Playing Our Cards Right
Why the evidence shows that high PTA is a risk factor for post-LASIK ectasia
Marcony Santhiago |
If percentage tissue altered (PTA – the combination of the flap thickness plus the ablation depth divided by the pre-operative central corneal thickness) is high, it represents a risk factor for post-LASIK ectasia (1)(2)(3)(4)(5)(6). And the higher the PTA, the higher the risk. I’d like to highlight three important points to go along with that statement: firstly, the concept of PTA comes from a solid theoretical foundation. Secondly, it is a risk factor and not a screening method. And thirdly, how risk factors are investigated is of utmost importance.
The creation of a LASIK anterior lamellar flap should not normally be associated with a significant loss in corneal biomechanical strength. However, corneal tensile strength is not uniform throughout the central cornea (posterior corneal stromal tissue is weaker than anterior stromal tissue – especially the posterior two-thirds of the cornea), meaning that the deeper the LASIK flap cut, and the greater the amount of tissue ablated, the weaker the remaining cornea becomes (7). Based on these structural differences, it is reasonable that a ratio or equation would be representative of post-LASIK changes, specifically of values of residual stromal bed or corneal thickness. And that’s why we proposed measuring PTA as a risk factor for post-LASIK corneal ectasia.
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