Assessment of the Shank-to-Vertical Angle While Changing Heel Heights Using a Single Inertial Measurement Unit in Individuals with Incomplete Spinal Cord Injury Wearing an Ankle-Foot-Orthosis

Abstract

Previous research showed that an Inertial Measurement Unit (IMU) on the anterior side of the shank can accurately measure the Shank-to-Vertical Angle (SVA), which is a clinically-used parameter to guide tuning of ankle-foot orthoses (AFOs). However, in this context it is specifically important that differences in the SVA are detected during the tuning process, i.e., when adjusting heel height. This study investigated the validity of the SVA as measured by an IMU and its responsiveness to changes in AFO-footwear combination (AFO-FC) heel height in persons with incomplete spinal cord injury (iSCI). Additionally, the effect of heel height on knee flexion-extension angle and internal moment was evaluated. Twelve persons with an iSCI walked with their own AFO-FC in three different conditions: (1) without a heel wedge (refHH), (2) with 5 mm heel wedge (lowHH) and (3) with 10 mm heel wedge (highHH). Walking was recorded by a single IMU on the anterior side of the shank and a 3D gait analysis (3DGA) simultaneously. To estimate validity, a paired t-test and intraclass correlation coefficient (ICC) between the SVAIMU and SVA3DGA were calculated for the refHH. A repeated measures ANOVA was performed to evaluate the differences between the heel heights. A good validity with a mean difference smaller than 1 and an ICC above 0.9 was found for the SVA during midstance phase and at midstance. Significant differences between the heel heights were found for changes in SVAIMU (p = 0.036) and knee moment (p = 0.020) during the midstance phase and in SVAIMU (p = 0.042) and SVA3DGA (p = 0.006) at midstance. Post-hoc analysis revealed a significant difference between the ref and high heel height condition for the SVAIMU (p = 0.005) and knee moment (p = 0.006) during the midstance phase and for the SVAIMU (p = 0.010) and SVA3DGA (p = 0.006) at the instant of midstance. The SVA measured with an IMU is valid and responsive to changing heel heights and equivalent to the gold standard 3DGA. The knee joint angle and knee joint moment showed concomitant changes compared to SVA as a result of changing heel height.

How does a systematic tuning protocol for ankle foot orthosis–footwear combinations affect gait in children in cerebral palsy?

“…This is supported by the absence of a significant relation between SVA and knee moments at MST on group level (Figure 4). This is surprising, since tuning is assumed to affect alignment of the knee joint centre with respect to the GRF, consequently leading to changes in the knee moment [19,20]. Our results, however, suggest children with CP might compensate for the more inclined shank by adaptations in other joints or the trunk, most likely to prevent high demands on knee extensor muscles.…”

“…A tuning-protocol using incrementing heel height to manipulate the heel-sole differential has been recommended for children with an EXT gait pattern [11,17,18]. Biomechanically, incrementing heel height of a fixed AFO results in a forward shift of the knee joint centre with respect to the GRF vector at midstance (MST), accompanied by increased internal knee extensor moments, knee flexion angle, and a more inclined shank [18][19][20]. Although the relation between incremented heel height, kinematics and kinetics seems consistent when investigating healthy adults [19], the beneficial effect of tuning in children with CP is variable [11,12,15].…”

“…However, this golden standard is mostly based on clinical experience or studies in healthy adults [17,23]. Up to now, little information is available on the strength of the relation between SVA at MST and other gait outcomes in patients, while this relation is not necessarily the same for atypical gait patterns [20].…”