Metallurgical analysis of aneurysm and microvascular clips. W. BRADFORD DELONG, M.D., AND ROBERT L. RAY, M.S. San Francisco and Oakland, California

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1 J Neurosurg 48: , 1978 Metallurgical analysis of aneurysm and microvascular clips W. BRADFORD DELONG, M.D., AND ROBERT L. RAY, M.S. San Francisco and Oakland, California The design of an aneurysm clip and the choice of its component materials require complex considerations of corrosion resistance, biocompatibility, desired performance, and manufacturing practicalities. This study analyzed 10 different types of aneurysm and microvascular clips for chemical composition and metallurgical characteristics. The opening force of each clip was determined with a spring gauge. All components of four multicomponent clips (three aneurysm clips and one microvascular clip) were manufactured from stainless steel, a strong precipitation-hardening grade of good general corrosion resistance. Two multicomponent clips (one aneurysm clip and one microvascular clip) each contained three components manufactured from different alloys. In an implanted clip, an increased rate of corrosion may result from electrolytic reaction between components composed of different alloys. This study demonstrated that stainless steel is a popular alloy for use in vascular clips, but it has several drawbacks. The authors suggest that manufacturers explore the use of PHI 5-7Mo instead. PH 15-7Mo is another precipitation-hardening stainless steel, but contains molybdenum, which renders it potentially more resistant to corrosion than. KEY WORDS 9 aneurysm clip 9 microvascular clip F EDERAL legislation has given the Food and Drug Administration firm control over surgical instruments and implants? 7 Neurosurgeons are establishing standards for neurosurgical devices through several organizations. 5 Few clinical data have accumulated upon which to base many of the required standards, and manufacturers of neurosurgical implants still must refer to orthopedic standards for guidance. As noted by the Utah Biomedical Test Laboratory, orthopedic standards "have rather limited applicability to neurosurgical implants. ''6 Orthopedic implants must withstand the stresses of weight-bearing. A stressed implant is subject to greater risk of corrosion failure than a non-stressed implant. In 1969, McFadden reviewed metallurgical principles applicable to neurosurgical practice, and in 1972 he published investigations of tissue reactions to various metallic implants placed within the intracranial and intraspinal compartments of dogs. 15'1e His work demonstrates that principles of corrosion apply to neurosurgical implants as well as to orthopedic implants, although the clinical impact of ignoring these principles in neurosurgical patients remains to be defined. Fox has described the array of vascular clips available? ~ We analyzed the chemical 6]4 J. Neurosurg. / Volume 48 / April, 1978

2 Metallurgy of vascular clips FIG. 1. The nine clips initially analyzed. composition and metallurgical characteristics of 10 types of aneurysm and microvascular clips to provide insight into methods currently being used in the manufacture of vascular clips. In the past, federal support for materials research and development has been applied largely to the development of alloys for military, space, and nuclear applications. Alloys for surgical implants have been adapted from these efforts incidentally. Considering the few data available to guide them, the manufacturers of aneurysm clips have produced remarkably reliable products. This article should not be construed as criticism of their efforts, and portions of it should not be used out of context for such a purpose. Although there is room for improvement in certain aspects of the design and manufacture of aneurysm clips, improvement must proceed at a practical pace and must be encouraged in such a way that the instrument manufacturers are not penalized for their past efforts. The authors have referred to each clip only by number. Materials and Methods Through commercial channels, we initially purchased nine types of clips from several distributors and manufacturers (Table 1 and Fig. 1). We purchased six clips of each type. One clip of each type was taken