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What You Should Know About Orthotic Materials And Torsional Stiffness

Many podiatrists choose orthotics materials based on personal preference or practices learned during training. This author highlights pertinent biomechanical and scientific principles in determining the role and effect that stiffness has in orthotic construction and function.

The use of in-shoe orthotics has become the backbone for treating multiple problems that afflict the musculoskeletal system. Clinicians have utilized orthotics in the successful treatment of many foot pain syndromes as well as ankle, knee and lower back pain.1-6 However, the research into orthotic function and treatment results is often conflicting. Many studies show significant changes in the kinetics of the foot and very mild changes in the kinematics of the foot.7-10 Other studies show little, if any, changes. As any clinician knows, there are wide variations in these changes, both subjectively and objectively.11

There is also a debate as to whether one needs to use custom orthotics for people with symptoms or whether inexpensive prefabricated orthotics are just as effective.12-14 In examining various studies on this subject, it is clear there are wide variations in the types of materials researchers are utilizing in fabricating orthotics. Most of the study authors classify the materials they use as either “rigid” or “semi-rigid.”

The materials that researchers have utilized in the studies include polypropylene, polyethelene, acrylic, fiberglass, carbon fiber composites, ethylene vinyl acetate (EVA), various proprietary materials as well as other types of cork, foams and laminates. Thicknesses of materials have ranged from 2.0 mm to 5.0 mm. In none of the research articles have study authors adjusted material type or thickness for patient weight, foot length or foot type. Additionally, none of the authors define the terms “rigid” or “semi-rigid.” 

Several years ago, while this author was preparing a grant proposal, he personally contacted 20 investigators who had published studies on orthotic function and asked them if they utilized some type of algorithm in selecting the orthotic material or the thickness of the material. None of the researchers contacted had any type of algorithm for doing so.

He then subsequently took a similar informal survey of 20 colleagues within the same health system and found that individual practitioners have only personal preferences for orthotic materials based on previous experience and the preferences of those under whom they had trained. In looking at the orthotic prescription forms used by many orthotic laboratories, one finds that some do give some guidance for orthotic material thickness based on patient weight but there is no research to back up these guidelines. Likewise, there is little research to set standards for material type and thickness based on foot deformity, symptoms, foot dimensions, foot morphology or biomechanical measurements.15

Understanding Orthotic Mechanics And Foot Contact

When looking at why an orthotic works or does not work, one has to consider that a patient is expected to put an insert into a shoe, and that insert has to push up under the foot in a way that changes its kinematics and kinetics so the patient walks with fewer symptoms and greater efficiency. The mechanics of the orthotic during the gait cycle are poorly understood. In considering basic mechanics, for any orthotic to push up against the foot, it first has to make contact with the foot. If it does not make contact with the skin, it cannot push. (See first photo above).

The non-weightbearing casting methodology was developed to capture the shape of the bottom of the foot when there is no force between the ground and the foot. This means there is no compression of the plantar soft tissues and the maximum distance of the cast is between the bottom of the heel and the highest point of the arch. When a person steps on this orthotic, the highest point of the arch makes contact at the same time with the bottom of the heel. If one makes the mold from a partial weightbearing cast of the foot, the soft tissues are compressed and there is less distance from the bottom of the heel to the highest point of the arch. This means that the bottom of the heel will hit the orthotic before the arch hits the orthotic, which markedly decreases the ability to start supporting the arch.

What You Should Know About The Stress-Strain Curve and Orthotic Materials

An important principle to consider for any material to start exerting force against the bottom of the foot is that as a material is loaded with force, it begins to deform. A basic tool for all material engineers is the stress-strain curve. This is a plot with stress on the y axis and strain on the x axis. The stress is usually expressed in terms of pounds per square inch (psi). In utilizing the metric system, one may use the term Pascals (Pa), which is 1 newton/square meter. An easy conversion is that 100 KPa is equal to 1 kg/cm2.

Strain, which is the same as saying deformation, is expressed as a percent of the original dimensions. For an elastic material, the stress-strain curve is a straight line, the slope of which is called Young’s modulus of elasticity. Given that only the y axis has units, Young’s modulus is also usually expressed in psi or Pa units. The importance of this to the podiatrist is that if one knows the amount of force being exerted on a material, one knows how much it deforms and vice versa.

Applying this principle to the functional foot orthosis, the orthotic must push up against the bottom of the foot. The only way it can do so is for it to deform from its original shape. If there is zero deformation, there is zero force being exerted by the orthotic against the foot. Although many practitioners call orthotics “rigid” or “semi-rigid,” there is no such thing as a rigid orthotic (i.e. one that does not deform at all when one steps on it).

