Exploring The Potential Of AFO Devices

Author(s): 
By Douglas Richie Jr., DPM

While several large orthotic laboratories have offered AFOs for over 20 years, the increasing demand for the devices has become a significant phenomenon in the podiatric field. Why have podiatrists turned to AFOs more and more in recent years? There are three key reasons that have caused this shift in treatment. First, there has been a meteoric rise in the number of patients who have challenging foot and ankle pathologies. Two of these pathologies, adult acquired flatfoot secondary to posterior tibial tendon insufficiency and diabetic Charcot’s arthropathy, have disappointing treatment results with traditional orthotic devices alone. Secondly, there has been increased inclusion of AFO, bracing and pedorthics topics into the curriculum of podiatric medical education and educational seminars. New “podiatric AFO” technologies have also emerged. These technologies are not only user-friendly, but tend to embody the traditions of podiatric biomechanics. Know The Differences Between AFOs And Traditional Foot Orthoses AFOs do provide mechanical advantages over traditional foot orthoses. AFOs provide a force system both above and below the major joint axes of the rearfoot and ankle. Depending on the AFO design , you can apply these forces in either the frontal, sagittal or transverse planes. Orthoses apply force to stabilize a joint via a three-point pressure system, which is defined as two forces applied to a body part opposed by a third force applied between the first two. In the majority of AFOs, multiple three-point force systems provide stability to one or more joints in one or more planes throughout the gait cycle. Traditional foot orthoses, particularly the Root Functional Orthosis, rely on ground reaction forces to stabilize a joint. You cannot establish a three-point force system with foot orthoses to control any of the major joints of the rearfoot. While the combination of proper footwear and a functional foot orthosis can potentially provide adequate three-point force systems, this is not always predictable and the effects of such a shoe-foot orthotic system are limited to certain phases of the gait cycle. The efficacy of functional foot orthoses to treat common lower extremity pathologies has been primarily validated by anecdotal reports in the medical literature. Today, there is considerable debate about how foot orthoses actually work. There is little evidence that these devices actually realign the skeleton and change joint position. Realizing the necessary design of an orthotic to change a joint position allows you to understand why traditional foot orthoses have demonstrated minimal ability to significantly change the position of the subtalar joint in gait. Foot orthoses may work by redirecting forces and changing neuromuscular patterns of the lower leg. Benno Nigg, et. al., are conducting research at the University of Calgary that will shed further light on this fascinating subject. Which Pathologies Can You Treat With AFOs? Podiatrists see several lower extremity pathologies in which changing a joint position is desirable in achieving a satisfactory treatment outcome. These conditions include adult acquired flatfoot secondary to posterior tibial tendon dysfunction, Charcot’s arthropathy and dropfoot. All of these pathologies require you to apply force to the tibia to adequately control rotation of both the ankle and the subtalar joints. Years ago, DPMs initially began using cam-walker style boots to treat these pathologies with great success. Yet once they reduced the acute symptoms, there was a reluctance to consider long-term treatment with a foot and ankle brace. Now it is recognized that the same stabilization provided by a cast-boot can be accomplished by an AFO with better function and patient compliance. How You Can Use AFOs To Treat Dropfoot In the case of dropfoot, there are many considerations in patient evaluation and AFO prescription, which can be confusing to the average practitioner. Critical assessment factors include range of motion, muscle weakness, spasticity, knee stability and the patient’s size and weight. Another critical concern is activity level and lifestyle of the patient. Footwear is critical to the success of any lower limb orthosis and particularly in dropfoot conditions. The shoe provides an essential component of the three-point force system. Prior to prescribing the AFO, you must discuss the footwear requirements of the particular brace and ensure the patient will be compliant in wearing the proper shoes. In many dropfoot conditions, pedorthic modifications — such as heel lifts, rocker soles and lateral wedging — are necessary to augment the control of the AFO. Therefore, you must have a good knowledge of footwear and pedorthic modifications, and have resources available for patient referral. A Review Of Five Helpful AFOs In treating dropfoot conditions, podiatrists have five basic choices of AFOs: short leg (podiatric) fixed and dynamic assist AFOs, full leg posterior leaf spring, solid ankle and dynamic assist (with or without posterior stop). Introduced in 1996, the Richie Brace® was the first podiatric AFO. The fixed hinge version of this AFO has been commonly prescribed for dropfoot conditions. This brace can maintain the foot in a neutral (90-degree) ankle position during swing phase. It can also help you control unwanted varus rotation of the subtalar joint, which can commonly accompany dropfoot in post-CVA patients and Charcot-Marie Tooth patients. The disadvantage of the fixed hinge version of this brace, as prescribed for dropfoot, is the fact that it does not allow plantar flexion or dorsiflexion, so some compensation in gait will develop. Also keep in mind that the brace is a short leg brace and will not have enough leverage to stabilize the dropfoot of a tall (over 6 feet 4 inches) or large patient (over 250 pounds). A short leg podiatric AFO known as the Dynamic Assist Richie