Optimising Ankle Foot Orthoses for children with cerebral palsy walking with excessive knee flexion to improve their mobility and participation; protocol of the AFO-CP study

Abstract

Background: Ankle-Foot-Orthoses with a ventral shell, also known as Floor Reaction Orthoses (FROs), are often used to reduce gait-related problems in children with spastic cerebral palsy (SCP), walking with excessive knee flexion. However, current evidence for the effectiveness (e.g. in terms of walking energy cost) of FROs is both limited and inconclusive. Much of this ambiguity may be due to a mismatch between the FRO ankle stiffness and the patient's gait deviations.The primary aim of this study is to evaluate the effect of FROs optimised for ankle stiffness on the walking energy cost in children with SCP, compared to walking with shoes alone. In addition, effects on various secondary outcome measures will be evaluated in order to identify possible working mechanisms and potential predictors of FRO treatment success.

Method/design: A pre-post experimental study design will include 32 children with SCP, walking with excessive knee flexion in midstance, recruited from our university hospital and affiliated rehabilitation centres. All participants will receive a newly designed FRO, allowing ankle stiffness to be varied into three configurations by means of a hinge. Gait biomechanics will be assessed for each FRO configuration. The FRO that results in the greatest reduction in knee flexion during the single stance phase will be selected as the subject's optimal FRO. Subsequently, the effects of wearing this optimal FRO will be evaluated after 12-20 weeks. The primary study parameter will be walking energy cost, with the most important secondary outcomes being intensity of participation, daily activity, walking speed and gait biomechanics.

Discussion: The AFO-CP trial will be the first experimental study to evaluate the effect of individually optimised FROs on mobility and participation. The evaluation will include outcome measures at all levels of the International Classification of Functioning, Disability and Health, providing a unique set of data with which to assess relationships between outcome measures. This will give insights into working mechanisms of FROs and will help to identify predictors of treatment success, both of which will contribute to improving FRO treatment in SCP in term.

Trial registration: This study is registered in the Dutch Trial Register as NTR3418.

Figure 1

Schematic representation of the study design. Following baseline measurements (T0), the subject’s will be prescribed an interventional FRO. The stiffness of this FRO will be varied (rigid, stiff and flexible) and the order of FRO stiffness will be block randomised. Accommodation time for each stiffness will last 4–8 weeks, after which effects will be evaluated (T1_K1, T1_K2, and T1_K3). Following these evaluations, an optimal FRO for the subject will be selected. Follow-up measurements (T2_Kopt) will be carried out at 12–20 weeks. B=Block; FRO= Floor Reaction Orthosis; K=AFO stiffness; K₁, K₂, and K₃ represent either rigid, stiff or flexible stiffness configurations.

Figure 2

Flowchart of the optimal FRO stiffness selection procedure. After sorting the different stiffness configurations based on peak knee extension angle in single support (KE-K_x), absolute differences in peak KE will be calculated. KE-K₂ and/or KE-K₃ will be excluded if this difference is more than five degrees. Otherwise, the remaining configurations will be sorted by net non-dimensional walking energy cost (NNC-K_x) (this can be either two or three remaining configurations). The stiffness that results in the lowest walking energy cost will be selected as the subject’s optimal FRO. K_1, K₂ and K₃ = rigid, stiff, or flexible FRO stiffness configurations; K_OPT = subject’s optimal FRO stiffness; KE = knee extension angle; KE-K_x = stiffness sorted by KE; NN_cost_%SMC = net non-dimensional walking energy cost relative to speed matched controls; NNC-K_x = stiffness sorted by NN_cost_%SMC; SS = single support.