Academy Society Spotlight: Gait-optimized AFO Selection

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By David Knapp, CPO

Custom-fitted carbon fiber AFOs are commonly prescribed for foot drop and transmetatarsal amputations. The application for foot drop addresses weakness of the dorsiflexors during swing phase, while transmetatarsal amputation requires accommodating for weak plantarflexors during stance phase to prevent drop-off, an excessive drop of the height of the body's center of mass. Selecting the right AFO is possible through a comprehensive evaluation of your patient that includes his or her stride length, weight, and gait pattern. This article evaluates these gait characteristics and their importance in selecting an AFO that will optimally benefit your patient.

AFO Use for Foot Drop

To prevent foot drop, the AFO must exert a dorsiflexion moment to support the foot and prevent the toe from catching on the floor. During swing phase, the required dorsiflexion moment at the ankle is small due to the lack of a ground reaction force and relatively small mass of the foot. The peak dorsiflexion moment during swing for an adult is approximately 1-2 Nm. The requirement is slightly higher in stance phase; to prevent foot slap, the AFO must also slow the foot as it plantarflexes during loading response. The peak moment during loading response in adults with a normal gait is 10-15 Nm of dorsiflexion. This is easily achieved because most custom-fitted carbon fiber AFOs can generate dorsiflexion moments in that range within the appropriate range of motion (Figure 1). Furthermore, this result is applicable over a wide patient base, since even a large variance in patient weight creates only a small change in the overall dorsiflexion moment demand at the ankle. Therefore, most commercially available, custom-fitted carbon fiber AFOs are capable of completely addressing foot drop and at least partially addressing foot slap.

If the AFO generates an excessive dorsiflexion moment during loading response, one possible adverse effect is an increase in knee extensor demand, which may cause your patient to report fatigue or burning in his or her quadriceps. Another adverse effect may be anterior/posterior knee instability that presents as excessive knee flexion followed by knee hyperextension. This knee wobble may be resolved through quadriceps strengthening, which will occur spontaneously if it is physiologically possible. Both of these problems can be resolved by using an AFO with a lower dorsiflexion moment.

Many patients who suffer from foot drop have concomitant hip and knee flexor weaknesses, which further impair swing phase clearance. There are several AFOs that store energy in terminal stance and release a portion of that energy during pre-swing. The released energy assists in hip and knee flexion to further assist swing phase clearance. To take advantage of this feature, the patient needs to have a regular cadence; ataxic or staccato gait patterns do not benefit from this type of energy return. Energy return can be estimated by the angular velocity in dorsiflexion that occurs during pre-swing (Figure 2). Because they are directly proportional, an AFO that releases a lot of energy will produce a high angular velocity during that period.

AFO Use for Drop-off

During terminal stance and pre-swing, the role of the AFO is to prevent drop-off. The demands on the AFO to produce a plantarflexion moment increase significantly in proportion with patient weight. A 200 lb. patient, for example, will require a peak plantarflexion moment of 140 Nm, while a 100 lb. patient will need only 70 Nm. This is significant since not all carbon fiber AFOs are capable of producing these large moments (Figure 3). Choosing an AFO without adequate stiffness will allow the patient's COM to drop during late stance and result in a loss of energy that will need to be compensated for through the use of other muscles. An AFO that is too stiff will produce an excessive extension moment at the knee that will induce a different set of gait deviations that will also reduce energy efficiency.

In selecting the optimal type of AFO for your patient, body weight and stride length are the most significant parameters. While the effect of body weight is obvious, incorporating stride length may be a new concept. As stride length increases, the moment at the ankle increases proportionately. A patient with a shortened step length may need significantly less moment at the ankle. The clinician must decide if the patient has the potential to increase his or her stride length and provide the most appropriate AFO based on that assessment (Table 1). If the desired value exceeds the options available, several manufacturers offer custom designs.

While most AFO choices are adequate to prevent foot drop and foot slap, additional care must be taken when choosing an AFO to supplement weak plantarflexors in late stance. By providing the correct amount of plantarflexion moment in late stance, the AFO will help to reproduce normal kinematics and prevent energy inefficient gait deviations. 

David Knapp, CPO, is the president of Connecticut Brace and Limb, Middletown, and chair of the American Academy of Orthotists & Prosthetists' (the Academy's) CAD/CAM Society. He received his P&O education at the Newington Certificate Program. 

Academy Society Spotlight is a presentation of clinical content by the Societies of the Academy in partnership with The O&P EDGE.