The design of a compliant composite crutch (2025)

The design of a compliant composite crutch

Dorota Shortell, MSME; Jeff Kucer, MSME; W. LawrenceNeeley, BSME; Maurice LeBlanc, MSME, CP

Stanford University, Department of Mechanical Engineering,Design Division, Stanford, CA 94305; Rehabilitation Research and DevelopmentCenter, Department of Veterans Affairs Health Care System, Palo Alto, CA 94304.

Abstract

Ambulation by crutches takes up to twice the energy of normalgait and can lead to injuries of the hands and arms. The compliant compositeforearm crutch described in this article seeks to address these problems. Thenew crutch is made of a single composite piece and has an S-curve in the mainbody to provide shock absorption and return of energy with the goal of reducingimpact and repetitive injuries. It is lighter and, we expect, more durable thancurrent crutches due to the lack of interfacing parts. The new forearm cuffdesign provides retention of the crutch on the arm without a pivot. Thecontoured forearm cuff with wrist supports and padding is intended to provideadded comfort and support. These features are integrated into an aesthetic,high-tech looking design in charcoal/black color. (See Figure 1). Initialtesting with seven users yielded favorable response in function and appearance.Quantitative analysis has identified improvements needed for future iterationin design. These include (1) making the wrist supports more narrow and placeddown one inch, (2) optimizing the stiffness of the top curve of the S-shape and(3) testing the crutch for motion and energy requirements in use.

Key Words: crutch, forearm, S-curve, compliance,shock absorption, composite material, cuff, spring return.

This project is based upon work supported by the Rehabilitation R&DCenter at the Department of Veterans Affairs Health Care System in Palo Altowith funding from the DOD Ballistic Missile Defense Organization. It wasconducted as a Stanford University mechanical engineering graduate class (ME310) design project.

Address all correspondence and requests for reprints to: Maurice LeBlanc,Rehabilitation Research and Development Center (MS-153), DVA Health CareSystem, Palo Alto, CA 94304: email: leblanc@roses.stanford.edu.

Figure 1. CAD model of the final crutch geometry

Introduction

Background

Crutches, in one form or another, have been used for 5000 years (1). Fromfallen tree branches used to assist balance and ambulation, they have evolvedinto their present configurations of underarm and forearm crutches. Thematerials have changed, but the overall design of the crutch is largely thesame. They are basically sticks with hand and underarm or forearm supports.

Current crutch designs present some problems for users:

• High energy expenditure (2): it takes about twice as much energy toambulate with swing-through crutch gait as it does for normal ambulation. Thisis due both to the upper body's reaction to the shock of impact and to thevertical movement needed to clear the feet in swing phase by users wearingknee-ankle-foot orthoses with locked knees. The user essentially is doing abody push-up with every step.

• Injuries caused by repetitive loads on the hands, wrists and arms duringambulation (3-7): These injuries affect many users and are simply stressinjuries to the upper limb caused by constant use of crutches. If users havearthritis or other conditions affecting the upper limb, then the effect iscompounded.

• Problems created by not standing and walking (8-10): If people do not usecrutches because they tire easily or acquire hand/arm problems, there arepossible consequences. There are many reasons - physiological and psychological- why it is good to stand and walk rather than sit and use wheeled mobility.These reasons include improved bone growth, improved blood circulation, reducedbladder infections, reduced pressure sores and prevention of contractures.

Objective

The goal of this project was to ameliorate the problems, as describe above,faced by users of crutches. The target user group chosen was permanent usersbecause they have long term and constant need for improvements in crutchdesign. (Most permanent users utilize forearm crutches whereas most temporaryusers utilize underarm crutches.)

Summary of Previous Investigations

This project was preceded by one which compared different kinds of underarmor axillary crutches (11). Four experimental designs of axillary crutches weretested by users and compared to standard underarm and forearm crutches. Thefour experimental designs were:

• The spring or "pogo" crutch (12-15) with a spring or air cylinderin the upright to dampen the shock of impact.

• The roller or rocker crutch (16-21) with an arc at the tip so that thecrutch rolls rather than pivots.

