstance. Injured hindlimbs were immobilized for 1 wk, followed by gradual return to activity and assessment of active and passive hindlimb function. Animals were euthanized at 3 or 6 wk postinjury to assess tendon structure, mechanics, and composition. Passive ankle stiffness and range of motion were superior in females at 3 wk; however, by 6 wk, passive and active function were similar in males and females but remained inferior in OVX. At 6 wk, female tendons had greater normalized secant modulus, viscoelastic behavior, and laxity compared with males. Normalized secant modulus, cross-sectional area and tendon glycosaminoglycan composition were inferior in OVX compared with females at 6 wk. Total fatigue cycles until tendon failure were similar among groups. Postinjury muscle fiber size was better preserved in females compared with males, and females had greater collagen III at the tendon injury site compared with males at 6 wk. Despite male and female Achilles tendons withstanding similar durations of fatigue loading, early passive hindlimb function and tendon mechanical properties, including secant modulus, suggest superior healing in females. Ovarian hormone loss was associated with inferior Achilles tendon healing.

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.

Background: Osteochondral allograft transplantation is an effective technique for repairing large lesions of the medial femoral condyle (MFC), but its use is limited by graft availability. Purpose/Hypothesis: The present study aimed to determine whether contralateral lateral femoral condyle (LFC) allografts can provide an acceptable surface match for posterolateral MFC lesions characteristic of classic osteochondritis dissecans (OCD). The hypothesis was that LFC and MFC allografts will provide similar surface contour matches in all 4 quadrants of the graft for posterolateral MFC lesions characteristic of OCD. Study Design: Controlled laboratory study. Methods: Ten fresh-frozen recipient human MFCs were each size-matched to 1 ipsilateral medial and 1 contralateral LFC donor (N = 30 condyles). After a nano–computed tomography (nano-CT) scan of the native recipient condyle, a 20-mm circular osteochondral “defect” was created 1 cm posterior and 1 cm medial to the roof of the intercondylar notch (n = 10). A size-matched, random-order donor MFC or LFC plug was then harvested, transplanted, and scanned with nano-CT. Nano-CT scans were then reconstructed, registered to the initial scan of the recipient MFC, and processed in MATLAB to determine the height deviation ( d RMS) between the native and donor surfaces and percentage area unacceptably (>1 mm) proud (% A proud) and sunken (% A sunk). Circumferential step-off height ( h RMS) and percentage circumference unacceptably (>1 mm) proud (% C proud) and sunken (% C sunk) were measured using DragonFly software. The process was then repeated for the other allograft plug. Results: Both MFC and LFC plugs showed acceptable step-off heights in all 4 quadrants (range, 0.53-0.94 mm). Neither allograft type nor location within the defect had a significant effect on step-off height ( h RMS), surface deviation ( d RMS), % A proud, or % A sunk. In general, plugs were more unacceptably sunken than proud (MFC, 13.4% vs 2.4%; LFC, 13.2% vs 8.1%), although no significant differences in % C sunk were seen between allograft types or locations within the defect. In LFC plugs, % C proud in the lateral quadrant (28.0% ± 26.1%) was significantly greater compared with all other quadrants ( P = .0002). Conclusion: The present study demonstrates that 20-mm contralateral LFC allografts provide an acceptable surface match for posterolateral MFC lesions characteristic of OCD. Clinical Relevance: With comparable surface matching, MFC and LFC allografts can be expected to present similar stresses on the knee joint and achieve predictably positive clinical outcomes, thus improving donor availability and reducing surgical wait times for matches.