Technology
Research is improving artificial body parts The fairly new science of biomaterials is pulling together interdisciplinary efforts on new materials and practical applications Ward Worthy C&EN, Chicago The search for materials to replace missing or defective body parts is probably almost as old as the practice of medicine. Technique has come a long way; compare George Washington's notorious wooden teeth, for example, to the polymers, alloys, ceramics, and other substances that provide today's remarkable replacements for bone, joints, teeth, blood vessels, and even entire organs. But progress, although considerable, has been largely empirical. If "biomaterials" is old as a technology, it is new as a science. Only lately have physicians, chemists, materials scientists, and engineers begun a concerted effort to unravel
Goat wears prosthesis composed of carbon/PTFE joined with the bone
the complicated interactions between materials and the demanding and often hostile environment of the human body in which they must function. Recently, the Society for Biomaterials held its annual meeting—it was only the fourth—in San Antonio. The meeting, hosted by Southwest Research Institute, was also the occasion of the 10th annual international biomaterials symposium. (The first was held in 1969 at Clemson University.) More than 125 papers were presented. Much of the work reported deals with basic research—characterization of materials, for example, or studies of tissue response and interfacing—or with detailed methods of preparation and fabrication. But other efforts, dealing with practical applications of the various materials, give ample evidence that the basic work is paying off. In orthopedic implants, for instance, particular attention has been given to improvements of hip joint replacements. The hip is a difficult joint to replace; it must carry a lot of weight, and at the same time allow a wide range of motion. In a "total hip arthroplasty," both the ball and the socket are replaced. One widely employed procedure involves amputation of the head and neck of the femur. The amputated parts are replaced by a ball and stem made of a strong, biologically inert chromium-cobalt-molybdenum alloy. The stem is held in place in the hollowed-out shaft of the femur with polymethyl methacrylate "bone cement." An artificial socket, made of high-density polyethylene, is substituted for the natural cartilage of the hip socket. Sometimes, however, only the ball is replaced; it articulates with the patient's natural socket. When successful, such procedures dramatically relieve pain and restore near-normal walking ability to people who have been crippled by arthritis, by femoral fractures that fail to heal, or by various other hip disorders. The success rate is pretty good—about 90% over 10 years, according to one orthopedic surgeon. Still, there are failures, both of technique and of material. Infection can set in, often necessitating removal of the prosthesis. The ball can grind its way through the socket. Debris caused by wear of the opposing joint surfaces can cause foreign-body reactions. One of the most frequent problems is that the stem becomes loose, sometimes intolerably so, in the shaft of the femur. Work continues to try to reduce the failure rate and to extend the useful life of the prostheses. One goal is the development of noncemented devices. The socalled "bone cement" is really just a grout,
not a true cement. Fixation of the stem in the femoral shaft is purely mechanical. For a number of reasons—including reaction to toxic products released by the polymer as it cures—bone can resorb around the stem, with loosening as the inevitable result. It obviously would be desirable if the stem could be held in place by ingrowth of bone into grooves, holes, or pores on the surface of the stem. To this end, a number of groups, notably in Europe, are exploring the use of bioceramic materials. For example, a team at the University of Heidelberg in Mannheim, West Germany, has been experimenting with all-alumina total hip replacements. The alumina articular surfaces have good wear characteristics—apparently superior to the metalpolyethylene joints—and a noncemented, screw-in alumina socket appears to be quite successful. But the Mannheim group has been unable to achieve satisfactory stabilization of an alumina stem, even though several designs have been tried. Attempts were made to increase bone bonding by coating the stems with a bioactive glass. (Dense pure alumina is biologically very inert; however, certain
The porous structure of carbon/PTFE allows for the ingrowth of tissues June 26, 1978 C&EN
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glasses, containing carefully controlled proportions of alkali and silica, are surface-reactive in the body.) Unfortunately, components examined some months later showed "gross dissolution" of the bioactive glass coating, with very little bonding and no stabilization. Relative movement between implant and supporting bone— the consequence of uncontrolled bending, shear, and rotational stresses in the femur—is cited as the main reason for nonstabilization. An "interim solution," consisting of the noncemented alumina socket and an alumina head attached to a metal stem, has proven very effective for partially cement-free implantation in humans, the Mannheim workers note. Another group, in Vienna, has had better results with all-alumina hip prostheses. In these researchers' procedure, however, the head of the patient's femur is left in place, but is covered after appropriate shaping with a "femoral cap." Their animal experiments showed that bone did indeed grow into grooves on the implants, and stable anchorage was achieved without cement. However, the femoral cap technique is not suitable for all patients. Of 15 similar prostheses implanted in human patients, 12 have remained stable over periods ranging from one to 20 months. Three became loose: one because of operative failure and two because degenerative changes in the femoral head apparently made the spongy bone unable to bear the load. This can be a particular problem with the porous, brittle bones of older people. Meanwhile, scientists in Houston, at Baylor college of medicine and the Methodist Hospital, have taken another approach. They apply a pyrolytic carbon coating to metal stems. Specifically, they apply a low-modulus, 80% porous material consisting of carbon fibers and polytetrafluoroethylene. It is essentially biocompatible and nonbiodegradable. Implanted, its open, interconnected pore structure allows adjacent tissue to grow completely into it, thus providing good stabilization. The carbon/PTFE material originally was developed at Methodist Hospital in the form of blocks that could be carved into appropriate shapes and used to repair defects in bones. But it also can be applied as a coating, fused to various substrates. In the Houston studies, a 1-mm coating of the carbon/PTFE material was fused to the metal stems of femoral head prostheses, which were implanted in human patients. Although long-term success has yet to be demonstrated, early results are encouraging. Four of the prostheses have had to be removed, for reasons unrelated to the material or to stem loosening. One, retrieved after 11 days, showed blood vessels at the tissue interface, but no organized fibrous tissue within the coating. Another was recovered at autopsy, complete with femur, six weeks after implantation. A 120-lb force was required to extract the stem from the femoral shaft. A third prosthesis had to be removed after nine weeks because of improper positioning. In that implant, blood vessels 24