• The suspension crutch (22-23) with a mountain climbing harness attached tothe top so that weight is borne partially by the harness and not entirely bythe hands.

• The prosthetic foot crutch (24) with an energy storing prosthetic foot atthe tip to absorb shock and provide spring action.

These four designs were tested with users and compared with standardunderarm and forearm crutches. Results showed no significant improvement inenergy consumption, and therefore none of the designs were selected for furtherdevelopment.

Methods

The project described in this article was conducted largely by Stanfordgraduate students as part of a three-quarter design class (25).

Needs Assessment: The first step was to conduct interviews toevaluate current crutch shortcomings. The interviews with crutch users helpedto generate a list of design requirements as follows:

1. Support the weight of the user: This is of utmost importance formaintaining safety, whether the user is standing, walking, running or climbingstairs.
2. Employ both shock absorption and energy return: The crutches shouldhave a means of absorbing shock and also have a way to return energy to theuser.
3. Durable: This correlates strongly with the weight bearing capabilityof the crutch and also the robustness of the interfaces between parts.
4. Lightweight: The crutch must be as lightweight as possible to allowease of maneuverability and low energy consumption.
5. Maximum mobility: The crutch cannot be bulky, must allow the user toeasily move the crutch tip in any direction and must easily detach from theuser in case of a fall.
6. Ease of object reach: The crutch must remain attached to the userwhile he or she is reaching for an object, opening a door, or shaking hands.(This function is performed in the present forearm crutch by a pivoting cuff.)
7. Comfort: Comfort between the arm and the cuff, and the hand and thegrip is important..
8. Silent Operation: One of users¹ biggest concerns with presentcrutches is that the pivoting elbow cuff and adjustment holes become loose andproduce a loud noises.
9. Support user self-esteem: The crutch should be attractive andstylish so that it is a personal accessory the user is proud of. [Some crutchesare now being made of Rosewood and other special materials. See ThomasFetterman, Inc. (26).]

Testing of Design Ideas

Several quick prototypes were built to test ideas. The first of these was anangled-cuff prototype. Angling of forearm support should serve todecrease the load placed upon the hands and wrists. Essentially, the amount ofload shifted from the hand and wrist to the forearm is a direct function of theangle at which the forearm is placed relative to the vertical. The angled cuffprototype (See Figure 2) allowed exploration of the effect of various placementangles upon the function of a single crutch. The prototype allowed adjustmentbetween 20 and 90 degrees in five-degree increments. This concept also has beenexplored by Nils Hagberg (27).

Figure 2. Angled cuff prototype

A wrist support prototype was produced in response to the pain feltin the hands and wrists of crutch users, caused by the force used to grasp thecrutch. As observed in user testing, high grip force is used to maintainstability when the maximum load is applied. To compensate, bracing elements(See Figure 3.) were positioned on either side of the wrist to stabilize it andallow a more relaxed grip on the handle. Though the design goal was achievedwith this prototype, user testing revealed concerns of possible injuryresulting from the inability of a user to disengage from the crutch during afall. (Future iterations of the prototype did not curve around the wrist, butrather were left open for easy arm removal.)

Figure 3. Wrist support prototype

A study was conducted to assess the geometry of the forearm supportto offer the user increased comfort and support. Working under the assumptionthat the cuff would take the general form of a sleeve or trough that cradledthe arm, plaster casts were created of forearms and used as molds to createpositive right and left models that were the size and shape of the forearms.Forearm supports were created by heating 0.2 inch thick rectangular sheets ofABS plastic and forming them over the models. The wrist supports eventuallywere integrated into the cuff. (See Figure 4).

Figure 4. Forearm support prototypes

An extensive spring study was conducted to determine the spring rateand travel that are optimal for a crutch. The users who were interviewedexpressed the need for shock absorption in their crutches. Thirty linearcompression springs, with constants ranging from 55 to 409 lb./in, wereevaluated by putting them into crutches and testing them with users. Thefollowing results were obtained:

1. Any spring that "bottoms out," or reaches the end of its travelso that the user feels an abrupt end to the compliance, is unacceptable.
2. Springs with spring constants greater than 170 lb./in. are too stiff anddo not feel different than a standard crutch.
3. Springs with spring constants less than 90 lb./in. are too compliant foradults.

