Philippe Cartier gravitated towards knee surgery very early in his career, and has since specialized in unicompartmental knee arthroplasty. He has himself designed a unicompartmental implant, which has given him an even deeper insight into the management of osteoarthritis of the knee. Maîtrise Orthopédique talked to him about his life, his work, and his treatment principles.

M.O. Mr. Cartier, where did you train?

P.C. I was Senior Registrar in the department headed by my own teacher, Prof. Cauchoix. He wanted me to get the post of a Reader in Paediatric Orthopaedics, at a hospital on the outskirts of Paris. At first, one of Prof. Cauchoix' close assistants had been envisaged as head of department, and I myself as Reader in Paediatric Orthopaedics. The vacancies were not due to occur for another two years. This is why Prof. Cauchoix sent me off to Berck, to spend the waiting time at the Institut Calot, which specializes in paediatric surgery.
M.O. Who were you working with there?
P.C. At Berck, I was working with Mr. Morel, a first-rate paediatric orthopaedist, whom I have always liked a lot and for whom I have the greatest admiration. One day, a sales rep of an implant firm came to the unit and wanted to show us a film about a unicompartmental knee prosthesis. I told him that since I was exclusively doing paediatric orthopaedics, I could not see how his device could be relevant to my work. Georges Morel wisely said, "Why don't you watch it, all the same?" That was back in 1973, a time when knee prostheses were enormous lumps of material that required extensive sacrifice of bone. The film was about Marmor's unicompartmental device, and we found it an eye-opener, because what it showed was exactly the sort of thing that we, as paediatric orthopaedists, had always felt to be of paramount importance: to restore function without causing destruction. I remember saying to Morel, "If ever I were to do knees, that is the sort of implant I would use."

M.O. Had you had any experience in this field?

P.C. None. I had worked under Merle d'Aubigné, under Judet, under Cauchoix. I had done pedicle screws under Roy-Camille; hips and septic cases under Letournel; hands with Tubiana; and feet with Maschas. I had, as it were, done everything except knees, by the time I went to Berck. Eventually, it was decided that the department of paediatric orthopaedics that I had been promised would not be opened; and three months before I was due to leave Berck, I found myself in a very nasty situation, without a job, and with nowhere to go.

M.O.How did you cope with this setback?

P.C. Very badly, I'm afraid. I had implicitly trusted certain people, and I had put in two years' work away from Paris. Being away from the capital had deprived me of my connections, of my practice, and of my friends. The day when they sprang the news on me that all my ambitions had come to naught was a black day all round: My car was towed away by the police from outside the restaurant where I had the news broken to me. I went home totally gutted. I remember sitting down astride a chair, like a cowboy in a Western, and hitting the whisky. I was not what you might call sober, but there was a thought that rose deep inside me: I said to myself, "They've done me down, but I shall get my own back: I shall have a name; a joint to my name; and I shall make it yet."

M.O. What, in fact, did you do to come back?

P.C. I started by phoning my friend Gérard Pouget, who had a post with Jacques Nègre in Orleans, which I had refused a few months earlier. Gérard saw that I was desperate, and told me of a private hospital in Rouen, where they were looking for an orthopaedic surgeon. I phoned that centre, which was run be colleagues from Paris: They welcomed me with open arms. There was a huge potential practice there. They said to me, "The spine and hands, that's what Moitrel does; Adrian does feet and hips; so there is just the knee left." I could do everything but the knee, but that little film I had seen "by accident" long ago had shown me the way, and I said, "Let's get go for it."

M.O. But, surely, you had to get some more training?

P.C. As far as joint replacements are concerned, the ones available at the time - Coventry's Geometric, the Guepar, and the Freeman - were not at all what I had in mind. I rushed off to see Leonard Marmor, who had been inserting "unis" in the States for the past two years. I was immensely privileged to benefit from his initial experience. He taught me from the word go that 6 mm tibial plateaus would lead to loosening, and that the size to use was 9 mm. So I was able to start out with the correct thickness, which is why I got above-average results at ten years. As far as the patella is concerned, I was able to benefit from my friendship with Christian Mansat. At the time, he was far ahead of the rest of the orthopaedic community, in his understanding of chondromalacia. I was able to spend a fortnight with him at Toulouse. He had just been to see Hughston, who had shown him a very modern concept of patellofemoral problems.

M.O. What sort of concept?

