Optical Biometry: Transforming Cataract Surgery for Surgeons

Cataracts are a common eye problem in which the lens becomes cloudy. They can lead to other issues like uveitis and glaucoma. Cataracts affect more than 22 million people in the United States and 95 million worldwide. Treatment is necessary to restore visual function and prevent serious complications.

Cataract surgery can improve vision for 95% of patients. However, there is a slight chance of complications. These complications include macular edema, cloudy vision, infection, and retinal detachment.

The surgeon must measure the eye size before cataract surgery to choose the right lens power for implantation. Surgeons make structural eye measurements before surgery using various methods, such as biometry or applying mathematics to biology. For example, we measured anterior chamber depth and lens thickness using ultrasound biometry.

Optical biometry uses light instead of sound and can obtain tired measurements with one device. Its use has become the gold standard, except in patients with a Grade 4 or higher or white cataract.

In this article, we review the history of optical biometry advancements and provide a technical introduction to the approach. We also compare optical biometry with traditional ultrasound biometry.

Brief History of Optical Biometry

Dr. D. Jackson Coleman first described the use of ultrasound biometry in the late 1960s, and its use for cataract surgeries began in the 1970s. The development of optical biometry prompted the need for higher accuracy of axial length measurements required for cataract surgery.

In the 1990s, optical biometry was introduced to address these needs. Partial coherence interferometry (PCI) was first developed by Dr. Adolf F. Fercher and Dr. Eckhard H. Roth at the University of Essen in Germany. The approach yielded an accurate and reproducible alternative to ultrasound biometry and was quickly commercialized by Carl Zeiss Meditec as the IOLMaster 500. The device provided a non-contact technique for measuring axial length, corneal curvature, and anterior chamber depth.

In the early 2000s, optical low-coherence reflectometry (OLCR) was introduced using principles similar to PCI’s. OLCR could measure more anatomical parameters with higher resolution, but the process took twice as long as PCI. Haag-Streit commercially introduced the Lenstar LS 900 soon after.

Imaging capabilities were further enhanced by the development of sweep-source optical coherence tomography (SS-OCT), which tunes the wavelength of light used to gather information, reduces the signal-to-noise ratio, and increases the speed at which data are collected.

Optical biometry has evolved rapidly over the past two decades, primarily due to advances in algorithms used to process the data. In practice, the choice of biometry approaches requires considering the patient’s clinical characteristics, the procedure’s cost, and the equipment’s availability.

Understanding Optical Biometry

Optical biometry measures the eye’s length and structure using light. More accurate and consistent than ultrasound biometry, which uses sound waves. Light has eight times the resolution of the 10 MHz sound waves in ultrasound biometry.

Optical biometry utilizes light-interference technology to generate interference patterns that it processes to calculate the distance between ocular structures. Here, we discuss the main types of optical biometry and their operational principles.

Laser Partial Coherence Interferometry

In partial coherence interferometry (PCI), an interferometer splits a 780 nm infrared laser into two beams, a sample beam and a reference beam. The interference pattern produced by light reflected from different eye structures is analyzed to determine distances. The approach can derive geometric intraocular distances with a resolution of 12 microns.

Optical Low-Coherence Reflectometry

Optical low-coherence reflectometry (OLCR), a standard Michelson interferometer setup with an 820 nm superluminescent diode, is capable of more measurements, including axial length, corneal curvature, retinal thickness, and central corneal thickness due to the alignment of the optical-path-length measurements. OLCR uses the same basic approach of splitting the light beam into sample and reference beams. The low coherence of the light over short distances is the basis of obtaining high spatial resolution.

Calculation Matters

The sophisticated algorithms used for signal processing further enhanced the accuracy and reliability of the instruments, correcting for eye movement and refractive index variations. Although optical biometry instruments yield accurate results, formulas are needed to calculate intraocular lens power.

One of the first formulas, the SRK formula, was a linear regression formula developed by Sanders, Retzlaff, and Kraff. This classical formula used axial length and corneal power, but more recent formulas consider additional variables to estimate influential lens position, increasing intraocular lens selection accuracy.

In the early 1990s, Dr. Kenneth J. Hoffer proposed that different formulas should be used for different axial lengths. According to Dr. Jack T. Holladay, president of Holladay Consulting and developer of the Holladay 1, 2, and Refractive Formulas, “My Hoffer Q formula worked better in short eyes; the SRK/T worked better in long eyes, and Holladay I worked best in medium-long eyes.

In average eyes, they were pretty much equally accurate. So, cataract surgeons became accustomed to choosing a lens power formula based on the axial length.”

Dr. Paul-Rolf Preussner and others further refined equations to consider the lens-shape constant. According to Dr. Holladay, these advancements resulted in eyes within about 0.5 D in 80% of eyes after surgery.

Artificial intelligence (AI) has further contributed to improvements in equations. The recently released Hill-RBF calculation method incorporates AI to yield results within 0.5 D. “All versions of the RBF method were created in tandem with the engineers and mathematicians at MathWorks, which employs some of the most sophisticated artificial intelligence experts in the world,” says Dr. Warren E. Hill, medical director of East Valley Ophthalmology in Arizona.

Biological sex affects all areas of biology, so it is not surprising that sex has been a recent consideration in lens power calculations. “I incorporated gender and race into a formula called the Hoffer H-5 some time ago, but it never caught on. More recently, Jack Kane, MD, from the University of Sydney, incorporated gender into his Kane formula, which uses AI. Now it’s becoming accepted that gender is an important part of lens power calculation,” reports Dr. Hoffer.

