Key Insights On Orthotic Materials
In order to help bring orthotic therapy into sharper focus, some of the top thinkers on biomechanics share their insights on various orthotic materials. They discuss the importance of addressing the patient’s specific activity and pathology in arriving at an appropriate orthotic prescription, and offer their views and experiences on the efficacy of various orthotic materials. Without further delay, here’s what they had to say to questions posed by Robert Phillips, DPM. Q: What type of feet do you feel almost always need rigid orthotics? What type of feet should never be put into a rigid orthotic? A: Christopher E. Smith, DPM, notes that he uses rigid orthotics for the mobile flatfoot to the rigid cavus foot. As long as there is free range of motion in the subtalar and midtarsal joints and one has taken a good neutral cast, Dr. Smith says these patients can “tolerate both extremes of biomechanical pathology.” If one captures the negative cast correctly and the lab does a good job with the positive cast correction, “a rigid material can be well tolerated by most foot types,” says Douglas Richie Jr., DPM. However, he believes that activity and pathology may have a greater role than foot type in determining the success of the rigid orthotic. William Olson, DPM, agrees that how one uses the orthotic goes a long way toward determining a successful outcome. As an example, Dr. Olson notes that runners do very well in rigid orthotics as running involves a linear, repetitive motion. In these applications, he notes the enhanced durability of a rigid device provides “a consistent degree of correction over an extended period of time.” While Dr. Smith likes devices to be rigid, he says he’ll use softer flexes for those who participate in linear sports and devices with frontal plane motion for side-to-side activities like court sports. Indeed, when it comes to treating athletes who participate in high-velocity, complex motion sports such as basketball, soccer and tennis, Dr. Olson says they require “more compliance in the device.” He points out that the position of the foot varies significantly from foot strike to foot strike as it contacts the supporting surface. Dr. Richie concurs. He has found that athletes who participate in sports such as tennis and volleyball emphasize lateral movement and ballistic muscular contractions so they do not tolerate rigid orthoses well. He also attributes this to the posture of the foot during these activities, noting that sports that require lateral movement and jumping place the foot in a plantarflexed position at the ankle more consistently than activities such as walking and running. As a result of this increased plantarflexion of the ankle, Dr. Richie says rigid orthoses seem to irritate the medial band of the plantar fascia more so than the semi-rigid devices. However, he also emphasizes that the rigid restriction of frontal plane movement of the rearfoot upon the forefoot is “unnatural” in jumping and lateral movement activities. “This intolerance is almost universal among all foot types,” explains Dr. Richie. Assessing The Impact Of Pathology Paul Scherer, DPM, says the type of pathology plays more of an essential role than foot type in determining whether rigid orthoses are indicated. He points out that pathologies such as plantar fasciitis, functional hallux limitus, hypermobile pediatric flatfoot and others produce symptoms because ground reactive force (GRF) places the foot in a position that results in dysfunction. Therefore, one’s treatment goals for these conditions should be geared toward capturing the shape of the patient’s foot without the effects of GRF, according to Dr. Scherer. He says one would then proceed to make a piece of rigid material close to the new shape and place it under the foot so GRF cannot change the foot position during stance and gait. Dr. Richie has also found that most intrinsic foot pathologies (such as plantar fasciitis and neuromas) “respond better to rigid orthoses” as opposed to non-rigid devices. Dr. Richie adds that lower extremity pathologies that are linked to impact shock may not respond as well to rigid orthotics as to semi-rigid orthotics. He says these pathologies include patellofemoral pain syndromes, degenerative arthritis of the knee as well as iliotibial band syndrome. According to Dr. Richie, one should also never use rigid orthotics to treat the rheumatoid foot or feet affected by fibromyalgia as these conditions “cause extreme sensitivity to the plantar surface of the foot.” Pressure peaks are another critical consideration when one is considering orthotic treatment for a given pathology. When treating pathologies such as metatarsalgia, hyperkeratotic lesions and ulcers, Dr. Scherer says the goal of orthotic therapy is to alter or redistribute the pressure peaks. “Rigid orthoses do a poor job of this where non-rigid orthoses succeed,” says Dr. Scherer. Dr. Smith says he is a purist who believes one of the fundamental principles of biomechanical treatment is to assess the foot to determine if the pathology