Chapter 7 Ocular Biometry

Brian K. Armstrong, Glauco Reggiani Mello, Ronald R. Krueger, Modified with permission from: Rocha KM, Krueger RK. Ophthalmic biometry. Ultrasound Clin 2008; 3(2):195–200.

Introduction

Cataract surgery and intraocular lens (IOL) implantation are currently evolving into a refractive procedure. The precision of biometry is crucial for meeting expectations of patients undergoing cataract surgery.¹ Moreover, the optimal results for new IOLs being developed, such as toric, multifocal, accommodative, and aspheric, all depend on the accuracy of biometry measurements. To meet these expectations, attention to accurate biometry measurements, particularly axial length (AL), is critical.² The fundamental points for accurate biometry include the AL measurements, corneal power calculation, IOL position (effective lens position [ELP]), the selection of the most appropriate formula, and its clinical application.

The measurement of axial eye length is one of the most important steps for IOL lens power calculation. An error in AL measurement of 1 mm can cause an error in IOL power of 2.5D (approximately). AL can be measured with a laser interferometer based system (IOL Master®) or with an ultrasound based system.

IOL Master®

Instrumentation and methods

Non-contact partial coherence laser interferometry (Zeiss IOL Master®, Carl Zeiss AG, Oberkochen, Germany) is used routinely by ophthalmologists worldwide to estimate IOL power before cataract surgery. It was developed to increase the accuracy of biometry measurements and has been shown to be more accurate and reproducible than ultrasound, using contact techniques.³ It is a non-contact and operator-independent method that emits an infrared beam, which is reflected back from the retinal pigment epithelium. The patient is asked to fixate on an internal light source to ensure coaxial alignment with the fovea. The reflected light beam is captured and the AL is calculated by the interferometer.

Because optical coherence biometry uses a partially coherent light source of a much shorter wavelength than ultrasound, AL can be more accurately obtained (reproducible accuracy of 0.01 mm).⁴ The IOL Master® also provides measurements of corneal power and anterior chamber depth, enabling the device to perform IOL calculations using newer generation IOL calculation formulas.⁴ As the patient must look directly at a small red fixation light during measurements with the IOLMaster®, AL measurements will be made to the center of the macula giving the refractive AL rather than the anatomic AL. For eyes with extreme myopia or posterior staphyloma, being able to measure to the fovea with the IOL Master® is an enormous advantage over conventional A-scan ultrasonography.⁵

Mechanism

The Michelson interferometer portion of the IOL Master® is used to create a pair of coaxial 780-nm infrared light beams with a coherence length of approximately 130 µm. Unlike the classic Michelson interferometer, for which the eye would have to be kept perfectly still, the use of a dual coaxial beam allows the IOL Master® to be insensitive to longitudinal movements and makes AL measurements mostly distance-independent. While one mirror of the interferometer is fixed, the other mirror is moved at a constant speed by a small motor. This process takes one of the light beams out of phase with the other by twice the displacement of the moving mirror. Both beams of light then illuminate the eye to be measured and are reflected at the level of the cornea and the retinal pigment epithelium. After passing through a polarizing beam splitter, all light beam components are combined together, producing interference fringes of alternating light and dark bands. The constant speed of the measuring mirror causes a Doppler modulation of the intensity of the interference pattern. An optical encoder is then used to sense the position of the moving mirror with great precision, which is then translated into an AL.⁵

A-constants and optimization

IOL constants for the IOL Master® are closer to those normally seen for the immersion technique and are typically higher than what would normally be used for the applanation technique, which is based on a falsely short AL due to corneal compression. In making the transition from applanation A-constants to IOL Master® A-constants, one should increase already optimized applanation IOL constants by 0.50 for the SRK/T formula and by 0.29 for the Holladay and Hoffer Q formulas. Failure to make this adjustment may result in approximately 0.50 diopters of postoperative hyperopia. The IOL Master® software comes with an IOL constant optimization feature that can subsequently be used to refine postoperative outcomes. Some lenses, like the Alcon SA60AT, show very little difference when compared to immersion A-scan ultrasonography, while others, like the Bausch & Lomb U940A, show a larger difference.⁵

Troubleshooting

Limited fixation

The IOL Master® requires the patient to fixate on the device’s internal light to allow for coaxial alignment with the fovea. Patients with retinal detachment or poor cooperation/fixation are unable to be tested with the IOL master and need to be evaluated by the ultrasonic methods.

Opaque media

In a series of studies, the IOL Master® (Carl Zeiss Meditec, Dublin, CA) failed to acquire AL in 8–22% of the patient population with cataract.⁶ Because the device is dependent upon an emitted light beam for AL calculation, the IOL Master® can be confounded by corneal scarring, mature or posterior subcapsular cataracts, or vitreous hemorrhage. These patients may require evaluation by ultrasonic methods.