A graph with suggested spring constants for body weight is shown in Figure5. This graph was developed by taking the qualitative results of user testingand fitting a trendline, called the target value. There is an upper and lowerbound to reflect different user preferences determined by maximums andminimums. A spring constant that addresses the needs of the majority of usersis 125 lb./in. Referring to the graph, this spring constant, while ideal for a157-lb. person, is also suitable for people weighing 117 to 198 lbs. This rangecovers 83% of the female population and 79% of the male population (28). The125 lb./in. spring will travel 0.628 inches for a 157 lb. person. 125 lb./in.and 0.6 in. travel were selected as the design target. The final crutch designassumes customized compliance for different weight classes.

Figure 5. Suggested Spring Constant for Adults

Integrating the Design Ideas into a First Prototype Crutch

The goal of the integrated prototype was to incorporate the desired designfeatures into a pair of functional, testable, prototype crutches. A pair offiberglass crutches that contained helical compression springs and a 45-degreeangle cuff was constructed (See Figure 6).

Figure 6. Integrated prototype crutch

The inner core of the prototype was made with a 125 lb./in metal springpress fitted between two wooden rods and secured with epoxy. A block of foamwas shaped to create the forearm cuff. The entire crutch core, except for thespring, was laminated with strips of fiberglass cloth dipped in epoxy resin.Concentric carbon fiber tubes shielded the spring. The rough surfaces andexposed fiberglass fibers were sanded and a final coat of resin was applied fora smooth surface finish.

A 45-degree forearm angle was chosen for this prototype in an effort toexamine the effects of a forearm cuff angle that was furthest from conventionalcrutches. The following conclusions were made from testing this firstprototype.

1. The spring constant of 125 lb/in. should be used for future iterations
2. A simpler way of incorporating shock absorption into the body must be found
3. The forearm cuff angle should be decreased from 45 degrees to 30 degrees toincrease stability
4. Support for bracing the wrist should be used on either side of the wristjoint
5. Cuffs should be padded with a soft material to prevent direct skin contactwith composite material and to increase comfort

Final Prototype

The final design was created in three-dimensional CAD. Features identifiedin the first prototype were incorporated into the final design. Additionaldesign requirements addressed were durability, light weight, ease of objectreach, comfort, quiet use, and aesthetics. Carbon fiber composite material wasused to fabricate the final design. This approach allowed the spring mechanismto be integrated into the body of the crutch itself, and the entire crutchcould be made from one part. We learned that Ergonomics, Inc. also has madeforearm crutches of composite material, but used standard round tubes (29).

For incorporating shock absorption directly into the body of the crutchwhile maintaining a single-piece design, inspiration was taken from prostheticfeet that use combinations of composite material leaf springs to achievecompliance (30). After trials with different geometry configurations, thedesign team decided on a S-curve design. Compliance was achieved by the twoarcs of the S-curve deflecting and acting as a spring. As force is applied,each curve compresses to absorb shock. As force is unloaded, the crutch returnsto its original position and returns kinetic energy.

Finite element modeling was used to determine the thickness of thecomposite cross-section and the amount of curvature that would give the desiredcompliance (31). Since the S-curved body has a rectangular cross-section, theproblem of attaching a conventional crutch tip was addressed by tapering theend of the crutch so that it transformed to a circular cross-section. Thehandle was incorporated as a simple rod, allowing handgrips of any style to beslipped over it.

The last step in the design was figuring out the geometry for the elbowcuff. The final solution has the two sides of the cuff extending up and aroundthe top of the forearm until the ends are about 2 inches apart. The sides areflexible to act as a quick-release mechanism. The posterior underside of thecuff is cut out, thus allowing the crutch to hang vertically as a user flexeshis/her elbow. The cuff is curved around the arm and padded to provide supportand comfort. The forearm support is at a 30-degree angle and has supportspositioned on either side of the wrist joint. The final CAD geometry of thecrutch and a close-up of the cuff area are shown in Figure 1.

