A New Modular Six-bar Linkage Trans-femoral Prosthesis For .
Prostheticsand OrthoticsInternational,1994, 18,98-108A new modular six-bar linkage trans-femoral prosthesis forwalking and squattingJ. K. CHAKRABORTY* and K. M. PATH.*** Department** Departmentof Applied Mechanics,of Applied Mechanics,Bengal Engineering College, Howrah,Indian Institute of Technology, Madras,AbstractFour-bar linkage mechanisms produced by manydesigners of knee joints for trans-femoralprostheses can provide knee rotation to permitwalking only. In Afro-Asian countries peopleare accustomed to a squatting posture in theirdaily activities. A six-bar linkage knee-anklemechanism trans-femoral prosthesis is describedwhich was developed and fitted to an amputee.The motion patterns of the ankle, knee and thighduring walking and squatting (obtained using aflickering light emitting diode system) for theabove prosthesis is compared with motionpatterns obtained for normal subjects. Thecloseness between both the patterns establishesthe suitability of the new modular trans-femoralprosthesis for producing near normal patterns ofmotion during walking and squatting. Theadditional facility of cross-legged sittingprovided in the prosthesis makes it functionallysuitable for Afro-Asian amputees.IndiaIndiafemoral prostheses are of single knee axis withsolid ankle except a few which have theprovision for ankle dorsiflexion (Radcliffe andLamoreux, 1972; Seliktar and Kenedi, 1976)and polycentric action at the knee joint(Radcliffe and Lamoreux, 1972; Cortesi, 1975;Cappozzo et al., 1980). The required amount ofankle dorsiflexion with knee flexion daringsquatting is not possible with any of the existingmodels except for one (Chaudhry et al., 1972)designed so far. But this model is an exoskeletalsingle axis prosthesis which makes it difficult toprovide a proper cosmetic cover. This being afixed, single axis knee, requires more effort atthe start of flexion during stance phase than thepolycentric knee prosthesis. This is because ofthe smaller effective lever arm of the single axisprosthesis as compared to the polycentric kneeaxis prosthesis.In normal walking two major muscle groupsof the lower limb control the swing phase ofwalking. The prosthetic leg without any swingphase control arrangement at the knee jointbehaves like a pendulum and without anycontrol produces an unnatural gait. In earlierdesigns this problem was solved by mechanicalmeans or by providing frictional resistance atthe knee joint (Murphy, 1964). Later differenthydraulic control systems (Lewis, 1965) wereintroduced. The disadvantage of a mechanicalfrictional resistance system lies in the fact thatthis system cannot produce natural gait.Moreover, these devices get damaged due towear. The hydraulic control unit can provide abetter control of swing but generally due to theIntroductionPeople in Afro-Asian countries are in the habitof sitting in a squatting posture for manyactivities starting from the use of the toilet tofarming operation and in cross-legged sittingposture for relaxation and during their dailyprayers. Amputees wearing usual trans-femoralprostheses are not in a position to perform theabove functions. Most of the models for transAllcorrespondencetobeaddressedtoDr. K. M. Patil, Eiepartment of Applied Mechanics,Bengal Engineering College, Howrah 711 103, WestBengal, India.98
New modularsix-bar linkage trans-femoralweight of the unit the prosthesis becomes heavyand also due to leakage of oil the prosthesisbecomes dirty. On the other hand, when apneumatic control is provided at the knee jointboth the above disadvantages can be removedand a better walking pattern can be achieved.The pneumatic swing phase control unit wasfirst developed in the Biomechanics Laboratoryat the University of California (Radcliffe andLamoreux, 1968; Zarrugh and Radcliffe, 1976).But this prosthesis is not able to provide theknee ankle coordinated motion necessary forsquatting. Therefore, it was felt necessary todevelop a new prosthesis which (i) can providethe basic requirement of a stance phase stabilitywith proper polycentric action (such that the hipankle reference line during stance phase passesin front of the variable knee axis), (ii) should beof modular design and will provide ease ofwalking and sitting in and rising from thesquatting posture, (iii) should be able to provideswing phase control.In this paper, details of a new modular sixbar linkage trans-femoral prosthesis, withfacilities for (i) swing phase control, (ii)coordinated motion between