Calculation of Residual Stromal Bed Thickness After LASIK
A lamellar laser refractive procedure such as LASIK involves creation of a corneal flap, ablation of the stromal bed, and replacement of the flap. The strength and integrity of the cornea postoperatively depend greatly on the thickness of the RSB. Thickness of the RSB is calculated by subtracting the sum of the flap thickness and the calculated laser ablation depth from the preoperative corneal thickness. For example, if the central corneal thickness is 550 μm, the flap thickness is estimated to be 140 μm, and the ablation depth for the patient’s refraction is 50 μm, the RSB would be 550 μm – (140 μm + 50 μm) = 360 μm. When the surgeon determines the RSB, the amount of tissue removed should be based on the actual intended refractive correction, not on the nomogram-adjusted number entered into the laser computer. For example, if a patient with –10.00 D myopia that is being fully corrected, the amount of tissue removed is 128 μm for a 6.5-mm ablation zone for a broadbeam laser. Even if the surgeon usually takes off 15% of the refraction for a conventional ablation and enters that number into the laser computer, approximately 128 μm of tissue will be removed, not 85% of 128 μm.
Most surgeons believe the RSB should be at least 250 μm. Others want the RSB to be greater than 50% of the original corneal thickness. If the calculation reveals a thinner RSB than desired, LASIK may not be the best surgical option. In these cases, a surface ablation procedure may be a better option, as this will result in a thicker RSB postoperatively.
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.