THE CLASSIC: Tibial Plateau Prosthesis

Abstract

Duncan Clark McKeever was born on September 13, 1905, in Valley Falls, Kansas. After attending local schools, he graduated from the University of Kansas Medical School in 1929. As a naval reservist, he spent the next four years in naval training centers, followed by a residency in pathology at St. Luke’s Hospital in Kansas City. While there, he fell under the influence of Drs. Frank Dickson and Rex Divley and became interested in orthopedics. After three years of association with them, he moved to Houston in 1939 to open a private practice. From 1941 to 1945, during World War II, he was back in the navy as chief of several hospitals. After the war, he returned to his private practice. McKeever’s knowledge of engineering principles led to his research interest in stress analysis as it applied to operative procedures on bones. His advanced ideas in orthopedic surgery led him to develop original procedures, and his exacting attention to details helped make them successful. His success led to additional innovative procedures, which included prostheses of the hip, patella, and tibial plateau. His continuing studies kept him in demand as a teacher. Frequent visits from his many friends included those from Latin American countries. Dr. McKeever enjoyed hunting and fishing, and he was always delighted to be at his ranch. McKeever was one of the founders of the Association of Bone and Joint Surgeons and became its third president. He was also a member and active participant in many orthopedic organizations and on local hospital boards and staffs. On a rainy evening, October 13, 1959, when driving someone else’s car, he ran out of gas; while filling the tank, he was struck by another car and killed. His untimely death was a great loss to orthopedics as well as a personal loss to his many friends. In the past, when a badly damaged knee joint lost any of its articular surfaces, we destroyed it. If the patella is rough, some surgeons take it out. Usually this is not necessary. If the condyles and the plateaus lose their articular surfaces, we arthrodese the knee. This is not an answer; it is an escape. A constructive solution must be found to replace this destructive one. Arthrodesis is an easy way out for surgeons and for patients who have trouble in only one knee, but what of those who have two bad knees? Arthrodesis is an admission of defeat. It is an answer that will be accepted less readily as knowledge of endoprostheses accumulates. The tibial plateaus present a special problem in endoprosthetic restoration. Mechanically, each plateau forms part of a separate joint. They must function synchronously, but the degree of damage of the two may not be identical. Within the same joint space the patellofemoral articulation must function. The knee joint has little structural stability. BIOMECHANICS There are several fundamental considerations applicable to all prostheses intended for functional restoration of joint surfaces. These factors should determine the design and the use of endoprostheses, and must always be given due consideration. The important fundamentals lie within the field of biomechanics. Prosthetic design need not continue to be developed solely by trial and error. A. There must be an optimal relation between surface area and the range of functional stress to be borne by the prosthesis and transmitted from it to bone. We can obtain a rough idea of the range of these stresses in normal joints by the application of simple mathematical formulas. From this application we can assume that the stresses must at times exceed 2,000 lbs. per square inch. In relation to the tibial plateau, the knee is a lever of the 2nd class. The point of action is between the applied force and the fulcrum. If the weight is 150 lbs., the femur is 18 inches long and the fulcrum is 1 inch from the center of application of the force on the tibial plateau, the force exerted is 17 × 150, or 2,550 lbs. If the area to which it is applied is 1 square inch, the load is 2,550 pounds per square inch. The object of an endoprosthesis is to achieve functional restoration. If we wish to restore normal function, we must make as close an approach as possible to the surface areas and contours existing in the normal joint, since in nature there is a correlation between these areas and the functional stresses imposed on them when in use. Their contour, design and density are determined by the effect of function during growth. B. An endoprosthesis must be self-retaining. It must be so designed and inserted that the normal forces existing in the joint in action hold it in place. Any screw, pin, flange or other retention device that functions as anything more than a guide to alignment or to retention of the prosthesis when the joint is at rest must eventually give way as a result of cyclic stress. C. The direction of stress transfer between the endoprosthesis and the bone on which it rests must be constant. The importance of this factor is very seldom appreciated. Bone will