Little difference in induced astigmatism seen with forceps, injector IOL insertion
Both the MemoryLens and the AcrySof fit through 3.2-mm incisions.
One of the advantages of sutureless small-incision cataract surgery is less induced corneal astigmatism. Many ophthalmologists prefer foldable, injectable lens implants for this reason. In theory, the more a wound is stretched during IOL implantation, the more induced corneal astigmatism would be expected. For this reason, we decided to compare induced corneal astigmatism with an injectable lens (Alcon AcrySof SA60AT) to a prerolled lens (CIBA Vision MemoryLens CV232). (Note: The CIBA Vision MemoryLens has since been renamed the CV232 SRE. The IOL, along with most of CIBA’s ophthalmic surgical business, was sold to the French company IOLTECH.) Both lenses can be inserted through a 3.2-mm incision. However, does one or the other cause more distortion of the surgical wound and hence more astigmatism? Keratometry was done pre- and postoperatively for both groups, and the changes in keratometry were compared.
Method of study
We randomly selected the charts of 60 patients who had received either an SA60AT or a MemoryLens within the past year. The SA60AT group had an age range of 27 to 93 years (average 68 years). The MemoryLens group had an age range of 51 to 88 years (average 70 years).
Preoperative corneal astigmatism in the SA60AT group ranged from 0 to 1.5 D (average 0.71 D), while the MemoryLens group ranged from 0 to 1.38 D (average 0.58 D). Patients with greater than 1.5 D of corneal astigmatism were excluded from the study. (I routinely do limbal relaxing incisions for preoperative corneal astigmatism greater than 1.5 D).
Javal keratometry had been done preoperatively for both groups. If postop keratometry readings were not available, the patient was brought in and postop keratometry readings were done. Keratometry readings were all done by the same technician on the same keratometer. The technician was masked as to the nature and design of the study so that all readings would be unbiased. All postoperative keratometry readings were taken at more than 6 weeks postop.
To determine if any astigmatism was induced postoperatively, I looked for a change in the amount of corneal astigmatism as compared to the preoperative value. To do this, the greater and lesser axis of curvature were measured and the difference in power recorded, as well as the axis. For example, if a patient had keratometry values of 44 D by 44.5 D at 90° preop and values of 44 D by 44.25 D at 90° postoperatively, the incision was considered to induce +0.25 D of corneal flattening at the steeper meridian. Since the surgical wound was at 135° and equidistant between 90° and 180°, flattening was considered to have occurred if the patient’s K values changed, for example, from 44.5 D by 44 D at 90° to 44 D by 44 D at 90°. If the meridian with the greater curvature became steeper, eg, from 44.5 by 44 D at 90° to 45 D by 44 D at 90°, then I considered this to be steepening of the meridian of greater curvature induced by the surgical wound.
Injectable IOL study group
Forceps-inserted IOL study group
Source: Nirenberg N
All operations were carried out under peribulbar anesthesia. The sclerocorneal wound was made at about the 135° meridian, as this oblique location is believed by some to induce lower amounts of astigmatism. The wound was formed by first incising the conjunctiva at the limbus with Westcott scissors from the 9:30 to 11:30 o’clock position. Hemostasis was performed with wet-field cautery.
A Thornton-Fine fixation ring was then placed on the eye opposite the incision site and a Sharpoint 3.2-mm microsurgical slit knife (Surgical Specialties No. 72-3261) was used to enter the eye. The tip was placed at the sclera 0.5 mm from the limbus. The blade was checked to be sure that its width was parallel to the wall of the eye. With slight downward pressure the tip of the blade was advanced into the sclera at the same time, making sure to angle the blade tip up and along the curve of the cornea so the blade passed within the corneal stroma as it was advanced forward. The blade was advanced 1 mm into clear cornea and then angled down into the anterior chamber, resulting in a total incision length of about 1.75 mm.
