Written and maintained by CASRAI Editorial Board
Last updated
General product and regulatory information only; this is not medical advice and does not describe any surgical technique.
Short answer
A Kirschner wire (K-wire) is a thin, stiff metal wire that is implanted in bone. A Steinmann pin is the same kind of device in a thicker size. In US federal regulation both fall under one entry, 21 CFR 888.3040, a Class II device. The tools used to cut, twist or crimp them, such as a wire cutter, wire twister or pin crimper, are a separate Class I category.
Definition and scope
The FDA regulation titled “Smooth or threaded metallic bone fixation fastener” (21 CFR 888.3040, checked 30 October 2026) describes the device as one “intended to be implanted” that “consists of a stiff wire segment or rod made of alloys, such as cobalt-chromium-molybdenum and stainless steel.” It may be smooth or fully or partially threaded, straight or U-shaped, and may be blunt pointed, sharp pointed, or have a formed, slotted head. Its classification is stated as “Class II.”
The regulation does not use the words Kirschner or Steinmann, and it sets no diameter at which a wire becomes a pin. Those names are clinical and catalogue conventions that sit under one regulatory description.
How K-wires and pins are used
The regulation lists four uses: fixation of bone fractures, bone reconstructions, a guide pin for insertion of other implants, and implantation through the skin so that a pulling force (traction) can be applied to the skeletal system. Radiopaedia, a secondary source, adds that the wires serve for temporary stabilisation and as an adjunct to plates and screws. Procedure and technique belong to surgical training and the manufacturer’s instructions for use, and are not covered here.
Kirschner wire vs Steinmann pin
The two names describe the same device family at different thicknesses. The table separates what the regulation says from what is convention and what one manufacturer lists.
| Question | What the source says | Source |
|---|---|---|
| Is there a legal diameter line between wire and pin? | No. 21 CFR 888.3040 describes one device type, a “stiff wire segment or rod”, with no diameter limit. | FDA regulation, 21 CFR 888.3040 |
| Conventional split | “Wire” is about 0.9 to 1.5 mm; “pin” is about 1.5 to 6.5 mm. | Radiopaedia (secondary, convention only) |
| One catalogue range, round K-wires | Round wires of 0.8 mm to 4.0 mm, with flat wires of 1.4 to 3.0 mm and double wires of 0.8 to 3.5 mm; lengths of roughly 65 mm to 900 mm. | Micromed product listing (manufacturer) |
| Origin of the names | Fritz Steinmann introduced his pin in 1907; Martin Kirschner described his wire in 1909. | Radiopaedia (secondary) |
The practical consequence: the catalogue range overlaps the conventional pin range, so a label that says “K-wire” or “Steinmann pin” does not by itself tell you the diameter. Read the diameter and length on the product record.
Related instruments are a different class
21 CFR 888.4540, “Orthopedic manual surgical instrument” (checked 30 October 2026), covers nonpowered hand-held devices for orthopedic surgery and names, among others, the pin crimper, wire cutter, wire twister and passer. These are Class I (general controls) and exempt from premarket notification, subject to the limitations in 21 CFR 888.9. The wire or pin itself is the implant; the cutter, twister and crimper are instruments. For the wider instrument family, see our guide to orthopedic surgical instruments and the osteotome entry.
| Item | Regulation | Class |
|---|---|---|
| Kirschner wire, Steinmann pin | 21 CFR 888.3040, smooth or threaded metallic bone fixation fastener | Class II |
| Wire twister, pin crimper, wire cutter, passer | 21 CFR 888.4540, orthopedic manual surgical instrument | Class I, exempt from 510(k) subject to 888.9 |
Where it matters for labs, clinics and supply records
A device record carries more detail than the trade name. The NLM AccessGUDID record for one listed product, “Steinmann Pins, single diamond, round end, 2.0 mm diameter, 12-inch (30 cm) long” (checked 30 October 2026), shows what a record can state: the tip style in the product name, an outer diameter of 2 mm, a length of 30 cm, FDA product codes HTY, HRS and HWC, packaged sterile: No, requires sterilization before use: Yes (moist heat or steam), and single-use: Yes. That is one example listing, not a rule for all pins; other products differ. The items worth reading on any record or label are:
- tip style (blunt, sharp, diamond) and whether the shaft is smooth or threaded, the features named in 21 CFR 888.3040;
- diameter and length, in millimetres, not only the “wire” or “pin” name;
- whether the product is supplied sterile or must be sterilised before use, and the single-use statement;
- the regulation and class, so that implants and the hand instruments are not mixed in one record.
For background on the regulator, see FDA and the Medical Device Amendments of 1976, which set up the device class system.
Common misunderstandings
- “K-wire and Steinmann pin are different regulated devices.” Not in 21 CFR 888.3040, which describes one device type; the split is by convention.
- “Wire twisters and cutters are implants.” They are hand instruments under 21 CFR 888.4540, Class I.
- “Orthopedic wire” means one product. The phrase is a loose search term; check the specific device name and regulation on the record.
FAQ
What is a Kirschner wire?
A stiff metal wire implanted in bone, covered by 21 CFR 888.3040 as a smooth or threaded metallic bone fixation fastener, Class II.
What are Kirschner wire sizes?
By convention about 0.9 to 1.5 mm (Radiopaedia). One manufacturer’s listing shows round wires from 0.8 to 4.0 mm and lengths from roughly 65 to 900 mm (Micromed, checked 30 October 2026).
What are Steinmann pin sizes?
By convention about 1.5 to 6.5 mm (Radiopaedia). An AccessGUDID example record lists a 2.0 mm by 30 cm pin.
Steinmann pin vs Kirschner wire: what is the difference?
Mainly thickness. The pin is the thicker of the two by convention; the regulation treats them as one device type.
What is an orthopedic wire cutter, wire twister or pin crimper?
Nonpowered hand-held instruments named in 21 CFR 888.4540, Class I and exempt from premarket notification subject to 21 CFR 888.9.
Sources
- US FDA, 21 CFR 888.3040, Smooth or threaded metallic bone fixation fastener, eCFR, checked 30 October 2026.
- US FDA, 21 CFR 888.4540, Orthopedic manual surgical instrument, eCFR, checked 30 October 2026.
- NLM AccessGUDID, Steinmann Pins device record, checked 30 October 2026.
- Micromed, Kirschner wire product listing, checked 30 October 2026.
- Radiopaedia, K-wire (secondary source), checked 30 October 2026.