withstand repeated applications of stress, and even increase in sectional density to offer increased resistance to the stress, provided that the stress is constant in direction. If there is an angular variation in direction of stress, absorption certainly will take place. The prosthesis cannot have just anatomic continuity with the bone; it must have functional continuity. D. The stress transfer from prosthesis to bone must take place at a single level. Any part of a prosthesis that passes this level will be nothing more than an alignment device to maintain a constant direction of stress. If a significant portion of the stress to be transferred from the endoprosthesis to the bone bypasses one part to reach another level of bone, absorption will occur and will continue until a balance is reached. This absorption will be in proportion to the amount of stress that bypasses the contact point. If all of it bypasses this point, total absorption will occur. Bone that is not functional as a stress-transmitting unit will disappear. We must not lose sight of the fact that endoprostheses transfer stress on two surfaces. The stress is transferred from one articular surface to the prosthesis, is transmitted through it and again is transferred to the bone. E. Complete functional restoration of the joint by a thorough surgical procedure must be the goal. A prosthesis may play a small, though vital, part in the result. Such problems as range of motion, stability, muscle balance and restoration of periarticular gliding surfaces must be given due attention individually and in relation to each other. CLINICAL CHOICE Case selection is an important consideration in the use of endoprostheses. It is a common error in surgical judgment to use a new procedure, or device, such as a prosthesis, in the most hopeless and difficult case that we can find. This attitude has been responsible for many discouraging failures of good surgical procedures; for instance, in the hip. I have done it, others have done it, and it is so natural that we probably shall continue to do it. But it is not logical. The proper case to select for the first use of an endoprosthesis is one in which the only functional deficit in the joint can be replaced by insertion of the prosthesis. This would suggest that the joint still is functional, or at least that it only recently has lost its function. The mental attitude of the patient, his tolerance to pain, his economic and psychological incentives to cooperate may be decisive. Some patients, through sheer will power, continue to get about on a joint that functionally is so deranged that others of weaker moral fiber and lower pain tolerance would long since have ceased to use it. Such people are good patients on whom to try a new surgical procedure. The physiology of the patient frequently is ignored. To do this is to invite failure. Prostheses are biomechanical problems. A functional unit that is satisfactory in a machine may fail in a living body. A machine cannot alter its structure to compensate for variations in stress; its margins of safety are constant. In a healthy body, bone can increase in density and in size to meet the additional strain if the stress is not applied too rapidly or in too great an amount. The direction of application should not change, but its margins of safety may be variable. In an unhealthy body, where the stress is applied too fast and in too great an amount or in a variable direction, bone will melt away. We must ensure a positive reaction to the prosthesis. Bone responds according to certain laws. We must know what they are and apply this knowledge. PHYSIOLOGICAL CONSIDERATIONS We cannot afford to assume that a patient’s physiology is normal; we must use every test at our command to detect any possible abnormality. Vital functions for which we have no laboratory or clinical test must be assumed to be subnormal. We should take steps to ensure their function at physiologic levels. Many reconstruction procedures have failed because the doctor did not realize the importance of the general health of the patient and did not take steps to improve it. All aging individuals, and many who have sustained an injury or have had other surgery, are in some degree of catabolism. The essence of degenerative change, the cardinal characteristic of aging, is that catabolism exceeds anabolism in rate. The body must be made to react positively to the prosthesis. This implies normal physiology, as expressed by rapid healing. Normal osteogenesis will ensure proper arrangement of stress lines for the transfer of strain from the prosthesis to bone and enable the bone to attain optimal cross-sectional density in a minimal time. Unless the patient is in a positive metabolic state, these positive reactions to the prosthesis cannot occur; ultimate failure then is certain. The metabolic phase of this problem must be considered in the light of the patient’s life expectancy. Optimal physiology must be maintained for the remainder of the patient’s life. Part of the surgeon’s job is to