The usual shape of the front surface of the eye (the cornea) is round. It allows light to focus on the back of the eye near the retina. If the corneal surface is oval instead of round, a condition known as astigmatism is present. Astigmatism causes blurred vision and is corrected with either glasses, contact lenses, and in some cases surgery.
Surgically placed relaxing incisions (small incisions on the corneal surface) cause the corneal curvature to change, becoming more rounded, thereby reducing astigmatism. The incisions differ in number and length according to preoperative measurements.
Relaxing incisions do not always totally eliminate astigmatism. In some cases there may be over-response or under-response. Seldom is corneal astigmatism completely corrected; however, significant reduction produces a more normal visual result. Small corneal perforations may occur but are uncommon with modern instrumentation.
When performed in combination with cataract and lens implant surgery, relaxing incisions can result in improved uncorrected visual acuity which in turn decreases the need for glasses after surgery. Many of the more experienced cataract surgeons in the United States use relaxing incisions routinely for patients with corneal astigmatism.