Accuracy of the Pentacam and IOL Master to Calculate the Effective Corneal Power After Corneal Refractive Surgery

The accuracy of IOL calculation is important for the visual outcome of patients undergoing
cataract extraction and IOL implantation. Different formulas such as Holladay I, HofferQ,
SRKT have been used with excellent results. All formulas use the corneal power among other
factors to calculate the IOL power. Corneal refractive surgery i.e. radial keratotomy (RK),
photorefractive keratectomy (PRK), and Laser in situ keratomileusis (LASIK) changes the
corneal power; therefore, it is difficult to measure the true corneal power after surgery by
any form of direct measurement, such as keratometry, or corneal topography. Keratometry and
topography assume a normal relationship between the anterior and posterior corneal
curvatures, and measure the anterior corneal radius. RK for myopia flattens both the anterior
corneal radius and the posterior corneal radius while PRK and LASIK for myopia flattens the
anterior corneal radius but leaves the posterior corneal radius mostly unchanged.

Standard keratometry measures an intermediate area and extrapolates the central power based
on some very broad assumptions. For this reason, keratometry, autokeratometry and simulated
keratometry by topography will typically over-estimate central corneal power following
keratorefractive surgery for myopia. This inaccuracy leads to an inability to meet the
patients' rising expectations and with the increasing popularity of refractive surgery,
calculating intraocular lens (IOL) power after refractive surgery is becoming increasingly
important.

Different methods to calculate the effective corneal power (keratometry) after refractive
surgery have been described (historical data, effective refractive power, modified Maloney
method, etc), however, intraocular lens power calculations in eyes with previous refractive
surgery remains difficult because of the inaccuracy of keratometry power measurements.

The Pentacam (Comprehensive Eye Scanner) is a non-invasive, diagnostic system created to take
photographs of the anterior segment of the eye by a rotating Scheimpflug camera measurement.
This rotating process supplies pictures in three dimensions. The center of the cornea is
measured very precisely because of this rotational imaging process. The measurement process
lasts less than two seconds and minute eye movements are captured and corrected
simultaneously. By measuring 25,000 true elevation points, precise representation,
repeatability and analysis are guaranteed. It provides a topographic analysis of the corneal
front and back surfaces that is based on the true elevation measurement from limbus to
limbus. Both corneal surfaces can be selected for analysis in axial (sagittal), tangential or
elevation representation modes.

The Pentacam using tomography calculates a virtual model of the anterior segment of the eye.
It is possible to move, zoom and rotate it to detect e.g. iris defects, cornea incisions e.g.
RK or size, location and shape of opacifications in the crystalline lens. The slicing
function in the three dimensions offers a detailed view of the different layers of the eye.
It includes the Holladay report that was developed to improve the calculation of IOLs for
patients which have undergone corneal refractive surgery.

The IOL Master is a non-contact optical coherence biometry that makes possible the exact
measurement of visual axis length. Its accuracy is not affected by high ametropia, pupil size
or state of accommodation. This non-contact technology makes it easy on the patient: no local
anesthesia, water bath, or contact probe. For IOL power calculation after previous refractive
corneal surgery, one can derive the effective corneal power using either the clinical history
method, or the rigid contact lens over-refraction method. It therefore permits selection of a
suitable lens after myopic LASIK/PRK without requiring refractive pre-LASIK data or
additional contact lens over-refraction.

The purpose of this study is to determine the accuracy of the Pentacam and IOL Master to
calculate the effective corneal power (keratometry, corneal curvature) in patients who have
undergone both corneal refractive surgery and lens extraction with intraocular lens
implantation.

Inclusion Criteria:

- Subject must have undergone corneal refractive surgery (RK, PRK or LASIK) prior to
cataract extraction.

- Age: 40 to 80 years old.

- Subjects must have undergone cataract extraction at least 4 weeks prior to enrollment
in this trial.

- Willing and able to comply with scheduled visits and other study procedures.

Exclusion Criteria:

- Preoperative ocular pathology: amblyopia, rubella cataract, proliferative diabetic
retinopathy, shallow anterior chamber, macular edema, retinal detachment, aniridia or
iris atrophy, uveitis, history of iritis, iris neovascularization, medically
uncontrolled glaucoma, microphtalmus or macrophtalmus, optic nerve atrophy, macular
degeneration (with anticipated best postoperative visual acuity less than 20/30),
advanced glaucomatous damage, etc.

- Uncontrolled diabetes.

- Use of any systemic or topical drug known to interfere with visual performance.

- Contact lens use during the active treatment portion of the trial.

- Any concurrent infectious/non infectious conjunctivitis, keratitis or uveitis.

- Any clinically significant, serious or severe medical or psychiatric condition that
may increase the risk associated with study participation or study device implantation
or may interfere with the interpretation of study results.

- Participation in (or current participation) any investigational drug or device trial
within the previous 30 days prior to the start date of this trial.

- Intraocular conventional surgery (other than cataract extraction) within the past
three months or intraocular laser surgery within one month in the operated eye.