Stainless Steel

Stainless steel is an iron-based alloy containing at least 10.5% chromium, the minimum needed for passivation. The medical grade is 316L, and it is the most common implant material for fracture fixation.

The trade-off. It is stiff, ductile enough to contour in theatre, and cheaper than titanium. Against that, it is the most corrosion-susceptible of the modern orthopaedic alloys, its stiffness causes stress shielding, and its nickel is an allergen.

The alloy. Iron is the base metal, about 60% of 316L, and each addition has a job:

• Chromium 17-20% forms the surface oxide layer (Cr₂O₃) that passivates the steel and gives it its corrosion resistance
• Nickel 12-14% stabilises the austenitic (FCC) phase at room temperature, which is what makes the steel ductile; it is also the allergen
• Molybdenum 2-4% resists pitting corrosion
• Carbon under 0.03%, the "low" in 316L, minimises carbide precipitation and so prevents sensitisation

Why the carbon is kept low. With more than 0.03% carbon, chromium carbides precipitate at the grain boundaries. The boundaries are depleted of chromium and the steel corrodes along them, intergranular corrosion. This process is sensitisation.

The crystal structure. 316L is austenitic, face-centred cubic (FCC). It is non-magnetic, ductile enough to be contoured or bent intraoperatively, and it work-hardens, getting stronger as you bend and shape it. It cannot be heat-treated for hardening.

Three crystal phases. Austenite is the stable phase in 316L at room temperature. Ferrite is present in ferritic steels, not in 316L, and martensite in hardened steels. Duplex steels are a mix of austenite and ferrite. The phase decides both the properties and the use:

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The passive layer. Chromium oxide (Cr₂O₃) forms spontaneously on the surface, 1-5 nanometres thick, and it is self-healing in an oxygen-rich environment. It is disrupted in low-oxygen crevices such as those under screw heads, which is where crevice corrosion (below) begins.

Weak points. Grain boundaries are potential corrosion sites, and inclusions (impurities) concentrate stress.

Implant-grade 316L is defined by ASTM and ISO specifications, which govern what the alloy contains and how it is processed.

Stiffness and stress shielding. Young's modulus is about 200 GPa, against 15-20 GPa for bone. That stiffness is good for rigid fixation, but the implant carries the load instead of the bone, and by Wolff's law unloaded bone resorbs. The result is stress shielding, and a risk of refracture after the plate is removed.

Strength depends on processing. Ductility is high, which is what allows plate bending, and fatigue strength is moderate. Yield strength depends on how the steel was made:

• Cold working deforms the steel at room temperature: yield strength rises to about 1000 MPa and ductility falls. Surgical implants are typically cold-worked for strength.
• Annealing is heat treatment: the steel is softer and more ductile, with a yield strength of about 200 MPa. Annealed stock is used for intraoperative bending.

Stainless steel is the most susceptible of the modern orthopaedic alloys to corrosion, and four mechanisms threaten it.

Crevice corrosion happens under screw heads, and it runs in sequence:

1. Oxygen is low in the crevice under the screw head
2. The Cr₂O₃ passive layer breaks down and the chromium cannot re-passivate
3. The crevice environment acidifies
4. Metal dissolution accelerates
5. Cr, Ni and Mo ions are released, producing metallosis

Fretting corrosion. Micromotion between plate and screw mechanically disrupts the passive film. Rigid fixation is the prevention.

Galvanic corrosion. Dissimilar metals in contact form a cell. Coupled with titanium, stainless steel is the anode (active) and corrodes, while titanium is the cathode (noble). Never mix metals.

Pitting corrosion. Chloride breaks down the passive film locally; the molybdenum in 316L resists it.

Prevention. Four measures reduce the risk:

• Never mix metals
• Minimise crevices at the screw-plate interface
• Use an electropolished surface, which enhances the passive layer, reduces surface roughness and improves corrosion resistance
• Avoid scratching the implant surface

Before choosing. Ask about nickel allergy (a rash from cheap jewellery), Type IV metal hypersensitivity and previous implant reactions, and whether the patient will need MRI.

Indications. Stainless steel suits:

• Fracture fixation with plates, screws and nails, and other temporary fixation devices
• Cost-sensitive settings
• Plates that need contouring intraoperatively
• Patients without a nickel allergy

When to avoid it. Choose another material for:

• Known nickel allergy
• A permanent implant, where titanium is preferred
• An existing titanium implant, because of galvanic corrosion
• A need for MRI follow-up

How firm is the nickel rule? Skin allergy does not strongly predict an implant reaction, and the Siljander knee arthroplasty cohort (below) found no revision difference between CoCr and nickel-free implants in nickel-allergic patients. In practice, take the history and consider titanium when it is severe.

