Page 20 - Maximizing the efficacy of ankle foot orthoses in children with cerebral palsy

20
Chapter I
outcome measures, giving insight the underlying working mechanism of AFOs, which may contribute to proving the efficacy of a treatment algorithm, i.e. how to prescribe a well-matched AFO[37].
Optimizing AFO treatment
Several factors that could improve AFO treatment in order to prescribe a well- matched AFO have been addressed in the literature, some of which will be discussed in this thesis.
Adjusting the AFO alignment
Firstly, literature suggests that an appropriate orientation of the shank (i.e. close to normal) during midstance is a main determinant for efficient walking. In normal walking, the shank is 10 to 12 degrees inclined with respect to the vertical at midstance, therewith positioning the knee joint in the middle of the base of support. It is assumed that this facilitates balance and appropriate ground reaction force alignment with respect to the knee and hip joints, and therefore contributes to conservation of energy[54]. Healthy individuals have the ability to adapt the orientation of the body segments to achieve adequate alignment independent from the footwear they are wearing as long as it allows ankle range of motion.
When wearing an AFO, for most AFOs the ankle is fixed into a pre-defined angle determined by the AFO’s neutral angle (i.e. the angle of the AFO when no force is applied). As ankle range of motion is restricted, the orientation of the shank at midstance is now defined by the combination of the AFO’s neutral angle, and the shoe’s heel-sole differential, i.e. the difference in height between the heel and the forefoot. Adjusting the heel-sole differential of the AFO-footwear combination is therefore expected to influence the orientation of the shank during walking. Subsequently, adjusting the AFO alignment could impact on the efficacy of the AFO, which is considered to be affected by the (mal-)alignment of the ground reaction force to the lower limb joint rotation centers[36,55,56]. Although recently more interest has been shown in the AFO alignment in CP, evidence for the effects are lacking, and evaluating the alignment is currently not completely incorporated into clinical practice[55].
To successfully implement a proper evaluation of the AFO’s alignment into the prescription process in CP, a parameter to quantify the AFO alignment seems required. The Shank-to-Vertical-Angle (SVA) has been proposed as a relatively simple control