Does A Weight-Bearing Scan Produce A Functional Foot Orthosis?

Years ago, Merton Root described the difference between a simple arch support and the true functional foot orthosis:¹

“An orthosis that is prescribed to resist specific abnormal forces identified by examination and is designed to promote improved function of the foot is called a functional orthosis. A functional orthosis does not support the arch of the foot. A functional orthosis does not ‘balance’ a foot. A functional orthosis does not hold a foot in any position. A functional orthosis only resists abnormal forces and promotes improved foot function. Functional orthoses are primarily designed to resist the abnormal direction of forces and to redirect those forces into a more normal direction within the foot”

Root and colleagues described a specific technique to perform neutral suspension impression casting of the foot as well as the creation of a positive model for fabrication of the functional foot orthosis.² That methodology focused on preserving specific shapes and contours on the plantar surface of the foot as well as capturing and supporting forefoot-to-rearfoot deformities.

Digital technology is now being used to simplify and expedite all aspects of foot orthotic manufacturing.³ In place of plaster of Paris impression casting, digital technology can capture the three dimensional (3D) shape of the leg, foot and ankle.⁴ This same technology creates a 3D model of the anatomy which one can then manipulate and modify in a similar fashion to traditional plaster of Paris positive casts, making the model appropriate for molding of a custom orthosis.⁵ Instead of thermal molding, digital technology can now program machines for milling of the orthosis or for additive manufacturing with a 3D printer.^6-8

However, with digital technology comes opportunity to take shortcuts and ultimately fail to follow the principles of true custom functional foot orthotic treatment. Devices for digital scanning originally required contact of the foot to the scanner, and therefore, the patient was either full or partial weight-bearing on the scanner.  However, serious flaws occur when implementing this process. Many “over ground” scanners only capture a two-dimensional image of the plantar surface of the foot, failing to incorporate the posterior aspect of the calcaneus and the three-dimensional alignment of the forefoot to the rearfoot.  All of these factors may lead to the fabrication of a custom arch support instead of a true functional foot orthosis.

In terms of the influence of weight bearing during scanning of the foot, Tsung and coworkers conducted an important study.⁹ They compared the shape of plaster casts of eight adult feet taken in non-weight-bearing, semi-weight-bearing  and full weight-bearing conditions. Compared to the non-weight-bearing foot shape, the semi-weight-bearing condition produced a six percent wider heel, a 15 percent decrease in arch height and a 21 percent lowering of arch angle. With a full weight-bearing cast, compared to a non-weight-bearing cast, the rearfoot width increased by eight percent, the arch height decreased 20 percent and the arch angle decreased 41 percent.⁹

The influence of weight bearing on capture of forefoot to rearfoot alignment during impression casting was reported by Jeff Root and this author.¹⁰ We compared the shapes and forefoot-to-rearfoot alignment obtained from impression casts of a single patient in three different weight bearing conditions: neutral suspension technique (non-weight-bearing); partial weight-bearing, neutral subtalar joint position; and full weight-bearing, neutral subtalar joint position. We demonstrated that when a patient with a true forefoot valgus deformity was allowed to bear any weight on their foot during the casting process, the forefoot-to-rearfoot relationship changed from valgus to varus. Furthermore, significant changes in geometry of heel width, arch width, arch height and calcaneal inclination were noted when comparing the three casting positions. With weight-bearing, the impression cast captured a wider heel and lower arch compared to the neutral suspension technique.¹⁰

The take home message is that scanning or casting in a weight-bearing position will result in a flatter, wider foot orthosis with a lower arch compared to a non-weight -bearing cast technique. While a functional foot orthosis is much more than an arch support, a well-designed functional orthosis must also support the proximal aspect of the medial arch in an effort to reduce excessive closed chain subtalar joint (STJ) pronation by supporting the talonavicular unit. A semi-weight-bearing or weight-bearing cast or scan captures a pronated foot shape with the talus in a more plantarflexed (ie compensated) position when compared to a non-weight-bearing, neutral suspension cast of the same foot. Actively resisting talar adduction and plantarflexion during closed chain STJ pronation, a functional foot orthosis can reduce the range of STJ pronation and reduce pathological forces. An orthosis made from a non-weight-bearing cast or scan captures the proximal, medial arch in its uncompensated, true anatomic shape thereby providing enhanced medial column support.

