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A Quantitative Comparison of Four Experimental Axillary Crutches

Maurice A. LeBlanc, MSME, CP
Lawrence E. Carlson, DEng
Teresa Nauenberg

ABSTRACT

Four experimental axillary crutches, along with conventional crutches, were tested by five disabled subjects and five normal subjects, with knees flexible and with knees immobilized to simulate paraplegic conditions. Experimental crutches included a suspension crutch, rocker crutch, spring crutch and prosthetic foot crutch. Active and resting heart rates were recorded, along with average velocity to calculate an energy expenditure index for each trial. No statistically significant energy savings were observed with the experimental designs. However, the suspension crutch was the most energy-efficient of the experimental crutches and is worth exploring further. In addition, immobilizing the knees of normal subjects is judged to be an effective method of simulating paraplegic crutch ambulation.

Introduction

Crutches have not changed significantly during their 5,000 years of use (1). There are many reasons-physiological and psychological-why it is good to stand and walk rather than sit and have wheeled mobility (2-4). These reasons include improved bone growth, improved blood circulation, reduced bladder infections, reduced pressure sores and prevention of contractures.

However, it takes roughly twice as much energy for a person with paraplegia to walk with crutches than an able-bodied person to walk normally (5). the consequence is most people with paraplegia opt to use wheelchairs because they are easier. The high energy rise of crutches is due in part to vertical rise of the body necessary to clear the ground with locked knees during swing phase and to the shock the arms and shoulders must absorb at ground impact. Basically, a crutch user is doing a push-up with every step, and our arms were not made for walking, so it is tiring.

Shoup et al. suggested, based on a kinematic analysis of crutch gait, that the energy of crutch gait was due to vertical height fluctuations of the body's center of mass, the shock of crutch strike and circumduction in moving the crutches forward (6). Rovic and Childress concluded the metabolic energy demands of supporting the body weight with the arms and shoulders were more significant (4). The crutches used in this study were designed to reduce the high-energy cost of crutch gait by focusing on one or more of these factors.

The purpose of this study was to test energy efficiency for functioning versions of each design. None of the designs was optimized. If any design demonstrated significant improvement in energy, redesign might be warranted to eliminate the disadvantages of that particular version.

Background

There have been three key literature searches on crutches. Shoup conducted a search to 1974, Carlson conducted a search to 1980 and LeBlanc conducted a search to 1989 (6-8). Most of the literature citations involve the use, history, prescription and adverse effects of using crutches. (Adverse effects include hand, arm, shoulder and axilla problems caused from the use of crutches by some people under certain conditions.) Some citations involve improved fitting techniques and minor design changes or modifications. A small number of citations actually addresses major changes in design that affect the dynamics of crutch walking (see Table 1 ).

From Table 1 , there are seven references to major design changes for crutches. However, only three separate ideas are represented. For this project, the authors selected the three separate design ideas and added a new one for a total of four concepts to test as described below.

Crutch Designs Tested

Weights of the experimental crutches, as well as summaries of their advantages and disadvantages, can be found in Table 2 .

Suspension Crutch

The basic concept of the suspension crutch is to relieve the upper body from full weightbearing during swing-through. The earliest known reference to this concept was the saddle crutch introduced by Taylor in 1883 (see Figure 1 ) (9). Designed as a walking aid for people with femoral fractures, it featured a saddle connected to suspenders that extended to the axillary pads. This allowed a person to bear weight on the perineal area rather than on the arms. Taylor reported satisfactory results by several patients with leg fractures. It apparently was not designed for swing-through gait.

This concept was adapted by modifying a rock-climbing harness and adding lateral straps that could be fastened to the axillary pads of standard crutches (see Figure 2 ). The main advantage of this design is significant weight relief. In fact, it is possible (although not very stable) to actually walk with a swing-through gait without using the hands at all. This design is the lightest of the experimental crutches tested with the harness adding only 625 g, which represents an increase of 36 percent in total weight over axillary crutches alone.

One disadvantage of this crutch system is a potential danger of still being attached to the crutches during a fall. In addition, they tend to be less stable than other crutches and are somewhat more difficult in tight turns. Furthermore, there is slightly greater lateral pressure on the torso from the axilla pads. The harness tested was designed to be adjustable to a wide range of sizes and is not very cosmetically attractive. However, it was envisioned that a custom suspension harness could be designed that would be worn under clothing with only a pair of straps protruding from the waist band.

