Corrosion behaviour of surgical implant materials: I. effects of sterilization

Corrosion Science, 1969, Vol 9, pp 755 to 762. Pergamon Press. Printed in Great Britain

CORROSION BEHAVIOUR OF SURGICAL IMPLANT MATERIALS: I. EFFECTS OF STERILIZATION* R. WINSTON REVIEt a n d NORBERT D. GREENE Corrosion Research Laboratory, Materials Research Center, Rensselaer Polytechnic Institute, Troy, New York, 12181 A~traet--This paper describes the effects of different sterilization procedures on the subsequent corrosion rates of four metals now used in the human body--type 316 and type 304 stainless steels, Ti and Vitallium (a Co-Cr-W-Ni alloy). Corrosion rates in isotonic NaC1 (0"9y. aqueous solution) were determined using the passive current method. Steam and dry heat sterilization produce a marked decrease in the corrosion rates of stainless steels, with a lesser effect on titanium, and negligible effect on Vitallium. Increasing time or temperature of sterilization results in a decrease in the subsequent corrosion rate. Rrsumr----Cetarticle drcrit les effets de diffgrentes techniques de st6rilisation stir la vitesse de corrosion ultrrieure de quatre mrtaux ou alliages utilisrs actuellement darts le corps humain: aciers inoxydables types 304 et 316, titane et Vitallium (un alliage Co-Cr-W-Mo). Les vitesses de corrosion darts une solution isotonique de NaCI (0,9 ~) ont 6t6 drtermin~s par le courant de passivation. Des st&ilisations la vapeur et/t la ehaleur srche provoquent une nette diminution des vitesses de corrosion des aeiers aciers inoxydables, un effet plus faible sur le titane, et un effet nrgligeable sur le Vitallium. L'accroissement de la dur~ ou de la temprrature de st&ilisation diminue la vitesse de corrosion ult6rieure. Zusammeafassung--Der Einflul3 unterschiedlicher Sterilisationsbehandlungen auf die Nachfolgende Korrosion yon vier Metallen, die im menschlichen K6rper eingesetzt werden, wird beschrieben (rostfreier Stahl Typ 316 und 304, Ti und die Co-Cr-W-Mo-Legierung Vitallium). Die Korrosionsgeschwindigkeiten in isotonlscher NaCI-Legierung (0,9 Gew. ~o) wurden durch Messung der passiven Aufl6sungsstromdiehte bestirmnt. Sterilisation mit Darnpf und Warmluft bewirkt einen deutliehen Abfall der Korrosionsgeschwindigkeiten der rostfreine Stihle und hat geringeren EinfluB auf Titan, vernaehlassigbaren EinfluB auf Vitallium. Zunehmende Sterilisationstemperatur und -zeit bewirken einen weiteren Abfall der nachfolgenden Korrosionsgeschwindigkeit. INTRODUCTION METALL1C implants are widely used in medical treatments---examples include suture wires and plates, nails a n d screws, to replace or temporarily reinforce damaged bones. I n addition to the rigorous strength requirements for sutures a n d orthopaedic appliances, all metallic materials placed in the body must be compatible with living tissue. C o m p a t i b i l i t y depends to a large extent o n the corrosion rate of the metal in b o d y fluids a n d the toxicity of the corrosion products. 1,~ Body fluids consist o f a n aerated solution c o n t a i n i n g 0.9 ~ NaC1, with m i n o r a m o u n t s o f other salts and organic c o m p o u n d s , a n d are a formidable corrosive e n v i r o n m e n t responsible for a high p r o p o r t i o n of i m p l a n t failures. I f excessive corrosion of a metallic i m p l a n t occurs, tissue irritation, infection, a n d bone resorption may result, a-5 Recent measurementsr, 7 have shown that when C-steel, stainless steels, Ti a n d other metals are surgically implanted in living tissue, their corrosion rates are initially *Manuscript received 11 September 1968; received in revised form I 1 February 1969. "l'Present address: Corrosion Laboratory, Massachusetts Institute of Technology, Cambridge, Massachusetts. 755

