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Biomedical applications of carbon fibre reinforced carbon in implanted prostheses
Curhon. 1977. Vol. IS. pp 33-37.
Pergamon Press
Printed in Great Elr~tain
BIOMEDICAL APPLICATIONS OF CARBON FIBRE REINFORCED CARBON IN IMPLANTED PROSTHESES GWYN M. JENKINS University College of Swansea, Swansea, Wales and FRANCISCOX. DE CARVALHO Centro Tecnico Aerospacial, SLo JosC dos Campos, Brazil (Received 12 May 1976) Abstract-Carbon fibre reinforced carbon has been manufacturedin both Brazil and Swanseawith strengthsand stznesses comparablewith metals in normal surgicaluse. The biocompatibilityof the carbon with living tissue, especially when implanted percutaneously, has been demonstrated and specific applications in bone adjustment and heart-valve fabrication are described.
1.INTRODUCTION
The biomedical applications of electrographite coated with a carbon pyrolytically deposited from gaseous hydrocarbons (LTI “Pyrolite”) have been summarized by Bokros [l]. A vitreous form of polymeric carbon, produced by the carbonization of various polymeric systems such as phenolic resin, has also been used for medical prostheses[2]. Carbon fibre in association with “Teflon” is used to coat the stems of prostheses for joint replacement (“Proplast”, marketed by Smith, Kline and French in the U.K.). The Orthopaedics Department of the Medical School of the University of Wales (under the direction of Prof. B. McKibbent) has been provided with a wide variety of carbons over the last few years and the biocompatibility of many forms of pure carbon has been well established (Private communications: Prof. B. McKibben, Drs D. H. R. Jenkins and I. Forster of the Cardiff Royal Infirmary). Early results on the use of carbon filament in repairing tendons and ligaments will be reported elsewhere[3]. The main drawbacks for many purposes are the mediocre strength and brittleness which pertain to most commerical forms of carbon. Carbon fibre reinforced carbon (CFRC) shows much improvement in these properties and, being wholly made of carbon, exhibits the expected biocompatibility. Medical applications have been explored in Card8 and in Brazil in close association with various hospitals in S6o Paulo. Two such adhibitions, to be described herein, are the manufacture of CFRC pins for bone adjustment and in the manufacture of heart valves. 2. MATERIALS TJXHNOLOGY CFRC is made in Swansea and Brazil from British high strength and high modulus fibres, supplied by both tProf. B. McKibben,Departmentof Orthopaedic& Traumatic Surgery,CardiffRoyal Infirmary,NewportRoad, Cards, Wales. 33
Courtaulds (“Grafil”) and Morganite (“Modmor”), derived from polyacrylonitrile fibre by suitable heat treatment. The fibres are dipped in a solution of a phenolic resin in ethyl alcohol and the alcohol allowed to evaporate off. The choice of resin is very important. In the U.K., we have relied on the close cooperation of B.P. Chemicals, Barry, for basic materials and know how. The coated fibres are then pressed together to form a composite by a “leaky mould” technique at 15O”C,between the heated platens of a hydraulic press, the phenolic resin having been converted thereby to a thermoset. In most cases micronized natural graphite is added to the resin, ostensibly to modify the enormous shrinkage which takes place on carbonization, but there are doubts about this explanation. The composite is heated typically at a rate of temperature rise of 0.01 K set-’ over the ranges where large differential internal stresses are experienced in the composite specimens. Accurate programming is essential in heating these specimens to temperatures in excess of 1300K in an inert atmosphere to produce CFRC. Further strengthening can be promoted by subsequent impregnation with resin, followed by further heat-treatment. This technique has been pioneered by Fitzer et al. in Karlsruhe[4] and Hill et a/. in Aldermaston[5]. Typical specimens made in Swansea and Brazil are rods of circular cross section, 250mm long, 4.5 mm in dia., with the component high modulus fibres aligned parallel to the axis of the rod. Such rods were fired in batches of four and the load-deflection results from flexure under 4-point loading for a given batch containing 65% fibre are shown in Fig. 1. The band indicates the small variation between specimens in the same batch. Results for a stainless steel Steinman pin of identical dimensions and made to medical implant specifications are shown for comparison. It is clear that CFRC rods consistently match values recorded for the metal, which is normally used for bone adjustment by the medical profession. The stiffness of CFRC calculated from the above data is
34
G.
