Sixth Edition. Implant Materials. Unalloyed Titanium.

Transcription

1 Sixth Edition Implant Materials. Unalloyed Titanium.

2 John Disegi Sixth Edition November 2008 About the Cover A portion of the Periodic Table depicts elemental titanium and various major implant alloy elements. Alpha crystal structure of unalloyed titanium. Acknowledgement The author wishes to acknowledge the technical contributions of Professor O. Pohler, Oberdorf, Switzerland.

3 Table of Contents Introduction 2 Basic Metallurgy 1. Composition 3 2. Microstructure 4 Properties 1. Physical 6 2. Tensile 7 3. Fatigue Corrosion Biocompatibility Surface 17 Clinical Features 18 References 19 Glossary 23

4 Introduction Titanium is the ninth most abundant element and the fourth most abundant structural metal in the earth s crust. Large deposits of rutile and ilmenite ore are located in the United States, Canada, and Australia. Titanium metal producers have access to large raw material reserves to ensure that the material requirements of the implant, aerospace, chemical processing, and other critical industries will be met in the future. A comparison of mineral reserves located in the United States indicates that titanium is much more abundant than other strategic minerals that are used to produce stainless steel and cobalt base implant alloys U.S. Mineral Reserves (%) Cobalt Chromium Nickel Titanium The United States mines about one-third of its titanium raw material requirement. The balance is imported mainly from Australia. 2 Titanium ore is treated with chlorine gas to produce an intermediate titanium chloride product. The titanium chloride is reduced with either magnesium or sodium metal to produce titanium metal granules. The resultant granules are pressed into a dense compact, and the compacts are welded together to form an electrode for vacuum melting. An electric current is passed through the electrode in a vacuum arc furnace to produce a titanium ingot. The ingot is normally double or triple vacuum arc melted to yield extremely pure and homogeneous titanium metal. The remelted ingot is further processed by conventional metalworking techniques to produce bar, wire, sheet, plate and tubular products. 3 2

5 Basic Metallurgy 1. Composition Four grades of unalloyed titanium known as Commercially Pure (CP) Grade 1, 2, 3, or 4 are available with a stable alpha phase microstructure. The primary difference in composition is related to oxygen content. The strength of unalloyed titanium increases as the oxygen content increases from Grade 1 through Grade 4. Small quantities of nitrogen and carbon tend to stabilize the alpha phase. The composition requirements of the four unalloyed titanium grades are precisely controlled and documented in ASTM F 67 specification for surgical implant material. 4 Composition limits are also specified for Grade 1 ELI (Extra Low Interstitial), Grade 1, Grade 2, Grade 3, and Grades 4A (annealed) and 4B (cold worked) in the ISO international standard. 5 The ISO limits are slightly different than ASTM F 67 limits and have been compared as follows: Unalloyed titanium composition limits for bar product according to ASTM F 67 and ISO specifications Grade 1 Grade 1 Grade 2 Grade 3 Grade 4, ELI 4A, 4B Nitrogen, max Carbon, max 0.10* 0.10* 0.10* 0.10* Hydrogen, * * * * max Iron, max Oxygen, max Ti Balance Balance Balance Balance Balance *ISO requirements for Grades 1, 2, 3, 4A and 4B 3

6 Basic Metallurgy continued Flat product such as sheet, strip, and plate are identical in composition except a maximum hydrogen content of 0.015% is specified for all grades in ISO Maximum hydrogen content of 0.010% must be met for billet in ASTM F 67 and ISO A billet is defined as a solid semi-finished section with a cross sectional area greater than mm 2 whose width is less than 5 times its thickness. Hydrogen content must be kept very low in titanium compositions. Titanium cleaning operations which use nitric-hydrofluoric acid solutions are carefully controlled to eliminate hydrogen absorption during pickling. A ratio of 10 parts nitric acid to 1 part hydrofluoric acid is recommended. 6 Unalloyed titanium usually contains a residual amount of iron in the form of TiFe. 7 This intermetallic compound increases the solubility limit of hydrogen but does not have a dramatic effect on mechanical properties. 8 The influence of iron content on the corrosion resistance can be significant. 9 ASTM F67 and ISO specifications permit a maximum iron content from 0.10% to 0.50% depending on grade. Synthes titanium bar product is specified to a lower maximum iron content, for enhanced corrosion resistance when compared to industry standards. Detailed information on the effect of iron content on corrosion resistance is included in the discussion on page Microstructure Unalloyed titanium microstructures do not contain nonmetallic inclusions due to the highly sophisticated double or triple vacuum melting practices that are used. Metallographic examination at 100X magnification typically reveals a complete absence of nonmetallic inclusions. ASTM F 67 does not define unalloyed titanium microstructural features but ISO requirements include a grain size of 5 or finer and the absence of inclusions or foreign phases when examined at 100X magnification. A fine grain size is generally desired to provide a good combination of tensile strength, ductility, and fatigue strength. Synthes unalloyed titanium products are produced to a fine grain size requirement for enhanced implant fabrication response and to provide an optimum combination of mechanical properties. The microstructure of unalloyed titanium can be altered by various metallurgical treatments. An annealed microstructure is obtained by heating the material to a defined temperature of around 700 C followed by a specific cooling cycle. The annealed micro-structure 4

