For a fixed cornea and a fixed lenticule, Cap + Lenticule + Residual stroma = CCT. A thicker cap preserves more of the strong anterior lamellae — but costs residual stroma in equal measure. This tool shows both quantities at once: tensile strength rises while the margin to the RSB floor shrinks.
Move the cap slider and watch the trade-off live.
Max thickness ≈ (OZ² · (|Sph| + |Cyl|))/3 + min. thickness (Z8-settable). Approximation — real Z8 CLEAR plan values differ. Uncheck the box above to enter a known lenticule value manually.
Depth, anterior (top) to posterior
Randleman approximation: strongest anteriorly, declining posteriorly. Area = load borne.
Your biomechanics are right: a deeper cap pushes the lenticule into a weaker zone and keeps more strong anterior lamellae intact → residual tensile strength rises. But because the sum is fixed, the residual stroma falls in lockstep toward the floor.
Ectasia is a bulging of the posterior bed, not a tensile failure. The intact cap “floats” on the partially decoupled interface — it cannot stabilise a razor-thin RSB. That is why the floor governs even though tensile strength looks better on paper. The Z8 input mask isn't wrong — it's conservative by design.
With a high ablation, the real lever isn't cap depth (fixed sum!) but lenticule volume — smaller optical zone / lower correction — or patient selection via CCT.