M. JENKINS and F. X.
DE
CARVALHO
x CFRPZ CFRC 2
0.50 Deflection
mm
I 50
1.00 for
span
of
Y
BOmm
Fig. 1. The force-deflection relationship for CFRC pins in bend over an 180mm span compared with that for stainless steel pins now in surgical use. The bar includes all the results taken with a set of four CFRC rods (av. dia. = 4.5 mm). 2OOk 10 GNm-’ as compared to - 210 GNm-’ for the
stainless steel. The maximum stress to fracture under such conditions is -370MNm-’ as compared with the yield stress of the steel under the same loading which is about 230MNm-*. Between batches, there is greater variability, especially in strength. When “high strength” carbon fibre is used in place of “high modulus” fibre, the bend stiffness is lowered to 160GNmd2for 70% carbon fibre content in the composite. However, the strength increases substantially to values in excess of 8OOMNm-*. Densities of 1.6 Mgn-’ are achieved after only one firing to 15OOK; subsequent impregnation and refiring are not necessary. The effect of heat-treatment and the addition of natural flake graphite to the resin is illustrated in Fig. 2. Carbonization of the plain resin transforms the composite into a stiffer material but with a much lower strain to fracture. The addition of graphite filler to the resin increases the stiffness of both resin and derived carbonbonded composites and, more importantly, allows the major part of the strength of the original resin composite to be retained after firing. It should be noted that all the composites exhibit linear Hookean elasticity up to the point of failure. Progressive splintering, as in wood, then occurs (this is not illustrated in Fig. 2, for the sake of clarity). This means that even after the main fracture occurs an appreciable proportion of the breaking load can still be sustained by the composite rod. No true yield, which in any way resembles the plastic deformation of stainless steel, is observed. Rings of CFRC may be made by winding carbon fibre coated with phenolic resin in an annular mould. Plates are made by using chopped fibre mat sandwiched between woven carbon cloth and pressed together in a suitable mould. However, we have concentrated on the production of uniaxial CFRC rods because of the essential simplicity of the technology of their fabrication. The other mechanical, physical and chemical properties of polymeric carbons in relation to their basic structure have been discussed in depth elsewhere[2]. tDr. D. H. R. Jenkins, Department of Orthopaedic &Traumatic Surgery, Cardii Royal Infirmary, Newport Road, Cards, Wales.
Strain
Fig. 2. The effect of carbonization and graphite filler on the stress-strain curves to fracture of flexed rods of phenolic resin reinforced with “hi& strenath” carbon fibre. CFRP, denotes composite with plain-resin; C-&P, denotes composite containing micronized graphite; CFRC, denotes CFRP, heat treated at 1500K; CFRG denotes CFRP2 heat treated at 15lXlK. 3. BIOCOMPATIBILITY OF CFRC on sheep and rabbits by Dr. D. H. R. Jenkinst at Cardiff have demonstrated some unique features of the biocompatibility of CFRC. When a rod of CFRC is inserted into soft living tissue and left there for a period of a few months with one end exposed, - 5 mm proud of the skin surface, it is found that the epithelium grows over the protruding end of the percutaneous implant and the tissue appears to adhere to the carbon surface. Very little change occurs in the surrounding tissue and no inflammation has been recorded. Porous carbons are penetrated readily by vascular and fibroblast material. It is presumed, therefore, that the strong bonding observed between the CFRC and living tissue may be attributed to this rapid in-growth; the nature of any intimate chemical adhesion at the true tissue: carbon interface has not been elucidated. The only disquieting phenomena encountered is the ease with which pieces of carbon are dislodged and removed by macrophages. Even here, however, the detritus is transported away from the implant without any trace of inflammatory response in the surrounding tissue [3]. Previous work reported by Benson[6], Mooney et al. [7] and Kadefors et al.[8] with other polymeric carbons as well as pyrolytic carbons set percutaneously showed that the attachment to soft tissue was good and increased with both pore dimensions and the size of anular grooves. Stainless steel and chrome-cobalt rods inserted in an identical manner to our CFRC rods very quickly induce the development of a thin fibrous lining as the living body attempts to sequester the metal implant and separate it from the surrounding tissue. Overgrowth of the epithelium is not normally observed in any metal. Rods of CFRC were skewered through soft tissue of an animal, leaving > 15mm protruding from the skin, and left Experiments
35
Biomedicalapplicationsof carbon fibre reinforcedcarbon for some months. On twisting these rods it is demonstrated (Fig. 3) that measurable adhesion of living tissue to the carbon has taken place. Steel pins inserted under the same conditions simply slide out in less than a month, indicating that the purely physical, wedging effect at the initial insertion soon relaxes as the surrounding living tissue retracts in response to the diffusion of microquantities of cytotoxic metal ions.
important because low stiffness would cause unnecessary tearing of soft tissue. X-rays revealed the presence of carbon only by its transparency compared with surrounding bone tissue. During the actual operation, four holes were drilled with a steel tool of slightly smaller diameter and the carbon rods pushed through afterwards. It was difficult to fabricate carbon rods hard enough to cut through bone using a standard drilling technique. The results reported from Brazil by Dr. Atlas indicate that the use of CFRC pins for such an operation is successful; no rejection or inflammation associated with infection was observed. 5. HEART
Fig. 3. A photograph of carbon fibre reinforced carbon rod skewered through living tissue, demonstrating the bonding of the tissue to the carbon which is sufficient to withstand shearing induced by a simple twist of the implanted rod. (Courtesy of D. H. R. Jenkins.)
4. CFRC
PINS FOR BONE STRETCHING
AND ADJUSTMENT
CFRC pins for bone extension were requested by Dr. S. Atlast of S&o Paulo. After certain diseases in children, notably poliomyelitis, the bone in one leg may not grow as much as in the other. At a certain age, between 12 and 14 yr, it is possible to correct this by breaking the bone in the shorter leg and pulling the two sections apart, 1 mm a day, until the two legs match. New bone is laid down progressively in the gap and densifies as load is gradually applied. To effect this separation, four steel pins are drilled through flesh and bone, one pair above and one pair below the fracture. These are set in a cradle which allows micro-adjustment of the separation between the two pairs of pins by means of a screw. The technique has been successful, but, unfortunately, living tissue retracts from the steel, especially at the skin/air interface, thus allowing infections ingress. Over the long period required-several months in many cases-before the pins can be withdrawn, infection and tissue reaction become serious problems. This can be remedied by the use of carbon. Sets of four CFRC pins were made, as described in Section 2 above, using “high modulus” fibre and supplied to Dr. Atlas. The stiffness and strength of these pins were tested beforehand and demonstrated to match the stiffness and yield strength of the stainless steel pins used previously. The loads required were surprisingly small as the pull is only against relaxed muscle and skin tissue so that the strength is more than adequate. The stiffness is tDr. Samoel Atlas, Department0 de Ortopedia e Traumatologia, Escola Paulista de Medecina, S5o Paulo, Brazil. ZProf. E. J. Zerbini and Dr. Seigo Tsuzuku, Instituto de Cora@o, Avenida Dr. Eneas de Carvalho Aguier, 44 SLo Paulo, Brazil.