Benefits of Optical Biometry

Optical biometry devices measures central corneal and lens thickness, horizontal diameter of the cornea, and macular and retinal thicknesses. Whole-eye biometry is also possible, making optical biometry the preferred approach of many eye care professionals.

There are many clinical advantages to using optical biometry for surgeons, including:

• Efficient and comprehensive surgical planning to ensure the appropriate selection of intraocular lens type and incision planning that depends on corneal curvature to optimize visual outcomes
• Enhanced precision and accuracy to improve intraocular lens power calculations.
• Reduced intraoperative surprises like capsular bag instability to help account for anatomical differences caused by prior refractive surgeries.
• Improved surgical outcomes. According to Dr. Steven C. Schallhorn, founder of the US Department of Defense refractive surgery program, “The latest-generation devices provide measurements for the most advanced power calculation formulas, and they make it easier to ensure the accuracy of their measurements. More accurate measurements and added parameters for certain formulas will collectively drive better outcomes.”

Despite its strengths, repeat measures may still be necessary when considering optical biometry readings. According to Dr. Robert Weinstock, a surgeon in Clearwater, FL, “There’s some thought that goes into it; it’s not a device for which you just accept the reading at face value. It’s no different than doing an IOLMaster and comparing that measurement to the Lenstar or doing a Lenstar and comparing it to immersion. It’s another data point to use in decision-making.”

Dr. Douglas D. Koch, professor of ophthalmology at Baylor’s Cullen Eye Institute, reports, “I recall one case in which I took a Lenstar reading on a patient; it was about 43.3 D. The IOLMaster 700 gave us no readings at all for this patient, with obvious mire distortion on the printout.

I looked back at a measurement I’d gotten with the Galilhen, the same patient initially presented three months earlier. That measurement was 44.4 D—more than a diopter of disagreement and with very di, different astigmatism measurements.

Upon more careful examination, I found dry spots on the cornea that were present during the Lenstar and IOLMaster measurements. So, I treated the dry eye. When I repeated the Lenstar and IOLMaster measurements, they matched the Galilei measurements.

The moral of the story is that validating your biometry with a second reading from another instrument can improve the quality and consistency of your measurements and avoid some poor outcomes.”

For patients undergoing cataract surgery, pre-operative optical biometry also has benefits, including:

• Better visual outcomes, including reduced dependence on corrective lenses and risk of re-operation to correct postoperative refractive errors caused by inaccurate IOL calculations
• Increased confidence in procedures
• Faster recovery times, including visual stabilization

Additionally, one of the most significant factors influencing patient satisfaction after cataract surgery is visual acuity, which is directly impacted by the accuracy of pre-operative biometry.

Comparative Analysis

No statistical difference exists in axial and anterior chamber lengths when measured by ultrasound and optical biometry. In clinical settings, using both approaches may be better for getting all the information needed for successful cataract surgery.

Optical Biometry

While optical biometry’s benefits are well-documented, potential drawbacks may impact its effectiveness and suitability in various clinical settings. Considering the benefits and drawbacks helps determine whether optical biometry is the right choice for the patient.

Advantages

• Requires fixation on a point of light, which enables relevant eye measurement to the fovea, rather than axial length, from the anterior to the posterior pole.
• Non-contact methodology prevents cross-contamination between patients and the potential for compression of the cornea during measurements.
• Easier to use
• Capable of obtaining more anatomic measurements, compared to ultrasound biometry
• No need for topical anesthetics
• Produces keratometry readings, so no additional devices are required
• Takes less time due to its highly automated measurements and user-friendly interface

Disadvantages

• Cannot produce accurate axial length measurement in eyes with posterior subcapsular cataracts
• Higher cost, which can be a barrier to use, especially in settings with limited resources
• Technical complexity requires specialized training and expertise
• Depends on a patient’s ability to be still and fixate on a spot of light
• May not perform as well with significantly shorter or longer axial lengths

Ultrasound Biometry

There is no one-size-fits-all ophthalmic measurement system. Although optical biometry has significant advantages regarding ease of use, sophistication, and speed, certain situations may warrant using ultrasound biometry.

Advantages

• Can perform measurements in patients with visual acuity worse than 20/200 or have eccentric fixation, macular degeneration, or corneal scarring
• Capable of obtaining measurements through dense cataracts or posterior capsule opacification
• Widely available due to lower cost
• Measurement approach flexibility, including contact and immersion methods that can decrease potential confounding factors introduced by corneal compression that can be up to 0.3 mm and produce errors up to 1.0 diopter
• Can measure extreme corneal lengths
• Can be adjusted manually so the operator has greater control, which can be particularly beneficial with challenging cases like unusual corneal pathologies
• Not very sensitive to slight eye movement

Disadvantages

• Greater inter-operator variability with accuracy largely dependent on the experience and skill of the operator
• Time-consuming, especially if additional preparation like anesthesia is required
• Less precise measurements
• Patient discomfort due to the contact nature of the process that often requires anesthesia

Conclusions

The development of biometry vastly and rapidly improved outcomes for patients with cataracts. Although optical biometry offers many advantages over ultrasound biometry, ultrasound biometry remains vital in specific clinical situations.

Healthcare providers should guide the choice of instrumentation based on the patient’s clinical needs, comfort, ability, and cost. Integrating these complementary methods may offer even greater precision and flexibility in cataract treatment as technology evolves.