Brace® was introduced in 2000 and offers the advantage of a hinged AFO for a more normal gait pattern with full ankle motion. This brace incorporates a pair of Tamarack® hinges that provide spring-like motion in the sagittal plane of up to 15 degrees of dorsiflexion. The advantage of this AFO over other traditional AFOs is the podiatric balanced foot orthotic for subtalar and midtarsal control, as well as the sport style design, which is preferable for some patients over the more bulky appearance of the longer posterior shell AFOs. The limitation of this brace is on tall and large patients as mentioned above. In these cases, a long length traditional hinged AFO with Tamarack hinges would be recommended. Articulated or hinged AFOs have limitations in dropfoot applications. First, you cannot use these braces when there is spastic contracture of the gastroc-soleus or significant equinus deformity. To prevent contracture in patients who do not have equinus, you may often combine a posterior stop with the dynamic assist ankle joint. You must use this combination with a full leg traditional posterior shell AFO. It is not available on the short leg podiatric AFO. Also keep in mind that you can only use articulated AFOs on patients who have relatively stable knees. Many dropfoot conditions are also accompanied by weakness of the posterior leg musculature gastroc-soleus. The soleus is the primary muscle of the lower extremity that controls flexion of the knee during stance. When there is significant weakness of the soleus, the knee will buckle into flexion after the contact period of stance. A solid full length AFO will provide retrograde stability to the knee in this case by limiting ankle joint dorsiflexion and anterior migration of the tibia on the talus. The result is a transfer of ground reaction force to knee extension moment. A hinged, articulated AFO cannot limit ankle dorsiflexion and therefore cannot provide retrograde knee stability. Therefore, in dropfoot conditions where there is also flexion instability of the knee, a full length AFO is recommended. When treating these patients, you have the choice of the posterior leaf spring AFO or the solid ankle AFO. The difference between these two is really based upon the thickness of the plastic as well as the trim lines, which determine the width and shape of the posterior shell. Posterior leaf spring AFOs are indicated when there is a need to prevent dropfoot and where there is no significant varus or valgus foot rotation. The narrower trim lines allow some dorsiflexion and plantarflexion of the ankle and thus require some degree of flexion stability of the knee. The advantage of the leaf spring is a smoother gait pattern compared to the less flexible solid ankle AFO. You can use the solid ankle AFO to resist spastic contracture of the posterior muscles. It also provides better knee extension moment than the posterior leaf AFO. With wider trim lines posteriorly, the solid AFO can control rotation of the tibia and provide a three-point force system to control varus/valgus rotations of the subtalar joint. While you can achieve better control of the dropfoot deformity with a solid AFO, be aware that some compensation may occur due to the restriction of normal plantarflexion at heel strike and dorsiflexion at midstance. Many experts recommend using a shoe with a rocker sole to counteract the restrictions imposed by the solid AFO and allow the patient to walk with a more normal gait pattern. Can AFOs Make An Impact For Charcot Deformities? The Charcot foot, from a biomechanical standpoint, is a combination of the adult acquired flatfoot and the dropfoot. Charcot feet have acquired equinus and have a dropfoot to some degree due to motor neuropathy affecting the anterior leg musculature. Lack of proper muscular “splinting” causes a foot slap during gait, which can be mitigated by an AFO. You can also control the significant deforming force of a tight heel cord with a solid ankle AFO. By decreasing sagittal plane movements, the AFO can protect the tarsometatarsal articulations and will greatly reduce plantar pressures on the metatarsal heads during terminal stance. You can control the valgus rotation of the heel, sagittal and transverse plane subluxation of the midtarsal joint with a custom, conforming orthotic foot plate if it is properly incorporated into the AFO. Semi-rigid, plastic and even graphite composite foot orthoses have been commonly used for neuropathic patients. A custom molded plastic AFO should not be contraindicated when there is sensory neuropathy. You can line the shell and foot plate with laminated multi-density foam to protect the skin. Expect the total contact design of the shell to reduce plantar pressures. Indeed, the plastic shell of the orthotic foot plate in an AFO may have less peak plantar pressures than a semi-rigid foot orthosis alone. The key is ensuring proper custom fabrication and fitting of the AFO along with careful monitoring of the patient after you’ve dispensed the AFO. Again, proper footwear is critical to the success of using AFO therapy to treat Charcot foot pathologies. The Arizona AFO™, designed by Ernesto Castro, CPed, is popular among podiatrists for treating Charcot deformities and adult acquired flatfoot. The device is a custom-made leather and plastic AFO that is designed to hold the relationship of the calcaneus to the long axis of the tibia and fibula so the calcaneus cannot move laterally and the talus cannot move medially. It also eliminates sinus tarsi impingement. Since the Arizona AFO wraps around the foot, you can use much thinner plastic and still have exceptional strength. In addition, you can extend the Arizona AFO up the leg like a conventional plastic AFO for added leverage and support, especially for lateral ankle instability that occurs in conditions such as Charcot-Marie-Tooth disease. Using AFOs To Conquer Adult Acquired Flatfoot Secondary To PTTD The greatest use of AFOs by podiatrists over the past