emphasize to the patient and his responsible relatives the importance of this factor, so that they will see to it that the regimen is continued after the patient has been discharged from direct medical supervision. Muscle function and balance must be restored with proper exercises. In the knee joint the function of the flexors is very important. The extensor mechanism cannot function normally unless it is balanced by hamstrings of good strength and resiliency. The hamstrings must be given adequate progressive exercises, for, paradoxically, the knee will not extend fully if the flexors are weak. Full extension must be restored. Full flexion is not essential, but good functional flexors are. Occasionally, arthroplasty of an ankylosed knee is indicated and justified, but there are many more knees in which restoration of one or both tibial plateaus for weight-bearing surfaces is indicated. Such restoration will avoid an arthrodesis and restore a functional range of pain-free motion not possible without it. In centrally or totally depressed tibial plateau fractures, restoration of position may not restore a smooth surface. In traumatic and degenerative arthritides, particularly in elderly individuals in whom a gradually developing flexion contracture precludes weight-bearing, a smooth plateau may restore function. Such conditions may follow trauma that occurred many years before. They may be the end result of osteochondritis dissecans, old untreated cartilage injuries, or the abnormal weight-bearing stresses occurring with a knock-knee or a bowleg. They may occur incidentally in rheumatoid arthritis. Many such cases are subjected needlessly to arthrodesis. DESIGN OF PROSTHESIS For some years I tried to design a prosthesis for application to the lower end of the femur. During this time I made several different drawings with a number of minor variations in each. Instinctively I felt that there was something wrong with them. After several years of study of the mechanical principles, during which time I made more and more application of these principles to the problems of endoprostheses in other locations, the basic fault of this approach to the problem finally occurred to me: Such a prosthesis violates one of the given principles. “There must be a constant direction of stress transfer from the prosthesis to the bone.” How does this apply to the knee joint? In the lower end of the femur, stress applied may vary through an arc up to 145° between the limits of flexion and extension. This precludes stress transfer from prosthesis to bone in a constant direction. In such a case extension produces a direct thrust. In flexion, the lower femur becomes the site of application of forces exerted through a lever. Bone will not withstand angular variations of stress at the point of contact with a prosthesis. The functional stress applied to the surface of the tibial plateau has a constant direction. It is in line with the axis of the tibial shaft no matter what position the knee is in. Any prosthesis applied to the knee and functionally similar joints-for example, the interphalangeal and the metacarpophalangeal joints-should be on the distal side of the joint. The restoration of the tibial plateau must be accomplished by two separate pieces, one for each tibial plateau. In many knees it is necessary to restore only a single plateau, in which case it is important to have a single-plateau type of prosthesis. Of importance also is the observation that there is a change in axis at the knee joint as flexion occurs. In many cases, this would cause either rocking or binding of a one-piece prosthesis made to cover both plateaus. The only way to avoid this with a one-piece prosthesis would be to have the lateral ligament sufficiently loose to prevent binding. Such a joint would be unstable in extension (figure not shown). The first prosthesis designed had exactly the same contact articular surface as the present prosthesis. This surface design was achieved by measuring 40 tibias of different sizes. These measurements disclosed that, while considerable variation existed in the overall diameters of the upper surfaces, there was little variation in the central weight-bearing areas. The largest tibia did not exceed the articular surface of the present prosthesis, and its dimensions were within the anatomic limits of the smallest adult tibia of those tested. The articular surface of the larger specimens was found to be an extension of the elliptical contour of the weight-bearing area of the smaller tibias. The central areas were almost identical. Furthermore, in practice, this contour has proven to be satisfactory. The original stem has been altered for greater ease of insertion. The prostheses are made in pairs. A pair will do both sides of either knee. For example, the prosthesis for the right medial plateau fits the left lateral plateau. They are labeled right and left. This is not an anatomic designation but refers to the right or the left side of the knee being operated upon as one faces it (figure not