directly from its commercial package and disassembled. The clip components were mounted in acrylic for microscopic examination and microhardness testing. Microscopic examinations were made in both unetched and etched (oxalic acid) conditions. The microhardness test values were converted from Knoop Hardness Numbers to the more familiar Rockwell "C" (HRC) scale (Table 1). With a Jonard dynamometer tension gauge, we determined the opening force at the tips of each of the remaining clips immediately upon removing it from its commercial package and again after stressing it by fully opening and closing it 100 times with its matched applier. ~ We also obtained four aneurysm clips of recent design, two bayonet-shaped with straight J. Neurosurg. / Volume 48 / April,

3 Clip No. Type of Clip TABLE 1 Details of clips investigated W. B. DeLong and R. L. Ray Length Chemistry and Hardness Approximate Length & Width Date Opening of Clip of Jaws Purchased Force* (mm) (ram) Jaws Pivot Spring (gm) 1 aneurysm 10 5 X 11~ 8/20/73 (Cb) HRC 41 HRC 20 HRC aneurysm 13 6 X 1~/6 3/15/73 (Cb) HRC 37 HRC 44 HRC aneurysm 12 8 X 1~ 12/4/75 HRC HRC 30 HRC aneurysm 8 4 X 1,1/4 2/14/ PH15-7Mo HRC 36 HRC 20 HRC 51 (spring in weakest position) 5 aneurysm 13 6 X 11~ 8/21/73 one piece t316 ( HRC aneurysm 12 5 X 2 4/23/73 one piece ( HRC microvascular 10 5 X 1 8/20/73 (Cb) HRC 33 HRC 50 HRC 50 t microvascular 10 4 X 1 2/20/73 one piece ( HRC microvascular /16/ HRC 47 HRC 44.5 HRC 49.5 (one clip opened at 200) 10 aneurysm 18 7 X 1 11/15/76 clip body & strut: Elgiloy a aneurysm 19 7 X 1 11/15/76 HRC *Note that the different clips vary in size (Fig. 1); therefore, we imply no comparison of the opening forces among the various clips. jaws (Clip 10), and two bayonet-shaped with curved jaws (Clip 10a). 2~ We measured the opening force of each of the four clips upon removing it from its commercial package, and then stressed one straight-jaw clip and one curved-jaw clip. We then submitted one clip of each configuration for metallurgical analysis and one for microhardness testing. Results Component Alloys and Microhardness Testing Each component of three aneurysm clips (Clips 1, 2, and 3) and one microvascular clip (Clip 7) was made of stainless steel, a strong semiaustenitic grade of good general corrosion resistance. The pivot of Clip 7 and the jaws of Clips 1 and 2 each contained a trace of columbium (niobium), a stabilizer sometimes added to stainless steel to counter carbide precipitation. The presence of columbium in some components and not in others probably means the components were made from different melts (batches) of steel. When stainless steel is properly processed, it will develop high strength and hardness while maintaining useful ductility. A hardness of Rockwell C 50 (HRC 50) in an austenitic stainless steel (300 series) would indicate almost nil ductility remaining, but 17-7PH stainless steel of the same hardness would still have good ductility. The pivot of Clip 1 had a hardness of HRC 20, much softer ~han expected from hardened. Each component of Clip 4 was made from a different type of stainless steel. The spring was fashioned from PH15-7Mo stainless steel, a molybdenum-bearing alloy of excellent corrosion resistance. The pivot and jaws were made from 301 and 304 stainless steel. This variation in component materials 6"16 J. Neurosurg. / Volume 48 / April, 1978

4 Metallurgy of vascular clips could result in an increased rate of corrosion through electrolytic reaction between the metallic components of the implanted clip. The low hardness of the 301 pivot (HRC 20) indicates low strength and wear resistance in this component. Clips 5 and 6 are both one-piece clips made from 316 stainless steel, another molybdenum-containing alloy of excellent corrosion resistance. Clip 8 was made from one piece of 301 or 304 stainless. The pivot of Clip 9 was made from 431 steel, a martensitic stainless which can be hardened to a high degree, but which has less corrosion resistance than the austenitic 300 series. The spring was made from 301 or 304 stainless. The jaws were not analyzed. If this clip is used only for temporary applications, the corrosive tendency bestowed upon it by the use of 400 series steel and by the variation in component materials probably has