five years has been in treating adult-acquired flatfoot secondary to posterior tibial tendon dysfunction (PTTD). This pathology is being recognized and treated in near epidemic proportions compared to 20 years ago. Patients commonly afflicted with this deformity are over 60 and usually present as poor surgical risks. However, using traditional foot orthoses to treat Stage II and III PTTD disorders has often been disappointing in arresting progression of the deformity and permanently alleviating symptoms. There’s a reason for that. In late Stage II and through Stage III PTTD, there is ligamentous disruption in the hindfoot that causes a loss of movement coupling between the foot and the leg. So the ability of a foot orthotic to direct ground reaction force against the foot cannot resist the significant internal rotation of the tibia, which results from talonavicular joint subluxation. Therefore, you need a three-point force system that controls tibial rotation in order to stabilize the adult-acquired flatfoot. A proper AFO design for PTTD must have the ability to control forces in the transverse and frontal planes, which dominate in this pathology. Traditional dropfoot design AFOs are not satisfactory as they emphasize control in the sagittal plane. Employing short leg AFOs, which incorporate a balanced podiatric foot orthotic, enable you to control movements of the hindfoot in the transverse and frontal planes. The medial and lateral orientation of the limb uprights in this type of brace are designed to restrict internal rotation of the tibia, which is coupled to medial displacement of the talus at the talonavicular joint. Clearly, the talonavicular joint has been identified as the pivotal area of deformity in the patient with PTTD. When you’re treating patients who need functional bracing, the podiatric short leg AFO (i.e. Richie Brace) would be recommended. In cases where further restriction of rearfoot and ankle motion is required, the Arizona AFO has been shown to be highly effective in treating PTTD. A recent study by Linn, et.al., revealed that using the Arizona AFO resulted in a statistically significant improvement overall in 18 out of 20 ankles (90 percent) diagnosed with PTTD.1 This is a marked improvement over prior studies that used medial posted orthotics, such as a UCBL or a rigid plastic AFO. In those studies, the results have been 67 to 77 percent, which is still favorable compared to outcomes with traditional foot orthoses.2 It has also been demonstrated that many patients can discontinue their AFOs after 12 months of treatment for PTTD. In these cases, the foot may stabilize and remain asymptomatic with proper footwear and a traditional foot orthosis. Over the past five years, thousands of patients with Stage II and III PTTD have been treated with the aforementioned advances in AFO technology with extremely favorable outcomes. As podiatrists share their experience using these AFOs with other colleagues, this treatment option has been used more frequently in lieu of surgical intervention. Clearly, a new standard of care has been established regarding the non-operative interventions for PTTD. What About Other Technologies? Another breakthrough making AFO therapy easier for the podiatric physician has been a casting technology known as the STS Casting Sock, which was developed by Richard Stess, DPM, and Peter Graf, DPM. You can use a fiberglass, tubular, circumferential sock impregnated with a water curable resin to obtain the impression of the entire lower leg. You simply roll this sock on the patient’s limb and properly position the foot, ankle and leg. This sock is manufactured in two lengths (17 inches and 30 inches) and various widths ranging from 2 inches to 3.5 inches in diameter. A newer development by STS has been the creation of fitted polyester socks of various lengths (e.g. ankle, mid-leg, Bermuda) that are also impregnated with water curable resins. The size of the fitted casting socks is determined by the patient’s shoe size. Since the resin impregnated material hardens quickly (three to four minutes) and captures a precise replica of the foot, ankle and leg, you can easily remove it without creating a mess during the casting process. No cast saw is required during removal. Orthotic fabrication laboratories actually prefer this impression casting material because of its accuracy and podiatrists have found that this technique is relatively simple to learn, reduces casting time and minimizes office clean up. For further information, the STS Web site is www.stssox.com. In Conclusion The increased use of custom AFOs has been a phenomenon in the podiatric profession in over the past five years. Yet understanding of this technology and widespread utilization among DPMs is still in its infancy. As newer podiatric AFO technology has become available, the interest and implementation of custom AFO therapy has been made easier and more efficacious in the hands of podiatrists. Clearly, any practitioner who professes to be the true “foot and ankle specialist” must understand and be prepared to implement this technology in the non-operative treatment of the most challenging lower extremity pathologies. Dr. Richie is a Director of the American Academy of Podiatric Sports Medicine.
 

 

References:

References 1. Lin SS, Momi KS, Berkman A, Janisse D, Castro E, Johnson JE: Nonoperative Treatment of Posterior Tibial Tendonitis Using the Arizona AFO™ (ongoing, unpublished study). Preliminary results were presented at the 2nd Combined Meeting of Foot and Ankle Surgeons in Venice, Italy, Sept. 16-19, 1998 and at the 66th Annual Meeting of the American Academy of Orthopaedic Surgeons in Anaheim, CA, Feb. 5, 1999. Excerpts and results are used with permission. The study can be found at www.ArizonaAFO.com. 2. Chao W, Lee TH, Hecht PJ, et. al.: Conservative management of posterior tibia tendon rupture. Orthopaedic Transactions 18: 1030, 1994-1995.

 

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