VALVES
In the human heart, the valves consist of tri-cuspid flaps of flexible living tissue. For various reasons, and especially after a rheumatic fever, this tissue hardens and so the valve becomes inefficient. The flaps must then be excised and replaced either with membrane cut from other parts of the same body or with an artificial heart valve. Numerous artificial heart valves have been designed and fabricated, but there are two main types: the ball-and-cage variety and the ring-and-disc variety made out of many materials including chrome-cobalt, pyrolytic carbon, polypropylene, silicon rubber and polyformaldehyde. A discussion of the relative merits of these possible materials for artificial heart-valve construction has been published already by Roschke[9]. Increasingly, however, the use of carbon has become popular because of its excellent biocompatibility with surrounding tissue and, more importantly, for this particular use, because of its good reputation in lowering the incidence of bloodclotting (thromo-embolism). Accordingly, we were requested by the heart surgery unit in S5o Paulo, under the direction of Dr. E. J. Zerbini,S to design and fabricate heart valves made entirely out of carbon. Liaison was maintained with Dr. Seigo Tsuzuku.S We were asked to adopt the disc-and-ring variety of valve because there would then be much less resistance to blood flow than for the ball-and-cage variety. The disc had to be highly polished and capable of tilting to angles approaching 90” to ensure the smooth outflow of the largest volume of blood. It was suggested the disc should be easily rotated about the axis though its centre perpendicular to its surface plane to avoid tissue overgrowth and localised wear. We were asked not to use hinges in which blood could be subjected to shearing forces-these would destroy the blood corpuscles and cause other damage, leading eventually to blood-clotting. Figure 4 shows the design of the first valve we constructed for the human heart. This was rejected because of the hinge objection mentioned above. Also, the fabrication process was rather complicated; the complex disc design, especially, caused difficulties in fabrication out of polymeric carbon. The valve eventually accepted is drawn in Fig. 5 with dimensions appropriate to a dog. It resembles the tilting disc valve prosthesis made by Bj@rk[lO] which was designed specifically for fabrication in metal and plastic; the disc was made out of polyformaldehyde and the rings and stops out of chrome-cobalt alloy. Since 1971, a
G.
M. JENKIM and F. X. DE CARVALHO followed by carbonization at 1000°C. The pins are cemented to the ring with a phenolic resin-micronized graphite mixture, subsequently fired at 1000°C. In the fury-assembled device, angles of opening in excess of 70” are possible. A photograph of the fully open, finished valve is shown in Fig. 6. This is now being tested by the Heart Surgery Unit in Ssto Paulo. It is fabricated in the Materials ~boratory of the Centro Tech&o Aeroespacial in Sb Jose dos Campos.
Detail “C” 0
0.5
IO
15
I I cm
2
,
Fig. 4. The rejected design of a carbon heart valve for a human heart.
Fig. 6. A photographof the final heart valve, illustrating the width of the aperture when fully open. 6. Dl~~lO~ ~~ ~LICA~ONS OF CFRC IN EWLANTEJI PROSTKFSRS
I
Detail “C”
Fig. 5. The carbon heart valve accepted for implantation in a dog.
