The genesis and evolution of acrylic bone cement

Orthop Clin N Am 36 (2005) 1 – 10

The genesis and evolution of acrylic bone cement Dennis C. Smith, PhD, DSc, FRSCan* Institute for Biomaterials and Bioengineering, University of Toronto, Toronto, Ontario, Canada

The commercial production of cast sheets of polymethylmethacrylate (Plexiglas, Rohm and Haas, Philadelphia; Perspex, Imperial Chemical Industries [ICI], Runcorn, UK; Lucite, DuPont, Philadelphia) in the early 1930s led to its development as a denture base and prosthetic material because of its characteristics of transparency, strength, and stability. Originally blanks of the material were molded under heat and pressure. In 1935 ICI introduced an injection molding technique for dentures in which the molten material was injected under hydraulic pressure into dried plaster molds. These techniques proved to be too cumbersome and critical and were not generally adopted. In 1936 it was discovered that a mixture of methyl methacrylate monomer and ground polymer produced a dough that could be molded in plaster molds and could be polymerized to a solid mass by heating using benzoyl peroxide as a polymerization initiator [1] In the next few years it was found that improved molding characteristics could be obtained using a powder that was a mixture of ground and spherical (bead) polymer particles obtained by suspension polymerization. The adoption of the dough molding technique led to the near universal use of these acrylic resins for dentures and prostheses (eg, cranioplasty) in the 1940s. It was discovered by German chemists in 1943 that the dough could be polymerized at room temperature if a tertiary amine such as dimethyl-ptoluidine was added with the benzoyl peroxide [2]. After 1945 this information became generally available and considerable research efforts were devoted

* Box 1321 RR1, 142 Northmount Crescent, Collingwood, Ontario, L9Y 3Y9, Canada. E-mail address: [email protected]

to producing heat-cured and cold-cured denture, prosthetic, and filling materials in the 1950s. The properties of acrylic resins for dentistry at that time were reviewed by Tylman and Peyton [3], Osborne [4], and Schwartz [5]. Although early applications had included cranioplasty and other prostheses such as the Judet femoral head, the major application was for dentures. In the United Kingdom the advent of the National Health Service in 1948 led to the provision of millions of dentures in the next few years. Concerns associated with an apparent high rate of fracture (midline fracture) and other perceived deficiencies of polymethyl methacrylate soon were expressed. Although there was considerable technologic improvement, however, there was little basic research into these problems. I had graduated in chemistry and then obtained a Masters degree in organic chemistry. Subsequently I joined the University of Manchester in 1952 as Assistant Lecturer in Dental Materials in the Dental School. This post was in the Department of Prosthetics under Professor Ernest Matthews, who had graduated originally in chemistry. Consequently there was a strong bias toward materials research in the Department and this became a career activity in the ensuing years. This, of course, was many years before the study of biomedical materials or biomaterials became established as a discipline. In 1954 I began a series of studies on denture base polymers with special reference to polymethyl methacrylate for my doctorate, which was awarded in 1957. I remained full-time in the Department and developed a career in dental materials. There were no comparable positions in medical schools. John Charnley was a frequent visitor in the Department from approximately 1948. Medicine and

0030-5898/05/$ – see front matter D 2004 Elsevier Inc. All rights reserved. doi:10.1016/j.ocl.2004.06.012

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Dentistry were part of the same Faculty in Manchester and there was a good deal of interaction in research. Charnley was well aware of the range of materials used in dentistry, because his father was a dentist. His boyhood friend, Fred Taylor Monks, was now a dental surgeon and was on the staff of the Dental School, which probably provided Charnley with an entree into the Dental School facilities. At first Charnley’s main contact was Henry Atkinson, now Emeritus Professor of Prosthetics in the University of Melbourne, then Senior Lecturer in Dental Prosthetics. Atkinson remembers that Charnley was bvery excited at having been given a 1/4 in. Wolf electric drill with which he had done all the household repairsQ and wondered if it could be used as a lathe. Charnley had a long experience of making his own instruments and gadgets and had bought a Myford lathe in 1946, which he had transferred to the Medical School. Atkinson had a Myford lathe and also a skilled technician in the Dental School, and Charnley was interested in his work on machining and thin sectioning of bones and rocks embedded in cast acrylic resin. There was also research into Judet prostheses and acrylic joints cast in alginate molds. Atkinson made several sections and models of joints for Charnley. He comments that bJohn was such a keen and stimulating personQ that his memory of the events is still clear after 50 years. Atkinson left for Australia in 1954; my laboratory inherited the Myford lathe and so John Charnley and I became acquainted more closely. We took to each other well, though I was much his junior. We were similar types, both short, stocky, large heads, argumentative, stubborn. In fact, at first I believed he was a Yorkshireman, because his characteristics were so similar to those found in that county and in my own, the adjacent Lincolnshire.