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and dense fibrous tissue had grown into the upper two thirds of the tissue-coating interface, but the lower portion of the coating contained only red blood cells and fibrin. The fourth implant was removed after two years because the head had penetrated the socket. Examination showed that dense, mature fibrous tissue had grown throughout the coating. Bone was prominent about 1 mm from the coating-tissue interface, but had not grown into the coating. When the prostheses were removed, separation was effected by shearing within and adjacent to the porous coating. No disbonding of the coating was observed in any of the four. Artificial teeth aren't nearly so "dramatic" as artificial hips, but they're a lot more prevalent. And as all the denture adhesive commercials on television point out, false teeth can sometimes cause a lot of trouble—slipping and sliding and restricting wearers to soft diets. So for years dentists and materials scientists have been trying to develop materials that could be implanted in the jawbone and used for firm attachment of dental prostheses, whether single teeth, bridges, or dentures. And for years it seemed as if almost all the accounts of research in the field were accounts of failure. Admittedly, it's a difficult task. The stresses of biting and chewing are tremendous; if the mechanical engineering isn't just right, the suffering bone will protest by resorbing in the vicinity of the implant, which then becomes loose. Also, the implant, to be useful, usually must protrude through the gum. If its material isn't biocompatible, the patient will have, in effect, a never-healing wound that will predispose him to infection, gum irritation, and other disorders. Recently there has been increasing evidence that success is near and that dental implants may soon become fairly commonplace. As with the hip joints, metals, ceramics, and carbon are among the substances that offer particular promise. At Brookdale Hospital Medical Center in Brooklyn, for example, a group of 21 toothless patients were fitted with mandibular implants of chromium-cobaltmolybdenum alloy, modified by addition of the "Brookdale bar," which provides external attachment points for dentures. All the patients were fitted with full lower dentures (attached to the implants) and opposing conventional upper dentures. To date there have been no problems from hemorrhage and only transitory problems from infection. The only significant adverse reaction—in three of the 21 patients—has been a moderate degree of bone loss under the implants' abutments. However,fiveof the implants have been in place for more than five years, and all the implants are still in place and clinically sound. The Brookdale technique lends itself mainly to attachment of bridges and dentures. Another group, at Battelle Memorial Institute's Columbus laboratories, is carrying out long-term studies of alumina "roots" for replacement of single teeth. To make the roots, which are cur-
rently installed in the mandibles of baboons, alumina powder is sintered into rods of uniform, fine-grain, high-strength ceramic. The rods are shaped by cutting on a computer-controlled milling machine. The resulting root structures have tapered, serrated surfaces that provide optimum stress distribution. The socket is revised to fit the artificial root, which is simply tapped into place. After about three months, histological examination reveals dense ingrowth of bone into the serrations, with no intervening tissue. When the root is firmly anchored, it is made functional by installation of a post, core, and gold crown. At the time of the meeting, 84 implants had been studied, with implant durations ranging from one to 28 months. The overall success rate was 63%, with most failures occurring during the early period when bone was growing into the serrations. Of the roots that have been in full function for more than six months, the success rate is 93%. Success, the Battelle workers note, is judged by dense bone ingrowth, resistance to movement, minimal gum irritation, and maintenance of proper occlusion. Still other materials with potential for use as dental implants are being investigated. These include carbon (low-temperature isotropic carbon used for structural components and also porous carbon coatings deposited on metallic substrates), bioactive glass-ceramic materials, and titanium. Another kind of implant research, still in the animal experiment stage, is currently going on at Southwest Research Institute, under the auspices of the Veterans Administration. The work deals with the development of improved artificial limbs; specifically, limbs that can be attached directly to load-bearing bone instead of being held in place by friction between a "sock" and the stump. The friction and presure of the sock can cause discomfort, and can sometimes even lead to ulceration of the stump. In the SwRI experiments, currently being performed on goats, the stump left after amputation is prepared in more or less the usual way, except that a sort of "cap," made of stainless steel with a porous carbon/PTFE coating, is inserted into the end of the bone. Three studs, radiating laterally from the cap and penetrating the skin, provide attachment points to which the external part of the prosthesis can be fastened firmly. At present, peg-leg goats are gamboling around SwRI's goat lot, seemingly unhindered by their artificial limbs. There are problems, however. Bone grows into the porous coating of the cap, anchoring it in a satisfactory manner. But the carbon doesn't work properly at the skin interface. Coating the studs with a polyester velour has proved somewhat more satisfactory, but less than completely so. Nevertheless, the SwRI workers are convinced that the principle is sound, that the right materials will be found, and that human amputees will eventually benefit from a superior "new generation" of artificial limbs. D
