Percent Tissue Altered (PTA) measures how much of the cornea’s total thickness is structurally affected during LASIK. It combines flap thickness and ablation depth relative to original corneal thickness.
Why does this matter?
Because corneal strength is not evenly distributed. The anterior stroma contributes disproportionately to biomechanical stability. ¹ Excessive disruption of this region weakens the cornea’s ability to resist outward pressure.
Santhiago and colleagues demonstrated a strong association between higher PTA values and postoperative ectasia, even in eyes with “normal” pre-operative topography. ²
In practical terms, when PTA approaches or exceeds recognised safety cut-offs, risk increases sharply. This applies even if residual thickness appears “acceptable” on paper.
At Blue Fin Vision®, PTA is a routine part of evaluation. It prevents cases that may look suitable superficially from crossing invisible biomechanical boundaries.
LASIK should never be reduced to “Do you have enough thickness?” The more appropriate question is:
How much of your structural integrity is being altered?
When PTA breaches safe parameters, we do not proceed.
References
- Randleman JB, Dawson DG, Grossniklaus HE, McCarey BE, Edelhauser HF. Depth-dependent cohesive tensile strength in human donor corneas. Invest Ophthalmol Vis Sci. 2008;49(10):4031-4036.
- Santhiago MR, Smadja D, Gomes BF, et al. Association between the percent tissue altered and post-LASIK ectasia. Am J Ophthalmol. 2014;158(1):87-95.
- Randleman JB, Woodward M, Lynn MJ, Stulting RD. Risk assessment for ectasia after corneal refractive surgery. Ophthalmology. 2008;115(1):37-50.
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