his chimney so that he could improve the combustion efficiency. Later he showed me with (rightly) considerable pride an expanding reamer he had made for preparing the acetabular seat. Sometime in late 1957 we got into a discussion about cementing protheses. I do not recall exactly the way in which this started. Unfortunately there was no amanuensis close by with a pencil and pad and certainly no tape recorder! Part of the stimulus may have been the laboratory in which the students were casting and cementing crowns. Because of his familiarity with dentistry, Charnley would be aware of the dental post crown that is cemented into the prepared enlarged root canal after removal of the diseased natural crown (Fig. 1). Nevertheless, after discussing the probable requirements for a suitable cement we agreed it should be possible to grout in the femoral prosthesis. This was an independent development. Although we were aware of the use of heat-cured acrylics for cranioplasty because such prostheses had been made in the Department, we were unaware of the work of Kiaer [6], who in 1951 had cemented in six Judet prostheses, nor that of Haboush [7] who had used acrylic cement as a seating compound on the cut surfaces of the femoral neck. Nor at that time did we know of the use of cold-curing acrylic material in cranioplasty by Spence [8], Blaine and Oliver [9], Woringer et al [10], and Robinson and MacAlister [11] or the investment of intercranial aneurysms

The genesis of acrylic bone cement Charnley came into the dental materials laboratory often, though not on a regular basis. I recall that he frequently would find me in the Junior Mechanics Laboratory, where the students practiced the fabrication of dental appliances. His questions were usually about the properties or fabrication of materials. At that time we were involved in research on acrylic polymers and on the casting of cobalt-chromium and gold alloys and a variety of other dental materials. Incidents that stick in my mind are John borrowing a 250°C thermometer, an expensive commodity at the time, to measure the temperature of the flue gases in

Fig. 1. Radiograph of a metal dental post crown cemented into the root canal.

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and spinal fixation by Dutton [12], Harris [13], and others. In those days communication was slow and access to literature was limited. Nevertheless these applications were later useful support for the safety of acrylic cement in low friction arthroplasty. Ironically we did not discover until a good many years later that Gluck in Germany had cemented total hip and knee replacements in 1890 [14]. Gluck used ivory components and a cement stated to be composed of colophony resin, pumice, and plaster of Paris. His degree of success, however, is unclear in his article. Our ab initio approach is underscored in that, after considering the necessity for low toxicity, autoclave sterilization, viscous consistency, and easy manipulation, my first suggestion was to use hard dental stone, ie, gypsum plaster—a calcium sulfate cement. I do not recall if there was any thought of cement resorption and replacement by bone. In the event the material proved unsatisfactory in trials because of extended setting in the presence of blood. Despite the unsatisfactory outcome, this first effort had provided a better definition of the requirements for clinical practicality, particularly for setting time, viscosity, and resistance to body fluids. From dental experience it was obvious that acrylic dough was a possibility, because it could reproduce fine detail and, when set, provided a rigid polymer that was stable under body conditions. My PhD work [15] had involved investigation of many aspects of the polymerization and the physical and mechanical properties of polymethacrylate materials, so we had a considerable foundation of relevant data and experience in this area. After due consideration and evaluation of several materials on which I had data for residual monomer, strength, and other properties discussed subsequently, I selected a denture repair material called Nu-Life. For clinical use the powder was dispensed into glass screw-top jars sealed with adhesive tape. Two Formagene tablets were placed in each jar for sterilization. The liquid monomer was filled into amber glass screw-top vials. Charnley undertook laboratory trials for handling, setting time, and packing consistency using a mock medullary canal that he called his stuffing box. He determined that optimum results were obtained using a viscous dough. I had drawn his attention to the fact that in the moulding of dentures in plaster molds, better flow and penetration into detail was achieved by moderate pressure and a dwell time rather than a rapid application of high pressure. Following these trials Charnley had faith in my foundation of data, experience, and judgment, and