7 represents the softest or lowest strength condition. A metallurgical process known as cold working can increase the strength of titanium by deforming the material at room temperature. Typical microstructures of Synthes titanium bar is shown in the series of transverse photomicrographs at 100X magnification. The photomicrographs were provided by University of Mississippi Medical Center. Annealed bar microstructure Cold worked bar microstructure The annealed bar material has an equiaxed alpha microstructure. This consists of a polygonal structure in which individual grains have equal dimensions in all directions. Equiaxed microstructures are usually developed by cold working followed by annealing above the recrystallization temperature. 1 This material exhibits a very fine grain size of ASTM 8.5. The cold worked bar material exhibits an elongated alpha microstructure that results from unidirectional cold working. An elongated alpha microstructure is typical of Synthes bar product that is used to manufacture bone screws. The amount of cold work must be closely controlled so that high strength and adequate ductility are obtained. Unalloyed titanium is completely nonmagnetic and remains nonmagnetic after severe cold working operations. Grade 1 through Grade 4 compositions in the annealed or cold worked condition exhibit no residual magnetism. 5

8 Properties 1. Physical Two important physical properties of unalloyed titanium for implant applications are the density and modulus of elasticity. A comparison 2 with wrought implant quality 316L stainless steel, wrought Co-28Cr-6Mo alloy, and wrought Ti-6Al-7Nb shows the properties set forth in the table below. The density of unalloyed titanium is 57% the density of wrought 316L stainless steel and about 53% the density of wrought Co-28Cr-6Mo alloy. The low density of titanium yields a weight reduction of nearly 50% when implants of similar dimensions are compared. The weight reduction represents a patient comfort factor especially for large sized implants. Modulus of elasticity, or Young s modulus, is a physical property of a material that describes the stress per unit strain in the elastic region. A material with a high modulus of elasticity will transfer less stress from the implant to the bone. This produces a condition known as stress shielding, which is undesirable because osteoporosis may develop and promote refracture of the bone when the implant is removed. 10 However, recent work 11 suggests that necrosis at the bone plate contact surface may be responsible for some of the clinical observations previously attributed to stress shielding. The modulus of elasticity of unalloyed titanium is 55 56% of 316L stainless steel and 42 43% of wrought Co-28Cr-6Mo alloy. Increased stress transfer to bone is desirable but modulus of elasticity properties are less important for fracture fixation implants with relatively small cross-sectional areas. Density Modulus of elasticity in Material (gm/cc) tension (MPa x 1000) Ti Grade Ti Grade Ti Grade Ti Grade L Stainless Steel Wrought Co-28Cr-6Mo Ti-6Al-7Nb

9 2. Tensile The tensile properties of unalloyed titanium are dependent on grade and type of metallurgical processing. ASTM F 67 specification outlines the minimum mechanical properties that must be met in the annealed or softest condition as a function of grade and product form. ISO international standard also covers annealed mechanical properties for Grades 1 ELI, 1, 2, 3, 4A, and includes Grade 4B in the cold worked condition. The minimum tensile properties of unalloyed titanium Grades 1 ELI, 1, 2, 3, 4, 4A, and 4B, implant quality 316L stainless steel (ASTM F 138), wrought Co-28Cr-6Mo alloy (ASTM F 1537), and Ti-6Al-7Nb alloy (ASTM F 1295) bar product in the annealed condition have been compared as follows: Minimum tensile properties for annealed bar product corresponding to industry standards Ultimate 0.2% tensile yield Elongation Reduction strength strength x 4D or 4W* of area** Material (MPa) (MPa) (%) (%) Ti Grade 1 ELI Ti Grade Ti Grade Ti Grade Ti Grade Ti Grade 4A Ti Grade 4B L Stainless Steel Wrought Co-28Cr-6Mo Ti-6Al-7Nb * Alternatively, a gauge length of 5.65 S o, where S o is the original cross-sectional area, may be used ** Not specified in ISO or ASTM F 138 Cold worked condition 7

