Implant Materials.
Wrought 18% Chromium– 14% Nickel– Nickel–2.5% 2.5% Molybde Molybdenum num Stainless Stainle ss Steel. Third Edition
John Disegi Third Edition Jan 2009
About the Cover
A portion of the Periodic Table depicts various major implant alloy elements. Face-centered-cubic (FCC) crystal structure of stainless steel.
John Disegi Third Edition Jan 2009
About the Cover
A portion of the Periodic Table depicts various major implant alloy elements. Face-centered-cubic (FCC) crystal structure of stainless steel.
Table of Contents
2
Introduction
Basic Metallurgy
Properties
1. Composition
4
2. Microcleanliness
5
3. Microstructure
6
1. Physical
9
2. Tensile
10
3. Fatigue
12
4. Corrosion
14
5. Biocompatibility
18
6. Surface
19
Clinical Feature Featuress
21
References
22
Glossary
26
Introduction
During World War II there was considerable interest in identifying satisfactory stainless steel implant materials to repair bone fractures associated with wartime injuries. Murray and Fink recommended Type 302 stainless steel 1 to the U.S. Army and Navy in 1943. After the war, Peterson (1947) reviewed the clinical performance 2 of plates and screws used for fracture treatment by the Army. He concluded that 18-8 SMo which contained 18% chromium + 8% nickel + 2% molybdenum provided the best combination of properties. In the early 50s, Blunt et al. evaluated various metals that were implanted in dogs 3 and Type 316 appeared to be the best choice. This landmark effort set the stage for the widespread use of Type 316/316L stainless steel for surgical implant applications. ASTM F 55-65T 4 was eventually published in 1965 to define the metallurgical requirements for Type 316 (Grade 1) and Type 316L (Grade 2) while ASTM F 56 for sheet and strip 5 was published one year later. The air-melted implant compositions performed reasonably well but it was recognized that premium melted material would be an advantage. Remelted material was capable of providing improved homogeneity, controlled microcleanliness, and better corrosion resistance. ASTM F 138 for special quality bar and wire was published in 1971 6 while ASTM F 139 for special quality sheet and strip 7 was issued in 1976. An AO Bulletin entitled Characteristics of the Stainless Steel AO Implants was published in 1975 by O. Pohler and F. Straumann 8. This bulletin documented the composition and metallurgical features of AO implant quality stainless steel that was used for surgical implants during this timeframe. Stainless steel melting practice typically includes electric arc melting to produce an electrode followed by argon oxygen decarburization (AOD) and premium remelting of the electrode into individual ingots. The ingots are hot pressed or rotary forged into intermediate-sized billets and blooms for further processing such as hot rolling. Hot rolled round, flat, and profiles may be further processed into bar, wire, sheet, strip, and special shapes using conventional wrought stainless steel metalworking methods. Specialty stainless steel cold finishing operations such as annealing, cleaning, and cold drawing are used for bar and wire while bar finishing may also include straightening, centerless grinding, and polishing. Wide flat rolled strip product may be straightened, slit, edged, and cut to length.
2
Other wrought stainless steels such as nitrogen strengthened high manganese alloys 9-10 may also be used for the manufacture of surgical implants but the present discussion will focus on AO 18% chromium14% nickel-2.5% molybdenum stainless steel, which is also known as implant quality 316L stainless steel in the United States and implant quality DIN 1.4441 stainless steel in Europe.
3
Basic Metallurgy
1. Composition
The chemical requirements for implant quality stainless steel are documented in ASTM F 138 11 and ISO 5832-1 12 specifications. A similar analysis of stainless steel is also available as ASTM A276 13 commercial quality alloy with different composition limits and microstructure features when compared to implant quality. The composition limits for implant quality and commercial quality stainless steel are compared.
Comparison Between Implant Quality and Commercial Quality Stainless Steel
Chemical Requirements (%)A Implant Quality
Commercial Quality
Element
ASTM F 138
ISO 5832-1
ASTM A 276
C
0.030
0.030
0.030
Mn
2.00
2.0
2.00
P
0.025
0.025
0.045
S
0.010
0.010
0.030
Si
0.75
1.0
1.00
Cr
17.00-19.00B
17.0-19.0B
16.00-18.00
Ni
13.00-15.00
13.0-15.0
10.00-14.00
Mo
2.25-3.00B
2.25-3.5B
2.00-3.00
N
0.10
0.10
0.10
Cu
0.50
0.50
—
Fe
Balance
Balance
(Balance)
A. Maximum, unless otherwise noted
B. %Cr + 3.3 X %Mo ≥ 26
The ASTM and ISO implant composition limits are nearly identical with slight differences in maximum silicon and molybdenum content. The following discussion highlights the relative importance of implant quality ASTM F 138 composition limits when compared to commercial quality ASTM A 276. Lower phosphorus content provides somewhat better ductility, especially for the majority of surgical implants that are moderately or highly 4
cold worked. The reduction in maximum sulfur content from 0.030% to 0.010% has a favorable effect on the volume fraction of sulfide inclusions. The importance of manganese sulfide (MnS) control in implant quality stainless will be reviewed later in the discussion regarding surface properties. Reduced silicon content is responsible for a decrease in silicate-type inclusions and provides better austenite stability. Higher chromium content has a favorable effect on corrosion resistance but has a negative influence on austenite stability. The nominal nickel content in implant quality stainless is significantly higher than in commercial quality and is primarily responsible for maintaining a completely austenitic microstructure. High nickel content minimizes the tendency to form delta ferrite ( δ-ferrite) while cold working response is decreased because of the inverse relationship with nickel content. The minimum 2.25% molybdenum content and higher chromium content ensures that the compositional requirement %Cr + 3.3 X %Mo ≥ 26 will be met. This is a unique requirement for implant quality stainless steel and will be described in more detail in the corrosion discussion. The limitation on copper content is a secondary method of controlling tramp elements that may be present in revert material that is used in the melting process. 2. Microcleanliness
ASTM and ISO standards establish nonmetallic inclusion limits for implant quality bar, wire, sheet, and strip when evaluated according to Method A of ASTM E 45 14. Microcleanliness limits are tabulated.
