Temporal Healing of Achilles Tendons After Injury in Rodents Depends on Surgical Treatment and Activity

Introduction: Achilles tendon ruptures affect 15 of 100,000 women and 55 of 100,000 men each year. 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 or activity on Achilles tendon healing and limb function after complete transection in rodents. Methods: Injured tendons were repaired (n = 64) or left nonrepaired (n = 64). The animals in both cohorts were further randomized into groups immobilized in plantar flexion for 1, 3, or 6 weeks that later resumed cage and treadmill activity for 5, 3, or 0 weeks, respectively (n = 36 for each regimen), which were euthanized at 6 weeks after injury, or into groups immobilized for 1 week and then euthanized (n = 20). Results: At 6 weeks after injury, the groups that had 1 week of immobilization and 5 weeks of activity had increased range of motion and decreased ankle joint toe stiffness compared with the groups that had 3 weeks of immobilization and 3 weeks of activity. The groups with 6 weeks of immobilization and no activity period 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 groups with 1 week of immobilization and 5 weeks of activity. Normalized comparisons between 1-week and 6-week postinjury data demonstrated that changes in tendon healing properties (area, alignment, and echogenicity) were maximized by 1 week of immobilization and 5 weeks of activity, compared with 6 weeks of immobilization and no activity period. Discussion: This study builds on an earlier study of Achilles tendon fatigue mechanics and functional outcomes during early healing by examining the temporal effects of different immobilization and/or activity regimens after initial postinjury immobilization. Conclusion: This study demonstrates how the temporal postinjury healing response of rodent Achilles tendons depends on both surgical treatment and the timing of immobilization/activity timing. The different pattern of healing and qualities of repaired and nonrepaired tendons suggest that two very different healing processes may occur, depending on the chosen immobilization/activity regimen.