Researchers have explored numerous offloading strategies to decrease plantar pressures in patients with diabetes, but few have studied whether making a deliberate change to a patient’s walking pattern might have a similar effect on pressure distribution.
by Mansoo Ko, PhD
Reducing peak plantar pressure (PPP) under the foot during walking has been a domain of research interest regarding persons with diabetes, since repetitive mechanical stress1,2 and loss of protective sensation3,4 on the plantar surface of the foot have been considered the most relevant factors in skin breakdown resulting in diabetic foot ulcerations. This article will review the biomechanical aspects related to normal foot function for plantar pressure distribution during gait cycle observed from healthy individuals. To facilitate an understanding of the mechanism contributing to ulceration of the diabetic foot, this article will also discuss forefoot plantar pressure associated with walking speed in diabetic patients. A case study will also be introduced to illustrate an adaptive strategy for off-loading during upright barefoot walking in diabetic patients with peripheral neuropathy.
Normal foot function
Human walking is a very complicated movement. The complexity of walking occurs because it is not a single joint movement, but it is associated with heterogeneous biomechanical mechanisms in relation to heel strike, foot flat, mid-stance, heel off, and toe off in the stance phase of gait cycle. During stance phase in healthy individuals, the heel contacts the ground in a supinated foot position and the subtalar joint of the ankle rapidly pronates after contact to minimize impact loading.5 In order to absorb potentially harmful impact forces, multi-axis structures of the foot allow not only rapid subtalar joint motion but also accurate control of the ankle dorsiflexors to form a curved beam (figure 1B) of the medial longitudinal arch (MLA) in early stance phase.6
The dorsiflexors, during the segment of the gait cycle from heel strike to foot flat, eccentrically decelerate foot drop, distributing plantar pressures generated in response to mechanical loading on the ground. At mid-stance, the load is more vertical and the MLA functions like a truss (figure 1C), with the plantar aponeurosis tightened for support in the foot.6 As the foot transmits the load vertically, the height of the MLA decreases to dissipate the shock of heel strike, with a navicular bone drop and eversion of the rear foot. Figure 1C shows a significant plantar pressure reduction at mid-stance in a healthy individual. Biomechanically, the center of gravity of the body is shifted forward and the foot proceeds quickly to heel off in preparation for the acceleration phase of the gait cycle. From heel off to toe off, the peroneous longus muscle exerts a large percentage of its force in stabilizing the base of the transverse arch against the vertical ground reaction forces directed upward against the plantar forefoot region of the foot.7 The amount of forefoot plantar pressure plays a role in generating a forward propulsive force before toe-off during gait cycle.2,8 Thus, the forefoot plantar surface is relatively exposed to high forces, which is one factor that contributes to the increased risk of foot ulceration in diabetic patients with peripheral neuropathy.
Elevated plantar pressure under the forefoot in diabetic patients when walking is associated with a force applied to a small area on the plantar surface. Forefoot structural changes, such as hammertoes and claw toes, and limited joint mobility have been thought to be the most relevant factors for increasing plantar pressure over a small area during walking.9 The force applied to a small area is associated with more harmful impact forces than the same force distributed over a larger area of the plantar foot surface.10 Hughes et al suggested that the increased force and pressure on the decreased contact area can cause foot ulcers under the first metatarsal head and hallux.11 During barefoot walking, peak plantar pressures in the diabetic foot have been shown to be higher at the forefoot than the rearfoot.1 Therefore, repetitive and/or excessive forefoot plantar pressure, coupled with foot structural changes, can contribute to the development of forefoot plantar ulceration.
Forefoot plantar pressure and diabetes
With respect to forefoot plantar pressure, previous studies have indicated that imbalance in pressure distribution exists between forefoot and rearfoot during walking in persons with diabetes. Foot plantar pressures ≥ 6 kg/cm2 (588.6 kPa) is considered an indicative pressure threshold, which can cause soft tissue damage during walking among high risk diabetic patients.3,12 Peak plantar pressure in diabetic patients with claw- or hammertoe deformity measured 621 kPa under second and third metatarsal heads, whereas a age-gender matched control group measured 363 kPa.9 In addition to the aforementioned foot pressure threshold of 6 kg/cm2, a forefoot-rearfoot peak plantar ratio greater than 2 also has been able to predict foot ulceration.3 In diabetic patients with severe neuropathy, the forefoot PPP was about 2.3 times higher than the rearfoot PPP, whereas it was 1.3 times higher in a mild neuropathic group.3 Mueller et al showed that PPP in diabetic patients with peripheral neuropathy were 2.6 times higher in the forefoot, where most skin breakdown takes place, compared to the rearfoot during barefoot walking.13 In that study, PPP in diabetic patients with a plantar ulcer was 10.1 kg/cm2 (988 kPa).13
