With any astigmatism correction system, accurate determination of the steep meridian is essential. The plus cylinder axis of the manifest refraction is used, as this accounts for corneal and lenticular astigmatism, which are “manifest” in the refraction. If the crystalline lens is to be removed at the time of the astigmatic incisional surgery (ie, LRI), the correction should be based on the steep meridian and magnitude as measured with corneal topography or keratometry. Intraoperative keratoscopy/aberrometry can be helpful in determining incision location and effect. The amount of treatment for a given degree of astigmatism employing LRIs can be determined from one of several nomograms, such as the one shown in Table 3-1.
Table 3-1 Sample Nomogram for Limbal Relaxing Incisions to Correct Keratometric Astigmatism During Cataract Surgery
Figure 3-5 600-μm preset diamond knife for creating limbal relaxing incision.
It is prudent to make reference marks, using a surgical marking pen, with the patient sitting up, preferably at the slit lamp (Fig 3-6). Marking with the patient in this position avoids reference-mark error due to cyclotorsion of the eyes. Studies have demonstrated that up to 15° of cyclotorsion can occur when patients move from an upright to a supine position. Concomitantly, during cataract surgery, AK incisions may be placed in pairs along the steep meridian, usually between the 7-mm and 9-mm optical zone and, because of induced glare and aberrations, no closer than 3.5 mm from the center of the pupil. LRIs are placed in the peripheral cornea, near the limbus. AK incisions used to correct post–penetrating keratoplasty astigmatism are often made in the graft or in the graft–host junction, but care must be taken to avoid perforation. When AK incisions are made in the host, the effect is significantly reduced. AK incisions in a corneal graft may require compression sutures at the meridian 90° away, and an initial overcorrection is desired in order to compensate for wound healing.
Figure 3-6 Marking the 6 o’clock axis of the limbus while the patient is sitting upright and looking straight ahead.
Excerpted from BCSC 2020-2021 series: Section 13 - Refractive Surgery. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.