is due to faulty mechanics. If so, Dr. Smith says one should take the best negative cast possible and fabricate a functional device. While Dr. Smith notes that most practitioners would avoid using rigid orthoses for the rigid flatfoot, he notes that he’ll use a semi-rigid device fabricated over a pronated and uncorrected cast. He adds that he’ll usually incorporate a soft top cover and accommodate as necessary. Q: How much rigidity should there be in an orthotic made for a diabetic foot condition? A: It depends upon the particular pathology, maintains Dr. Scherer. Do you wish to change the position of the patient’s foot or the pressure peaks affecting the plantar aspect of the foot or both? Dr. Scherer says the more specific you can be on the pathology, the more selective you will be in choosing an appropriate orthotic. In theory, Dr. Richie says a rigid device would be “very desirable” in treating progressive Charcot arthropathy and ulceration in diabetic patients. After all, total contact casting represents the ultimate form of rigid stabilization of the hindfoot and midfoot, and is widely accepted as the gold standard for the treatment of diabetic Charcot arthropathy, notes Dr. Richie. However, he points out that once the orthotic is designed to only contact the plantar surface of the foot, most DPMs would be reluctant to employ rigid materials to limit motion and dissipate plantar pressures. While Dr. Olson’s practice is exclusive to sports medicine, he notes that determining whether the patient has a compromised sensory status is more important than his or her diabetes per se when it comes to employing orthotics. If the diabetic patient has normal sensation and normal proprioceptive capabilities, “it would seem that using a relatively rigid device would be perfectly appropriate,” according to Dr. Olson. However, if there is a question as to whether the patient has adequate sensation, then Dr. Olson says it would be more appropriate to use an orthotic that would yield under focal areas of load. He says the device could be made of laminate or multidensities of plastazote. When it comes to prescribing orthotics for patients who have diabetic sensorimotor neuropathy, Dr. Richie emphasizes that you need to be cognizant of the severe forces that are transmitted through the foot. “If you are not controlling ankle and leg rotation, then you can be sure that the foot of the diabetic patient will rotate on the surface of the foot orthosis,” explains Dr. Richie. Therefore, he says one must design the orthosis to “give” or accommodate shear stress and ground reactive forces. Ideally, Dr, Richie says you would use a semi-rigid orthotic with adequate padding in the top cover and extension. For these patients, Dr. Smith recommends having as much rigidity as necessary to control the biomechanics of the foot. However, he notes that he uses soft, full-length, top cover materials for an accommodative interface such as poron or expanded rubber. Dr. Richie says a foam-type interface can mitigate much of the impact and shear on the skin surface while underlying semi-rigid material can provide the needed resistance to abnormal moments exerted on the pedal joints. Q: Many laboratories are offering milled polypropylene orthotics. What differences have you found in the performance properties of orthotics made from milled polypropylene versus those made from heat-forming techniques? A: Milled orthotics, if they are made properly by a laboratory, should be identical in shape as vacuum-formed devices, according to Dr. Scherer. He says the only difference is that milled orthotics are more rigid at the same thickness because the material has never been heated to be shaped. Dr. Scherer points out that a 1/8-inch milled orthotic (3.1 mm) has the same strength and flexibility as a 5/32-inch vacuum-formed orthotic (4.7 mm). He says this is a “huge advantage” for milled orthotics because you get the same flexibility as compared to a vacuum-formed device, but with a thinner orthotic. There are two other distinct advantages to milled orthotics, according to Dr. Olson. One of the benefits is enhanced shape retention. He points out that a thermoformed polypropylene device will tend to “creep” under repetitive load or return to its original flat shape over time. Dr. Olson says the rate of creep is determined by several factors including the degree and number of force repetitions applied, the thickness of the material, the use of extrinsic posting and the purity of the polypropylene. On the other hand, a milled polypropylene orthotic has no memory of a different original shape and is under no strain in its final shape, explains Dr. Olson. He also notes that with milled devices, you can leave certain areas of the device that are subject to maximum strain and resulting shape deformation thicker in order to provide more rigidity to those areas. Dr. Olson says this is not possible with thermoformed polypropylene devices unless the extra reinforcement is laminated on as a separate step. Dr. Olson notes that fiber-reinforced polypropylene devices are available