“Pyrolite” carbon disc occluder for the Bjork valve has been offered as an alternative to pol~orm~dehyde [l]. Our valve also bears some resemblance to the LilleheiKaster pivoting disc valve [ 1l] which has been offered on the market with the alternative of a silicon-alloyed LTI “P yrolite” carbon disc in a titanium cage since 1971. These valves had to be re-designed to take into account the peculiarities of polymeric carbons. The finally accepted device consists of a ring of carbon, channelled at the outside to receive a standard dacro~s~icone rubber ring to which the surrounding tissue is stitched. The disc is quite plain and featureless with no sharp edges-a shape which may be fabricated easily out of glassy carbon. The disc is free to rotate but is restricted in its opening by round pins of CFRC set into the outer ring. The ring is made of a fine-grained electro-graphite strengthened and hardened by repeated impregnations with phenolic resin
It has been demonstrated that CFRC has a role to play in the design of medical implants. An important future appli~tion for CFRC rods would be in the fixation of artificial limb extensions to the stumps of amputated limbs as was first envisaged by Mooney et al.[7]. After amputation, soft tissue and associated nerves are allowed to grow over and cover the exposed bone. In present day usage the artificial extension to the limb is moulded to fit the new contours and so all external stresses are transmitted to the general body frame via this newly grown and sensitive soft tissue, thus causing considerable pain in many cases. A percutaneous shaft of CFRC could, conceivably, allow the artificial limb extension to be locked directly into the remaining bone, thus producing a strong permanent fixation which would avoid the transmission of stresses via nervous soft tissue. The degree of skeletal attachment of other polymeric carbons is claimed to be far better than that for ceramics (alumina, apatite) in common use. Staritski and MooneyllZ], for instance, have shown that direct osseous attachment to a vitreous carbon bone bridge is possible provided grooves of appropriate dimensions are present in the implanted prosthesis. They do state, however, that the glassy form of polymeric carbon lacks su~cient strength to bear functional loading. This limitation can easily be overcome with CFRC. The use of CFRC plates for bone-mending in particularly gaping wounds affords an appreciable advance in surgical technology. However, it is ditlicult to conceive of a technique for attaching the plates to the living bone without recourse to the metal screws in current use. A
Biomedical applications of carbon fibre reinforced carbon
galvanic cell could then be set up and cytotoxic metal ions forced into solution in the all-pervading aqueous body fluids. By constructing our own galvanic cell with saline solution as the conducting medium and measuring the net electro-motive force, we have found that this effect is most marked in the cases of stainless steel and chrome-cobalt alloy. The effect with titanium is negligible in that the e.m.f.s observed between carbon and titanium electrodes were less than 0.1 mV and so titanium screws are recommended for the rapid fixation of CFRC plates to bone. A possible important application is for total joint replacement~speci~ly hip art~oplasty. It is estimated (I.G. Turner, Private communication), that 30,ooOsuch operations were carried out in the U.K. alone in 1973.The annual increase in the incidence of this operation has been nearly 30% in recent years. A basic problem in the use of CFRC for this adhibition is the difficulty of achieving a hard, highly polished surface and also its rather poor resistance to certain kinds of wear, although Dunlop, for instance, have successfully incorporated it in brakelinings for high speed aircraft [2]. The wear resistance of other forms of intrinsically hard carbon, e.g. glassy polymeric carbon and silicon-alloyed “PyroIite”[l], is much greater, and the friction in a ball-and-socket joint, for instance, much less. It is conceivable, therefore, that a new replacement for the head of the human femur would consist of a ball of the harder carbon bonded to a stem of CFRC which would be pushed into the marrow cavity of the shaft of the femur. The biocompatibility of the CFRC would ensure good and permanent fixation. It is clear, however, that more experience in CFRC technology and surgical practice with carbon is required before more sophisticated devices can be promoted. Current work at Swansea and Cardiff is directed towards this end under contract to the U.K. Medical Research Council. Preliminary results are to be reported elsewhere [3]. Acknowledgements-The major part of this work was carried out in 1973when C.M.J. was Visiting Professor at the Aeronautical
37
Institute attached to the Brazihan Aerospace Centre on leave of absence from the University of Wales, sponsored jointly by the United Kingdom Ministry of Overseas Development and the Centre. G. M. Jenkins is indebted to Dr. D. H. R. Jenkins of the Medical School of the University of Wales, Cardiff, for permission to use more recent examples from his work and to the Medical Research Council for their present support in financing the employment of a technical assistant, C. Grigson, to manufacture CFRC implants at Swansea.
REFERENCES 3. C. Bokros, L. D. La Grange and F. J. Schoen, In Chern~sf~ and Physics oj Carbon (Edited by P. L. Walker, Jr,), Vol. 9. Marcel Dekker, New York (1972). G. M. Jenkins and K. Kawamura, Polymeric CarbonsCarbon Fibre, Glass and Char. Cambridge University Press, Cambridge, England (1976). D. H. R. Jenkins, I. Forster, G. M. Jenkins, J. Hill and S. Carlton, Fil~entous carbon as a tendon and ligament prosthesis, In Proceedings of the Conference on “~a~e~a~s for use in Medicine and Biology”. Biological Engineering Society, Brunei University (1976). 4. (a) E. Fitzer and A. Burger, Proc. Intern. Conj. on Carbon Fibres, their Comp. andAppl., London, Paper 36 (1971);(b) E. Fitzer and B. Terwiesch, Carbon 10,383 (1972);(c) E. Fitzer and B. Terwiesch, Carbon 72, Baden-Baden, Paper 34 and We~sfo~fechn~ck 5,53 (1974);(d) E. Fitzer, M. Heym and K. Karlisch, Prepr. int. Coni. on Carbon and Graph~fe, S.C.I., London, Paper 106 (Session III) (1974). 5. J. Hill, E. J. Walker and C. R. Thomas, Paper FC-19, U.S. Carbon Conference, Gatlinburg (1973). 6. J. Benson, Presuruey on biomedical applications of carbons. North-American-Rockwell, Rocketdyne Report R-7855 (1969). 7. V. Mooney, P. Predecki, J. Renning and J. Gray, J. Biomed. Muter. Ref. Symp. No. 2, Part 1, 143 (1971). 8. R. Kadefors, .I. B. Reswick and R. L. Martin, Med. & Biol. Engng 8, 129 (1970). 9. E. J. Roschke, Biomafer. Med. Devices Art. Organs 1, 249 (1973). 10. V. 0. Bjork, Scan. J. Thor. Cardiovasc. Sug. 3, 1 (1969). il. R. L. Kaster, C. W. Lillehei and P. J. K. Starek, Trans. Am. Sac. Artif Intern. Organs 16, 233 (1970). 12. C. Staritski and V. Mooney, Mater. Des. Considerations Attachment Prostheses Musculo-Skeletal Syst. Symp., Clemson Unio. S.C., U.S.A. (1972).