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so he operated on the first case in Manchester in 1958. He reported the preliminary results of six cases in the British Journal of Bone and Joint Surgery in 1960 [16]. It is important to appreciate that this advance was not simply the use of acrylic cement but rather a conscious recognition of its ability to fill completely the medullary canal and adapt to the bone interface, so facilitating stress transfer, minimizing local stresses, and thereby stabilizing and anchoring the prosthesis. It was a new technique and this provided the basis for the development of Charnley’s concept of low friction arthroplasty [17] over the next decade.

The development of acrylic bone cement This clinical success aroused great interest and some skepticism in part because Charnley was at this time regarded as antiestablishment and partly because it was not appreciated that Charnley and I had a substantial foundation of scientific data and technologic experience that we had considered before embarking on this new approach to arthroplasty. In this context, I had been able to visit the firm of Kulzer and Company in Germany in 1958, where I was well received and shown their technology for milling and blending methacrylate polymers for their dental products, Paladon, Palapont, and Palavit. The scientific background was a consequence of the extensive basic research on new polymers in the 1940s and 1950s following discoveries during World War II. There was considerable activity on acrylic materials in the United Kingdom in particular at ICI at Blackley, north of Manchester, and at Welwyn Garden City. We received excellent collaboration from their Plastics Division, in particular from E.A.W. Hoff [18] who was carrying out research that was highly relevant to our problems with dental polymers. Hoff was studying the influence of polymer composition and structure in polymethacrylates on mechanical properties and glass transition temperature. His investigations on mechanical losses (or internal friction), for example, showed that in dynamic stressing polymethyl methacrylate exhibited a secondary loss peak at approximately 40° C and 5 Hz. In addition, considerable amounts of data on tensile behavior were made available to us from Dietz and McGarry at Massachusetts Institute of Technology (MIT). This basic information and technical support was important to our studies of dental polymers.

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Only a few examples can be cited here to illustrate some of the data we had accumulated by 1958 as a basis for the cement application [15].

Properties of polymethyl methacrylate polymers We had developed analytic techniques using infrared spectrophotometry and bromination chemical analysis for the detection and estimation of residual monomer (RM) in polymethyl methacrylate after curing with reference to its potential effect on toxicity and physical properties [19,20]. In heat-cured materials the RM was a function of the temperature and duration of cure. At 100°C the RM rapidly decreased to an equilibrium level of approximately 0.2%, whereas at 70°C many hours were required to attain the same levels (Fig. 2) [21]. Short or inefficient cures thus could leave 2% – 4% RM or even more in the material. Long-term analyses of specimens maintained in water at 37°C showed that little of this RM was eluted from the polymer. On the other hand, there was a significant reduction in RM in the material, suggesting a slow, continued, diffusion-controlled polymerization [19,21]. These changes corresponded to the rate of thermal decomposition of the benzoyl peroxide as determined by ultraviolet (UV) spectrophotometry [22]. The initial benzoyl peroxide concentration of 0.9% in the test materials rapidly decreased to near zero at 100°C, but many hours were required at 70°C for a similar reduction. Similar experiments on cold-cure materials such as Nu-Life showed initial concentrations of 1% – 2% benzoyl peroxide and dimethyl paratoluidine in the respective powder – liquid components. One hour after

Fig. 2. Decrease in residual monomer in polymethylmethacrylate cured at 70°C (upper curve) and at 100°C (lower curve).

