Instrumented immobilizing boot paradigm quantifies reduced Achilles tendon loading during gait
Introduction
Over the past 3 decades, acute Achilles tendon ruptures have increased 10-fold (Lantto et al., 2015), leading to functional deficits performing heel raises – a clinical measure of generalized plantar flexor function – in two out of three patients one year after injury (Brorsson et al., 2017) and decreased plantar flexion at push-off during gait (Agres et al., 2015). Tendon elongation and shorter plantar flexor fascicles explain these poor functional outcomes in patients (Hullfish et al., 2019, Silbernagel et al., 2012) and are likely caused by suboptimal loading throughout healing (Hillin et al., 2019, Williams, 1990). To protect the healing tendon from excessive loading, patients wear immobilizing boots for 8–12 weeks following the initial treatment of the Achilles tendon rupture, whether treated surgically or nonsurgically (Willits et al., 2010). However, the mechanical loads applied to the Achilles tendon during ambulation in these immobilizing boots are unknown, limiting the potential efficacy of rehabilitation protocols.
To address this unmet clinical need, we developed an instrumented immobilizing boot paradigm to quantify Achilles tendon loading and boot load sharing during ambulation. Using an immobilizing boot currently prescribed in our clinical practice, we secured a 3-part load sensitive insole inside the boot to quantify loads transferred through the foot by the Achilles tendon and instrumented the posterior strut with a load cell to quantify boot loading. In this technical note, we describe the technical aspects we considered as we developed this instrumented immobilizing boot paradigm and present experiments establishing its validity.
Section snippets
Instrumented immobilizing boot
We developed an instrumentation paradigm to quantify Achilles tendon loading during ambulation in an immobilizing boot used to treat Achilles tendon ruptures (Fig. 1). To do this, we decided to leverage a commercially available immobilizing boot (Vacoped, Oped, Munich, Germany) that is currently prescribed in our clinics. Our framework relies on two assumptions: 1) that internal plantar loading measured by the instrumented insole is generated by the Achilles tendon acting to plantar flex the
Results
During walking at a self-selected speed with the immobilizing boot constrained to 0 degrees plantar flexion, subjects consistently loaded their Achilles tendons (Fig. 2). By iteratively adjusting the geometric parameters of the instrumented insole, we decreased our objective function by 78% (uncalibrated RMSE 13.6%, calibrated RMSE 3.0%, Fig. 2 bottom). This calibration decreased expected instrumented insole errors of peak tendon loading approximately 16%. After applying these calibration
Discussion
In this study, we developed an instrumented immobilizing boot paradigm that addresses an unmet clinical need. By using a commercially available instrumented insole and load cell, we can now quantify loads carried by both the biologic tissue and immobilizing boot during ambulation. Further, this paradigm logs data continuously, which permits patient loading data to be captured away from the clinic or research lab. Monitoring patient loading provides clinicians and researchers critical new data
Authors’ contributions
TH and JB designed the experiment, TH performed the experiment and fabricated the instrumented boot, TH and JB analyzed the data, TH and JB drafted the manuscript, KO provided clinical input into the immobilizing boot paradigm, all authors revised the intellectual content of the manuscript; all authors approved the final version of the manuscript
Declaration of Competing Interest
The authors declared that there is no conflict of interest.
Acknowledgements
We thank Oped Medical, Inc for providing the immobilizing boot used in this study.
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