Clemson
Pergamon Press
Printed in Great Elr~tain
BIOMEDICAL APPLICATIONS OF CARBON FIBRE REINFORCED CARBON IN IMPLANTED PROSTHESES GWYN M. JENKINS University College of Swansea, Swansea, Wales and FRANCISCOX. DE CARVALHO Centro Tecnico Aerospacial, SLo JosC dos Campos, Brazil (Received 12 May 1976) Abstract-Carbon fibre reinforced carbon has been manufacturedin both Brazil and Swanseawith strengthsand stznesses comparablewith metals in normal surgicaluse. The biocompatibilityof the carbon with living tissue, especially when implanted percutaneously, has been demonstrated and specific applications in bone adjustment and heart-valve fabrication are described.
1.INTRODUCTION
The biomedical applications of electrographite coated with a carbon pyrolytically deposited from gaseous hydrocarbons (LTI “Pyrolite”) have been summarized by Bokros [l]. A vitreous form of polymeric carbon, produced by the carbonization of various polymeric systems such as phenolic resin, has also been used for medical prostheses[2]. Carbon fibre in association with “Teflon” is used to coat the stems of prostheses for joint replacement (“Proplast”, marketed by Smith, Kline and French in the U.K.). The Orthopaedics Department of the Medical School of the University of Wales (under the direction of Prof. B. McKibbent) has been provided with a wide variety of carbons over the last few years and the biocompatibility of many forms of pure carbon has been well established (Private communications: Prof. B. McKibben, Drs D. H. R. Jenkins and I. Forster of the Cardiff Royal Infirmary). Early results on the use of carbon filament in repairing tendons and ligaments will be reported elsewhere[3]. The main drawbacks for many purposes are the mediocre strength and brittleness which pertain to most commerical forms of carbon. Carbon fibre reinforced carbon (CFRC) shows much improvement in these properties and, being wholly made of carbon, exhibits the expected biocompatibility. Medical applications have been explored in Card8 and in Brazil in close association with various hospitals in S6o Paulo. Two such adhibitions, to be described herein, are the manufacture of CFRC pins for bone adjustment and in the manufacture of heart valves. 2. MATERIALS TJXHNOLOGY CFRC is made in Swansea and Brazil from British high strength and high modulus fibres, supplied by both tProf. B. McKibben,Departmentof Orthopaedic& Traumatic Surgery,CardiffRoyal Infirmary,NewportRoad, Cards, Wales. 33
Courtaulds (“Grafil”) and Morganite (“Modmor”), derived from polyacrylonitrile fibre by suitable heat treatment. The fibres are dipped in a solution of a phenolic resin in ethyl alcohol and the alcohol allowed to evaporate off. The choice of resin is very important. In the U.K., we have relied on the close cooperation of B.P. Chemicals, Barry, for basic materials and know how. The coated fibres are then pressed together to form a composite by a “leaky mould” technique at 15O”C,between the heated platens of a hydraulic press, the phenolic resin having been converted thereby to a thermoset. In most cases micronized natural graphite is added to the resin, ostensibly to modify the enormous shrinkage which takes place on carbonization, but there are doubts about this explanation. The composite is heated typically at a rate of temperature rise of 0.01 K set-’ over the ranges where large differential internal stresses are experienced in the composite specimens. Accurate programming is essential in heating these specimens to temperatures in excess of 1300K in an inert atmosphere to produce CFRC. Further strengthening can be promoted by subsequent impregnation with resin, followed by further heat-treatment. This technique has been pioneered by Fitzer et al. in Karlsruhe[4] and Hill et a/. in Aldermaston[5]. Typical specimens made in Swansea and Brazil are rods of circular cross section, 250mm long, 4.5 mm in dia., with the component high modulus fibres aligned parallel to the axis of the rod. Such rods were fired in batches of four and the load-deflection results from flexure under 4-point loading for a given batch containing 65% fibre are shown in Fig. 1. The band indicates the small variation between specimens in the same batch. Results for a stainless steel Steinman pin of identical dimensions and made to medical implant specifications are shown for comparison. It is clear that CFRC rods consistently match values recorded for the metal, which is normally used for bone adjustment by the medical profession. The stiffness of CFRC calculated from the above data is
34
G.
