Roger Weber CPO, Ronan Reynolds, Samuel Landsberger Sc.D., Donald McNeal PhD.
Los Amigos Research and Education Institute (LAREI)
Rancho Los Amigos National Rehabilitation Center
Downey, California
Ankle Foot Orthoses (AFO's) with dorsiflexion resistance have been used for a long time to increase the stability of patients with weak calves and quadriceps. This has been commonly accomplished using springs, stops, and flexible plastics. Unfortunately, the amount of dorsiflexion resistance needed by the patient has been hard to predict, and hard to control.
Our goal was to create an orthosis with dynamic dorsiflexion control similar to what has been achieved by many devices in lower limb prosthetics. We used biomechanics and analysis methods that have been used extensively in prosthetics to develop these functions for orthotics.
The question we asked ourselves: "What does an energy storing 'graphite' prosthetic system do?" We found that generally it has two main functions. One function is control weight acceptance during initial heel contact. The second function is to controllably deflect in dorsiflexion and store energy from mid-stance to toe-off. Additionally, hybrid prosthetics generally have some inversion, eversion and rotary control.
The well-known importance of soft (appropriate) initial heel contact in prosthetics is even more significant in orthotics because many orthosis wearers have proximal weakness. If the ankle forces at initial contact are not managed, a flexion moment may be generated at the knee, and at the hip, which the wearer may not be able to control. It is therefore important to have a relatively soft heel or allow plantarflexion during loading response to manage these forces .
Due to the usual pattern of weak or absent dorsiflexors exhibited in these patients, plantarflexion resistance of the orthosis is required to be at least strong enough to maintain the foot in a neutral position for swing-thru. Additional aids to this process may include adding a SACH heel, and/or trim a separation in the molded orthosis between the heel plantar surface and the proximal segment.
During mid-stance, the ankle rotates from plantarflexion into dorsiflexion and the moment across the ankle increases non-linearly. This is because as the dorsiflexion angle increases, the moment arm of the body's mass about the ankle joint increases. Increased, nonlinear dorsiflexion resistance is then needed to counteract this non-linear ankle moment. This dorsiflexion resistance is shown in Figure 1.
Our orthosis also uses ground reaction forces to stabilize the knee. We use a graphite strut to create dynamics. As in the prosthetic model, usually the graphite shanks/struts that control dorsiflexion are viewed as "energy storing". Dorsiflexion forces create a strut deflexion resulting in an anteriorly directed concavity. Then at toe-off, when these forces are released thru unweighting, the strut will straighten causing a relative plantarflexion force. This has been termed "active push-off". This process can be viewed differently when applied to an orthosis. In this application, the resistance of the strut into dorsiflexion from mid-stance to toe-offis used as a dynamic ground reaction force to stabilize the knee. Unloading the orthosis at toe-offcan give some active "push-off"(plantarflexion), which will aid the hip to initiate swing-thru, but the main dynamics focus on knee control. As previously stated, the resistance to dorsiflexion must increase as the dorsiflexion angle increases so that the patient does not experience "knee collapse" prior to toe-off.
Our challenge was to develop a dynamic orthosis that is light in weight and cosmetically appealing, while allowing independent control of plantarflexion and dorsiflexion resistance, with dorsiflexion resistance being both dynamic and non-linear. Additionally, we wanted an orthosis that could be custom modified (engineered) for an individual's height, weight, motor deficit, and activity level. Prosthetics has handled these challenges before, but with orthotics we have additional challenges such as strict space and weight limitations, and limitations on a patient's "gadget tolerance".
At Rancho, an orthosis has been developed we term "The Vertebral Orthosis". The name reflects the nature of the structure of the orthosis that somewhat replicates the spine's vertebral column. The orthosis has several distinct components. First, the main support, consisting of a premanufactured graphite strut that is selected based on patient requirements (weight, activity, etc.). This ideally will conform to the proper plantar-resist/dorsi-assist requirements of the patient. If this is not optimal, a mechanical posterior articulation is incorporated with an integrated dorsiflexion assist. Alternating graphite struts, with some crossing the ankle joint, and some not, can also be used to create plantarflexion and dorsiflexion resistances independent of each other.
In order to achieve the correct amount of dorsiflexion resistance for specific dorsiflexion angles and input loads, a quantitative process has been developed. Using this process, we can predict the required dorsiflexion resistance for a particular patient and custom engineer the orthosis to generate this resistance. This is achieved by utilizing segmented sections between the foot shell and calf; these replicate "vertebral bodies". Thru the posterior area passes the graphite strut, linking the segments together with a flexible structure. Along the medial and lateral parameters a channel is created. This channel can be used with elastic bands for increased dorsiflexion assist, as-well-as aid in the alignment of the vertebral bodies during loading. Based on the required dorsiflexion resistance, spacing between vertebral bodies that sequentially close under load, has been optimally calculated to allow for non-linear dorsiflexion resistance without terminal collapse as seen in Figure 3. This therefore creates a dynamic ground reaction knee stabilizing orthosis.
We have designed several versions of the "Vertebral Orthosis" which allow each orthosis to have independent plantar/dorsiflexion in a controlled and quantifiable manner. Clinical trials have been performed with five subjects. They have been evaluated after one to two months of usage in a pathokinesiology lab and the results are currently being analyzed.
This work was supported through a grant from the National Institute on Disability and Rehabilitation Research of the Department of Education.
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