Ophthalmic Blades, Knives and Trocars: Keratome Sizes, Crescent, Side-Port, MVR and Vitrectomy Gauges Explained
Last updated: November 2026
Ophthalmic blades are small, precise and single use. Each has a job: the keratome sets the size of the main incision, the crescent makes scleral tunnels, the side-port knife makes paracenteses, and the MVR blade and trocars enter the vitreous cavity. Using the right blade gives a self-sealing, astigmatism-neutral wound.
Blades for cataract surgery
| Blade | Typical sizes | Used for | Tips |
|---|---|---|---|
| Slit (keratome) knife | 2.2, 2.4, 2.6, 2.75, 2.8, 3.0, 3.2 mm | Clear corneal main incision for phaco; enlarging for IOL | Match to the phaco tip and IOL cartridge; enter parallel to the iris plane after a short tunnel |
| Side-port (15°) knife | 15° angle, about 1 mm wide | Paracentesis (side port) for second instrument and injections | Make it 60–90° from the main incision; keep it short and watertight |
| MVR (microvitreoretinal) blade | 20 G, 23 G | Side port, sclerotomy, paracentesis in many procedures | Sharp spear tip; 20 G about 1.2 mm wide |
| Crescent knife | Bevel-up, angled; 2.0–2.5 mm wide | Scleral tunnel dissection in SICS and trabeculectomy | Keep a uniform depth (about one-third to half scleral thickness); go wider at the internal lip (funnel shape) |
| Keratome for SICS | 5.2–6.5 mm (or enlarging) | Entering the chamber and enlarging the internal opening of the tunnel | Internal opening wider than the external one |
| Stab knife (e.g. 15° or 30°) | — | General stab incisions, trabeculectomy flaps | — |
| Diamond knives | Fixed or adjustable depth | Precise clear corneal, limbal relaxing incisions | Reusable; costly; handle carefully |
Vitrectomy gauges
| Gauge | Outer diameter (about) | Notes |
|---|---|---|
| 20 G | 0.9 mm | Older standard; sclerotomies need sutures |
| 23 G | 0.6 mm | Common; usually sutureless with angled entry |
| 25 G | 0.5 mm | Sutureless; smaller instruments, now very common |
| 27 G | 0.4 mm | Smallest common gauge; less wound leak, more flexible instruments |
Trocar-cannula systems: angled (beveled) entry about 3.5–4 mm behind the limbus (phakic eyes further back than pseudophakic), parallel to the limbus, then perpendicular, to create a self-sealing tunnel.
Making a good clear corneal incision
Steps
- Stabilise the globe with a fixation ring or forceps opposite the incision.
- Enter the cornea just inside the vascular arcade with the keratome tip.
- Advance in the corneal plane for about 1.5–2 mm (tunnel), then dimple down and enter the chamber parallel to the iris.
- Withdraw along the same path to avoid enlarging or irregular edges.
- At the end, hydrate the stromal edges and check the wound does not leak.
Common errors: too short a tunnel (leak, iris prolapse), too long (oar-locking, corneal striae and poor view), premature entry, and Descemet membrane tear from a blunt blade.
Pearls
- Match keratome size to your phaco sleeve and IOL cartridge before the case.
- Make the side port before the main incision so you can fill the chamber with OVD first.
- In SICS the tunnel is a funnel: narrow outside, wide inside.
Common mistakes
- Using a blunt or reused blade (Descemet tears, ragged wounds).
- Short tunnels that leak.
- Entering the chamber too steeply.
Test yourself: flip cards
Quick quiz
Related: Phacoemulsification Step by Step: A Beginner's Guide, IOL Injectors and Cartridges for Residents: Alcon A, B, C, D, Preloaded Systems, J&J Unfolder, Zeiss, Medicel and Indian Lenses, Sutures, Needles and Sterilisation in the Eye Theatre, Ophthalmic Viscosurgical Devices (OVDs): Cohesive, Dispersive, Viscoadaptive, Brands, Soft-Shell Technique and Removal, Retina.
Key sources
- Fine IH. Clear corneal incisions. Int Ophthalmol Clin. 1994;34:59-72
- Gogate PM. Small incision cataract surgery: complications and mini-review. Indian J Ophthalmol. 2009;57:45-49
- Fujii GY, et al. Initial experience using the transconjunctival sutureless vitrectomy system. Ophthalmology. 2002;109:1814-1820
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