False positive readings

On rare occasions, reflections from the surface of an IOL in the pseudophakic eye may produce an AL reading falsely short by as much as 4.0 mm. This sometimes occurs if the measurement is taken directly through a reflection off the IOL. This phenomenon has been seen in pseudophakic eyes with PMMA, silicone, and acrylic IOLs and should be suspected if there is a large difference in AL between the right and the left eyes. This can easily be avoided by taking measurements from multiple areas within the measurement reticule and with the focusing spot moved away from any IOL reflection.⁵

Biometric A-scan ultrasound

An A-scan is widely used for biometric calculations. See Clip 7.1 It should be remembered that ultrasonic AL measurement is actually determined by calculation. The ultrasonic biometer measures the transit time of the ultrasound pulse and, using estimated velocities through the various media (cornea, aqueous, lens, and vitreous), calculates the distance.¹ In some cases the precision of the measurements can be optimized by use of B-scan, so we will discuss some of those clinical scenarios. Clinical decisions can be made during dynamic examinations. A-scan biometry includes two main techniques: contact method and immersion technique.

Contact

In the contact (applanation) method, the ultrasound probe directly touches the cornea. The contact technique is completely examiner dependent because it requires direct contact and anterior compression of the cornea. See Clip 7.2 Previous studies have demonstrated a mean shortening of AL by 0.1 to 0.33 mm using the contact technique compared with immersion technique.⁷^–¹⁰ In the echogram for the axial eye length measurement, the first spike represents the probe tip placed on the cornea, followed by the anterior lens capsule, posterior lens capsule, vitreous cavity, retina, sclera, and orbital tissue echoes (Figure 7.1). The corneal spike is a double-peaked echo to represent the anterior and posterior surfaces of the cornea. The retinal spike is generated from the anterior surface of the retina. This echo needs to be highly reflective with a sharp 90° take-off from the baseline. The scleral spike is another highly reflective spike just posterior to the retinal spike. The orbital spikes are low reflective spikes behind the scleral spike.

Figure 7.1 Contact A-scan of a normal phakic eye. The spikes correspond to corneal surface (C), anterior (AL) and posterior lens capsule (PL), and retina (R).

Reproduced with permission from: Rocha KM, Krueger RK. Ophthalmic biometry. Ultrasound Clin 2008; 3(2):195–200.¹

Immersion

Because the immersion method eliminates compression of the globe, this technique has been shown to be more precise than contact biometry (Figure 7.2). See Clip 7.3 In the immersion technique, a scleral shell filled with fluid is placed over the cornea while the patient lies supine. The most commonly used scleral shells are the Hanson shells (Hansen Ophthalmic Development Laboratory, Coralville, IA, USA) and the Prager shells (ESI Inc., Plymouth, MN, USA). The shells come in different sizes, although the 20 mm shell is the most versatile. The probe is immersed in the fluid overlying the cornea. Clinically, this method is important in eyes with a small AL (high hyperopia, microphthalmos, nanophthalmos). Phakic AL measurement spikes using the immersion technique:

• The initial spike is the tip of the probe, which has no clinical significance. This echo is now moved away from the cornea in immersion and has become visible.

• The other spikes, from left to right on the screen, represent anterior lens capsule, posterior lens capsule, retina, sclera, and the orbital fat.

Figure 7.2 Immersion A-scan of a normal phakic eye. The spikes correspond to waterbath (W), anterior and posterior cornea surface (C), anterior (AL) and posterior lens capsule (PL), and retina (R).

Reproduced with permission from: Rocha KM, Krueger RK. Ophthalmic biometry. Ultrasound Clin 2008; 3(2):195–200.¹

Settings

Most instruments offer the choice of either a manual or automatic (“pattern recognition”) measurement mode. To obtain an AL measurement in manual mode, the examiner determines the A-scan to be measured and depresses a foot pedal to take the measurement. This is the preferred methodology because the examiner can examine the spikes and make sure that they are properly aligned and appropriately gated. In the automatic mode, the machine is programmed to recognize spikes that occur within a preset range from the probe. When the display has a series of spikes which the software recognizes, the instrument will record that measurement. When the automatic mode is in use, the instrument is prone to making errors and giving inadequate measurements. There is also an option of a contact or immersion measurement mode. Make sure this setting is correct, otherwise your measurements will be erroneous. Biometers also have phakic, pseudophakic, and aphakic settings that can be chosen based on the lens status of the patient. Some instruments also have a dense cataract setting. The dense cataract setting is used when the examiner is having difficulty displaying a distinct, high spike from the posterior lens capsule and the retina.¹¹

Velocity settings

Accurate measurements of AL require the use of appropriate sound velocity settings. Sound waves travel at different speeds according to the physical properties of the medium. The ultrasound velocity varies in relation to the medium within the eye and IOL materials (Tables 7.2 and 7.3).^12,¹³ In a normal phakic eye, the average ultrasound velocity is 1555 m s⁻¹. In eyes with a short AL (<20 mm), it is 1560 m s⁻¹, whereas in longer eyes it is 1550 m s⁻¹. This difference is due to an inverse proportional shift in the axial ratio of solid to liquid as the eye increases in length.