knee and ankle(provided by a six-bar linkage), (iii) squattingand (iv) cross-legged sitting, are described. Theprosthesis is fitted to an amputee and his motionpatterns(obtainedexperimentally)arecompared with normal patterns.MethodologyThe prosthesis designed and developed fordifferent functional improvement is shownschematically in Figure 1. Different functionalcapabilities of the prosthesis are describedbelow.Polycentricaction at kneeThe trans-femoral prosthesis has a six-barlinkage arrangement at the knee (Fig. 2) bywhich the motion from the thigh can betransmitted to the foot during squatting actionand during the swing phase of walking. Thefour bars 'a', 'b', 'c' and 'd' form a four-barlinkage mechanism with a short posterior link'c', designed after several trials, so as to createan instantaneous centre in full extension locatedwell above a corresponding single axis kneecentre and posterior to the hip ankle line. Thisresults in stability of the prosthesis duringstance phase. With little effort of the hip99prosthesisFig. 1. Schematic diagram of the new modular six-barlinkage trans-femoral prosthesismuscles, flexion can be initiated and theinstantaneous centre moves rapidly down to thenatural position of the anatomical knee joint. Upto 10 of knee flexion, the centre of rotation iswell above the location of a single axis kneejoint, thus the amputee will be able to controlboth extension and flexion voluntarily over thiscritical range of motion. With this linkagearrangement a flexion angle of 150 can beachieved for squatting ngTo achieve the coordinated knee-ankle motionduring sitting in the squatting posture two links'e' and ' f (Fig. 2) are arranged in such afashion that the links 'e' and ' f with the fourbar linkages 'a', 'b', 'c' and 'd' form a six-barlinkage mechanism consisting of two loopsCABDC and GFEBDG. An analysis of relativerotations between the linkages was carried outby solving two vector loop equations obtainedfrom the dimensions and orientations of thelinkages. With thigh motion, the relativerotation obtained between the link 'b' and link'f is transferred to the ankle by connecting a
100J. K. Chakrabortyand K. M. Patiloff. With knee flexion after heel off, the ankleangle starts increasing by rotation of the footpivoting at the toe. Dorsiflexion at the anklejoint after midstance facilitates a smooth patternof walking. In the case of a SACH orconventional foot flexion at the ankle joint isvery limited and the amputee has to follow anunnatural gait by raising the hip with muchphysical effort. In the present design to providefacilities of dorsiflexion after midstance andplantarflexion after heel off, the lower end ofthe connecting rod 'g' is allowed to move in aslot (Figs. 1 and 6) provided in the foot unit.Normally when the prosthesis is straight i.e.when knee flexion is zero the pin connected atthe lower end of the rod 'g' touches the uppersurface of the slot thus restricting the relativemovement between the shank and the foot andthus behaves almost as a single foot-shank unitduring the early phase of stance. The initialplantarflexion at heel strike is obtained by thecompression of a rubber bumper providedbehind the ankle joint. A spring (marked 2 inFigure 1 and marked 12 in Figure 6 inside theslot) and the rubber bumper facilitate aFig. 2 Six-bar linkage knee-ankle mechanism used in thenew prosthesisrod 'g' in parallel with the pylon connecting theanterior end of the link 'f' to the foot unit. Theposition of the pivotal point E and dimensionsand orientations of links 'e' and 'f' wereselected after several trial solutions of the abovementioned loop equations so as to have avariation of knee and ankle angles with thethigh angles similar to those of normal persons.The position of links in the mechanism andthe locus of instantaneous knee centre forrotation of the thigh unit relative to the shank isshown in Figure 3. During squatting the upwardmovement of the lower end of the connectingrod 'g' is restrained by the upper end of a slotprovided at the foot and the shank rotatesforward with knee flexion to provide the coordinated flexion of the ankle joint.Ankle dorsiflexionwalkingand plantarflexionduringDuring normal walking after heel strike theankle undergoes plantarflexion of about 8 untilthe foot flat position, alter which the shankrotates forward with foot flat on the ground toprovide a dorsiflexion of about 13 , before heelFig. 3. Locus of instantaneous knee centre for rotation ofthigh relative to shank up to 150 of flexion.