P.C. The important point was that patellar problems were not related to the cartilage as such, but had to do with patellofemoral imbalance. Managing the condition was, therefore, a matter of restoring the balance: Once the patella was sitting squarely at the right height, and the cartilage had been tidied up, everything would be fine, even in advanced chondromalacia. The trouble is that an investigation of the patella is a complex matter that requires great experience.

M.O. Overall, that equates to the concept of a tibial tubercle transfer ...

P.C. Most certainly not: This is soft tissue surgery. For instance, in cases of patellar instability with a flat trochlea, I have by now done more than 120 transfers of the medial third of the patellar tendon, so-called Mansat II procedures, in adults, with good results. We are also reviewing these cases. Whatever the disorder, there is one fundamental principle in orthopaedic surgery, and that is that one should try to find out what is causing the imbalance, and then to restore a balanced pattern, rather than cause even more destruction. That is also how I see things. Everything that the good Lord made works, and one will never improve on it. So one must not destroy what He has made, in the vain hope of doing better - that would be presumptuous. One has to make the best of what is left, of what has not been destroyed.

M.O. How did you manage to get into knee surgery?

P.C. I just had to knuckle down, since there were a lot of patients with bad knees and not that many surgeons to attend to them. I did osteotomies, and still do quite a few nowadays. I then started inserting unicompartmental knee replacements. However, to this day, I would not do a uni in cases where an osteotomy is truly indicated. The problem with joint replacements is that everything was being thrown together pell-mell, and no consideration was being given to where these devices were indicated and where they were not. I came to unicompartmental replacement with Marmor's experience in mind; I then developed my own tibial cutting guide, and went on to work with Villiers to establish a system for the calculation from radiographs of the correct tibial plateau thickness, as well as a system for the positioning of the femoral component. All these exercises taught me a lot. I very soon found out that unis are a valid concept. However, just as I was finding this, Insall stated that unis did not work. I was in an invidious position: After all, if that nobody Cartier in Rouen says one thing, and the great Insall in New York says something else, there is no prize for guessing whom people will believe. This led to me being one day in a very funny situation. Henri Dejour had made it possible for me to become a member of the International Knee Society, and I was in the States, sitting at a conference with a thousand other participants, where the great Insall was showing his failed unis. I got up, and in very bad English said, "Zeez protheses are badly put, and it is for zeez that zeez does not work." I was just 35, and completely unknown, but I asked for the floor because I found Insall's statements unacceptable. So Insall said to me in front of the whole audience, "Do you mean to say that I am a poor surgeon?" I very much wanted to mitigate my reply, but was let down by my extremely poor English; so I plucked up courage and said, "Yes," and sat down again. Everybody laughed. I shall remember this scene to my dying day, and Insall still remembers it 20 years on. Since then, we have become good friends, and our views on rotating platforms have converged; we find ourselves speaking at the same conferences; and there are no hard feelings whatsoever between us.

M.O. Where was your disagreement with Insall?

P.C. When I read Insall and Walker's first articles, I was struck by two things: Firstly, they had a lot of loosening and rapid wear. That was with 6 mm plateaus, from which Marmor's experience had preserved me. Secondly - and that made me think a lot - they found that where unis had been used in varus knees, the opposite compartment would degenerate rapidly. There would be early wear, and microscopy would show birefringent particles in the tibial and femoral cartilage. However, this pattern was not seen after arthroplasty for genu valgum. Unis had been condemned by the experts - and yet, my simple Auvergnat peasant way of thinking told me that if a device is rubbish, it will be rubbish regardless of which side is was put in. I gave the matter a lot of thought, and eventually came up with the following conclusion: In a valgus knee, most of the wear and the loss of bone stock are in the femoral condyle. However, the early femoral components were only 4 mm thick, and could not possibly compensate for the condylar dysplasia in these knees. This meant that in nine cases out of ten, the valgus deformity would be left undercorrected, with the knee still in valgus even if a chunky tibial component had been inserted. However, when treating a medial compartment, Insall and Walker had used the sort of reasoning employed with osteotomies or total knee replacements: They had done a medial release. They did not know that the length of the medial collateral ligament must not be interfered with. As my friend Deschamps says (and I wholeheartedly agree with him), the length of the medial collateral ligament is our safety factor; all we need to do is fill in the medial joint space. The American surgeons are, to me, past masters of knee replacement surgery; however, they have one problem: They will use TKR concepts when doing uni surgery. A uni, however, is only a spacer, and with a sufficient thickness of polyethylene and with a good anterior cruciate, undercorrection will be tolerated without any ill effects.

