IEEE TRANSACTIONS ON NEURAL SYSTEMS AND REHABILITATION ENGINEERING, VOL. 14, NO. 3, SEPTEMBER 2006 361

Design and Evaluation of a Stance-ControlKnee-Ankle-Foot Orthosis Knee Joint

Terris Yakimovich, Member, IEEE, Jonathan Kofman, Member, IEEE, and Edward D. Lemaire

Abstract—Conventional knee-ankle-foot orthoses (KAFOs)are prescribed for people with knee-extensor muscle weak-ness. However, the orthoses lock the knee in full extension and,therefore, do not permit a natural gait pattern. A new electro-mechanical stance-control knee-ankle-foot orthosis (SCKAFO)knee joint that employs a novel friction-based belt-clampingmechanism was designed to enable a more natural gait. TheSCKAFO knee joint allows free knee motion during swing andother non-weight-bearing activities and inhibits knee flexionwhile allowing knee extension during weight bearing. A prototypeSCKAFO knee joint was mechanically tested to determine themoment at failure, loading behavior, and wear resistance. Themean maximum resisting moment of the SCKAFO knee joint overfive loading trials was 69 Nm 4.9 Nm. The SCKAFO knee-jointstrength and performance were sufficient to allow testing on a90 kg subject at normal walking cadence. Proper function of thenew electromechanical knee joint was verified in walking trials ofan able-bodied subject.

Index Terms—Design, evaluation, knee-ankle-foot orthosis(KAFO), knee joint, orthosis, stance control.

I. INTRODUCTION

I NDIVIDUALS with weak quadriceps are usually prescribeda knee-ankle-foot orthosis (KAFO) that locks their knee in

constant full extension. KAFO users must adopt abnormal gaitpatterns to compensate for the knee motion constraints imposedby the brace. Compensatory gait patterns include raising thepelvis on the side of the swinging leg to allow the leg to clear thefloor (hip hike), swinging the leg forward around the side of thebody (circumduction), plantar flexing the ankle of the contralat-eral foot (vaulting), or assuming an exaggerated lateral trunksway [1], [2]. These abnormal compensatory patterns can lead tosoft tissue injury and joint dysfunction at the hip and lower backthat cause pain and reduction in range of motion [2]. Walkingwith an immobilized knee also reduces walking efficiency by24% [3] thereby leading to premature fatigue that limits the dis-tance a user can walk. Surveys have shown that increased energy

Manuscript received August 25, 2005; revised December 16, 2005; acceptedDecember 27, 2005. This work was supported by a University of Ottawa In-terfaculty Collaborative Research Award and by a Natural Sciences and Engi-neering Research Council of Canada (NSERC) Idea to Innovation Grant.

T. Yakimovich was with the Department of Mechanical Engineering, Uni-versity of Ottawa, Ottawa, ON K1N 6N5, Canada. He is now with the OttawaHealth Research Institute, The Rehabilitation Centre (Ottawa Hospital), Ottawa,ON K1H 8M2, Canada (e-mail: terrisy@gmail.com).

J. Kofman was with the Department of Mechanical Engineering, Universityof Ottawa, Ottawa, ON K1N 6N5, Canada. He is now with the Department ofSystems Design Engineering, University of Waterloo, Waterloo, ON N2L 3G1,Canada (e-mail: jkofman@engmail.uwaterloo.ca).

E. D. Lemaire is with the Institute for Rehabilitation Research and Devel-opment, The Rehabilitation Centre (Ottawa Hospital), Ottawa, ON K1H 8M2,Canada and also with the Faculty of Medicine, University of Ottawa, Ottawa,ON K1H 8M5, Canada (e-mail: elemaire@ottawahospital.on.ca).

Digital Object Identifier 10.1109/TNSRE.2006.881578

demand from KAFO use is one of the major reasons that KAFOrejection rates can range from 60% to nearly 100% [4]. StandardKAFOs offer relatively poor cosmetic appeal as they force theuser to walk in an unnatural manner. Conventional KAFOs alsolimit user mobility. Walking on uneven ground, snow, stairs, in-clined surfaces, or even stepping onto a curb is complicated bythe KAFO user’s inability to flex their knee.

Recently, a new type of KAFO, known as a stance-controlknee-ankle-foot orthosis (SCKAFO), has emerged on the or-thotics market. A SCKAFO is designed to allow free knee mo-tion in swing while providing knee support in stance. SCKAFOadvantages, compared to a fixed knee KAFO, are improved gaitsymmetry, improved gait kinematics, improved mobility, lesscompensatory movements [2], [5], and less energy expenditurewhen walking [3], [6]. Many KAFO users with sufficient lower-limb strength can benefit from a SCKAFO, including the elderlyand people with multiple sclerosis, muscular dystrophy, polio,postpolio, incomplete spinal injury, unilateral leg paralysis orparesis, trauma, congenital defects, or isolated quadriceps weak-ness.