New modularsix-bar linkage trans-femoralrestraining action at ankle during plantarflexion.A relatively strong tension spring (marked 1 inFigure 1) connected to the pylon at a pointposterior to the ankle joint facilitates risingfrom the squatting position and is also useful inminimising the inherent knee instability duringflexion by resisting ankle dorsiflexion.Although a total of eight links are used fortransferring motion from thigh to foot, theprediction of motion of the prosthesis andestimation of forces in the pneumatic damperhave been obtained from the kinematic analysis,of the two loop equations for a six-bar linkageknee mechanism consisting of the linkages 'a','b', 'c', 'd', 'e' and 'f'.Swing phasecontrolDuring the normal swing phase of walking themotion of the foot and shank is controlled byquadriceps muscles during flexion and byhamstring muscles during extension of knee.Quadriceps action restricts excessive heel riseafter push off and provides acceleration in theinitial part of swing phase followed bydeceleration by hamstrings so as to have smoothentry (heel strike) into the next stance phase. Inthe trans-femoral prosthesis to achieve similarcontrol during swing phase, a pneumaticdamper may be provided between thigh andshank at the knee joint. This damper is basicallyloiprosthesisa double acting cylinder with a piston movinginside. With knee flexion and extension, the airinside the cylinder is compressed and providesresistance to pendulum motion of the shank. Onthe top and bottom end of the cylinder,provisions are made for air leakage to achieveresistance characteristics similar to resistingmoments developed in natural knee joint duringswing phase.Cross-leggedsittingSitting on the ground in cross-legged posture isa regular habit in Afro-Asian countries. Aprovision is made in the present design forcross-legged sitting with the help of a turntablelocated above the knee joint (in the thighportion) of the prosthesis. A lock is provided toprevent motion of the turntable during walking.Before sitting in cross-legged posture the lock isoperated manually to allow the rotation of theshank about the thigh axis.Modular and endoskeletaldesignThe prosthesis is made of high strengthaluminium and the different parts fabricatedseparately, are assembled together to provide anendoskeletal structure. The modular design byits fabrication facilitates mass production andreplacement of parts of the prosthesis. Thedimensions of different linkages are selectedsuch that the endoskeletal structure can beprovided with a soft cosmetic cover.Details of the mechanical design of theprosthesisThe mechanical design description of differentparts of the prosthesis are given below.Knee unitFig. 4. Details of knee unit.The knee unit (Fig. 4) consists of two sets ofaluminium linkages (part numbers 1 and 2)connecting the upper and lower portions of thejoint. The anterior links (1) are longer than theposterior links (2). The upper ends of thelinkages are connected to two L-brackets (3) byspecially designed internally threaded pins andscrews (4), so that the joints can providemobility during motion. The L-brackets arefitted to the back of the turntable (5). A thinplate is fitted to the top of the turntable toconnect the whole unit with the socket (7). Thesocket is made of moulded resin cast from thepositive mould of the amputee's stump.
102J. K. ChakrabortyThe lower portion of the knee unit is fitted tothe shank and it consists of an aluminium block(8) to which the pylon (9) is fitted. Twospecially shaped aluminium plates (10) arefitted on both sides of the lower block. Thelower ends of the linkages for knee rotation areconnected to these plates. The planes of rotationof the shorter links and longer links areseparated by spacers (11), so that the upper partof the knee joint can move freely relative to thelower part of the knee mechanism up to 150 ofknee flexion without any interference from linkmotion. Space is provided in between the twoL-brackets for fitting the swing phase controlunit (12) which is connected to the upperportion of the knee unit at the desired positionby a bracket (13). The lower portion of theswing phase control unit is connected to theblock (8) of the lower portion of the knee unit.To transmit the knee motion to the ankle,another set of links (14) and (15) are connectedbetween the upper portion and the lower portionof the knee unit. The links (14) are connected atsuitable selected points on the L-brackets andlinks (15) are pivoted at intermediate points ofthe pylon vertical axis. The two sets of links areconnected together at their ends. The projectedends of the link (15) are connected to the footand ankle unit by a connecting rod (16). Anextension stop (17) is provided to stophyperextensior) of the prosthesis during stancephase.TurntableThe turntable consists of two aluminiumcircular discs; the upper one fitted inside thegroove of the lower one to provide sufficientbearing surface (Fig. 5). Steel balls providedbetween the two discs act as ball-bearings andminimize friction and facilitate smooth relativerotation between them. A tension spring is fittedinside along a circular groove, connecting thetop and bottom discs. When the bottom discrotates counter clockwise relative to the top discduring cross-legged sitting, the spring isextended and helps, when it is released, thelower portion of the prosthesis to come back toits initial position. The turntable can rotate 110 of axial rotation for ease of sitting in the crosslegged position. In order to avoid any accidentalrotation of the: prosthesis about the long axisduring walking a locking arrangement isprovided. In the normal position of theand K. M. PatilFig. 