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Revisiting Supine Non-Weight Bearing Casting For Fabrication Of Functional Foot Orthotics And AFOs

In this article, the author provides a refresher to practitioners as to how best to position themselves and their patients regardless of material or technology utilized when capturing a model of the foot and ankle with the neutral suspension technique.

Numerous articles over fifty-plus years discuss various techniques to capture a 3D replica of the plantar foot shape for the fabrication of custom foot orthotic devices. This article will not discuss the creation and origins of these techniques, nor will it address the various biomechanical theories of neutral position suspension casting as articulated by Root, Weed, Orien1 Kirby2, and others. Since 1971, many podiatrists in the US, Australia3,4 and others looked at neutral suspension techniques, at times questioning their validity or influence upon foot function.3,5,6 Some advocated that one can improve control of the foot’s biomechanical function by casting the patient in a prone position, fully- or partial-weight bearing using either foam boxes, digital scanners, plaster or STS casting socks.7,8 The same discussion exists regarding casting for custom functional ankle-foot orthotic (AFO) devices. This direction of this review is only toward the non-weight bearing neutral suspension casting technique where one seats the patient with the foot in a supine position.

Within the last five to 10 years, I have observed more frequent discussion of the accuracy of digital scanning versus more traditional casting methodologies and materials for custom foot orthotic devices. I’ve noticed a gradual emphasis on the best technologies to capture foot anatomy as opposed to an emphasis on foot position maintained during the utilization of any of these modalities in the the neutral position casting (NPC) technique. This article should serve as a refresher to practitioners as to how best to position themselves and their patients regardless of the material or technology when incorporating the neutral suspension technique for capturing a model of the foot/ankle. If the goal is to capture an image of the external dimensions of the foot, then one could just as easily go to a retail establishment for scanning or casting at a fraction of the cost of seeing a biomechanically-trained physician. If, however, a practitioner wishes to utilize the Root technique, or its modifications, then a review of some pertinent casting principles and foot positions can, in my experience, assist and lead to more consistent and improved clinical outcomes.

Thoughts On Correct Foot Position For Supine Non-Weight Bearing Casting For Orthotics Or AFOs

1. Position the patient on an examining table with the knee slightly flexed in order to minimize the effect of the gastrocnemius soleus complex. This is particularly important if patient has limited dorsiflexion secondary to gastrocnemius equinus.  Sometimes a small pillow on the posterior aspect of the knee will achieve the slightly flexed position.

Step 1

2. Place the foot vertically and rotate the tibia so the subtalar joint (STJ) is in neutral position.

Step 2

                                                       

3. If the patient is seated in a supine position, the practitioner should position as close as possible to the patient’s foot, at approximately chest level. Being positioned too far from the patient’s foot, as I have seen with certain scanners, often results in an excessively supinated foot position.

Step 3

4. If the patient is seated on the examining chair, and the limb is rotated and abducted, then reposition the hip to internally rotate leg, placing the foot in a more vertical position.

5. With the thumb and forefinger placed within the sulcus of the fourth and fifth metatarsal phalangeal joints or on the fourth and fifth metatarsal heads, move the subtalar joint through its complete range of motion (ROM), from its maximal position of supination to that of pronation. Attempt to place the STJ in the desired neutral (neither pronated nor supinated), pronated or other predetermined position based on the components of patient’s foot anatomy determined during the biomechanical examination.

                                                           

Step 5.

6. An alternative method to determine, possibly concurrently, STJ neutral position, according to Elveru and colleagues9 involves palpating the medial and lateral aspects of the head of the talus. Then, move the STJ to its maximum inversion and eversion position in the frontal plane. The subtalar joint is considered at neutral where the talar head protrudes equally on the medial and lateral sides.9

Step 6.

7. Once one establishes STJ neutral position, one then grasps the proximal phalanges of the fourth and fifth toes, at the sulcus, with the thumb and forefinger. Jeff Root describes “that position of the hand grip is likened to a salute with the wrist straight, fingers and forearm should parallel the angle of the sulcus”10 The practitioner should then slightly rotate their hand into a better grasping position so that when making an attempt to “lock” or stabilize the midtarsal joint, the hand does not slip and he or she does not lose the established position. Sometimes a two-inch gauze pad can assist in grasping the sulcus and prevent slipping.                                                   

Step 7.

8. While maintaining the grasp at the sulcus of toes four and five, exert a slightly upward force in order to stabilize the midtarsal joint, sometimes referred to as “locking” of this joint. At the point at which the midtarsal joint is stabilized or “locked,” the practitioner’s forearm should stay in a position that parallels the axis of the STJ. 

