Selection of stainless steels for surgical implants
Introduction
Stainless steels for surgical implants are specified in ISO 5832. Part 1, (1997), covers two steel types, compositions D and E. Part 9, (1992), covers a single high nitrogen grade.
Both these ISO standards are incorporated into British Standards as BS 7252, parts 1:1997 and 9:1993, respectively. The other parts of these standards cover other alloys for implants, including titanium, nickel and cobalt alloys.
Stainless steel grades for surgical implants
The three grades listed in these standards are austenitic types with specific compositions for these special applications. The grades in Part 1 are sometimes wrongly referred to as the ‘316 implant grades’, although the chromium, nickel and molybdenum contents do overlap that of some of the commercially available 316 steels.
THERE ARE NO EQUIVALENT STEEL GRADES IN BS EN 10088 OR THE OBSOLETE BS 970 STANDARDS.
The compositions, in percent, are shown below:
| Elements |
ISO 5832-1 ‘D’ |
ISO 5832-1 ‘E’ |
ISO 5832-9 |
| Carbon – C |
0.030 |
0.030 |
0.08 |
| Silicon – Si |
1.0 |
1.0 |
0.75 |
| Manganese – Mn |
2.0 |
2.0 |
2-4.25 |
| Phosphorus – P |
0.025 |
0.025 |
0.025 |
| Sulphur – S |
0.010 |
0.010 |
0.01 |
| Nitrogen – N |
0.10 |
0.10-0.20 |
0.25-0.5 |
| Chromium – Cr |
17.0-19.0 |
17.0-19.0 |
19.5-22 |
| Molybdenum – Mo |
2.25-3.5 |
2.35-4.2 |
2-3 |
| Nickel -Ni |
13.0-15.0 |
14.0-16.0 |
9-11 |
| Copper – Cu |
0.50 |
0.50 |
0.25 |
| Niobium – Nb |
– |
– |
0.25-0.8 |
Figures are maxima, unless a range is shown.
Additional requirements for implant stainless steels
Composition
ISO 5832-1 also requires that the basic pitting index, expressed as C=3.3 Mo + Cr, is 26 minimum for steel compositions D and E.
In addition to the special composition limits, there are additional requirements that preclude the use of standard ‘316’ grades, produced by normal, commercial, steelmaking methods.
Grain size and ferrite level
All steels must have an ASTM grain size finer, (i.e. a higher number), than No. 4 and be free from detectable ferrite
Non-metallic inclusion levels (steel cleanness)
Both standards require that the non-metallic inclusion levels are measured when the steel is in the billet form and set limits for sulphide, alumina, silicate and oxide inclusion types. Although not made mandatory by the standards, vacuum or electroslag re-melt steelmaking methods are suggested to meet the specified steel cleanness levels.
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