Inclusion Limits for Implant Quality Stainless Steel
Maximum Limits Inclusion Type
Thin
Heavy
A (Sulfide)
1.5
1.0
B (Alumina)
1.5
1.0
C (Silicate)
1.5
1.0
D (Globular Oxide)
1.5
1.0
Bar and wire inclusion ratings are usually performed on billet or bar samples while sheet and strip are rated at an intermediate hot rolled stage. In the past, double vacuum melting such as vacuum induction melt (VIM) + vacuum arc remelt (VAR) have been used to ensure that 5
Basic Metallurgy continued
the inclusion limits would be met. The implant quality suppliers developed a significant database over the years that verified double vacuum melting was not required to meet the inclusion limits. Alternate melting practices were established for implant quality stainless which included electric arc + argon oxygen decarburization (AOD) refining + VAR in the United States. European melt practice was similar but included electric arc + AOD or vacuum ladle refining + electroslag remelt (ESR). VAR practice tends to minimize Type D globular oxides because oxygen content is reduced during vacuum remelting. The volume fraction of Type A sulfides are usually lower in ESR ingots because the slag layer can be formulated to desulfurize the arc melted electrode. 3. Microstructure Carbides
Carbides of the M6C type have been observed 15 in the higher carbon Type 316 alloy after prolonged heating in the 800 –1200°F sensitization range. M6C tends to precipitate intergranularly and has an adverse effect on intergranular corrosion resistance. This observation provided technical justification for the eventual widespread use of low carbon 316L rather than higher carbon 316 for surgical implants. Each lot of implant quality stainless steel must be capable of passing Practice E of ASTM A 262 16 intergranular corrosion susceptibility test. Delta Ferrite/Chi/Sigma
ASTM F 138, ASTM F 139 and ISO 5832-1 implant standards specify that implant microstructures must contain no δ-ferrite, chi, or sigma phases when examined at 100X magnification. Longitudinal specimens may be etched in a mixture of copper chloride, hydrochloric acid, and ethanol to reveal the presence of δ-ferrite. An electrolytic etch in potassium hydroxide may also be used to confirm δ-ferrite which will be visible as a blue stain. Delta ferrite is considered an objectionable secondary phase in implant quality stainless steel because of inferior corrosion resistance and magnetic permeability when compared to the austenitic matrix. Chi and sigma phases may be identified according to methods outlined in ASTM E407 Practice for Microetching Metal and Alloys. The specialty steel producers may also use weld metal constitution diagrams such as Delong or Schaeffler to estimate the amount of δ-ferrite in each remelted ingot. Numerous calculations over the years have indicated that δ-ferrite will typically not be observed in the micro-
6
structrue of homogeneous remelted material when the calculated delta ferrite content is less than 1.5% based on chromium and nickel equivalents. The calculated δ-ferrite values represent a preliminary estimate since finish mill products must be metallographically examined to verify that δ-ferrite is not present in the microstructure. Grain Size
Implant quality bar, wire, sheet, and strip must meet a grain size of five or finer according to prevailing ASTM standards. The actual grain size area becomes smaller as the ASTM grain size number increases. A fine grain size is desirable to provide a good combination of tensile and fatigue properties. The specialty stainless suppliers measure and certify the ASTM grain size after the last annealing operation. However, the majority of implant quality stainless steel is ordered in the moderate or highly cold worked condition. The implant standards specify that the grain size should be measured on a transverse sample if the material is rated in the cold worked condition. This compensates somewhat for the grain elongation that occurs in the longitudinal plane during unidirectional cold working. Implant device manufacturers may also specify restrictions on grain size uniformity. For example, a grain size of ASTM 7.5 or finer may be acceptable as long as the material does not exhibit duplex grains that differ by more than 2.5 ASTM grain size ratings. Photomicrographs of annealed implant quality stainless steel with an ASTM grain size of 6.5 and moderately cold worked implant quality stainless steel with a grain size of ASTM 6.5 are shown. The photomicrographs were provided by Dr. Lyle Zardiackas, University of Mississippi Medical Center.
Etched Transverse Microstructure of Annealed Implant Quality Stainless Steel (100X)
Etched Transverse Microstructure of Cold Worked Implant Quality Stainless Steel (100X)
7
Basic Metallurgy continued
Medical device manufacturers commonly recertify lots of implant quality stainless steel as part of their Quality Assurance program. Some small diameter wire products may be ordered in the extra hard condition with an ultimate tensile strength around 1400 MPa. It is extremely difficult to accurately measure austenite grain size in material with this amount of microstructure distortion. In these instances, the recertification laboratory will normally document the grain size rating certified by the supplier. Magnetic Permeability
The microstructure and composition will also influence the magnetic permeability of stainless steel. Some types of austenitic stainless steel will become magnetic during cold working or fabrication. The metal deformation that occurs during cold working can create a solid state austenite to martensite phase transformation. Martensite is a ferromagnetic or highly magnetic phase. An increase in magnetic permeability due to cold working or the presence of a secondary magnetic phase such as δ-ferrite are not desirable since patients with implanted devices may be subjected to magnetic resonance imaging (MRI) procedures. Microstructures must be completely nonmagnetic to avoid implant heating or movement of the implant during MRI. Stainless grades which contain molybdenum will not become magnetic during cold working because the stabilized austenitic microstructure is resistant to strain induced martensite phase transformation at room temperature. Also, implant quality stainless steel does not contain secondary magnetic phases in the microstructure. Magnetic permeability measurements 17 have confirmed that negligible magnetic response is obtained with highly cold worked (extra hard) AO implants.