Step 8.

                                              

9. If prior to casting, one determines a patient has a hypermobile first ray, creating an appearance of an elevated first ray, at this point during casting or scanning, one should place the forefinger on the first metatarsal shaft just proximal to the MPJ. Then, exert a mild plantar force on this location to place it in on the same horizontal plane as the second metatarsal. If, however, the first ray is in a slightly plantarflexed position, apply an upward force to the first metatarsal attempting to align it with the adjacent metatarsal. If these modifications during casting do not take place, in my experience the most likely outcome will be less than optimal control of the foot.

Plantar force on the first metatarsal. Dorsal force on the first metatarsal.                                    

10. The practitioner should maintain the foot in this position during the entire casting/scanning period until the cast is firm. I recall John Weed, DPM, former biomechanics instructor at the California College of Podiatric Medicine (California School of Podiatric Medicine at Samuel Merritt University), while instructing on casting technique would often observe a helpful subtle change in the skin lines on the anterior ankle joint, in such that they are horizontal and without any distortions or deviations.

11. If during your biomechanical examination you determine that the patient has a mild gastrocnemius equinus, it is my experience that placing the foot in a slightly pronated position (not maximally pronated) during casting the patient will tolerate a rigid foot orthotic more easily.

Finally as Doug Richie, DPM points out in his 2019 article in Podiatry Today,5 a study by Becerro de Bengoa Vallejo and coworkers11  stated the fact that the Root-style functional foot orthoses actually improved function of the foot.11 It is my hope that, by following the above-described method of non-weight bearing neutral position suspension casting, regardless of the casting material or technology, that one can fabricate effective, reproducible custom functional foot orthotic devices or AFOs that resolve the patients symptoms and satisfy the practitioner's expectations.                                     Finished cast.  Finished cast.                                                                                    

Dr. Stess is President and Co-Founder of the STS Company in Mill Valley, Calif. He is the former (retired) Chief of Podiatry and Residency Director of the VA Medical Center in San Francisco, a former (retired) Associate Professor at the California School of Podiatric Medicine at Samuel Merritt University and a former (retired) Assistant Clinical Professor at Hadassah Medical Center in Jerusalem, Israel.

Online Exclusives
By Richard Stess, DPM
References
  1. Root ML, Orien WP, Weed JH. Neutral position casting techniques. Los Angeles:Clinical Biomechanics Corp;1971.
  2. Kirby K. Demonstration of neutral suspension negative casting technique. Available at: https://www.facebook.com/kevinakirbydpm/videos/580679709206494 . Published April 10, 2020. Accessed April 21, 2021.
  3. Chuter V, Payne C, Miller K. Variability of neutral position casting of the foot. J Am Podiatr Med Assoc. 2003;93(1):1-5.
  4. Payne C, Munteanu S. Position of the subtalar joint axis and resistance of the rearfoot to supination. J Am Podiatr Med Assoc. 2003;93(2):131-135.
  5. Richie Jr D. Study supports root theory principle of neutral suspension casting for orthoses. Podiatry Today. Available at: https://www.podiatrytoday.com/blogged/study-supports-root-theory-principle-neutral-suspension-casting-orthoses . Published November 06, 2019. Accessed April 2, 2021.
  6. Lee WC, Lee CK, Leung AK, Hutchins SW. Is it important to position foot in subtalar joint neutral position during non-weight-bearing molding for foot orthoses? J Rehabil Res Dev. 2012;49(3):459-66.
  7. Kennedy S. Casting for foot orthotics – what works the best? The O & P Edge. 2004. Available at: https://opedge.com/Articles/ViewArticle/2004-08_08. Accessed April 29, 2021.
  8. Williams B, Fuller E. Scanner Casting: Is It Better Than Plaster Impression Casting? Podiatry Today. 2015;28(6):50-55.
  9. Elveru RA, Rothstein JM, Lamb RL, Riddle DL. Methods for taking subtalar joint measurements. A clinical report. Phys Ther. 1988;68(5):678-682.
  10. Root J. Improving Negative Casts for Orthosis. Pedorthic Association of Canada PAC Symposium 2015. April 17-18, 2015. Vancouver, BC. Available at: http://www.pedorthic.ca/wp-content/uploads/2014/10/2015-PAC-Program.pdf . 
  11. Becerro de Bengoa Vallejo R, Sanchez Gomez R, Iglesias ME. Clinical improvement in functional hallux limitus using a cut-out orthosis. Prosthet Orthot Int. 2016;40(2):215–223.

 

 

 

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