Newer scanners capture a three-dimensional view of the foot including the posterior aspect of the calcaneus to determine true forefoot-to-rearfoot relationships.  These scanners do not require contact of the foot on the device and allow the practitioner to follow the long accepted method of positioning the foot for neutral suspension imaging.  This non-weight-bearing condition allows accurate capture of the plantar foot contours to assure optimal position of the medial longitudinal arch, heel width and calcaneal inclination as well as accurate forefoot-to-rearfoot alignment.  Contour and support of these key anatomic structures enables the functional foot orthosis to resist abnormal forces in a way that custom arch supports cannot.

Indeed, lower extremity clinicians can employ new technologies to improve accuracy and expedite the fabrication of foot orthoses.  At the same time, the critical lessons learned about the subtle nuances of foot positioning cannot be abandoned.  Failure to follow time-tested methods of foot positioning will result in the production of a custom arch support instead of a true functional foot orthosis.  It is the true functional orthosis which has stood the test of time as a reliable and efficacious modality for the treatment of a myriad of lower extremity conditions.^11,12

Dr. Richie is an Adjunct Associate Professor within the Department of Applied Biomechanics at the California School of Podiatric Medicine at Samuel Merritt University in Oakland, Calif. He is a Fellow and Past President of the American Academy of Podiatric Sports Medicine. Dr. Richie is a Fellow of the American College of Foot and Ankle Surgeons, and the American Academy of Podiatric Sports Medicine. Dr. Richie is the author of a new book titled "Pathomechanics of Common Foot Disorders," which is available from Springer at https://www.springer.com/us/book/9783030542009 . He discloses that he is Founder and President of Richie Technologies.

References

1. Root ML. Understanding functional foot orthoses. Pacesetter Magazine. 1982;2(1):6-10.

2. Root M, Weed J, Orien W. Neutral Position Casting Techniques. Los Angeles:Clinical Biomechanics Corporation;1971.

3. Dombroski CE, Balsdon MER, Froats A. The use of a low cost 3D scanning and printing tool in the manufacture of custom-made foot orthoses: a preliminary study. BMC Res Notes. 2014;7(1):443.

4. Houston V, Mason C, Beattie A, et al. The VA-Cyberware lower limb prosthetics-orthotics optical laser digitizer. J Rehabil Res Dev. 1995;32(1):55–73.

5. Wand M, Adams B, Ovsjanikov M, et al. Efficient reconstruction of nonrigid shape and motion from real-time 3D scanner data. ACM Trans Graph. 2009;28(2):15.

6. Chen RK, Y-a J, Wensman J, Shih A. Additive manufacturing of custom orthoses and prostheses - a review. Addit Manuf. 2016;12:77–89.

7. Y-a J, Plott J, Chen R, Wensman J, Shih A. Additive manufacturing of custom orthoses and prostheses–a review. Procedia CIRP. 2015;36:199–204.

8. Walbran M, Turner K, McDaid A. Customized 3D printed ankle-foot orthosis with adaptable carbon fibre composite spring joint. Cogent Eng. 2016;3(1):1227022.

9. Tsung BY, Zhang M, Fan YB, Boone DA. Quantitative comparison of plantar foot shapes under different weight-bearing conditions. J Rehabil Res Dev. 2003;40(6):517-526

10. Root J, Richie D.  A comparison of negative casting techniqes used fo the fabrication of custom ankle foot orthoses.  Podiatry Management.  Sept 2007;129-136

11. Harradine P, Bevan L. A review of the theoretical unified approach to podiatric biomechanics in relation to orthoses theory. ^{J Am Podiatr Med Assoc.} 2009;99:317-325.

12.  Landorf K, Keenan A, Rushworth L: Foot Orthosis Prescription Habits of Australian and New Zealand Podiatric Physicians. J Am Podiatr Med Assoc. 1991;91:175-183.