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ORIGINAL RESEARCH ARTICLES

Plantar Pressure Changes with Use of an Intrepid Dynamic Exoskeletal Orthosis

Stewart, Julianne PT, DPT; Djafar, Tatiana MS; Miltenberger, Richard MPO; Kingsbury, Trevor MA; Wyatt, Marilynn PT, MA

Author Information
Journal of Prosthetics and Orthotics: January 2020 - Volume 32 - Issue 1 - p 59-64
doi: 10.1097/JPO.0000000000000281
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Abstract

Of the injuries sustained in recent US military conflicts, the largest category is composed of injuries involving the limbs.1 The mechanism of these injuries includes improvised explosive device, gunshot, grenade, mortar, and shrapnel, and the damaged systems commonly involve skin, muscle, peripheral nerves, osseous structures, and vasculature.2 In addition, many service members sustain lower-limb trauma during training accidents or motor vehicle accidents. Service members are left with the difficult choice between amputation and attempting limb preservation after sustaining a critical limb injury. Although limb preservation procedures have become a much more common alternative to amputation after critical limb trauma in the civilian sector,3,4 high levels of disability have been reported with both amputation and limb preservation.5,6 The debate between amputation and limb preservation continues to be an area of great concern and extensive study4,7,8 due to advances in both fields including new bracing techniques for patients who have undergone limb preservation procedures.

The Intrepid Dynamic Exoskeletal Orthosis (IDEO) is a custom passive dynamic ankle-foot orthosis that was initially developed at Brooke Army Medical Center, San Antonio, Texas, for use with service members who have sustained critical lower-limb injuries and is now available to civilians. It consists of a custom proximal cuff, posterior carbon fiber struts with multiple available levels of stiffness, and a custom carbon fiber footplate with medial-lateral support (Figure 1). The design and development of the IDEO are described extensively by Patzkowski and colleagues.9 Notably, the design of the IDEO is fundamentally different from that of an unloading-type brace as there is no clearance between the user's heel and the footplate of the device. The IDEO was designed to enable performance of high-level activities such as running or jumping and aims to transfer loading from the foot to the anterior proximal tibia via deformation of the carbon fiber struts and custom-molded tibial cuff that occurs during midstance to terminal stance.9 The IDEO has been shown effective in decreasing pain levels during activity and improving functional performance in high-level tasks.10 In combination with the return-to-run clinical pathway,11,12 use of the IDEO has been shown to improve self-selected gait speed and functional performance measures, as well as rates of return to duty.13 Use of the brace has also been shown to decrease preference toward amputation among service members with critical lower-limb injuries.14 Despite these very successful outcomes, little is known about the mechanical effects of the IDEO on those who use it. Two studies have compared the gait of IDEO users to healthy controls.15,16 However, it is unknown how plantar pressures are impacted by use of the IDEO in an injured population.

F1
Figure 1:
The IDEO brace with custom-molded carbon fiber footplate, carbon fiber struts, and a custom-molded, hinged tibial cuff.

Dynamic plantar pressure can be measured during walking using pressure-sensitive insoles or mats, and it indicates the pressure experienced at various points on the foot, temporal components of this pressure, area of contact, and the progression of the center of pressure. Plantar pressure measurement has been used extensively in both clinical and research settings to examine contact area, loading, and pressure to the foot after injury, with bracing, and in gait deviations caused by disease. Significant changes in peak plantar pressure have been noted in subjects with foot injuries.17–19 Many researchers have studied the effects of bracing or orthosis use on plantar pressures to validate the assumption that these devices can help correct distribution of pressure in the foot or decrease plantar pressure peaks. Birke et al.20 demonstrated decreased plantar pressure in all parts of the foot while wearing a walking boot. Others noted up to 47.6% reduction in peak overall plantar pressure while using a patellar tendon bearing brace.21 Medial to lateral shifts in hindfoot pressure have been demonstrated with custom foot orthosis use.22 Although each of these devices shares some characteristics with the IDEO, it is not known how the IDEO impacts plantar pressure variables. Pain during weight bearing is often the chief complaint of those with critical limb injury; therefore, it is crucial to understand the plantar pressures experienced by an injured population during weight bearing both with and without an IDEO.

The aim of this study was to evaluate the effect of an IDEO on total foot peak plantar pressure and peak plantar pressure in each of the eight regions of the foot in an injured population in both shod (without IDEO) and IDEO conditions for the affected and unaffected limbs. It is hypothesized that when comparing shod to IDEO conditions, peak pressure will be decreased in the affected limb in the IDEO condition and will be unchanged in the unaffected limb.

