By John O'Connor Vanden'Cll Fellow 111 Enginecnng Science, Professor of Engmccnng Science, Research Director, Oxford Orthop aedic Engineenng Centre
1
IN 1966, I HAD BEEN A U NNE RSITT LEcruRER for nearl y two years and a Fell o w of St. Peter's for almost one when John Goodfellow came to see me . My interests were in tribology, how surfaces of machine and structural components transmit load to each other by friction and pressure He too was a recent appointment, as a Consultant Orthopaedic Surgeon at the uffield Orthopaedic Ce n tre. H e was interested in the contribution of possible mechanical factors to the onse t of osteoanhritis o f the hi p, a fo rm of degenerative disease which manifests itself us ually in th e seve nth decade of life, witho ut o bvio us cause. H e believed that the articulating surfaces of the healthy yo ung hum an hip are not preci ely the spherical ball and socket of the anatomy texts , surfaces which remain full y in contact in all positions o f the joint, an d tha t a slight mismatch was critical to the function of the joint. He was seeking collaboration .
We were able to show that the surfaces of the young hip are indeed slightly mismatched, that they make contact over only part of the available surface area , but tha t the co n tact areas grow with increasing load . We speculated that osteoarthritis is a consequence of failure of tl1e cartilage layers in regions of habitual contact , those parts of the surfaces which are in contac t at small as well as large loads . To give such a theory credence , it was necessary to show that it could appl y witl1 equal force to other joints , in particular to the knee in which osteoartl1ritis is even mo re common. The difficul ty was tl1at the articulating surfuces of the femur ( the thigh bone ) and the tibia ( the shin bone ) are distinctly m ismatched , the lower femur having a pair o f more or less spherical condyles which make contact witl1 mo re or less flat tibial plateam: If these surf.ices alone were responsible for load transmission , tl1e contact areas between mem would be much smaller man mose at th e hip and me level s of contact pressure would be much highe r, unlikel y beca use me cartilage layers at both joints are very similar in nature .
It occurred to us that the menis ci ofme knee , two semi -lunar cartilage elements which are interposed between the surfaces oftl1e bones, could have an important role in load bearing We s howed mat m e meni sci transmit most of tl1e compressive load from tl1e femur t o me tibia, bringing m e surfaces into much closer conformity, wim large contact ar eas and low contact pressures, quite similar in function to m e hip .
At mis stage, we lost interest in theo ries of osteoarmritis because it was obvious m at man, in designing an artificial Jenee, should seek to e mulate Nature and s hould use analogues ofme natural menisci. In m e 1970s, me thrust in design was to use surfaces which mimic ked in shape those of me nan1ral joint, \vim the same disadvantage of small contact areas and large contact pressures and m e fear mat high wear rates would be inevitable We merefore developed w hat has come to b e called th e Oxford M eniscal Knee, figure 1 This uses a spherical me tal component (co balt chrome ) to replace each femoral condyle and a flat metal component to replace each tibial plateau Large contac t areas can be ac hieved by interposing a p lastic ( ultra high molecular weight polye m ylene ) m eni scal bearing between the metal surfaces, with a spherical socket at me upper interface to match m e spherical femoral s urface and a flat surface below to match the tibial su rface.
Figure 2
The menisca l bearing is not attac hed to either bone but is held in position by tension in the ligam ents whic h hold the bo nes together.
Clinical ex perience has shown that o ur device is best used to treat unicompanm ental osteoanhrius, whe n the cartilage in the com partm ent o n the in side of the knee has degenerated while that in the outer compartment remain s reasonab ly intact . This allows for conservative treatment . Figure 2 compares x-ra y pictures of a knee treated wi th an Oxford Uni and a knee tre ated with the more common Total Kn ee Replacement, a much more invasive procedure.
It has to be said that our ideas have been slow to gain acceptance and it is onl y now that implant manu fac turers have begun to develop meniscal bearing kn ee replacements which exploit some of the ideas .
A feature which has attracted a lot of recent interest has been the development by a young colleague, Da\id Murra y, ofa minimally invasive technique fo r the implantation of the Oxford U ni through a 6cm incision, with no division of mu scle ti ssue. The patient can walk only three hours after surgery so that the technique has the potential for out -patient treatment , as opposed to the more normal week to formight of hospital stay. Our co lleagues recentl y performed this operation at a conference in Sydney. When they telephoned tl1e hospital the fo llowin g day, they we re told that the patient had gone home . When they telephoned the patient's ho me , t he y were to ld t hat she was o ut shopping.
H owever, despi te excell ent clinical results and ve 1y low wear ra tes of retrie ved co mpo nents, it is clear that some centres have not been ab le to reproduce tl1e success of otl1ers, tha t careful ele ction of appropriate pati ents and very precise surgical skill s are needed . We therefore nm regular instructional course to make surgeons aware of tl1e hazards.
While it has been nece ssary to maintain a co ntinu ing contribution to the clinical wo rk, in the de\·elopment of surgical tools and fix t ures, da ta-bases for the recordin g of the clinical data and the identification of the risk-fa ctors, my main interest has been in the sc ientific and engineering paths whic h ha\·e been opened for me. When John Goodfellow and I first implanted our protot)rpe prosthes is into J post -mortem specimen, we found tl1 at th e meni scal bearin g ca me to tl1e front of the joint when the knee was straightened and thJt th ey moved towards the back when t he knee was bent . We concluded that the hu man knee beh.wes rather like a cockpit simul ato r, in whi ch two massive bodies (t he bones) are g uided o ver e,ich other by ligJments, thin bands of collage n fibre which hold th e bones together . We devel oped experiment which define the path s o f movem ent of the bo nes quite precise ly and mathematical models which have proved to be predi cti ve of tl1e results of the ex periments and which explain o ur ori ginal observations of the movements of tl1e meni scal bearin gs It beca me cl ear that th ese explanations co uld o n! )' be p,trtial, unless we took account of the actions of t he muscles, the main stabilisers and mobilisers of the skeleton. We built apparatus to .illO\\
Fig1m3
us simulate the effects of muscle activity on specimens from the post -mortem room but needed also to make observations on the living.