10 Properties continued The minimum tensile strengths of unalloyed titanium Grades 3, 4, and 4A in the annealed condition are similar to 316L stainless steel, and the minimum yield strengths are superior to 316L stainless steel in the annealed condition. The minimum yield strength of Grade 4B is similar to Co-28Cr-6Mo alloy. Moderately to highly stressed implants are normally fabricated from cold worked material. Cold worked implant quality 316L stainless steel bar must meet a minimum UTS of 860 MPa and a minimum elongation of 12%. CP titanium Grade 4B in the cold worked condition must meet a minimum UTS of 680 MPa and a minimum elongation of 10% according to ISO ASTM F 67 also specifies that grades may be cold worked but a minimum 10% elongation must be met. Titanium can be cold worked to produce high tensile properties that are nearly equivalent to cold worked 316L stainless steel. Refined titanium compositions and unique metallurgical processing have been developed by the Synthes group to increase the stress resistance of unalloyed titanium. 12 The minimum tensile strength and minimum elongation for selected grades of conventional CP titanium are compared to Synthes unalloyed titanium in the diagram below. The mechanical property diagram reveals that the minimum tensile strength of Synthes unalloyed titanium is significantly greater than conventional industry grades. Higher strength capabilities and excellent ductility are achieved with Synthes unalloyed titanium because of the specialized metallurgical processing that has been developed. Minimum Tensile Strength (MPa) Synthes Industry Minimum Elongation (%) 8

11 Certain small diameter Synthes 316L stainless steel implants such as Kirschner wires, Steinmann pins, and Schanz screws are produced to a very high tensile strength that exceeds 1,350 MPa to resist bending deflection. 12 Unalloyed titanium is generally not capable of attaining extremely high levels of tensile strength. Consequently, these small diameter Synthes implants are also available in Ti-6Al-4V or Ti-6Al-7Nb alloy with a moderately high tensile strength of around 920 MPa. ASTM F 67 requirements for UTS, minimum 0.2% yield strength, and minimum elongation are identical for bar, wire, sheet, strip, and plate. Additional mechanical property requirements for unalloyed titanium sheet, strip, and plate include maximum 0.2% yield strength, minimum bend test values, and property limits applied to the transverse and longitudinal directions. A billet is defined as material with a forged cross-sectional area >10,322 mm 2 whose width is less than five times the thickness. Mechanical properties of billets shall be negotiated between the manufacturer and the purchaser. ISO has similar bend and tensile property requirements for annealed Grade 1, 2, 3, and 4A but does not specify a maximum 0.2% yield strength for sheet and strip. These additional properties are not specified for implant quality 316L stainless steel, and Co-28Cr-6Mo alloy is not capable of being manufactured in sheet or strip form. The maximum 0.2% yield strength and bend test requirements for unalloyed titanium sheet, strip, and plate according to ASTM F 67 industry standard are specified below: Additional mechanical property requirements for unalloyed titanium sheet, strip, and plate in the annealed condition Bend test mandrel diameter (mm) ASTM F 67 Maximum 0.2% Under 1.8 mm 1.8 mm to 4.75 mm grade yield strength (MPa) thick thick T* 4T T 5T T 5T T 6T *T = the thickness of the bend test specimen 9

12 Properties continued Maximum yield strength and bend test requirements ensure that titanium flat mill products can be fabricated into various implant shapes in both the transverse and longitudinal planes. The bend test criteria is also a measure of good formability, since the material must be free of cracks after bending to the required radius. The composition, grain size, and fabrication of Synthes sheet is tightly controlled to provide maximum ductility for exceptional bone plate contourability. ASTM F 67 also specifies tensile property requirements for annealed wire sizes. Wire diameters 3.18 mm are identical to annealed bar product. Ultimate tensile strength for annealed wire sizes <3.18 mm are also the same as for annealed bar product. The major differences for annealed wire sizes less than 3.18 mm diameter are related to minimum 0.2% yield strength, minimum elongation, and the absence of reduction of area requirements as follows: Minimum yield strength and elongation requirements for annealed titanium wire < 3.18 mm diameter Yield strength (0.2% offset), Diameter (mm) Grade MPa Elongation % <3.18 to 1.58, incl <1.58 to 0.51, incl <0.51 to 0.13, incl Size variations and out-of-round tolerances are also compiled for wire diameters < 3.18 mm in ASTM F

13 3. Fatigue Fatigue is defined as the process of progressive, permanent structural change occurring in a material that is subjected to alternating stresses and strains. 13 The alternating stress and strain effects are usually localized and may produce cracks or complete fracture after a sufficient number of cycles. Unalloyed titanium fatigue life, or number of loading cycles sustained before failure, is influenced by many factors including composition, 14 grain size, 15 processing history, 16 surface\finish, 17 residual surface stress, 18 and ultimate tensile strength. 19 Major test dependent variables include type of alternating load (rotating-beam; plane bending; tension-compression), specimen geometry, frequency, and test environment. Fatigue testing of orthopaedic implants is generally performed at a low frequency of 3 5 cycles/sec and may include a 0.9% saline or biological test environment. This is somewhat different than the test procedures typically used for fatigue characterization of bar and sheet product. Because of the complexity of fatigue testing, only a brief overview will be presented. The following data documents the plane bending fatigue data of unnotched, 1.0 mm thick unalloyed titanium sheet as a function of tensile strength: Plane bending fatigue data for unnotched 1.0 mm thick unalloyed titanium sheet 19 Ultimate tensile Condition Plane bending strength (MPa) Annealed Cold rolled fatigue strength (MPa) 371 x x x x x x x x x x x 383 Frequency: 58 cycles/sec Test Environment: Air 11

