VALIDATION OF A DYNAMIC ANKLE ORTHOSIS TO REDUCE TIBIAL BONE STRAIN COMPARED TO A STANDARD OF CARE WALKING BOOT

Introduction: Overuse injuries of the lower extremity are the most prevalent, with tibial bone stress injuries (TBSI) accounting for 29-74% in military cadets (1,2) and 21.9% in recreational runners (2). Clinicians prescribe a walking boot (WB) to be worn daily for 3 to 26 weeks (3-5). The WB is meant to immobilize the ankle joint and stabilizes the lower limb, reduce the loading environment, and promote bone healing. However, the rigid design of a WB results in reduced ankle mobility, decreasing muscle activity (6,7) and causes significant muscle atrophy to occur within 4 days of boot wear, delaying the recovery period. Further, the effects of a WB on mechanical loading of the tibia remains unknown. Our lab has developed and evaluated a custom Dynamic Ankle Orthosis (DAO) that applies a distractive force to offload the lower leg and ankle while retaining sagittal ankle motion during walking. Previous treadmill walking study of acute brace wear found the peak tibial compressive force (10.9%) and Achilles’ tendon force (12%) were moderately reduced in the DAO compared to a clinical WB. Sagittal plane ankle motion during stance phase significantly increased by 54.9% with the DAO compared to the WB (8). The objective of this study was to validate offloading the tibial by 12% Bodyweight provides a significant reduction in tibial bone strain between the DAO and walking boot in a human cadaver model. We hypothesize the DAO will reduce the vertical reaction force leading to reduced tibial bone strain compared to a clinical walking boot.

Materials and

Methods: Seven fresh frozen cadavers (6 males, 1 female; 58 ± 12yrs; right limb) were positioned in a custom robotic testing platform in two brace conditions: DAO and WB (Össur FormFit Walker Air). Force insoles (100Hz, Loadsol, Novel) measured in-shoe vertical reaction force (VRF). A rosette (800Hz, Kyowa Strain Gauges) and linear (800Hz, Micro-Measurements) strain gauge were attached to the distal and midshaft of the tibia, respectively. Strain gauges recorded tibial bone deformation. Specimens underwent two axial loading conditions up to 900N: stationary vertical loading (Figure 1A) and simulated dynamic stance (5 degrees of plantarflexion and 8 degrees of dorsiflexion; Figure 1B). The DAO was engaged for a targeted offloading value of 12% body weight. Wilcoxon Signed Rank (p < 0.05) and effect sizes (r) were used to assess mean differences between the DAO and WB.

Results, Conclusions, and Discussions: Stationary vertical loading and simulated dynamic stance results are presented in Table 1.During stationary loading, the DAO largely reduced in-shoe VRF (17.3%), this leading to large reductions in tibial bone strain for both the distal (20.4%) and midshaft (41.1%) strain gauges compared to the WB. During the plantarflexor phase of the simulated dynamic loading, there was no difference for the in-shoe VRF (1.3%) or midshaft (3.2%) strain gauge between the DAO and WB. However, there was a large reduction at the distal bone strain (15.9%) wearing the WB compared to the DAO. During the dorsiflexion phase of the simulated dynamic loading, the DAO largely reduced in-shoe VRF (23.4%), leading to large reductions in tibial bone strain at the distal (22.3%) and midshaft (46%) tibia with the DAO compared to the WB.

Our findings partially support our hypothesis that the DAO was able to reduce the tibial bone strain compared to a WB for both stationary and dynamic simulated loading conditions. These findings confirmed 12% offloading provided by the DAO was effective at reducing tibial bone strain compared to a WB. Moving forward, we propose two studies: 1) will assess the daily comfort level of wearing the DAO for one week in healthy adults. 2) Will be clinical investigation that compares treatment outcomes in a TBSI runners population wearing either the DAO or WB. The aims for Study Two will be to assess (1) bone healing quality and (2) functional ability (muscle strength, mobility, muscle atrophy) between the injured and uninjured limb.

Acknowledgements (Optional):

References:
1. Cosman F, Ruffing J, Zion M, et al. Determinants of stress fracture risk in United States Military Academy cadets. Bone. 2013;55(2):359–66.
2. Armstrong DW, Rue J-PH, Wilckens JH, Frassica FJ. Stress fracture injury in young military men and women. Bone. 2004;35(3):806–16.
3. Liem BC, Truswell HJ, Harrast MA. Rehabilitation and Return to Running After Lower Limb Stress Fractures. Current Sports Medicine Reports. 2013;12(3):200.
4. Kahanov L, Eberman LE, Games KE, Wasik M. Diagnosis, treatment, and rehabilitation of stress fractures in the lower extremity in runners. Open Access Journal of Sports Medicine. 2015;6:87–95.
5. Nattiv A, Kennedy G, Barrack MT, et al. Correlation of MRI Grading of Bone Stress Injuries With Clinical Risk Factors and Return to Play: A 5-Year Prospective Study in Collegiate Track and Field Athletes. Am J Sports Med. 2013;41(8):1930–41.
6. Kadel NJ, Segal A, Orendurff M, Shofer J, Sangeorzan B. The Efficacy of Two Methods of Ankle Immobilization in Reducing Gastrocnemius, Soleus, and Peroneal Muscle Activity during Stance Phase of Gait. Foot Ankle Int. 2004;25(6):406–9.
7. Powell D, Clowers K, Keefer M, Zhang S. Alterations in neuromuscular activation patterns associated with walking in short-leg walking boots. Journal of Sport and Health Science. 2012;1(1):43–8.
8. Johnson PA, DiAngelo, DJ, Paquette, MR, A Dynamic Ankle Orthosis Reduces Tibial Compressive Force and Increases Ankle Motion Compared With a Walking Boot, Medicine & Science in Sports & Exercise. 2023;55(11):2075-82.