M.O. You mean, leave the knee in varus?

P.C. When I first suggested leaving a varus deformity undercorrected, I was told that this would lead to loosening. This is why I kept a low profile for a few years, trying to get sufficient clinical follow-up. We have now published our results obtained with the Full Poly Marmor device, at more than ten years: Undercorrection by between 0° and 5° has proved to be the solution to the problem; it does not cause major polyethylene wear, neither does it lead to a high rate of loosening. Thus, the picture at over ten years is very reassuring. If the big players in the States had put in 9 mm plateaus at the time, and left the tibial bony varus by not tampering with the medial collateral ligament, everybody would be using unis by now. My own findings are very close to those of the Swedish surgeons, who are very much in favour of unicompartmental replacement. Unfortunately, the history of the uni has been further bedevilled by the fact that when the uni was finally coming again in the States, the model used was the PCA, which suffered from a poor design.

M.O. What was wrong with it?

P.C. Several things. At the tibial level, the design encouraged subluxation. At the femoral level, unduly extensive resection was required, and the implant was not self-locking. This meant that, with the recommended cementless devices, the femoral component would be extruded or would loosen rapidly. Once again, the concept was too close to that of a total knee prosthesis, whereas a uni is only a "resurfacing" device.

M.O. One thing is still not very clear - the importance of the thickness of the polyethylene plateau. Surely, 6 mm plateaus may have a lot of wear, but 9 mm ones will also wear?

P.C. The problem is this: All unis start out by bedding down. That is to say that over the first two years, there will be one or two millimetres' wear, as the femoral component makes itself at home in the plateau. So, if one starts with a 9 mm plateau, there will still be 7 mm left after this running in, and there won't be a problem. If, on the other hand, one starts out with 6 mm, there will be only 4 mm left, and then the stresses resulting from undercorrection will be such as to cause loosening or, as Marmor puts it, avulsion of the prosthetic plateau. What I am talking about is, of course, a benign imprint in correctly sterilized unis - unfortunately, there are more modern sterilization techniques in use nowadays, such as gamma irradiation, which have had disastrous effects from which the unis, too, have suffered.

M.O. Where would you use unis?

P.C. There seems to be a certain segregationism about nowadays, which I find very upsetting, because it runs counter to the logic of surgery for unicompartmental OA. It is well known that there are contraindications to tibial osteotomy. In patients over 60, with advanced (Stage III or IV) OA, with very major degeneration, osteotomy will not work for more than five years. However, for what one might call ideological reasons, osteotomies are done, which produce a few degrees of valgus. In a few years, this will have made a suitable candidate for a uni into a patient who - at 65 years of age - can only be managed with total knee replacement. I am very much against that sort of thing. Also, a "straight" tibia will only very rarely produce a good result after osteotomy. An osteotomy of a tibia that does not have a varus curvature will give an oblique joint line. In obese females aged between 45 and 55 (the usual patient pattern), this will necessitate total knee replacement after five years, i.e. when these ladies are at most 60 years old. In such cases, it is easier to do a uni, which, incidentally, is infinitely easier to revise compared with the insertion of a TKR on an oblique joint line. Osteotomies have also been known to fail because the surgeon was unaware of deformities in the sagittal plane. It is totally wrong to decide on an osteotomy without having seen lateral X-rays taken in full extension. If, in full extension, there is subluxation, then this condition will not be corrected by an osteotomy, and the knee will degenerate rapidly.

Regardless of whether there is posterior subluxation of the femur on the tibia, with a posterior "dished" defect in the tibia, or whether there is anterior subluxation of the femur on the tibia, as a result of osteophytes, the correct solution is a uni, if the ACL is sound, or a total knee replacement in the ACL-deficient knee.

M.O. There are also problems with fixed flexion deformities ...