Numerous attempts have been made over the last threedecades to design a practical SCKAFO that permits flexionduring swing while maintaining support during limb loading.These design efforts have integrated hydraulic [7], friction[8], elastic [9] or impingement [10]–[12] based mechanisms,or conventional unidirectional clutches [4], [5], [13]–[15] orbrakes [16], [17] into the orthotic knee joint. However, mostof these designs have not led to a clinically or commerciallypractical working model due to poor function, requirements ofspecific knee or ankle movements to engage knee locking, orexcessive size, weight, or cost.

Four SCKAFO designs have emerged on the orthotics marketin the past three years: the Stance Control Orthotic Knee Jointby Horton Technology Inc., the 9001 E-Knee by Becker Or-thopedic, the Swing Phase Lock by Fillauer and the UTX byBecker Orthopedic, also marketed as the FreeWalk by OttoBock HealthCare. Unfortunately, both the UTX and SwingPhase Lock require the knee to be fully extended to engagethe knee-joint lock; the 9001 E-Knee creates ratchet noise and,along with Horton’s Stance Control Orthotic Knee Joint, is tooheavy and bulky for many users. Users wearing these orthosesare given insufficient support in many activities, are limited inwhere they can walk, or require increased energy expendituredue to heavy and cumbersome designs that may have poorcosmetic appeal. All four SCKAFO knee joints are relativelyexpensive, limiting their coverage by insurance systems.

A need exists for a SCKAFO knee joint that provides thebasic functions of permitting free knee motion during swing andinhibiting knee flexion while allowing knee extension at any

1534-4320/$20.00 © 2006 IEEE

362 IEEE TRANSACTIONS ON NEURAL SYSTEMS AND REHABILITATION ENGINEERING, VOL. 14, NO. 3, SEPTEMBER 2006

knee angle in stance. The device should also be slimmer, lighter,and more affordable than current commercial SCKAFO kneejoints. This paper presents the design, fabrication, mechanicaltesting and functional evaluation of a novel electromechanicalSCKAFO knee joint that addresses these needs.

II. DEVICE DESIGN

A. Design Criteria

In order to address the functional, structural, cosmetic, andcost limitations of current commercial devices and meet theneeds of KAFO users, the new SCKAFO knee joint was de-signed based on the following criteria.

1) Basic functions:a) allow free knee motion when not weight-bearing;b) inhibit knee flexion at any knee angle when weight-

bearing;c) allow knee extension at all times.

2) Functional requirements:a) have a reaction time of less than an estimated 6 ms

for switching stance-swing modes (since the onset ofpush off occurs within 1% of stride time, approxi-mately 12 ms, optimal device activation should occurwithin half of the onset duration, 6 ms);

b) permit a range of knee flexion of 110 (so that userscan tuck their legs under a chair when sitting).

3) Cosmetic requirements:a) be compact with dimensions not exceeding 18 50

120 mm;b) generate no perceivable noise.

4) Structural and safety requirements:a) resist a knee-flexion moment of 77 Nm per joint (es-

timate for 90 kg user in stair ascent [18]);b) resist failure when loaded by a user of up to 90 kg

(static-load factor of safety greater than 3, dynamic-load factor of safety greater than 2);

c) must default to knee-support mode in case of control-system failure;

5) Service requirements:a) have a mean time between servicing comparable to

commercial SCKAFOs (six months).The dimension constraints and 90 kg maximum user weight

specified above were selected to ensure that the new SCKAFOhas dimensions below or equal to those of existing designs andthat it can accommodate a user of the same weight. This wouldensure that the SCKAFO remain competitive with existing com-mercial SCKAFO designs.

B. SCKAFO Components

The SCKAFO knee joint was designed to be installed as apair, into a custom modular SCKAFO (Fig. 1) by a certified or-thotic technician. A SCKAFO includes knee joints, polypropy-lene thigh and AFO shells, upper and lower uprights, a pair ofmedial and lateral SCKAFO knee joints, a control system (onlysolenoids shown), and a footplate. The new SCKAFO knee jointintegrates a novel friction-based belt-clamping mechanism toprovide knee flexion resistance while allowing knee extensionat any knee angle. To initiate swing, an electronic control systemautomatically triggers the knee joint to allow free knee motion.

Fig. 1. Photographs of a pair of SCKAFO knee joints integrated into a modularSCKAFO system: (a) side view and (b) front view.

