Post-Refractive Ectasia & Residual Bed (RSB): The 300µm Limit & PTA Formula

In excimer laser ablation, the depth of stromal tissue vaporized ($t$, in microns) to correct a myopic prescription of power $D$ across an optical zone diameter ($S$, in millimeters) is governed by Munnerlyn's Formula:

$$t \approx \frac{S^2 \times D}{3} \quad \longrightarrow \quad \text{For a } 6.5\ \text{mm zone: } t \approx \mathbf{14\ \mu\text{m per Diopter of Myopia}}$$

Correcting a $-7.00\ \text{D}$ prescription vaporizes approximately $100\ \mu\text{m}$ of corneal stroma.

The Residual Stromal Bed (RSB) is the thickness of un-ablated, pristine corneal tissue remaining beneath the flap:

$$\text{RSB} = \text{Central Pachymetry (CCT)} - [\text{Flap Thickness} + \text{Ablation Depth}]$$

While historical guidelines tolerated RSB down to $250\ \mu\text{m}$, international consensus now mandates a strict minimum safety threshold of $\ge 300\ \mu\text{m}$. Violating this threshold leaves the posterior stroma incapable of resisting physiological intraocular pressure ($15\ \text{mmHg}$), leading to progressive mechanical blowout.

Dr. Marcony Santhiago proved that ectasia risk is governed by the proportion of total corneal thickness biomechanically altered:

$$\mathbf{PTA} = \frac{\text{Flap Thickness} + \text{Ablation Depth}}{\text{Preoperative CCT}} \times 100\%$$

Large multi-center trials confirmed that a $\text{PTA} \ge 40\%$ is the single most robust independent predictor of iatrogenic ectasia, even in eyes with normal preoperative topography! If a treatment plan yields $\text{PTA} \ge 40\%$, LASIK is strictly contraindicated, and the patient must be redirected to PRK or ICL (Phakic IOL).