Scheimpflug Corneal Power Measurements for Intraocular Lens Power Calculation in Cataract Surgery




Purpose


To compare the keratometric (K) readings from the Pentacam-HR (High Resolution) unit with the automated K values from the IOLMaster keratometer (K IOLM ), and to evaluate them in the commonly used intraocular lens (IOL) power calculation formulas for routine cataract surgery.


Design


Prospective, comparative observational study.


Methods


setting : Private practice, Lynwood, California. study population : Fifty cataractous eyes scheduled for surgery between July and August 2012. observation procedure : The K readings from the Pentacam-HR unit taken at the 2-, 3-, 4-, and 5-mm zones and the 2-, 3-, 4-, and 5-mm rings, respectively, from 3 different maps: sagittal corneal front (K F ), true net power (K TNP ), and total refractive power (K RP ) are compared with K IOLM . IOL power calculations were performed with each of the 25 sets of K readings. main outcome measures : The IOL prediction median absolute error (MedAE) obtained with each measurement.


Results


K F averaged 0.03-0.13 diopter (D) higher than K IOLM ( P > .05), K TNP averaged 1.16-1.21 D lower than K IOLM ( P > .001), and K RP averaged 0.23-0.72 D lower than K IOLM ( P > .001), with large variations in the measurements. The MedAE obtained with the different Pentacam K readings ranged from 0.44-0.64 D vs 0.52 D obtained with K IOLM ( P > .05). MedAE was lower in all categories when the pupil was 3 mm or smaller.


Conclusion


The Pentacam K F values were the closest to K IOLM and the K F readings from the 2-mm ring yielded the best results for IOL power calculation.


Automated keratometric (K) readings provided by the partial coherence interferometry device (K IOLM ) IOLMaster (version S.2, Carl Zeiss Meditec, Dublin, California, USA) have been the most widely used values for intraocular lens (IOL) power calculation and have long been considered the gold-standard keratometric measurements. The keratometer within the IOLMaster measures corneal power by analyzing reflections of light projected at 6 points on the cornea over a 2.3-mm paraxial radius.


Anterior segment imaging by the Pentacam Comprehensive Eye Scanner (Oculus, Inc., Wetzlar, Germany) is playing more and more a vital role in preoperative evaluation of cataract patients. The Pentacam uses Scheimpflug technology to acquire detailed images of the cornea and of the anterior chamber. It uses a rotating camera that scans 138,000 true elevation points over both the anterior and posterior corneal surfaces, forming a 3-dimensional image of the cornea. These values are then analyzed by the incorporated software to obtain corneal pachymetry, topography, keratometry, and anterior chamber photography. It also provides a measure of the pupillary diameter and lens densitometry.


As a keratometer, the Pentacam yields multiple sets of data according to different parameters. The most commonly used value is the simulated K (SimK), derived from images taken exclusively from the anterior corneal surface, over a 3-mm ring, using a 1.3375 index of refraction to estimate the corneal power.


The new software of the Pentacam-HR (high-resolution professional model) allows measurement of the corneal power taken from 3 different maps:


The Sagittal (axial) Power map is a placido-style map of the front corneal surface. The K readings (K F ) are derived from a simple conversion formula:


KF=(n1)(1000)/raKF=(n1)(1000)/ra
K F = ( n − 1 ) ∗ ( 1000 ) / r a
where K F is the corneal power in diopters (D), r a is the anterior corneal radius in mm, and n is the corneal index of refraction (1.3375).


The True Net Power map shows the optical power of the cornea by using sagittal curvature values from the anterior and posterior surfaces, and by using the correct refractive indices of 1.376 for the anterior corneal interface and 1.336 for the posterior corneal interface. The K readings (K TNP ) are calculated:


KTNP=[(1.3761)(1000)/ra]+[(1.3361.376)(1000)/rp]KTNP=[(1.3761)(1000)/ra]+[(1.3361.376)(1000)/rp]
K T N P = [ ( 1.376 − 1 ) ∗ ( 1000 ) / r a ] + [ ( 1.336 − 1.376 ) ∗ ( 1000 ) / r p ]

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Jan 9, 2017 | Posted by in OPHTHALMOLOGY | Comments Off on Scheimpflug Corneal Power Measurements for Intraocular Lens Power Calculation in Cataract Surgery
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