Phacoemulsification was carried out using a divide-and-conquer technique. Following cortical cleanup and polishing of the posterior lens capsule, a posterior chamber lens implant was inserted. This was either a MemoryLens CV232 inserted directly through the incision with fine smooth forceps or an Acrysof SA60AT injected using the Monarch II injector. After checking the wound for watertightness, the conjunctiva was closed with simple coaptation cautery.
Ciloxan (ciprofloxacin HCl, Alcon) eye ointment was placed on the eye, and the eye was patched and shielded. On postop day 1, patients began a 1-drop dose of both Ciloxan and Pred Forte 1% (prednisolone acetate, Allergan) four times a day. The Ciloxan was discontinued after 5 days and the patients were slowly tapered off the Pred Forte after week 4. Postoperative astigmatism was measured by Javal keratometry at 6 weeks to 6 months postop.
In the SA60AT group, the absolute mean change in corneal astigmatism was 0.19 D, range +0.5 to –0.5 D, variance 0.04. In the MemoryLens group the absolute mean change in corneal stigmatism was 0.16 D, range +0.5 to –0.37 D, variance 0.02. In both groups, most changes clustered about 0, with a 0.25 D or less change to either side. There was only a 0.03 D difference in induced corneal astigmatism between the SA60AT and MemoryLens groups. This was not statistically significant (P = .0021).
The amount of change in corneal curvature varied from an increase in steepening of 0.5 D at the axis of greater curvature to 0.5 D of flattening of the previous axis of greater curvature. Most changes were 0.25 D or less.
A small steepening of the cornea could be attributed to two causes. It could be that excessive cautery for hemostasis caused shrinkage of collagen fibrils and a small steepening in the axis of the incision. The other possible explanation could be that, since most changes fluctuated between –0.25 and +0.25 D, there is no real change in corneal curvature; the readings, clustered around the true value of 0, are just small errors to the plus or minus side due to tolerable error of the instrument or measuring system. This is the more likely explanation since a small amount of steepening or flattening was seen regardless of preoperative axis of greatest curvature.
As expected in an elderly population, most patients were found preoperatively to have their greater axis of curvature at 180° (against-the-rule astigmatism). However, some had the steeper curve in the 90° meridian (with-the-rule astigmatism). On review of the data it was interesting to observe that in both groups there were small amounts of flattening or steepening clustered about 0, indicating a relatively neutral sclerocorneal wound. In only a few instances were the axis of greater or lesser corneal curvature changed by any meaningful amount. Patients who had a small amount of astigmatism at 90° or 180° tended to remain that way postoperatively, with no steepening, flattening or change in the axis of their corneal astigmatism.
In theory, stretching of the surgical wound would be expected to cause a flattening of the cornea at the axis of the wound, inducing corneal astigmatism. There was a surprisingly small amount of surgically induced astigmatism in both the SA60AT group and the MemoryLens group. It appears that there is no clinical difference in the amount of wound stretching and induced corneal astigmatism with insertion of either the MemoryLens CV232 or the Acrysof SA60AT IOLs.
For Your Information:
- Neal Nirenberg, MD, FACS, is in practice with Warren Hill, MD, FACS, at East Valley Ophthalmology, 7525 E. Broadway Rd., Ste. 6, Mesa, AZ 85208; 480-981-6111; e-mail: Nirenberg@A-Scan.Net. Dr. Nirenberg has no direct financial interest in the products mentioned in this article, nor is he a paid consultant for any companies mentioned.
- Jaffe NS, Clayman HM. The pathophysiology of astigmatism after cataract extraction. Trans Am Acad Ophthalmol Otolaryngol. 1975;79:615-630.
- Leen MM, Ho CC, Yanoff M. Association between surgically induced astigmatism and cataract incision size in the early postoperative period. Ophthalmic Surg. 1993;24:586-592.
- Gills, JP. Cataract Surgery: The State of the Art. Slack Incorporated; 1998:30-31.
- Beltrame G, Slavetat M, Chizzolini M. Corneal topographic changes induced by different oblique cataract incisions. J Cataract Refract Surg. 2001;27:721-727.