Rocker Crutch

Rocker crutches were introduced by Joll in 1917 and by Hall in 1918 (see Figure 3 and Figure 4 ) (10,11). Joll's roller was about 11 inches long with a two- to three- foot radius. His stated intention was to increase the rate of progression and have more secure contact with the ground. Hall cited advantages of longer stride length, more even motion and more secure contact with the ground. The cited disadvantages of this design are that it is more cumbersome, heavier and more complicated. Hall proposed a clever folding feature to reduce the rocker size when necessary, such as when going up and down stairs. Lumex Inc. currently makes the Sure-Gait Axillary Crutch, which has a small roller bottom the same width as the double uprights (i.e., about six inches) (11).

The version tested was designed by Carlson while at the Bioengineering Centre, University College, London in 1979 (see Figure 5 ). It was patterned after an unpublished design by Austin Isherwood, MD, and features sectors of a 60-inch radius (1.5 m) arc at the bottom of the crutch.

The basic concept is to minimize the change in vertical displacement of the axilla pads, thus reducing the vertical displacement of the body during ambulation. Its other advantages include a smooth gait and increased stride length. In addition, this design is very stable: One can balance on the crutches with the feet off the ground.

The main disadvantage is the rocker bottom's awkwardness on stairs, in aisles, etc. The version tested weighed 3.0 kg per pair, 73 percent heavier than the axillary crutches tested.

Spring or Pogo Crutch

Spring crutches were described by Lewin in 1928 and by Shoup in 1980 (see Figure 6 and Figure 7 ) (12,13). In Lewin's design the double uprights telescoped up and down, and in Shoup's design the spring was put in the single bottom tube. The presumed advantages are that it absorbs shock at ground contact and gives energy return at push-off. The disadvantages appear to be that it lowers the body during swing-through, thereby increasing the need for vertical rise, and has moving parts that are subject to wear, noise and breakdown. Also, the crutches are not rigid when used as braces for balance.

Studies by Shoup and Parziale showed reduction in shock but did not measure energy expenditure (13,14). The version of this crutch implemented for this project is one Carlson's students at the University of Colorado designed (15). It incorporates a 3/8-inch pneumatic cylinder in the bottom single tube that can be adjusted for body weight by changing the air pressure (see Figure 8 ).

Prosthetic Foot Crutch

One of the recent concepts in lower-limb prosthetics that is proving to be very successful is an "energy-storing" foot that uses elastic deformation of spring elements within the foot to absorb shock and ostensibly return energy to the user during toe-off. Although extensive clinical testing has not demonstrated any statistically significant energy savings, amputee reaction has been very positive (16).

This new idea for a crutch was proposed by LeBlanc. The concept is to have a crutch that "walks." An energy percent more than axillary), which is concentrated at the end of the crutch and must be moved forward by the user storing prosthetic foot is attached to the bottom of the crutch (see Figure 9 .) In this case, a Hosmer Dorrance Quantum Foot (with medium-strength spring) was used because it adapted well to the crutch (17). The action of this crutch is like the roller/rocker crutch in that it provides greater stride length and like the spring/pogo crutch in that it has springiness. It offers some shock absorption at heel-strike, some flexing during stance phase and presumable some energy return at push-of. The main disadvantage is extra weight (2.7 kg per pair, 56 percent more than axillary), which is concentrated at the end of the crutch and must be moved forward by the user during stance phase.

Previous Related Work

Gillespie et al. Tested a wooden rolling crutch on 20 normal subjects and reported 9 percent lower oxygen uptake with the rolling crutch (18). Basford et al. Tested similar crutches on 150 hospitalized patients who were not expirenced crutch users and found no significant differences in energy based on heart rate (19). Nielsen et al. Evaluated the Sure-Gait crutch, which is commercially available crutch with a small radius arc at the bottom. Based on the oxygen uptake of 24 normal females, no significant differences were found (20).