P.C. The problem with fixed flexion deformities is that everybody - and especially the Americans - will equate a fixed flexion deformity of more than 15° with the need for a TKR. This is very unfortunate. More often than not, the flexion deformity will not be capsular but bony in origin. It is the result of anterior bony impingement of the tibial plateau, the tibial spine, of osteophytes, and of the slope of the worn plateau. When all these defects are corrected, lo and behold, a uni can be used to manage the knee, providing that the ACL is sound. If, however, the flexion deformity is corrected by an osteotomy using a forward sloping cut, there will be a forward sloping joint line, which usually causes instability in the patient and rapid osteoarthritic degeneration in the joint. So, if there is a fixed flexion deformity of bony origin, from intercondylar or notch impingement, plus osteophytes, and the patient is young, osteotomy and open or arthroscopic debridement of the notch may be used; however, if the patient is aged 60-65 or above, the causes of the deformity have to be removed, and a uni inserted. This is better than doing an osteotomy, which would be less than optimal, or a TKR, which would be overkill.

M.O. How exactly do you remove the causes of the flexion deformity?

P.C. It's very simple. We do an arthrotomy or an arthroscopic procedure; remove the anterior phytes from the tibial spine, going into the notch; a thorough debridement is performed; and the posterior capsule is detached with a dissector, since the capsule is not retracted but simply stuck to the phytes. This spring-cleaning plus an osteotomy in younger subjects, and plus a uni in the elderly, will produce good extension in the majority of cases.

M.O. What would you say to those who maintain that putting in a uni is courting disaster, and that it would be better to do a TKR straight away?

P.C. There is one thing that is terrifying and insufficiently appreciated - and that is escalation. In the States, surgeons are worried about litigation and compensation, but, astonishingly enough, they are not worried about escalation. At any conference in the States nowadays, two thirds of the time is given over to total joint replacements; the uni is given a couple of minutes in the interval, when the cleaners come in; and, as sure as God made little apples, one third of the time is devoted to revision surgery after total joint replacements. The escalation produced by the failure of total joint prostheses is frightening. This is very major surgery, with problems of bone grafting, with thromboembolic complications and delayed healing, sometimes even infection. People may disagree with my views, but I have been doing knee surgery for the past 22 years, and I have not got anywhere near the revision rate the Americans have after total joint replacement. I would never have enough material to write a book describing my trials and tribulations in revising prostheses I put in at any time since I first started doing knee arthroplasty.

M.O. You do not seem to be happy with the conventional unconstrained TKR.

P.C. Not very happy, no. I have always been a believer in preserving as much as could be preserved; so, early on, whenever both compartments were destroyed and the ACL was in reasonable condition, I would put in two unis, one for each compartment. I have by now done some 400 of these procedures, and the follow-up is extensive. I therefore had every right to think that TKRs were too destructive. About ten years ago, at a meeting of the International Knee Society in Tokyo, an implant manufacturer was showing a cruciate-retaining "meniscal bearing" device, as well as a revision model with a rotating platform, in which the cruciates were sacrificed. I thought that this platform was an ingenious idea, and was among the first to use the device - without correcting the tibial varus, without any ligamentous release, retaining the posterior cruciate, and almost always without a patellar implant. In recent years, I have had the posterior part of the plateau notched, to permit PCL retention. Using this device has confirmed to me the lessons learnt from the "bicompartmentals", viz. letting the components that make up for the degenerative wear slide will produce superb results. The rotation of the knee joint is the key to the problem - where it is facilitated, and where the back of the patella is not replaced (although there are some instances where we would use a patellar implant), the patient will be given a knee which, as I tend to say, he will "forget about." I think that this "forgotten knee" is a useful term, since none of the conventional, symmetrical total joint replacements inserted with a release will allow the patient to forget that he or she has undergone knee arthroplasty. At present, 98% of my patients are managed with unis or with rotating platform devices.

M.O. However, present-day TKRs are reasonably anatomical in their design ...

P.C. There is one thing that people do not realize, although Dejour drew attention to it a long time ago, and that is asymmetrical OA. From managing the two compartments with separate arthroplasties, I know that one may, on occasions, use a small medial plateau and a very wide lateral one, with excellent results. I am now convinced that the 20% of clinical failures (not radiological failures - hence the encouraging survival curves) of TKRs have occurred in cases of asymmetrical OA managed with standard implants. I know that, in the large studies, the mean ten-year survival rates are close to 94%; however, if you discuss this in detail with the surgeons concerned - no matter which country they are from -, you will find this pattern of well-inserted prostheses, with beautiful X-rays, but with patients who are in pain and who are disappointed by the result of surgery. Roughly speaking, this pattern is seen in one out of every five cases.

M.O. Would the rotating platform device solve the problem?

