Foot Orthosis Biomechanics in Footwear

All participants wore standardised thin cotton socks with their most commonly used footwear during testing.
The five conditions analysed were: (Figure 1):
(i) Shoe alone (control),
(ii) Customised foot orthosis,
(iii)  Contoured polyethylene sham foot orthosis,
(iv) Contoured ethylene vinyl acetate (EVA) sham   foot orthosis,
(v)  Flat ethylene vinyl acetate (EVA) sham foot orthosis.
All feet were cast with the participant prone, using a commonly used technique to gain a neutral impression of the foot [25 (link)]. The customised foot orthosis was a modified Root style device balanced to the neutral calcaneal stance position as it is the most commonly prescribed orthosis by Australian and New Zealand podiatrists [25 (link)]. The orthotic shell thickness was either 3.0 mm, 4.0 mm or 4.5 mm polypropylene which was determined by each participant’s bodyweight and foot posture [17 (link)]. Participants with a neutral or pronated foot (FPI > 0) and bodyweight under 75 kg were prescribed a 4.0 mm polypropylene shell, while participants with a bodyweight 75 kg or over were prescribed a 4.5 mm shell. Participants with a cavoid foot (FPI < 0) were issued a 3.0 mm shell if their weight was less than 75 kg, while a 4.0 mm shell was issued if their weight was greater than 75 kg. Each customised foot orthosis was manufactured by a commercial orthotic laboratory (Footwork Podiatry Laboratory Pty Ltd, Melbourne, Australia) using a computer-aided design and computer-aided manufacturing process (CAD-CAM), whereby each orthosis was moulded against a positive cast that was milled from a timber composite block.
Following this, the two contoured sham orthoses were moulded against the same positive cast as the customised foot orthosis. One of the moulded sham devices was made from 1 mm polyethylene. The other moulded sham device was made from 3 mm EVA (90 kg/m²). These materials were chosen as both were expected to collapse under minimal force, thus providing minimal effect on plantar pressures (see Additional file 1: Figures S1 and S2 for basic information on the deformation characteristics of the foot orthoses). Both of these devices received minimal modification post-moulding, and the contoured orthosis had minimal grinding under the heel. The final flat sham orthosis was made from 3 mm EVA (90 kg/m²) and was only bevelled at the anterior margin just proximal to the metatarsophalangeal joint line. The contoured [14 (link),17 (link)] and flat EVA sham foot orthosis [10 (link)] were also selected as similar devices have been used in previous randomised controlled trials or trials in progress. All sham and customised orthoses were covered with the same thin vinyl material.