from certain podiatry labs. While these devices are thermoformed, he notes that they do tend to resist creep due to the enhanced strength provided by the fiber reinforcement. Q: What orthotic materials do you use most often in your practice? A: Dr. Smith says he uses carbon/fiberglass/resin composites exclusively at Northwest Podiatric Laboratory. One can make the material flexible or rigid depending on the type of laminants you use, the angulation of the individual layers (toward or away from the midline of the device) and by adding or subtracting a layer, according to Dr. Smith. Given the total flex range available with this material, Dr. Smith says he does not need or use any other material. He emphasizes that the flexibility/rigid quality is “essentially independent of thickness.” With a top cover, the devices are approximately 3 mm thick and they are less than 2 mm without the top cover, notes Dr. Smith. He adds that they are designed to have a firmer lateral column and a more flexible medial column to facilitate physiologic pronation. As Dr. Smith noted previously, he employs devices with frontal plane motion for those who participate in side-to-side sports. For the majority of their orthotic prescriptions, Drs. Olson and Richie use TL-2100 composite core material (Performance Materials). Dr. Richie cites the ability to choose variable thickness and the rigidity of the material, which he says is “reliable and predictable.” Dr. Olson, who developed the material, says the main strengths are the minimal weight and bulk of the final device and the device’s durability. In a 1993 study of the TL-2100 material, Dr. Richie notes that he and Dr. Olson showed that athletes had a “significant preference” for the thin composite TL-2100 material over thicker polypropylene.1 When treating athletes who participate in ballistic, jumping sports, Dr. Richie uses the semi-flexible TL-2100 and employs the rigid TL-2100 for all other patients. In occasional, specific circumstances, Dr. Olson utilizes homogenous polypropylene devices and has employed fiber-reinforced polyolefin orthotics in the past as well. When cushioning and pressure dispersion is of primary importance (in cases such as inferior calcaneal periostitis), Dr. Olson says foam laminates are “quite useful and very well tolerated.” Occasionally and particularly with rheumatoid patients, Dr. Richie uses a thin polypropylene shell with a poron bottom fill and soft plastazote-type top cover. He adds that these patients do not tolerate rigid orthoses. Dr. Scherer emphasizes that his orthotic prescriptions are geared toward addressing the requirements of the specific pathology of the patient. In some cases, he notes that you can alter the malpositioning effects of GRF and reduce the pressure peaks with a specific top cover and forefoot extension on a rigid device. Q: Is the combination of cork and leather ever your first choice in orthotic material? A: Drs. Olson and Richie never prescribe devices made from these materials. While the combination of cork and leather is never his first choice, Dr. Scherer notes that some DPMs are able to adapt these materials to treat certain pathologies, particularly those related to neuromuscular disorders. Dr. Smith says he can use a cork and leather orthotic to accommodate a plantar prominence or ulcer. However, he prefers to use a semi-rigid shell that is molded over an uncorrected cast and will add accommodations as necessary. He points out that the thin shape of the composite shells facilitates an easier fit of the device into the shoe. Q: Have you utilized any of the carbon fiber products for making orthotics? What type of feet is most suited for orthotics made from carbon fibers? What type of feet does poorly with carbon fiber orthotics? A: As Dr. Olson noted earlier, he uses TL-2100 materials for the majority of the orthotics that he prescribes. He says the thermoplastic composite materials can be easily and repeatedly adjusted and modified. Dr. Olson also cites the TL-2100’s three levels of rigidity, which can be properly used “in a variety of clinical applications.” While Dr. Smith primarily uses carbon/fiberglass/resin composites, he firmly believes there isn’t one particular material that is better suited for treating a particular foot or foot type. He says it all comes down to the adequacy of the negative cast. “If you start with a good negative cast, then one may utilize a more rigid material,” points out Dr. Smith. “If the negative cast is improperly aligned, then one should use a more flexible material with less control but more ‘forgiveness.’” He maintains that the idea of one material being intrinsically more comfortable than another is erroneous. “Theoretically, if one material of a given strength controls the mechanics of the foot, then a different material of the same strength should do the same,” explains Dr. Smith. Dr. Olson says all devices that use some sort of fiber reinforcement, whether it’s carbon-graphite (the strongest), fiberglass or other types of woven fibers, offer a stronger and thinner product in comparison