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 Series 8 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. Tangential Power Map Analysis Day 1 to 5 Reversal Day 12 to 112
Peripheral flattening
Mid peripheral steepening
Central flattening
Reversal
Tangential
Power
Map
Analysis
Day 12 to 112
Peripheral steepening
Mid peripheral flattening
Central steepening

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

6. 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

7. 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

8. 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

9. 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
∆ Pif
∆ Tissue Volume
Residual Stromal Bed
Decreased Compliance
Tissue thins
Normal Compliance
Normal Thickness
LASIK Flap
Increased Compliance
Tissue thickens

10. Biomechanical Dynamic
Inflammatory cytokines drive Pif negative
Flap has low tension and high compliance
Flap imbibes fluid, swells and thickens
Inflammatory cytokines
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

11. 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

12. 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