M. JENKINS and F. X.
DE
CARVALHO
x CFRPZ CFRC 2
0.50 Deflection
mm
I 50
1.00 for
span
of
Y
BOmm
Fig. 1. The force-deflection relationship for CFRC pins in bend over an 180mm span compared with that for stainless steel pins now in surgical use. The bar includes all the results taken with a set of four CFRC rods (av. dia. = 4.5 mm). 2OOk 10 GNm-’ as compared to - 210 GNm-’ for the
stainless steel. The maximum stress to fracture under such conditions is -370MNm-’ as compared with the yield stress of the steel under the same loading which is about 230MNm-*. Between batches, there is greater variability, especially in strength. When “high strength” carbon fibre is used in place of “high modulus” fibre, the bend stiffness is lowered to 160GNmd2for 70% carbon fibre content in the composite. However, the strength increases substantially to values in excess of 8OOMNm-*. Densities of 1.6 Mgn-’ are achieved after only one firing to 15OOK; subsequent impregnation and refiring are not necessary. The effect of heat-treatment and the addition of natural flake graphite to the resin is illustrated in Fig. 2. Carbonization of the plain resin transforms the composite into a stiffer material but with a much lower strain to fracture. The addition of graphite filler to the resin increases the stiffness of both resin and derived carbonbonded composites and, more importantly, allows the major part of the strength of the original resin composite to be retained after firing. It should be noted that all the composites exhibit linear Hookean elasticity up to the point of failure. Progressive splintering, as in wood, then occurs (this is not illustrated in Fig. 2, for the sake of clarity). This means that even after the main fracture occurs an appreciable proportion of the breaking load can still be sustained by the composite rod. No true yield, which in any way resembles the plastic deformation of stainless steel, is observed. Rings of CFRC may be made by winding carbon fibre coated with phenolic resin in an annular mould. Plates are made by using chopped fibre mat sandwiched between woven carbon cloth and pressed together in a suitable mould. However, we have concentrated on the production of uniaxial CFRC rods because of the essential simplicity of the technology of their fabrication. The other mechanical, physical and chemical properties of polymeric carbons in relation to their basic structure have been discussed in depth elsewhere[2]. tDr. D. H. R. Jenkins, Department of Orthopaedic &Traumatic Surgery, Cardii Royal Infirmary, Newport Road, Cards, Wales.
Strain
Fig. 2. The effect of carbonization and graphite filler on the stress-strain curves to fracture of flexed rods of phenolic resin reinforced with “hi& strenath” carbon fibre. CFRP, denotes composite with plain-resin; C-&P, denotes composite containing micronized graphite; CFRC, denotes CFRP, heat treated at 1500K; CFRG denotes CFRP2 heat treated at 15lXlK. 3. BIOCOMPATIBILITY OF CFRC on sheep and rabbits by Dr. D. H. R. Jenkinst at Cardiff have demonstrated some unique features of the biocompatibility of CFRC. When a rod of CFRC is inserted into soft living tissue and left there for a period of a few months with one end exposed, - 5 mm proud of the skin surface, it is found that the epithelium grows over the protruding end of the percutaneous implant and the tissue appears to adhere to the carbon surface. Very little change occurs in the surrounding tissue and no inflammation has been recorded. Porous carbons are penetrated readily by vascular and fibroblast material. It is presumed, therefore, that the strong bonding observed between the CFRC and living tissue may be attributed to this rapid in-growth; the nature of any intimate chemical adhesion at the true tissue: carbon interface has not been elucidated. The only disquieting phenomena encountered is the ease with which pieces of carbon are dislodged and removed by macrophages. Even here, however, the detritus is transported away from the implant without any trace of inflammatory response in the surrounding tissue [3]. Previous work reported by Benson[6], Mooney et al. [7] and Kadefors et al.[8] with other polymeric carbons as well as pyrolytic carbons set percutaneously showed that the attachment to soft tissue was good and increased with both pore dimensions and the size of anular grooves. Stainless steel and chrome-cobalt rods inserted in an identical manner to our CFRC rods very quickly induce the development of a thin fibrous lining as the living body attempts to sequester the metal implant and separate it from the surrounding tissue. Overgrowth of the epithelium is not normally observed in any metal. Rods of CFRC were skewered through soft tissue of an animal, leaving > 15mm protruding from the skin, and left Experiments
35
Biomedicalapplicationsof carbon fibre reinforcedcarbon for some months. On twisting these rods it is demonstrated (Fig. 3) that measurable adhesion of living tissue to the carbon has taken place. Steel pins inserted under the same conditions simply slide out in less than a month, indicating that the purely physical, wedging effect at the initial insertion soon relaxes as the surrounding living tissue retracts in response to the diffusion of microquantities of cytotoxic metal ions.
important because low stiffness would cause unnecessary tearing of soft tissue. X-rays revealed the presence of carbon only by its transparency compared with surrounding bone tissue. During the actual operation, four holes were drilled with a steel tool of slightly smaller diameter and the carbon rods pushed through afterwards. It was difficult to fabricate carbon rods hard enough to cut through bone using a standard drilling technique. The results reported from Brazil by Dr. Atlas indicate that the use of CFRC pins for such an operation is successful; no rejection or inflammation associated with infection was observed. 5. HEART
Fig. 3. A photograph of carbon fibre reinforced carbon rod skewered through living tissue, demonstrating the bonding of the tissue to the carbon which is sufficient to withstand shearing induced by a simple twist of the implanted rod. (Courtesy of D. H. R. Jenkins.)
4. CFRC
PINS FOR BONE STRETCHING
AND ADJUSTMENT
CFRC pins for bone extension were requested by Dr. S. Atlast of S&o Paulo. After certain diseases in children, notably poliomyelitis, the bone in one leg may not grow as much as in the other. At a certain age, between 12 and 14 yr, it is possible to correct this by breaking the bone in the shorter leg and pulling the two sections apart, 1 mm a day, until the two legs match. New bone is laid down progressively in the gap and densifies as load is gradually applied. To effect this separation, four steel pins are drilled through flesh and bone, one pair above and one pair below the fracture. These are set in a cradle which allows micro-adjustment of the separation between the two pairs of pins by means of a screw. The technique has been successful, but, unfortunately, living tissue retracts from the steel, especially at the skin/air interface, thus allowing infections ingress. Over the long period required-several months in many cases-before the pins can be withdrawn, infection and tissue reaction become serious problems. This can be remedied by the use of carbon. Sets of four CFRC pins were made, as described in Section 2 above, using “high modulus” fibre and supplied to Dr. Atlas. The stiffness and strength of these pins were tested beforehand and demonstrated to match the stiffness and yield strength of the stainless steel pins used previously. The loads required were surprisingly small as the pull is only against relaxed muscle and skin tissue so that the strength is more than adequate. The stiffness is tDr. Samoel Atlas, Department0 de Ortopedia e Traumatologia, Escola Paulista de Medecina, S5o Paulo, Brazil. ZProf. E. J. Zerbini and Dr. Seigo Tsuzuku, Instituto de Cora@o, Avenida Dr. Eneas de Carvalho Aguier, 44 SLo Paulo, Brazil.