C. Friction-Based Belt-Clamping Mechanism

The SCKAFO knee joint belt-clamping mechanism consistsof four main components: the disc, belt, hammer, and anvil(Fig. 2). The disc, hammer, and anvil pivot about their respec-tive pins. The disc is attached to the upper upright and the faceplates are attached to the lower upright. The disc and face platespivot with respect to each other at the disc pin, the axis of kneerotation. The two face plates house the joint’s functional com-ponents. The belt (Megadyne America, Pineville, NC 28134;MegaFlat T-155), 5 mm wide and made of a polyester carcasscoated in neoprene, is anchored to the disc by machine screws.The belt wraps around the disc one and a half times before itpasses over the hammer, between the anvil and hammer, and fi-nally connects to a belt recoil spring that keeps the belt taught atall times. The distal end of the belt recoil spring is anchored tothe lower upright by a bracket. The belt is wrapped around thedisc to produce a friction force in order to reduce the belt tensionat the belt-disc connection site. The gap between the anvil andhammer is just wide enough to allow the belt to travel betweenboth components without resistance. The hammer is biased torotate in the counter-clockwise direction (Fig. 2) by the hammerrecoil (extension) spring. A push-type solenoid (Saia-BurgessInc., Vandalia, OH 4537-0427; Part #195203-231), connected tothe lower upright by a bracket, is used to selectively immobilizethe hammer via a hammer screw. A compression spring biasesthe solenoid plunger downward to allow unobstructed hammermotion when the solenoid is deactivated. The extension stop pinbutts against the face plate, to prevent hyperextension. The up-right brackets butt against the face plates to limit knee flexion to110 . The main components of the belt-clamping mechanismare machined from 7075-T651 aluminium plate. The pins aremachined from 17-4 PH precipitation-hardened stainless steel.

D. Control System

A control system is used to deactivate the solenoid, to permithammer motion whenever the limb is weight bearing, andto activate the solenoid, to demobilize the hammer when the

YAKIMOVICH et al.: DESIGN AND EVALUATION OF A STANCE-CONTROL KNEE-ANKLE-FOOT ORTHOSIS KNEE JOINT 363

Fig. 2. Component diagram of the new SCKAFO knee joint design (face plateremoved). Knee joint is in stance mode with the solenoid plunger in the downposition, allowing the hammer to rotate clockwise.

leg is non-weight-bearing. A series of force-sensing-resistor(FSR) pressure sensors (Interlink Electronics, Camarillo, CA93012; Part #402), adhered to the footplate of the orthosis,are used to determine when the braced leg is weight bearing.A computer-based logic program acquires signals from thepressure sensors and deactivates the solenoid whenever thepressure of any one of the three sensors is above its prede-termined threshold (indicating weight bearing) and activatesthe solenoid whenever the pressure of all three sensors arebelow their set thresholds (indicating non-weight-bearing). Toachieve versatility during SCKAFO development, the controlsystem is preliminary and employs a desktop computer andalternating current/direct current (ac/dc) power supply (SystronDonner, Concord, CA 94518–1399; Model #TL3-QV). Thesystem’s control logic is simulated with LabVIEW 7.0 soft-ware and is interfaced with the pressure sensors and solenoids

via a National Instruments Data Acquisition Board. Furtherdevelopment of the control system will use a dedicated logiccircuit and compact rechargeable battery to replace the desktopcomputer and ac/dc power source.

E. SCKAFO Belt Clamping and Knee-Joint Function

The SCKAFO knee joint’s main attribute of providingweight-bearing/stance limb support and autonomouslyswitching to allow swing free-knee motion is achieved asfollows. When the pressure on any one of the foot pressure sen-sors is above its predetermined threshold due to weight bearing,the solenoid is deactivated and the solenoid plunger adoptsthe down position, out of the path of the pivoting hammer andhammer screw (Fig. 2). The knee joint is thus ready for stancephase. A knee flexion moment (counter-clockwise) onthe upper upright and disc initiates a small counter-clockwiserotation of the disc. However, this creates tension in the beltwhich produces a deflection force on the curved upper endof the hammer. The deflection force creates a clockwisemoment on the hammer about the hammer pin that overcomesthe relatively small opposing counter-clockwise moment on thehammer produced by the hammer recoil spring force . Thelower end of the hammer clamps the belt against the anvil withclamping force (reaction force on hammer shown). Withthe belt clamped against the anvil, the belt cannot travel and thedisc is prevented from further rotation in flexion.

At any knee angle during stance, with the solenoid still deac-tivated and the hammer free to rotate, knee extension creates aclockwise rotation of the disc and reduces the belt tension. Thedeflection force on the hammer decreases with the reductionin belt tension. When the moment from is less than the mo-ment from the hammer recoil spring , the lower end of thehammer rotates away from the anvil (Fig. 2). This rotation elim-inates the clamping force on the belt and provides enoughspace for the belt to pass between the hammer and anvil withoutresistance. As the knee extends, the belt recoil spring keeps thebelt taught at all times. In this state, the belt is ready to apply aforce on the hammer if there is any knee flexion.