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R. Wn~STONR~'vm and NORBERTD. GREENE

very high and they decrease with time. Any pretreatment which would lower these high initial rates would be beneficial. Before implantation in the body, all surgical implants must be subjected to sterilization, a pretreatment which destroys microbial life (bacteria, spores, fungi and viruses). Steam sterilization is most commonly employed, but dry heat sterilization may also be used. Chemical disinfection ("cold sterilization") is frequently utilized in animal experiments. Previous investigators of surgical implant corrosion have used unsterilized specimenss,9 or have not reported the specific sterilization procedure, including temperature and time. ~°.n This research has studied the effects of various sterilization methods on the corrosion behaviour of four metals used in the human body--type 316 and type 304 stainless steels, Ti and wrought Vitallium (Haynes 25). EXPERIMENTAL

Isotonic NaCI (0.9wt. ~o aqueous solution) was used in this investigation since it qualitatively resembles both the composition and corrosivity of body fluids as discussed elsewhere/Corrosion rates in this environment were measured by the passive current method. 12 Passive current-time relationships were determined by immersing the specimen in N2-saturated solution and immediately, with a potentiostat, controlling the potential at a constant value in the central part of the passive region, -- 0.100V vs. SCE for type 316 and type 304 stainless steels and Vitallium, and ÷ 0.500V vs. SCE for Ti. The applied anodic current is the passive current, which is equivalent to the corrosion rate in an oxygenated solution./2 For simplicity, corrosion rates are reported in terms of c.d. To convert to the more familiar mils penetration per year (mpy), the following conversion factor coincidentally applies to all the metals utilized in this study: l~A/cm 2 = 0.33mpy.* The chemical composition and metallurgical treatment of each metal is listed in Table 1. All specimens were cylindrical and about 10cm 2 in projected surface area. The sample preparation involved abrasion with 2/0 emery paper, mounting with the Stern-Makrides compression gasket assembly, ~4 ultrasonic cleaning for 3min in detergent solution, rinsing in distilled water, sterilization as indicated, and immersion in isotonic NaCI maintained at 38.6 ° 4- 0.2°C (101.5°F). This corresponds to the average body temperature of dogs, one of the more common experimental animals employed for in vivo testing of implant materials. Water was double-distilled and had a minimum resistivity of lmegohm-cm. Conventional cells and electrochemical apparatus were employed3 s Sterilization followed recommended procedures. ~s Except where noted, steam sterilization was done at 132°C (270°F) for 3min xe.17 and dry heat sterilization at 160°C (320°F) for 2h. ~s Chemical disinfection was accomplished by immersion in either zephiran chloride tinted tincture 1 : 750 for 30min ~s or Bard-Parker formaldehyde germicide for 3h, ~9 followed by rinsing in distilled water. RESULTS

Figure 1 shows the effects of sterilizationon the corrosion behaviour of type 316 stainless steel. In the unsterilized condition, this alloy demonstrates typical passive *The metal valency changes necessary for these calculations are determined from the potentiostat potential, the solution pH (6--7), and the appropriate potential-pH diagram, is

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behaviour, z° That is, the passive corrosion rate decreases as an inverse function of time, and a logarithmic plot yields a nearly straight line with a slope of approximately 1. Both steam and dry heat sterilization lower the corrosion rate considerably, with the dry heat method being somewhat more effective. Chemical disinfection with either zephiran chloride or Bard-Parker formaldehyde germicide has a negligible effect on corrosion rate. Steam and dry heat sterilization cause a marked lowering of corrosion rate and thus a decrease in the amount of metal transformed to metal compounds. For example, steam sterilization of type 316 stainless steel results in a lowering of the amount of charge transferred from 2.4mC/cm 2 to 0.3mC/cm ~ during the period of exposure from 0.2 to 7.0h. Figure 2 illustrates that steam and dry sterilization also lower the passive corrosion rate of type 304 stainless steel. The effects of sterilization are more pronounced than those observed with type 316 stainless. Again, the dry heat method is the most effective in reducing subsequent corrosion. Sterilization pretreatments have very little influence on the passive corrosion characteristics of Ti and, except for very short exposure periods, virtually no effect on -