In 1992, John Kenwright was appointed as the new Professor of Orthopaedic Surgery. He invited me to move my research group from laboratories at the Engineering Schoo l in Parks Road and to take on the role of research director at the Orthopaedic Engineering Centre at the Nuffield Orthopaedic Centre. A former researc h student, Julian Morris (SPC 197 0 -74), gave us a gift of th e parts of the video - based gait analy is system which his company manufactures, leaving us to provide only the computers and other peripherals This equipment allows us to quantify the movements of the limb segments of the locomotor system, the foot, the shank, the thigh and the pelvis, and to determine the angles of rotation at each of the joints. Force plates flush ,vith the floor allow us determine the loads applied. Electro des over the motor units of the muscles allow us detect phasic activity in the muscles.
All this data is not sufficient to determine the leve ls of force applied by individual mu des. We have therefore developed a mathematica l model of the human locomotor system , figure 3, which is u ed to calculate from the experimental data how the lines of actio n of the muscle and ligament forces and the contact forces at the joints vary during activity. We can th e n estimate how the muscle force level al o vary. We have been able to compare the e estimates with measurement telemetered from living patients in whom instrnmented hip repl.lcements had been implanted
Several exciting possibilities arise from this model. To cu tomise the model to individu al patients , it is necessary to input numbers which describe the shapes of the bones and their articular surface , together with the areas of origins and insertions of muscks an d ligaments, the socalled parameters of the model. These quan tities differ from individual to individual and are precisely th e quan tities which orthopaedic surgeons change when perfo rmi ng jo int rep lace m ents, osteotomies, tendon transfers. By changing model parameters, si mulation of the con sequences of co mplex surgery might be feasible. Our laboratory is used for up to five sessions per week to assess cerebral palsy patients to help the difficult problem of evaluating possible surgical treatments We are planning ways of developing customised models for these children in order to perform the complex surgical experiments on the computer rather than on the c hild.
We are designing a family of knee replacements for the younger patient, which rvill repair the worn surfaces of the joint while preserving all the ligaments.
. . • w e designing a famil y of kn ee replacements fo th Work on JOmt rep lacements conunues. e ar r e younge r
• 1 • h . 11 • ti orn surfaces of tl1c J·oint while preservin g all tile li gaments My St p patient, w uc w1 repair 1e w · . . . • - eter's d • d 1• es for preclim cal tes un g of kn ee replaceme t Th colleague Stu art Turnbull has es1gne mac 1m n s. ese
• . d d • ll y by a Manch es ter co mpany. Rece ntl y surg fi machmes are now bemg pro uce commema . . , eons rorn the Instituto Ortopedici Rizzoli in Bologna asked me to design an ankl e P: 0s t1l es,s. They provided a research student, Alberto Leardini , to do the work . We performed expenm ents ID tile P0stm0nem room and developed a matllematical model to explain tl10se mo vements. We used til e matllem atical model to d • d f 1 . . es,gn a prostl1esis and , in April 1999, implanted prototype components '.11 a e O P as~c ID tile postmortem room. The friction was too hi gh and components di~ not move as predi cted ; iliey did ~ot ~ ove at all. One of rny new colleagues devised a Mediterranean so luu on to our problem . A doll o p of olive 011.
Anoilier Fellow of St. Peter's, Hamish Simpson , tile Clinical Reader in Oriliopaedic Surgery, has collaborated wiili our engineers for years in studying tile effects of acti vity on tile rate at which fractures of tile long bones repair. They use similar techniques for treating leg le ngili disc repanci es by sawing the short bone in half and using a framework to move tl1e bone fragm ents apart at a rate of 1mm per day. ature kindly fills tile growing gap wiili tissue which eventually ossifies . The onl y limitation to me additional lengili gain is me degree of stretch which me soft tissues can tolerate .
Finally, observation ofilie osteoariliritic lesions in tile knees ofJo hn Goodfellow's patients showed that tl1 occur on iliose surfaces which make contact when tile leg is straight, as it is at tile moment of heelstrike C} during level walking . We have specu lated mat tile sub-population at risk of the disease may be mose who transmit very high impulsive loads between foot and floor at heelstrike Caucasians , especially women, are known to be more at risk tl1an Chinese. We have shown iliat hee lstrike loading rates in a large population of Caucasian subjects are higher man in a sinlilar group of Chinese and iliat Caucasian women ha, e higher loading rates man Caucasian men. My original objective, to define me chanical factors in the developme nt of disease of me human musculo-skeletal system, still motivates me work .
Acknowledgements: The word "We" is ttsed in t he above when describing the co11trib11tio11s of research studmts including Seth Greenwald, Nigel Shrive, Ed Biden, Jim Collins, Da vid Fir:.Patrick, Amy Zavatsky, David Wilson, Richie Gill, Tung-Wu Lu, Marian Harrington, Wm Ling Chen, Je11111fer Fe-ikes, Me lissa Cimo11 , Ahmed Imran. The work has been mpported with gmnts, mainly fro m the Arthritis and RJm, matim, Co,mril. The components of our knee replacements are manufactured by Biomet Ltd, windon, 1vho also support our research work.