To terminate stance mode in the knee-joint mechanism, whenthe foot pressure on all sensors drops below the predeterminedthreshold, the control system activates the solenoid and pushesthe solenoid plunger into the path of the hammer screw (Fig. 3).The hammer is prevented from rotating counter-clockwise andclamping onto the belt because the solenoid plunger now blocksthe hammer screw. With the plunger providing a resisting reac-tion force , the belt cannot be clamped and the knee joint canflex and extend freely throughout swing. To allow the solenoidplunger to engage and block the hammer screw, the hammershould be in the fully open (counter-clockwise) position. Relax-ation of the knee-flexion moment as the limb is unloaded priorto swing will reduce the belt tension and deflection forceand allow the hammer recoil spring to pull the hammer into theopen position (Fig. 3). At the beginning of the next heel-strike,the solenoid is once again deactivated and the solenoid com-pression spring pushes the solenoid plunger down and out of thepath of the hammer screw. The hammer can then pivot freely,as required for the new stance phase. In the case of control

364 IEEE TRANSACTIONS ON NEURAL SYSTEMS AND REHABILITATION ENGINEERING, VOL. 14, NO. 3, SEPTEMBER 2006

Fig. 3. Drawing of the new SCKAFO knee joint in swing mode, with the sole-noid plunger in the up position, blocking clockwise rotation of the hammer bythe hammer screw butting against the solenoid plunger.

system failure, the solenoid compression spring pushes the sole-noid plunger down, thereby defaulting to limb-support (stance)mode.

When providing knee flexion resistance in stance, the beltcomponent experiences some elastic deformation, leading to alow level of knee flexion. This behaviour allows the SCKAFOknee joint to provide a more natural load response, to provideshock absorption with resisted knee flexion at the onset of limbloading.

F. Knee-Joint Structural Features

In addition to the important functional features of theSCKAFO knee joint, the device also has several notable struc-tural features. The SCKAFO knee joint measures 20.77 cmlong, 5.88 cm wide, 1.64 cm thick, and weighs 339 g withoutthe solenoid. The joint’s discreet size makes it 30% thinner and7% lighter than the Horton Technology Stance Control OrthoticKnee Joint, the only commercial SCKAFO knee joint thatoffers comparable performance. To allow the inner componentsto be easily accessed for orthotic servicing, the lateral faceplate maintains a sliding fit with the knee-joint assembly andis attached by three machine screws which allow for simpleremoval. High-performance polytetrafluoroethylene-lined com-posite bushings (GGB North America, Thorofare, NJ 08086;Part # 0806DU) are used to protect the pivoting bearing surfacesof the hammer and disc from wear against their respective pins.

G. Design Analysis

Force analysis was performed on the SCKAFO belt-clampingmechanism to verify that the belt would not slip when beingclamped by the hammer and anvil under peak knee-flexionloading for the current device dimensions. The analysis wasbased on the belt tension above the clamp site in belt

Fig. 4. Drawing of the SCKAFO knee-joint components used in clamping ofthe belt.

section (Fig. 4), the anvil/hammer-belt friction force ,and the hammer clamping force at the clamp site. Todetermine , the belt tension in belt section wasfirst calculated by

(1)

where is the maximum knee flexion moment appliedto the SCKAFO knee joint in stance by a 90 kg user, andis the perpendicular distance from the knee-joint axis to thecenterline of the belt (at ).

Belt tension was then determined using the equation forthe reduction of tension of a flexible member wrapped around acurved surface [19]

(2)

where is the static coefficient of friction of the belt with thehammer, estimated experimentally [20], is the angle of wrapof the belt around the curved surface of the hammer, andare the initial and final contact points of the belt on the hammerin the curved region of the hammer surface, respectively, and

is the point of hammer-belt contact where clamping begins(Fig. 4).

The hammer clamping force was determined from a sum-mation of moments on the hammer about the hammer pin atpoint , (Fig. 5). To simplify analysis, an estimatedbelt-on-hammer force was assumed to act at point [20],the midpoint of belt-hammer contact between points and(Fig. 4); and the hammer clamping force was assumed to actat point , the midpoint of contact between the initial and final

YAKIMOVICH et al.: DESIGN AND EVALUATION OF A STANCE-CONTROL KNEE-ANKLE-FOOT ORTHOSIS KNEE JOINT 365

Fig. 5. Free-body diagram of the hammer component showing the forces actingon it.

clamping points and , respectively, of the hammer and anvilon the belt. was calculated from the summation of momentsby

(3)

where and are the and components of the belt-on-hammer force, respectively; is the belt friction force on thehammer at point and is determined below; and , , ,and are the perpendicular distances between point and

, , , and , respectively.The belt friction force in (3) is determined by considering

that the belt is acted upon by two identical friction forcesfrom the anvil and hammer. Since the anvil and hammer aremade of the same material and have the same surface properties,these components have identical coefficients of friction with thebelt. To prevent belt slip when the belt is clamped, the sum ofhammer and anvil friction forces on the belt must balancethe belt tension . Upon rearranging, is determined by

(4)

In order to prevent belt slip between the anvil and the hammer,the hammer dimensions were selected such that the calculatedclamping force , determined by (3), satisfied the followingequation:

(5)

where is the anvil-belt and hammer-belt static coefficient offriction.