5. Details of turntable used for cross-legged sitting.prosthesis this lock will hold the two discstogether. At the time of cross-legged sitting thelock has to be manually operated for unlocking.The upper disc and the lower disc are connectedby a central bolt.PneumaticdamperThe pneumatic damper consists of analuminium cylinder inside which a brass pistonmoves. The cylinder is closed by twoaluminium caps on each end. The caps are eachprovided with a one way check valve to allowthe air to enter into the chambers when thepiston is moving away from the ends. When thepiston moves towards the cap after compressingthe air for a specified distance, a spring-loadedpin is pressed to leak the air through a throttlingvalve. The lower connecting point of thedamper is adjusted so that the piston can befully extended up to 60 of knee flexion afterwhich the extension of piston will cease forfurther flexion of the knee during squatting. Theforce developed inside the cylinder can beadjusted by the flexion throttling screw or theextension throttling screw (as the case may be),thus providing the necessary resistance forcontrol of swing. The fully extended length ofthe damper is 140 mm.Foot and ankle unitThe conventional foot used in the trans-femoralprosthesis has generally a wooden solid anklewith the provision of heel cushioning and toeflexion by rubber blocks. In the present design
lVew modularsix-bar linkage trans-femoralprosthesis103Fig. 6. Details of foot and ankle unit.the foot has been modified to provide ankleflexion daring walking and squatting (Fig. 6).An ankle unit fabricated separately, is fitted in agroove made in the wooden ankle block of thefoot unit.An U-shaped aluminium upper block (1) restson two guide plates (2) which are embedded ina rectangular groove (3) on the foot unit at theankle position. This upper block is connectedwith the foot unit by a central bolt (4) and a tiebar (5). A steel pin (6) connecting the upperblock, central bolt and a tie-bar is supported onboth sides by the walls of the guide plates. Thecentral bolt is fitted with the foot from thebottom side by a nut. The tie-bar is screwed tothe wooden portion of the foot on the posteriorside. This arrangement helps in resisting thehorizontal force transmitted to the ankle jointduring motion. A hard rubber block (7) isplaced at the posterior side of the upper unit toresist the ankle plantarflexion. The pylon (8) isconnected to the upper U-block. The lower endof the connecting rod (9) from the knee unit isconnected to the ankle unit by a steel pin (10)which is allowed to move along a slot (11)made on both guide plates to allow ankledorsiflexion and plantarflexion during walking.To provide additional resistance to ankleplantarflexion at heel strike, the free end of therod (9) is loosely connected to the foot by atension spring (12) and a plate (which iscushioned by a rubber block (13)). A tensionspring (14) is connected from the pylon rod tothe back side of the ankle unit by a nut and boltarrangement (15) to provide the necessaryresisting force to ankle dorsiflexion duringsquatting and to help the foot to rise after heeloff during walking. The total mass of theprosthesis is 4 kg which is much less than themass of the lost limb of the amputee.ResultsFigure 7 shows computed variations of kneeand ankle rotations with thigh rotation asindependent variable during squatting posturefor the selected dimensions of the links of theprosthesis. The analytical graphs for theprosthesis mechanism for knee and anklerotations are obtained by giving the thighrotations obtained experimentally from normalwalking as input to the loop ed on Figure 7 and it is observedthat the pattern, given by the prostheticmechanism closely follows the normal patternof squatting.The walking pattern of the amputee, wearingthe trans-femoral prosthesis was recorded usinga modified method of flickering light emittingdiode system originally proposed by Soderbergand Gabel (1978) and as detailed below.Flickering light emitted diodes (LEDs) are fittedat hip, above and below the knee joint, anklejoint, toe and heel. In addition four LEDs (ofdifferent colour) are fitted at the pivoting points
104J. K. ChakrabortyFig. 7. Knee and ankle angle variations during squatting.of the four-bar linkages (a b c d) to record themotion of the relative centre of rotation of theshank and thigh, required for the dynamicanalysis of the prosthetic motion. The LEDsconnected with the toe and heel switches areand K. M. Pαtilattached to thigh and shank portions of theprosthesis to record the temporal factors ofwalking. When the amputee is walking, motionsof flickering LEDs are recorded on a singleframe of a colour film using a still camerawhose shutter is kept open in a dark roomduring one complete cycle of walking. Thedisplacement pattern of hip, knee, ankle jointsand toe of the prosthesis, recorded on the film,are projected on a screen and a stick diagram isdrawn.The direction of the line passing through thehip and centre of contact of the foot with theground (obtained from the force platform,recorded simultaneously with the walkingpattern) and the position of instantaneous kneecentres
The trans-femoral prosthesis has a six-bar linkage arrangement at the knee (Fig. 2) by which the motion from the thigh can be transmitted to the foot during squatting action and during the swing phase of walking. The four bars 'a', 'b', 'c' and 'd' form a four-bar linkage mechanism with a short posterior link
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