little clinical importance. At first glance, Clips 10 and 10a appear to be one-piece clips, but they are really multicomponent clips, since they possess a small reinforcing strut (Fig. 2). Both components were made from Elgiloy, a cobalt-based superalloy. Elgiloy and MP35N, another superalloy, potentially have great strength and hardness. However they are difficult alloys with which to work, and there is little room for error during processing. When processed optimally, Elgiloy may reach a hardness of HRC 58 while still possessing great strength and excellent spring characteristics. The clip we tested possessed a hardness of only HRC 43-47, a level that probably reflects suboptimal processing. Surgical implants containing several components may corrode more readily if the components are made from different alloys. Dissimilar metals in close proximity after implantation may form galvanic cells. Even if its components are made from the same type of alloy, a clip with multiple parts may have a greater tendency to corrode than a clip of the one-piece variety, because corrosion may begin in the interfaces between the components of the clip as the result of electrolyte stagnation and poor oxygen supply in these areas. Microscopic Analysis The microstructures observed in the components correlated well with the results of the FIG. 2. Aneurysm Clip 10. The presence of a small reinforcing strut (arrow) makes this a multicomponent clip. chemical analyses and microhardness testing, thus confirming our conclusions regarding the alloys used in the components. The methods used to manufacture the clip components were not known to us. Microscopic examination at X l00 to X1000 revealed porosity and prominent non-metallic inclusions in most of the components studied. The inclusions were judged to be oxides and silicates, typical of commercial AISI and ASTM stainless steels, normally manufactured in the electric furnace. More refined melting processes, such as vacuum arc remelting (VAR) or electroslag remelting (ESR), will produce alloys which are cleaner and contain fewer inclusions in their microstructures. Clip 9 showed clean microstructures in each of its three components. The components of Clip 1 contained a few light inclusions, and were cleaner than the components of most of the other clips, although still not so clean as would be expected had the steel been manufactured by vacuum arc remelting. The components of Clips 10 and 10a, made from Elgiloy, were free from prominent inclusions. Opening Force and Stress Results The opening force of Clips 1 and 7 was in accord with the tensions advertised by the distributor (45 to 50 and 110 to 130 gm, respectively). Clip 2 opened with a force of 60 to 65 gm, which may be lower than desirable in an aneurysm clip. Clip 5 had an opening force of 170 to 240 gm. The larger version of this clip, which we did not analyze metallurgically, had an opening force of approximately 250 gm. Clip 8 had considerably more strength than required in a clip designed for temporary occlusion of small vessels (170 to 220 gm). This clip rapidly became deformed with J. Neurosurg. / Volume 48 / April,

5 Alloy W. B. DeLong and R. L Ray TABLE 2 Chemical compositions of some of the alloys used for surgical implants Composition (maximum 7o or range) Fe Cr Ni Co Mo C Mn AI Ti Other 301 balance balance balance L balance balance L balance balance PH15-7Mo MP35N balance balance Elgiloy balance cast CoCrMo balance wrought CoCr balance Ti6A14V Be 0.04 Si Si,Wl balance V repeated full opening and closing within the jaws of the matched applier. One clip we tested opened initially with a force of 190 gm; opening and closing it once in the applier reduced the force to 100 gin; four times gave 70 gm. It was completely sprung after 30 cycles. On the other hand, we fully opened and closed an aneurysm clip of similar design 100 times within the limits of its matched applier, and noted no change in the opening force (130 to 140 gm). The packages of Clip 10 bore the notation "Spring Pressure 115." We measured the opening force of these clips at 100 gm. After opening and closing one Clip 10 in the matched bayonet applier 30 times, it became sprung and the jaws would no longer close completely. The packages of Clip 10a bore the notation "Spring