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How To Create A Twisted Plate Orthotic From A Flatbed Scan Of The Foot

In my last blog post, I talked about the twisted plate orthotic discussed by Sarrafian and my use of lateral forefoot postings on those devices.1 Doug Richie, Jr., DPM, FACFAS then informed me that McConaill first mentioned this concept in 1945.2 

Regardless of who came up with the concept first, I would like to explain this month how you can create a twisted plate device using a flatbed scan of the foot. To do that, I’d like to briefly explain the difference between neutral forefoot casting on a flatbed scanner versus casting in a non-weightbearing neutral subtalar joint (STJ) position. 

I have touched on this concept a few times in previous articles comparing 3D scanning versus plaster casting for foot orthotics.3-5 Essentially, when you cast on a flatbed scanner, the lateral forefoot cannot maximally pronate or dorsiflex when the STJ is in the neutral position. 

Here one can see the pre-casting position of a foot without lateral column dorsiflexion while the subtalar joint is neutral. This is the same position as one would capture with the foot in partial weightbearing on a flatbed scanner.

Obviously, the scanner glass is flat, which keeps the plantar aspect of the foot flat as well during the scanning/casting process.

In both the photo immediately below and the photo above, you can see the off-weightbearing foot with the lateral column maximally dorsiflexed as well as what the same foot position would look like from the plantar aspect of a flatbed scanner. In the non-weightbearing process, practitioners usually will maximally dorsiflex the lateral column to resistance while holding the STJ in neutral position (see photo below). This means essentially dorsiflexing the lateral column of the foot, usually at the fifth MPJ to resistance without moving the STJ out of neutral position. 

In this photo, one can see the position of the foot prior to casting in a neutral non-weightbearing position with lateral column dorsiflexed.

These same practitioners, especially if they have been casting with this method for many years, usually balk at trying to cast the foot in a partial- or full-weightbearing position because of this detriment in the casting process or for other reasons. They can usually figure out how to do this for a non-contact 3D scan but most clinicians, if not all, are at least initially uncomfortable with the partial-weightbearing technique with full contact of the foot to the scanner.

In the photos immediately below, you can see that the foot on the scanner has a three mm felt wedge under the fifth ray, simulating the amount of dorsiflexion one can accomplish in the non-weightbearing cast technique. Obviously, some patients have more flexible/less stiff lateral columns that are able to achieve dorsiflexion greater than three mm. 

This photo illustrates lateral wedging of the fifth ray with three mm of felt on a flatbed scanner with foot in partial weightbearing and neutral subtalar joint position.In this photo, one can see a lateral view of three mm felt wedging of the fifth ray in partial weightbearing and neutral subtalar joint position.

What I am advocating for here is that if you know how much range of motion the lateral column has in comparison to the stable third MPJ, you can then wedge the lateral forefoot for approximately that amount during the production of the orthotic itself, and it will give you essentially the same result as the non-weightbearing cast modified by the lab. 

Yes, we know the foot will elongate slightly when one casts with partial weightbearing on a scanner and that the medial arch may also be lower in comparison to non-weightbearing casting. However, keep in mind that orthotic labs add between three and six mm of cast or arch fill to orthotics to account for this non-weightbearing cast issue. In my opinion, I don’t think I usually find a need to adjust my casts quite as much as when I cast in partial weightbearing. Regardless, it is up to the practitioner and his or her labs to make the final judgement in most cases.

Remember that Kogler and colleagues showed that lateral wedging of the forefoot of a foot orthotic worked better at reducing plantar aponeurosis strain than a medial heel post (see illustration below).6 

This illustration demonstrates a lateral forefoot post modification to a shoe insole. (Illustration courtesy of Kevin Kirby, DPM)
Illustration courtesy of Kevin Kirby, DPM

Next month. I will discuss in detail how to measure the excursion of the range of motion of the lateral column. Once we all understand that, then adding the correct amount of lateral wedging becomes a much easier process. 

References

  1. Sarrafian SK. Functional characteristics of the foot and plantar aponeurosis under tibiotalar loading. Foot Ankle Int.1987;8(1):4-18. 
  2. MacConaill MA. The postural mechanism of the human foot. Proceedings of the Royal Irish Academy. 1945;L(B):265-278.
  3. Williams B. Key insights on digital casting techniques. Podiatry Today. 2010;23(1). Available at: https://www.podiatrytoday.com/key-insights-on-digital-casting-techniques . Published December 30, 2009. Accessed June 8, 2020.
  4. Williams B, Fuller E. Scanner casting: is it better than plaster impression casting? Podiatry Today. 2015;28(6):50-55. 
  5. Williams B, Root J. Is Scanning More Effective Than Casting For Custom Orthoses? Podiatry Today. 2020;33(5). https://www.podiatrytoday.com/scanning-more-effective-casting-custom-orthoses . Published May 18, 2020. Accessed June 8, 2020.
  6. Kogler GF, Veer FB, Solomonidis SE, Paul JP. The influence of medial and lateral placement of orthotic wedges on loading of the plantar aponeurosis. J Bone Joint Surg Am. 1999;81(10):1403-1413.
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