PERCENT TISSUE ALTERED CALCULATOR

PTA is the percentage of corneal thickness removed or cut through in a laser procedure: flap or cap plus ablation, divided by the thinnest pre-operative pachymetry. Enter four values below; the calculation runs in your browser.

WHY THE CEILING IS NOT A SINGLE NUMBER

The commonly quoted 40% threshold comes from eyes with normal pre-operative topography, where percent tissue altered ≥40% was present in 97% of eyes that went on to develop ectasia, with an odds ratio of 223 — far stronger than any other single variable in that series.^[1] RLES AI applies the ceiling as a function of corneal steepness: 40% when Kmax is 46 D or above, 45% below it. A steeper cornea gets the tighter limit.

The threshold was derived and validated in LASIK. In lenticule extraction and surface ablation, PTA is calculated and reported but is not treated as a pass/fail test, because no validated threshold exists for those procedures; there, the residual stromal bed and residual corneal thickness floors decide.

WHAT PTA DOES NOT TELL YOU

PTA is one of three independent columns. Residual corneal thickness is controlled separately, and it behaves differently: it equals pachymetry minus ablation, so it is independent of flap thickness. Thinning the flap improves PTA and the residual bed but leaves RCT untouched — only reducing ablation repairs an RCT violation. Abnormal topography, residual bed thickness, age and pre-operative corneal thickness are all part of published ectasia risk models,^[2] and none of them appears in a PTA figure.

FROM ONE NUMBER TO A PLAN

This page computes one value. A signature-ready plan tests roughly 300 items for the same eye — refraction, topographic and tomographic indices, biomechanics, device nomogram rows, hinge geometry, the post-operative keratometry window, medication and chronic-disease screening — and names every item that was not reported. See the full data set and the evidence behind it.

FREQUENTLY ASKED QUESTIONS