P.C. The solution will come from two kinds of prostheses: rotating platform devices, where rotation is used to compensate for anatomical asymmetry; and semi-constrained asymmetrical prostheses. The position of the implant should not be guided by the landmarks suggested by the manufacturer - the tibial crest, etc. -, but by the middle of the intercondylar notch of the implant component. In practice, I open up the knee, and I look at the state of the femoral condyles, without worrying about the tibia. I put in the femoral component in such a way as to have the notch exactly in the middle. I put the knee into extension, and observe, from the centre of the notch, where the midpoint of the tibia is. That's what it's all about. When one does it this way, and puts the knee into flexion, one can understand why some total knee replacements have failed. In some knees, one plateau is twice as wide as the other. If the surgeon does not realize that, he will leave the patient with a reduced ROM or a painful knee. When such patients come for re-operation to improve the ROM, a ruler placed at the midpoint of the implant notch, with the knee extended, will often be seen to be a centimetre away from the midpoint of the prosthetic tibial plateau. This asymmetry can be compensated for by a rotating platform. However, if a conventional design is to be used, and the centre of the tibial plateau is to match the centre of the notch, a large selection of asymmetrical tibial platforms would have to be available. I personally favour this approach, but the financial implications are such as to make manufacturers consider rotating platforms rather than asymmetrical plateaus.

M.O. What has been the contribution of meniscal bearings?

P.C. Nowadays, surgeons worldwide are convinced that a device inserted with a tibial varus is doomed to loosening. We have reviewed all our rotating platforms, after a minimum of five years' follow-up, and come up with the following finding: A rotating platform device, whether cemented or cementless, without medial release, and not eliminating the varus deformity, will not loosen or subside. Also, a rotating platform truly rotates. I have seen this myself, in the handful of cases where an initially unimplanted patella had to be given a prosthesis at some time after the initial procedure. And if one wants the patient to "forget" about his or her knee arthroplasty, then one must not detach the MCL. A knee prosthesis inserted with a release will never score ten out of ten. The only way of avoiding a release is to insert an implant without doing anything to the ligament. However, if one implants a knee with a bony varus deformity and does not do anything about the MCL, the implant must be able to rotate; otherwise there will be trouble.

M.O. Why should there be trouble if it does not rotate?

P.C. Because rotation reduces stress. It resolves the dual problem of how to improve congruency and reduce stress.

M.O. What are your views on the continuing argument about whether or not to retain the posterior cruciate ligament?

P.C. Quite honestly, I have not been using any posterior stabilized devices at primary surgery, for the last eight years. I am not interested in that design any more. It is no longer something I would consider as a management principle. I sometimes see some old farmers, some backwoodsmen, with arthritic knees with flexion and varus deformities the like of which one does not normally see any more nowadays. They have been walking like this for years, shambling along like gorillas. Of course, even I would use posterior stabilized prostheses in such cases, with a medial release and sacrifice of the PCL. However, that sort of patient is rare nowadays in a city practice; and if the question is whether an intact PCL should be sacrificed or retained, I must say that this is an argument that is way behind the times, and that surgeons should wake up to fact that there are now rotating platform prostheses around.

M.O. The first series of rotating platform devices were not all a success ...

P.C. Why is there trouble with the rotating platforms in the States? Because over there, with the exception of the designers of these implants, surgeons are doing too much of a release, so the rotating platform goes round like a teetotum. The rotating platform is not a novel implant; it is a novel philosophy.

M.O. Are you still doing many osteotomies?
P.C. I cannot give you a very precise answer, because, at present, my patient selection is affected by the pattern of my practice. I am seeing more and more failed osteotomies or unis, as well as patients needing revision after total knee replacement. One thing, however, I am sure of: Overall, of the under-60s, 60% should be managed with osteotomy, 20% with a uni, and 20% with a TKR; while over the age of 60-65, unis would be indicated in 30%, osteotomies in 20%, and TKRs in the rest.

M.O. How do you perform your osteotomies?

