Horizontal Corneal White to White Diameter Measurements Using Calipers and IOLMaster

Purpose: To determine if the mean horizontal white-towhite (WTW) corneal diameter, measured intraoperatively with calipers, of a population of cataract surgery patients differs from the currently accepted normal WTW range of greater than 11.0 and less than 13.0 mm. In addition, we compared manual caliper measurements with those with IOLMaster. Setting: Cincinnati Eye Institute, Cincinnati OH. Design: Retrospective cross-sectional study involving chart review. Data collected included age, gender, intraoperative WTW caliper measurements and pre-operative IOLMaster WTW measurements. Methods: Data was collected retrospectively from 516 subjects undergoing cataract surgery aged 19 97 years (average = 73.7). Statistical analysis was performed with Statistical Package for Social Sciences ® versions 21.0 and/or 23.0 (Armonk, NY). Primary outcome analysis was determination of normal WTW range derived from caliper measurements. Secondary outcome analysis involved comparison of caliper versus IOLMaster WTW measurements. Results: Five hundred and sixteen patients studied provided 829 and 977 WTW measurements from calipers and IOLMaster, respectively. Average WTW was 12.22 mm with calipers and 12.12 mm with IOLMaster. Normal range (mean ±2 SD) was 11.2 13.2 with calipers. Caliper measurements were an average of 0.1 mm larger than IOLMaster measurements (p < 0.001). Conclusions: Based on our obtained WTW values using calipers, the normal corneal WTW horizontal diameter is wider than the current accepted range. Additionally, this study found that the IOLMaster provided smaller measurements than calipers. Introduction The corneal diameter as determined by the horizontal white to white (WTW) is an important measurement that has diverse clinical applications within the field of ophthalmology. In the clinic, microphthalmia, relative anterior microphthalmia, microcornea, macrophthalmia, and macrocornea are examples where the measured corneal diameter is important. In the operating room, surgeons have relied upon the measured corneal diameter for sizing of some types of intraocular lenses. Yet, exact estimation of the normal horizontal corneal diameter range is vague, and the current definitions of microcornea and macrocornea referenced in many different textbooks vary from source to source. Some define microcornea in adults as anywhere from 10.0 [1-6] to 11.0 mm [7-8] or less. Similarly, the definition of macrocornea can vary anywhere from 12.5 [6] to 13.0 [1-5,8] mm or greater, although there is general consensus among clinicians that the current accepted standards of normal WTW is greater than 11.0 mm and less than 13.0 mm. The ranges referenced in textbooks are based on measurements taken with manual calipers, as automated devices are a relatively new development that have only appeared within the last two decades. Few studies have sought out to re-evaluate this parameter and attempt to more precisely define the “normal” WTW diameter. A study published by Rufer, et al in 2005 used Orbscan® to evaluate corneal diameters of 390 subjects found that normal limits, defined as the mean ± 2 standard deviations, were between 10.7 and 12.6 mm [9]. This study was limited by one measuring technique and did not include comparison with the IOLMaster, which is the device regarded as having the highest reliability in measuring corneal diameter as compared to other methods [10]. Another recently published study in rural China by Fu, et al, used the IOLMaster to evaluate corneal diameters in 1721 subjects. The authors found a general WTW mean of 11.75 mm and a standard deviation of 0.40 mm. The range resulting from the mean ± 2 standard deviations was 10.95 mm to 12.55 mm [11]. Neither study included measurements taken with manual calipers. In our study, we evaluated horizontal WTW corneal diameter measurements Research Article