Format of Stent Stability Testing (Accelerated and Long Term Aging)

A

arvindp

#1
Can any one provide me a format of stability testing (accelerated and long term aging ) protocol for stent ? Any report of result ?
 
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R

Roland Cooke

#3
I can't really bring any expertise to this discussion, but gnerally the tests will be the same for both. I can't remember where the incubation formula can be found, there's bound to be a relevant standard somewhere.

For the tests, at the very least the physical integrity of the stent, its delivery system, and its sterile pack will all need to be confirmed. In addition, if the stent is drug-eluting, all the stability factors pertaining to the pharmaceutical agent will need to be considered.
 

Sam Lazzara

Trusted Information Resource
#5
Here is some very recent related information that you might find useful.
Please note that this is not a drug-coated stent - it is a nickel-titanium (NINTINOL) self-expanding stent used in the peripheral vasculature.

Side Note: ASTM F1980 is the most commonly used standard I have seen for accelerated aging (AA) of medical devices, and it points out the need to also perform real time aging to confirm the AA results.

FDA PMA Summary of Safety and Effectiveness: ev3 EverFlex(tm) Self-Expanding Peripheral Stent System

For those involved with NiTi self-expanding devices (particularly implantable ones), this PMA summary has some useful information.
http://www.accessdata.fda.gov/cdrh_docs/pdf11/P110023b.pdf

Highlights
- Biocompatibility tests performed and rationale for tests not performed
- Animal and in vitro test results including acceptance criteria
- Clinical study design and results
- High fracture rate seen in animal studies - swine and sheep (>50% rate up to 180 days)
- Relatively low fracture rate in human experience at 1 year (1 out of 260 patients - trans-axial spiral fracture)
- Human fracture analysis table breaks down different fracture modes into 5 classes
- Existence of a "Stent Fracture Committee"
- Significant post-market study commitments (2 studies)
--- new study requires 169 patients followed up to 36 months post-procedure (sample size determined to detect stent fractures)
--- following patients from earlier studies up to 36 months post-procedure

Excerpt
High fracture rates were observed in both animal studies; however, animal studies
often present worst case conditions for fracture. For this PMA, the animal testing was
comprised of long length stenting and overlapped configurations representing severe
anatomic conditions. In addition, experimental studies in small research undulates
are often limited by anatomic challenges such as angularity, curvature and taper of the
femoral and iliac arteries. Because of the high fracture rates observed in these animal
studies and in light of the limitations posed, the available clinical data on stent
fracture for the EverFlex stent weighed heavily in the evaluation of fracture rate.
 
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B

bharatbuk

#6

Ronen E

Problem Solver
Staff member
Moderator
#7
The use of any "canned" protocols for accelerated life testing of higher-than-low-risk medical devices is IMO DANGEROUS.

True accelerated life testing is complicated, science-based engineering, and sometimes an art... It's not a case of one-size-fits-all!

How can a method for accelerated testing of sterile packaging be directly applied to the mechanical reliability of a complex metal implant?...
 

Sam Lazzara

Trusted Information Resource
#8
I would be more concerned about polymers/adhesives than metals since they are generally more susceptible to aging effects.

Since packaging is typically comprised of polymers, and since there are not any standards I am aware of that are similar to ASTM F1980 for the device inside the package, the industry tends to apply its concepts to the device itself. The most common practice I have seen is to assume Q=2, meaning that every increase in temperature of 10 degrees C doubles the aging rate. Oversimplified? Perhaps, but it is a reasonable and conservative approach that is generally applied by device companies and accepted by regulatory agencies. Otherwise, we would not be able to implement any state-of-the-art devices.

Most device companies don't have the resources to do the mad science needed to determine the real "Q" for their device. I think they would go out of business quickly if they saddled themselves with that type of work.
 
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Ronen E

Problem Solver
Staff member
Moderator
#9
I would be more concerned about polymers/adhesives than metals since they are generally more susceptible to aging effects.

Since packaging is typically comprised of polymers, and since there are not any standards I am aware of that are similar to ASTM F1980 for the device inside the package, the industry tends to apply its concepts to the device itself. The most common practice I have seen is to assume Q=2, meaning that every increase in temperature of 10 degrees C doubles the aging rate. Oversimplified? Perhaps, but it is a reasonable and conservative approach that is generally applied by device companies and accepted by regulatory agencies. Otherwise, we would not be able to implement any state-of-the-art devices.

Most device companies don't have the resources to do the mad science needed to determine the real "Q" for their device. I think they would go out of business quickly if they saddled themselves with that type of work.
Thanks for this commentary.

All this is well known to me -- my previously expressed opinion is despite all that, or maybe because of all that...

The real problem is -- as you have put it -- that "there are not any standards I am aware of that are similar to ASTM F1980 for the device inside the package" (I share this view). The application of Arrhenius-equation-based models (simplified or not, e.g. the simplified Q10 model you referred to) seems to be the industry/regulators status-quo, however that doesn't automatically make it scientifically-sound or even right. You described the situation as industry applying the same base concepts to device life testing, but IMO this is not true. Arrhenius equation (and thus all its simplified derivatives) apply to ageing mechanisms based on chemical reactions of order 0 or 1; yet, I've seen countless cases where such models were applied to ageing / failure mechanisms that have very little to do with chemical reactions of whatever order, not to mention that in many many cases developers don't even ponder what the failure mechanisms of concern are, and whether / how they can be accelerated / enhanced for reliable life testing purposes. So, I'd dare and say that the de-facto industry situation has significantly deviated from original concepts & intentions.

Sure, current practices definitely make proper pseudo-engineering appearance that allows regulators to clear innovative devices and thus maintain a politically-sound stance ("allowing innovation while maintaining public safety"); however, IMO this is a direct contributing factor to the many cases we see, on an almost regular basis, of cleared devices recalls due to reliability / durability "surprises" long before the anticipated expiry. An illustrative example: http :// www .fda.gov/Safety/MedWatch/SafetyInformation/SafetyAlertsforHumanMedicalProducts/ucm315718.htm - OBSOLETE BROKEN 404 LINK(s) UNLINKED

You state that the simplified Q10 (with a factor of 2) is "reasonable". For a specific device, maybe it is and maybe it isn't; who knows, if no real analysis is held beforehand, or at the very least - a parallel real-time ageing study is held, to validate the accelerated ageing model?... If lack of resources / knowledge are acceptable arguments to justify such pseudo-engineering, anything goes. IMO it's an attitude issue first, and I strongly disagree that it's impossible / impractical - in 100% of the cases - to develop an appropriate real-engineering model. True, it's sometimes challenging and in some cases it is impractical; but between the current state of things and the reasonably-achievable there is vast room for improvement. You may call it "mad science"; I call it sound engineering (and yes I'd like to think engineering is about 90% science and 10% art :)).

In particular, I think that a rationalization such as "polymers / adhesives are generally a greater concern than metals, because..." is a step in the right direction (away from one-size-fits-all thinking), but it's only a first step. My main theme is that the accelerating model should be tailored to the particular application, accounting for specific design, failure modes, intended use and environmental characteristics. Before getting to advanced topics such as "Q=?", basic questions such as "what are the failure modes that concern us?", "what types of processes govern / promote them?", "can these processes be reliably accelerated or enhanced at all?" etc. should be looked into. Asking this type of questions doesn't cost much and may help engineers get into a more responsible state-of-mind.

Cheers,
Ronen.
 
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