Pressure 195." The opening force we measured was 140 gm. After opening and closing one Clip 10a 100 times in the applier, the opening force was 80 gm. We stressed the other clips by fully opening and closing them 100 times with the applier designed for use with the clip being tested. None of these clips exhibited appreciably altered opening force afterward. Discussion Corrosion is the destruction of a metal by the loss of ions from its surface or from its intergranular zones. The metal is ionized either by direct attack by chemical agents or, as in the case of surgical implants, by electro- chemical action. Minute local-action galvanic cells form. The anodes of such cells are located at sites of metallic imperfections and contaminants, or in areas of electrolyte stagnation and oxygen deprivation (such as pits and crevices). Metal is oxidized at the anode, and the substance of the implant is depleted as products of corrosion accumulate. These products may incite significant tissue reaction, may be toxic, or may be allergenic?,s,g,13a 3 Depending on the application, an alloy suitable for surgical implantation must be able to resist several types of corrosion including uniform attack, pitting corrosion, crevice corrosion, intergranular corrosion, galvanic corrosion, stress corrosion cracking, and corrosion fatigue. 21,22 Surgical implants are currently being made from certain stainless steels, titanium and its alloys, tantalum, and nickel- or cobalt-based superalloys, such as MP35N, Elgiloy, and Vitallium (cast CoCrMo or wrought CoCr alloys). The chemical compositions of these alloys are listed in Table 2?,xs,lg For use in an aneurysm clip, an alloy must possess favorable spring characteristics. This consideration eliminates tantalum (which is too soft), Vitallium (which is too brittle), and probably alloys of titanium (which are extremely difficult to process). Certain types of austenitic and precipitation-hardening stainless steels may be suitable, and some of the superalloys possess properties that are extremely attractive. 618 J. Neurosurg. / Volume 48 / April, 1978

6 Metallurgy of vascular clips Stainless Steel There are four general types of stainless steel, distinguished by differences in crystalline structure and chemical composition: ferritic, martensitic, austenitic, and precipitation-hardening. All contain iron and chromium. The chromium is thought to combine with oxygen to form a barrier insulating the metal from its environment. This barrier is either physical in the form of a tenacious surface film of chromium oxide (oxide-film theory of passivity) or chemical in the form of an adsorbed layer of oxygen molecules (adsorption theory). 22 An alloy that has such a surface film protecting it from corrosion is said to be "passivated," and behaves in a less active manner than would be predicted from its electrochemical or thermodynamic characteristics. The ferritic grades are not generally used in the manufacture of surgical instruments or implants because they cannot be adequately strengthened either by work hardening or by heat treatment. The martensitic grades can be hardened and strengthened to a high degree through quench-hardening and tempering. Because of its strength, martensitic stainless is used in cutlery and surgical instruments. However, its corrosion resistance is relatively poor, and it is not generally accepted for surgical implants. The martensitic grades of stainless steel are contained in the 400 series. The "300" grades are primarily austenitic, and their corrosion resistance is generally good. The austenitic grades must be strengthened through cold working, a process that cannot achieve the hardness of the heattreated martensitic grades. Molybdenum used in some alloys of the 300 series (316, 316L, 317, 317L) imparts further corrosion resistance to these grades, possibly by stabilizing the chromium oxide surface film. For use in permanent stainless steel surgical implants the ASTM F4 Committee on Medical and Surgical Materials and Devices has specifically approved only molybdenum-containing austenitic stainless? There is no commercially available grade of stainless steel corresponding exactly to ASTM standards for surgical implants but 316, 316L, 317, 317L come close. 