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Board Review Orthotics & Prosthetics

Casting for Neutral Orthotics

Robert Phillips, DPM

Robert D Phillips, DPM shows the technique for casting for a traditional neutral position foot orthotic as originally described by Dr. Merton L. Root. It is traditionally accepted that capturing the correct shape of the foot is the most important step in manufacturing an orthotic that is comfortable and effective. Dr Phillips pays particular attention to many of the small details in positioning the patient and also the casting person when followed makes it easy to capture an accurate cast. He also goes through the evaluation process after casting to ensure accuracy.

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Goals and Objectives
  1. Describe how to position a patient so that they feel relaxed and so that minimal effort is needed by the practitioner to place the subtalar joint in neutral position and lock the midtarsal joint
  2. Describe the proper arm and hand position for the practioner casting the foot
  3. Describe how improper practioner techniques create errors in the cast.
  4. Describe how to evaluate a cast to ensure that the neutral foot shape has been correctly captured.
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  • CPME (Credits: 0.75)

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    Release Date: 03/16/2018 Expiration Date: 12/31/2018

  • Author
  • Robert Phillips, DPM

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  • Lecture Transcript
  • Male Speaker: Hello. I'm Dr. Phillips, team leader for the podiatry section at the Orlando VA Medical Center in Orlando, Florida. Today, we will be discussing the casting process for a standard root type orthotic. The casting process for neutral orthotics is the most important step in creating both a functional and a comfortable foot orthosis. Most cases of a patient not feeling that an orthotic is completely comfortable are due to inaccuracies in the casting process. Therefore, it is critical that practitioners consistently work on refining their casting technique. There are four critical steps in the casting process that we will review in this lecture. Very little time will be spent on the first as I have discussed this in a previous lecture. The second is the pre-plaster application phase in which you will get the patient properly positioned and practice your casting. The third is the plaster application itself in which I will show you some of the secrets I have found to make it easier. The fourth is the evaluation of the cast to make sure you have a good cast. Before beginning the casting process, you must identify any bow stringing of the plantar fascia, dorsiflex the hallux of each foot and look for any signs of medial plantar fascia bow stringing. If you see such, mark the medial and lateral borders of the bow stringing with the pant, so that it will transfer to your cast and the lab can make a plantar fascial band accommodation in the orthotic. If you fail to do this, you run the risk that your orthotic may induce a plantar fibroma. The second step in the casting process is to properly position the patient. If you do this correctly, you will have a very easy time with the casting process. Before you actually apply the plaster, you will practice holding the foot in the correct position to make sure that you will have no difficulties after applying the plaster. The casting for the orthosis relies first and foremost on the patient feeling comfortable and relaxed. Because there are no uniarticular extrinsic or intrinsic muscles of the foot, any muscular contraction by the patient while casting for orthotics is going to distort one or more of the joints of the foot and give you an inaccurate cast. You should have a patient sitting comfortably in a casting chair or lying comfortably in the supine position. If the patient is sitting, the knees should be 20 degrees flexed, which takes the stretch off the hamstrings and makes the patient feel comfortable. Then tilt the chair backwards until the patient tells you they are very comfortable. If you have a chair with an adjustable headrest, position it for maximum patient comfort. If you do not, then a pillow to support the head and neck will help your patient to feel more relaxed. Again, I emphasize that you cannot produce an accurate cast of the foot if the patient, in any way, tries to help you. I found that a great many people when they feel you lift their foot, they automatically contract the anterior tibialis or other dorsiflexion muscles of the ankle. This seems to be more of a reflex. And I have found that the greater the dorsiflexion force against the forefoot, the more likely you are to trigger this reflex. You will learn in this lecture that you do not have to dorsiflex the ankle joint to 90 degrees and that it takes only a small amount of force to lock the midtarsal joint. And here are some other things you could to help your patient relax. You may need to flex the knee more than 20 degrees. Instead of asking the patient to relax the foot, ask them to concentrate on relaxing the knee, ask the patient to close their eyes during the casting process, create a relaxing atmosphere in the office. This means decreasing the noise level, decreasing intensity lighting and maybe playing some soft music. The next important factor in creating a good cast is for the person taking the cast to exert minimal effort and not to fatigue. This means that you, the cast-taker, need to find the position of maximum comfort. And then you will adjust the patient's foot to match your comfort. To do this, hold your arm in front of you at a comfortable height with the wrist and fingers straight. Find that position that requires minimal effort, a position that you can hold for about five minutes without your arm or hand tiring.

