Journal of Refractive Surgery

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BIOMECHANICS SECTION 

Biomechanics of Corneal Refractive Surgery

Dan Z Reinstein, MD, MA, FRCSC; Cynthia Roberts, PhD

Abstract

In this issue of the Journal of Refractive Surgery, we introduce a Biomechanics Section, which will feature one to two original articles per issue, highlighting current work devoted to investigating the biomechanics of the cornea. A new era is dawning in biomechanical research that comes with the ability to not only measure corneal biomechanical properties in vivo,1,2 but also determine the internal structure of the cornea layer by layer.3,4 This is analogous to the early days of topographers or wavefront sensors, when the devices and their capabilities were not well understood. Optical aberrations initially provided an explanation for the patient with visual complaints, despite excellent visual acuity. Ultimately, wavefront customization grew from the ability to objectively document image quality on an individual basis. Similarly, individual variations in biomechanical properties may offer the ability to initially explain, and ultimately predict, corneal response to a surgical procedure, leading to the next generation of biomechanical customization.

Some of the evidence pointing to the impact of corneal biomechanical properties on surgical outcomes lies in the measurement of intraocular pressure (IOP), both before and after refractive surgery. It is well known that measured IOP is reduced, on average, following a refractive procedure. It has been assumed that this is the result of reduced curvature and thickness in myopic procedures. However, Chang and Stulting5 performed a retrospective review of over 8000 myopic LASIK patients, and determined that although measured pressure was reduced on average by approximately 2 mmHg, the range of change was approximately + 10 to -15 mmHg. Every patient in this population had reduced thickness and curvature, and yet almost half of them had an increase in measured IOP. Clearly, the artifact in IOP measurement cannot be explained by thickness alone, and "correction" of measured IOP postoperatively using a linear correction factor based on thickness is problematic.6 This leads to the conclusion that refractive surgery likely alters the fundamental biomechanical properties of the cornea.

Reinstein et al have demonstrated that after LASIK, peripheral thickening of the stromal layer occurs outside of the zone of tissue ablation,4 and this has been corroborated by topography studies demonstrating peripheral steepening after LASIK.7 Therefore, as our understanding of corneal biomechanics improves, surgical outcomes may also improve.8"11

The Biomechanics Section is initiated in this issue with two original articles that present finite element based models of corneal behavior. Deenadayalu et al offer a model that predicts central flattening and a subsequent hyperopic shift after lamellar flap creation that supports clinical data in the literature. Fernandez et al offer a model of corneal response to transplantation, including lamellar and penetrating procedures. Both articles advance our knowledge of corneal behavior.

1. Luce DA. Determining in vivo biomechanical properties of the cornea with an ocular response analyzer. / Cataract Refract Surg. 2005;31:156-162.

2. Grabner G, Eilmsteiner R, Steindl C, Ruckhofer J, Mattioli R, Husinsky W. Dynamic corneal imaging. / Cataract Refract Surg. 2005;31:163-174.

3. Reinstein DZ, Silverman RH, Trokel SL, Coleman DJ. Corneal pachymetric topography. Ophthalmology. 1994;101:432-438.

4. Reinstein DZ, Silverman RH, Raevsky T, Simoni GJ, Lloyd HO, Najafi DJ, Rondeau MJ, Coleman DJ. Arc -scanning very highfrequency digital ultrasound for 3D pachymetric mapping of the corneal epithelium and stroma in laser in situ keratomileusis. / Refract Surg. 2000;16:414-430.

5. Chang DH, Stulting RD. Change in intraocular pressure measurements after LASIK: the effect of the refractive correction and the lamellar flap. Ophthalmology. 2005;112:1009-1016.

6. Liu J, Roberts C. Influence of corneal biomechanical properties on intraocular pressure measurement: quantitative analysis. J Cataract Refract Surg. 2005;31:146-155.