8
Properties
1. Physical
Important physical properties18 are compiled for implant quality stainless steel. Physical Properties of Implant Quality Stainless Steel
Density (g/cm3)
Modulus of Elasticity in Tension (GPa)
Electrical Resistivity (microhm•mm)
Mean Coefficient of Thermal Expansion From 293 –873°K (10-6 /K)
7.95
186.4
740
18.5
Thermal Conductivity at 373°K (W•m/m2•K) 16
The density of stainless steel is not critical since the weight of relatively small-sized fracture fixation implants is not considered a major material factor. The modulus of elasticity is a measure of the stress per unit strain in the elastic region. The relative stiffness of an implant material is directly related to the modulus of elasticity. The stiffness increases as the modulus increases. The modulus of stainless steel is about 80% greater than unalloyed (Commercially Pure) titanium. Stainless implants will be significantly stiffer than titanium implants of the same general dimensions. In addition, the implant design will also influence the stiffness or flexibility of an implant system.
9
Properties continued
2. Tensile
The tensile properties of implant quality stainless steel bar and wire are specified in the latest revisions of ASTM F 138 and ISO 5832-1. Although the size ranges may differ, the tensile property limits specified in ASTM and ISO material standards are essentially the same. Tensile Properties of Implant Quality ASTM F 138 Bar and Wire and ISO 5832-1 Bar
Standard
Condition
Diameter or Thickness (mm)
Ultimate Tensile Strength (MPa)
Minimum 0.2% Yield Strength (MPa)
Minimum Elongation in 4D or 4WA (%)
ASTM F 138 ISO 5832-1
Annealed Annealed
≥ 1.60 All
min 490 490–690
190 190
40 40
ASTM F 138 ISO 5832-1
Cold Worked Cold Worked
1.60– 38.1 ≤ 22
min 860 860–1100
690 690
12 12
ASTM F 138 ISO 5832-1
Extra Hard Extra Hard
1.60–6.35 ≤8
min 1350 min 1400
— —
— —
A. 4D = 4 X diameter; 4W = 4 X width. Alternately, a gage length corresponding to ISO 6892 19 may be used.
The ISO bar standard includes a maximum limit for ultimate tensile strength in the annealed or coldworked conditions, which is not a requirement in the ASTM specification. The size differences are primarily related to product definitions since ASTM defines a size less than 1.60 mm diameter as fine wire while ISO defines all sizes less than 2 mm diameter as wire. Additional tensile requirements for ASTM fine wire and various ISO wire sizes are specified in the respective material standards. The annealed condition represents the lowest strength condition. This condition is generally preferred for the manufacture of cerclage wire and reconstruction plates where a low strength is satisfactory but a maximum amount of ductility is needed. Annealed tensile properties for cerclage wire are specified in a separate ASTM F 1350 specification20. The cold worked condition is an intermediate strength condition used for the manufacture of bone screws, bone plates, intramedullary nails, etc. The extra hard condition is the highest strength condition used primarily for small diameter wire products such as Kirschner wire and 10
Schanz screws where increased resistance to permanent bending deformation (high yield strength) is most important. Mechanical properties for sheet and strip are covered in ASTM F 139 21 and ISO 5832-1 industry standards. Sheet and strip minimum tensile properties are tabulated as follows. Tensile Properties of Implant Quality Stainless Steel Sheet and Strip
Standard
Condition
Ultimate Tensile Strength (MPa)
Minimum 0.2% Yield Strength (MPa)
Minimum Elongation in 50 mmA (%)
ASTM F 139 ISO 5832-1
Annealed Annealed
min 490 490–690
190 190
40 40
ASTM F 139 ISO 5832-1
Cold Worked Cold Worked
min 860 860–1100
690 690
10 10
A. Or 5.65√ So, where So is the original cross sectional area, in mm 2.
A comparison of ASTM F 139 with ISO 5832-1 indicates identical minimum tensile properties for sheet and strip in the annealed or cold worked condition. The ISO requirements for sheet and strip also include a maximum ultimate tensile strength for the annealed or cold worked conditions. Implant quality stainless steel may be cold worked to increase the ultimate tensile strength. Data has been published by Carpenter Technology 18 that defines the mechanical property relationships for wire product with different levels of cold work. The ultimate tensile strength, 0.2% yield strength, and hardness increase as the percentage of cold work increases. The % elongation tends to decrease as the % cold work increases. The % reduction of area is actually a better measure of the ductility than % elongation and the decrease in the reduction of area is more consistent as a function of increasing cold work. These types of mechanical property relationships are very typical for austenitic stainless steels.
11
Properties continued
Typical Tensile Properties and Hardness of Implant Quality Stainless Steel as a Function of Cold Work
Condition
Cold Work (%)
Ultimate Tensile Strength (MPa)
0.2% Yield Strength (MPa)
Annealed
0
586
Cold Worked
35 48 52 60 70 80
862 1000 1034 1103 1172 1241
Elongation (%)
Reduction of Area (%)
Hardness (HRC)
248
57
88
88 HRB
793 827 848 883 896 945
18 16 16 16 17 13
72 69 65 62 60 57
26 32 34 36 38 40
3. Fatigue
Fatigue is the phenomenon leading to fracture under repeated or fluctuating stresses having a maximum value less than the ultimate tensile strength of the material. The fatigue loads are well below the level which normally would be required to cause fracture within a single load cycle. The fatigue cycle is the time interval during which the stress is regularly repeated. There are many material variables that influence fatigue test results such as grain size, processing history, surface finish, and degree of cold work. Testing variables include type of alternating load, specimen geometry, frequency, and test environment. A common method of expressing the fatigue properties of a material is known as the S-N curve. The S-N curve is a best-fit plot of the individual test values where S refers to the applied cyclic stress and N is the number of cycles required to fracture the test specimens. S-N curves for AO implant quality stainless steel are reproduced from previous research work performed by O. Pohler 22 at Stratec Medical. The fatigue curves describe the stress concentration (MPa) versus the number of cycles to failure (N) for fully reversed bending fatigue of recrystallized (annealed) or cold worked implant quality stainless steel specimens in air and in Ringer’s solution. Ringer’s solution is a physiological solution containing a mixture of sodium, potassium, and calcium chlorides plus bicarbonates that approximates the electrolyte and corrosive environment in the human body at a pH of 6.5. The fatigue test results 12
confirm that the cold worked condition provides a higher fatigue life when compared to the annealed or softest condition. This trend is generally observed for most non-brittle engineering materials that can be cold worked to develop higher ultimate tensile strengths. The use of a more severe testing environment such as Ringer’s solution, which contains chloride ions, shows a general decrease in the cycles to failure for a given stress concentration when the material is in the cold worked condition. The aggressive Ringer’s test environment does not have a significant effect on fatigue results for specimens in the annealed condition.
