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Keys To Assessing Orthotic Function

Sharing insights from his clinical experience as well as relevant studies, this author discusses pertinent principles in evaluating orthoses, ranging from the longitudinal arch contour and hallux dorsiflexion to heel cup fit and midtarsal joint motion.

How many podiatrists assess and evaluate orthotics before their patients try them out? Very few, I would guess. Is there an advantage for both the doctor and the patient to a five-minute assessment? What criteria could better predict if patients will be comfortable and if the orthotic will reduce their symptoms?

If I could take you back to your training in the clinic, your clinician probably asked you, after dispensing orthotics to your patient, to describe the “fit and function” of the devices. I believe it is important to follow this procedure in your practice now, not only to be better prepared to predict the outcome but also to avoid patient complaints on the return visit.

There are only a few “fit and function” evaluations for a new orthotic: longitudinal arch contour and contact; hallux dorsiflexion with and without the device; heel cup fit; and reduction of midtarsal joint motion in gait. These are easy to relearn, of great value in developing patient confidence in your devices and easy to teach to staff.

Evaluating Longitudinal Arch Contour And Contact
This is sometimes called the “Goldilocks” criteria simply because the arch of the orthotic should not be too low as it would create a gap between the device and the foot. The arch of the orthotic also should not be too high as this can lead to blanching of the skin in the arch when the patient stands on the device.

The most common causes of a gapping at the longitudinal arch are poor selection of positive arch fill technique, poor negative casting technique or orthotic laboratory shortcuts in making the device.

Most orthotic laboratories have three arch fills. This is the amount of plaster or virtual plaster in a digital system that is over the original positive cast. More fill (maximum fill) lowers the arch away from the foot. Minimum fill raises the orthotic into the foot for better control. Pathologies like severe osteoarthritis, severe equinus or diabetic neuropathy require the orthotic to be slightly less “tight” and the resulting space between the orthotic and the foot is usually filled in with a softer material.  

Inadvertently supinating the midtarsal joint during casting or scanning, which is also inverting the forefoot on the rearfoot when it should be pronated, will force your orthotic lab to lower the arch of the orthotic away from the foot when the patient stands on it. If you have more forefoot varus in your cast than the patient has in the foot, you have supinated the midtarsal joint and your orthotic will be too flat (or, as some people say, “sloppy and uncomfortable”) at the posterior area of the arch where it enters the heel cup. If you do not support the forefoot properly, all the weight will shift to the rearfoot, causing proximal arch pressure and pain. The standard in most cases is no gapping and no perceived pressure, or as Goldilocks said, “just right.”  

For example, as one can see in the photos at left, by inadvertently inverting the forefoot when casting a patient for orthoses, this supination of the midtarsal joint produces a contour that gaps at the arch and moves the pressure proximally. The medial arch of the orthoses should match the contour of the distal medial arch.

In order to keep the price of orthotics low, many orthotic labs have converted to non-custom or library systems without telling their doctors. Those systems (rather than adding lateral expansion, arch fill and balancing forefoot to rearfoot) select a “close enough” shape from a library of pre-existing shapes that are supposedly balanced and corrected. These systems often produce an orthotic that has a low longitudinal arch, which is supposed to make your patients’ orthotic more comfortable. Statistically, I have found only a third of the time is the arch contour just right. Some are better, some are worse, some work and some do not. Ask your lab if it uses a library system for your account.

What The Literature Reveals About Hallux Dorsiflexion And Orthotic Assessment
Assessing hallux dorsiflexion is vital when treating patients with plantar fasciitis or functional hallux limitus. The literature demonstrates that if your orthotic stabilizes the midtarsal joint, dorsiflexion of the hallux should increase substantially in stance.1,2

Roukis and colleagues presented a quantitative analysis of the effect that first ray position has on the motion of the first metatarsophalangeal joint (MPJ).1 Using a goniometer to measure the degrees of first MPJ joint dorsiflexion with the first ray in three positions, the study authors noted the first MPJ dorsiflexion decreased 19 percent as the first ray moved from the weightbearing resting position to 4 mm dorsiflexed, 19.3 percent as the first ray moved from 4 mm dorsiflexed to 8 mm dorsiflexed and 34.7 percent as the first ray moved from the weightbearing resting position to 8 mm dorsiflexed.

In a study of 48 feet in 27 patients, my colleagues and I examined whether orthoses made from a cast with the first ray plantarflexed and a 4 mm medial skive could increase the maximum degree of dorsiflexion in patients with functional hallux limitus in stance and gait.2 We casted patients for orthoses with the first ray plantarflexed and in the customary neutral rearfoot position with a locked midtarsal joint. We saw significant changes in the mean maximum dorsiflexion in stance and in the mean maximum sub-hallux pressure in gait with orthoses.

Preventing the midtarsal joint from supinating is what prevents stretch on the planar fascia and compression on the first MPJ. Kogler and colleagues implanted a differential variable reluctance transducer (strain gauge) in the plantar fascia of several cadaveric limbs and applied a weightbearing load to the tibia.3 They found when they applied a varus wedge to the forefoot, therefore supinating the midtarsal joint, the strain increased on the plantar fascia. Conversely, when they applied a valgus wedge, pronating the midtarsal joint, they decreased the strain. A properly casted and made orthotic should pronate the midtarsal joint and relieve tension on the plantar fascia, therefore allowing greater dorsiflexion of the hallux.

