The Effect of the Scotchcast Boot and the Aircast Device on Foot Pressures of the Contralateral Foot

Introduction Offloading devices are commonly used to facilitate wound healing in the diabetic foot. These devices work on the principle of reducing pressure loading at the active wound site, thereby allowing the wound to heal without the constant pressure applied to the wound. Several offloading devices have been developed over the years that include commercial off-the-shelf and custom-made devices. Offloading devices are often bulky and heavy in order to allow offloading, which may potentially lead to asymmetrical gait or problems with maintaining balance during walking. A history of foot ulceration is one of the strongest risk factors for future foot ulceration. Consequently, closely monitoring the contralateral foot when providing an offloading device to a patient is important. Although several authors have shown that offloading devices are effective in wound healing and reducing the pressure at the ulcerated foot,[1–8] few have shown the effects offloading devices may have on the contralateral foot. Lavery and colleagues reported on pressures measured in the contralateral foot in patients wearing total contact casts for offloading their foot ulcers.[9] The authors reported that reducing the peak pressure with a total contact cast on the ulcer site did not increase pressures on the contralateral foot. The purpose of this study was to examine the effect of two different offloading devices on foot pressures on the contralateral foot. The two devices tested were the Scotchcast boot (SCB), which is a custom-made up-to-ankle-level device, and an Aircast device (AIR) (Aircast Inc., Lincolnshire, United Kingdom), an off-the-shelf, below-knee device. The authors hypothesized that there may be a different effect on the contralateral foot between an ankle- and a knee-level offloading device caused by differences in limitation of movement at the ankle joint. The results presented in this paper are part of a larger study looking at the efficiency of the two offloading devices. Methods Twenty-two consecutive diabetic patients with active or recently healed plantar ulcers were recruited from the Manchester Diabetes Centre. All patients were using the SCB as part of their foot ulcer treatment. The study was approved by the local ethics committee; all patients received full information about the study and gave informed consent before any testing was carried out. Foot pressures were measured in three different footwear conditions, as follows: the post-op sandal (SAN) (Benefoot UK Ltd, Prestwich, United Kingdom)was used as a control condition and the SCB and the AIR were used as the two offloading devices. The offloading devices were worn on the affected feet only. In each footwear condition, a SAN was worn on the unaffected foot. No stockings were worn during testing on either foot in order to standardize the pressure measurements. Dressings were worn at the ulcer site throughout the experiment as necessary. The SAN was used as the control condition. It features a dual density 3/16-inch insole, a flared outsole with a rocker bottom, a reinforced heel counter, a padded collar and top-line, and an upper shoe made from mesh nylon. The SCB is custom made for each patient and has been used for more than 15 years at the Manchester Foot Hospital and Manchester Diabetes Centre.[10,11] It is made by first applying ribbed stockinette from beyond the toes to above the ankle. Nine-millimeter–thick orthopedic felt (at half thickness) is then wrapped around the ankle, crossing over the top of the foot and securing it with tape. Felt is placed under the sole of and around the foot. Orthopedic wool is applied over the felt, and over this layer, a layer of foam roll is applied. Fiberglass tape is finally applied, making the boot solid and durable. The boot is reinforced under the sole with a sheet of fiberglass. A windowed area is cut to relieve pressure from the ulcer site. The SCBs used in this study were removable by cutting open the dorsum of the cast and securing the open edges with 10cm strips of elastoplast adhesive strapping. The SCB was secured to the foot using Velcro straps and was worn with a cast sandal. The AIR is a commercially available contact cast offloading device and is also known as the Pneumatic Walker™ Diabetic System (Aircast Inc.). The AIR extends from the toes to just under the knee. It features a multicell support for total contact fit, a rocker sole, a regulated air-cell inflation with a hand-bulb pressure gauge, and a semirigid shell with full air-cell coverage, enabling maximum plantar unloading, regulating shear stress, and protecting and immobilizing the limb. The three types of footwear conditions were tested in a random order. The subjects were allowed to practice walking in each footwear condition until they felt comfortable. The Pedar In-Shoe Pressure Measurement System (Novel GmbH, Munich, Germany) was used for the pressure measurements. The insoles were