Abstract
Traumatic knee injuries in automobile accidents and sports often lead to damage of the peroneal nerve. A lack of control of muscles innervated by the peroneal nerve due to this damage, results in the inability to dorsiflex and evert the foot and to extend the toes. This condition is commonly known as foot drop. Foot drop reduces the stability in the body while walking and running and may also cause injury due to lack of foot clearance during the swing phase of the gait. Traditionally, an ankle foot orthosis (AFO), comprised of a moulded sheet of plastic that conforms around the posterior calf and distally contains all or part of the calcaneous as well as the plantar foot, is used to treat foot drop. The intent of this orthosis is to dorsiflex the foot to provide clearance during the swing phase of walking and running. Traditional AFO results in increased pressures due to a decrease in dorsiflexion range of motion at the ankle and make the orthosis increasingly uncomfortable to wear. Several other existing designs of foot drop AFO suffer from similar inadequacies. To address these issues, a new AFO was developed. The device was successfully used by one person with foot drop without issues for more than one year. This new design conforms to the lower anterior shin and dorsum of the foot using dorsiassist Tamarack ankle joints to allow for greater plantar and dorsiflexion range of motion. While still limiting ankle inversion it does allow for more ankle eversion. This orthosis can be discretely worn inside shoes due to its smaller size, and can be worn for a longer period of time without discomfort.
Introduction
When there is damage to a nerve in the leg, muscular control may be lost. In the case of peroneal nerve, those muscles include the tibialis anterior, extensor digitorum longus, extensor hallux longus, peroneus tertius, peroneus longus and peroneus brevis. This damage can occur in a variety of ways. The most common cause is traumatic knee injuries sustained during automobile accidents and sporting accidents. The common peroneal nerve that often gets injured is a branch of the Sciatic nerve that splits just above the knee and runs on the lateral side of the knee directly across the fibular head. A lack of control of the above-mentioned muscles means that the person cannot dorsiflex (elevate) or evert (abduct) their foot. This condition is known as foot drop. This places the foot in a continuously ‘dropped’ state, hence the name of the condition. This condition makes walking and running awkward as the toe often drags and touches the walking surface potentially leading to injury due to increased frequency of tripping and reduced ankle stability. The gait pattern known as ‘steppage gait’ (deliberate high hip and knee flexion during the swing phase) is a consequence of drop foot.
A brace known as an Ankle Foot Orthosis (AFO) is commonly used to treat foot drop. An AFO is designed so that the limp foot is supported by coupling the foot with the lower leg.
[1]
An AFO is also traditionally prescribed for hemiplegic patients after stroke or head injury, paraplegic patients with a spinal cord injury, and flaccid paretic patients with foot drop.
[2]
The traditional thermoplastic flexible AFO can be seen in Figure 1. It is comprised of a moulded sheet of plastic that wraps around the posterior of the leg and under the foot with fabric straps across the ankle to secure the heel in place. The brace is most often made from polypropylene or polyethylene plastic. An articulated AFO retains the solid foot piece and leg piece of the standard AFO, but flexes at small ankle joints built into the brace instead of relying on the leaf spring action of the brace itself.
[3]
A most recent improvement in the design of AFO is the one made up of carbon fibre and polypropylene shell, a metal hinge joint, and two artificial pneumatic muscles to activate ankle movement.
[4]
A traditional ankle foot orthosis (AFO).
The existing brace designs have some major shortcomings. Firstly, most of them are large enough to make shoe fit a challenge. The traditional AFO, as shown in Figure 1, extends proximally on the calf to approximately 2 cm below the distal fibular head. The foot section is as wide as the patient's distal metatarsal width and at the heel cup will be at least the width of the heel plus the thickness of the thermoplastic times two. This means that it is only possible to wear this style of brace with certain shoes. In general, only larger or soft, flexible shoes like oversized running shoes can be worn. The brace leads to foot discomfort and cramping most likely due to the reduction of dorsiflexion range of motion at the ankle over the first year of wearing the AFO. This increases the contact pressure at the metatarsal head and calf as well as dorsal ankle strap pressure as the heel will not sit comfortably in the AFO. These excessive pressures make wearing the AFO intolerable after 30–45 min. The foot cramp is also caused due to the fact that the anterior end of the foot is unsupported due to the shortness of the foot plate. The foot plate has a 3 mm thick lining for cushioning and a metatarsal pad to support the transverse arch. This, added to the 3 mm thickness of the thermoplastic foot section adds up to a depth of approximately 10 mm. This causes the toes to abnormally flex (to touch the ground) during the propelling phase of the gait. The difference in height between the ground and the toes also means that the toes can no longer support the amount of weight that they normally do, and the pressure is correspondingly increased on the leading edge of the foot pad. The final major problem of this style of brace is the ambient temperature. Due to the large amount of the leg that is covered in thermoplastic, it gets uncomfortably hot quickly during hot summer days, or when doing physical activity. This is emphasized since a tall sock must be worn under the brace to reduce rubbing between the skin and the plastic.
The aim of this research was to develop an AFO that would address the above mentioned problems. We had a goal to develop a brace that would be functionally supportive for a drop foot (provide dorsiflexion motion and inversion control in swing phase of gait) but also would (i) not result in foot cramp or blisters on the heel, and (ii) be comfortable even if worn for a long amount of time while performing routine recreational activities. In addition to the above requirements, the brace should also be less bulky so that it can be worn inside a variety of normal sized shoes. The study was performed on the first author (DB) who had foot drop.