VALVES
In the human heart, the valves consist of tri-cuspid flaps of flexible living tissue. For various reasons, and especially after a rheumatic fever, this tissue hardens and so the valve becomes inefficient. The flaps must then be excised and replaced either with membrane cut from other parts of the same body or with an artificial heart valve. Numerous artificial heart valves have been designed and fabricated, but there are two main types: the ball-and-cage variety and the ring-and-disc variety made out of many materials including chrome-cobalt, pyrolytic carbon, polypropylene, silicon rubber and polyformaldehyde. A discussion of the relative merits of these possible materials for artificial heart-valve construction has been published already by Roschke[9]. Increasingly, however, the use of carbon has become popular because of its excellent biocompatibility with surrounding tissue and, more importantly, for this particular use, because of its good reputation in lowering the incidence of bloodclotting (thromo-embolism). Accordingly, we were requested by the heart surgery unit in S5o Paulo, under the direction of Dr. E. J. Zerbini,S to design and fabricate heart valves made entirely out of carbon. Liaison was maintained with Dr. Seigo Tsuzuku.S We were asked to adopt the disc-and-ring variety of valve because there would then be much less resistance to blood flow than for the ball-and-cage variety. The disc had to be highly polished and capable of tilting to angles approaching 90” to ensure the smooth outflow of the largest volume of blood. It was suggested the disc should be easily rotated about the axis though its centre perpendicular to its surface plane to avoid tissue overgrowth and localised wear. We were asked not to use hinges in which blood could be subjected to shearing forces-these would destroy the blood corpuscles and cause other damage, leading eventually to blood-clotting. Figure 4 shows the design of the first valve we constructed for the human heart. This was rejected because of the hinge objection mentioned above. Also, the fabrication process was rather complicated; the complex disc design, especially, caused difficulties in fabrication out of polymeric carbon. The valve eventually accepted is drawn in Fig. 5 with dimensions appropriate to a dog. It resembles the tilting disc valve prosthesis made by Bj@rk[lO] which was designed specifically for fabrication in metal and plastic; the disc was made out of polyformaldehyde and the rings and stops out of chrome-cobalt alloy. Since 1971, a
G.
M. JENKIM and F. X. DE CARVALHO followed by carbonization at 1000°C. The pins are cemented to the ring with a phenolic resin-micronized graphite mixture, subsequently fired at 1000°C. In the fury-assembled device, angles of opening in excess of 70” are possible. A photograph of the fully open, finished valve is shown in Fig. 6. This is now being tested by the Heart Surgery Unit in Ssto Paulo. It is fabricated in the Materials ~boratory of the Centro Tech&o Aeroespacial in Sb Jose dos Campos.
Detail “C” 0
0.5
IO
15
I I cm
2
,
Fig. 4. The rejected design of a carbon heart valve for a human heart.
Fig. 6. A photographof the final heart valve, illustrating the width of the aperture when fully open. 6. Dl~~lO~ ~~ ~LICA~ONS OF CFRC IN EWLANTEJI PROSTKFSRS
I
Detail “C”
Fig. 5. The carbon heart valve accepted for implantation in a dog.
