Measuring Axial Length

Immersion biometry and optical coherence biometry offer clinical advantages.

By Thomas C. Prager, PHD, MPH, and David R. Hardten, MD

In the early days of IOL implantation prior to commercially available biometers, the accepted strategy for achieving emmetropia with a posterior chamber IOL was simply to add +18.0 D to the preoperative refraction. Equally primitive were the early IOL power calculation formulas, which often produced large refractive errors for long and short eyes.

Forty years later, both technology and patient expectations have progressed dramatically. Gone are the days when just being close to the refractive target was good enough. For a cataract surgeon implanting presbyopia-correcting IOLs, one important part of the procedure's overall success requires accurate preoperative measurement of axial length.

Applanation Biometry's Downsides

Applanation, or contact, biometry begins by placing the ultrasound probe directly on the corneal surface. Even the most experienced technician may encounter parallax problems, causing a measurement that's off-axis by small or large amounts.

When attempting to center the probe on the cornea, there will always be some degree of corneal compression, especially for patients with reduced intraocular pressure. It is this unavoidable artifact of variable corneal compression that generally makes the applanation technique inadequate for the accuracy required for presbyopia-correcting IOLs.

A Better Option: Immersion Biometry

The immersion technique — in contrast to applanation biometry — does not involve touching the cornea with the ultrasound probe. Instead, the probe is coupled to the ocular surface through a fluid interface. This fluid coupling removes the artifact of variable corneal compression and thereby yields more consistent measurements.

Immersion biometry may be performed with an open cylinder using a Hansen Shell (Hansen Ophthalmic Development Laboratory, Coralville, Iowa) filled with Goniosol. Alternatively, the operator may use a fixed immersion shell, the most common being the Prager Shell (ESI Inc.). Modified from a 1977 design by Jackson Coleman, MD, the Prager Shell (Figure 1) is the only one cleared by the FDA and has a patented probe auto-stop as well as a Luer fitting to facilitate changing contaminated tubing between patients.

Figure 1. Prager Shell with Auto-stop and Luer fitting

Using a fixed immersion shell is a one-handed procedure, and is much easier to master than the open shell technique. The latter requires dexterity to position the probe at an appropriate distance from the cornea while remaining perpendicular to the retina and directing sound waves through the center of the cornea and lens. The fixed immersion technique, with a short learning curve, minimizes significant variables such as corneal compression, alignment of the ultrasound beam, and probe insertion. This leads to faster acquisition and more reproducible results.

Appropriate fixation while performing an immersion scan may be a problem due to light scatter as a consequence of a dense cataract. This can be overcome by using patient proprioceptive feedback, e.g., having the patient fixate on his or her thumb at arm's length.

When judging the quality of the axial length measurement by immersion biometry, the corneal, lenticular, and retinal spikes should be high and steeply rising, indicating that the incident sound wave is perpendicular to the acoustic interface (Figure 2). Spikes that are poorly formed (stair-stepped) or of low amplitude suggest poor alignment with the visual axis and may be associated with errors in axial length (Figure 3).

Figure 2. Typical immersion A-scan. a = probe tip; b = double peak of the cornea. c and d = the anterior and posterior lens capsule. e = retina spike. f = orbital fat.

Figure 3. Stair-stepped or low-amplitude spikes suggest poor alignment with the visual axis and may be associated with errors in axial length.

When using the immersion technique, if the tops of the spikes appear flattened, as depicted in Figure 3, this suggests that the amplifier gain is too high, which may result in inaccurate measurements. With very long eyes such as in those with a staphyloma, the macula may be located on the sloping wall of the protrusion, and the retinal spike may be much lower than the corneal spikes. In normal eyes, the retinal-scleral spikes often match the height of the corneal spikes.

Detection of orbital fat spikes behind the retina is necessary to ensure that the measurement was taken through the macula and not through the optic nerve head. In my opinion (Dr. Prager), measuring patients with silicone replacement of vitreous gel often results in inaccurate axial lengths. These eyes should be measured instead with coherent light instruments.

While immersion biometry has cost advantages over optical coherence biometry, a study from 34 centers showed that the former may transfer microorganisms from patient to patient. In the study, 53% of swabs from the shell or tubing grew organisms associated with endophthalmitis or keratitis. These concerns readily can be eliminated by soaking the shell in alcohol for five minutes and using new tubing for each patient. You can avoid this potential problem altogether by using optical coherence biometry (described below).

For normal eyes there is little clinical difference in outcomes. However, optical coherence biometry may be more precise for very long eyes with a peripapillary posterior staphyloma.

Immersion biometry has the advantage of affordable instrumentation and clinical accuracy equivalent to that of optical coherence biometry. The immersion method can measure the axial length through almost any axial opacity, which is a limitation of coherent light technology in patients with dense cataracts.

Another Option: Optical Biometry

Even as reliable as immersion ultrasound has been, clinicians have increasingly used optical coherence biometry for intraocular lens calculations. Minimal patient cooperation is necessary for optical biometry, which can be performed in the fully upright position by technicians with adequate training. Software and hardware have advanced significantly in recent years. The Carl Zeiss IOL Master uses 780-nm light to measure the distance from the retinal pigment epithelium to the corneal surface. It's quick, accurate, and doesn't require contact with the corneal surface.