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Vitallium as shown in Figs. 3 and 4. As in the preceding experiments, the 2h dry heat sterilization technique produces greater reductions in corrosion rate than the shortterm steam sterilization. To determine the relative importance of time, temperature and type of sterilization, a series of tests were performed with type 316 stainless steel and some results are plotted in Fig. 5. In general, increasing either time or temperature improves corrosion resistance; however, there appears to be a limit to the effectiveness of long sterilization times, as shown in Fig. 5. Increasing the time of dry heat sterilization from 2 to 10h does not decrease corrosion rate significantly, whereas increasing steam sterilization from 3min to 2h decreases the rate considerably. DISCUSSION The most important observation of this investigation is that sterilization treatments can produce significant changes in corrosion resistance depending on the metal and the procedures employed. Vitallium is essentially unaffected by conventional sterilization methods, while the austenitic stainless steels are markedly improved by such pretreatments. These changes in relative corrosion rates caused by sterilization indicate that, in the development and comparison of alloys for surgical implant applications, specimens for corrosion rate measurements should be sterilized prior to testing. Also, the importance of using the same procedure throughout an experimental programme is evident, since differences in times and temperatures can produce large differences in subsequent corrosion rates.

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762

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3. Steam sterilization at 132°C (270°F) for 2h is suggested to reduce corrosion rates o f type 316 stainless steel implants. Acknowledgement--This investigation was supported by Public Health Service Grant No. 12661 from the National Institute of General Medical Science.

REFERENCES 1. D. C. LUDWXGSON,J. Metals 16, 226 (1964). 2. N. D. GRE~m and D. A. JONES,J. Mater. 1, 345 (1966). 3. J. Co~n~Nand G. HAMMOND,J. Bone Jt Surg. 41A, 524 (1959). 4. J. T. SCAI.~, G. D. WINTERand J. T. SHIRLEY,Brit. reed. J. ii, 478 (1961). 5. J. H. HIcKs, Chem. Ind. 28, 1240 (1964). 6. V. J. COLANGELO,N. D. GREENE,D. B. KET'rELKAMP,H. ALEXANDERand C. J. CAMPBELL,jr. biomed. Mater. Res. 1, 405 (1967). 7. R. W. ~ and N. D. GREENE,Comparison of the in vivo and in vitro corrosion of 18-8 stainless steel and titanium, to be published. 8. T. P. HOARand D. C. MEARS,Proc. R. Soc A294, 486 (1966). 9. J. COHEN,J. Bone Jr Surg. 44A, 307 (1962). 10. P. G. LAING,A. B. FERGUSONand E. S. HODGE,J. biomed. Mater. Res. 1, 135 (1967). 11. A. B. FEROUSON,P. G. LArNGand E. S. HODGE,J. Bone Jt Surg. 42A, 77 (1960). 12. D. A. JONESand N. D. GREENE,Corrosion 22, 198 (1966). 13. M. POURUAIX,Atlas of ElectrochemicalEquilibria in Aqueous Solutions. Pergamon Press, New York (1966). 14. M. STERNand A. C. MAKRIDF.S,J'. electrochem. Soc. 107, 782 (1960). 15. N. D. GREENE,Experimental Electrode Kinetics. Rensselaer Polytechnic Institute, Troy, New York 1965). 16. J. J. PERKINS,Principles and Methods of Sterilization. Thomas, Springfield, Illinois (1956). 17. Wilmot Castle Company, Rochester, N.Y., Model "7" Castle Super Speedclave Instructions. 18. Winthrop Laboratories, New York, Instructions for use of zephiran chloride. 19. Bard-Parker Company, Inc., Danbury, Conntecticut, Directions for use of Bard-Parker formaldehyde germicide. 20. M. STERN,J. electrochem. Soc. 106, 376 (1959).