to similar products that do not have the reinforcement system. “It has been my clinical experience that the bulk and weight reduction of composite orthotic devices is immediately noticed by patients who have previously worn non-reinforced orthotic products,” notes Dr. Olson. With the exception of rheumatoid feet, Dr. Richie says composites are “almost universally applicable for most foot orthotic prescriptions,” given the availability of various thicknesses and stiffness. Dr. Scherer reiterates that his choice of material would be dependent upon the given patient’s pathology. However, he says an additional criteria is the shoe environment. He notes that carbon fiber is an advantage in the ski boot and ice skate. Q: Do you utilize plastazote as an orthotic material? How long does the orthotic last? A: Dr. Smith does not use plastazote because “it bottoms out very quickly.” While Dr. Richie notes that plastazote can be an excellent component of a foot orthotic, he agrees with Dr. Smith that it is a poor core material that bottoms out “unpredictably.” “Athletes will destroy a plastazote orthosis in less than 30 days,” points out Dr. Richie. When treating patients who have diabetes or polyneuropathies, Dr. Smith prefers to use poron. He says he emphasizes mechanical control with accommodations to offload plantar lesions. According to Dr. Scherer, several authors have demonstrated in the literature that plastazote is “an extremely effective orthotic material” when it comes to redistributing pressure peaks in feet. In particular, Dr. Scherer says combining different durometer plastazote material with poron materials may help facilitate the reduction of pressure peaks. Dr. Olson uses a plastazote laminate for patients who require cushioning and pressure dispersion, and are willing to sacrifice long-term control and durability. He says these devices work well in treating conditions such as inferior calcaneal periostitis, certain stress fractures of the lower extremity and certain pelvic and spinal conditions of mechanical etiology. The devices last about six months, according to Dr. Olson. After multiple refurbishings of old devices and repeated fabrication of new devices, Dr. Olson says patients eventually ask about more durable materials. At this point, Dr. Olson says they are generally ready to tolerate and benefit from more durable orthoses. Q: If you could design the perfect orthotic material, what would be its characteristics? A: Dr. Richie says the profession is almost there with certain composites. The only missing feature, according to Dr. Richie, is the ability to orient the construction of the material to resist forces in certain planes, depending upon the pathology. “It would be great to order an orthotic that has more frontal plane versus sagittal plane rigidity or vice-versa,” offers Dr. Richie. Dr. Smith would emphasize minimal thickness so a new shoe is not necessary to accommodate the bulk. He also advocates sufficient strength to control the mechanics of the foot with a firmer lateral column and a somewhat more flexible medial column to facilitate physiologic pronation. Dr. Smith says he would also want a durable material that did not creep or flatten out with usage. He adds that it should also be easily moldable and inexpensive. “The optimal orthotic material would provide resistance to imposed forces over tens of thousands of cycles without failure, whether that failure is due to creep or microfracture,” suggests Dr. Olson. “At present, I believe the thermoplastic composites come the closest to achieving these standards of the orthotic materials currently available.” He says the ideal orthotic material should also be thin and lightweight for optimal performance and comfort, allow for the use of cushioned top covers and be available in a number of different rigidities to ensure an appropriate material match to the patient’s activity and/or pathology. Dr. Phillips is the Director of Podiatric Residency at the Coatesville Veterans Affairs Medical Center in Coatesville, Pa. Dr. Olson is a member of the attending staff at the Center for Sports Medicine at St. Francis Memorial Hospital in San Francisco. He is a Past President of the American Academy of Podiatric Sports Medicine. Dr. Richie is a Director of the American Academy of Podiatric Sports Medicine. He is also an Adjunct Clinical Professor of Biomechanics at the California School of Podiatric Medicine at Samuel Merritt College. Dr. Scherer is the Chairperson of the Department of Applied Biomechanics at the California School of Podiatric Medicine at Samuel Merritt College. He is also the Podiatric Medical Director of ProLab Orthotics. Dr. Smith is the Vice President and Podiatric Medical Director of Northwest Podiatric Laboratory. He served on the faculty of the California College of Podiatric Medicine for 24 years before retiring in 1992 with the title of Emeritus Professor of Biomechanics.
References 1. Richie DH, Olson WR. Orthoses for Athletic Overuse Injuries: Comparison of Two Component Materials. JAPMA. Volume 83, Number 9, pp.492-498, September 1993.