Courtemanche et al reported that, compared to an age matched control group, diabetic patients with peripheral neuropathy developed a more cautious walking pattern of decreased walking speed and increased double support time.14 In the walking trials, walking speed was 1.06 m/s in long term diabetic patients (64±9 years) and 1.27 m/s in the healthy elderly group (63±6 years).15 Speed of walking is positively associated with PPP on the plantar surface of the foot in persons with diabetes.8,16 Thus, it seems logical to assume that the diabetic patients with a slower, cautious walking pattern could have reduced PPP in the plantar surface of the foot compared to healthy individuals. However, forefoot PPP remains much greater in diabetic patients with neuropathy, even during cautious walking, than in a healthy control group. Mueller et al indicated that the forefoot PPP was 34 % greater in the diabetic patients with neuropathy than in the control group.13 Bus et al also showed that, at the metatarsal head region, PPP was 1.7 times higher in an age- and gender-matched group.9 This is because biomechanical constraints such as limited joint motion, claw- and hammertoe deformity, and callus can cause higher forefoot pressures even during slower walking speed in diabetic patients with neuropathy.17-19 Bus et al indicated that claw/hammer toe deformity can cause distal migration of the fat-pad cushion and plantar pressure transfers proximally under the metatarsal heads in diabetic patients.9 Reduced plantar tissue thickness is associated with higher PPP, indicating greater risk of forefoot ulceration.10
In addition, limited motion at the ankle joint caused by a tight Achilles tendon can elevate forefoot PPP during the earlier heel-off phase of the gait cycle.20,21 Cronin et al reported that Achilles tendon length changes during walking were limited in long-term diabetic patients compared to an age-, height-, and body mass matched group.15 Zimny et al indicated that limited joint motion at the ankle is significantly associated with higher plantar pressure at the forefoot in diabetic patients with peripheral neuropathy.22 Mueller et al have shown that lengthening of the Achilles tendon attenuates the recurrence of neuropathic plantar ulcers of the forefoot by 52%, compared with total contact cast therapy. 23 Orendurff et al have found that Achilles tendon lengthening can reduce peak forefoot plantar pressures by 27% in persons with diabetes.8
Preliminary data (not published) shows diabetic patients without biomechanical constraints have a significantly slower walking speed (0.94 m/s) than an age- and gender-matched healthy group (1.17m/s), but results in a significant reduction of PPP at the fore and rear foot during barefoot walking. There were no locally elevated plantar pressures during a cautious walking pattern in the participants with diabetes. This finding indicates that diabetic patients without the aforementioned biomechanical constraints might be less exposed to the risk of soft tissue damage during walking.10
Some specific walking patterns have been investigated to reduce forefoot peak plantar pressures in people with diabetic neuropathy.24-26 Brown et al indicated that the use of a step-to gait may be beneficial for diabetic patients who need to reduce plantar pressures under the forefoot.25 Similarly, Drerup et al suggested that the step-to walking pattern provides PPP reduction under the forefoot on the side of the leading foot during walking.26 However, the step-to walking pattern requires a stopping moment to prepare for the next step,24 which results in a slower and asymmetrical gait.25 Few studies have documented the effect of postural reeducation training on plantar pressure during gait. The following case study shows that simple interventions, such as correcting the postural alignment of trunk during gait, can result in immediate effects on the reaction forces on the plantar surface of the foot in a diabetic patient with peripheral neuropathy.
The participant was a 71 year old Caucasian male diagnosed with type II diabetes eight years earlier. The participant had no history of foot ulceration. Decreased sensation to monofilament and vibration, and formation of a callus on the second metatarsal head was observed from clinical examinations (figure 2). The participant was instructed to perform walk at self-selected speeds over a 15-m walkway under three conditions: 1) normal, 2) leaning forward, and 3) upright. In the normal condition, the participant was instructed to walk normally as he would in the community. In the leaning forward condition, the participant was instructed to walk while maintaining a hip flexion angle of 25°. In the upright condition, the participant was instructed to walk as if a string was pulling his chest straight up toward the ceiling, keeping his head facing forward. Muscle activity and the kinetic and kinematic data from laboratory experiments identified that 1) PPP in the forefoot matched the area of the callus on the second metatarsal head for all three conditions; 2) a smooth reciprocal inhibition of the soleus muscle occurred, while the tibialis anterior muscle became activated for upright walking condition, and 3) a significant reduction (15%) of vertical ground reaction force was observed under the forefoot during upright walking compared to the other two conditions.
The results of this case study show that correcting postural alignment may absorb impact from heel strike and dissipates shock over greater area of plantar surface at mid stance during walking. It was noted that contact area during upright walking (115 cm2) was greater than during forward bending (102 cm2) and normal walking (108 cm2). The significant reduction of vertical ground reaction force may be associated with increased contact area during barefoot walking. This increased contact area allows vertical ground reaction forces to be dissipated over a larger plantar surface.
Diabetic patients show an increase in plantar pressure under the heads of the metatarsals in the forefoot and thus have a higher incidence of plantar ulcers in this region.13 Therefore, decreasing the plantar pressures in the forefoot region may reduce the chance of skin breakdown and plantar ulcers under the metatarsal heads in the diabetic patient. Many strategies are currently used to decrease PPP during walking for diabetic patients with biomechanical constraints such as casting, insoles, rocker shoes, and customized shoes.24,27 These devices work to prevent localized pressure on the foot and increase the area of weight-bearing force. Our findings suggest that modifying postural alignment of the trunk may have a potential effect for distributing the vertical force over a larger area of the plantar surface in ambulatory individuals with diabetic peripheral neuropathy during walking.
Mansoo Ko, PhD, is an assistant professor in the department of physical therapy at Angelo State University in San Angelo, TX.
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