Since the advent of phacoemulsification, the incision size used in cataract surgery has steadily decreased, leading to faster visual recovery for the patients. With foldable IOLs, the incision size shrunk to 4 mm or less, and now with injectable IOLs, we are down to 3 mm or less. There are even reports of a complete sub-1 mm cataract surgery, including the placement of a thin optic IOL. But what is the ideal incision size?
Because fluid flow is related to the size of the phaco needle, going to sub-1 mm incisions reduces the flow rate and makes surgery significantly slower. Incisions more than 3 mm tend to have a greater flattening effect on the cornea at that meridian. In addition, the incision must seal effectively to prevent leakage as well as tear film influx, which could increase the risk of endophthalmitis. So the ideal incision size for cataract surgery is likely to be somewhere between 1 mm and 3 mm. The two primary factors to consider are astigmatic effect of the incision and the risk of infection due to leakage or tear film contamination.
A corneal incision tends to cause flattening at that meridian with a corresponding steepening 90° away. With temporal clear corneal incisions, this can lessen against-the-rule astigmatism but worsen with-the-rule astigmatism. This should be considered when using other methods such as limbal relaxing incisions to address pre-existing astigmatism at the time of cataract surgery.
Wider clear corneal incisions tend to have more astigmatic effect than narrower ones, and longer tunnel lengths tend to seal more effectively as well as have less astigmatic effect. So initially it seems like smaller is better, but there is more. Smaller incisions may pose a higher risk for ultrasonic wound burns if the fit of the phaco needle is too tight. With IOL insertion, a tight incision is likely to rip and tear and not stretch. This torn incision has ragged edges and is likely worse than if the incision had been slightly enlarged using a sharp blade. So we do not want the incision to be too small.
For most surgeons, the first step is to measure the astigmatic effect of your current incisions. This may be done by taking serial keratometric measurements before and after the cataract surgery and then analyzing the results. For most incisions between 2.5 mm and 3 mm, there is a flattening of about 0.5 D; more for larger incisions and less for smaller incisions. The astigmatic difference between a 3.5 mm incision and a 2.5 mm incision is significant, whereas the difference between 2.5 mm and 1.5 mm is far less. The effect of your incision must be taken into consideration when planning a limbal relaxing incision (Figure 1).
Risk of infection
An incision that leaks is more prone to infection because tear film contaminants are more likely to enter the eye in the postoperative period. Although smaller width incisions may seal better than wider incisions, a more important aspect may be the tunnel length and architecture of the incision. Certainly, incisions that are torn after attempted stretching often have irregular architecture, focal Descemet’s detachments and poor integrity — a 2.8-mm clean-cut incision is certainly far better than a 2.2-mm incision that is torn because of attempted stretching.
|Before surgery, the patient has 1 D of with-the-rule corneal astigmatism. This is slightly worsened by the flattening of the temporal clear corneal incision. Limbal relaxing incisions to treat 1.5 D of with-the-rule astigmatism are used to fully treat the astigmatism, resulting in a spherical cornea and better vision for the patient. |
Images: Devgan U
|The wound-assist method allows a smaller corneal incision; however, there may be exposure to tear film contaminants. By placing the injector tip into the anterior chamber, the Tecnis 1-piece acrylic IOL (Advanced Medical Optics) is injected into the capsular bag. Using this slightly larger incision allows a smooth, controlled delivery while keeping the IOL away from the tear film and associated contaminants.|
Another risk of infection is introduction of tear film contaminants at the time of IOL insertion. Using folding forceps to insert the IOL allows the tear film to come into contact with the optic as the lens is placed in the eye. This can be avoided by using an IOL injector and ensuring that the injector tip is placed in the anterior chamber before the IOL is inserted. This creates a closed system in which the IOL is shielded from any potential tear film contaminants at the time of implantation.
Recently, the technique of “wound-assisted” lens injection has been described, which entails abutting the injector tip to the incision and then pushing the rolled-up IOL through the corneal incision. This is typically done with a single-piece acrylic lens because of its rigidity when rolled in the injector cartridge. This allows the IOL to go through a smaller incision because the added bulk of the injector tip is not placed inside the corneal incision. However, there may be exposure of the IOL to the tear film, and this may introduce bacteria into the anterior chamber at the conclusion of the case.
I have used the Tecnis 1-piece acrylic lens (Advanced Medical Optics) via this wound-assisted method to place the lens via a 2.2- to 2.4-mm incision, with good results. However, my preference is to use a slightly larger 2.8-mm incision and place the injector tip inside the incision to prevent contact of the IOL with the tear film contaminants (Figure 2).
The move toward smaller incisions has benefited both patients and surgeons, and we are currently at the sweet spot, in which the incisions are large enough for controlled, efficient surgery and IOL insertion but small enough to seal well and produce minimal astigmatic effects. When evaluating your own patients and surgical results, make sure that the incisions seal well and have a predictable astigmatic effect.
For more information:
- Uday Devgan, MD, FACS, is in private practice at Devgan Eye Surgery in Los Angeles, Beverly Hills, and Newport Beach, California. Dr. Devgan is Chief of Ophthalmology at Olive View UCLA Medical Center and an Associate Clinical Professor at the Jules Stein Eye Institute at the UCLA School of Medicine. Dr. Devgan can be reached at 11600 Wilshire Blvd., Suite 200, Los Angeles, CA 90025; 800-337-1969; fax: 310-388-3028; e-mail: firstname.lastname@example.org; Web site: www.DevganEye.com. Dr.Devgan is a consultant to Abbott Medical Optics and Bausch & Lomb, and is a stockholder in Alcon Laboratories and formerly in Advanced Medical Optics.
- Agarwal A, Agarwal A, et al. Phakonit: phacoemulsification through a 0.9 mm corneal incision. J Cataract Refract Surg. 2001; 27(10):1548-52.
- Fine IH, Hoffman RS, Packer M. Profile of clear corneal cataract incisions demonstrated by ocular coherence tomography. J Cataract Refract Surg. 2007;33(1):94-97.
- May W, Castro-Combs J, et al. Analysis of clear corneal incision integrity in an ex vivo model. J Cataract Refract Surg. 2008;34(6):1013-1018.