Shoup performed displacement and impact tests on five normal children using spring crutches (13). He found no correlation between crutch flexibility and vertical displacement of the shoulder, which would indicate potential energy difference. However, he did measure shock values that were reduced by half or more with spring crutches. Parziale and Daniels measured the shock loads in wooden spring-loaded axillary crutches in seven normal subjects who were experienced crutch users and found 22 percent smaller shock and 24 percent smaller peak stresses (14). None of the studies on spring crutches attempted to measure actual energy consumption by the user.

Methods

Energy Expenditure Index (EEI)

The two most common methods to quantify energy expenditure during ambulation are the rate of oxygen uptake and heart rate. Oxygen uptake is generally considered to be the most accurate measure; however, it requires subjects to wear a face mask that channels all the transpired gases to the appropriate instrumentation, which is usually mounted on a trolley that must follow the subject. A more recent system uses a portable instrument that can be carried by a subject, but it weighs 2.6 kg and still requires the use of the face mask (21). This is inconvenient and may introduce artifacts, especially with disabled subjects.

Heart rate is also used as a measure of energy expenditure (22-24). Most investigators have used maximum heart rate for this purpose. However, this does not take into account different subjects may have different resting heart rates nor the effect of different velocities of walking on energy expenditure.

The measure of expenditure that was used for this study is the energy expenditure index (EEI), which is calculated by (25):

EEI has been shown to correlate well with oxygen uptake for submaximal levels of cardiovascular activity (25-26).

Heart rate was measured with a Hewlett-Packard 78207C electrocardiogram monitor using surface electrodes with a telemetry system. The transmitter was small enough to be carried unobtrusively in a small belt pack. Heart rate was recorded for 30 seconds after subjects had walked for two minutes. Subjects rested five minutes after each test at which time the resting heart rate was taken for the next run.

Subjects

Five regular crutch users with differing disabilities (post-polio, above-knee amputation, spinal cord injury) participated in the study (see Table 3 ). The main criterion for participation was the ability to perform swing-through ambulation on crutches. In addition, five normal subjects were tested both under completely normal conditions and with their knees immobilized with either plaster casts or knee splints to simulate KAFOs with locked knees.

Subjects walked around a 24-in indoor oval track at a self-selected comfortable speed. Each lap was timed with a stopwatch; average lap time was used to calculate average velocity. In addition to the experimental crutches, each subject was tested using standard axillary crutches. For comparison purposes, disabled subjects were also tested on their own crutches, which were usually forearm style. Two tests were performed with each type of crutch. The order of crutches tested was randomly mixed.

Analysis

Data were analyzed for differences using the Student t-test (27). Differences for crutch type within each test group were analyzed using a paired t-test while differences for the same crutch between test groups were analyzed using the t-test for independent means.

Results

The EEI and average velocity for all subjects on all crutches are shown in Figure 10 . The solid lines indicate EEI values for normal children as a point of reference (mean +/- standard deviation). Mean values and standard deviations of EEI for the three test groups (disabled, normal with flexible knee, normal with rigid knee) are shown in Figure 11 and listed in Table 4 . Averages and standard deviations of velocity are plotted in Figure 12 and tabulated in Table 5 .

None of the experimental crutches tested demonstrated any statistically significant savings in energy expenditure compared with axillary crutches. Among the experimental crutches, EEI values for the suspension crutch were significantly lower (p<.l0) than the others for normal subjects with both flexible and rigid knee conditions.

The EEL for normal subjects with rigid knees was significantly higher for each crutch as compared with the same crutch used with subjects with flexible knees (p<.l0, except for the rocker crutch, = .13). Comparing normal subjects with disabled subjects, the only significant difference in EEL was that it was lower for the suspension crutch used by normals with flexible knees (.78 vs. 1.07 b/in).

The only significant difference in average velocity (which was self-selected) for disabled subjects was their velocity using axillary crutches was higher than when using the suspension crutch (70.2 vs. 59.6 in/mm, p<.05). Normal subjects (flexible knee) walked faster with both the pogo and axillary crutches than with the rocker, prosthetic foot (both p<.l0) or suspension crutches (p<.l2). With their knees immobilized, normal subjects showed no significant differences in their average velocities among crutch types. However, the velocity of normal subjects using the suspension crutch was significantly higher with flexible knees than with rigid knees (59.3 vs. 52.6 in/mm, p<.0l).