14 Properties continued The fatigue strength is the maximum stress that can be sustained for a specific number of cycles without failure, the stress being completely reversed within each cycle unless otherwise stated. The plane bending fatigue strength of the 1.0 mm unalloyed titanium sheet increases as the ultimate tensile strength increases. The fatigue strength varies between 45 66% of the UTS, and this index is known as the endurance ratio. Many metallurgical factors and specific test variables exert a strong influence on the fatigue strength that is obtained for implant materials. The fatigue properties of unalloyed titanium, implant quality 316L stainless steel, and a wrought cobalt base implant alloy have been documented. 20 In this study, Grade 3 titanium had a higher fatigue life than 316L stainless steel in the low load range. The fatigue results were highly dependent on loading conditions. The endurance limit is the maximum stress below which a material can presumably endure an infinite number of stress cycles. Comparative fully reversed flexural fatigue results for various implant materials have been compiled 21 as follows: Typical fully reversed flexural fatigue results for various implant materials Endurance limit at Material Condition 10 7 cycles (MPa) CP Ti Annealed L Stainless Steel Cold worked Co-Cr-Mo Cast Ti-6Al-4V Annealed

15 4. Corrosion The superior corrosion resistance of unalloyed titanium and titanium alloys compared to iron or nickel based implant alloys is well documented in the literature. 22 The referenced study measured the anodic polarization behavior of a number of implant materials versus a Saturated Calomel Electrode (SCE) in a physiological solution. Tabulated corrosion results as follows: Breakdown potential for implant metals in Hanks solution at 37 C Breakdown Potential (Volts) L Co-Cr-Mo Ti-6Al-4V Ti Corrosion resistance increases as the anodic breakdown potential increases. The results indicate that titanium has superior corrosion resistance by a large margin over the other implant materials that were tested. 22 R. Solar has also studied the corrosion resistance of titanium 23 and concluded that titanium and some of its alloys may be the most biocompatible and corrosion-resistant metallic implant materials in present use. Titanium readily forms a passive surface film which provides a high degree of immunity against attack by most mineral acids and 2 chlorides. The ability of a passive film to repassivate readily if the film is scratched, abraded, or disrupted is considered an important feature of any highly corrosion resistant material. Repassivation studies have shown that the surface oxide film on titanium is much more stable than 316L stainless steel or Co-Cr-Mo alloy. 22 A low iron content in the unalloyed titanium microstructure has been shown to improve the stability of the protective oxide film. Unalloyed titanium with a low iron content of 0.020% has demonstrated a 40% increase in anodic breakdown potential in 3.5% sodium chloride at 25 C when compared to a composition containing 0.150% iron. 13

16 Properties continued Iron contamination on the surface of titanium or the presence of iron in the titanium microstructure can also decrease the corrosion resistance in reducing acids. Accelerated laboratory tests have documented the effect of microstructure iron content on the corrosion rate in reducing acid solutions as follows. 9 Effect of iron content on the corrosion rate of unalloyed titanium in 10% HCl at 21 C Corrosion rate (gm/m 2 /hr) Material Iron Content (%) For many materials, localized corrosion may occur within crevices and small cavities on metal surfaces exposed to aggressive solutions containing dissolved oxygen and chloride ions. The data below compares 316L stainless steel with unalloyed titanium Grade 2 under accelerated exposure conditions. 24 Crevice corrosion resistance in geothermal brine with 100 ppm oxygen at 232 C for 15 days Corrosion Rate (micrometer/year) 316L Stainless Steel 6200 Ti Grade 2 0 The results demonstrate the excellent corrosion resistance of titanium in an accelerated crevice corrosion environment. No crevice corrosion was detected for the Grade 2 titanium, while severe crevice corrosion was measured for the 316L stainless steel. Fretting corrosion is a form of corrosion that can occur when the protective passive film is mechanically disrupted as a result of fretting or abrasive action. This type of corrosion is frequently encountered with implant screws and plates due to the relative motion between the underside of the screw head and the contact surface of the plate. Analysis of retrieved Synthes titanium implants has shown that no 14

