A Comparison of Three Methods of Measuring Central Corneal Thickness in Normal and Thinned Corneas

A COMPARISON OF THREE METHODS OF MEASURING CENTRAL CORNEAL THICKNESS IN NORMAL AND THINNED CORNEAS A Thesis Presented in Partial Fulfillment of the Requirements for The Degree of Master of Science in the Graduate School of The Ohio State University By Amber J. Colling, B.A. Graduate Program in Vision Science * * * * * The Ohio State University 2010 Master’s Examination Committee: Dr. Mark Bullimore, MCOptom, PhD, Advisor Dr. Jeffrey Walline, OD, PhD Dr. Karla Zadnik, OD, PhD Copyright by Amber J. Colling 2010 ABSTRACT The thickness of the cornea is an important parameter in Goldmann applanation tonometry, corneal disease, contact lens wear, and refractive surgery. Pachymetry is the measurement of corneal thickness, and currently ultrasound pachymetry is the gold standard. Several non-contact instruments, the Orbscan II (Orbtek, Inc.) and the Visante anterior segment-optical coherence tomographer (Carl Zeiss Meditec) have been introduced that provide more information about the cornea including corneal thickness topography maps. Both of these give corneal thickness measurements across the entire surface of the cornea in contrast to ultrasound pachymetry, which only provides information at one point and requires topical anesthetic and corneal contact. Each of these instruments is based on different principles. Ultrasound is based on the reflection of sound from the anterior and posterior corneal surfaces The Orbscan II is a corneal topographer that measures corneal thickness by analyzing reflections from the reflecting surfaces of the cornea via slit-scanning technology and videokeratography. The Visante AS-OCT uses a Michelson-type interferometer where scanning infrared spots move across the corneal surface with multiple axial scans to form high-resolution cross- sectional images. The technology was first applied in ophthalmic use as retinal OCT. ii The anterior segment OCT uses a longer wavelength of 1310 nm versus 820 nm of the retinal OCT allowing for better penetration through tissues. Little information is available comparing the Ultrasound, Orbscan II and the Visante in normal corneas and thin corneas. The goal of the present study is to compare central corneal thickness measurements from three instruments in normal corneas and corneas that have been thinned due to refractive surgery or keratoconus. To our knowledge, this is the first study to examine central corneal thickness measured by these three instruments in normal and thinned corneas. The aim was to determine repeatability of the instruments, to assess agreement between the instruments, and to assess the effect of corneal thickness on repeatability and agreement. Because each instrument is based on different principles, measurements may be biased when normal versus abnormally thin corneas are measured. Forty-five subjects (15 normal, 15 post-LASIK, and 15 keratoconus) were examined on two occasions, separated by one-to-seven days. Three measurements (later averaged) were made on the right eye of each subject using the three techniques. The subjects were analyzed as three groups: normal corneas, post-LASIK and keratoconus. The data were entered into a Microsoft Excel spreadsheet and were analyzed using SPSS statistical software. Mixed model analysis of variance was used to compare between instruments, visit, and within groups. For each technique and cornea type, the difference between instruments and visits was determined for each subject and analyzed using Bland-Altman analysis to produce the mean, standard deviation, and 95% limits of agreement (LoA). A paired two-sample t test was also conducted for validity and iii repeatability. For normal corneas, mean central corneal thickness was as follows: 550 ± 34 μm for ultrasound, 541 ± 44 μm for Orbscan II, and 532 ± 38 μm for Visante. For post-LASIK corneas, mean central corneal thickness was as follows: 502 ± 40 μm for ultrasound, 495 ± 52 μm for Orbscan II, and 485 ± 42 μm for Visante. For keratoconic corneas mean central corneal thickness was as follows: 529 ± 46 μm for ultrasound, 498 ± 44 μm for Visante, and 490 ± 53 μm for Orbscan II. Mixed model analysis showed a significant interaction between instrument and cornea types (p=0.012). That is, the instruments not only measured significantly different from each other (p < 0.001) but also significantly different when measuring the various cornea types. Repeatability was poorer for all techniques in the keratoconic corneas and for ultrasound. For normal corneas, the Visante was the most repeatable: 95% LoA of –8 to +8 μm followed by the Orbscan: 95% LoA of –11 to +12 μm, and ultrasound: 95% LoA of –15 to +19 μm. For post- LASIK corneas, the Orbscan was the most repeatable: 95% LoA –11 to +11 μm followed by the Visante: 95% LoA of –10 to +13, and ultrasound: 95% LoA –24 to +16 μm. For keratoconic corneas, the Visante was the most repeatable: 95% LoA –36 to +37 μm followed by the Orbscan: 95% LoA –31 to +47 μm and ultrasound: 95% LoA –40 to +53 μm. The results of this study show that there was a significant interaction between the different instruments and the types of corneas that were measured. The Orbscan II and Visante underestimate corneal thickness compared to the ultrasound in normal and post- LASIK corneas. The ultrasound did give a higher thickness than the Visante and the Orbscan II in keratoconic corneas; however the Visante gave a higher central corneal iv thickness than the Orbscan II. Though ultrasound has been considered the gold standard, its limitations have long been recognized including the need for topical anesthetic and subsequent contact with a probe. It only yields a single point measurement of which the accuracy and repeatability may be affected by inappropriate probe positioning. The optical techniques provide more information about the entire cornea and do not require contact. The data acquired by these techniques can be invaluable to clinicians, enabling them to detect sub-clinical keratoconus, follow the progression of disease, and assess pre- and post- keratorefractive and keratoplasty outcomes. It is important to note that in regards to pachymetry, central corneal thickness acquired by the Visante and Orbscan are not yet considered interchangeable with ultrasound. v ACKNOWLEDGEMENTS First and foremost, I would like to thank my co-advisors Drs. Mark Bullimore and Kathryn Richdale. Dr. Bullimore gave me the opportunity to do research. His belief in me gave me the confidence to expand on my T35 summer research project and pursue the Master of Science. This accomplishment means more to me than he may ever know! Dr. Richdale has been my ever-supportive mentor and friend. She is an exemplary clinician and researcher, and I am forever indebted to her for her unwavering guidance and support. I would also like to thank Drs. Jeffrey Walline and Karla Zadnik for their time and effort spent in reviewing and enhancing this study. Their encouragement motivated me to continue the course and achieve the goal! I would also like to thank my parents and my sister for their never-failing love, prayers, and support. They have always taught me to reach higher and never settle for mediocre. And last but not least, I thank Landon Colling for his love and patience. He has always encouraged me to follow my dreams and never give up. vi VITA April 22, 1983 Born – Olathe, Kansas 2004 B.A. Biology, Olivet Nazarene University 2010 Doctor of Optometry, The Ohio State University FIELDS OF STUDY Major Field: Vision Science vii TABLE OF CONTENTS Abstract ii Acknowledgements vi Vita vii List of Figures x List of Tables xii Chapters: 1. Historical Review 1.1 Introduction 1 1.2 Corneal Anatomy 3 1.3 Factors Influencing Corneal Thickness 4 1.3.1 Age, Gender, Ethnicity 4 1.3.2 Keratoconus 5 1.3.3 Refractive Surgery 8 1.4 Pachymetry 10 1.4.1 Ultrasound 10 1.4.2 Orbscan II 11 1.4.3 Visante 11 1.5 Comparative Studies 12 1.5.1 Studies of Normal Corneas 12 1.5.2 Studies of Post-LASIK Corneas 14 1.5.3 Studies of Keratoconic Corneas 14 1.6 Repeatability Studies 15 1.7 Summary 15 2. Objectives 20 3. Methods 3.1 Overview 21 3.2 Subjects 21 3.3 Instruments 22 3.4 Summary of Study Protocol 22 3.5 Statistical Methods 24 viii 4. Results 4.1 Overview 26 4.2 Mean Central Corneal Thickness 26 4.3 Between Session Repeatability 29 5. Discussion 5.1 Overview 43 5.2 Central Corneal Thickness 44 5.3 Repeatability 45 5.4 Ultrasound Pachymetry 46 5.5 Orbscan Pachymetry 47 5.6 Visante Pachymetry 50 5.7 Summary 51 5.8 Limitations 52 6. Conclusions 57 Appendix: Tables of Individual Subject Data 59 List of References 105 ix LIST OF FIGURES Figure 1: Picture of ultrasound pachymeter 17 Figure 2: Picture of Orbscan topographer and pachymetry map 18 Figure 3: Picture of Visante AS-OCT with anterior segment picture and pachymetry map 19 Figure 4: Results comparing mean central corneal thickness in each of the three subject groups and for each of the three techniques 34 Figure 5: Validity: Visante versus Ultrasound for all cornea types. 35 Figure 6: Validity: Orbscan versus Ultrasound for all cornea types 35 Figure 7: Validity: Visante versus Orbscan for all cornea types 35 Figure 8: Validity: Visante versus Ultrasound for normal corneas 36 Figure 9: Validity: Orbscan versus Ultrasound for normal corneas 36 Figure 10: Validity: Visante versus Orbscan for normal corneas 36 Figure 11: Validity: Visante versus Ultrasound for post-LASIK corneas 37 Figure 12: Validity: Orbscan versus Ultrasound for post-LASIK corneas 37 Figure 13: Validity: Visante versus Orbscan for post-LASIK corneas 37 Figure 14: Validity:

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