Research & Presentations
Choosing the Optimal PD-AFO for Your Patient By David Knapp, CPO
Introduction Passive dynamic ankle-foot orthoses (PD-AFOs) are nonarticulated AFOs that are designed to flex during the stance phase of gait and return to their neutral angle during swing phase. They are most commonly fabricated using carbon-fiber composite laminates, and consist of a cuff and a footplate that attach together with a strut. The strut flexes to accommodate ankle motion, and the footplate flexes to accommodate metatarsophalangeal joint motion. PD-AFOs are designed to replicate the functions of the ankle muscles during gait. The gait cycle can be divided into seven segments based on function (see Figure 1). Throughout almost all of stance phase, PD-AFOs accurately mimic the direction of the torques that the muscles would normally produce. During pre-swing, however, the normal ankle will produce a plantarflexion moment as it plantarflexes to about 20 degrees, while the PD-AFO stops plantarflexing at its neutral angle and stops producing a moment. The interval where this occurs is brief—approximately 3 percent of the gait cycle—and it results in an early toe-off from the ground. Singer1 demonstrates that this has a negligible effect on gait efficiency. Perry2 refers to this period in normal gait as roll-off, rather than push-off; the distinction is important because it reflects the fact that the plantarflexor muscles do not show any electromyography (EMG) activity at this point in the gait cycle.
22 O&P News | March 2018
Figure 1
Normal ankle kinematics for normal gait. Line segments in green indicate good congruity between normal kinetics and those of the PD-AFO; red and yellow lines represent large and small disparities respectively. (Adapted from Perry)
Three factors need to be considered when designing a PD-AFO for a patient: 1. Strut and footplate stiffness: This determines the magnitude of the moments the AFO can produce. 2. Neutral angle (the angle at which the AFO produces no moment in either direction): This can be used to tune the timing of the moments. 3. The amount of energy return: This is important in the transition from stance to swing phase. The stiffness and neutral angle together will determine the functional range of motion for the AFO.
Stiffness The term quasi-stiffness is used by Shamaei3 to describe the behavior of the ankle during normal gait. Active muscles generate moments about the ankle in a repeated pattern during each step. Figure 2 shows the normal ankle during gait. The counterclockwise pattern indicates that the muscles are actively feeding energy into the system. PD-AFOs, because they are passive devices, produce a clockwise path on this type of plot, indicating that energy is lost in the deformation of the orthosis. Although it changes throughout the