2,8 The austenitic grades of stainless steel are especially susceptible to carbide precipitation and intergranular corrosion. The use of low carbon stainless steel grades is effective in minimizing carbide precipitation. The "L" grades contain less than 0.03% carbon (for example, 316L and 317L). In the late 1940's, stainless steels utilizing the process of precipitation-hardening were developed. " Precipitation-hardening " " " " " stainless steels carry the "'PH" designation and are semi-austenitic. They can be hardened and strengthened through heat treating while retaining useful ductility and good corrosion resistance. The grades currently being used for surgical instruments and implants (17-7PH and PH15-7Mo) contain aluminum as the main "PH" element. These grades are austenitic after the initial step of processing and the manufacturer fabricates his product at this stage. Further processing after fabrication attains strength and hardness by converting the austenitic microconstituents to a stronger martensitic form by precipitating aluminum-nickel compounds within the grains of the metal? These compounds form "interference particles" and inhibit deformation. The precipitation-hardening grades of stainless steel are particularly versatile from the standpoint of the manufacturer. A variety of processing programs is available, each program bestowing specific metallurgical properties upon the finished product. The manufacturer can choose the program that best fits his needs. The corrosion resistance of the precipitation-hardening grades is generally considered to be slightly less than that of the 300 series? Corrosion resistance can be enhanced by the use of molybdenum (for instance, PH15-7Mo). From the standpoint of corrosion resistance, one would expect to be less suitable for implantation than some of the other available alloys, such as 316, 316L, 317, 317L, MP35N, and perhaps Elgiloy and PH15-7Mo. However, clips made from 17-7PH have had a favorable implant history dating from at least TM One clip designer believes his clips possess superior corrosion resistance because they are processed by means of a patented form of electropolishing. To the best of our knowledge, this process has not yet been subjected to independent scientific scrutiny. Hayakawa, et al.? 1 note the recurrence of an internal carotid J. Neurosurg. / Volume 48 / April,

7 W. B. DeLong and R. L. Ray aneurysm previously shown to be satisfactorily obliterated by a clip made of. The recurrence was noted 7 months after the initial craniotomy and was associated with spontaneous fracture of one of the clip jaws. Superallo ys The superalloys were developed for hightemperature space-age applications. Some of the nickel- and cobalt-based molybdenumcontaining alloys have high corrosion resistance along with metallurgical characteristics which permit their use in springcontaining surgical implants. They are difficult alloys to process, and suboptimal processing will compromise performance of the finished product, as seems to have been the case in the Elgiloy clips we tested. A one-piece clip has been fabricated from MP35N? 4 The manufacturer appears to have overcome the technical problems associated with the processing of this alloy, but the clip is not yet commercially available. We could not obtain samples for testing and analysis. MP35N has been used for several years in the femoral stem of total hip prostheses, and its implant history demonstrates superior corrosion resistance in stressed conditions? 8,24 Biocompatibility The implant histories of the various aneurysm clips used through the years have not demonstrated evidence of local or systemic toxicity resulting from clip materials. It appears that alloys used thus far in the manufacture of aneurysm clips are biocompatible. Potential allergenicity of metallic surgical implants has recently received attention. A surprisingly high incidence of sensitivity to metal, particularly cobalt and nickel, has been found in patients with metal-to-metal Vitallium hip prostheses?,8,9,13 Sensitization to the metallic components of the prosthesis may have contributed to loosening of the implant in some instances. Metal sensitization probably occurs only when an implant is subjected to the stress of weight-bearing and metallic particles are released