Results

Manufacturing

Sparta Inc. (32), an R&D company with offices in San Diego, CA thatdesigns and develops composite material products, made the Stanford studentdesigned crutches using a wet layup method with three different graphitematerial weaves:

• Hexcel 282 cloth: a balanced 0/90 plain weave,
• XC1131: 90% of graphite in vertical direction, for primary bendingstiffness
• CBX 1200: +/- 45 degree stitched fabric, for torsional stiffness

A mold was constructed by precision cutting a piece of wood using a CNCmilling machine to the geometry given in the CAD model of the crutch . Thefibers were placed on the mold and coated with resin (See Figure 7).

Figure 7. Fully layed-up crutch on the model

The final crutches are 0.32 inches thick in the curved section and weigh 20oz. each without tips or padding. Since the lay-up was not pressurized in thecuring, approximately 33% less fiber volume and weaker material properties wereachieved than if production methods had been used. Production methods wouldyield a higher fiber volume, meaning that the same performance could beachieved with a thinner crutch. It is estimated that a final production crutchwould weigh only 16 oz.

Once the main body was finished, tips, handgrips, and padding for the cuffwere attached. Conventional rubber tips and handgrips were used. Custom-madepadding was attached to the forearm support using spray adhesive. The finalcrutches are shown in Figure 8.

Figure 8. Final crutches

User Testing

Quantitative testing: weight, thickness, spring constant, center ofweight, bending profile, etc. were measured in the laboratory before usertesting. Both static and dynamic loads of up to 250 lbs. were successfullyapplied to the crutches prior to user testing.

User profiles: age, disability, type of crutches, problems in use,etc. was acquired from the subjects before they tested the crutches in use. Allusers were aged 41-55 with an average age of 46.5 years. Five of the six usershad post polio and one user had cerebral palsy. All were users of forearmcrutches.

User testing: the six subjects were given time to get acquaintedwith the new crutches and test them in use. Then a questionnaire wasadministered with 15 topics using a Likert Scale of 1 to 7 with 7 being thebest. Highlights of their feedback are as follows:

• How do you like the shock absorption? Most liked that the crutch has shockabsorption (5.5 average score) but felt somewhat uncomfortable with the amountof movement (3.4 average score) since they were used to rigid crutches.
• How do you like the one-piece design? Most of the subjects very much likethe one-piece design (6.2 average score).
• How do you like the appearance of the crutch? Five were keen on theappearance, and two did not like it being so different (5.1 average score).
• How does the weight and weight distribution feel to you? Most were verypleased with the light weight (6.1 average score).
• How do you like the amount of padding in the cuff area? All wereappreciative of having padding to increase the comfort (6.3 average score).
• How do you feel about how quiet the crutches are? All very much liked thatthey are not noisy like their present crutches (6.8 average score).
• How do you feel about the stability of the crutches? Most felt somewhatapprehensive about the movement and stability of the crutches (3.2 averagescore) because they are so much different that their present crutches with noflexibility. Dimensions of the fabricated crutch were best suited for a 100 to130 lb. person. Heavier users said that the crutches were too compliant.

Overall, user feedback indicated that the general design appears to be animprovement over current forearm crutches, but improvements are necessary tomeet their all their needs.

Biomotion Laboratory Testing

Experimental testing was done in the Stanford University BiomotionLaboratory (33). The crutch was loaded from zero to 100 pounds over a period oftwo seconds. The force applied to the ground and the three-dimensional positionof the photo-reflectors attached to the crutch were measured. From this data,it was possible to calculate the stiffness of the entire crutch and theS-curved portion as 43 lb./in. and 211 lb./in., respectively.

The resulting stiffness values showed that the crutch is deflecting in twodifferent manners. The first deflection is in the S-curve and is what wasoriginally planned for. The second deflection is due to the bending of the cuffposteriorly. This latter of deflection causes the crutch to be more compliantand creates a feeling of instability in heavier users.

