State of the Prescription Process for Dynamic Ankle-Foot Orthoses

Influence of custom dynamic orthoses on tibiotalar joint reaction force and contact stress: A cadaveric study

Post-traumatic osteoarthritis (PTOA) often develops following tibial pilon fractures. Evidence suggesting PTOA development is driven by elevated articular contact stress from residual malreduction has led surgeons to strive for precise articular reduction, typically at the cost of extended operative time. Post-operative bracing using carbon fiber custom dynamic orthoses (CDOs) offers another means to decrease tibiotalar joint reaction force (JRF) and contact stress. The purpose of this cadaveric study was to measure how CDO stiffness influences ankle JRF and contact stress over the stance phase of gait. A servohydraulic load frame was used to test five cadaver ankles, with axial loading (240-330 N) and pneumatic actuation of the Achilles tendon (50-436 N) serving to quasi-statically model multiple points in the stance phase of gait. Three CDO rotational stiffness conditions were tested: (1) No CDO-0 Nm/deg, (2) low stiffness CDO-1.8 Nm/deg, and (3) moderate stiffness CDO-2.3 Nm/deg. JRF and contact stresses were measured using a piezoresistive pressure sensor inserted into the tibiotalar joint. An insole plantar pressure sensor placed between the cadaveric foot and CDO footplate measured limb/device interactions via the plantar center of pressure (COP). As limb loading progressed through stance, the plantar COP progressed from hindfoot to forefoot, as it would in normal gait. Both CDOs demonstrated decreases in JRF, reaching as high as 32% for the low CDO and 26% for the moderate CDO, with associated decreases in contact stress. This suggests that post-operative bracing could lessen PTOA risk after pilon fractures.

Metatarsophalangeal Joint Dynamic Stiffness During Toe Rocker Changes With Walking Speed

Dynamic joint stiffness (or simply "stiffness") is a customization criteria used to tune mechanical properties of orthotic and prosthetic devices. This study examines metatarsophalangeal (MTP) joint stiffness during the toe-rocker phase of barefoot walking and establishes baseline characteristics of MTP joint stiffness. Ten healthy individuals walked at 4 speeds (0.4, 0.6, 0.8, and 1.0 statures·s-1) over level ground. MTP sagittal plane joint angles and moments were calculated during the toe-rocker phase of stance. Least-squares linear regressions were conducted on the MTP moment versus angle curve to determine joint stiffness during early toe rocker and late toe rocker. Multilevel linear models were used to test for statistically significant differences between conditions. Early toe rocker stiffness was positive, while late toe rocker was negative. Both early toe rocker and late toe rocker stiffness increased in magnitude significantly with speed. This study establishes baseline characteristics of MTP joint stiffness in healthy walking, which previously had not been examined through a range of controlled walking speeds. This information can be used in the future as design criteria for orthotic and prosthetic ankle and ankle-foot devices that can imitate, support, and facilitate natural human foot motion during walking better than existing devices.

Comparison of Existing Methods for Characterizing Bi-Linear Natural Ankle Quasi-Stiffness

Natural ankle quasi-stiffness (NAS) is a mechanical property of the ankle joint during motion. NAS has been historically calculated as the average slope (linear regression) of the net ankle moment vs. ankle angle during discrete phases of stance. However, recent work has shown that NAS is nonlinear during stance. Specifically, during the loading phase (~10-60% of stance), plantarflexion moment increases at an accelerating rate compared to dorsiflexion angle. Updated models have been developed to better capture this inherent nonlinearity. One type of model is called bi-linear NAS (BL-NAS) divides the loading phase of stance into two sub-phases, called early loading (EL) and late loading (LL) NAS. Two papers, written by Crenna & Frigo in 2011 and Shamaei et al. in 2013, outline different BL-NAS models. Both models fit measured data better (lower RMSE) than standard single linear NAS (SL-NAS) models, but have not been widely adopted, possibly because of methodological discrepancies and lack of applicability to physical devices at the time. This paper compares these existing BL-NAS models and translate those findings to possible orthotic device designs. Results showed that both BL-NAS models had lower RMSE than SL-NAS, EL-NAS was not significantly different across walking speeds, and LL-NAS increased significantly at faster walking speeds. These improved NAS models better approximate natural human movement than commonly used SL-NAS models, and provide a basis to design ankle-foot devices with multiple stiffness properties to emulate and facilitate natural human motion.

