A Nitinol Strut Fractures at 382 Million Cycles
A Nitinol strut fractures at 382 million cycles. The patient feels nothing yet. Here’s the sequence, step-by-step, and why a Hazard Analysis is essential before first-in-human.
A Nitinol strut fractures at 382 million cycles. The patient feels nothing yet.
Here’s the sequence, step-by-step, and why a Hazard Analysis is essential before first-in-human.
The device is a self-expanding Nitinol stent in the superficial femoral artery, a mechanically hostile environment for the implant. Every stride, knee bend, and hip flexion loads it. Ten years of walking is roughly 400 million cycles (ISO 25539-2 sets the fatigue-durability expectation).
The Foreseeable Sequence of Events, in action verbs only:
- Laser cutting creates undetectable surface defects
- Stent expands against the arterial wall.
- The limb flexes, millions of times.
- A strut accumulates fatigue. (Note: this could be expressed as subcritical crack growth)
- The strut fractures from high impact event.
- The fragment separates.
- Blood carries the fragment downstream.
Stop there. The sequence ends before the injury.
The Hazardous Situation is the moment of exposure (ISO 14971:2019, 3.5): The distal vessel wall is vulnerable to the migrating fragment, and the lumen is partially blocked. No "causing," no "leading to." Just the state.
The Harm is a clean noun or noun phrase (ISO 14971:2019, 3.3), examples: distal embolization. tissue ischemia. limb ischemia or worse.
The hazard is not the broken strut. It is the mechanical energy of a foreign body moving where blood flows (ISO 14971:2019, 3.4).
The control comes first at the source (Clause 7.1): a durability product requirement for 400+ million cycles as a device lifetime, an electropolishing manufacturing process to reduce the probability strut crack formation, verified by an ISO 25539-2 durability test to the full cycle count (plus a safety factor). Naturally, that path is ID-traced to a product requirement in the Design Input and Output Matrix.
Why it matters: the fragment does not care whether the analysis was thorough. The patient does.
Read your last stent Hazard Analysis. Can you point to the row where the strut cracks from impact or fatigue, and the control that stops it?
#RiskManagement #ISO14971 #ISO25539-2 #HazardAnalysis #MedicalDevices #DurabilityTest