Mechanism and prevalence. Nickel hypersensitivity is a Type IV (delayed) reaction. Nickel allergy affects 10-15% of females and 2% of males, but symptomatic metal hypersensitivity to an implant is less than 1%. The European and US figures on the evidence cards below are rates of skin sensitisation in the general population, not of implant reaction.

Signs after implantation. Dermatitis overlying the implant, chronic unexplained pain and a persistent effusion.

Patch test or LTT. Patch testing detects cutaneous hypersensitivity and has poor predictive value for deep implants. The lymphocyte transformation test (LTT) detects systemic sensitisation and is more specific for implant reactions, although, as the controversies section notes, it is not widely validated or available.

Management. Remove the implant after fracture union, replace it with titanium if needed, and debride any metallosis tissue.

Imaging. Plain radiographs assess implant position and loosening, with no difficulty from stainless steel. CT has metal artefact but can still assess fixation. The steel is non-magnetic, so MRI is possible, but it gives significant artefact for soft-tissue assessment, where titanium gives minimal artefact.

Blood and fluid. Each test answers a different question:

• Serum metal ions (Cr, Ni, Mo) when metallosis is suspected; they are elevated in corrosion
• CRP and ESR if infection is suspected
• Aspiration if there is an effusion

The failed implant. A removed implant and its tissue can be analysed:

• Sonication for biofilm bacteria
• Histology to distinguish metallosis from infection
• ICP-MS to quantify metal ions

A patient with a painful, swollen, or draining stainless steel implant has a narrow but high-stakes differential. The cardinal task is to exclude infection before attributing symptoms to metal.

Contouring. Stainless steel is ductile and can be bent intraoperatively, and work hardening strengthens the bent area with each bend. Avoid excessive bending because of notch sensitivity.

Screw insertion. Match the screw type to the plate system, avoid cross-threading and maintain a uniform torque.

Complications. Corrosion-related problems occur in 1-5%, and stress shielding is variable. Metallosis shows as tissue staining and osteolysis.

Why it remains the trauma standard. Union rates are equivalent to titanium's, at a significant cost advantage. The steel contours on the table, performs reliably as a temporary implant that can be removed after healing, and most patients tolerate its nickel without issue.

Why its use is declining. Titanium is preferred for permanent implants, and MRI compatibility is driving titanium adoption. Newer nickel-free stainless steels are in development; the nitrogen-strengthened grades are described below.

Routine follow-up. Serial radiographs track fracture healing and implant loosening, and the patient is watched for signs of a metal reaction.

Removal is optional once the fracture has healed, and it reduces long-term corrosion exposure. The indications:

• Symptomatic metal hypersensitivity, including nickel sensitivity that develops
• Prominent hardware
• Infection
• A healed fracture in a young patient
• Refracture risk from stress shielding

Timing. Wait at least 12-18 months after ORIF for full remodelling, protect for 6-8 weeks after removal, and counsel the patient on refracture risk.

The standards card, guidelines and controversies all name a "higher-nitrogen, low-nickel" or "nickel-free" grade (ASTM F1586 / ISO 5832-9) as an alternative, and a viva mentions using nitrogen or manganese to stabilise the austenite - but what these steels are and why they exist is never explained.

• Why they were developed. Conventional 316L relies on nickel to hold the austenitic (FCC) phase, but nickel is the commonest contact allergen and only a modest strengthener. Adding nitrogen (an interstitial element) does both jobs better: it is a strong austenite stabiliser (so nickel can be cut or removed) AND a potent solid-solution strengthener.
• The two families. (1) High-nitrogen, reduced-nickel wrought grades (ASTM F1586 / ISO 5832-9; e.g. "REX 734") keep some nickel but add roughly 0.3-0.9% nitrogen for markedly higher yield and fatigue strength than 316L. (2) Essentially nickel-free, high-manganese/high-nitrogen austenitic steels (e.g. "BioDur 108") use manganese plus nitrogen to hold the austenite with almost no nickel, for the genuinely nickel-sensitive patient.
• What you gain. Higher yield and fatigue strength (allowing thinner, stronger implants), better pitting and crevice-corrosion resistance (nitrogen stabilises the passive film), and lower nickel content and ion release.
• The caveats. They are more expensive, less universally stocked, and - as the controversies note - have limited long-term clinical outcome data, so they are not yet standard of care; most nickel-allergic patients are still served well by titanium or even standard alloys.