Alternative methods for measuring ankle-foot orthosis alignment in clinical care

BACKGROUND

Changes in gait due to an ankle foot orthosis (AFO) have been shown to be impacted by the sagittal plane alignment of the AFO, but there is variability in practice and lack of consensus as to how this alignment should be measured. The neutral angle is a measure of AFO alignment that has the potential to be used by various specialties that prescribe, provide, and analyze AFOs. Currently, a lack of validated measurement methods prevents the neutral angle from being used in various clinical settings. Two experimental neutral angle measurement methods are proposed to address this shortcoming: a portable low-cost method for use during AFO fabrication and fitting, and a laboratory-based method for use during dynamic three-dimensional gait analysis (3DGA).

RESEARCH QUESTION

What is the concurrent validity of the two experimental neutral angle measurement methods against the gold standard?

METHODS

The gold standard neutral angle measurement (NA_GOLD) was prospectively collected during a static 3DGA trial for 19 pediatric AFOs from 10 individuals. While NA_GOLD was being collected, the neutral angle was simultaneously measured using digital differential inclinometers (NA_INCL). Within the same 3DGA session, the neutral angle was also measured during the swing phase of gait (NA_SWING). The NA_INCL and NA_SWING measurements were compared to NA_GOLD using repeated measures ANOVA, ICC, and bootstrapped errors-in-variables regressions.

RESULTS

Repeated measures ANOVA indicated no differences between measurement methods (p = 0.43) and ICC analysis indicated good absolute agreement (ICC(A-1) = 0.85). Mean absolute deviations between the NA_INCL and NA_SWING with NA_GOLD measurements were 2.4 ° and 1.9 °, with standard deviations of 2.9 ° and 2.7 °, respectively. Maximum observed differences were less than 7 °. The NA_INCL and NA_SWING methods explained 74 % and 81 % of the variance in NA_GOLD, respectively.

SIGNIFICANCE

The concurrent validity of two new neutral angle measurement methods provides alternative means to assess AFO alignment in the clinic.

Nonlinear net ankle quasi-stiffness reduces error and changes with speed but not load carried

BACKGROUND

Natural ankle quasi-stiffness (NAS) is a key metric used to personalize orthotic and prosthetic ankle-foot devices. NAS has traditionally been defined as the average slope (i.e. linear regression) of the net ankle moment vs. ankle angle curve during stance. However, NAS appears to have nonlinear characteristics. Characterizing nonlinear NAS across a wide range of tasks will enable us to incorporate these attributes into future orthotic and prosthetic ankle-foot device designs.

RESEARCH QUESTION

Does nonlinear NAS change across multiple intensities of walking, running, and load carriage tasks?

METHODS

This observational study examined 22 young, healthy individuals as they walked, ran, and walked while carrying a load at three intensities (speed or load). Linear, quadratic, and cubic regressions were done on the net ankle moment vs. ankle angle curve over three phases of stance: impact, loading, and push-off. RMSE between regressions and measured data were computed to determine regression accuracy, and multilevel linear models (MLMs) were used to determine significant differences between coefficients across intensities.

RESULTS

Quadratic and cubic regressions of NAS had significantly lower RMSE than linear NAS for all phases of stance. Because of diminishing reductions in RMSE between quadratic and cubic regressions, only quadratic regression coefficients were further analyzed. Most first (linear) and second (nonlinear) order coefficients of quadratic regressions exhibited clear trends with respect to changes in walking or running speed, but not to increases in load.

SIGNIFICANCE

This was the first study to our knowledge to thoroughly characterize nonlinear NAS across multiple gait tasks and intensities. This study provides an advanced understanding of the characteristics of nonlinear NAS for the design of future prosthetic and orthotic ankle-foot devices.