ISO 7153-1:1991-Surgical instruments - Metallic materials-Stainless steels

 

iso7153

Medical devices are subject to a set of uniform laws throughout the European Union. In this context, the EC Directive concerning medical devices places rigid quality demands on surgical instruments as well. Any manufacturer must heed and meet these requirements, as non-complying products are barred from being placed on the European single market. Similar regulations are in place in the United States and Japan. Other countries have started preparing and implementing their corresponding regulations.

This part of ISO 7153 contains a survey and a selection of stainless steels available for use in the manufacture of surgical, dental and specific instruments for orthopaedic surgery.

NOTE 1 When selecting the grade of steel and the shape, dimensions and delivery conditions of the raw material for manufacturing surgical instruments, it is necessary to take into account factors, such as the design of the instrument or the production facilities of the manufacturer, that are not covered by this part of ISO 7153. For this reason, it is not intended, nor is it possible, that the information given in this part of ISO 7153 should remove the decision-making responsibility from the instrument manufacturer for selecting an appropriate raw product with suitable properties; nor is it intended to preclude the use of other types of steel in the manufacture of instruments, such as the use of carbon steel for cutting instruments. International Standards for surgical instruments, when published, should be observed when making this decision as they may contain additional or new information to be taken into account when selecting appropriate steel grades.

Selection of stainless steels for surgical instruments

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.

There are no harmonised standards in place that Surgical Instruments can be assessed against to prove quality, specific to their functionality. Standards such as ISO9001 and 13485 may be referenced, but they do not demonstrate quality, they signify a formalised quality system.¬ The BS 5194 standards document guidelines for each group of surgical instruments, specification covered are:

  • Steel Type
  • Instrument Hardness
  • Instrument Finish
  • Corrosion Resistance

Any company selling surgical instruments, should address the following standards in their technical documentation:

  • BS EN ISO 13485 2003 Medical devices QMS Requirements for regulatory purposes
  • BS EN ISO 14971 2007 Medical devices Application of risk management to medical devices
  • 93/42/EEC 1993 Official Journal of the European Communities L169 volume 36
  • 2007/47/EC 05/09/07 Amending Council Directive 93/42/EEC L247/21
  • MEDDEV 2.12-1 rev 5 04/2007¬ ¬ Medical devices : Guidance document Guidelines on a medical devices vigilance system
  • MEDDEV 2.12-2 05/2004 Post Market Clinical Follow-up
  • BS EN 980 2008 Graphical symbols for use in the labelling of medical devices
  • BS EN 1041 2008 Information supplied by the manufacturer of medical devices
  • BS 5194 Part 1 ISO 7153-1 1991 Surgical instruments. Specification for stainless steel
  • BS 5194 Part 2 1989 Surgical instruments. Specification for instruments with pivot joints
  • BS 5194 Part 3 1985 Surgical instruments. Specification for dissecting forceps
  • BS 5194 Part 4 1985 Surgical instruments. Specification for scissors, shears & other jointed cutting instruments

All of these factors should be covered by the manufacturer, so adherence to these guidelines should be sought, when purchasing new instrumentation.

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 ISO13402, 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.

Benefits of being certified to ISO 7153?

  • Provides an effective way of guaranteeing the safety of end user. thus increasing the customer base of hospital,doctors…..
  • A charter that helps to develop the necessary risk mitigation measures that prevent potential surgical disasters in the future.
  • ISO 7153 has been critical in promoting the development of an inclusive risk protection and safety policy for manufacturing surgical instruments.
  • Through a good risk assessment on the production processes, there has been considerable development of cost-effective production
  • Consumer satisfaction
  • Enhanced reputation
  • and edge over competitor