Embracing additive manufacture: implications for foot and ankle orthosis design

Abstract

Background: The design of foot and ankle orthoses is currently limited by the methods used to fabricate the devices, particularly in terms of geometric freedom and potential to include innovative new features. Additive manufacturing (AM) technologies, where objects are constructed via a series of sub-millimetre layers of a substrate material, may present the opportunity to overcome these limitations and allow novel devices to be produced that are highly personalised for the individual, both in terms of fit and functionality.Two novel devices, a foot orthosis (FO) designed to include adjustable elements to relieve pressure at the metatarsal heads, and an ankle foot orthosis (AFO) designed to have adjustable stiffness levels in the sagittal plane, were developed and fabricated using AM. The devices were then tested on a healthy participant to determine if the intended biomechanical modes of action were achieved.

Results: The adjustable, pressure relieving FO was found to be able to significantly reduce pressure under the targeted metatarsal heads. The AFO was shown to have distinct effects on ankle kinematics which could be varied by adjusting the stiffness level of the device.

Conclusions: The results presented here demonstrate the potential design freedom made available by AM, and suggest that it may allow novel personalised orthotic devices to be produced which are beyond the current state of the art.

Figure 1

Process schematic for SLA (left) and SLS (right).

Figure 2

Process schematic for FDM process.

Figure 3

Foot orthosis fabrication. FO being printed in polylactide (PLA) on a low cost FDM machine (RapMan; Bits from Bytes, Clevedon, UK).

Figure 4

FO with adjustable metatarsal support elements. CAD model (left) and fabricated device (right). 2^nd to 4^th adjusters not shown in CAD model for clarity. Sections of the adjustable elements and their corresponding holes in the FO are threaded, allowing their height to be easily adjusted with a screwdriver.

Figure 5

Adjustable stiffness ankle foot orthosis. A) In the lower stiffness condition, when the gas spring on the medial side compresses to provide resistance to plantarflexion, the disengaged spring on the lateral side is free to slide down its support bracket without giving any resistance. B) The slider component provides the upper attachment point of the gas springs and is held in place by two M6 bolts (one above and one below). By adjusting these bolts the slider can be moved up and down, and this alters the shank to foot angle. The adjustment range is approximately 6° of anterior and posterior tilt. C) The shank section is mounted on runners to allow it to move up and down freely. This is intended to reduce friction between the calf and this component of the device during gait.

Figure 6

Kinematics. Mean ankle and knee kinematics in the sagittal plane for normal (shod) walking and high and low stiffness AFO conditions. Positive angles indicate (dorsi)flexion in the sagittal plane.

Figure 7

Kinetics. Ankle and knee kinetics in the sagittal plane for normal (shod) walking and high and low stiffness AFO conditions. Positive angles indicate: an internal dorsiflexion moment at the ankle; and an internal extension moment at the knee. %BWxH: percentage of the participant’s bodyweight multiplied by their height.