The solid ankle cushioned heel (SACH) foot is not the only foot design assigned to the Medicare Kl category; nonflexible keel feet and single-axis feet are included as well. However, the SACH foot is probably the most common foot design selected for prosthetic foot studies because of its widespread popularity worldwide and because it has served as a "gold standard" for prosthetic feet for so many years. Conversely, when discussing dynamic response feet, it appears that the Flex-Foot is the most common K3 prosthetic foot design included in research publications. This is probably the result of the Flex-Foot being the first prosthetic foot with the unique J-shape design and the first to be constructed with carbon fiber materials. When the author began reviewing the literature in an attempt to discuss why such significant differences were found among the prosthetic feet in terms of function and speed of walking, many interesting observations were found by other investigators who were comparing these two foot designs as well as several other prosthetic foot designs. Many of these observations may assist in explaining why there was such a considerable improvement among the categories of prosthetic feet.
The SACH foot has long been considered the standard of care for low functioning and elder amputees. First designed in 1958 by Eberhart and Radcliffe, this simple design has a wooden keel enveloped in rubber with a rubber cushioned heel that supposedly absorbs ground reaction forces and permits plantar flexion of the foot. For decades, the SACH foot has been the most commonly prescribed foot and a favorite of clinicians for its simplicity and low cost.
Although the SACH foot is a favorite with many clinicians, the simple design that makes it so popular has been found to present several potential shortcomings. Compression of cushioned heeled feet combined with rigid ankles tends to increase the time from initial contact to loading response,1 suggesting that rather than promoting plantar flexion, the cushion heel design actually delays the plantar flexion motion necessary to achieve loading response. The delay in progressing to loading response can be associated with the increased double support time that is frequently observed with amputee gait.2–4 A consequence of the prosthetic foot's delay to loading response is that the sound limb is simultaneously in late stance waiting for the prosthetic foot to reach foot flat. As the intact forefoot remains on the floor for a prolonged period, the ground forces are directed to an area of the foot that has the highest risk of ulceration in diabetic amputees. Although there is no evidence that increased foot ulcers do occur, the potential may exist.
To compensate for the lack of energy generated during late stance by the plantarflexor muscles, the hip extensors generate greater work from initial contact to mid-stance.2 However, it has been determined that there is no significant difference in the pattern or magnitude of power output generated by the knee and hip between dynamic-response feet and nondynamic-response feet.1,5 Interestingly, Torburn et a1.6 found that the biceps femoris and possibly the gluteus maximus appear to have considerably greater effort during early stance with all feet except the Flex-Foot.
Dynamic response feet with a J-shaped pylon design appear to promote a faster walking speed that can be attributed to a couple of reasons. For instance, the Flex-Foot has repeatedly demonstrated the greatest peak dorsiflexion moment of any prosthetic foot.6–8 The increased dorsiflexion occurs because of the larger moment arm created by the pylon and foot plate being constructed as a single section permitting the body's weight to progress over the stationary foot and allowing the pylon to mimic the tibia's forward progression. Because of the greater dorsiflexion there is greater movement of the center of pressure farther forward.7,8 The ability of the center of pressure to move farther forward is also a product of the foot plate extending to the toe of the prosthetic foot. Because the body's weight passively produces the force that dorsiflexes the J-shaped foot, the amputee does not have to provide any additional muscular effort during single limb support. As a result of the increased dorsiflexion and the extended foot plate, the rate of progression of the center of pressure moves faster from the rearfoot to the forefoot at both slow and fast walking speeds.6,9
The extended foot plate design appears to have several advantages that can directly affect both walking speed and the contralateral limb. Traditionally, most keels of a prosthetic foot are similar to the SACH, extending approximately to the mid-foot with the forefoot primarily constructed of a rubber material that deforms to replicate the metatarsal break that occurs in the human foot. Because of the shorter keel, the body tends to progress more rapidly to terminal stance as the center-of-mass essentially runs out of base-of-support to progress over. Torbum et a1.6 found that the SACH and single-axis feet had a more rapid progression during terminal double-limb support or preswing than the Flex-Foot. Essentially, it appears that the amputee is falling off the end of the foot.
Macfarlane et al.10 reported that the Flex-Foot had a significantly longer duration of early and late swing with the contralateral limb. The greater swing time suggests that greater stability exists when standing on the Flex-Foot, because the ability to balance longer over the prosthetic foot during late stance permits the sound limb to take a slower and longer step. They concluded that the roll-over shape is influenced by the mechanical properties of the prosthetic foot, such as the shape of the keel or foot plate and the materials of the foot. The authors also suggested that prosthetists tend to align dynamically to achieve the preferred walking pattern by trying to achieve a favorable roll-over relationship. The toe characteristics of the longer foot plate designs allow the center of pressure to move further on the toe extending the "roll-over" shape as a result possibly reducing the forceful early loading on the sound limb.12
The falling onto the contralateral limb with SACH was confirmed by Lehman et al.,8 who found that the difference of aft shear impulse on the prosthetic side and the fore shear impulse on the sound side showed the smallest value for the Flex-Foot and the greatest value for the SACH foot. There is also a greater vertical force loading peak, a greater plantar flexion moment, and an increased knee flexion moment on the sound limb while using a SACH, which may explain the greater transfer of loading to the sound limb during initial contact, due to the ineffective push-off on the prosthetic side. Because of the amputee's marked inability to push-off after foot-flat, the "drop-off" phenomenon becomes even more evident when amputees try to walk fast or run with conventional keel prosthetic feet.13 As a result, some amputees may limit their walking speed to reduce the drop-off effect and reduce the forces placed on the sound limb.
When compared with the human foot, the passive nature of dynamic response feet offers limited energy return,5 primarily due to the loss of plantarflexion muscle action. However, the J-shape design of the Flex-Foot has been demonstrated to produce 70% to 90% energy return as compared with the 20% to 40% return of the SACH foots: a twofold to threefold difference between the two feet. This is not to say that there is conservation in the metabolic cost of walking between dynamic-response and nondynamic-response feet. To date, it has been fairly well established that only a 4% difference exists between dynamic-response and nondynamicresponse prosthetic feet.8,14,15
Gait symmetry has a considerable influence on walking speed because equal stride lengths provide a uniform baseof- support permitting better control of the displacement of the body's center-of-mass, ultimately improving balance. Amputees, however, tend to take a slightly longer step with the prosthetic limb3,4 with increased asymmetry as walking velocity increases.16,17 Schneider et a1.9 found that the Flex- Foot offered fewer asymmetries than the SACH foot, which may be attributed to improved balance on the prosthetic foot during late stance over the extended foot plate. Moreover, a longer stride was noted with the Flex-Foot with fewer steps per minutes than with the SACH foot, yet with similar walking speed.11 However, the same group also found that the Flex-Foot facilitated faster walking and more normal values at self selected walking speeds.
Correspondence to: Robert Gailey, PhD, PT, Miami Veterans Affairs Medical Center, Functional Research Laboratory, University of Miami Miller School of Medicine, Department of Physical Therapy, 5915 Ponce de Leon Boulevard, 5th Floor Plumer Building, Coral Gables, FL 33146; e-mail: .
ROBERT GAILEY, PhD, PT, is affiliated with Miami Veterans Affairs Medical Center, Functional Research Laboratory, University of Miami Miller School of Medicine, Department of Physical Therapy, Coral Gables, Florida.