17 microscopically visible corrosion attack is detectable between screw heads and plates. Only mechanical wear and oxidized wear particles are found. 25 In vitro fretting results at room temperature have been reported for plates and screws 26 when tested according to ASTM F 897 test procedure: Fretting volume loss in 0.9% saline solution after 14 days Fretting volume loss (mm 3 ) Material Plate Screws Total Ti ± ± ± L Stainless Steel ± ± ± The weight loss results in the referenced study have been converted to fretting volume to account for the density differences. Each screw and each two-hole plate was individually weighed, in addition to each three component combination. Hence, there is a slight difference between plate + screw cumulative volume loss and total plate/screw volume loss. The fretting resistance of the titanium plates and screws was superior to 316L stainless steel plates and screws in this study under accelerated laboratory fretting conditions. An accelerated form of corrosion known as galvanic corrosion can occur in a mixed metal system due to the difference in the electrochemical potential between the two materials. Emergency clinical situations may be encountered that may require the use of 316L stainless steel and unalloyed titanium for multicomponent device applications. Consequently, a clinical study was conducted with Synthes implant quality 316L stainless steel screws and Synthes unalloyed titanium bone plates. 27 No clinical disadvantage was observed in the study for this specific combination of implants. The extent of galvanic corrosion that may be experienced with a mixed metal system is difficult to predict on the basis of values extracted from the electromotive series. This is highlighted in a report by Kruger 28 that states Even if one uses a more relevant series of potentials, there is no assurance that one can reliably predict the extent of corrosion caused by a bimetallic couple. Solar concluded that implant quality 316L exhibited multiple pitting in all tests when coupled with itself, a titanium alloy, or cast Co-Cr-Mo. 23 According to Solar, similar results were obtained by Levine and Staehle who concluded problems can occur with any of the widely used metals, however, especially if metals are mixed. Galvanic corrosion effects are also dependent on the relative ratio of the anodic to cathodic areas. 15

18 Properties continued The mixing of unalloyed titanium with implant quality 316L stainless steel should be avoided to eliminate any possibility of galvanic corrosion or accelerated fretting corrosion. Many of the advantages of unalloyed titanium such as improved corrosion resistance, improved biocompatibility, and absence of allergic response may not be realized with a mixed metal system. The possibility of increased product liability exposure must also be considered when a mixed implant system is used. 5. Biocompatibility As early as 1940, Bothe and colleagues concluded that unalloyed titanium pegs were well tolerated in an animal model and the biocompatibility was similar to stainless steel or Co-Cr-Mo alloy. 29 Leventhal worked with rabbits and rats and reported in 1951 that titanium was inert and appeared to be ideal for fraction fixation. 30 Brunski concluded that good tissue response was obtained when pure titanium dental implants were evaluated in beagle dogs. 31 Organ culture studies at the Laboratory for Experimental Surgery. 32 have demonstrated the excellent growth development of embryonic rat femora implanted with titanium rods. A study by Williams and Meachim 33 involved a large series of retrieved human implants during the period of 1967 to No corrosion was noted in any of the 49 titanium implants but 54% (64 out of 119) of the stainless steel implants exhibited varying degrees of corrosion. About 90% of the endosteal implants inserted worldwide are titanium and Weiss has reported that unalloyed titanium dental implants are being used routinely with clinical success. 34 Metal sensitivity reactions must also be considered. Dobbs and Scales have reported that to their knowledge there are no reports which suggest that metal sensitivity or adverse reactions of any kind are associated with titanium implants. 35 This is in contrast to various clinical studies which have shown that metal sensitivity reactions have been observed with 316L stainless steel and Co-Cr-Mo implants. Preoperative patch testing of 212 patients undergoing total hip replacement indicated that 6.6% were sensitive to nickel, cobalt, and chromium. 36 There was some indication that metal sensitivity was provoked in four patients after implantation. A more sophisticated metal sensitivity test known as leukocyte migration has shown 18% sensitive to nickel, 15% sensitive to cobalt, and 3.5% sensitive to chromium in a study with 629 patients. 37 Synthes unalloyed titanium implants are recommended in situations where metal sensitivity is preoperatively verified or where 316L stainless steel implants have provoked an allergic patient response. Unalloyed titanium also exhibits unique biocompatibility properties 38, 39 which include soft tissue and bone adhesion to the titanium surface. 16

19 The bonding of biomolecules to the titanium surface has been analyzed by sophisticated analytical techniques. 40 Branemark has documented the direct apposition of bone to unalloyed titanium dental implants in a 15 year follow-up study. 41 A major advantage of tissue integration at the surface has been the possibility of less bacterial colonization and reduced infection. 42 Synthes titanium implants have been used for fracture treatment of over 5000 cases since Excellent biocompatibility has been clinically observed in this large patient population Surface Titanium that is exposed to air or water spontaneously forms a titanium oxide film which is about 0.5 to 0.6 nanometers thick. 43 A nanometer is an extremely small unit of length equal to 1 x 10-9 meter. The exceptional stability and corrosion resistance of this passive layer has been previously discussed. Synthes unalloyed titanium implants are chemically treated in an electrolytic process known as anodizing to increase the thickness of the naturally occurring titanium oxide film. The anodized film consists of titanium oxide and is about nanometers thick. Various colors can be produced in the anodizing process and this is a function of the oxide thickness. Light interference within the oxide film is responsible for the color that is obtained. Standard Synthes titanium implants are anodized in a manner that creates a consistent and reproducible gold appearance, although other colors can be produced. Iron contamination may be present on the titanium surface as a result of the implant machining or fabricating operations. Corrosion pitting failures have been attributed to this type of surface contamination. 2 The chemical conditioning treatments that are a part of the anodizing process remove surface contaminants that may be present from the various manufacturing operations. Rahn and coworkers 44 have investigated the relative amount of tissue growth on Synthes titanium discs. The results indicated that mechanically polished, chemically polished, and anodized surfaces promoted similar animal cell growth patterns. It is expected that similar cellular adhesion properties would be obtained for these relatively smooth titanium surfaces. The color readily distinguishes Synthes titanium implants from Synthes 316L stainless steel implants and this is considered an additional benefit derived from the anodizing treatment. The gold appearance is aesthetically appealing, and the implants display reduced external visibility when epidermal coverage is minimal. 17

