Gait responses to modifying the spring stiffness of a dorsiflexion stopped ankle-foot orthosis in a polio survivor with plantar flexor weakness

Abstract

Introduction and aim: In patients with post polio syndrome (PPS), gait is frequently hampered by excessive and/or abrupt ankle dorsiflexion, caused by weakness of the plantar flexors. This can lead to instability and fatigue due to a reduced walking efficiency [1]. To provide stability and increase walking efficiency, patients with plantar flexor weakness can be prescribed with a spring-like dorsiflexion stopped ankle-foot orthosis (DS-AFO). In patients with stroke and MS presenting spastic leg paresis, it has been shown that walking with such an AFO can optimize walking efficiency by choosing the correct ankle stiffness [2]. A similar principal might also apply to patientswith flaccid paresis. The aimof this studywas to evaluate the effect of different degrees of DS-AFO ankle stiffness on walking efficiency and gait biomechanics in patients with PPS and plantar flexor weakness. Patients/materials and methods: One male patient with PPS and mild plantar flexor weakness participated (age: 67 years, MRC score plantar flexors: 4+, gait pattern: excessive ankle range of motion combined with persistent knee flexion during stance). For this patient a custommade foot partwith calf casingwas fabricated, in which five interchangeable carbon leaf springs with a different degree of stiffness (k) could be placed (range: k1: 0.8Nm/deg to k5: 7.3Nm/deg, as measured with the BRUCE device [3]). The patient was first assessed for gait biomechanics (Vicon MX, AMTI force plates) at comfortable speed while walking with the DS-AFO (5 springs, tested in random order). Each measurement started after the patient indicated to be customized to the new spring. After each condition, satisfaction was rated with a visual analogue scale. Subsequently, walking efficiency was assessed (Cosmed) during a 6-min walk test for the most satisfying DS-AFO spring, the stiffest and themost compliant spring. Gait biomechanics andwalking efficiency were also assessed for the patients’ own DS-AFO and for walking with shoes alone. Results: The patient rated DS-AFO-k3 (2.5Nm/deg) asmost satisfying: ‘It supported, butdidnot constrain toomuch’.Walkingwith this DS-AFO resulted in the highest comfortable gait speed (10% higher compared to shoes alone) and it increasedwalking efficiency with 2.5% compared to walking with shoes alone and to walking with his ownDS-AFO (Fig. 1). However,most efficient gaitwas seen with the stiffest spring (k5 =7.3Nm/deg): +4.1% compared to walking with shoes alone. Regarding gait biomechanics, ankle range of motion (range: k1: 33◦ to k5: 19◦) and ankle power during push-off (range: k1: 2.1W/kg to k5: 1.2W/kg) reduced with increasing stiffness. Only the three stiffest springs resulted in a decreased knee flexion during terminal stance (all about −4.8◦), compared to the two most compliant springs. Discussion and conclusions: The results of this case study indicate that for this patient with PPS and plantar flexor weakness optimizing DS-AFO ankle stiffness was useful for improving gait with respect to walking efficiency. Walking with the stiffest DSAFO (k5 =7.3) resulted in the most efficient gait, and, although the improvements were small, gait efficiency and reduction of knee flexion seemed to be related. However, the most efficient DS-AFO wasnot considered themost satisfyingDS-AFO. Sinceplantar flexor weakness from themeasuredpatientwas onlymild, a high stiffness may not be optimal. This is the first result from an ongoing project; we expect that a broader range in plantar flexorweaknesswill yield more precise relations onhowDS-AFOmechanics andpatient characteristics interact towards gait performance. Ultimately, this will serve DS-AFO stiffness selection in patients with PPS.