Table 1 Residual monomer and peroxide in Nu-life Time days/weeks

Residual monomer %

Residual peroxide %

0.04 2 7 4 12 20 400

1.95 1.80 1.45 1.70 1.64 1.70 1.60

0.35 0.50 0.60 0.55 0.50 0.40 0.46

mixing and polymerization the RM was approximately 2%, with little decrease with time even after 8 years and similarly for the peroxide levels (Table 1). These data, dental experience, studies on sensitivity [20], animal studies [23], and the (later) reported experience in neurosurgery [12,13] supported our view that the risks from methyl methacrylate were small. Studies on mechanical properties demonstrated a marked effect of RM on tensile and bending strengths and moduli and on Rockwell hardness, especially greater than 3% – 4% [15]. At the higher levels of RM, pronounced deformation and reduction in strength was observed (Fig. 3). This deformation was recoverable on annealing the specimens, indicating that it was largely high elastic strain rather than viscous flow. The tensile strength of these strained and annealed specimens was low nevertheless, suggesting that microcracks did not heal [24]. Strength and deformation properties also were found to be affected by molecular weight and water sorption, which was approximately 2% by weight on the cured polymer at saturation. Considerably improved mechanical properties were obtained by the incorporation of glass fiber or cloth, but molding properties were adversely affected [25].

Fig. 3. Change in stiffness and deformation with residual monomer content in dental polymethylmethacrylate.

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Increased RM was found to substantially reduce glass transition temperature [26].

Studies of dynamic failure in polymethyl methacrylate

Fig. 4. Flexural fracture surface of commercial cast polymethylmethacrylate (Perspex) showing origin of fracture (top right) and parabolic figures arising from secondary crack initiation.

Our clinical studies had shown that service failure in dentures was caused by fatigue or impact failure. Studies of service fractures and subsequently of model systems provided new information on the mechanisms of failure [27]. This work was the subject of a communication to Nature in 1958 [28]. This fracture analysis provided insights into the mechanisms of service fractures. As an example, the flexural fracture of Perspex (Fig. 4) illustrated how the main fracture originated at some major surface flaw, initiating secondary cracks at flaws ahead of the main front as it propagated through the cross-section. These secondary cracks intersected with the main crack, forming parabolic or hyperbolic figures, depending on the relative crack velocities. A comparison of commercial cast acrylic sheet (Perspex), laboratory cast sheet (more flaws caused by dust particles), and the molded monomer – poly-

Fig. 5. Comparative tensile fracture surfaces of polymethylmethacrylate (A) commercial cast sheet, (B) laboratory cast sheet, and (C) dough molded dental acrylic material.

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mer material showed dramatic differences. The flexural fracture of Perspex showed a smooth surface with an initial failure from a surface flaw and the characteristic markings produced by initiation of secondary cracks ahead of the main crack front (Fig. 5A). Increasing numbers of micro flaws led to more complex fracture patterns (Fig. 5B) and a ridged structure in the dough-molded material (Fig. 5C). Examination of the relevant variables showed more brittle behavior at lower temperatures and at higher strain rates in contrast to plasticization and weakening in the presence of water or RM [15,28]. I found the structure of the dough-molded material could be delineated by etching with nitric acid [15]. This technique showed that, for example, increase in the monomer to polymer ratio in the cement resulted in better solvation and consequently improved properties [24]. Dynamic properties, namely impact and fatigue strengths, were observed to be affected strongly by microstructure and structural flaws, in addition to the variables of RM, strain rate, temperature, and water content [15,24,27].

Fatigue failure Flexural fatigue testing was undertaken on a 10-station cantilever machine (Fig. 6A,B) at 2 Hz and 37°C on wet and dry annealed specimens of

Fig. 7. Flexural fatigue data for pink acrylic resin at 20°C and 50% RH (upper line) and at 37°C and 100% RH (lower line).

acrylic and other polymers. The results showed that Perspex had superior resistance to failure compared with the pink or clear dough molded material [15,24,25]. Fatigue resistance was significantly lower in the water saturated specimens than in the dry specimens (Fig. 7). At low stress levels there was some evidence of long-term resistance to failure. Incorporation of ethyl methacrylate improved fatigue resistance [15]. The fracture surface of the fatigue specimens exhibited a change at longer lives to a conchoidal, multi-ridged pattern typical of slow interrupted fracture [27]. At higher magnification, crack nucleation could be seen to occur at polymer bead boundaries [27]. The laboratory fractures were comparable to the appearance observed in the in vivo denture fracture surface that itself was analogous to a classic metal fatigue failure (Fig. 8A,B) [27]. These findings clearly indicated the need for maximum thickness of bone cement under (desirably) compressive stresses in the clinical setting.

