The effect of thickness variation on the rigidity of ankle foot orthoses provided to the NHS: A case for the need for quality control

Background
Drape-forming is a cost-effective method used worldwide to manufacture bespoke ankle foot orthoses (AFOs). It involves draping a heated polymer material sheet over a positive cast of the user’s limb. Previous research has shown that the manual nature of drape-forming can influence the thickness of the final AFO and even lead to structures that are inadequately rigid to be clinically effective. This study assesses the difference between the prescribed and the final thickness of AFOs meant for use by UK’s National Health Service (NHS) and estimates its potential impact on AFO rigidity.

Methods
A clinically relevant method to measure AFO thickness as part of the manufacturing process was developed and validated. This method was used by three major UK manufacturers for all bespoke rigid AFOs they provided to the NHS within a predefined period. A validated finite element model was used to estimate the impact of the observed difference between prescribed and final thickness on AFO stiffness.

Results
86 AFOs were assessed in total. Final thickness was between 4.0% and 35.5% lower than the prescribed one (median thickness reduction= 17.4%). This discrepancy in thickness led to a relative reduction in AFO stiffness ranging between 7.0% and 80.0% (median stiffness reduction= 30.7%).

Discussion
The adequacy of AFO thickness cannot be judged based on prescription thickness. Measurements of final thickness as part of standard practice should be considered to enhance the provision of bespoke AFOs. Further research is needed to establish thresholds of acceptable manufacturing-induced deviation from the prescribed AFO thickness.