⥠Key takeaways
- Two very different categories: equipment fasteners (304/316) for housings and instruments, and implant fasteners (titanium or cobalt-chrome) for bone screws and plates.
- 316L is the medical standard stainless. Low carbon, higher molybdenum, resistant to body fluids, specified under ASTM F138.
- Implants use Ti-6Al-4V, chosen for biocompatibility, low density and an elastic modulus closer to bone than steel.
- Every medical part is passivated, and implants are also electropolished to Ra âĪ 0.4 Ξm.
- No burrs and no cracks. Any surface that contacts the body must be free of defects.
- Cleanroom packaging: medical-grade cleaning plus double bagging, compatible with ethylene oxide or gamma sterilisation.
- ISO 13485 is a quality system, not a product standard. It qualifies the supplier, and it is a precondition rather than a specification.
1. Equipment versus implant
2. Material selection
3. Surface finishing
4. Cleaning and packaging
5. Quality systems and certification
6. Decision tree and FAQ
1. Equipment versus implant
| Dimension | Equipment fasteners | Implant fasteners |
|---|---|---|
| Application | CT, ultrasound and operating table housings | Bone screws, bone plates |
| Material | 304 / 316 | Ti-6Al-4V, cobalt-chrome-molybdenum |
| Surface | Passivated | Electropolished and passivated |
| Body contact | No | Yes |
| Certification | ISO 13485 supplier | FDA, CE or NMPA device approval |
| Relative cost | 1Ã | 10â100Ã |
The cost gap is worth understanding before specifying. Roughly half of it is material and the rest is traceability: lot-level documentation, cleanliness verification, sterility validation and the cost of maintaining a certified quality system.
2. Material selection
| Material | Application | Standard |
|---|---|---|
| 304 | General equipment housings | GB/T 1220, ASTM A276 |
| 316L | Body fluid contact, surgical instruments | ASTM F138, ISO 5832-1 |
| Ti-6Al-4V ELI | Bone screws, bone plates | ISO 5832-3, ASTM F136 |
| Cobalt-chrome-molybdenum | Joint replacement | ISO 5832-4, ASTM F75 |
3. Surface finishing
- Passivation: citric or nitric acid to remove free iron from the surface. Without it, a 316 part can still rust in a steriliser.
- Electropolishing: brings the surface to Ra âĪ 0.4 Ξm, which reduces bacterial adhesion and improves corrosion resistance by removing the disturbed layer left by machining.
- No coating on implants. A plated layer can spall into tissue, and there is no coating system with the long-term biocompatibility data that an implant requires.
4. Cleaning and packaging
- Ultrasonic cleaning with a medical-grade detergent.
- Rinse with medical-grade purified water, with conductivity monitored.
- Double-bag in PE, with the inner bag sealed and labelled for traceability.
- Sterilise by ethylene oxide or gamma irradiation, validated to ISO 11135 or ISO 11137.
5. Quality systems and certification
A medical device supplier must hold ISO 13485. Implants additionally require device approval â FDA 510(k) or PMA in the United States, CE under MDR in Europe, NMPA in China. A general fastener plant cannot simply start making medical parts, because the certification is on the process and the system, not on the drawing.
6. Decision tree and FAQ
Step 1: does it contact the body?
No â 304 or 316. Yes â titanium alloy, with device-level certification.
Step 2: is certification required?
Export â FDA or CE. Domestic China â NMPA. Equipment only â ISO 13485 supplier is the baseline.
FAQ
Q: What is the difference between 316 and 316L? The L means low carbon, 0.03 % maximum, which prevents chromium carbide precipitation at grain boundaries during welding and therefore intergranular corrosion. For medical use, L is mandatory.
Q: Can implants be made from stainless? Short-term fixation can use 316L, and historical implants did. For long-term implantation, titanium is preferred for corrosion resistance and biocompatibility.
Q: Can medical screws be plated? Implants cannot. Equipment housings can, provided the coating is not on a patient-contact surface and the biocompatibility of the finished device is assessed.
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