Further development after 1960

Fig. 6. Flexural fatigue apparatus (A) general view showing 10 stations and (B) close-up showing waisted cantilever specimen under load. Environmental control box parts removed.

These data and other results that have not been discussed provided an informed basis in 1960 for the clinical use of acrylic bone cement and pointed the way to future progress and development in areas such as polymerization systems and copolymers [15]. This did not come about as rapidly as might have been expected with the benefit of present day hindsight. There were several reasons for this. One was, as mentioned previously, the pressure on John to document the success of the concept clinically, which, of course, took time. There was less interest in the scientific materials foundation in the context of the clinical orthopedic scene at that time, because the emphasis was on trial of clinical concepts

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Fig. 8. Fracture surfaces (A) clinical midline fracture of upper acrylic denture and (B) steel turbine shaft.

and there was little concern or funding for basic materials research and few such researchers. Another factor was that I assumed naı¨vely that because much of the work had been published that this would be sufficient documentation to justify further research. It was only much later that I realized that orthopedists did not read dental journals! Further, because my knowledge of the magnitude of the clinical problem was still elementary at that time, I did not foresee how rapidly the situation would develop and the demand would grow, necessitating the need for larger-scale research. After 1960 the immediate concerns therefore were technologic, involving dispensing, mixing, sterilization, and clinical technique. Further research and development was interrupted by a fortuitous chain of events that inhibited the preparation of a joint comprehensive report in the orthopedic literature. This was a consequence of the fact that we both had broad interests and long agendas. In 1960 – 1961 I was Visiting Associate Professor at Northwestern University in Chicago. When I returned after this hiatus, John was grappling with the problems of the polytetrafluoroethylene (PTFE) socket. He also was developing the greenhouse enclosure against infection and coping with the many inquiries and training visits to Wrightington, whereas I had a rapidly expanding Dental Biomaterials Unit with increasing graduate research activity. Nu-Life continued to be used by Charnley until 1964 when he decided he would prefer a nonpigmented cement with no additives. It turned out

that the Nu-Life material, supplied by the Cottrell Company of London, was actually manufactured by the Dental Manufacturing Company in Blackpool not far from Wrightington. Further, Frank Hawtin, the Managing Director, was a patient of Charnley. Both he and the Works Manager, George Thurman, were already known to me. An active collaboration orchestrated by Charnley then ensued in which a variety of trial compositions were evaluated in comparison with Nu-Life. Principal problems were dough-time, viscosity, sterilization procedure, radiopacity, and packaging. This work extended into 1966 and resulted in the production of CMW Bone Cement. By this time there were competitive cements. McKee at Norwich had begun to use acrylic cement after a visit to Wrightington by his colleague WatsonFarrar in late 1960. McKee used Simplex, which had been used for several years for cranioplasty. Simplex acrylic materials were manufactured by Dental Fillings Ltd. of London, whose Managing Director was the redoubtable Dr. W.L. Rawitzer. This company marketed filling and prosthetic materials. Simplex Pentocryl, a denture material, had been used for direct cranioplasty by Robinson and McAlister [11] in 1955. Later this material was designated Simplex-P (a moldable paste) and a second form designated Simplex-C (a cream consistency). The former was recommended for cranioplasty and arthroplasty and the latter for neurosurgical applications such as intracranial aneurysms [12,13]. These cements then were marketed by a related company,

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North Hill Plastics, Ltd. The material Palacos R from Kulzer Co. also became available in the United Kingdom around this time. Comparative data were obtained for these materials, but Charnley preferred the CMW material made to his specification until late in his career. The observations from his post mortem specimens demonstrated convincingly his ability to achieve excellent cement penetration with his technique using thumb pressure (Figs. 9 – 11). After 1964 a further obstacle to new work on bone cement was that I had invented a new type of dental cement that was adhesive to calcified tissue— the polyacrylate or polycarboxylate cements [29]. The patenting, development, and clinical use of these cements was a major activity for the next few years. Some years later in Toronto, Drs. Robert Jackson and Walter Peters and I investigated the usefulness of the zinc polyacrylate cement for orthopedic use, but they were toxic to bone because of the zinc. Their later offshoots, the glass ionomer cements, also have received trial for orthopedic and craniofacial applications. The refinement of the implants and the stabilization of the bone cement formulation led us to draft a comprehensive report for publication in 1966 and an outline paper in 1968, which are still extant. A major problem, however, was to combine 30 pages of the

Fig. 9. Early (1963) hip joint showing pink acrylic cement (Nu-Life) retaining femoral and acetabular components after 2.5 years in situ. (Courtesy of John Charnley.)