7. Dupps WJ Jr, Roberts C. Effect of acute biomechanical changes on corneal curvature after photokeratectomy. / Refract…

In this issue of the Journal of Refractive Surgery, we introduce a Biomechanics Section, which will feature one to two original articles per issue, highlighting current work devoted to investigating the biomechanics of the cornea. A new era is dawning in biomechanical research that comes with the ability to not only measure corneal biomechanical properties in vivo,1,2 but also determine the internal structure of the cornea layer by layer.3,4 This is analogous to the early days of topographers or wavefront sensors, when the devices and their capabilities were not well understood. Optical aberrations initially provided an explanation for the patient with visual complaints, despite excellent visual acuity. Ultimately, wavefront customization grew from the ability to objectively document image quality on an individual basis. Similarly, individual variations in biomechanical properties may offer the ability to initially explain, and ultimately predict, corneal response to a surgical procedure, leading to the next generation of biomechanical customization.

Some of the evidence pointing to the impact of corneal biomechanical properties on surgical outcomes lies in the measurement of intraocular pressure (IOP), both before and after refractive surgery. It is well known that measured IOP is reduced, on average, following a refractive procedure. It has been assumed that this is the result of reduced curvature and thickness in myopic procedures. However, Chang and Stulting5 performed a retrospective review of over 8000 myopic LASIK patients, and determined that although measured pressure was reduced on average by approximately 2 mmHg, the range of change was approximately + 10 to -15 mmHg. Every patient in this population had reduced thickness and curvature, and yet almost half of them had an increase in measured IOP. Clearly, the artifact in IOP measurement cannot be explained by thickness alone, and "correction" of measured IOP postoperatively using a linear correction factor based on thickness is problematic.6 This leads to the conclusion that refractive surgery likely alters the fundamental biomechanical properties of the cornea.

Reinstein et al have demonstrated that after LASIK, peripheral thickening of the stromal layer occurs outside of the zone of tissue ablation,4 and this has been corroborated by topography studies demonstrating peripheral steepening after LASIK.7 Therefore, as our understanding of corneal biomechanics improves, surgical outcomes may also improve.8"11

The Biomechanics Section is initiated in this issue with two original articles that present finite element based models of corneal behavior. Deenadayalu et al offer a model that predicts central flattening and a subsequent hyperopic shift after lamellar flap creation that supports clinical data in the literature. Fernandez et al offer a model of corneal response to transplantation, including lamellar and penetrating procedures. Both articles advance our knowledge of corneal behavior.

REFERENCES

1. Luce DA. Determining in vivo biomechanical properties of the cornea with an ocular response analyzer. / Cataract Refract Surg. 2005;31:156-162.

2. Grabner G, Eilmsteiner R, Steindl C, Ruckhofer J, Mattioli R, Husinsky W. Dynamic corneal imaging. / Cataract Refract Surg. 2005;31:163-174.

3. Reinstein DZ, Silverman RH, Trokel SL, Coleman DJ. Corneal pachymetric topography. Ophthalmology. 1994;101:432-438.

4. Reinstein DZ, Silverman RH, Raevsky T, Simoni GJ, Lloyd HO, Najafi DJ, Rondeau MJ, Coleman DJ. Arc -scanning very highfrequency digital ultrasound for 3D pachymetric mapping of the corneal epithelium and stroma in laser in situ keratomileusis. / Refract Surg. 2000;16:414-430.

5. Chang DH, Stulting RD. Change in intraocular pressure measurements after LASIK: the effect of the refractive correction and the lamellar flap. Ophthalmology. 2005;112:1009-1016.

6. Liu J, Roberts C. Influence of corneal biomechanical properties on intraocular pressure measurement: quantitative analysis. J Cataract Refract Surg. 2005;31:146-155.

7. Dupps WJ Jr, Roberts C. Effect of acute biomechanical changes on corneal curvature after photokeratectomy. / Refract Surg. 2001;17:658-669.

8. Roberts C. The cornea is not a piece of plastic. / Refract Surg. 2000;16:407-413.

9. Roberts C. Future challenges to aberration-free ablative procedures. } Refract Surg. 2000;16:S623-S629.

10. Roberts C, Dupps WJ. Corneal biomechanics and their role in corneal ablative procedures. In: MacRae SM, Krueger RR, Applegate RA, eds. Customized Corneal Ablation: Quest for Supervision. Thorofare, NJ: SLACK Incorporated; 2001:109-132.

11. Roberts C. The impact of corneal biomechanics on outcomes in laser refractive surgery. In: Buratto L, Brint S, eds. Custom LASIK: Surgical Techniques and Complications. Thorofare, NJ: SLACK Incorporated; 2003:489-491.

10.3928/1081-597X-20060301-14

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