Discussion

Finding appropriate disabled test subjects turned out to be more difficult than anticipated. The target population was individuals who had long-term disabilities that impaired their mobility but who were also able to walk with a swing-through gait on crutches. There was a wide range of disabilities among the disabled subject group, which perhaps accounts for the large standard deviation shown in the data for that group. In addition, not all subjects could successfully use each type of crutch, so some of the test comparisons had smaller sample sizes.

The above-knee amputees tested were the most successful users (lowest EEI and highest velocity) of all types of crutch, probably because they have normal neuromuscular control, both motor and sensory, and are lighter in weight. The other subjects were encumbered by the extra weight and rigid knees of KAFOs. Spinal cord-injured paraplegics had the most difficulty with crutch ambulation. Two potential subjects were not used; one because of spontaneous spasticity in the hip muscles and the other due to excessive fatigue.

With normal subjects, the data were more tightly grouped. The suspension crutch had significantly lower EEL values than the other experimental crutches but not significantly lower than the axillary crutch. However, limited tests with one normal subject, after some training, showed 15 percent lower energy expenditure. The noticeable weight relief of the suspension crutch might offer the greatest potential for energy savings as suggested by Rovic and Childress (4).

Training is undoubtedly a factor in crutch use that was not accounted for in this study. Disabled subjects were, on average, 13 percent to 32 percent more energy-efficient with the crutch they used routinely although not with statistical significance. It is quite possible energy expenditure would decrease and average velocity increase with practice, especially with more unusual designs.

Most disabled subjects regularly used forearm crutches because they felt the crutches were lighter and more effective when rising from a seated position because the elbow can flex normally. The reason for using axillary upper support for each crutch in this study was the suspension crutch requires a relatively high point of support, and keeping all the crutches consistent seemed appropriate. Also, this consistency would minimize the training effect since most subjects were new to axillary crutches. Given the subjects' preference for forearm crutches; however, and considering that both forearm and Canadian crutches appear to be more energy-efficient than axillary crutches, it may be worth considering adapting some of the crutches to forearm or Canadian styles (23).

Subjective comments by test subjects regarding the relative merits of each crutch were mixed. The prosthetic foot crutch was judged to be very stable, but most subjects commented on the extra weight. Some subjects deemed the rocker crutch to be very smooth and comfortable while others found it hard to turn and difficult to stabilize. The shock absorption of the pogo crutch was noticed by many subjects, but so was the difficulty in clearance during swing-through.

There was a preponderance of positive comments regarding the suspension crutch. Most subjects perceived considerable weight relief, but they also noted that it felt slower and less stable. One subject referred to the suspension crutch as requiring the most skill but the least effort.

Restricting the knee motion of normal subjects appears to be an effective means of simulating a disabled condition. Subjectively, all normal subjects observed that having rigid knees required considerably more hip flexion for ground clearance. The EEL increased significantly from 22 percent to 35 percent, which suggests an orthosis with a flexing knee could have potential for paraplegics.

Conclusion

Immobilizing the knees of normal subjects seems to be a good way to simulate the energy expenditure of paraplegics walking on crutches. The suspension crutch appears to offer the most potential for savings in energy expenditure although it requires the most skill to use and would therefore probably require some practice and training.

Acknowledgments

This study was supported by the National Institute on Disability and Rehabilitation Research, Innovation Grant No. H133C90065-89. The authors would also like to thank Jessica Rose, RPT, MA, for her assistance in measuring EEL; Theresa Buckley, MS, for her help in performing tests; and Doug Williams, PhD, for his statistical expertise. They would also like to thank Terry Shoup, PhD, now dean of engineering at the University of Santa Clara, for his sharing of ideas and for the loan of his pogo crutches for examination.

Maurice LeBlanc, MSME, CP, is director of research at the Packard Children's Hospital, 725 Welsh Rod, Palo Alto, CA 94304; (415) 497-8192

Lawrence E. Carlson, DEng, is associate professor of mechanical engineering at the University of Colorado at Boulder, Campus Box 427, Boulder, CO 80309.

Teresa Nauenberg is a graduate sutdent in mechanical engineering at Stanford University.

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