Methodology
Ethics approval is not required for this project. The design methodology followed the traditional design process of brainstorming different ideas, evaluating them and arriving at the best possible design. It was decided to have articulating mechanism rather than the posterior flexure strut as shown in Figure 1. It was decided it made more sense to have the articulating center of rotation in-line with the ankle joint rather than behind the ankle joint. This would eliminate any discomfort caused by rubbing of the foot skin against the orthosis caused as a result of offset in the joint centers of rotation between the ankle and the orthosis. The dorsiflexor moment needed to keep the foot at the maximum dorsiflexion range of approximately 90° during the swing phase of the gait was calculated by extending the patient's knee and applying the force to the plantar 5th metatarsal head until the foot is dorsiflexed to 90° (measured with a goniometer). This was found to be approximately 3.1 Nm. This moment was to be provided by the Tamarack Dorsiflexion Assist joint with two durometers (model 742- 75, large size) [5] providing a total of 3.4 Nm moment. These flexible joints located at the ankle provided the maximum available range of motion and compliance. It was also decided to eliminate the hard heel cup and supporting plantar foot section completely and instead support the foot from the top allowing the circumferential tightening of the shoe to hold the foot and brace in place. The anterior tibial shell is reduced in height as compared to the original AFO and is a semi-flexible wrap around design utilizing a shin strap for suspension to the leg. This would eliminate the discomfort caused by the difference in the height between the toe and the footpad and the rubbing at the heel cup. The strap that secures the brace to the leg goes around the front of the leg instead of the rear to avoid having a plastic seam in the brace. The struts run down the sides of the leg (instead of the front or back) to eliminate rubbing through the range of motion. The foot piece is twisted to contour to the shape of the top of the foot surface, and it remains on the top of the foot to reduce discomfort. It should be noted that this new design relies on being tightly tied into the shoe against the foot in order to work.
The brace prototype was then custom-made by a certified orthotist. The brace is made up of a co-polymer made by mixing 85% polypropylene with 15% polyethylene. This material is strong but flexible and light. A soft foam layer 4 mm thick was added to the entire inside of the brace. This layer will reduce the hardness of the brace where it contacts the skin and eliminate any points of pressure that may cause discomfort.
The brace was worn by the first author DB (inside his shoe) all day (8–10 h), almost every day. DB also continued with his daily routines such as walking, biking and jogging for more than a year. The subject is 188 cm in height, 25 years old and weighs 190 lbs. The subject anecdotally noted the general level of discomfort caused, any blister development and any cramps after wearing it for several hours.
Results
The final design, which incorporates all the necessary features to make this brace functional, can be seen in Figure 2. The AFO properly fits inside an athletic shoe and successfully supports the foot at 90° during the swing phase of the gait as shown in Figure 3. Minor discomfort was experienced during the acclimatization period of about one week, but after that, the brace was worn all day without any discomfort. Walking and running on a treadmill was assessed while wearing this AFO. Successful foot clearance and body balance were attained. There were no blisters (because of the absence of heel cup and plantar foot section) or foot cramps even after wearing it for several hours while engaging in normal walking. This is due to the ability of the AFO to function in plantarflexion through to neutral dorsiflexion. The brace was successfully worn for more than one year with the subject involved in routine walking and running.
A prototype of the new brace: (a) Sagittal view, (b) anterior view. The new AFO inside an athletic shoe (during swing phase of the gait). It can be seen that the brace successfully prevents the foot from dropping.
Discussion and conclusion
The new AFO eliminates most of the shortcomings of the older designs. It is more comfortable to wear and is less bulky. This means that the new design can be worn with a regular sock underneath instead of always using a tall soccer style sock. The brace holds the foot in maximum dorsiflexion, high enough to swing through without toe contact and that is the main goal of any AFO. It also holds the foot high enough that the heel contacts first on foot strike. The AFO allows natural contact of the heel and underside of the foot with the footwear eliminating the chance of blisters caused due to plantar foot section and heel cup. The prototype brace also prevents excessive ankle eversion simply by not being flexible in that direction. This ensures a safe landing for each stride during running. After a few days of acclimatization, the brace can be worn for multiple (8–10 h) hours without discomfort. The patient used the AFO without any problems for one year, after which it had to be remade because of wear and tear due to overuse.
It should be noted that the brace, while much shorter, still requires space in the shoe and therefore will not work with all shoes, but it is an improvement over the original design. It should also be noted that the flexible ankle joints protrude slightly (in the medial and lateral directions) and this may cause space issues with a taller shoe or a boot. Nevertheless, the new design would be a more comfortable and less restrictive alternative for the existing designs, especially for young foot drop patients who would be interested in activities such as running.
While this design is a significant improvement compared to previous designs that proposed hinged AFO instead of rigid AFO, [6] the study has some limitations. We have tested this on only one subject. The design should be tried on several foot drop patients to make sure this brace is comfortable for a variety of patients. A kinematic study using motion analysis system can be used to quantify the extent of support the brace provides. Future study is needed to investigate the effects of new design on the biomimetic roll over shape [7] of the ankle-foot when compared to older designs. It is possible that depending on the biomechanics of the foot the use of a custom foot orthosis may be beneficial to increase function, comfort and compliance with this dorsal AFO design.