Finite Element Model

The goal of the finite element analysis after user testing was to optimizethe bending behavior for the crutches. It was found that the level ofdeflection in the S-curve was dwarfed by the bending moment created by the cuffat the top of the crutch. The finite element software ANSYS was used to run theanalysis (31).

There were four possible parameters that could reduce cuff bending:increasing the thickness of the upper curve, decreasing the cuff angle,reducing the amount of curvature of the top curve, and changing the apex of thetop curve. Changing the thickness of the upper curve was undesirable since itwould complicate the manufacturing process. Decreasing the cuff angle wasunfavorable because the hands would have to carry more load. Therefore, thesolution was chosen to change the curvature and apex of the top curve. Thefinal optimization moved the top curve up 1.0 inches and inward 0.5 inches.

Figure 9. Two-dimensional finite element representation oforiginal curve of crutch (left) and optimized curve (right).

Discussion

The final design of the compliant composite crutch addresses some of theneeds expressed by permanent crutch users. Its single-piece composite designreduces noise. Shock absorption and energy return are addressed with theS-shaped body that acts like a spring. The energy that is stored in thebeginning of the gait cycle theoretically is returned to the user at the end ofthe gait cycle. Energy testing is needed to confirm that assumption. Theforearm cuff design gives added support and comfort, while allowing users tomaneuver and reach for objects. Users liked the charcoal/black, high-techappearance of the crutches.

Future Improvements

The positive responses from testing by crutch users of the compliantcomposite crutch are encouraging. However, this design must go throughadditional design iteration before it is ready as a product. Neededimprovements include:

• Refinement of the direction of shock absorption in the top curve using theoptimized geometry developed in the finite element model.

• Modified geometry for the wrist supports so that they do not interfere withwristwatches. We recommend moving them down by one inch and reducing them totwo-thirds their original width.

• Testing of energy consumption in ambulation by crutch users.

Vision of Final Product

Ultimately, the crutch may be a semi-custom product that is selected foreach individual, much like the FlexFoot in lower limb prosthetics (30). TheS-curve of the crutches can be made in different thicknesses to achieve desiredcompliance for users of different weights, such as light, medium and heavy. Thecrutches would be cut to length on the height of the crutch from floor tohandgrip. I. e., all crutches could be manufactured at the longest possiblelength and then cut to the size for each user. The fitting of the cuff could beaddressed by making a large cuff with accommodation for smaller forearms byusing extra padding. Users could specify handgrips and tips of choice.

The relatively high cost of manufacturing the crutches is a potentialproblem. Composite materials are typically high in cost. To keep the crutchesreasonably priced, manufacturing processes would have to be further explored.Possibilities include using fiberglass instead of carbon fiber and making theS-curve of composite material with the forearm cuff made separately by aone-piece injection molded part.

An idea for improving the design of the crutch is to make it foldable. Manycrutch users want their crutches to be compact for travel or storage. Thisdesign could be adapted to fold in half by adding a hinge between the twocurves, at the point of lowest stress, so that one curve nests in the othercurve. Another suggestion by one of the crutch testers is to place a smallreflector on the end of the handgrip and on the back of the forearm cuff forvisibility at night.

Conclusions

The compliant composite crutch prototype addresses many of the concerns ofcrutch users. Their positive feedback is encouraging. However, the potentialbenefits must be proved and documented with further study. Design improvements,as discussed above, need to be implemented. The goal is that through furtherredesign, analysis, and testing, the compliant composite forearm crutch willoffer improvement to permanent crutch users.

Acknowledgements

We would like to thank:

• S. Eric Cregger for his early work on the project and advice throughout
• Prof. Mark Cutkosky, Katie Broughton and the Stanford ME 310 teaching team
• Joel Zuieback, Tom LaCombe, and Walt Whatley at Sparta Inc. for ideas andfabrication
• Guy Hammer at BMDO for funding and information on composite materials
• Dr. Dudley Childress, Laura Miller, Dr. Scott Yerby, George Bennett, Dr.Eric Sabelman, Eric Topp, and Prof. Sheri Sheppard for their input.
• The permanent crutch users who tested the crutches and offered theirthoughts.

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