In patients with staphyloma, the operator can improve accuracy by fixating the light onto the macula where fixation is occurring. The keratometer measures the corneal curvature at 2.5 mm, which is narrower than the typical 3.2 mm of the manual keratometer; this may be an advantage when calculating patients with prior refractive surgery. Also built into the software is the Haigis L formula, which likewise may help with patients who have received refractive surgery. The software also contains other useful formulas.

The Haag Streit Lenstar offers a new way to perform optical biometry. The keratometry measures curvature at 1.65-mm and 2.3-mm diameters. It also allows measurement of axial length, anterior chamber depth, and other dimensions without contacting the cornea. The Lenstar is userfriendly with multiple IOL calculation formulas built into the software. Data acquisition consists of l6 individual full eye scans and four individual keratometric scans, taken on two concentric rings, along the patient's visual axis.

When using optical or immersion biometry, perform additional measurements in difficult eyes, such as those with axial lengths that are different between the two eyes, or when there is poor signal-to-noise ratios on the scans. Some patients with very dense cataracts may still require immersion ultrasound.

Corneal Curvature Is Critical

Measurement of the corneal curvature is important in addition to accurate measurement of the axial length in patients with cataract. Typically, the clinician uses manual keratometry with a calibrated keratometer. When measurements are performed by a trained operator, the values produce quite accurate calculations. Automated keratometers have improved significantly over the last several years, and are a viable method to measure corneal curvature for use in IOL calculation formulas.

With increased use of toric IOLs and corneal relaxing incisions — and especially since we now have the femtosecond laser to perform the incisions — it has become increasingly important to detect irregular corneal astigmatism. Corneal topography is therefore an important part of the preoperative evaluation in patients with high expectations for astigmatic control after cataract surgery. Topography can help to confirm the axis found on manual or automated keratometry measurements. It can also detect corneal irregular astigmatism in conditions such as keratoconus and anterior basement membrane dystrophy.

Increasing numbers of patients undergoing cataract surgery have previously had corneal refractive surgery such as LASIK or PRK. In these patients, measurement of the corneal curvature is even more complicated, because the corneal shape is now altered. Typically, it will show an oblate shape with a flatter zone centrally and mid-peripheral steepening. With standard manual or automated keratometry, the corneal curvature is measured as steeper than actual, and the result is a hyperopic surprise after cataract surgery.

This error can be reduced by using additional methods to calculate the corneal power used in the IOL formulas. One useful source with a compilation of these formulas is the ASCRS IOL calculator (www.iolcalc.org). We utilize several formulas in our practice, with targets that have been developed through measurement of past results of patients with LASIK or PRK. We use a worksheet to keep the results organized.

Still, despite several methods of measurements designed to enhance accuracy, there is often disagreement with the results of the formulas. In that case, the surgeon must choose an IOL based on one of the formulas and proceed with the surgery despite the potential for residual refractive error. The patient should be prepared for potential residual myopia or hyperopia with the potential for another procedure such as laser vision correction or intraocular lens exchange if significant refractive error remains.

Obtain More Precise Outcomes

Regardless of whether the axial length is measured by the immersion technique or optical coherence biometry, several simple steps in reviewing the data will result in more precise outcomes.

For the great majority of patients, both eyes should show an axial length within 0.3 mm. If the difference between eyes is greater than this, have a different person repeat the measurements for confirmation. Following confirmation of this difference, place a note in the chart stating that the “measurements are outside normal physiological findings.”

If in fact repeat measurements show a significant difference in axial length between the eyes, re-measure. Before measuring axial length, use the current glasses prescription to estimate the anticipated axial length difference. The average eye axial length is approximately 23.30 mm. Assuming the central corneal power is the same for each eye at normal axial lengths, for every 1 mm of axial length difference you can anticipate a 3.0 D difference in the refractive error.

Corneal curvature measurements are typically very accurate with automated or manual keratometry. But with irregular astigmatism, corneal topography or tomography can help recognize the problem so you can devise potential treatment options. Cases where the patient has previously undergone refractive surgery need additional thought and testing.

Finally, assuming no lens-induced myopia, the axial length should correspond with the preoperative refractive error. For example, it would be very unlikely for a +4.00 D refractive hyperope to have an axial length of 26.0 mm.

Measure for Measure

The precise measurement of axial length requires either a fixed immersion technique or optical coherence biometry using the IOLMaster. Both techniques have their advantages and disadvantages. Yet, to enhance patient satisfaction and prevent a possible lens exchange, clinicians should avoid applanation biometry when implanting presbyopia-correcting intraocular lenses. OM

	Dr. Prager, who contributed the section on immersion biometry, is a clinical professor at the University of Texas Medical Branch Department of Ophthalmology and Visual Science in Galveston, Texas.
	Dr. Hardten, who wrote about optical coherent light biometry and keratometry, is an adjunct associate professor of ophthalmology at the University of Minnesota, and director of refractive surgery at Minnesota Eye Consultants in Minneapolis.