AFOs and Interfaces: The Important Role of Socks
AFOs and forces during gait
Although the primary purpose of an AFO is to facilitate a gait pattern that is as close to normal as possible in a patient who is unable to walk or run unassisted, inevitably the device changes gait dynamics. There is very little in the literature on the exact nature of these changes, but several studies provide insights into the altered biomechanics.
In a recent study Silver-Thorn et al tested the effect of ankle orientation on heel loading and knee stability in eight individuals who had experienced a stroke. 4 The investigators applied experimental AFOs (molded polypropylene articulated AFOs fitted with pressure sensors) and tested three ankle orientations (5° of plantar flexion, 5° of dorsiflexion, and neutral). Results showed consistently greater peak heel contact force during loading response in the plantar flexed position than in the neutral or dorsiflexed positions.4 The findings suggest clinicians should be aware of the potential for increased stress at the interface between the orthosis and the heel, especially because, as stated by the study authors, “guidelines regarding AFO ankle position…are not well defined.” 4
In a study of healthy volunteers, Balmaseda et al found that the use of an AFO was associated with a 20% increase in vertical force magnitude at the end of push-off, a more posterior location of impact at heel strike, and a more lateral position of the trajectory of the center of pressure throughout stance. 5 McHugh 6 observed that the body-device interface forces in the lower leg are greatest when the purpose of the AFO, which in this case was designed to assist with dorsiflexor insufficiency and plantar flexor insufficiency, is to compensate for plantar flexor insufficiency in late stance. When an AFO is used in patients who lack dorsiflexion power during swing, the forces are relatively small. He also noted that, in the presence of spasticity, it is possible for body-device interface forces to be significantly higher.
These observations suggest that pressure and impact patterns on the plantar aspect of the foot can change dramatically when an AFO is used to address lower extremity motor and biomechanical issues. This is important for AFO design in terms of function and the types and strengths of materials used. By implication, these observations also demonstrate the need for soft tissue protection from the changing dynamics in the feet and lower leg, irrespective of AFO type or functionality.
Alignment and interfaces
Design and function are not the only factors that affect AFO interfaces with the skin and soft tissues of the feet and lower legs. Proper alignment is a significant factor, as well, especially with articulated AFOs; in particular, it is essential to achieve proper alignment between the components of the AFO and the anatomical joints. 7
According to a recent study by Gao et al, alignment of the AFO superior or posterior to the anatomical joint was associated with increased stresses at the interface of the foot and the foot plate of the orthosis—either more normal stress (in the case of superior alignment) or more tangential stress (in the case of posterior alignment).7 However, as a clinician, I understand that maintaining the best possible alignment can be a challenge, and therefore I recommend mitigating the effects of these stresses to help reduce or eliminate the possibility of soft tissue injury.
The need for protection
Given the potential damage to the skin and soft tissue that can be associated with AFO use, I believe clinicians should provide a consistent, robust system of protection for the feet and lower legs of the patients who wear them. I’ve also learned in my practice that many patients simply are not aware of the need for this protection and cannot be expected to take the necessary precautions on their own. For example, I recently I saw a patient who had had an AFO fitted at another facility several years earlier. I explained the expectations for her new device and the importance of monitoring her skin and communicating with us if a problem arose. She said, “I thought it was normal for my ankles to be bright red all the time.”
What is needed for skin and soft tissue protection, especially in cases where foot pathologies may render the foot more vulnerable to increased stresses? Doug Richie, DPM, has suggested that footwear requirements for the pathologic foot include the protective benefits of padded socks. 8 Although his comments were made specifically in regard to the feet of people with diabetes and peripheral neuropathy, I believe similar requirements also are appropriate for the feet of patients who wear AFOs—at least some of whom may have diabetes and peripheral neuropathy.