By Bruce E. Williams, DPM
Recently, on the Facebook page of Craig Payne, creator of Podiatryarena.com, there was discussion about a published study that compared running biomechanics and perceived comfort between a 3D-printed orthotic and a traditionally manufactured orthotic. The study showed there were no differences between the two devices. This is as most of us would suspect, provided both orthotics were made from the same cast/scan of the foot, that they both utilized the same prescription, and that the materials used were essentially similar in overall stiffness.
What drew my interest was that many of the commenters appeared to believe there was no real difference in the production processes of the two different devices, and that 3D printing essentially offers no improvement to the overall production process or potential function of custom foot orthotics.
What many practitioners fail to understand when comparing traditional manufacture versus additive manufacturing (aka, 3D printing) of foot orthotics is how we currently control, through computer numerical control (CNC) manufacturing, overall and specific segmental regions of stiffness or flexibility in orthotics, and how the 3D printing process could improve upon that ability.
Stiffness in current manufacture of CNC-milled custom foot orthotics is typically controlled by altering the thickness of the polypropylene used in the production of the device: For example, using 4mm poly for most of your devices, but when wanting something more flexible, downgrading to a 3mm thickness material. Arch-fill is another tool that can alter the stiffness of a specific region of the foot orthotic, specifically the medial arch. We can decrease the arch-fill in flat flexible feet to essentially “raise” the arch or bring more conformity of the orthotic to the foot. In high arch feet that have different degrees of flexibility, we can increase the arch fill to effectively lower the orthotic’s medial arch so it no longer matches up with the foot’s actual shape, ie, less conformity and less stiffness. In other words, traditional manufacturing requires the computer component, but then extensive handwork by a craftsman who is at least one step away from the clinician who designed the device and may not be available when a new orthotic is required. Additive manufacturing allows for the exact replication of the same orthotic as many times as the patient requires.
With 3D printing, the regional stiffness of a device can be altered in increments as the practitioner who’s assessing the patient and designing the device sees fit. If a more conforming device is needed for a higher arch foot, decrease the medial arch stiffness so that it is much more comfortable for the patient. The opposite can be done for patients with flatter medial arches, ie, increase the stiffness of the medial arch to add a level of increased control.
Additive manufacturing will eventually offer opportunities for changing the positioning of heel and forefoot wedges as well. As Dr. Kevin Kirby has shown, the area of the heel that allows a varus heel post to work effectively is rarely, if ever, in the “usual” position where we traditionally add varus heel wedges or medial heel skives. In the future, additive manufacturing should allow us to place those wedges in tighter, more specific, and potentially more useful areas.
3D printing for foot orthotics is just in its infancy—but like an infant, it’s potential is vast. The degree to which additive manufacturing can and will improve our current production of foot orthotic devices is really up to us: If we can dream of a better way to assist the mechanics of our patient’s foot function, additive manufacturing should allow us to produce a device specifically to assist those needs!
Bruce E. Williams, DPM, served as a practicing podiatrist specializing in sports medicine, biomechanics, orthotics, athletic shoes, and gait and movement analysis for more than 27 years, most recently with the Weil Foot & Ankle Institute in Chicago, Illinois. In January 2019, he embraced 3D printing of foot orthotics when he joined Go 4-D as Medical Director, and as a consultant to HP’s FitStation. Dr. Williams is a long-standing member of the LER Editorial Advisory Board.