Table 7.2 Sound velocities for axial length measurements.

Medium	Velocity (m s⁻¹)
Soft tissue	1550
Cornea	1641
Aqueous / vitreous	1532
Crystalline lens	1641
Silicone oil	980

Reproduced with permission from: Rocha KM, Krueger RK. Ophthalmic biometry. Ultrasound Clin 2008; 3(2):195–200.

Table 7.3 Average sound velocities according to lens status.

Eye types	Velocity (m s⁻¹)
Phakic	1555
Aphakic	1532
Pseudophakic (PMMA)	1556
Pseudophakic (acrylic)	1549
Pseudophakic (silicone)	1476
Phakic (gas)	534
Phakic (silicone oil)	1139
Aphakic (silicone oil)	1052

Reproduced with permission from: Rocha KM, Krueger RK. Ophthalmic biometry. Ultrasound Clin 2008; 3(2):195–200.

The electronic gates

Electronic gates allow ultrasound units to provide an electronic read-out of the AL in millimeters. Biometers are equipped with two or four gates. The two main gates are the “corneal gate” placed in the region of the corneal spike and the “retinal gate” placed in the region of the retinal spike. Instruments equipped with four gates allow positioning of these gates over the leading edges of the echoes generated from the anterior surface of the cornea, the posterior surface of the cornea, the anterior surface of the lens, the posterior surface of the lens, and the anterior surface of the retina.¹⁴ These gates need to be properly monitored and adjusted during AL measurement because certain clinical situations can interrupt normal gating (i.e., dense cataracts, IOLs, etc.). Improper placement of gates can read the measurement between two unrelated surfaces instead of between the cornea and the retina.

Troubleshooting

Errors in an AL measurement are due to improper technique yielding shorter or longer measurements.^15,¹⁶ One of the commonest errors is misalignment of the ultrasound probe with the visual axis or macular surface. When the retinal, lenticular or corneal spikes are of high amplitude and steeply rising without sloping or spikes, the ultrasound beam is most likely on axis. The scleral echo should easily be identified and the orbital fat echoes should descend quickly and at a steep angle. The retinal spike will be present and of high amplitude and can even appear steeply rising, but, if the scleral spike is not as high in amplitude as the retina, the sound beam is misaligned along the nerve. No sclera is present at the optic nerve: If there are no scleral or orbital fat echoes visible, the ultrasound beam is most likely aligned with the optic nerve rather than the macula.¹⁷ See Clip 7.4 Biometry units that are not equipped with an oscilloscope or a screen that displays the actual scan have a high error rate and are definitely not recommended.¹⁴

Comparison of IOL Master® and immersion A-scan

Optimal use

Clinical situations such as a mature or darkly brunescent lens, central posterior subcapsular plaques, corneal scars in the visual axis, and vitreous hemorrhages may interfere with the partially coherent light beams and decrease the signal to noise ratio to the point that may preclude a meaningful measurement. In these cases, immersion A-scan may be the best biometric technique. On the other hand, very difficult immersion ultrasonography measurements, such as eyes with posterior staphylomata or eyes in which the vitreous cavity has been temporarily filled with silicone oil, are more routine with the IOL Master®.⁵

Measuring specific conditions – challenging eyes

Aphakic

The IOL Master® and most ultrasound machines have a setting for aphakic eyes. When setting the measurement mode to aphakic, two gates will be present (on the respective corneal and retinal surfaces), and the biometer will calculate the distance at a velocity of 1532 m s⁻¹, the correct velocity for the aqueous and vitreous.

Pseudophakic

IOLs occasionally need to be exchanged after cataract surgery because of surgical complications or postoperative refractive surprise.¹⁸ Patients who had received older-generation IOLs might request IOL exchange for restoration of more visual function by, for example, correction of presbyopia, astigmatism, glare, etc. Indication for IOL exchange aiming to reduce residual refractive errors increased from 13.9% in the late 1980s to 30–40% in the early 2000s.¹⁸ Ocular biometry in pseudophakic eyes is thus even more important than previously expected.⁶ During the measurement of pseudophakic eyes, the first spike represents the lens implant, followed by multiple signals. IOL implantation causes multiple echoes within the vitreous cavity (Figure 7.3). The first spike (IOL echo) should also be aligned along the visual axis and should be of maximum height. Adjustments should be made according to the ultrasonic velocity of the IOL material. Nevertheless, the identification of retinal spikes can be difficult in some cases because of the proximity of the multiple echoes to the retina spike. In these cases, the examiner should decrease the gain for better identification of the retina spike. Holladay and Prager described a conversion factor to improve the accuracy of the AL measurements in pseudophakic eyes.¹⁹ They considered the implant composition, the center thickness, and the amount of vitreous and aqueous crossed by the ultrasonic beam. The conversion factor was obtained by multiplying the center thickness of the IOL by a factor related to the implant’s ultrasonic velocity.