Selection of stainless steels for surgical instruments

Introduction

Stainless steels for surgical instruments are specified in BSENISO 7153-1:2001, which incorporates BS 5194-1:1991. This edition renumbered the previous ISO 7153-1:1991 as the BSENISO 7153-1:2001. This covers metallic materials for surgical instruments, part one covering only stainless steels.

Grades

The standard gives each steel grade a 'Reference Letter', with only cross references to ISO 4957 and ISO 683-13 standard grades. There are 11 martensitic steels, 1 ferritic steel and 4 austenitic steels in Table 2 of the standard. Most instrument manufacturers regard these stainless steel grades as generic and tend to also refer to European or national standards.

The compositions are summarized below.

Steel grade Chemical composition %
Ref ISO4957 ISO683-13 C Si max Mn max P max S Cr Mo Ni Others
Martensitic steels
A - 3 0.09-0.15 1 1 0.04 0.03x 11.5-13.5 - 1x -
B 27 4 0.16-0.25 1 1 0.04 0.03x 12-14 - 1x -
C 28 5 0.26-0.35 1 1 0.04 0.03x 12-14 - 1 -
D - - 0.42-0.50 1 1 0.04 0.03x 12.5-14.5 - 1x -
E - - 0.47-0.57 0.5 1 0.03 0.025x 13.7-15.2 - 0.5x -
F - - 0.6-0.7 0.5 1 0.03 0.025x 12-13.5 - 0.5x -
G - - 0.65-0.75 1 1 0.04x 0.03x 12-14 0.5x 1x -
H - - 0.35-0.4 1 1 0.045 0.03x 14-15 0.4-0.6 - V:0.1-0.15
I - - 0.42-0.55 1 1 0.045 0.03x 12-15 0.45-0.9 - V:0.1-0.15
K 30 - 0.33-0.43 1 1 0.03 0.03x 15-17 1-1.5 1x -
R - - 0.85-0.95 1 1 0.045 0.03x 17-19 0.9-1.3 - V:0.07-0.12
Ferritic steels
L - 8a 0.08x 1 1.5 0.06 0.15-0.35 16-18 0.06x 1x -
Austenitic steels
M - 11 0.07x 1 2 0.045 0.03x 17-19 - 8-11 -
N - 17 0.12x 1 2 0.06 0.15-0.35 17-19 8-10 -
O - 14 0.15x 1 2 0.045 0.03x 16-18 - 6-8 -
P - 20 0.07x 1 2 0.045 0.03x 16.5-18.5 2-2.5 10.5-13.5 -

These grades do not match steel numbers in EN 10088-1, but the following comparisons may be useful in identifying these steel types. These alternatives are for guidance only and must not be used when specifying instruments to BSENISO 7153-1:2000

BSENISO 7153-1 BS (AISI) Type / EN 10088-1 Nearest
A 410S21 / 1.4006
B 420S29 / 1.4021
C 420S45 / 1.4028
D 1.4034
I 1.4116
K 1.4122
L 1.4105
M 304S31 / 1.4301
N 303S31 / 1.4305
O 301S21 / 1.4310
P 316S31 / 1.4401

Applications

Table 1 of the standard gives examples of the applications for which each of the grades is suitable. These are under general headings of 'cutting instruments', 'non-cutting instruments' and 'fitting parts and other assemblies'. The standard should be consulted for this comprehensive list of preferred uses.
The instrument types mentioned are suitable for both dental and surgical instruments.

Grades A, B and C would be generally described as 410 / 420 types and are used extensively for dental and surgical instruments. They offer moderate corrosion resistance in comparison to other types of stainless steel (eg austenitic and duplex grades). They are used for applications where cutting edges, wear resistance and strength are required. A good combination of corrosion resistance and a range of mechanical strength via heat treatment can be expected from these grades.

Long service lives should be expected from martensitic stainless steel dental and surgical instruments, properly manufactured and subjected to appropriate cleaning procedures. For example, dental extraction forceps usually have an average service life of 15 years. There are some examples where such instruments have given 30 years service life. Other more delicate instruments and those with cutting edges may be expected to have a much shorter service lives, but they should not be expected to suffer corrosion damage.

Corrosion resistance

Corrosion problems associated with martensitic stainless steels tend to be related to either process deficiencies or substances encountered in clinical practice. Process deficiencies include incorrect heat treatment (usually apparent from the distribution of carbides in the microstructure), iron contamination from grinding/finishing operations, over-heating during grinding operations or selection of an inappropriate grade.

Corrosion testing

Most of the ISO product standards for dental and surgical instruments refer to ISO 13402, which specifies corrosion tests based on the methods of sterilisation commonly encountered by these products. Martensitic stainless steels should meet the requirement of ISO product standards with ease (ie resistance to autoclaving, corrosion and thermal exposure).

Sterilization practice

Corrosion problems associated with clinical practice tend to be associated with contact with aggressive substances (eg chloride-containing disinfectants) and/or inappropriate exposure times to such substances. For example, steam sterilisers (autoclaves) should use distilled, de-ionised or sterile water and not tap water for sterilisation or there is a risk of corrosion.

Prior to sterilisation, contaminated instruments may be soaked in a disinfectant. On one occasion, neat Milton solution (approx. 16% sodium chloride) was used and the instruments were soaked over a weekend. Extensive corrosion occurred in this case.

Acknowledgement

This article has been prepared with assistance from Tony Newson previously at AvestaPolarit (now Outokumpu Stainless Ltd). Mr Newson is now General Manager - Stainless Steel Producers Group, Eurofer, Brussels

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