1000
125
Cold worked in air Cold worked in Ringer’s Recrystallized in air Recrystallized in Ringer’s
900
a P M , n o 700 i t a r t n e c 500 n o c s s e r t 300 S
100
75
50
25
i s k , n o i t a r t n e c n o c s s e r t S
0
100 3
10
10
4
10
5
10
6
7
10
Number of cycles to failure
13
Properties continued
The endurance limit is the maximum stress below which a material can presumably endure an infinite number of stress cycles. Fully reversed (R=-1) endurance limit values have been published 23 for implant quality stainless steel as follows: Endurance Limit for Implant Quality Stainless Steel
Condition
Cold Work (%)
Flexural Endurance Limit at 107 Cycles (MPa)
Annealed
0
179
Cold Worked
30 60 80
379 448 483
The flexural fatigue test results verify that the endurance limit increases as the percentage of cold work increases. There is a limitation regarding the amount of cold work that can be introduced into implant quality stainless steel material. Cross-sectional area is a factor since only small- to moderate-size cross sections can be obtained in the highly cold worked condition. Large cross-sectional areas cannot be cold worked the same amount due to processing and equipment limitations. Many fracture fixation implants require a certain balance between strength and ductility (i.e. ability to contour a bone plate) so that it is not always desirable to specify the highest attainable tensile strength. 4. Corrosion Pitting
The Composition section referred to a term known as the compositional index %Cr + 3.3 X %Mo ≥ 26 which is a unique feature of implant quality stainless steel material. The index was derived from a study published by Steinemann in 1980 24 which determined that adequate pitting resistance in the body could only be assured if this relationship was satisfied. The increased chromium and molybdenum content of implant quality stainless steel provide improved corrosion properties in a chloride-containing physiological environment. Calculations indicate that implant quality stainless steel with a nominal 17.50% chromium content must contain at least 2.60% molybdenum to meet the compositional formula.
14
Stress Corrosion Cracking
Stress corrosion cracking (SCC) is a form of corrosion failure that can occur under the combined action of corrosion and stress. The stress may be the result of residual stress in the material and/or the applied stress related to the end-use application. One common test method to evaluate SCC involves the use of smooth hour-glass-shaped specimens that are subjected to slow strain rate tensile tests in very aggressive chemical solutions at elevated temperatures. Typical SCC tests may be performed in boiling solutions of 45% magnesium chloride at 154°C. The presence of high chloride concentrations and elevated temperatures accelerate the occurrence of SCC. Implant quality stainless steel has been evaluated for SCC by Sheehan et al. 25 under laboratory conditions that are more representative of the chloride concentration and temperature that are relevant for surgical implants. Electropolished hour-glass specimens of cold worked implant quality were pulled to failure in lactated Ringer’s solution at 37°C and strain rates of 10 –5, 10 –6, and 10 –7 cm/cm/sec. The slow strain rate tension test results reported by Sheehan were as follows: Slow Strain Rate Test Results for Implant Quality Stainless Steel
Strain Rate (cm/cm/sec)
Ultimate Tensile Strength (MPa)
Elongation (%)
Reduction of Area (%)
10 –5 10 –6 10 –7 10 –6 10 –6
956 947 930 934 947
27 31 24 28 27
43 49 47 43a 46b
a. HCl acidified to pH of 2
b. FeCl3 acidified to pH of 2
A material is considered to be susceptible to SCC if a decrease in strength and/or ductility is observed with a decreasing strain rate or at lower pH values. The results indicate there is no significant change in strength or ductility as a function of strain rate or pH level. SCC did not occur with cold worked implant quality stainless under these test conditions which were meant to simulate a neutral and acidified in vivo corrosion environment. Examination of fractured specimens confirmed that the fractures were ductile cup-cone fractures with no evidence of secondary branching associated with SCC.
15
Properties continued
Fretting
The excellent corrosion resistance of stainless steel is related to the presence of a chromium oxide layer at the surface of the alloy. This corrosion resistant layer is referred to as the passive film. Fretting corrosion is a form of corrosion that can occur when the passive film is mechanically disrupted or removed. The removal of the passive film due to mechanical action in a corrosion environment tends to accelerate the overall corrosion rate and this effect is known as fretting corrosion. Fretting corrosion can occasionally be observed when bone plates and screws are clinically retrieved. Typically, the contact of the underside of the screw head with the bearing surface surrounding the plate hole creates a fretting corrosion condition due to localized passive film disruption. S. Brown and K. Merritt 26 have reported laboratory fretting weight loss results and standard deviations for stainless steel DCP plates and screws. The tests were performed with two-hole plate specimens that were cut from DCP implants and cortical bone screws at one cycle per second for a total of 806,000 cycles. Each two-hole plate and each screw was weighed, in addition to each three-component combination. Hence, there is a difference between the plate + screw versus total weight loss values.