shown). OPERATIVE TECHNIQUE Through a median parapatellar incision the semilunar cartilage, or its remnant on the involved side, is removed. The femoral condyle may be flattened if the weight-bearing surface is worn away badly. This necessitates the removal of a portion of the posterior part of the condyles to restore the elliptical contour of the articular surface and permit smooth flexion (figure not shown). With a reciprocating saw, a triangular piece of bone is removed from the tibial plateau and the tibial spines. An anteroposterior is made inch from and to the where the triangular piece of bone was removed. A then is made at right to the anteroposterior and inch from the of the plateau (figure not shown). It medial to the anteroposterior and then lateral to it. These need not be but they must the bone (figure not shown). The prosthesis then is inserted so that the anteroposterior flange on the prosthesis rests in the anteroposterior It is or back the knee until the flange on the prosthesis the It may be necessary to the joint in to do this (figure not shown). may be by of the or by a in the With the on the prosthesis in position the the knee is The prosthesis will as the joint in extension. of the joint then can be tested. If it is smooth and the joint is in the insertion is satisfactory. The patella may or If it is badly it should be restored with a prosthesis. The other tibial plateau may be restored in exactly the same Any necessary of the of the condyles or of the remainder of the joint should be out. I of the that these should be than The surface will be if this is The articular margins of the condyles should be in this If it is necessary to the tibial plateau to or the prosthesis should be inserted The ligament and are continuity with the on the tibial A should be made the prosthesis. I to it with an The plateau, in which the prosthesis is is and the piece of bone may be removed and to the The plateau should be in this position by a bone from a of with the at the tibial (figure not shown). of the cases in which this prosthesis has been would have been subjected to an arthrodesis. least one of them not have been in so as one is to be with both knees knees of the were involved in a very advanced rheumatoid the degenerative of which had been by to of of The first case was operated on in This was an almost hopeless joint, due to an advanced This was such a case as I have should not be for trial of a new device or had been in flexion for and on for had a restoration of both tibial plateaus by a prosthesis, a prosthesis and an joint was to restore the periarticular gliding surfaces and the after had a smooth range of motion from of flexion to extension. had almost of flexion and a very of active extension This patient had in a In of continued to be 1 had a range of motion, and from to with a the and without a in the years after the had lost some had all and had had an of general years after medical was without a or has of flexion and extension did not have any pain unless was on all it is considered that this patient, had a of and a rheumatoid and degenerative with almost of all joint surfaces of the knee, that had been on for several and that had a flexion contracture when first this result satisfactory. is still without a or a and case was a of had had rheumatoid for had 150 of for a steps with had advanced of the patella and of the joint surfaces. There was flexion contracture in both also of on the left knee and about on the right knee. On a and of the semilunar were out on the left knee. A lateral tibial plateau prosthesis was and the plateau was to the as as A prosthesis was in medical regimen were and all of after the first the right knee was operated on in a similar a lateral tibial plateau prosthesis and a prosthesis being and were It was not considered necessary to the tibial plateau on this side because the prosthesis produces some and it in this knee. The result have been if it had been to the The patient about without or a up and with some is time as a has had no of rheumatoid in of very stress due to the of has continued to most of the load of have been out on other To I have inserted plateaus in 40 In most of prostheses have been in with the plateau All of them were badly damaged knee and of and of the of the condyles were out. of one or both semilunar was necessary in every There has been one failure due to of an old This the removal of both plateau prostheses and the prosthesis, and the patient has an All the other cases are without or though some of the patients are to a for All have a satisfactory functional range of motion, from extension to or more of In one patient pain has it is by a of 1 every or this pain is to be of functional stress other cases are in of but are not considered to have an With this prosthesis it is possible to restore satisfactory function to most of the badly damaged knee joints that would be subjected to an arthrodesis. If this prosthesis will function in these damaged knee it will function in any case other than that with an