« The Modified CTR | Main | Multifocal lenses and children »
May 28, 2006
Where Is the Axis?
CRS Today May 2006
The surgical correction of astigmatism.
Bruce Wallace III, MD, FACS
Central corneal surgery such as PRK, Epi-LASIK, and LASIK can reduce unwanted postoperative astigmatism, but these procedures are relatively expensive alternatives to incisional, peripheral, corneal surgery such as limbal relaxing incisions (LRIs). Excimer laser ablation also means another surgical event for patients, and some of these patients may have other ocular conditions such as dry eye, which might adversely affect their postoperative outcome. Most surgeons have found that LRIs provide a safe and effective means of correcting unwanted cylinder with remarkable postoperative stability. For these reasons, LRIs have become a popular method to reduce astigmatism.
Astigmatic procedures require the surgeon to select the proper axis of astigmatism. With toric IOLs and LRIs, the plus axis is the reference point. With corneal laser correction, the minus axis is the reference point.
AXIS LOCATION STUDY
My colleagues and I performed a study to decipher the best way to locate the proper axis for astigmatic correction.1 We found a surprising variance between refractive, keratometric, and corneal topographic axis locations among patients with 2.00D or less of measurable cylinder. However, for eyes with more than 2.50D, we began to see a stronger correlation of the axis' location with all methods used to measure corneal cylinder.
Most of my patients in need of astigmatic correction benefit from LRIs. When planning the incisions, I pay most attention to the axis' location and cylindrical pattern as displayed by the corneal topographic map. If I find a significant disparity between corneal topography and keratometry (ie, more than 30°), I may elect to postpone the LRI until after the lens' replacement and usually will offer any needed astigmatic correction for this eye in tandem with lens surgery on the fellow eye.
Posted by aman at May 28, 2006 10:51 PM