    [05:06]

    If you have a suitable casting chair, it is best to stand while casting. However, if you are exceptionally tall or don't have adequate height control with your chair, you'll have to sit while casting. Next, you must find out at what height the foot has to be. To do this, you need to hold your arm in its most comfortable position in front of you and now, elevate the chair until the web space of the foot is at the same level as the web space between your thumb and forefinger. Now, move the subtalar joint through its range of motion. Put your thumb onto the fourth and fifth metatarsal heads. If you're casting the patient's left foot, use your right hand. And if you're casting the right foot, use your left hand. To pronate the foot, place some straight abduction force against the forefoot. To supinate the foot, place a straight adduction force against the forefoot. As you move the subtalar joint through its range of motion, you'll feel it move through an arch of motion. Neutral position is that point where it feels that you have reached the bottom of the arch of motion. An experienced clinician can become very sensitive to this bottom of the arch feeling and could find neutral position with his or her eyes closed without needing to palpate for congruency of the talonavicular joint or the subtalar joint. I'd like to make a pointer about moving the subtalar joint and finding neutral position. I would recommend that you not try to dorsiflex or plantar flex the foot when moving the subtalar joint. On the left, you see the correct technique of moving the subtalar joint with a straight abduction and adduction force. On the right, you see an incorrect method of moving the subtalar joint by using a plantar flexing and dorsiflexing force. Remember, there's very little sagittal plane of motion in the subtalar motion. And when the subtalar joint moves in the transverse plane, it will concurrently move in the sagittal plane. Making the error of moving the ankle joint with the subtalar joint will markedly decrease your sensitivity in finding neutral position. The standard root orthotic requires that the midtarsal joint be pronated to the end of its range of motion. This is then by dorsiflexing the fourth and fifth rays to their end range of motion. Now, how much force is needed to pronate the midtarsal joint to its end range of motion? The average foot weighs between one and one-and-a-half percent of the total body weight. So that means then average persons will have a foot that weighs between two and three pounds. This means that you only need to apply about one-half percent of body weight against the fourth and fifth rays to fully pronate the midtarsal joint. For an average person, it takes only eight to 16 ounces of dorsiflexion force against the forefoot to pronate the midtarsal joint. Many people put too much force against the midtarsal joint and over-pronate it and produce a faults forefoot valgus. Now, that you know where subtalar joint neutral position is, one of the most critical parts of the whole setup process is to get the leg properly positioned. So that when the subtalar joint is in neutral position, the foot is vertical to the ground. Most of the casting errors that occur can be prevented by this one very important step. Put the subtalar joint in neutral position. And most people will show the foot abducted from being vertical. Therefore, on most people, you want to internally rotate the leg until the foot is vertical to the ground when the subtalar joint is in its neutral position. Now, you want the people to be able to hold this hip and leg position while staying completely relaxed. For many people, this leg rotation can be maintained comfortably by merely having them lean on the arm of the chair that is opposite the one you are casting.

    [10:04]

    Other people have limitations of internal rotation. And for those people, I put a pillow or some towels under the hip that I'm casting. Remember again that the patient needs to be completely relaxed when the leg is rotated into this correct position. Now, that you have the leg properly rotated, you can easily pronate the midtarsal joint by gently lifting and dorsiflexing the foot vertical to the ground. If you have properly rotated the leg, you can do this with your eyes closed because any vertical force under the fourth and fifth metatarsal heads will automatically bring the subtalar joint to its neutral position. While many people do cast by pushing upward against the fourth and fifth metatarsal heads, I do not recommend it because you can dorsiflex the fifth metatarsal above it's neutral position, which runs the risk of creating Tailor's bunion pain. The most desirable bones to hold are the fourth and fifth proximal phalanges. I have outlined where the proximal phalanges are. That area between the metatarsal heads and the web space. Here, you see me holding the fourth and fifth proximal phalanges between the side of my thumb and my index finger. A common error that is made is to hold the toes distal to the web space. If you do this, you will be grasping the middle and distal phalanges. And if you do this, you are more likely than not going to end up plantar flexing the fourth and fifth metatarsals. Plantar flexing the fourth and fifth metatarsals will resolve in a cast that has an inverted forefoot deformity. While mildly dorsiflexing the forefoot against the rearfoot, plantar flex the fourth and fifth MPJs back to their neutral position. You will notice that I can do this by rotating my wrist slightly as if I was looking at my watch. I may also do this by a small movement at my first metacarpal joint. Sliding the metatarsophalangeal joint downward until the toes in the same position that it is when the fifth metatarsal is dorsiflexed. While holding the foot in the casting position, it is important that your entire arm be in line with the sulcus of the toes. Note the one, I look at the bottom of the foot. The sulcus, my thumb, fingers and elbow are all in a straight line. A common mistake is to lift the arm too high. If you raise your elbow higher than the sulcus line, you will produce an adduction force against the forefoot, which will supinate the oblique axis of the midtarsal joint. The arm and hand are also in line with the sulcus when you look at the foot from the distal ends of the toes. In order for you, the person taking the cast, to remain comfortable with the arm in this position, you may need to stand with your body within about six inches of the bottom of the foot. A common mistake that is made is to have the wrist flexed and the arm held plantar to the sulcus line. This would result in the long axis of the midtarsal joint being supinated and will produce an inverted forefoot deformity in your cast. Last, before I apply the plaster of Paris, note that I practice the casting process several times, note that my body is very close to the foot which prevents my arm from fatiguing. And also note that I don't have to lift the foot and dorsiflex the ankle joint very much to fully pronate the midtarsal joint. Now, that you have your patient correctly positioned and you know how the foot feels when you will be holding it, you are now ready to apply the plaster of Paris. Some people like to use two 30-inch plaster splints to cast the foot. I used to do this. However, for the convenience of our facility, I now use a roll of five-inch of plaster splint and cut it to fit the foot as you note here. I wrap it around the back of the heel and measure it just to be on the ends of the toes. After measuring, I then double the length and cut one piece. You will need two layers of plaster to get the proper strength of the cast. If the plaster is old or doesn't feel very stiff, I will use three layers.

