Toric Correction of High Astigmatism
Correcting astigmatism during cataract surgery can be achieved by three methods. The surgeon may place an incision, perform an astigmatic keratotomy before or after cataract surgery, or implant a toric IOL. If the astigmatism is not too high, the location and type of the incision used may reduce the amount of aberration by 2.00 to 3.00 D. The results for astigmatic keratotomies are rather unpredictable, and the reduction in astigmatism does not usually exceed 3.00 D.
Although one option in toric IOLs, toric plate haptic silicone lenses (STAAR Surgical, Monrovia, CA), has been used for several years with fairly good results, these lenses are only available with a cylinder of 2.00 or 3.50 D. Also, because they can rotate after implantation, the surgeon may wish to use them in combination with an intracapsular ring, although this step may complicate the operation and incorporate additional costs. A plate haptic lens may also cause delayed postoperative complications, such as decentration and dislocation, especially in eyes with pseudoexfoliation or following a posterior capsulotomy.
A NEW TORIC LENS
Dr. Schmidt Intraocularlinsen GmbH (St. Augustin, Germany), a company of the group HumanOptics AG (Erlangen, Germany), has worked during the past few years to develop a three-piece, foldable toric IOL. The MicroSil Toric IOL is available in powers of between -3.00 and 31.00 D and cylindrical powers ranging from 2.00 to 12.00 D (Figure 1). This IOL, type MS 6116 TU, is a posterior chamber lens (PCIOL) and features stable PMMA haptics in a z-design, as well as a 6-mm optic made of silicone. The IOL's overall diameter is 11.6 mm.
The optic of the lens is marked with two peripheral lines, which indicate the steep axis. The formation of the haptics impedes the spontaneous rotation of the IOL after implantation. The surgeon folds the lens within the box, thus avoiding unnecessary manipulation or false positioning. Implanting the MicroSil Toric lens is somewhat more difficult than inserting a conventional PCIOL because of the shape of its haptics. The surgeon should begin with a capsulorhexis large enough to accommodate them and then insert the inferior haptic into the capsular bag in the usual manner. To insert the superior haptic, he should use an implantation forceps. Then, he must rotate the lens so that its lines align with the steep meridian, which he marked preoperatively at the slit lamp. Manual rotation is possible, even after aspirating the viscoelastic material. With the MicroSil Toric IOL, a good refractive result is best achieved by paying close attention to the wound architecture, the IOL calculation, and the influence of rotational stability within the capsular bag.
DR. GERTEN'S EXPERIENCE
My colleagues and I conducted a retrospective study1 in which we determined the implantation of PMMA toric IOLs to be a promising procedure. Our results indicated, however, that improving the procedure depended on reducing its rate of surgically induced astigmatism by using smaller incisions. This need led to the demand for foldable toric IOLs.
Implantation
First, I use a special marking instrument to mark the patient's axis of corneal astigmatism prior to administering peribulbar anesthesia and while the patient sits upright. After performing phacoemulsification, I implant the IOL through a 3.2- to 3.4-mm incision (Figure 2). Finally, I rotate the lens in the correct axis until the lines in the lens' optic match the marks on the cornea.
The z-design of the MicroSil's haptics significantly increases the IOL's rotational stability compared with conventional C-loop haptics. This design helps to balance any mechanical forces during postoperative capsular bag shrinkage. Additionally, the lens' z-haptics are similar to the conventional C-loop haptics of the PMMA toric IOL initially used for our study in that they undulate in the zones of contact with the capsular bag, thereby further increasing the lens' postoperative rotational stability.
Study Results and Conclusion
My colleagues and I implanted 24 MicroSil Toric IOLs following phacoemulsification in cataractous eyes that displayed corneal astigmatism of between 2.00 and 6.00 D.2 Twenty-two of the patients had congenital astigmatism, and two had astigmatism induced by a keratoplasty procedure. All of the patients' astigmatism was stable, regular, and orthogonal.
After implanting the MicroSil MS 6116 TU, the patients' mean refractive astigmatism decreased from -3.15 D (±1.61 D) preoperatively to -0.36 D (±0.80 D) postoperatively. Because no eye experienced IOL rotation of more than 10º, no realignments were necessary beyond 3 to 6 weeks postoperatively. The lenses rotated about 2.6º on average, which decreased the astigmatic correction by approximately 9%. This rotational result represents a significant increase in the behavior and stability of the toric lens, and therefore in the correction of the astigmatic error compared with previous lenses used in the study.1 This experimentally determined rotational stability, which we specifically attribute to the lens' z-haptic design, shows that implanting this type of foldable, three-piece toric IOL enables the stable correction of corneal astigmatism.
DR. SCHIPPER'S EXPERIENCEHigh Astigmatism
In addition to providing an option for correcting astigmatism in normal cataract surgery, this lens is superb in allowing corrections of very high astigmatism, such as that found after corneal perforation or keratoplasty. My staff and I im-planted the MicroSil Toric lens in two patients with 12.00 D of corneal astigmatism and in one patient with 18.00 D. Although these patients' keratometry values remained un-changed following the operation, their refractive astigmatism was reduced to values of between zero and 1.00 D. These patients are immensely satisfied and now enjoy UCVAs of 20/30 to 20/25. We have not observed any rotation or decentration of the implant in 2 years of follow-up.
An Alternate Implantation Method
Although the lens is designed for implantation into the capsular bag, we placed it into the ciliary sulcus in one case of traumatic aphakia. As in the previously described cases, we have not found any postoperative rotation or decentration of the lens. Of course, this single case cannot justify the regular implantation of the MicroSil into the ciliary sulcus; each case must be evaluated carefully before the surgeon takes this approach.
Summary
In our experience, the MicroSil toric IOL has proven safe and effective. We find it useful in cases of medium-to-high astigmatism. Its implantation is easy to learn, and patients are satisfied with their outcomes.
Isaak Schipper, MD, is head of the Eye Clinic in the Kantonsspitals in Luzern, Switzerland. He is the first surgeon in Switzerland to implant the MicroSil Toric IOL. He holds no financial interest in any product mentioned herein. Dr. Schipper may be reached at isaak.schipper@ksl.ch.
1. Gerten G, Michels A, Olmes A. Toric intraocular lenses. Clinical results and rotations stability. Ophthalmologe. 2001;98:715-720.
2. Gerten G, et al. Foldable toric IOL. Paper presented at: The ESCRS XIV Congress; September 2001; Amsterdam, The Netherlands.
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