20 Clinical Features Synthes unalloyed titanium implants offer major clinical advantages which have been summarized as follows: Features and clinical advantages of Synthes unalloyed titanium implants Material feature Excellent corrosion resistance Unique biocompatibility Nonallergenic Low density Good ductility Four grades Clinical advantage Permanent implants Tissue attachment for enhanced fixation Complete absence of metal sensitivity Lightweight implants Easily contoured Implant design versatility Sterilization Implants may be sterilized by any of the standard methods such as steam autoclave, ETO, gamma radiation, electron beam, and RF discharge. Implant handling Excessive fingerprint contamination from handling may produce slight discoloration after repeated steam autoclave cycles. No adverse effects are related to this change in surface appearance. Diagnostic imaging X-radiography, Magnetic Resonance Imaging, CT Scans, and PET scans can be utilized. MRI scan resolution is superior to 316L stainless steel because titanium produces less starburst, or signal interference. Implant retrieval Occasional black deposits (wear debris) may be observed at implant removal sites. No adverse tissue reaction is associated with this clinical observation

21 References 1. Bannon, B. and Mild E., Titanium Alloys for Biomaterial Application: An Over view, in Titanium Alloys In Surgical Implants, ASTM STP 796, 1983, pp Donachie, M., (ed), Titanium A Technical Guide, ASM International, Metals Park, OH, IMI Titanium Properties and Applications, Technical Brochure, IMI Titanium Limited, Birmingham, England. 4. ASTM F 67 Standard Specification for Unalloyed Titanium for Surgical Implant Applications (UNS R50250, UNS R50400, UNS R50550, UNS R50700), American Society for Testing and Materials, Philadelphia, PA. 5. ISO Implants for Surgery, Metallic materials, Part 2: Unalloyed Titanium, International Organization for Standardization. 6. ASTM B 600 Standard Recommended Practice for Descaling and Cleaning Titanium and Titanium Surfaces, American Society for Testing and Materials, Philadelphia, PA. 7. Hülse, K., et al., Influence of small additions of Fe, Cr, Ni, on the recrystallization behavior of commercially pure titanium, Internal Report, Deutsche Titan Gmbh, Essen, West Germany, RMI Titanium Metallography, Technical Brochure, RMI Company, Niles, OH. 9. Low iron, commercially pure titanium a standard product of the RMI Company, Technical Brochure, RMI Company, Niles, OH. 10. Park, J., Hard Tissue Replacement Implants, Chapter 12, in Biomaterials Science and Engineering, Plenum Press, 1984, pp Perren, S., et al., Early Temporary Porosis of Bone Induced by Internal Fixation Implants: A Reaction to Necrosis, Not to Stress Protection?, Clinical Orthopaedics and Related Research, Number 232, July Disegi, J. and Wyss, HJ, Implant Materials for Fracture Fixation: A Clinical Perspective, Orthopedics, Volume 12, Number 1, January 1989, pp Fatigue Design Handbook AE-10, Second Edition, Society of Automobile Engineers Inc., Warrendale, PA, 1988, p