“Pyrolite” carbon disc occluder for the Bjork valve has been offered as an alternative to pol~orm~dehyde [l]. Our valve also bears some resemblance to the LilleheiKaster pivoting disc valve [ 1l] which has been offered on the market with the alternative of a silicon-alloyed LTI “P yrolite” carbon disc in a titanium cage since 1971. These valves had to be re-designed to take into account the peculiarities of polymeric carbons. The finally accepted device consists of a ring of carbon, channelled at the outside to receive a standard dacro~s~icone rubber ring to which the surrounding tissue is stitched. The disc is quite plain and featureless with no sharp edges-a shape which may be fabricated easily out of glassy carbon. The disc is free to rotate but is restricted in its opening by round pins of CFRC set into the outer ring. The ring is made of a fine-grained electro-graphite strengthened and hardened by repeated impregnations with phenolic resin
It has been demonstrated that CFRC has a role to play in the design of medical implants. An important future appli~tion for CFRC rods would be in the fixation of artificial limb extensions to the stumps of amputated limbs as was first envisaged by Mooney et al.[7]. After amputation, soft tissue and associated nerves are allowed to grow over and cover the exposed bone. In present day usage the artificial extension to the limb is moulded to fit the new contours and so all external stresses are transmitted to the general body frame via this newly grown and sensitive soft tissue, thus causing considerable pain in many cases. A percutaneous shaft of CFRC could, conceivably, allow the artificial limb extension to be locked directly into the remaining bone, thus producing a strong permanent fixation which would avoid the transmission of stresses via nervous soft tissue. The degree of skeletal attachment of other polymeric carbons is claimed to be far better than that for ceramics (alumina, apatite) in common use. Staritski and MooneyllZ], for instance, have shown that direct osseous attachment to a vitreous carbon bone bridge is possible provided grooves of appropriate dimensions are present in the implanted prosthesis. They do state, however, that the glassy form of polymeric carbon lacks su~cient strength to bear functional loading. This limitation can easily be overcome with CFRC. The use of CFRC plates for bone-mending in particularly gaping wounds affords an appreciable advance in surgical technology. However, it is ditlicult to conceive of a technique for attaching the plates to the living bone without recourse to the metal screws in current use. A
Biomedical applications of carbon fibre reinforced carbon
galvanic cell could then be set up and cytotoxic metal ions forced into solution in the all-pervading aqueous body fluids. By constructing our own galvanic cell with saline solution as the conducting medium and measuring the net electro-motive force, we have found that this effect is most marked in the cases of stainless steel and chrome-cobalt alloy. The effect with titanium is negligible in that the e.m.f.s observed between carbon and titanium electrodes were less than 0.1 mV and so titanium screws are recommended for the rapid fixation of CFRC plates to bone. A possible important application is for total joint replacement~speci~ly hip art~oplasty. It is estimated (I.G. Turner, Private communication), that 30,ooOsuch operations were carried out in the U.K. alone in 1973.The annual increase in the incidence of this operation has been nearly 30% in recent years. A basic problem in the use of CFRC for this adhibition is the difficulty of achieving a hard, highly polished surface and also its rather poor resistance to certain kinds of wear, although Dunlop, for instance, have successfully incorporated it in brakelinings for high speed aircraft [2]. The wear resistance of other forms of intrinsically hard carbon, e.g. glassy polymeric carbon and silicon-alloyed “PyroIite”[l], is much greater, and the friction in a ball-and-socket joint, for instance, much less. It is conceivable, therefore, that a new replacement for the head of the human femur would consist of a ball of the harder carbon bonded to a stem of CFRC which would be pushed into the marrow cavity of the shaft of the femur. The biocompatibility of the CFRC would ensure good and permanent fixation. It is clear, however, that more experience in CFRC technology and surgical practice with carbon is required before more sophisticated devices can be promoted. Current work at Swansea and Cardiff is directed towards this end under contract to the U.K. Medical Research Council. Preliminary results are to be reported elsewhere [3]. Acknowledgements-The major part of this work was carried out in 1973when C.M.J. was Visiting Professor at the Aeronautical
37
Institute attached to the Brazihan Aerospace Centre on leave of absence from the University of Wales, sponsored jointly by the United Kingdom Ministry of Overseas Development and the Centre. G. M. Jenkins is indebted to Dr. D. H. R. Jenkins of the Medical School of the University of Wales, Cardiff, for permission to use more recent examples from his work and to the Medical Research Council for their present support in financing the employment of a technical assistant, C. Grigson, to manufacture CFRC implants at Swansea.
REFERENCES 3. C. Bokros, L. D. La Grange and F. J. Schoen, In Chern~sf~ and Physics oj Carbon (Edited by P. L. Walker, Jr,), Vol. 9. Marcel Dekker, New York (1972). G. M. Jenkins and K. Kawamura, Polymeric CarbonsCarbon Fibre, Glass and Char. Cambridge University Press, Cambridge, England (1976). D. H. R. Jenkins, I. Forster, G. M. Jenkins, J. Hill and S. Carlton, Fil~entous carbon as a tendon and ligament prosthesis, In Proceedings of the Conference on “~a~e~a~s for use in Medicine and Biology”. Biological Engineering Society, Brunei University (1976). 4. (a) E. Fitzer and A. Burger, Proc. Intern. Conj. on Carbon Fibres, their Comp. andAppl., London, Paper 36 (1971);(b) E. Fitzer and B. Terwiesch, Carbon 10,383 (1972);(c) E. Fitzer and B. Terwiesch, Carbon 72, Baden-Baden, Paper 34 and We~sfo~fechn~ck 5,53 (1974);(d) E. Fitzer, M. Heym and K. Karlisch, Prepr. int. Coni. on Carbon and Graph~fe, S.C.I., London, Paper 106 (Session III) (1974). 5. J. Hill, E. J. Walker and C. R. Thomas, Paper FC-19, U.S. Carbon Conference, Gatlinburg (1973). 6. J. Benson, Presuruey on biomedical applications of carbons. North-American-Rockwell, Rocketdyne Report R-7855 (1969). 7. V. Mooney, P. Predecki, J. Renning and J. Gray, J. Biomed. Muter. Ref. Symp. No. 2, Part 1, 143 (1971). 8. R. Kadefors, .I. B. Reswick and R. L. Martin, Med. & Biol. Engng 8, 129 (1970). 9. E. J. Roschke, Biomafer. Med. Devices Art. Organs 1, 249 (1973). 10. V. 0. Bjork, Scan. J. Thor. Cardiovasc. Sug. 3, 1 (1969). il. R. L. Kaster, C. W. Lillehei and P. J. K. Starek, Trans. Am. Sac. Artif Intern. Organs 16, 233 (1970). 12. C. Staritski and V. Mooney, Mater. Des. Considerations Attachment Prostheses Musculo-Skeletal Syst. Symp., Clemson Unio. S.C., U.S.A. (1972).
Clemson