PTA Is Not the Way
We need to rethink the concept of PTA as a predictor of post-LASIK iatrogenic ectasia risk
Alain Saad |
Percent tissue altered (PTA) has been presented as a significant predictor of post-LASIK ectasia risk (1)(2). But I don’t believe that it is – and I’d like to explain why.
There are two major drawbacks that limit the applicability of PTA in our daily practice. Firstly, the way PTA has been computed does not reflect the true biomechanical instability induced by LASIK. Secondly, a risk factor should not be used as a screening metric.
1) The core limitation of PTA:
In their 2014 preliminary paper, Santhiago et al. (1) achieved 97 percent sensitivity and 89 percent specificity for PTA ≥40 as a predictor for ectasia in post-LASIK patients who had normal pre-operative topography. But there was no external validation of these findings until recently. We have made it our objective to evaluate the PTA metric in independent patient populations. We first performed a retrospective analysis of 593 eyes with normal pre-operative topography that underwent LASIK surgery and had a minimum of two years follow up (3). Not a single case of iatrogenic ectasia was found – despite 126 eyes (21 percent) having a PTA ≥40, and with 19 eyes (3.2 percent) having a PTA >47 – a value which Santhiago et al. (1) reported as having 100 percent specificity. Thus, our independent study did not confirm the specificity of PTA as a predictor for ectasia. In fact, the 126 cases would have been unnecessarily rejected for LASIK surgery if PTA>40 was applied as a screening tool in this population. A recent study by Djodeyre et al found similar results with 20 percent of their cases having a PTA>40 and none developing iatrogenic ectasia (4).
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