    [15:02]

    Now, you are ready to wet the plaster. I use very warm water because I want my plasters to set quickly and I also want my patient to be as comfortable as possible. The colder the water you use, the slower the plaster will set and the more likely your patient will recoil when you first put it on the skin. The plaster sprint needs to be creamy smooth. However, you want to make sure that you lose the least amount of plaster into the water to maximize the strength of the plaster and also minimize the drying time. Note that in this movie that I just touched my plaster splint to the top of the water. I do not submerge it to the bottom of my bucket. I can assure everyone that the water at the top of the bucket is as warm and wet as the water at the bottom of the bucket. Also note in the movie that I leave about three inches at the end of the plaster splint dry. When I accordion-fold the splint after getting it wet and squeeze it, the excess water in the splint is squeezed into the dry ends and I have very little water and plaster dripping back into my ball. This preserves the plaster and the splint and also makes it for a very few plaster drips onto the floor, which will make clean-up a lot easier. Just squeeze in the plaster in your accordion-fold in your hand, pull it out straight and fold about one quarter to one half inches of the top over on itself to create a four-layer section at the top of the cast. This will make it much easier to take the plaster splint off your patient without distorting the cast. Note that we start by putting the plaster around the back of the heel, making sure that the top edge is brought up to the bottom of the malleoli. By bringing it this high, you will be capturing the full height of the calcaneus, which will make it much easier for the laboratory to accurately bisect the calcaneus. I gently bring the plaster forward and drape it around the distal ends of the toes. So it doesn't fall off the foot, initially. I then start smoothing the plaster into the medial arch so that it adheres to the skin. I like to start on the medial side because the plaster needs to be conformed more on the side. Make sure that you do not have too much tension on your plaster as you have brought it forward as this will cause the splint to pull away from the skin. Now smooth the lateral side of the plaster into the foot. If you have the medial side well adhered, you'll have little problem with the lateral side. Make sure there's a little overlap of the medial and lateral sides in the center so that the entire plantar surface of the foot is covered. When you are done smoothing the plaster on the bottom of the foot, smooth the excess plaster around the back of the heel. The excess plaster at the front is bunched up and into and around the sulcus as you see me do here on the right. Now, you are ready to hold the foot. Grasp the fourth and fifth proximal phalanges as you have already practiced and gently dorsiflex the ankle joint to resistance while slightly lifting the leg upward. Hold this as you have already practiced and has been described before until the plaster sets. Gently stroking the plaster on the bottom of the foot will help smooth it into the skin and also will help it set a little faster. While the plaster is setting up, watch the dorsal aspect of the foot. It is natural for some patients to want to help you hold their foot. They seem to think you're not strong enough to hold it and they will contract the anterior tibialis muscle. If you detect this, remind them to relax. We've already given you some hints for helping your patient relax. If they cannot relax or you did not detect this contraction, the long axis of the midtarsal joint will be supinated in your cast, which will mean that you will get a faults inverted forefoot deformity in your cast. If this happens, you have to recast the patient. After the plaster is set, you will let go of the foot. As you release the foot, you should feel the foot immediately drop. If you do not feel the ankle joint plantar flex, then you should assume that the patient has contracted one or more of the pretibial muscles and you will have to recast the foot.

