Achilles Tendon Loading During Walking Differs Between Commonly Used Immobilizing Boots
Authors: Baxter Josh R. , Hullfish Todd J. , Farber Daniel C. , Humbyrd Casey J. 
Published: November 2022

Category: Sports; Basic Sciences/Biologics Introduction/Purpose: Achilles tendon ruptures have increased 10-fold in the past three decades, leading to long-term functional deficits in nearly two-thirds of patients. To counter this, rehabilitation protocols have been developed to strike a balance between protecting the healing tendon from re-rupture while allowing patients to return to activities of daily living as early as possible. However, Achilles tendon loading is neither prescribed nor quantified during immobilization and there is no standardized method of immobilization. Therefore, the purpose of this study was to quantify the amount of tendon loading experienced while walking in 3 different kinds immobilizing boot with respect to prescribed ankle angle and walking speed. Methods: Two healthy-young adults (2M; Age: 29.6 +- 2.3 y/o; BMI: 30 +- 7) walked over flat ground while wearing 3 different immobilizing boots and athletic shoes after providing written informed consent in this IRB approved study. We calculated Achilles tendon loading while subjects walked in 3 boots that positioned the ankle in 30 degrees of plantar flexion (Figure A, Air Cam Walker, United Ortho; AirSelect Walker, Aircast, VACOped, OPED). In all three boots, an instrumented insole (Loadsol, Novel) was placed under the foot to quantify load experienced by the tendon. We calculated the Achilles tendon loading profiles during gait for each immobilizing boot and compared them against the normal shod condition. We also provided the surgical repair threshold of 0.5 bodyweights to visualize the potential efficacy of each immobilizing boot for early rehabilitation when the tendon is early in the healing phase. Results: As expected, tendon loading was highest in the shod condition with no ankle bracing and lowest in the fully plantarflexed adjustable boot (Figure B). On average, the immobilizing boots reduced tendon loading decreased by 68% compared to unsupported walking in shoes but there were differences among the boots. The 3 different immobilizing boots provided differing levels of support. The 2 walking boots that use heel wedges to support the ankle in plantar flexion provided the least support, reducing loading by an average of 60-68%. The more rigid boot that constrained ankle angle using a posterior strut reduced tendon loading by 77% compared to gait in a normal walking shoe. Based on biomechanical studies of Achilles tendon repair strength in cadaveric experiments, only the rigid boot with the posterior strut reduced tendon loading biomechanics to levels that could be resisted by the surgical repair (Figure B). Conclusion: In this study, we used an instrumented insole to quantify Achilles tendon loading in a variety of immobilizing boots used by rupture patients. Our initial findings show that tendon loading is varies greatly between boot types and patients, highlighting the importance of understanding how these loads change in patients. Our future work centers around identifying the tendon loading profiles that promote tendon healing and optimize patient outcomes. These loading profiles will then provide the quantitative data needed to personalize loading for patients and guide rehabilitation to improve outcomes.