P.C. In young and active subjects with a varus curvature, I always do a medial opening wedge, with a graft and plaque fixation. There are several reasons for this procedure, the chief one being that I think that the patient may require joint replacement in the future, and that it is preferable to add rather than to remove bone. However, in borderline cases, in middle-aged ladies who do not yet require a joint replacement, I frequently do lateral wedge closing osteotomies, which are held together with a staple. If the case is not borderline, I overcorrect by 4° with reference to 0°, and use a plate. If the case is borderline, I correct to normal alignment, so that a uni may be used later on without having to remove hardware; in such cases, I do not plate. There is a third, very interesting pattern - that of a patient well into his or her sixties, with Stage IV OA, where an osteotomy would not work but where, equally, a uni would be contraindicated because of a massive tibial varus curvature. For the past two years, I have been doing what the Aubaniac team and Jean-Noel Argenson have suggested - osteotomy to correct to normal alignment, and a uni. It took me some time to get used to that principle, but now I am very happy with the technique, and am myself developing it further all the time.

M.O. Why did you leave Rouen?

P.C. I left Rouen because my work on unicompartmental replacement required me to travel. It was just not possible to be in Rouen, in Paris, and travelling all the time. Perhaps it was also my own personal development that took me back to Paris. At the time, I knew Lemaire. And I'll tell you how he came to take me as his partner. I went to watch him operate, because I was keenly interested in his ACL technique. Third time round, he says to me, "You have been doing knees for years; you should be able to pick this up a bit faster." So I say to him, "Awfully sorry, you must think I'm incompetent. But for as long as I have not understood an operation, I won't do it. Perhaps I'm not as good as the others." A few days later, Lemaire suggested that I go into practice with him. Which goes to show that self-criticism can be a good thing. I had an excellent relationship with Lemaire, and over ten years, we have built up a very unified team - the pun, by the way, is quite unintentional.

M.O. Why did you leave the Clinique des Maussins?

P.C. Lemaire was very well known; but my work, too, was taking off in a major way, and we were, perhaps, beginning to crowd each other. So I thought that I might set up my own centre, together with a young partner that I had at the time. I went to the Clinique des Lilas, where they were trying to put a team together. I asked that everything should be organized as I wanted it, and the management said yes. Since then, I have taken on three partners. Nowadays, I only operate on OA and chondromalacia cases, while all the ligament surgery and the sports medicine side are looked after by my team. In five years, we have built up a flourishing centre, where 80% of the case load is knees. The remainder is shoulder and hip surgery.

M.O. Why did you go into orthopaedic surgery?

P.C. I went into medicine because my father wanted me to go into something that was safe. In those days, children did as their parents told them. I then went into surgery, because I was lucky enough to belong to a small group of students who were taught by Cabrol and Cordier, from their first year at medical school. Orthopaedics I went into for a very simple reason: Every time I opened up an abdomen, I felt sick. I was overcome by some inexplicable angst that should really be investigated by a shrink. And there was another reason. I started my specialty training under Chigot, where I was exposed to the full spectrum of paediatric surgery. The really awful thing was hernia surgery. I was all right doing fusion for flat feet, but these hernias, with their hundred and one leaflets, took me three hours every time. I thought that I was making such a hash of the innards of these little kids that I chickened out and went into orthopaedic surgery. Obviously, these were not very noble reasons, but that's the way it went, early on in my career.

M.O. Talking of technique, what is your opinion of the development of instrumentation for arthroplasty?

P.C. I advocate minimalism in instrumentation. That does not mean that I want to get rid of instruments altogether. It's just that too many young surgeons nowadays lack experience, but are reassured by a huge range of instruments. The main problem is lack of knowledge, and too much instrumentation means not enough knowledge. Once a surgeon has been properly trained, he will need less and less instrumentation, and will replace extra ironmongery with extra thinking.

M.O. Is it not all because technology has advanced too rapidly?

P.C. There is no instrument around nowadays that would allow me to compensate for asymmetrical OA. A surgeon who uses only his instruments will overlook this problem, and will finish up with a poor result. The absolutely essential thing is an examination of the knee before tibial resection is commenced. Leaving the ligaments intact is not something that can be taught by the instruments, and the same goes for flexion and extension gaps. If a knee is destroyed before surgery, i.e. if the medial compartment is released, there will be a certain instability, and the whole flexion and extension pattern will be corrupted. If the knee is not destroyed, if proprioception is preserved, then a mild residual instability in flexion as compared with extension may be tolerated: I know from ten years' follow-up experience that the instability will decrease, and that the knee will stabilize, because its physiological ligament potential has been preserved. It's the battle of biology against the instruments. I think that we need to be able to manage the biology, the instruments, and our thought processes, so as to be able to make maximum use of what technology is offering us. On no account, however, should the instruments have priority over the human mind.