22 References continued 14. Beevers, C. and Robinson, J., Some Observations On The Influence Of Oxygen Content On The Fatigue Behavior of Alpha Titanium, Journal of the Less-Common Metals, Volume 17, 1969, pp Beevers, C. and Halliday, M., On the Formation of Internal Fatigue Damage in Association with Twins in Alpha Titanium, Metal Science Journal, Volume 3, 1969, pp Gollard, D. and Beevers, C., Some Effects of Prior Deformation and Annealing on the Fatigue Response of Alpha Titanium, Journal of the Less-Common Metals, Volume 23, 1971, pp Thomasson, L., et al., The Influence of Surface Treatment on the Fatigue Properties of Titanium and Titanium Alloys, WADC Technical Report , Part 1, February Shot Peening Applications, Technical Brochure, Sixth Edition, Metal Improvement Company, Inc., Paramus, NJ, Royal Aircraft Establishment, The Fatigue Properties of Titanium and Titanium Alloys. Part IV. The Effect of Stressing Mode, Report RAE Library Trans 1533, Royal Aircraft Establishment, Farnborough, England, Pohler, O., Study of the Initiation and Propagation of Fatigue and Corrosion Fatigue of Orthopedic Implant Materials, Dissertation Abstract International 44, Ohio State University, March Physical and Mechanical Properties of Orthopaedic Alloys, Zimmer Technical Monograph , Rev. 1/10MZ, Zimmer Inc., Fraker, A., et al., Surface Preparation and Corrosion Behavior of Titanium Alloys for Surgical Implants, in Titanium Alloys In Surgical Implants, ASTM STP 796, American Society for Testing and Materials, 1983, pp Solar, R., Corrosion Resistance of Titanium Surgical Implant Alloys: A Review, in Corrosion And Degradation Of Implant Materials, ASTM STP 684, American Society for Testing and Materials, 1979, pp Crevice corrosion resistance of titanium and titanium alloys to aqueous salt solutions, Technical Brochure, RMI Company, Niles, OH. 20

23 25. Pohler, O., Stratec Medical, Waldenburg, Switzerland, Private Communication. 26. Brown, S., and Merritt, K., The Effects of Serum Proteins on Corrosion Rates In-Vitro, in Clinical Applications Of Biomaterials, John Wiley and Sons, Ltd., 1982, pp Ruedi, T., Titanium and Steel in Bone Surgery, Journal for Trauma Healing, Journal 123, 1975, Springer-Verlag, (English Translation). 28. Kruger, J., Fundamental Aspects of the Corrosion of Metallic Implants, in Corrosion and Degradation of Implant Materials, ASTM STP 684, 1979, pp Bothe, R., et al., Surgery, Gynecology and Obstetrics, Vol. 71, 1940, p Leventhal, G., Journal of Bone and Joint Surgery, Vol. 33A, No. 2, 1951, p Brunski, J., et al., Composition and Morphology of Material Attached to Titanium Bladevent Dental Implants in Beagles, presented at the Second Annual Meeting of the Society for Biomaterials, Philadelphia, PA, April 12, Gerber, H. and Perren, S., Evaluation of Tissue Compatibility of in vitro Cultures of Embryonic Bone, in Evaluation of Biomaterials, John Wiley & Sons Ltd., 1980, pp Williams, D and Meachim, G., Journal of Biomedical Materials Research Symposium, No. 5, (Part 1), 1974, p Weiss, C., et al., The Successful Use of Precompacted and Coined Titanium for Physiologically Designed Endosteal Blade Implants The Need for Statistical Analysis, presented at the Fifth Annual Biomaterials Symposium, Clemson University, April 18, Dobbs, H. and Scales, J., Behavior of Commercially Pure Titanium and Ti-318 (Ti-6Al-4V) in Orthopedic Implants, in Titanium Alloys In Surgical Implants, ASTM STP 796, American Society for Testing and Materials, 1983, pp Deutman, R., Metal Sensitivity before and after Total Hip Arthoplasty, Journal of Bone and Joint Surgery, Vol. 59-A, No. 7, October 1977, pp

24 References continued 37. Merritt, K. and Brown, S., Biological Effects of Corrosion Products from Metals, in Corrosion and Degradation of Implant Materials: Second Symposium, ASTM STP 859, American Society for Testing and Materials, 1985, pp Steinemann, S., et al., Adhesion of Bone to Titanium, Biological and Biomechanical Performance of Biomaterials, Proceedings of the Fifth European Conference on Biomaterials, Paris, France, September 1985, pp Kennedy, J., et al., Insoluble complexes of amino-acids, peptides, and enzymes with metal hydroxides, J.C.S. Perkin I, 1976B, pp Gold, J., et al., XPS Study of Retrieved Titanium and Ti Alloy Implants, presented at the 8th European Conference on Biomaterials, Heidelberg, Germany, September, Branemark, P., et al., A 15-year study of osseointegrated implants in the treatment of the edentulous jaw, Int. J. Oral Surg., Vol. 10, 1981, pp Gristina, A., Biomaterial-Centered Infection: Microbial Adhesion Versus Tissue Integration, Science, Vol. 237, September 1987, pp Gold, J. and Brunski, J., Surface Characterization Of Ti-Coated Coverslips For Use In A Cell Adhesion Study: A Comparison With Titanium Dental Implant Surfaces, Transactions of the Third World Biomaterials Congress, Vol. XI, 1988, p Rahn, B., et al., Cultured Cells Contacting Implant Material of Different Surface Treatment, in BIOMATERIALS, John Wiley and Sons Ltd., 1982, pp