Stainless Steel DCP Fretting Weight Loss After 14 Days
Solution
Number of Samples (N)
Plate (mg)
Screw (mg)
Total (mg)
0.9 % saline + 10 % serum
5 4
1.23 ± 0.37 0.14 ± 0.07
1.21 ± 0.30 0.22 ± 0.08
2.22 ± 0.60 0.32 ± 0.11
Results indicate a significant reduction in weight loss when 10% calf serum was added to the 0.9% saline solution. The weight loss reduction may be a result of a lower coefficient of friction at the fretting interface because of the lubricating quality of the serum. The referenced study also reported that fretting torque loads of 260 N and 560 N were statistically equivalent but fretting torque loads of 860 N provided significantly lower weight loss. The torque load anomaly was explained on the basis of minimal fretting at low loads while the equipment design tended to minimize relative motion at the highest torque
16
load. The laboratory fretting study highlights the observation that fretting corrosion is a complicated phenomena which is influenced by many variables. Proper design of implant dimensional features and optimum surface finishing techniques can minimize the occurrence of clinical fretting corrosion of stainless steel plates and screws. Galvanic
The well known AO study published in 1975 by Dr. Thomas Ruedi 27 examined the clinical implications of mixing stainless steel and titanium implants. The retrospective study was based on a group of 519 human fractures of the upper and lower limbs that utilized titanium DCP plates + stainless screws. A follow-on study involved a series of 75 tibia platings that evaluated various metal and mixed metal combinations. The relative advantages and disadvantages were summarized on the basis of clinical, radiological, and implant tissue retrieval studies. Less titanium wear debris was observed in surrounding tissues for stainless screws + pure titanium plates when compared to pure titanium plates and screws. This suggested that fretting corrosion resistance was improved for this specific mixed implant combination. However, stainless steel corrosion and tissue discoloration were experienced with the mixed metal system. It was concluded that the unalloyed titanium monosystem demonstrated significantly better tissue biocompatibility when compared to the stainless screw + unalloyed titanium plate system. Galvanic corrosion may occur as a consequence of the existing potential difference between dissimilar biomaterials. The following information has been extracted from a 1991 publication by M. Barbosa 28. Two methods commonly used to estimate the effect of mixing metals include the use of mixed potential theory and current flow measurements. The application of mixed potential theory relies on the interpretation of polarization diagrams and is beyond the scope of this review. Current flow measurements or in vitro accelerated corrosion tests can be used to rank materials in terms of corrosion resistance and to decide whether is is safe or unsafe to use various dissimilar couples. From several published papers it is possible to conclude that most materials coupled with implant quality stainless steel are unsafe. The data has been summarized by M. Barbosa as follows:
17
Properties continued
Predicted Behavior of Galvanic Couples
Biomaterial Couple
Behavior
Ti-6Al-4V + Carbon Ti-6Al-4V + Vitallium Stainless Steel + Carbon Stainless Steel + Ti-6Al-4V Stainless Steel + Vitallium
Safe Safe Unsafe Unsafe Unsafe
Vitallium is a cobalt based alloy that is primarily used for total joint prostheses. It should be noted that stainless steel + pure titanium is also viewed as unsafe because the corrosion resistance of pure titanium and titanium alloys (including Ti-6Al-7Nb) are very similar and produce nearly equivalent galvanic reactions when coupled with implant quality stainless steel. The predicted behavior is based on laboratory experiments and is a first approximation of whether clinical problems may be experienced. Mixing of stainless steel implants with unalloyed titanium, titanium alloy, and cobalt alloy implants should be avoided for implants that are in contact with each other. This recommendation is consistent with a critical review of the best available information on the use of dissimilar biomaterials for orthopaedic applications. The use of mixed metals in the same bone is not a problem as long as the implants are not in direct contact with each other. 5. Biocompatibility
In 1974, Desai and Sinkford 29 inserted screw-shaped stainless steel, gold, and platinum alloy implants into rat femurs. The authors found the thinnest connective tissue layer at the stainless steel interface after a 5-week implantation. The stainless steel implants did not provoke an inflammatory response. Pfluger 30 in 1980 implanted different materials in rabbit tibia which included porous and grooved stainless steel, tantalum, and niobium implants. Histological results between 3-week and 6-month implantation revealed that all metals were “equally biocompatible.” Millar et al. performed a histological study 31 in 1990 that evaluated the effect of bone tissue reactions to stainless steel and titanium screws implanted in canine skulls. No significant differences were observed in the tissue reactions between stainless steel and titanium screws after 4-, 12-, and 24-week implantation periods.