Classical stress analysis [19] was also performed on theSCKAFO to determine the static-load and repeated-loadinginfinite-life factors of safety of the individual components.With the exception of the belt, all knee-joint components had astatic-load factor of safety greater than 3.1 and an infinite-lifefactor of safety greater than 2.3. Details of the analysis arefound in [20] while details of mechanical testing of the belt aregiven in Section III.

Fig. 6. Photograph of the experimental setup used in mechanical testing of theSCKAFO.

III. MECHANICAL AND FUNCTIONAL EVALUATION

A. SCKAFO Knee-Joint Mechanical Testing

The SCKAFO knee joint was mechanically loaded to deter-mine the highest flexion moment that could be supported by theSCKAFO knee joint before failure, the amount of knee flexionpermitted by the joint as a function of the applied joint flexionmoment, and the amount and type of wear that occurs with re-peated loading.

A SCKAFO knee-joint prototype was mounted in a 4482 In-stron Material Testing Machine so that a controlled momentcould be applied to the joint about the knee-joint axis (Fig. 6).The machine was controlled via a computer terminal using In-stron 9 Series software. The solenoid and control system werenot connected to the SCKAFO knee joint and therefore not usedduring mechanical testing. The knee joint was installed on a setof -in stainless steel orthosis uprights (Becker Or-thopedic, Troy, MI, 48083-4576 USA; Part #1001-A6S #7, 8)and the ends of the uprights were pinned to custom machinedclevises fixed to the testing machine mounts. A plumb line wasfixed to the center of the knee-axis pin and hung in front of alevel metal ruler that was fixed to the bottom mounting clevis.The plumb line indicated the horizontal distance between theknee-joint axis and the line of compressive force applied tothe pinned ends of the uprights. A protractor was adhered to

366 IEEE TRANSACTIONS ON NEURAL SYSTEMS AND REHABILITATION ENGINEERING, VOL. 14, NO. 3, SEPTEMBER 2006

Fig. 7. SCKAFO knee-joint resistance moment versus knee-joint flexion angle for five separate trials loading the SCKAFO knee joint to failure. The dashed lineindicates the expected peak load applied to the joint by a 90 kg user in normal cadence [21].

the distal half of the knee joint, centered on the knee axis pinwith the 0 angle line aligned with the longitudinal axis of thejoint. A mark on the center of the upright bracket indicatedthe knee-joint angle on the protractor. The instantaneous mo-ment applied to the SCKAFO knee joint was determined by

, where is the compressive force applied by thetesting machine and is the moment arm, the perpendicular dis-tance from the knee-joint center to the line of compressive force.The relationship between the knee-joint angle and the mate-rials testing machine crosshead vertical displacement was deter-mined by measuring knee-joint angles at 1 cm vertical intervalsprior to testing. The relationship between the moment armand materials testing machine crosshead vertical displacementwas determined by measuring the distance , at the same 1 cmintervals prior to testing. Knee angles and moments for the loadcurves were based on these relationships (Fig. 7).

The SCKAFO knee joint was set to a preload flexion angle of10 before every vertical-loading test to prevent the knee jointfrom buckling and to initiate flexion about the knee-joint axispin. The initial 10 flexion angle was also necessary to hor-izontally offset the plumb line weight from the lower mount(Fig. 6). The SCKAFO knee joint was loaded with a downwardcrosshead velocity of 508 mm/min, the maximum speed of thetesting machine. To simulate knee-flexion velocity in slow ca-dence, the downward crosshead velocity would have to be 6000mm/min, based on 114 ms loading time, knee range of motionof 15 [21], and a moment-arm length of 284.5 mm. Verticaldisplacement of the upper testing-machine mount and compres-sive load values were sampled at a rate of 20 Hz, the maximumsampling rate of the machine.

Two separate tests were performed on the SCKAFO kneejoint.

1) Strength and loading behavior test: The SCKAFO kneejoint was loaded to failure five times. For each of thefive trials, a new 5-mm-wide, factory-cut T-155 MegaFlatbelt was installed in the SCKAFO knee joint. The kneejoint’s maximum resisting moment at failure, the applied

SCKAFO knee-joint flexion moment, and correspondingjoint flexion angle were recorded for each trial.