to surrounding tissue. To the best of our knowledge, there are no reports of allergic reactions to nickel release from 316 stainless steel implants through the years. This alloy contains 10% to 14% nickel. MP35N contains 33% to 37% nickel, but studies have shown that in a bath of Ringer's solution at 40 ~ C, MP35N releases 2.7 times less nickel than 316 stainless steel. TM It seems highly unlikely that aneurysm clips manufactured from any of the alloys with a currently established implant history would cause problems related to allergenicity. Acknowledgment Metallurgical analyses were performed in the laboratories of Testing Engineers, Inc., Oakland, California. References 1. American Society for Testing and Materials, Annual Book of ASTM Standards, 1975 Edition, Part 46, Designations F55-71 (pp ) and F56-71 (pp ). Published by ASTM, 1916 Race Street, Philadelphia, PA American Society for Testing and Materials, Annual Book of ASTM Standards, 1975 Edition, Part 4, Designation A (pp ). Published by ASTM, 1916 Race Street, Philadelphia, PA Armco Advanced Materials. Technical Data Manual. Armco and PH15-7Mo Stainless Steel Sheet and Strip, Pamphlet LA Published by Armco Steel Corporation, Advanced Materials Division, P.O. Box 1697, Baltimore, MD 21203, 1969, 119 pp 4. Benson MKD, Goodwin PG, BrostoffJ: Metal sensitivity in patients with joint replacement arthroplasties. Br Med J 4: , Burton CV, McFadden JT: Neurosurgical materials and devices. Report on regulatory agencies and advisory groups. J Neurosurg 45: , Daniels AU, Price NH, Cosgrove WE: Final Report: Neurosurgical Implant Literature Search. TR July 12, Published by Utah Biomedical Test Laboratory, University of Utah Research Institute, 520 Wakara Way, Salt Lake City, UT Delong WB: A simple method of measuring aneurysm clip tension. Technical note. J Neurosurg 47: , Elves MW, Wilson JN, Scales JT, et al: Incidence of metal sensitivity in patients with total joint replacements. Br Med J 4: , Evans EM, Freemen MAR, Miller A J, et al: Metal sensitivity as a cause of bone necrosis and loosening of the prosthesis in total joint replacement. J Bone Joint Surg 56B: , J. Neurosurg. / Volume 48 / April, 1978

8 Metallurgy of vascular clips 10. Fox JL: Vascular clips for the microsurgical treatment of stroke. Stroke 7: , Hayakawa I, Sasaki A, Fujiwara K, et al: Ein ungew6hnliches Aneurysmarezidiv nach Verclippung ein struktureller Fehler des Clips? Acta Neurochir 35: , Heifetz MD: Technical suggestion: a new intracranial aneurysm clip. J Neurosurg 30:753, Jones DA, Lucas HK, O'Driscoll M, et ah Cobalt toxicity after McKee hip arthroplasty. J Bone Joint Surg 57B: , McFadden JT: Aneurysm clips. J Neurosurg 46:129, 1977 (Letter) 15. McFadden JT: Metallurgical principles in neurosurgery. J Neurosurg 31: , McFadden JT: Tissue reactions to standard neurosurgical metallic implants. J Neurosurg 36: , Public Law Medical Device Amendments of th Congress, S. 510, May 28, Semlitsch M, Niederer PG, Ware DO: Wrought CoNiCrMoTi-AIIoy: Protasui-10. Report DePuy Technical Bulletin, July, Warsaw, Ind: DePuy, Simmons WF, Gunia RB: Compilation and Index of Trade Names, Specifications, and Producers of Stainless Alloys and Superalioys. American Society for Testing and Materials Data Series DS45A. Published by ASTM, 1916 Race Street, Philadelphia, PA 19103, Sugita K, Hirota T, Iguchi I, et ah Comparative study of the pressure of various aneurysm clips. J Neurosurg 44: , Sary P: Corrosion Behaviour of Cast and Forged Implant Materials for Artificial Joints, Particularly with Respect to Compound Designs. Sulzer Technical Review, Research and Development, 1974, pp Uhlig HH: Corrosion and Corrosion Control: An Introduction to Corrosion Science and Engineering, ed 2. New York: John Wiley and Sons, 1971, 419 pp 23. Winter GD: Tissue reactions to metallic wear and corrosion products in human patients. J Biomed Mater Res 8 (No. 5, Part 1):11-26, Younkin CN: Multiphase MP35N alloy for medical implants. J Biomed Mater Res 8 (No. 5, Part 1): , 1974 This study was funded in part by a grant from Edward Weck& Co. Mr. Ray is a consulting metallurgical engineer in Oakland, California. Address reprint requests to." W. Bradford DeLong, M.D., 909 Hyde Street, San Francisco, California J. Neurosurg. / Volume 48 / April,