18
Gerber and Perren 32 developed an embryonic bone culture test to evaluate metal-related cellular behavior of an extensive array of metals, including stainless steel. In this series of experiments, implant quality stainless steel was well tolerated as evidenced by the appearance of normal cartilage cells near the implant. Soft tissue reactions around stainless steel and titanium LC-DCP bone plates in sheep were evaluated by Ungersbock, Pohler, and Perren 33. A thicker soft tissue capsule layer was observed for the stainless plates when compared to titanium. However, all implants demonstrated good biocompatibility with a low number of round cells, mast cells, macrophages, and polymorphonucleated leukocytes in the soft tissue covering the plates. ASTM F 138 implant quality stainless steel 6 has been successfully used for human implants for nearly 30 years. The alloy has a well characterized level of local biological response and is specified as a control material in ASTM F 981 Standard Practice for Assessment of Compatibility of Biomaterials for Surgical Implants with Respect to Effect of Materials on Muscle and Bone 34. The possibility of metal sensitivity reactions must be considered when evaluating the overall biocompatibility of implant quality stainless steel. The most common metal sensitivity test for human patients involves paravertebral patch testing on the skin of the back. The standard allergens include 2.5% nickel sulfate, 0.5% potassium dichromate, and 1% cobalt chloride solutions dispersed in petrolatum. The test patches remain in situ for 48 hours and the skin reactions are recorded after 48, 72, and 96 hours. Test results reported by S. and G. Hierholzer 35 for 208 human patients who experienced uncomplicated healing with stainless implants indicated about 4% were allergic to nickel and/or chromium. Slightly over 10% of 497 patients who had aseptic or septic complications displayed evidence of metal sensitivity while nickel accounted for greater than 90% of the allergic reactions. 6. Surface
Electropolishing has emerged as one of the most common surface treatments for stainless steel implants. The first step in the electropolishing process consists of securing the implants to special fixtures or loading them into wire baskets. The fixtures or baskets are placed on a copper bar known as the anode in the middle of the process tank. The anode is surrounded by one or more stainless steel plates known as the cathode. An electrical current is applied to the fixtured implants which are immersed in an acidic solution. The electropolishing process
19
Properties continued
removes a measurable amount of metal from the implant surface at a controlled rate and produces a very smooth surface. The amount of metal that is removed is primarily dependent on the applied current and the immersion time. The electropolishing process usually includes a pre-cleaning step to remove oil, grease and loose organic or inorganic surface debris that may be present from the various manufacturing operations. The actual electropolishing treatment tends to remove superficial foreign material, improves the corrosion resistance 36, provides a low friction surface, and creates a passive film 37 on the implant surface. Various types of foreign material may include free iron from machining tools, contact films from various sources, and imbedded particles from surface texturizing operations. A smooth electropolished surface roughness of around 5 microinch AA (Arithmetic Average) is normally obtained for stainless implants and this provides a low coefficient of friction. A reduced coefficient of friction can be an advantage for multicomponent implants where fretting corrosion is a consideration. Sulfur content greater than 0.005% can promote surface blemishes on electropolished stainless steel implants. The manganese sulfides present at the surface of the implant will be preferentially attacked during electropolishing and cosmetic features may be compromised. Uniform surface appearance is not as problematic when bead blasting or shot peening are used to produce a textured surface before electropolishing. Typical implant quality compositions rarely exceed 0.002% sulfur content and electropolishing problems will be minimized regardless of whether a smooth or matte finish is desired. Implants which are not electropolished must be chemically passivated to restore maximum corrosion resistance. Various nitric acid concentrations and temperatures for the chemical passivation of stainless steel surgical implants are compiled in ASTM F 86 specification 38. Surface operations such as electrochemical etching or laser marking may be used to identify stainless steel implants. Selective nitric acid repassivation is generally required to improve the corrosion resistance of the electrochemically or laser marked area.
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Clinical Features
Biocompatibility
Successful human implantation for over 30 years. Galvanic corrosion
Stainless steel implants should not be in contact with titanium-based or cobalt-based implants. Mechanical
Stainless steel implants have a good combination of strength and ductility. Metal sensitivity
A small percentage of patients may be allergic to nickel. MRI
Starburst or signal void effects may interfere with diagnostic imaging. Sterilization
Steam autoclave, ETO, gamma radiation, electron beam, and other methods may be used. Surface
Electropolishing provides low frictional surface properties.
21
References
1.
Laing, P. G., “Clinical Experience with Prosthetic Materials: Historical Perspectives, Current Problems, and Future Directions,” Corrosion and Degradation of Implant Materials, ASTM STP 684, B. C. Syrett and A. Acharya, Eds., American Society for Testing and Materials, 1979, p. 202.
2.
Peterson, L., “Fixation of Bones by Plates and Screws,” Journal of Bone and Joint Surgery, Vol. 29, 1947, pp. 335-347.
3.
Blunt, J. W., Jr., Hudack, S. S., and Murray, C. R., “Metals and Plastics in Orthopedic Surgery and General Surgery,” Clinical Congress, American College of Surgeons, New York, 1952.
4.
ASTM F 55-65T Standard Specification for Stainless Steel Bar and Wire for Surgical Implants, American Society for Testing and Materials, West Conshohocken, PA.
5.
ASTM F 56-66 Standard Specification for Stainless Steel Sheet and Strip for Surgical Implants, American Society for Testing and Materials, West Conshohocken, PA.
6.
ASTM F 138-71 Standard Specification for Stainless Steel Bar and Wire for Surgical Implants (Special Quality), American Society for Testing and Materials, West Conshohocken, PA.
7.
ASTM F 139-76 Standard Specification for Stainless Steel Sheet and Strip for Surgical Implants (Special Quality), American Society for Testing and Materials, West Conshohocken, PA.
8.
Pohler, O. and Straumann, F., “Characteristics of the Stainless Steel ASIF/AO Implants,” Institute Straumann AG, Waldenburg, Switzerland, September 1975.
9.
ASTM F 1314 Standard Specification for Wrought Nitrogen Strengthened-22 Chromium-12.5 Nickel-5 Manganese-2.5 Molybdenum Stainless Steel Bar and Wire for Surgical Implants, American Society for Testing and Materials, West Conshohocken, PA.
10. ASTM F 1586 Standard Specification for Wrought Nitrogen Strengthened-21 Chromium-10 Nickel-3 Manganese-2.5 Molybdenum Stainless Steel Bar for Surgical Implants, American Society for Testing and Materials, West Conshohocken, PA. 11. ASTM F 138 Standard Specification for Wrought 18 Chromium14 Nickel-2.5 Molybdenum Stainless Steel Bar and Wire for Surgical Implants (UNS S31673), American Society for Testing and Materials, West Conshohocken, PA.