• Work hardening (the mechanism). Plastic deformation occurs by the movement of dislocations (line defects) through the crystal lattice. Deforming the metal multiplies dislocations and tangles them, so they increasingly obstruct one another's motion; more stress is then needed to keep deforming - the metal becomes stronger (higher yield) but less ductile. This is why bending a plate strengthens the bent zone, and why over-bending or repeated re-bending embrittles it.
• Cold working. Deforming (rolling, drawing, forging) below the recrystallisation temperature work-hardens the steel throughout, raising yield strength toward ~1000 MPa at the cost of ductility. Implants are supplied cold-worked when strength matters.
• Annealing. Heating above the recrystallisation temperature lets new strain-free grains nucleate and grow (recrystallisation), dissolving the tangled dislocations - the steel returns to a soft, ductile state (yield ~200 MPa). Annealed stock is chosen when the surgeon needs to contour the implant on the table.
• The trade-off in theatre. Ductility lets you contour a plate, but every bend work-hardens and locally embrittles it; contour smoothly, avoid sharp bends over screw holes, and never bend-then-reverse - the accumulated cold work plus a stress-concentrating notch is a classic site for fatigue failure.

Related pages: Titanium Alloys is the direct comparator throughout this page - lower modulus, better biocompatibility, no nickel, but poorer notch sensitivity and a different failure mode; Cobalt-Chrome Alloys for the third structural alloy, and for the point the hypersensitivity cards make - cobalt-chrome is NOT a reliable nickel-free alternative, because it contains residual nickel and cross-reacts; Corrosion Mechanisms for the electrochemistry in full, including the crevice and galvanic processes that dominate plate-and-screw constructs rather than the trunnion corrosion the Ude review describes; Trunnionosis and Taper Corrosion for the modular-junction problem that review is actually about; Implant Fracture Biomechanics and Screw Biomechanics for fatigue failure, stress concentration and why a stiff construct fails where it does; Ceramics and Polyethylene (UHMWPE and XLPE) for the non-metallic bearing materials that avoid ion release entirely.

• Cutaneous allergy vs deep implant failure: Whether a positive skin patch test predicts symptomatic peri-implant hypersensitivity remains unresolved. Most nickel-allergic patients tolerate stainless steel and CoCr implants; the Siljander 2023 TKA cohort found no difference in revision between CoCr and nickel-free implants in nickel-allergic patients. Routine use of costly nickel-free implants for skin allergy alone is not strongly supported.
• Best test for implant metal allergy: Patch testing reflects skin sensitisation, not necessarily deep-tissue reactivity; the lymphocyte transformation test (LTT) may be more specific but is not widely validated or available. No gold-standard diagnostic test exists.
• Routine implant removal after union: Whether to remove stainless steel hardware after fracture healing (to reduce long-term corrosion exposure and stress shielding, balanced against refracture and re-operation risk) is debated and largely driven by symptoms and patient factors rather than firm evidence.
• Stainless steel vs titanium for trauma: No high-quality RCT demonstrates a difference in union rates; choice is driven by cost, MRI needs, intraoperative contourability and allergy rather than proven outcome superiority.
• Nitrogen-strengthened low-/nickel-free steels: Higher-strength, reduced-nickel austenitic grades exist but have limited long-term clinical outcome data and are not yet standard of care.

1. Ahlström MG, Thyssen JP, Wennervaldt M, Menné T, Johansen JD. Nickel allergy and allergic contact dermatitis: a clinical review of immunology, epidemiology, exposure, and treatment. Contact Dermatitis. 2019;81(4):227-241. PMID: 31140194. doi:10.1111/cod.13327
2. Siljander BR, Chandi SK, Debbi EM, McLawhorn AS, Sculco PK, Chalmers BP. A comparison of clinical outcomes after total knee arthroplasty in patients with preoperative nickel allergy receiving cobalt chromium or nickel-free implant. J Arthroplasty. 2023;38(7 Suppl 2):S194-S198. PMID: 37100098. doi:10.1016/j.arth.2023.04.048
3. Ude CC, Dzidotor GK, Iloeje K, Nair LS, Laurencin CT. Corrosion of metals during use in arthroplasty. ACS Appl Bio Mater. 2023;6(6):2029-2042. PMID: 37261398. doi:10.1021/acsabm.2c01082
4. Chung CY. A simplified application (APP) for the parametric design of screw-plate fixation of bone fractures. J Mech Behav Biomed Mater. 2017;77:642-648. PMID: 29101896. doi:10.1016/j.jmbbm.2017.10.025
5. Kounis NG, Koniari I. Hypersensitivity to metallic implants: pathophysiologic and diagnostic considerations. Acta Biomed. 2018;89(3):428-429. PMID: 30333472. doi:10.23750/abm.v89i3.6718
6. ASTM F138/F139; ISO 5832-1 / ISO 5832-9. Standard specifications for wrought 316L and nitrogen-strengthened stainless steel for surgical implants.