Fig. 10. Vertical section through femoral shaft showing adaptation of acrylic cement to stainless steel and bone. (Courtesy of John Charnley.)

scientific approach that I preferred with the clinical exposition that inevitably reflected John’s strongly held opinions. We also had slight disagreements on the advisability of simple addition of barium sulfate to the powder and on the expansion of the cement on setting. Our respective schedules allowed little time to homogenize these matters, and we were still trying when I moved to Toronto in 1969. There many new responsibilities arose in starting a new department, and our joint article was stillborn. So John published

Fig. 11. Horizontal section after prosthesis removal showing penetration of cement into cancellous bone. (Courtesy of John Charnley.)

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his clinical monograph in 1970 [30] and I published only a summary of the materials aspects [31]. Our correspondence continued until the late 1970s, but of course by then acrylic bone cement had become well accepted and a host of new implants and techniques had come forth. Looking back it seems obvious that I should have drawn together all the unpublished data and material published in disparate locations and produced a review in the orthopedic literature. It did not happen for the reasons outlined, however, and so after 1970 when acrylic bone cement was reclassified in the United States for general use, various investigators treated it as a new material and the prior history and dental experience was overlooked or forgotten. Our work from a decade earlier thus was repeated with much the same results and new original work was further delayed. Over the years I have noted that, for example, studies of residual monomer continue to be made with increasingly more complex and expensive equipment. The results, however, are not much different from those we obtained with a pipette, a burette, and a conical flask. Now, of course, substantial changes in composition are inhibited by regulatory requirements. The Clemson Award for Basic Research in Biomaterials was awarded to me by the Society for Biomaterials in 1976 and to John in 1977. At the latter meeting John spoke generously of our collaboration and said of the concept, bReally, it was all done in half an hour.Q Although there was some literal truth in that, it did not obscure the effort and the dedication required to change the course of clinical practice. In that regard there were of course many other contributions to the end result just as many streams run together to form one river. It will be interesting to see how acrylic bone cement survives in the new millennium. Given the demographics in many countries, it seems likely that there must be a short-term future of at least two decades, even with the progress in biologic fixation and tissue engineering discussed later in this issue. Whatever the long-term future, a new and improved bone cement must surely rest on basic research into structure – property relationships of acrylic or other polymers in the context of clinical performance.

Epilogue The epilogue to this history contains its own ironies. Only 20 years after the development, John Charnley died on Thursday, August 5, 1982, having

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revolutionized hip surgery, with his faith in lowfriction arthroplasty fully justified. About that time I developed osteoarthritis myself, and in the fullness of time I underwent arthroplasty on both hips. Thanks to Charnley’s vision and the skill of Dr. Allan Gross I have had nearly 14 years of a vigorous life and a recent successful revision promises to allow me a continuation of a normal life until old age overtakes me. So it turns out that, in the end, I did my research really for myself. So you never know what the future will bring. But that’s another story.

Summary The genesis and evolution of acrylic bone cement In 1957 John Charnley and I began to discuss the question of cement fixation of femoral prostheses. This was an independent development, because we had no knowledge of any other work in this area. Our conclusion was to grout in the prosthesis with a suitable cement. I selected a cold-curing acrylic denture repair material called Nu-Life as most appropriate. This material proved satisfactory in laboratory and clinical trials, and in 1958 Charnley undertook hip replacement using a cemented femoral prosthesis in six patients. This clinical success aroused great interest and some skepticism, because it was not always appreciated that Charnley and I had a substantial body of scientific data that justified this new approach to arthroplasty. Not surprisingly, the clinical aspects received most emphasis in the next decade. The scientific data were published mostly in the dental literature and were forgotten by the time cementation arthroplasty was generally accepted and so were rediscovered in subsequent years.

