Same refractive effect (ΔK) — different force redistribution: PRK · FemtoLASIK · CLEAR/SMILE
Anteriorly the result is identical by definition (K1/K2, ΔK = treatment target). The model compares how the IOP load is redirected through the altered architecture — load paths, stress density per load-bearing lamella, and the modelled posterior shape response (TTS model after Reinstein/Randleman).
Force on the posterior surface. Ocular pulse = the physiological 1–3 mmHg IOP fluctuation with every heartbeat, caused by pulsatile choroidal blood inflow (measurable as ocular pulse amplitude, OPA).
Laplace: wall stress ∝ IOP × radius — a larger cornea deforms more at the same pressure. Applies equally to all three procedures (and matters for ICL sizing).
Mean K sets the radius of curvature: flatter corneas (larger r) carry more wall stress (Laplace). K1/K2 also drive the drawn corneal profile live — steeper K = visibly rounder dome (schematic), and treatment flattening shows as well.
K2 − K1 = corneal cylinder, cross-checked against the refractive cylinder (mismatch → lenticular astigmatism). Steep K2 or high corneal cylinder triggers an ectasia-screening flag.
myopic correction
SE = sphere + ½·cylinder → SE −7.0 D ≙ tissue depth ≈ 97 µm (Munnerlyn, at current OZ ≈ 13.8 µm/D)
Munnerlyn: ablation depth ∝ OZ² — reducing the OZ saves tissue when the residual bed is tight, at the cost of halo/night-vision risk (check the scotopic pupil). Enlarging the OZ costs depth but improves optical quality.
No slider — the contribution can only be estimated, not measured. Applied as a fixed penalty for PRK only (centrally destroyed). Not applied to FemtoLASIK (anterior decoupling is already fully contained in the flap term — no double counting) and not applied to CLEAR (Bowman remains intact).
preoperative pachymetry
360° side cut → flap carries no tensile load
only a 2–4 mm incision → cap remains load-bearing
All three in one view — same cornea, same IOP, different give
The three deformed profiles overlaid at identical scale (untreated cornea as dashed reference). Deflection schematically exaggerated; it scales live with the IOP slider and the ocular pulse.
Threshold map: corneal thickness × correction — where does the CLEAR advantage prevail?
Each point = one treatment (x-axis: spherical equivalent incl. astigmatism). Colour = difference in remaining tensile strength (ΔTTS in percentage points, incl. the fixed Bowman penalty for PRK only). Click into the map to set sphere & pachymetry — cylinder and flap/cap sliders update the map live.
CCT (µm)
Spherical equivalent SE = sphere + ½·cylinder (D, myopic) · tissue depth ≈ SE·OZ²/3 (Munnerlyn, current OZ)
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Remaining total tensile strength (TTS model, preoperative = 100 %)
Anteriorly identical (same ΔK) — the procedures do not differ in refractive effect, but in the geometry of force redistribution and the modelled posterior shape stability.