2) Repeated loading and wear test: To determine the resis-tance of the belt to wear and failure in repeated loading,a new 5-mm-wide, factory-cut T-155 MegaFlat belt wasinstalled in the SCKAFO knee joint. Using the same beltfor the entire test, the joint was repeatedly loaded to anaverage flexion moment of 52 Nm until the belt failed. Aloading moment of 52 Nm was considered a safe and real-istic value for repeated joint loading because this momentwas 10% higher than the mean peak moment plus one stan-dard deviation estimated for a 90 kg user in normal cadence[21]. A factor of safety of 1.1 for a 90 kg user in normal ca-dence is implied by this loading. For each loading cycle, theSCKAFO knee joint remained at maximum load for a pe-riod of at least 10 s before the load was released. Followingbelt failure, the belt was inspected for modes of wear.

B. Functional Evaluation

In order to verify the electromechanical function of theorthosis in deactivating and activating the solenoid at theappropriate instants in gait, a functional evaluation was per-formed. A custom SCKAFO that incorporated two prototypeSCKAFO knee joints was fabricated by a certified orthotist andthe temporary control system described above was used. Usinga 4-camera video-based motion analysis system (APAS) (ArielDynamics Inc., Trabuco Canyon, CA 92679), unilateral kine-matic gait analysis was performed on a 58-year-old able-bodiedmale subject walking at a normal cadence along a level 8-mrunway for two test conditions: wearing no brace and wearingthe new SCKAFO.

IV. RESULTS

A. SCKAFO Knee-Joint Mechanical Testing

For each strength-testing trial, the belt failed in tension be-tween the disc and the hammer, labeled as points and ,

YAKIMOVICH et al.: DESIGN AND EVALUATION OF A STANCE-CONTROL KNEE-ANKLE-FOOT ORTHOSIS KNEE JOINT 367

respectively, in Fig. 4. This result was expected since the belttension is always greatest between these two points. All otherknee-joint components showed no signs of failure or yieldingthroughout testing. The SCKAFO knee-joint resisting momentwith respect to the knee-joint flexion angle, for all five loadingtrials, is plotted in Fig. 7. For all knee-joint mechanical testingresults, the knee-joint flexion angle represents knee-joint flexionfrom the 10 flexion starting position rather than the absoluteknee-joint angle. The five failure moments had a mean valueof 69 Nm Nm . The greatest moment at failure was73.4 Nm or 95% of the 77 Nm design moment. This design mo-ment was based on a 90 kg user in stair ascent, and exceedsthe moments required for fast and normal cadence walking.The lowest moment at failure was 60.7 Nm (Trial 4). The max-imum belt tension at failure, determined by (1), averaged 2488N Nm . An estimated peak belt tension of 1700 Nmay occur for a 90 kg user in normal cadence.

As shown in Fig. 7, an inconsistency existed between theSCKAFO joint flexion angle and the joint resisting moment overthe five loading trials due to the friction the belt imposed on it-self around the disc. Following loading, the belt would oftenremain very tightly wrapped around the disc, held against itselfby belt friction. Throughout testing, the belt tightness on thedisc fluctuated between the tightly bound state and the looser,belt-spring-tightened state. The belt tightness on the disc di-rectly affected the SCKAFO knee-joint flexion angle achievedunder a given moment and was responsible for the discrepancyin the SCKAFO joint loading behaviour during strength testing.As shown in Fig. 7, an initial SCKAFO knee-joint flexion ofapproximately 5 resulted in negligible joint flexion resistance.This “free” knee flexion was the result of the disc rotating asthe hammer moved from the fully open position to the fullyclamped position. Beyond the initial 5 of joint flexion, therewas a near-linear relationship between joint moment and jointflexion angle for the SCKAFO knee joint (Fig. 7). In this phase,knee flexion may have been due to the combination of belt elon-gation, possible early belt slip in initial clamping, and belt slipabout the disc.

For the repeated loading test, the neoprene belt coating failedfollowing 66 loading cycles and failure occurred at the beltclamp site only. Abrasive wear on the thin neoprene coat was ev-ident, eventually allowing the bottom corner of the anvil to peelthe coat from the polyester carcass as the belt traveled across theanvil. Other less critical forms of belt damage included crackingand plastic deformation of the neoprene coat and delaminationof the neoprene coat from the polyester carcass at the clamp site.Excluding the belt, no other knee-joint component showed anyvisual signs of wear or permanent deformation from loading.During each load cycle, the knee-joint angle remained constantfor the 10 s that the joint was held loaded at the maximum mo-ment.

