A better way to predict sulcus size for posterior phakic IOLs

The standard measure, measuring limbus size with a caliper, may yield to extrapolating sulcus size from sphere and mean corneal power.

MONTREAL — With the advent of posterior chamber phakic IOLs, it has become necessary to measure the size of the sulcus to fit the phakic IOL properly. Because direct measurement of the sulcus is impossible, ophthalmologists have traditionally measured the limbus size with a caliper, and then adjusted by adding 0.5 mm for myopic eyes and subtracting 0.5 mm for hyperopic eyes.

Recent studies have suggested, however, that most posterior chamber phakic IOL complications are the result of improper sizing. In addition, no method of estimating sulcus size through limbal measurement has been tested.

In this study by Mihai Pop, MD, ultrasound biomicroscopy (UBM) was used to correlate caliper sizing of the limbus. However, his study found that there is no perfect measurement device or equation for the job. The standard measures of anterior chamber depth, axial length, pachymetry, keratometry and refraction were also researched.

Study methods

The study evaluated 43 eyes of 42 patients. The mean age of the 13 men and 11 women was 34 years. The limbus was not measured in 10 eyes. All patients were measured during a preoperative evaluation for phakic anterior or posterior IOL implantation to correct myopia or hyperopia. The manifest reaction and best corrected visual acuity were evaluated.

The UBM examinations were performed using a Humphrey Instruments model 840 biomicroscope with a 50-MHz transducer. The biomicroscope has a probe with an axial and lateral resolution of 50 µm, according to the manufacturer, and provides a sampling resolution on scans of 5 µm.

Before the measurements were taken, topical proparacaine 0.5% was used to anesthetize the cornea. An eyecup was inserted between the upper and lower eyelids and was filled with gonioscopic solution. The patient was asked to fixate on a ceiling target with the fellow eye to maintain accommodation and fixation, according to the study.

Cross-sectional images were taken through the cornea, iris, ciliary body and lens, from nasal to temporal and from superior to inferior, providing 360-degree coverage of the anterior chamber on a central axis. Using the UBM’s software image calipers, the examiner recorded two-dimensional echograms, except for sulcus size.

UBM was used to measure anterior chamber depth from the corneal endothelium to the anterior surface of the lens capsule and corneal thickness from the central endothelium.

The sulcus was evaluated by a technician, who took measurements from three axial images using the software calipers of the ultrasound biomicroscope. The nasal cross-section was measured from the nasal iris root at the ciliary process to the nasal edge of the iris. The central cross-section was measured from both edges of the iris. The temporal cross-section was measured from the tem poral edge of the temporal iris root at the ciliary process. The nasal, central and temporal sections of the anterior chamber were printed and assembled on paper, and sulcus size was calculated 1:17 scale conversion.

Using the data recorded by the UBM computer, the sulcus size was estimated by another technician. This technician also performed the measurements with another set of recorded echograms.

Axial length was determined by ultrasonography. The probe directly touched the surface of the cornea. The axial length was measured from the corneal epithelium to the macula.

Anterior chamber, vitreous depth and lens thickness were also evaluated using ultrasonography. The anterior chamber depth was measured from the corneal epithelium to the macula.

The limbus size was measured using calipers with a lowest unit of measure of 1 mm.

Central corneal thickness was measured with a pachymeter. The pachy meter directly touched the surface of the anesthetized cornea. Three measurements were averaged for each eye.

Results

Dr. Pop found that sulcus size was not significantly correlated with limbus size. The coefficient of variation of the UBM measurement for the sulcus size was 4.6%, which was slightly greater than the coefficient variation of the caliper measurement of the limbus size, 4.2%.

The results of the study found that the traditional estimation of sulcus size through limbal measurement is inadequate because the limbus size alone cannot predict sulcus size. The sulcus, even though measured by UBM, represented greater variations that the measurement of the limbus performed with a caliper.

Dr. Pop’s study found that using a backward elimination multiple regression performed with all measures to predict sulcus size resulted in the following equation: Sulcus size = 18.9 + –0.023 × sphere + –0.15 × mean keratometry. With this equation, there was a greater chance of accurately predicting the actual sulcus size, he said.

Significant negative power correlations were found between corneal power and the sulcus, limbus and sphere. Dr. Pop believes corneal power may be a good indicator of the eye’s proportions.

The difference between the sulcus and the limbus was 0.6 ± 0.7 mm for myopia and 0.3 ± 0.8 mm hyperopia, with a difference of 0.3 mm between the two groups. Therefore, the traditional approach of subtracting 0.5 mm for myopia and adding 0.5 mm for hyperopia was considered inadequate by this study. In general, Dr Pop said, sulcus size showed a low correlation with the other parameters.

For Your Information:

Mihai Pop, MD, can be reached at Michel Pop Clinics, 9001 l’Acadie Boulevard North, Suite 900, Montreal, Quebec H4N 3H5, Canada; +(1) 514-381-2020; fax: +(1) 514-381-2144.

References:

Pop M, Payette Y. Predicting sulcus size using ocular measurements. J Cataract Refractive Surg. 2001;27:1033-1038.