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Brian R Will, MD Adjunct Clinical Professor of Ophthalmology

Biomechanics of Hyperopic Shift in Stage 4 Diffuse Lamellar Keratitis and Central Toxic Keratopathy. Brian R Will, MD Adjunct Clinical Professor of Ophthalmology Loma Linda University School of Medicine Loma Linda, California Medical Director Will Vision and Laser Centers

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Brian R Will, MD Adjunct Clinical Professor of Ophthalmology

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  1. Biomechanics of Hyperopic Shift in Stage 4 Diffuse Lamellar Keratitis and Central Toxic Keratopathy Brian R Will, MD Adjunct Clinical Professor of Ophthalmology Loma Linda University School of Medicine Loma Linda, California Medical Director Will Vision and Laser Centers Vancouver, Washington No Financial Interest

  2. Patient Series8 eyes with Stage 4 DLK • All eyes studied using time matched • Pentacam imaging • Tangential Power Map • Elevation Map • Scheimphlug photographic images • Optical Coherence Tomography • Hi Resolution Corneal images • Imaging began at presentation of Stage 4 DLK • All eyes ultimately recovered 20/20 BSCVA

  3. Mid peripheral steepening Mid peripheral flattening Central flattening Peripheral flattening Central steepening Peripheral steepening Reversal Day 1 to 5 Tangential Power Map Analysis Day 12 to 112

  4. Flap initially thin - Day 1 of Stage 4 DLK Flap markedly thickens by Day 12 Flap resumes normal thickness over time Residual Stromal Bed normal - Day 1 Residual Stromal Bed marked thinning Residual Stromal Bed normal over time Flap initially normal thickness Thickens by 40 microns or more Then becomes normal over time Residual stromal bed normal Thins by 50 microns or more Then becomes normal over time

  5. With injury / epithelial cell removal • Anterior cornea swells • Posterior cornea thins • Caused by changes in interstitial fluid pressure • Change induced by inflammatory cytokines • Did not study the reversibility of the phenomenon • Failed to recognize the biomechanical impact of these fluid shifts • Incorrectly identified keratocyte apoptosis as the mechanism for changes in Pif

  6. Key Concepts - Biomechanics of the Hyperopic Shift • Induced by effects of inflammatory cytokines on interstitial fluid pressures (Pif) • Corneal shape is controlled by the complex relationship between • Localized changes in Pif • Localized changes in tissue tension • Localized changes in tissue compliance • Flap edema created by compliance mismatch between flap and RSB

  7. Local Control of Corneal Pif • Complex interaction between keratocytes and ECM modulated by • Inflammatory cytokines • Transmembrane proteins - Integrins • Actin cell cytoskeleton • Outside-In transmembrane signaling • Pif is Not uniform in the cornea • Central cornea 5X more negative than limbus • Endothelial function cannot create this gradient • Pif gradient drives fluid from periphery to center

  8. Residual Stromal Bed Decreased Compliance Tissue thins Normal Compliance Normal Thickness ∆ Pif LASIK Flap Increased Compliance Tissue thickens ∆ Tissue Volume Relationship between Pif and Tissue tension • Tissue compliance determines the volume of fluid (∆Vol) a tissue can hold at a given Pif (∆Pif) • Compliance is affected by tissue tension • Higher tension - less compliant • Lower tension - more compliant

  9. Inflammatory cytokines drive Pif negative Flap has low tension and high compliance Flap imbibes fluid, swells and thickens Inflammatory cytokines Inflammatory cytokines Inflammatory cytokines drive Pif negative Lowered Pif pulls fluid from the limbus Mid peripheral cornea imbibes fluid, swells and thickens Inflammatory cytokines drive Pif negative Lowered Pif pulls fluid from the limbus Mid peripheral cornea imbibes fluid, swells and thickens As mid peripheral cornea swells the RSB exhibits increased tension + compression Tension and compression decreases compliance of the RSB RSB markedly thins centrally Biomechanical Dynamic

  10. Etiology of Hyperopic Shift • Biologically modulated reversible biomechanical event • Controlling elements include: • Inflammatory cytokines • Change in local interstitial fluid pressures • Local tissue edema • Local tissue tension • Local tissue compliance • Mid-peripheral edema causes localized steepening and central flattening of RSB • Flap edema caused by compliance mismatch between flap and RSB

  11. Clinical Applications • Pentacam and Optical Coherence Tomography demonstrate no evidence of tissue necrosis • Tissue necrosis is a theory not supported by any credible data and should be categorically rejected • Roberts model of biomechanics is demonstrated to be incorrect • Similar biomechanical events occur to some degree in nearly all cases of PRK, LASIK and LASEK • A better understanding of the impact of alteration in local Pif, tissue tension and tissue compliance will have significant effects on predictability of refractive endpoints and the avoidance and management of complications

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