When one examines the literature on this subject, most of the authors report that orthotics do not push the foot to its neutral position when the patient stands on it.16 However, this should not be surprising to any materials engineering student. This is because the orthotic may have a perfect conformity to the bottom of the neutral foot when there is no weight being placed on it. However, as soon as one places weight on the orthotic, it deforms from its original “neutral-foot” shape. By the time the equilibrium point is reached when the force being applied to the orthotic matches its deformation as described by Young's modulus, there may be a high degree of change in the orthotic shape (see second image above). Therefore, it is impossible for just about all orthotics to push the foot all the way to neutral because the orthotic shape changes with any force being applied. The only way for an orthotic to bring a pronated foot to its neutral position when full weight is placed on it is to start with an orthotic shape that captures the shape of the foot when it is in a mildly supinated position.

Understanding the stress-strain characteristics of an orthotic helps one understand how an orthotic works inside of a shoe. As one measures the amount of force being placed on an orthotic inside the shoe, during the contact period of gait, an average person will load the orthotic with approximately 120 percent of his or her body weight. This should cause the orthotic to deform from its neutral shape and if the orthotic is theoretically ideal, with 120 percent of body weight, it will be the shape of the foot when the foot is about four degrees pronated.

During the midstance period of gait, the total force on the orthotic starts decreasing until it is between 70 to 80 percent of body weight when the center of body mass is directly over the center of the foot. In this time period, the foot should have stopped pronating and started its movement back toward a neutral position. So the decrease in force on the orthotic allows it to start springing back toward its neutral position shape. At about the time that the heel starts its upward velocity, a little before it actually comes off the ground, the foot should start accepting more weight. However, the weight is now going forward onto the ball of the foot and the orthotic bears less weight until one is at the point of full heel off. At this point, the subtalar joint should be back in its neutral position and the orthotic should be back to that same neutral position shape.

Evaluating The Roles Of Material Length And Thickness In Orthotic Prescriptions

Orthotic flexibility is a crucial part of the prescription process and while there is currently not much research on the subject, the practitioner should be aware of some of the basic principles that determine flexibility. I tell patients that if they can easily flex an orthotic with their hands, it probably is doing very little in retaining its proper shape when the patient stands on it with 50 to 150 pounds of force on each orthotic.

The simplest principle to understand is how thickness of the material determines flexibility. This basic principle says that rigidity is proportional to the cube of the thickness (see third graphic above). In comparing an orthotic made from 3/16” polypropylene and an orthotic made from 1/8” polypropylene, one notes that the 3/16” device is 150 percent thicker than the 1/8” device. Therefore the 3/16” polypropylene device is going to be (1.5)3 more rigid, which means that the 3/16” device is 337 percent more rigid than the 1/8” device. Comparing a 1/4” thick device with a 3/16” thick device, one notes that the 1/4” insert is (1.33)3 more rigid, which calculates to the 1/4” device being 237 percent more rigid than the 3/16” device and eight times more rigid than the 1/8” device.

Orthotic flexibility is also dependent on the cube of the length. If the length of a beam is doubled, there is an eightfold increase in the beam’s flexibility (see fourth graphic above). I have a size 10 foot and wear an orthotic made from acrylic. The length of the orthotic from the center of the heel to the anterior edge is 13 cm. If one adds a heel post and extends it one cm distal to the center of the heel so the distance from the anterior edge of the heel post to the anterior edge of the orthotic is only 12 cm, then the rigidity of the orthotic increases by (13/12)3 which means that the rigidity is increased by 127 percent. If the clinician extends the heel post one cm further so the distance from the anterior edge of the heel post to the anterior edge of the orthotic is only 11 cm, the rigidity is 165 percent greater.   

For those patients with longer feet, one can easily see that the orthotic needs to be thicker than for those with shorter feet. To date, laboratories, while asking for the size of the foot on their prescription forms, do not tell clinicians whether they account for the length of the foot in their recommendation for material thickness.

Understanding The Impact Of Curvature On Orthotic Rigidity

A third variable that determines orthotic rigidity is the curvature. The rigidity is inversely proportional to the radius of the curvature. For an orthotic that has a high curvature, there is much greater rigidity than for an orthotic with low curvature. When one looks at the curvature of an orthotic, one notes that for most people, the medial arch is higher than the lateral arch. This means that it takes more force to flex the medial side than the lateral side of the orthotic. If there is equal force on the medial and lateral sides of the orthotic, then the lateral side of the orthotic will flex more than the medial side.

