Foot Orthoses For Chronic Ankle Instability
In light of an increasing body of research on the
Is a Virtual Cast Equivalent to a Plaster Cast?
A lot of practitioners are hesitant to switch from the plaster casting technique to a laser scan. That thinking is perfectly understandable. A plaster cast of a foot in a non-weight bearing neutral subtalar joint position with the midtarsal joint locked, demonstrates a corrected position of the foot with an exact shape of the plantar surface. This has always been the gold standard for the fabrication of orthotic devices in the podiatry profession. In fact, plaster casting has differentiated podiatrists and chiropodists from other practitioners, who have taken on orthotic prescription. It is common practice to have patients stand on a weight-bearing pressure pad, which shows a technical computer graphic of the pressure points of the foot on the screen.
The problem with that methodology is that the foot is captured in a deviated position and not a corrected position. A pronated foot when weight-bearing won’t show the higher arch shape required as would a non-weightbearing cast. The orthotics created in those cases of a weight-bearing impression are often based on guesswork. Hence the plaster cast has been the most accurate casting methodology. Orthotics fabricated from an improper casting technique can result in the continuance of overuse syndromes. Frequently patients present to an office complaining that their orthotics haven’t been helpful. If you place the orthotic up against the patient’s foot, you may see a big gap between the arch height of the patient’s foot and the device itself. Likely this is due to their orthotics being fabricated from an improper cast. With plaster casting, biomechanical foot imbalances are corrected since the foot is captured in a nonweightbearing neutral position.
So what about a scan for casting?
Is a virtual cast equivalent to a plaster cast?
When the Sharpe Shape laser scanner came out about 15 years ago, I was skeptical since having trained in California, and indoctrinated into the plaster casting methodology, a scanner seemed too easy and not as accurate. Dr. Glenn Cornwell, who founded Paragon Orthotic Laboratory, convinced me to try out a laser scanner. My suggestion at the time was that we do an informal study to compare results with both plaster casts for a patient and a laser scan and see how, or if, the results would differ. We did two pairs of orthotics- one pair from a laser scanner and the other from the plaster casts submitted. I discovered that the results were the same!
The key with the laser scan or any 3D scan (as with plaster casting) is to hold the foot in a corrected non-weightbearing, neutral subtalar joint position, and by dorsiflexing the fifth metatarsal head you can lock the midtarsal joint to represent the reactive force of gravity. That should result in an accurate virtual cast. Basically, the foot is held in the same neutral position as you would when casting with plaster splints. Some practitioners ask the patient to simply hold their foot in this position while they take the scan. The patient can the see the virtual cast of their foot on the screen, which you can move around to show how the foot pronates when weight-bearing. Many patients are impressed by the technology, and understand that the image on the screen would indeed be a virtual representation of a real cast of their foot. The virtual cast will also show forefoot varus or valgus or a plantarflexed first ray. The patient’s plantar foot shape should be readily visualized in the virtual cast.
After 15 years of using only a laser scan to have orthotics fabricated, I can tell you that the results with the orthotics from Paragon Labs have been consistently excellent, and no different from when I was using a plaster cast exclusively for so many years prior.
The there are numerous advantages of a scanner for your practice:
- Quick and easy, and there is no plaster mess (that’s a huge advantage)
- No waiting for casts to dry. We used to add alum powder to the water in order to accelerate the drying process, but it was still time consuming waiting for the casts to try enough so as to not get distorted with removal.
- No draping the floor with paper sheets
- No longer a need for a plaster trap under the sink. A messy apparatus that has to be cleaned out periodically (if you use a plaster casting method without a plaster trap, it’s just a matter of time before your pipes are clogged).
- Plaster splints are costly
- No longer a need to carefully package a box of plaster casts and ship them , which adds time and money, risk of loss in the post.
- Faster turnaround time
Following a scan, you simply email the scans to the lab and your results will be just as good as when you took plaster casts. Nothing else changes with respect to your biomechanical exam, measurements and the Rx for the lab.
Once you switch, you’ll wonder why you didn’t sooner. It’s interesting that many patients are now well informed and read that they should have a plaster cast when getting orthotics. However, they can readily understand when you explain to them that a non-weightbearing neutral position virtual cast, and plaster cast, are the same (just without the mess). Of course, some practitioners are comfortable with the way they do things, happy with the results, and don’t want to change. Nonetheless, switching to a 3D scanner would be an easy change to make, and something worth considering to make your life easier.
By Dr. Lloyd Nesbitt, DPM
Click to download the ParagonPro Scanner app.
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