    [20:06]

    After you have let go of the foot, allow a couple of minutes for the plaster to continue to set up before starting to loosen the cast from the foot. Tell the patient to just stay relaxed and not move. As the plaster sets up, the patient should feel the plaster start to separate from the bottom of the foot. This is totally normal. After a couple of minutes, assist the separation of the skin from the cast by gently pulling on the skin around the edges of the cast. After the cast has been totally separated from the foot, you are ready for the final step to evaluate your cast. When you first take the cast off the subject's foot, examine the inside of the cast to make sure that you do not have any separation of the plaster from the skin. Now, the picture on the right shows a close up of a portion of the inside of the cast, showing the transfer of the skin lines into the plaster mold. The entire inside of the cast should look like this. If you do not see the skin lines, you should recast the foot. The next step is to make sure that the forefoot to rearfoot relationship matches that which you measured before casting your patient. To do this, set the cast on a level surface with the toes hanging over the edge of the surface. Now, draw the bisection line on the posterior aspect of the calcaneus and measure the relationship of the calcaneal bisection to the ground. If the patient has a forefoot valgus, then the cast should be sitting with the heel inverted the same degree as the forefoot valgus. If the patient has a forefoot varus, then the cast should be sitting with the heel everted at the same degree as the forefoot varus. If your cast is not within two degrees of your measurement, then you should either recast or remeasure your patient as one of them is wrong. These two casts are the same patient who has a perpendicular forefoot to rearfoot relationship. On the left, the cast is with the midtarsal joint pronated. Note that the heel bisector is perpendicular to the ground. On the right, the cast has been supinated around the long axis of the midtarsal joint. This has caused the cast to sit on the ground with the calcaneal bisector everted from the perpendicular. In this particular cast, the practitioner has dorsiflexed the fourth and fifth toes at the metatarsophalangeal joints. This has resulted in the fourth and fifth metatarsals being plantar flexed, which is induced in the inverted forefoot deformity into the cast. The practitioner needs to recast the patient making sure that he or she grasps the proximal phalanges of the toes to prevent the toes from dorsiflexing when he or she dorsiflexes the ankle joint. Now, place the foot back into its casting position and look at the bottom of the foot, holding your cast up against the foot. Compare the lateral border of the cast with the lateral border of the foot. They should mirror each other. If the lateral border of the cast is more adducted than the lateral border of the foot, then you know you have supinated the oblique axis of the midtarsal joint. If the lateral border of the cast is more abducted than the lateral border of the foot, then you know that you have pronated the subtalar joint. Now, do the same thing looking at the lateral side of the foot. Place the foot in its casting position and compare the curvature on the lateral side of the arch with the lateral curvature on the cast. If the arch is higher on the cast, then you have supinated the oblique axis of the midtarsal joint. If the arch is lower, then you have pronated the subtalar joint. Again, look at the metatarsal head prominences and make sure that the roundness plantarly matches the foot in its casting position. In this particular case, note that the metatarsal heads have too much curvature, which means they are plantar flexed. If an orthotic is made over this cast from any type of hard material, the cast will feel too high in the arch.

    [25:03]

    The final evaluation is the most difficult because it involves a good deal of subjective evaluation. You are going to evaluate a cross-sectional shape of the cast at the mid-arch. For this illustration, I have cut the cast to make it easier to see. On the left, I have a cast in which the subtalar joint was neutral and the midtarsal joint was pronated. I visually divide the cast into thirds. The lateral third is now evaluated as having a very smooth convexity from medial to lateral, which approximates the area under the cuboid. On the right is a cast where the long axis of the midtarsal joint was supinated. Again, the cast is divided into thirds. In this case, the lateral side curvature is much narrower and the lateral side appears to come more to a point. Also note that the arch is higher in the supinated midtarsal joint cast. Your patient will feel this arch height and complain of discomfort. As this evaluation is very much an art, the practitioner should practice taking casts with the midtarsal joint pronated and supinated to become familiar with the difference in the cross-sectional curvatures across the cuboid when the joint is in these two different positions. In summary, I'd like to make the following points. Number one, very small differences in the shape of an orthotic can mean very big differences between orthotic success and failure. Precision and accuracy of the cast can make it very successful and non-precision sets you up for failure, which all the posts and skives cannot correct. I would trust that the resident should give as much care to learning to practice casting as they do to learning surgery. Number two, I hope I have emphasized the importance of proper patient positioning and also the proper holding techniques of the foot. If you find yourself fatiguing while casting, then you should consider that either you or the patient are not properly positioned. Number three, casting a foot is not something that you can learn by doing once or twice. As the years progress, you will refine your technique and refine your sense of detecting the perfect amount of force needed to be placed on the foot so that you can get the perfect cast every time. I would like to thank Present and also the sponsor of this lecture for the opportunity of giving this presentation. I welcome your questions and comments at any time. Good luck to you all.