INTRAOCULAR LENS CALCULATIONS

  • 21/10/2021
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I. INTRODUCTION

Choosing the appropriate intraocular lens (IOL) power is a major determinant of patient satisfaction with cataract surgery. The 3 main factors to consider include taking accurate measurements (biometry), selecting appropriate calculations (formulas), and assessing the patient’s needs and expectations to determine the postoperative refractive target (clinical considerations).

II. BIOMETRY

At minimum, 2 measurements are required to calculate the appropriate implant power: the axial length and the corneal curvature (keratometry) of the eye. Precise measurements are critical given that an error of only 0.3 mm in axial length will result in a 1-D error in IOL power.

A. Axial Length

1. A-scan ultrasound

Axial length has traditionally been obtained utilizing A-scan ultrasound to measure the distance between the anterior surface of the cornea and the fovea. This measurement is determined by calculation; an ultrasound pulse is applied and the transit time through the eye is measured. Using estimated velocities of ultrasound waves through various media (ie, cornea, aqueous, lens, and vitreous) the distance traveled through the eye is then calculated.

1. The instrument should have an oscilloscope screen to differentiate a good measurement from a poor one. Characteristic echo peaks or spikes should be observed when the probe is aligned properly (Figure 16-1). These include the following: a tall peak for the cornea, tall peaks for the anterior and posterior lens capsule, tall peak for the retina, moderate peak for the sclera, and moderate-to-low peaks for orbital fat. If these spikes are not well seen, then the probe may be misaligned.

2. The contact A-scan technique must be performed carefully, as compressing the cornea will result in a shorter-than-expected measurement. It is recommended that measurements are taken to the nearest hundredth of a millimeter, and multiple measurements should be taken and averaged. If several are taken and differ by a significant amount, they should be discarded until consistent readings can be obtained. It is also prudent to measure both eyes for comparison.

3. Measurements must be adjusted for specific clinical situations— silicone oil in the posterior chamber, aphakia, dense cataract, etc. Be sure to choose the machine settings appropriate for the eye being mea- sured (ie, phakic, aphakic, pseudophakic). Additionally, the machine should be regularly calibrated, checking measurements against an eye of known axial length.

2. Immersion technique

  1. The immersion technique of Ossoinig is thought to more accurately represent the true axial length because there is no corneal compression.
  2. In this technique, the patient lies in the supine position and a scleral shell is placed on the eye and filled with Goniosol. The ultrasound probe is placed in this solution and the beam is aligned with the macula by having the patient look at the probe tip fixation light.
  3. Although the immersion method may be strongly advocated by some users, applanation A-scan is the more commonly used method and has provided the data for the overwhelming majority of IOL calculation research. IOL-specific A-constants and ACD-constants are based on applanation techniques and need to be adjusted if immersion measurements are used.

3. Optical biometry: IOLMaster

In the last decade, the technique of optical coherence biometry was introduced by Haigis, which utilizes light rather than ultrasound to measure the length of the eye. The first device introduced was the IOLMaster (Zeiss), based on the principle of partial coherence interferometry using a 780-nm multimode laser diode.

  1. There are 2 advantages to the IOLMaster:(1) Measurements are taken without contact to the patient’s eye, thus eliminating variability due to an examiner technique. (2) The distance measured lies between the anterior surface of the tear film and the retinal pigmented pigment epithelium(rather than the anterior surface of the cornea and the internal limiting membrane), which may be more physiologically accurate (refractive rather than anatomic axial length).
  2. To use this instrument, the patient is asked to focus on a small red fixation light, and the examiner maneuvers the focusing spot within the measurement reticule, sampling areas until the best peak pattern is obtained. Five to 20 measurements are then obtained until the readings differ by less than 0.1 mm. Maximal axial length measured is 40 mm.
  3. The primary disadvantage of this optical device is that any significant axial opacity, such as a corneal scar, dense posterior subcapsular plaque, darkly brunescent cataract, or vitreous hemorrhage, will reduce the signal-to-noise ratio (SNR) to the point that reliable measurements are not possible.

4. Optical biometry: LENSTAR

The second optical biometry device introduced was the LENSTAR (Haag-Streit). This unit utilizes optical low coherence reflectometry with a superluminescent diode laser of 820 nm.

  1. Because of the spectral characteristics of this laser, a high level of resolution can be achieved and reflective structures within the cornea, anterior chamber, lens, and retina can be detected. This allows simultaneous measurements of the axial length, central corneal thickness, anterior chamber depth, and lens thickness.
  2. Although both the IOLMaster and the LENSTAR measure axial length using optical biometry (maximal axial length measured is 32 mm), the LENSTAR also uses this technology to measure anterior chamber depth, while the IOLMaster uses slit lamp imagery, which is considered to be slightly less accurate.

5. Data validation

A-scans should be re-measured under the following conditions:

  1. Axial length is less than 22 mm or greater than 25 mm in either eye.
  2. The difference between the 2 eyes is greater than 0.3 mm.
  3. The measurements do not correlate with the patient’s refraction (ie, hyperopes should have shorter eyes, and myopes should have longer eyes).

B. Corneal Curvature

The general principle of keratometry is the following: an illuminated object (such as a ring) is placed near the eye, and the cornea acts as a convex mirror reflecting light off of its surface, producing a virtual image. The size and position of this image are measured. Knowing the object size, the image size, and the distance between the object and the reflective surface, the radius of the reflective surface can then be calculated. It should be noted that neither manual keratometers nor automated keratometers measure the central curvature directly. Multiple intermediate areas are measured and the central corneal power is calculated.

1. Manual keratometry

The classic Bausch & Lomb keratometer is the gold standard for manual keratometry, which measures 2 reflected mires at approximately 3 to 3.2 mm. Unusual readings obtained via other methods should be checked manually.

  1. The eyepiece should first be carefully focused to the examiner’s eye. Failure to do so may result in errors greater than 1 D in power.
  2. The keratometry measurements should be performed prior to the A-scan, as applanation may result in corneal irregularities.
  3. A single reading is not satisfactory; repeat until 2 identical measurements are taken in both meridians. Measurements from both eyes should be compared and any unusual readings should be rechecked.

2. Automated keratometry

Automated keratometry is based on the same principles, utilizing optical sensors and computerized technology to measure the cornea at the flat and steep meridians. Both the IOLMaster and LENSTAR analyze a pattern of LEDs imaged on the front surface of the cornea to determine the corneal curvature.

  1. The IOLMaster measures curvature based on the relative position of 6 projected light reflections at a 2.5-mm optical zone.
  2. The LENSTAR makes use of 32 reference points, that are arranged on 2 concentric rings with 16 measuring points each. The outer circle is projected in a 2.3-mm diameter ring, and the inner circle is projected in a 1.65-mm diameter ring. The small diameter can provide useful data in patients who have undergone a refractive laser procedure that has altered the central corneal curvature.

3. Topographic keratometry

Topographic analysis measures the corneal curvature over the surface at hundreds to thousands of data points. These measurements are then used to calculate a simulated keratometry value (Sim-K). This type of analysis may provide greater accuracy than keratometers in corneas with irregular astigmatism; however, the derivations vary between different topography units due to variable settings and calibrations. Therefore, care should be taken before using these interchangeably with measured keratometry readings.

4. Data validation

Manual keratometry readings should be repeated under the following conditions:

1. Corneal curvature is less than 40 D or greater than 47 D.
2. The difference is greater than 1 D between 2 eyes.
3. The keratometry measurements correlate poorly with the refractive corneal cylinder.

*Dikutip dari Buku Essentials of Cataract Surgery 2nd Ed, halaman 157-161

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