Contact duration (ISO 10993-1) vs. body contact days (ISO TS 21726)

Dirk S

One of our customers got a pushback by a reviewing authority on the categorization of the contact duration based on ISO 10993-1:2018. According to our client, the device has a cumulative prolonged contact duration and endpoint consideration (incl. chemical characterization) was performed accordingly.
However, the reviewing authority required to classify the device as a permanent contact device, based on ISO TS 21726:2019 Clause 5.2 NOTE 2:
“A medical device that contacts the body once per week for a life-time (i.e. 9,9 years = 1 contact d/week × 52 weeks/year × 70 years) ÷ 365 d/year) would result in the selection of 10 μg/d as the TTC value.”
Based on this, endpoint consideration and chemical characterization testing would need to be redone, said the reviewing authority.
ISO TS 21726 is released since 2019 and this is the first time in the last 4 years, that cumulated contact duration categorization according ISO 10993-2:2018 was not accepted because of this NOTE 2 of ISO TS 21726:2019.
Is there a common understanding, that contact duration of e.g. 5 min per week over a time of 30 years are counted as a whole body contacting day would end up as 1,560 device contact days and permanent contact duration and not (as according to ISO 10993-1:2018) as 5.42 days and prolonged contact duration?
I would appreciated your thought on this matter, as this argumentation of the reviewing authority has a huge impact on device classification and biological endpoint consideration. It would mean, that ISO 10993-1:2018 and contact duration calculation according Clause 5.3 would be overruled by ISO TS 21726.


Involved In Discussions
It seems to me as an example, how to choose needed TTC for boarding values. Calculation is for the whole life once per week contact, and calcualtion time for life length of 70 years is 9.9 years. And according to table 1 from cl.5.3, for less than 10 years time of contact you should take TTC of 10 μg/d, even though it should be 1,5 μg/d for lifetime contact.
As for 5 min per week over a time of 30 years contact, note a to table 1 says, that prolonged contact category is applied only for those devices, which are classified as prolonged contacted by ISO 10993-1. For 5 min per day contact, device is is not classified so accroding to ISO 10993-1 (I am writing by memory, may be you need to recheck classification in ISO 10993-1).


Super Moderator

the example calculation in ISO TS 21726:2019 Clause 5.2 NOTE 2 assumes 1 full day ("1 contact d") = 24 hours per week over a total device lifetime of 70 years.

Your device is intended for 5 minutes = 5/(60*24) = 0.003 days per week over a total lifetime of 30 years, which is distinctively shorter that the above cited example calculation. Bottom line: your classification appears correct to me.
It would mean, that ISO 10993-1:2018 and contact duration calculation according Clause 5.3 would be overruled by ISO TS 21726.
There is an additional formal argument: ISO 10993-1:2018 is the governing, soon to be MDR-harmonised and USFDA-recognised leading standard for biocompatibility evaluation for medical devices. ISO TS 21726 is not even a standard, but a "technical specification", that is hierarchically below a full-blown standard. "NOTES" in such documents are generally recognised to be non-normative and non-mandatory, only further illustrating and explaining the related requirement. So a non-normative note in a technical specification cannot overrule the normative text in a parent state-of-the-art standard.

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