It only takes a moment to stand the patient up on a platform without the orthotic, forcibly dorsiflex the hallux and approximate the range of motion. Then place the orthotic under the foot and repeat the procedure. The hallux should dorsiflex a greater number of degrees. If this test is not positive, your outcomes with heel pain and first MPJ pain will be very poor. Test the hallux dorsiflexion without the device first and then test it again with the patient standing on the device (see right photo).

What You Should Know About Heel Cup Fit
Heel pad expansion on weightbearing in comparison to non-weightbearing is difficult to predict. The literature actually says it is a roll of the dice and varies so greatly that there is no relationship when one uses a non-weightbearing cast or image to make an orthotic.4 An orthotic should obviously have the same medial to lateral heel cup width as the weightbearing width of a heel. It is not within the orthotic laboratory’s control to predict the heel width every time. Therefore, it is important to check if the heel fits into the orthotic before dispensing the device.

A simple solution to improve heel width weightbearing is to measure the heel with calipers or by having the patient stand on a paper copy of a ruler and writing the width on the prescription. Orthotic laboratories with digital custom orthotic design capabilities can then dial in the correct width of the heel cup.

Emphasizing The Importance Of Midtarsal Joint Motion
A properly casted, balanced and corrected orthotic should stabilize the midtarsal joint in gait. If there is anything you should do to see if your orthosis is working properly, for most mechanically-induced pathology, it is watching the patient walk after dispensing the device.

The midtarsal joint in most pathologies continues to move after forefoot contact but before heel lift. This is a major biomechanical fault that causes stress and trauma. Observing that this motion has been reduced or eliminated with the orthotic in the shoe is valuable in determining the effectiveness of the orthotic and the appropriateness of the shoe gear. Watch to see if the foot remains immobile after forefoot contact but before heel lift. This means the device is in proper contact with the foot.

These four simple observations have the potential of not only improving your practice protocols that deal with orthoses but also may greatly improve your patients’ clinical outcomes.
    
Casting Observations And Hints To Ensure Better Fitting Devices
There are several things any clinician can do that will avoid a poor fitting orthotic when casting or scanning a foot. An effective, proper and comfortable orthotic fit begins with the casting. Here is some advice and the reasons this advice works.

Do not use foam box casting. Regardless of what you have been told, you cannot receive the same orthotic if you use a foam box for casting as you would with neutral position casting. The upward pressure of the foam as the patient pushes down inverts the forefoot on the rearfoot, creating a fake forefoot varus when there is none. The artificial forefoot varus, when the orthotic laboratory corrects it, will produce an orthotic that is very comfortable but totally ineffective because the arch of the device will gap from the arch of the foot distally.

Also, it is almost impossible to find the bisection of the heel in the foam since what looks like the back of the heel is actually the plantar aspect of the heel passing through the foam on its journey down. If a laboratory cannot bisect the heel, it cannot balance or correct for the forefoot to rearfoot deformity, and you will wind up with a device that basically holds the foot in its pathological position.

Dorsiflex the hallux or gently plantarflex the first ray. It is often human nature when someone grasps your toes or foot to contract your tibialis anterior. Try it and watch the tendon bow at the anterior ankle. This contracture dorsiflexes the first ray of your patient’s foot and plantarflexes the hallux. A dorsiflexed first ray affects the contour of the medial arch and, as I described earlier, creates a higher forefoot varus or a false forefoot varus. Slightly dorsiflexing the hallux with your other hand prevents this deforming contracture and will produce a more effective and comfortable orthotic.

In the photos at the left, note the difference in the rearfoot and forefoot osseous alignment with and without an orthotic.

Dorsiflex the forefoot on the rearfoot. In the non-weightbearing position, the forefoot can dorsiflex and plantarflex on the rearfoot. If you do not consciously dorsiflex the forefoot until the ankle joint is loaded when casting or scanning, the resulting device will have a medial to lateral arch that is much too high to be comfortable. It is very difficult for an orthotic laboratory to see this mistake and the lab will make a device with a much higher arch.

Final Thought
The key to easy orthotic dispensing and avoiding adjustments is to pay attention first to the position of the foot when casting or scanning.

Dr. Scherer is a Clinical Professor at the Western University of Health Sciences College of Podiatric Medicine in Pomona, Calif. He is also the CEO of ProLab Orthotics/USA.

References

  1.     Roukis TS, Scherer PR, Anderson CF. Position of the first ray and motion of the first metatarsophalangeal joint. J Am Podiatr Med Assoc. 1996; 86(11):538-46.
  2.     Scherer PR, Sanders J, Eldredge D, Duffy S, Lee R. Effect of functional foot orthoses on first metatarsophalangeal joint dorsiflexion in stance and gait. J Am Podiatr Med Assoc. 2006; 96(6):474-81.
  3.     Kogler GF, Veer FB, Solomonidis, SE Paul JT. The influence of medial and lateral placement of orthotic wedges on the loading of the plantar aponeurosis. J Bone Joint Surg Am. 1999; 81(10):1403-13    
  4.     Fuller E, Hogge JD. Measurement of the expansion of the calcaneal fat pad upon weightbearing. J Am Podiatr Med Assoc. 1998; 88(1):12-16.
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