METHODS

Study Design and Subjects

The within-subjects study protocol was approved by the Naval Medical Center San Diego Institutional Review Board in compliance with all applicable federal regulations governing the protection of human subjects. Seventy-four potential subjects were identified via retrospective review of a registry of clinical patients at a major military treatment facility spanning from the start of the IDEO program at this facility in June 2013 to August 2016. Subjects were screened for inclusion if they had been prescribed a unilateral IDEO for lower-limb injury causing significant deficits to gait and function, had received clearance to use this device by an orthopedic physician, and were able to walk on level surfaces for approximately 15 minutes in each shod (without IDEO) and IDEO-donned conditions. Subjects were then excluded if they had bilateral lower-limb injury, spine or pelvis injury, or central neurological disorder (moderate or severe traumatic brain injury, cerebral palsy, cerebrovascular attack, or other neurological disorder). Forty-two eligible subjects (40 males) aged 29.7 ± 8.1 years at the time of the study were identified (height 178.6 ± 7.8 cm, weight 92.6 ± 15.4 kg). Each subject had been fit for a custom IDEO brace by a single orthotist (R.M.) and had received this brace within 2 weeks of the study and had not yet received any training in the use of this device. Subject characteristics are described in greater detail in Table 1.

T1
Table 1:
Subject characteristics

Equipment

The novel Pedar-x in-shoe plantar pressure measurement system (Novel, Munich, Germany) was used to record peak plantar pressure and contact area during gait. The Pedar-x system has 99 capacitive sensors in each insole and collects data at up to 101 Hz. Novel pedar systems have been used extensively in studies examining plantar pressure in injured populations18,19,23 and have shown good validity and reliability.24

Data Collection

Each subject was fit with the pair of regularly calibrated, pressure-sensitive insoles that best matched his or her foot and shoe size. For the shod condition, the insoles were placed in the subject's shoes directly underneath his or her feet. For the IDEO condition, the insole on the affected side was placed between the subject's foot and the IDEO footplate. The insoles were connected via a cable to a subject waistband containing a transmitter that enabled wireless data transmission to the data collection computer. All cables were secured to the subject's body to minimize safety hazards. A calibration trial was collected before each condition with the subject's shoes loosely donned with shoelaces untied and feet elevated off the floor one at a time while in a seated position to achieve a baseline pressure level. Following this, a standing static trial was performed to verify correct placement of the insoles under the feet. For each condition, the subject walked for two 35-m trials at a self-selected velocity while plantar pressure data were collected at 50 Hz and transmitted to a collection computer via Bluetooth. These trials were performed in a flat, linear hallway without turns or obstacles.

Data Processing

Novel pedar software was used to create masks dividing the foot into eight regions. These regions were defined as the hindfoot (posterior 31% of foot), midfoot (subsequent 29%), forefoot (subsequent 20%), and toes (anterior 20%) (Figure 2). These regions were further divided into medial and lateral sections (50% of medial-lateral width of each of the forefoot, midfoot, and hindfoot was used as dividing line, the toe region was divided 33% from the medial side). Peak pressure was calculated for the total foot and for each of the eight regions. Percentage decrease from shod condition to IDEO condition was calculated with a positive percentage indicating a reduction in pressure and a negative percentage indicating an increase in pressure.

F2
Figure 2:
Definitions of the length (L) and width (W) divisions of the total foot used in this analysis and location of sensors in insoles.

Statistical Analysis

Due to nonnormal distribution of some of the data, pairwise Wilcoxon signed-rank tests were performed to compare IDEO and SHOD conditions in the total foot and in each of the eight sections of the foot previously described for both the affected foot and unaffected foot with an initial significance level of P < 0.05. This resulted in 18 pairwise Wilcoxon signed-rank tests and an adjusted significance level of P < 0.0027.

RESULTS

In the analysis of peak plantar pressure, pressures were significantly decreased in the all regions of the affected foot except the medial midfoot while using the IDEO with up to 63.8% reduction seen at the forefoot and toes. As a result of these decreased pressures in the forefoot and toes, the line of progression drawn by the movement of the center of pressure during walking became much shorter on the affected side in the IDEO condition (Figure 3). Increases in peak pressure of up to 23.7% were seen in some regions of the unaffected foot and in the total foot (Table 2). Averaged peak plantar pressures across the entire subject group for both shod and IDEO conditions are shown in Figure 3.