B. Functional Evaluation

The SCKAFO functioned as designed during the trials, pre-venting knee flexion while allowing knee extension in stanceand providing free knee motion in swing. The control systemoperated effectively, deactivating and activating the solenoidsto cause belt clamping and belt free motion, respectively, at the

Fig. 8. Graph of the mean sagittal knee angle over three walking trials, whilewalking with no brace (solid black) and walking with the SCKAFO (solid gray).The dashed line represents one standard deviation either side of the mean forthe no brace condition. The vertical dotted lines indicate the average instants ofsolenoid deactivation and activation in the gait cycle.

appropriate instants of the gait cycle (Fig. 8). Gait analysis re-vealed that walking with the SCKAFO had a desired minimaleffect on the subject’s gait pattern as shown by similar meanknee-angle curves during gait for the no-brace and SCKAFObraced conditions (Fig. 8). The subject was able to maintainhis full range of knee motion throughout the gait cycle whenwalking with the SCKAFO.

V. DISCUSSION

This paper presented the design, fabrication, mechanicaltesting, and functional evaluation of a novel electromechanicalSCKAFO knee joint that automatically provides knee support instance at any knee angle, knee extension in stance, uninhibitedknee motion in swing, and addresses the functional, structural,cosmetic and cost limitations of current commercial SCKAFOknee joints. The new SCKAFO knee joint is 30% slimmer and7% lighter than commercial SCKAFO knee joints that deliversimilar performance. This is an important improvement overexisting designs as the weight and bulkiness of an orthosis maybe enough reason for a user to not wear their prescribed device[22].

For the SCKAFO knee joint, all components except the beltwere designed to have met or surpassed the target factors ofsafety of 3 for static-load and 2 for repeated-loading infinite-life. The lowest calculated component factors of safety were3.1 for static-load and 2.3 for repeated-load infinite-life. Thesecalculations were based on a peak load of 77 Nm expected fora 90 kg user in stair ascent [18]. Mechanical testing was limitedto 73.4 Nm or 95% of the design load because of the limitedbelt strength, and all knee-joint components excluding the belt,resisted failure up to this 73.4 Nm load. Considering that theexpected loading by a 90 kg user in normal cadence walkingwould be 47 Nm, (based on the average peak moment plus onestandard deviation) [21], all SCKAFO knee-joint componentsexcluding the belt demonstrated to resist up to 1.6 times theexpected loading for normal cadence walking. The belt resistedup to 60.7 Nm or 1.3 times the expected loading for this walkingcondition.

368 IEEE TRANSACTIONS ON NEURAL SYSTEMS AND REHABILITATION ENGINEERING, VOL. 14, NO. 3, SEPTEMBER 2006

As the belt failed after 66 loading cycles due to wear of theneoprene coat of the belt at the clamp site, a superior belt ma-terial must be developed with greater tensile strength and wearresistance. A stronger belt carcass material, such as Kevlar orVectran, is expected to increase the maximum resisting mo-ment of the SCKAFO knee joint by 3–5 times. A more ade-quate coating material, that will not permanently deform underrepeated loading and that is more resistant to abrasive wear thanneoprene, may also be used to increase the cycle life to approxi-mately 500 000 cycles (approximately six months use). Modifi-cations to clamp the belt over a larger area will reduce compres-sive stresses on the belt. Modifications to the clamping compo-nent surfaces will also be explored. The belt-clamping mecha-nism successfully prevented belt slip during joint loading sincethe knee-joint angle remained constant for the 10 s that the jointwas held at maximum moment during repeated-load testing.

Regarding the mechanical function of the knee joint, changein belt tightness around the disc with each mechanical loadingtrial resulted in different flexion angles for the same applied mo-ment on the SCKAFO knee joint (Fig. 7). SCKAFO knee-jointflexion angles of 24 –30 occurred in mechanical testing at47 Nm. A smaller flexion angle may be desired for limb sup-port in weight bearing and might be achieved by reducing beltslack around the disc. This could also result in less variabilityin the SCKAFO knee-joint loading behavior. Use of a belt witha higher tensile elastic modulus than the T-155 MegaFlat beltmay also be used to reduce the knee-joint flexion angle if nec-essary. However, the functional evaluation suggested that thejoint flexion angles may actually be ideal and that no modifica-tion is necessary. Gait evaluation demonstrated proper electro-mechanical function of the orthosis regarding solenoid deactiva-tion and activation, belt clamping, and free motion. This properknee-joint function permitted normal knee-joint angles with theSCKAFO. Testing the SCKAFO with a single able-bodied sub-ject was able to demonstrate that the SCKAFO prototype couldachieve the designed function parameters. A study involvingclinical testing of the SCKAFO with KAFO users with quadri-ceps weakness is required to evaluate the clinical efficacy of theorthosis for the target population.