References [1] Kulzer Co. DRP Patent 737 058 1936. [2] Kulzer Co. DRP Patent 973 590 1943. [3] Tylman SD, Peyton FA. Acrylics and other synthetic resins used in dentistry. Philadelphia7 JB Lippincott; 1946. [4] Osborne J. Acrylic resins in dentistry. Oxford7 Blackwell; 1948. [5] Schwartz JR. The acrylic plastics in dentistry. Brooklyn7 Dental Items of Interest Publishing Co.; 1950. [6] Kiær S. Hip arthroplasty with acrylic prosthesis. Acta Orth 1953;XX11(2):126 – 45.

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[7] Haboush EJ. A new operation for arthroplasty of the hip based on biomechanics, photoelasticity, fast-setting dental acrylic and other considerations. Bull Hosp Joint Dis 1953;14:242 – 77. [8] Spence WT. Form-fitting plastic cranioplasty. J Neurosurg 1954;11:219 – 25. [9] Blaine G, Oliver LC. Acrylic cranioplasty. Br J Surg 1952;39:371 – 2. [10] Woringer E, Schweig B, Brogly G, Schneider J. Nouvelle technique ultra-rapide pour la refection de breches osseuses craniennes a la resine acrylique. Advantages de la resine acrylique sur la tantale. Rev Neurol 1951;85:527 – 35. [11] Robinson RG, Macalister AD. Acrylic cranioplasty. A simple one-stage method using a cold curing material. Br J Surg 1954;42:312 – 5. [12] Dutton J. Intercranial aneurysm. A new method of surgical treatment. Brit Med J 1956;8:585. [13] Harris P. Spinal fixation using onlay of Simplex-P. Paper no. 36. Washington7 Excerpta Med International Congress Series; 1961. p. E81 – 2. [14] Gluck T. Referat u¨ber die durch das moderne chirurgische experiment gewonnenen positiven resultate, betreffend die naht und den ersatz von defecten hoherer gewebe, sowie u¨ber die verwerthung resorbirbarer und lebendiger tampons in der chirurgie. Langenbecks archiv fur klinische chirurgie 1891;41: 187 – 239. [15] Smith DC. Studies in denture base materials with special reference to polymethyl methacrylate. PhD Thesis. Manchester, England7 University of Manchester; 1957. [16] Charnley J. Anchorage of the femoral head prosthesis to the shaft of the femur. J Bone Joint Surg 1960; 42B:28 – 30. [17] Charnley J. Arthroplasty of the hip by the low friction technique. J Bone Joint Surg 1961;43B:601.

[18] Hoff EAW. Dynamic mechanical properties of polymethacrylates. J Appl Chem 1952;2:441 – 50. [19] Smith DC, Bains MED. Residual methyl methacrylate in the denture base and its relation to denture sore mouth. Brit Dent J 1955;98:55 – 8. [20] Smith DC, Bains MED. The detection and estimation of residual monomer in polymethyl methacrylate. J Dent Res 1956;35:16 – 24. [21] Smith DC. The acrylic denture base. Residual monomer. Br Dent J 1958;105:86 – 91. [22] Smith DC. The acrylic denture base. The peroxide concentration in dental polymers. Br Dent J 1959;107: 62 – 6. [23] Wiltse LL, Hall RH, Stenehjem JC. Experimental studies regarding the possible use of self-curing acrylic in orthopaedic surgery. J Bone Joint Surg 1957;39B: 961 – 72. [24] Smith DC. The acrylic denture base. Mechanical evaluation of dental polymethylmethacrylate. Br Dent J 1961;111:9 – 17. [25] Smith DC. Recent developments and prospects in dental polymers. J Prosth Dent 1962;12:1066 – 78. [26] Smith DC. The acrylic denture base. Some effects of residual monomer and peroxide. Br Dent J 1959;106: 331 – 6. [27] Smith DC. The acrylic denture. Mechanical evaluation, midline fracture. Br Dent J 1961;110:257 – 67. [28] Smith DC. Fracture markings in polymethylmethacrylate. Nature 1958;182:132 – 3. [29] Smith DC. A new dental cement. Br Dent J 1968;125: 381 – 4. [30] Charnley J. Acrylic cement in orthopaedic surgery. Edinburgh7 Livingstone; 1970. [31] Smith DC. Medical and dental applications of cements. J Biomed Mater Res Symp 1971;1:189 – 205.