F3
Figure 3:
Group averaged peak plantar pressures (in kPa) with shod on left and IDEO on right. The average center of pressure progression line is also delineated by the red line on each foot.
T2
Table 2:
Peak pressure mean values ± standard deviations in kPa in the total foot and eight foot regions in the affected foot and unaffected foot for each shod and IDEO conditions

DISCUSSION

These findings help provide insight into the mechanism behind the improvement in function and decrease in pain that has been seen with use of the IDEO.10 Interpreting the change in the line of center of pressure progression seen in the IDEO (Figure 3) and the regions of greatest peak plantar pressure reduction from the IDEO (Figure 2) in the context of normal gait, the greatest offloading occurs from midstance through terminal stance. For subjects with foot and ankle pathology, midstance and terminal stance are often painful and difficult due to the high dorsiflexion moment typical of these phases causing stress on injured tissues. This dorsiflexion moment is created by the long lever arm of the foot combined with the progression of the body's center of mass to a position anterior to the ankle joint center. Although the forefoot is ordinarily loaded from midstance to terminal stance, the dorsiflexion moment experienced during this time creates stress on the brace itself due to the rigidity of the IDEO footplate, causing deformation of the posterior struts and a transfer of normal forefoot loading to the proximal tibial cuff.

A slight increase in peak plantar pressure was seen in the lateral hindfoot of the unaffected foot with use of the IDEO. This may be partially due to increased gait velocity while using the IDEO. Velocity was self-selected and was not measured during plantar pressure testing. However, gait velocity data were collected during an instrumented gait study immediately before plantar pressure data collection for each subject. These unpublished data showed velocity was significantly higher (P < 0.001) with the IDEO (1.19 ± 0.17 m/s shod vs. 1.26 ± 0.14 m/s IDEO). Small, region-dependent increases in plantar pressure with this 0.07 m/s increase in velocity could be explained by previous findings, but the expected increases would be approximately 4.1% at the heel.25 Considering these findings, much of the increase in plantar pressure is not explained sufficiently by changes in velocity.

It is plausible that the increased peak pressure in the lateral hindfoot of the unaffected foot is due to overloading the unaffected limb to adjust for unfamiliarity with the IDEO as these subjects had yet to undergo training in use of the brace at the time of testing. In any case, increases in loading on the unaffected limb are less than half the magnitude of the unloading seen in the forefoot of the affected limb, so it is unlikely that overloading the unaffected foot is contributing substantially to the changes in plantar pressure seen in the affected foot.

There are several limitations of this work, most notably including a lack of measured gait velocity during plantar pressure data collection. The effect of gait velocity on plantar pressure discussed previously may be confounding the results, particularly the increases in plantar pressure noted on the unaffected side. During this study, plantar pressure was measured outside of a gait laboratory environment, and total loading was not measured outside the shoes. This limits understanding of the potential contributions of purposeful or habitual unloading of an injured limb. An additional limitation is the fact that all IDEO braces used in this study were made by a single orthotist. This ensures good consistency throughout this study but limits the applicability of the findings to IDEO braces fabricated by other orthotists. Future work on this topic would benefit from measuring or controlling gait speed, using force plates to measure total loading, and including IDEOs fabricated by other orthotists. In addition, the effect of training in use of the IDEO on plantar pressure variables should be further examined as only preliminary work in this area has been done.26

CONCLUSIONS

Despite a growing body of evidence demonstrating improvement of function with use of the IDEO, this is the first study to confirm that this brace can decrease peak plantar pressure during walking, particularly in the forefoot and toes of the affected foot. This outcome provides insight to clinicians seeking a device to improve painful gait, particularly for conditions of the foot or ankle that produce pain in weight-bearing. In addition, although the IDEO was designed to enable performance of high-level activities in a population with critical limb injury, this study provides support for using the IDEO for walking by those with a diverse collection of injuries.

ACKNOWLEDGMENTS

The authors would like to acknowledge the invaluable help of Keith Qualls with IDEO fabrication; Jenny Anne Maun, Kimberly Rowe, and Katherine Sharp with data collection; Robert Riffenburgh with statistical analysis; and John-David Collins with statistical analysis interpretation. This work was supported with resources provided by Extremity Trauma and Amputation Center of Excellence. The Geneva Foundation provided contract management and support services for two of the authors on this article (J.S., T.D.).

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Keywords:

orthosis; rehabilitation; limb preservation; trauma; injury; ankle; foot

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