25 Glossary ALLOY. A metallic substance composed of two or more elements at least one of which is metal. ALLOYING ELEMENT. An element, added to and remaining in a metal, that changes the metal s structure and properties. ALPHA. The low-temperature form of titanium with a hexagonal close-packed (hcp) crystal structure. ANNEALING. A metal-softening operation in which the metal is heated to and held at a specified temperature, followed by cooling at a controlled rate. ANODIC REACTION. An oxidation reaction that produces electrons at the anode of an electrochemical cell. When dissimilar metals are coupled, the anode usually experiences increased corrosion. ANODIZING. An electrolytic process that increases the thickness of the protective oxide film on titanium. BETA. The high-temperature form of titanium with a body-centered cubic (bcc) crystal structure. BETA TRANSUS. The temperature which designates the alpha-tobeta phase transformation of unalloyed titanium. BODY-CENTERED CUBIC. A unit cell which consists of atoms arranged at cube corners with one atom at the center of the cube. BRITTLENESS. The tendency of a material to fracture without first undergoing significant permanent deformation. CATHODIC REACTION. A reduction reaction that consumes electrons at the cathode of an electrochemical cell. When dissimilar metals are coupled, the cathode usually undergoes reduced corrosion. COLD-WORKED MICRO-STRUCTURE. A microstructure resulting from cold working the material. COLD WORKING. Permanently deforming a metal or alloy at room temperature to increase its strength. CRYSTAL. A solid composed of atoms that repeat in a pattern of regular intervals in three dimensions. DESCALING. Chemically or mechanically removing the thick oxide layer that is formed on metals during high temperature processing. DUCTILITY. The ability to permanently deform before fracturing. ELECTRODE. A cylindrical metal compact that is suitable for vacuum arc melting or a metal ingot that is suitable for remelting. 23

26 Glossary continued ELONGATED ALPHA. A fibrous type of microstructure that results from unidirectional cold working of unalloyed titanium. ELONGATION. A term that describes ductility by measuring the amount of extension that a material undergoes during tensile testing. EQUIAXED STRUCTURE. A micro-structure feature that consists of polygonal shaped grains with equal dimensions in all directions. FATIGUE. The phenomenon leading to fracture under repeated or fluctuating stresses having a maximum value less than the ultimate tensile strength of the material. FATIGUE LIFE. The number of cycles of stress or strain of a specified character that a given specimen sustains before failure of a specified nature occurs. FATIGUE STRENGTH. The maximum stress that can be sustained for a specific number of cycles without failure, the stress being completely reversed within each cycle unless otherwise stated. FRETTING CORROSION. An accelerated form of corrosion that can occur when the protective passive film is mechanically abraded. The relative motion of the underside of a bone screw head with the contact surface of a bone plate is a typical example. HEXAGONAL CLOSE-PACKED. A unit cell which consists of a hexagonal arrangement of atoms in a plane surrounding an atom followed by three atoms in the next horizontal plane. HOT-WORKED MICRO-STRUCTURE. A microstructure resulting from hot working the material. HOT WORKING. Permanently deforming metal at an elevated temperature that is usually above the recrystallization temperature. INCLUSION. A particle of foreign material in a metallic microstructure that is usually considered undesirable. INGOT. A metal casting that is suitable for remelting or hot working. INTERMETALLIC COMPOUND. A phase in an alloy system that has a well-defined composition and limited solubility. LONGITUDINAL. Parallel to the principle direction of hot or cold working. MICROSTRUCTURE. The structure of metals as revealed by microscopic examination of a specimen. MODULUS OF ELASTICITY. A measure of the stress per unit strain in the elastic region before permanent deformation occurs. 24

27 NANOMETER. An extremely small distance equal to 1 x 10-9 meter. PASSIVATION. The process of changing the chemical activity of a metal surface to a less reactive state, usually to increase the corrosion resistance. PICKLING. Chemical removal of the thick oxide layer that is formed on metals during high temperature processing. POLYGONAL STRUCTURE. A closed planar shape bound on at least three sides. RECRYSTALLIZATION. A change from one crystal structure to another that occurs during heating or cooling through a critical temperature range. REDUCTION IN AREA. A tensile testing measure of ductility that equals the original area minus the area after fracture divided by the original area, expressed as a percentage. SOLUBILITY. A measure of the amount of a substance that can be dissolved in a metal or alloy. STRAIN. Change in length per unit length in the direction of the applied stress. STRESS. Force per unit area. TRANSVERSE. Perpendicular to the principle direction of hot or cold working. TWINNING. A microstructure feature that describes mirror image positions across a planar interface. ULTIMATE TENSILE STRENGTH. In tensile testing, the maximum load at fracture divided by the original cross-sectional area. UNALLOYED TITANIUM. Single phase titanium metal that does not contain major alloying additions. VACUUM ARC REMELTING. A melting process in which an electric arc is used to remelt an electrode inside a vacuum chamber. YIELD STRENGTH. In tensile testing, the stress at which the stress-to-strain ratio exhibits a specified deviation, usually designated as 0.2% offset. 25

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