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Stainless steels for surgical instruments, and related components, are specified in BS EN ISO 7153-1:2016, which incorporates BS 5194-1:1991. The second edition renumbered the previous ISO 7153-1:1991, as BS EN ISO 7153-1:2001, which itself has now been superseded by the third edition, i.e. BS EN ISO 7153-1:2016. This covers all metallic materials for surgical instruments, not just stainless steels. However only references in the standard to stainless steels will be described here.
This new edition of standard gives each steel grade a new ‘Reference Letter’, (the corresponding previous, obsolete, reference letters used in BS EN ISO 7153-1:2001 are provided in the current standard for information purposes only and should no longer be used), the cross references to ISO 4957 and ISO 683-13 standard grades have also been removed, instead there is reference to grade classifications, i.e. numbers and names, from BS EN 10088-1:2014. There are 18 martensitic steels, 2 ferritic steels, 2 Precipitation hardening steels, and 5 austenitic steels in Table 4 of the standard, which is summarised below. Most instrument manufacturers regard these stainless steel grades as generic and tend to also refer to European or national standards. In the new standard, Table 5, there is an indication of where steel compositions are identical to the previous version of the standard, (‘I’), where they are similar but not identical, (‘N’), and where they differ substantially, (‘W’).
| New Reference
in Rockwell (HRC)
in Vickers HV5/
|p||1.4543||X3CrNiTiNb 12-9||selectable||selectable||precipitation hardening|
Footnotes to Table
a. The conversion from Rockwell hardness to Vickers hardness is done according to ISO 18265:2003, Table A.1.
To the calculated HV5/HV10 1 HRC (value based on experience for stainless steels) was added.
b. Not included in EN 10088-1. Where the material number is given in brackets, the material was only included in the steel-iron-list.
c. Historic material number which is used for different fields of application than 1.4121, but is designated to the same material (as 1.4121). 1.4197 is used for rotating instruments.
Table 1 of the standard gives examples of the surgical applications for which each of the grades is suitable. The standard should be consulted for this comprehensive list of preferred uses, and a separate Table, A.1, in Annex A describes which stainless steel grades are commonly used for dental instruments.
Grades b, a, g, and c would be generally described as 410 / 420 types, with ‘m’ being in the same sub-family, but with a high carbon level similar to that of steels used for razor blades. These grades are used extensively for dental and surgical instruments. They offer moderate corrosion resistance in comparison to other types of stainless steel, (e.g. 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 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.
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, (i.e. resistance to autoclaving, corrosion and thermal exposure).
Corrosion problems associated with clinical practice tend to be associated with contact with aggressive substances, (e.g. 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.
The original article was 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), but he has not been involved in the preparation of this updated version.