carefully taped to the subjects’ feet in order to standardize sensor contact with the ulcer site between the different footwear conditions for each patient. Subjects were asked to walk a 10-meter distance four times. The first and last steps were not used for pressure analysis; thus, only the mid-gait footsteps were used, and the effect of acceleration and deceleration were discarded. The time taken for an eight-meter walk was used to ensure that walking speed did not confound results. For the pressure analysis, each foot was divided into five masks, including the heel, midfoot, first metatarsal head, lateral metatarsal heads, hallux, and lateral toes. Peak pressure, pressure time integral, and contact time were calculated for each mask. Statistical analysis was carried out using SPSS version 10. A repeated measure ANOVA was used to compare the three different footwear conditions and walking speed in each condition. Results The majority of the patients participating in the study were male (82%, 18/22), had type 2 diabetes (59%, 13/22), and had long duration of diabetes (20.6±14 yrs), and all had active or recently healed ulcers at the plantar surface of the foot. Twenty-three ulcer sites were included in the study. One study patient had bilateral ulcers. Ulcer sites were at the midfoot (n=4), first metatarsal head (n=7), lateral metatarsal head [metatarsal head 2–5] (n=8), great toe (n=2), and lateral toes (n=2). Figures 1 and 2 display the mean peak pressures and pressure-time-integral under the different regions of the plantar surface of the contralateral foot for the three different footwear conditions. Peak pressure and peak pressure time integral were not different in the contralateral foot between the three different footwear conditions. Figure 3 displays the contact time for the offloaded and contralateral foot in the three different footwear conditions. There was no difference in foot contact time between the three different footwear conditions for both the offloaded and the contralateral foot. However, a nonsignificant trend was noted toward a longer contact time for the contralateral foot compared to the offloaded foot for the sandal, SCB, and AIR condition (p=0.30, p=0.15, and p=0.05, respectively). Discussion Offloading is a key component to the healing process of a diabetic foot ulcer. Hence, several different offloading devices have been developed and are being used in clinical practice. The pressure reducing efficacy of the majority of these devices have been reported; however, very little attention has been paid to the effect of offloading on gait or on foot pressures in the contralateral foot. The results of the present study show that there were no differences in peak pressure or peak pressure time integral in the contralateral foot between the three different footwear conditions. However, there was a trend for a shorter contact time on the offloaded ulcerated foot compared to the contralateral foot. The results confirm previous findings reported by Lavery and colleagues,[9] who described that wearing a total contact cast did not affect foot pressures on the contralateral foot. In the present study, the authors compared two different offloading devices, one extending from the toes to the ankle and the second extending to just below the knee. They anticipated that there could possibly be a different effect on the contralateral foot caused by a difference in ankle movement restriction by the offloading device. Comparable results were reported by Veves and colleagues, who investigated foot pressures on the contralateral foot in diabetic and nondiabetic amputees and in diabetic nonamputees.[12] The authors showed that the diabetic amputees did not have increased foot pressures compared to the nonamputees; however, foot pressures were increased compared to the nondiabetic amputees. These results indicate that wearing an artificial limb did not increase foot pressures on the contralateral foot, which is in agreement with the results from the present study. Nevertheless, it was interesting to note that there was a trend for a shorter contact time on the offloaded foot compared to the contralateral foot, indicating an asymmetric gait pattern with the strongest trend observed for the AIR. Although the difference did not reach statistical significance, this difference could potentially lead to overloading the contralateral foot due to asymmetric gait or lead to problems with maintaining balance during walking. The majority of patients wearing offloading devices suffer from peripheral neuropathy, which consequently leads to reduced proprioception, putting these patients at risk for falling. In conclusion, wearing the SCB and AIR was not related to increased peak pressure and peak pressure time integral in the contralateral foot in the investigated group of patients. However, trends for increased contact time on the contralateral foot were observed, which highlight the importance to closely monitor the contralateral foot in patients wearing offloading devices.