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12. ISO 5832-1, Implants for Surgery-Metallic materials-Part 1: Wrought stainless steel, International Organization for Standardization, Geneva, Switzerland. 13. ASTM A 276 Standard Specification for Stainless Steel Bars and Shapes, American Society for Testing and Materials, West Conshohocken, PA. 14. ASTM E 45 Standard Test Methods for Determining the Inclusion Content of Steel, American Society for Testing and Materials, West Conshohocken, PA. 15. Mills, K., Davis, J. R., Dieterich, D. A., Crankovic, G. M., and Frissell, H. J., “Metallographic Techniques and Microstructures: Specific Metals and Alloys,” Metals Handbook Ninth Edition: Metallography and Microstructure, American Society for Metals, Metals Park, OH, 1985, pp. 284-289. 16. ASTM A 262 Practices for Detecting Susceptibility to Intergranular Attack in Austenitic Stainless Steels, American Society for Testing and Materials, West Conshohocken, PA. 17. Disegi, J. A., “Magnetic resonance imaging of AO/ASIF stainless steel and titanium implants,” Injury, AO/ASIF Scientific Supplement, Volume 23, Supplement 2, 1992, pp S1-S4. 18. Carpenter Technology Corporation Alloy Data, Biodur™ 316LS Stainless, Medical Implant Alloys, Carpenter Technology Corp., Reading, PA, 2/95. 19. ISO 6892 Metallic Materials-Tensile Testing, International Organization for Standardization, Geneva, Switzerland. 20. ASTM F 1350 Standard Specification for Stainless Steel Surgical Fixation Wire, American Society for Testing and Materials, West Conshohocken, PA. 21. ASTM F 139 Standard Specification for Wrought 18 Chromium14 Nickel-2.5 Molybdenum Stainless Sheet and Strip for Surgical Implants (UNS S31673), American Society for Testing and Materials, West Conshohocken, PA. 22. Pohler, O., “Failures of Metallic Orthopedic Implants,” Metals Handbook, Ninth Edition, Volume 11 Failure Analysis and Prevention, American Society for Metals, Metals Park, OH, 1985,
pp 683-688. 23. Shetty, R. H., et al. Metals in Orthopedic Surgery, Encyclopedia Handbook of Biomaterials and Bioengineering, Part B Applications, Volume 1, Marcel Dekker, 1995, p. 536. 23
References continued
24. Steinemann, S. G., “Corrosion of surgical implants: in vivo and in vitro tests,” Evaluation of Biomaterials, G.D. Winter, J.L. Leray, and K. deGroot, Eds., John Wiley and Sons Inc., New York, 1980, pp 11-34. 25. Sheehan, J. P. et al., “Study of Stress Corrosion Cracking Susceptibility of Type 316L Stainless Steel In Vitro,” Corrosion and Degradation of Implant Materials: Second Symposium, ASTM STP 859, A. C. Fraker and C. D. Griffin, Eds., American Society for Testing and Materials, Philadelphia, 1985, pp 57-72. 26. Brown, S. A., and Merritt, K., “Fretting Corrosion of Plates and Screws: An In Vitro Test Method,” Corrosion and Degradation of Implant Materials: Second Symposium, ASTM STP 859, A. C. Fraker and C. D. Griffin, Eds., American Society for Testing and Materials, Philadelphia, 1985, pp 105-116. 27. Ruedi, T. P., “Titan und Stahl in der Knochenchirurgie,” Heft 123, Springer-Verlag, 1975. 28. Barbosa, M. A. “Corrosion Mechanisms of Metallic Biomaterials,” Biomaterials Degradation: Fundamental Aspects and Related Clinical Phenomena, Chapter 9, M. A. Barbosa (Ed.), Elsevier
Science Publishers BV, The Netherlands, 1991, pp 250-253. 29. Desai, R. J. and Sinkford, J. C., “Tissue response to intraosseous implants in albino rats”, Oral Surgery, Oral Medical Oral Pathology, Volume 37, 1974, p. 26. 30. Pfluger, G., et al., “Bone reactions to porous and grooved stainless steel, tantalum, and niobium implants,” Biomaterials, G. D. Winter, D. F. Gibbons, and H. Plenk Jr., (Eds.), 1980, pp 45-50. 31. Millar, B. G., Frame, J. W., and Browne, R. M., “A histological study of stainless steel and titanium screws in bone”, British Journal Oral Maxillofacial Surgery, Volume 28, 1990, pp 92-95. 32. Gerber, H. and Perren, S. M., “Evaluation of Tissue Compatibility of in vitro Cultures of Embryonic Bone,” Evaluation of Biomaterials, G. D. Winter, J. L. Leray, and K. deGroot, (Eds.), 1980, pp 307-314. 33. Ungersbock, A., Pohler, O. E. M., and Perren, S. M., “Evaluation of soft tissue reactions at the interface of titanium limited contact dynamic compression plate implants with different surface treatments: an experimental sheep study,” Biomaterials, Vol. 17, No. 8, 1996, pp 797-806.
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34. ASTM F 981 Standard Practice for Assessment of Compatibility of Biomaterials for Surgical Implants with Respect to Effect of Materials on Muscle and Bone, American Society for Testing and Materials, West Conshohocken, PA. 35. Hierholzer, S. and Hierholzer, G., Internal Fixation and Metal Allergie, Thieme Medical Publishers, New York, 1992. 36. Irving, C. C., Jr., “Electropolishing Stainless Steel Implants,” Corrosion and Degradation of Implant Materials: Second Symposium, ASTM STP 859, A. C. Fraker and C. D. Griffin, (Eds.),
American Society for Testing and Materials, Philadelphia, 1985, pp 136-143. 37. ASTM A 967 Standard Specification for Chemical Passivation Treatments for Stainless Steel Parts, American Society for Testing and Materials, West Conshohocken, PA. 38. ASTM F 86 Standard Practice for Surface Preparation and Marking of Metallic Surgical Implants, American Society for Testing and Materials, West Conshohocken, PA.
