Playing Our Cards Right
Why the evidence shows that high PTA is a risk factor for post-LASIK ectasia
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 (
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 (
When we first started trying to determine new methods of assessing corneal biomechanics with BJ Dupps, we also looked at which intraoperative variables induced changes in these parameters. PTA was one of those investigated and, to our surprise, it had the highest number of significant correlations with changes in the biomechanical variables under assessment. As ectasia likely represents a reduction in biomechanical integrity below the threshold required to maintain corneal shape and curvature, we wanted to understand if there was an association between PTA and ectasia. We identified that PTA was significantly higher in a group of patients who developed ectasia after LASIK compared with a group of patients who hadn’t developed any complications three years after surgery (
Our case-control study was appropriately designed to investigate PTA as a risk factor – and this matters. Saad et al. (
Another flaw was the actual design of the study; cohort studies are for common outcomes with a rare risk factor. Ectasia is not a common outcome – the authors themselves cited it as having an incidence of 0.04–0.6 percent (
We’ve not finished our work on PTA as a risk factor for ectasia – we’ll shortly be publishing our tenth paper. We understand that flap thickness is different to ablation depth so we are working on an equation that gives constants for those variables. There will be people with a PTA higher than 40 percent who will never develop ectasia, because ectasia is a rare adverse event. But our take home message is very simple: high PTA is a risk factor for ectasia. And we’ve proved it through a correctly-designed case-control study that demonstrated a high odds ratio (
- MR Santhiago et al., “Changes in custom biomechanical variables after femtosecond laser in situ keratomileusis and photorefractive keratectomy for myopia”, J Cataract Refract Surgery, 40, 918–928 (2014). PMID: 24726160.
- MR Santhiago et al., “Association between the percent tissue altered and post-laser in situ keratomileusis ectasia in eyes with normal preoperative topography”, Am J Ophthalmol, 158, 87–95 (2014). PMID: 24727263.
- MR Santhiago et al., “Role of percent tissue altered on ectasia after LASIK in eyes with suspicious topography”, J Refract Surg, 31, 258–265 (2015). PMID: 25884581.
- 4. MR Santhiago et al., “Relative contribution of flap thickness and ablation depth to the percentage of tissue altered in ectasia after laser in situ keratomileusis”, J Cataract Refract Surg, 41, 2493–2500 (2015). PMID: 26452462.
- MR Santhiago. “Percent tissue altered and corneal ectasia”, Curr Opin Ophthalmol, 27, 311–315 (2016). PMID: 27096376.
- MR Santhiago et al., “Ectasia risk factors in refractive surgery”, Clin Ophthalmol, 10, 713–720 (2016). PMID: 27143849,
- 7. I Schmack et al, “Cohesive tensile strength of human LASIK wounds with histologic, ultrastructural, and clinical correlations”, J Refract Surg, 21, 433–445 (2005). PMID: 16209440.
- A Saad et al., “Evaluation of the percentage tissue altered as a risk factor for developing post-laser in situ keratomileusis ectasia”, J Cataract Refract Surg, 43, 946–951 (2017). PMID: 28823442.
- World Health Organization. “Global health risks. Mortality and burden of disease attributable to selected major risks”. Available at: bit.ly/WHOrisks. Accessed January 12, 2017.
- JB Randleman et al., “Risk factors and prognosis for corneal ectasia after LASIK”, Ophthalmology, 110, 267–275 (2003). PMID: 12578766.
- M Miraftab et al., “A modified risk assessment scoring system for post laser in situ keratomileusis ectasia in topographically normal patients”, J Ophthalmic Vis Res, 9, 434–438 (2014). PMID: 25709767.
Marcony Santhiago is Professor of Ophthalmology at the Federal University of Rio de Janeiro and the University of Sao Paulo, Brazil; and Adjunct Professor of Ophthalmology at the University of Southern California, Los Angeles USA.