When this author measured his own orthotic, he found that the height of the medial arch was 22 mm and the height of the lateral arch was 10 mm. When calculating the radius of curvature for these two arch heights, I found that the medial side would have a radius of curvature of 73 mm and the lateral side would have a radius of curvature of 117 mm. This would mean that the medial arch would be 161 percent more rigid than the lateral arch. When I adjusted this factor by the factor that the lateral side was a little shorter than the medial side, the final calculation was that the medial side was 123 percent more rigid than the lateral side.

The difference in the rigidity of the orthotic under the medial side versus the lateral side may make the orthotic feel hard under the medial arch. It can also cause patients to feel that they are rolling toward the outside. Of course, the greater the difference between the height of the lateral column and the height of the medial column of the foot, the more likely the patient will have some of these problems.

Many clinicians will tell the patient they just have to get used to this discomfort, which they term “the break-in period.” Some patients will do this but others cannot or will not. The clinician can alleviate this problem with a simple orthotic addition. By adding additional material, such as cork, under the lateral column of the orthotic, one can help ensure the rigidity of the lateral column is a little closer to the rigidity of the medial column. When it comes to the relationship between curvature and rigidity, another consideration for clinicians to remember is that an orthotic for a person with a low arch will flex more than one for a person with a high arch.

Applying Principles Of Rigidity And Flexibility To Custom Orthoses

Therefore, when prescribing orthotics, if the person has a low arch, the clinician will want to order a material that is a little thicker. For a person with a high arch, the clinician will want to order a material that is a little thinner. With the advent of 3D printing technology, laboratories are encouraged to develop algorithms that will allow clinicians to order orthotics that have variable thicknesses. Areas of low curvature could be made more thicker and more rigid, and areas of the orthotic with high curvature could be made thinner. A great amount of research is needed in this area.

Orthotics not only flex in the longitudinal direction but they also twist around the longitudinal axis of the orthotic. In stance, if the rearfoot everts, the forefoot has to invert relative to the rearfoot to keep the metatarsal heads in contact with the ground.  This is often called the “twisted plate” concept of foot function. In order to resist pronation of the rearfoot, an orthotic must not only provide an inversion torque on the heel but it must also provide an eversion torque against the forefoot. Steindler advocated doing this wedging on the outside of a shoe.17 Root’s non-weightbearing casting technique, by molding the foot with the forefoot everted to the rearfoot at its end range of motion, took the Steindler concept from the outside of the shoe and put it inside of the shoe.18 Therefore, in order to resist pronation of the foot, the orthotic has to resist bending in the medial and lateral columns, but also has to resist torsion. The resistance to torsion is proportional to the material’s polar moment of inertia. Polar moment of inertia of a plate is proportional to the formula ab(a2 + b2) in which “a” is the width of the plate and “b” is the thickness of the plate. Materials like polypropylene and acrylic are fairly homogenous, and their torsional stiffness is closely related to their bending stiffness.

However, in the past 30 years, different composite materials that meld fiber with the basic plastic or resin materials have been introduced into the orthotic business. When one incorporates a fabric or fiber into the orthotic material, there is usually much greater stiffness in the direction of the fibers, and much more flexibility in a direction that is 45 degrees to the fiber direction. To resist torsion, orthotics should then have fibers running diagonally to the long axis of the orthotic as well as fibers running longitudinally. New 3D printing techniques will add to the possible complexity of resisting various torsional forces on various types of feet.

In Conclusion

In summary, the clinician needs to remember that in order for an orthotic to work properly, rigidity is an important part of the prescription. In resisting deformation, the orthotic is able to push against the foot in ways that will change its kinematics and kinetics. This is one of the least studied aspects of orthotic function but it can make all the difference as to whether an orthotic is able to achieve its objectives or not. I often explain to patients that if a patient weighs 150 pounds, the orthotic has to push up with 150 pounds of force, whether it is made from the hardest concrete or the softest foam.

When patients complain that orthotics are “too hard,” it does not mean that the material itself is necessarily too rigid. It usually means that the force on top of the orthotic is not evenly distributed. Changing the flexibility properties of the orthotic and increasing the thickness in the parts where the patient does not feel any abnormal pressure is many times very effective in alleviating the abnormal pressure areas.

It is hoped that researchers and orthotic labs will make a much greater effort to research what happens to the shape of an orthotic when a person actually steps on it.

Dr. Phillips is affiliated with the Orlando Veterans Affairs Medical Center in Orlando, Florida where he is program director for the Podiatric Medicine and Surgery Residency.  He is a Diplomate of the American Board of Foot and Ankle Surgery, and the American Board of Podiatric Medicine. Dr. Phillips is a clinical volunteer faculty Professor of Podiatric Medicine with the College of Medicine at the University of Central Florida. He is also a member of the American Society of Biomechanics.

By Robert “Daryl” Phillips, DPM


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