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Glossary
ALLOY. Metallic substance composed of two or more elements, at least one of which is a metal. ALLOYING ELEMENT. An element added to and remaining in a metal that changes the metallic structure and properties. ANNEALING. Metal-softening operation in which the alloy is heated to and held at a specific temperature, followed by cooling at a controlled rate. ANODIC REACTION. Oxidation reaction that produces electrons at the anode of an electrochemical cell. ARGON OXYGEN DECARBURIZATION (AOD). Stainless steel refining process in which a mixture of argon and oxygen gas is injected through the molten alloy to reduce the carbon content and to remove impurities. ASEPTIC. Free from septic matter. AUSTENITE. The normal face-centered cubic crystalline structure of implant quality stainless steel with good formability and nonmagnetic properties. BILLET. Semifinished round or square product that has been hot worked by forging, rolling, or extrusion. BLOOM. Semifinished rectangular hot rolled product that is usually larger than a billet. 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. Reduction reaction that consumes electrons at the cathode of an electrochemical cell. COLD WORKED MICROSTRUCTURE. A microstructure resulting from cold working the material. COLD WORKING. Permanently deforming a metal or alloy at room temperature to increase its strength. DESCALING. Chemically or mechanically removing the thick oxide layer that is formed on alloys during high temperature processing. DELTA FERRITE. A secondary magnetic phase that can be present in austenitic stainless steel but is not permitted in implant quality stainless steel. DUCTILITY. The ability to permanently deform before fracturing. 26
ELECTRODE. Cylindrical metal ingot that is suitable for remelting. ELECTROPOLISHING. Metal finishing process in which a smooth, highly polished surface is produced by an electrochemical anodic reaction in an electrolytic cell. ELECTROSLAG REMELTING (ESR). Consummable electrode melting process in which an electrode is melted into an ingot by the passage of electric current through a conductive slag layer at the surface of a mold. ELONGATION. A term that describes ductility by measuring the amount of extension that a material undergoes during tensile testing. ENDURANCE LIMIT. The maximum stress below which a material can presumably endure an infinite number of stress cycles. EXTRA HARD. Metalworking term used to describe material that has been subjected to a high amount of cold work. 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 specific nature occurs. FATIGUE STRENGTH. The maximum stress that can be sustained for a specified 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 at the surface of a metal or alloy is mechanically disrupted in a corrosive environment. The relative motion of the underside of a bone screw head with the contact surface of a bone plate is a typical example. GALVANIC CORROSION. Corrosion associated with the current of a galvanic cell consisting of two dissimilar conductors in an electrolyte. GRAIN SIZE. A measure of the area or volume of grains in a metal or alloy. HOT WORKED MICROSTRUCTURE. Microstructure resulting from hot working the material. HOT WORKING. Permanently deforming a metal or alloy at an elevated temperature that is usually above the recrystallization temperature.
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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. LONGITUDINAL. Parallel to the principal direction of hot or cold working. MACROPHAGE. A cell having the ability to ingest bacteria and foreign particles. MARTENSITE. Generic term for an acicular microstructure formed by a diffusionless phase transformation. A martensitic stainless steel microstructure is magnetic. MAST CELL. Connective tissue cell. MICROCLEANLINESS. A relative measure of the amount of nonmetallic inclusions that are present in a metal or alloy. MICROSTRUCTURE. The structure of metals and alloys as revealed by microscopic examination of specimens. MODULUS OF ELASTICITY. A measure of the stress per unit strain in the elastic region before permanent deformation occurs. PARAVERTEBRAL. Alongside or near the vertebral column. PASSIVATION. The process of changing the chemical activity of a metallic surface to a less reactive state, usually to increase the corrosion resistance. PASSIVE FILM. Protective surface film formed on a metal or alloy as a result of a passivation reaction. PATHOGEN. A microorganism or substance capable of producing disease. PERMEABILITY. A generic term which is the ratio of the induction to the magnetic force under specific magnetizing conditions. PICKLING. Chemical removal of the thick oxide layer that is formed on metals and alloys during high temperature processing. PITTING CORROSION. A corrosion process that forms small sharp cavities in a metallic surface. POLYMORPHONUCLEATED LEUKOCYTE. White blood cell with a nucleus composed of two or more lobes or parts. RECRYSTALLIZATION. A change from one crystal structure to another that occurs during heating or cooling through a critical temperature range.
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REDUCTION OF AREA. The difference, expressed as a percentage of original area, between the original cross-sectional area of a tensile test specimen and the minimum cross-sectional area measured after complete separation. ROUND CELL. Spherical or globular shaped cell. SENSITIZATION. A condition of being made sensitive to a specific substance such as a protein, pollen, or metal. SEPTIC. Pertaining to pathogenic organisms or their toxins. SIGMA PHASE. A hard, brittle, nonmagnetic secondary phase with a tetragonal crystal structure that can be present in austenitic stainless steel. STAINLESS STEEL. An iron base alloy that contains at least 10% chromium as the principal alloying element. STRAIN. Change in length per unit length in the direction of the applied stress. STRAIN RATE. The time rate of straining for the usual tensile test. STRESS. Force per unit area. STRESS CORROSION CRACKING. Failure of metals and alloys by cracking under combined action of corrosion and either residual and/or applied stress. TRANSVERSE. Perpendicular to the principal direction of hot or cold working. ULTIMATE TENSILE STRENGTH. In tensile testing, the maximum load at fracture divided by the original cross-sectional area. VACUUM ARC REMELTING (VAR). Consummable electrode melting process in which an electric arc is used to remelt an electrode into an ingot in a vacuum chamber. VACUUM INDUCTION MELTING (VIM). A process for melting and refining metals in which the metal is melted inside a vacuum chamber by induction heating. WROUGHT. A metal or alloy that has been shaped by pressing, forging, rolling, or some other elevated temperature deformation process. YIELD STRENGTH. In tensile testing, the stress at which the stress to strain ratio exhibits a specified deviation, usually designated as 0.2% offset.
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