The two solenoids currently installed in the SCKAFO drawa combined current of 1.64 A at 8.6 V. Power for the SCKAFOfor a 16-h day would require a 9-V battery that offers 10.5Ah. At present, a battery with these specifications would be toolarge and heavy to be incorporated into a practical SCKAFOsystem. A more efficient electromechanical actuator such as adirect current micro-motor or a mechanical pushrod actuated bya ground-reaction force on the foot may substitute the solenoidto minimize or eliminate the power requirements of the currentcontrol system. A control algorithm that minimizes solenoid ac-tivation, such as early deactivation in terminal swing, could helpreduce power consumption; however, additional sensors may berequired. Based on the manufacturer’s specifications, the FSRsare expected to function for more than 10 million actuations.Various arrangements of the FSRs in the circuit and algorithmswill be investigated to ensure system reliability and user safety,including use of redundant sensors in case of any FSR failure.

In the prototype design, solenoids produced a clicking noisewhen the plunger contacted the inner solenoid housing. A soft

intermediate material could be placed between the contact sur-faces to reduce this noise during solenoid activation.

The SCKAFO joint is longer than current commercialSCKAFO knee joints. This extra length could provide difficultywhen attaching the joint to the lower AFO section for peoplewith a short tibia as there is limited length for bending the loweruprights to connect the joint to the AFO section. The length ofthe prototype knee-joint can be shortened by minimizing thehammer length and improving upright-bracket integration.

VI. CONCLUSION

A novel SCKAFO knee joint was designed to provide kneeflexion resistance at any knee angle while allowing knee exten-sion in stance and free knee motion in swing. The SCKAFOknee joint is lighter, slimmer, and has the potential to offer morenatural function over current commercial SCKAFO knee joints.Future development will aim mainly to increase the cycle lifeand factor of safety for the SCKAFO knee-joint belt compo-nent.

ACKNOWLEDGMENT

The authors would like to thank Dr. M. Munro, C. Ayranci, L.Goudreau, J. Tomas, B. Cotter, P. Deegan, R. Kalsi, M. Russell,and D. Nielen for their valuable contributions to this research inorthosis fabrication and testing. The authors would also like tothank the anonymous reviewers for their comments.

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Terris Yakimovich (S’05–M’06) received theB.Eng. degree in aerospace engineering from Car-leton University, Ottawa, ON, Canada, in 2002 andthe M.A.Sc. degree in mechanical engineering fromthe University of Ottawa, Ottawa, ON, Canada, in2005.

He is a Research Engineer with the Ottawa HealthResearch Institute, The Rehabilitation Centre (TheOttawa Hospital), Ottawa, ON, Canada. His researchinterests include product design and rehabilitation en-gineering.

Mr. Yakimovich was awarded third place in the 2005 Student Design Compe-tition of the IEEE EMBS. He was also a winner of the 26th Annual Rehabilita-tion Engineering and Assistive Technology Society of North America (RESNA)Student Design Competition, and received the 2005 Clifford Chadderton Awardfor Prosthetic and Orthotic Research, awarded by the International Society forProsthetics and Orthotics (ISPO) Canada and The War Amputations of Canada.

Jonathan Kofman (M’97) received the B.Eng. de-gree from McGill University, Montréal, QC, Canada,in 1982, the M.A.Sc. degree from the Ecole Poly-technique, Montréal, QC, Canada, in 1987, and thePh.D. degree from the University of Western Ontario,London, ON, Canada, in 2000, all in mechanical en-gineering.

From 1983 to 1995, his main area of research wasin orthotics, prosthetics, biomechanics, and rehabili-tation, mostly as a Research Engineer. From 1987 to1991, his research in Jönköping, Sweden, included

development of laser-camera range sensors and CAD/CAM systems for pros-thetics, which were commercialized by CAPOD Systems AB, Sweden, in 1991.From 2000 to 2004, he was an Assistant Professor in the Department of Mechan-ical Engineering, University of Ottawa. He is currently an Assistant Professorin the Department of Systems Design Engineering, University of Waterloo, Wa-terloo, ON, Canada. His research interests include prosthetic and orthotic de-sign, biomechanics, rehabilitation, human-machine interfaces and human-robotinteraction, intelligent biomechatronic and optomechatronic systems, and com-puter vision. He has recently coauthored four international conference awardpapers.

Dr. Kofman is a Member the American Society of Biomechanics and Inter-national Society for Optical Engineering (SPIE), and he holds licenses from theProfessional Engineers of Ontario and Ordre des Ingénieurs du Québec.

Edward D. Lemaire received B.Sc. (kinanthro-pology) and M.Sc. (biomechanics) degrees fromthe University of Ottawa, ON, Canada and thePh.D. degree from the University of Strathclyde,Bioengineering Unit, Glasgow, U.K.

He is a Research Associate with the Institutefor Rehabilitation Research and Development atThe Rehabilitation Centre (The Ottawa Hospital,Canada), an Assistant Professor with the Universityof Ottawa, Faculty of Medicine, and President ofthe Canadian National Society of the International

Society for Prosthetics and Orthotics. He has extensive experience withresearch involving prosthetic, orthotic, and other mobility devices, especiallyin the areas of computed aided technologies and telerehabilitation applications.