Handheld corneal hydrometer provides accurate measurement of refractive index

The device takes some of the guesswork out of designing optimal treatment plans for patients.

Issue: April 2010

A novel handheld hydrometer accurately measures refractive index and water content in the corneal stroma, enhancing the predictability of laser refractive surgery, according to proponents of the device.

The VCH-1 Corneal Hydrometer (Index Instruments) takes the guesswork out of measuring water content and refractive index, researchers Jorge L. Alió, MD, PhD, and Sudhir Patel, PhD, FCOptom, FAAO, told Ocular Surgery News in e-mail interviews.

“Excimer laser correction is now over 20 years old, yet we still face many unknowns and barriers to overcome, eg, in relation to the exact mechanism of healing and why we get undercorrection in some and the opposite in others,” Dr. Patel said. “We have used the current and earlier versions of VCH-1 on about 500 patients.”

Dr. Alió said surgeons cannot make a correct compensation of differences in water content, noting that “only direct measurement may do it.”

Jorge L. Alió

Accurate measurement of corneal water content is critical to the accuracy of existing and future photoablation procedures. “All such techniques work in a watery environment, and hydration is one of the main issues involved in the effectiveness of the refractive correction,” Dr. Patel said.

Drs. Patel and Alió led the first clinical trials for the VCH-1 at the Vissum Institute of Ophthalmology in Alicante, Spain. The studies supported theories of an inverse relationship between refractive index and water content.

Real-time gauge of hydration

Optical coherence tomography, slit lamp, ultrasound and other existing technologies provide vital biometric data but are limited in their ability to estimate water content of the corneal stroma, Dr. Patel said.

Sudhir Patel

“To estimate the water content using these techniques requires an assumed value for corneal refractive index and/or density,” he said. “Our work has shown the [refractive index] does vary and this impacts on the accuracy of these techniques to estimate water content.”

Most refractive procedures incorrectly assume that the cornea is optically homogeneous, with a constant refractive index of about 1.376, Dr. Patel said. Refractive index increases with age, and stromal water content is lower in older corneas.

“The refractive index of the corneal stroma in vivo reduces from Bowman’s layer to the endothelium,” he said. “For the midstroma, [refractive index] increases as we become older. Sound theoretical arguments support the notion that [refractive index] is inversely proportional to stromal water content or hydration, and our in vitro study firmly supports that theory.”

The VCH-1 measures refractive index using familiar Abbé refractometry, Dr. Patel said.

“The measured [refractive index] value is converted to water content using well-established hypotheses,” he said. “It only takes moments for the user to calibrate and check the validity of the instrument. Excimer laser photoablation definitely dehydrates the stroma, and the VCH-1 has allowed us to quantify by how much, in real-time, in real individual cases.”

Reduced need for re-treatment

Drs. Patel and Alió found that postoperative over- or undercorrection correlated with preoperative water content.

“We all know, the drier the tissue, the greater the amount of photoablation per unit dose of the excimer laser,” Dr. Patel said. “So, dry tissue with relatively high [refractive index] will have a tendency toward overcorrection and conversely, wet tissue with relatively low [refractive index] will have a tendency toward undercorrection.”

Femtosecond laser flap creation also contributes to dehydration of the corneal stroma, Dr. Patel said.

In a review of more than 350 cases, investigators found a strong correlation between attempted correction of sphere and expected correction — but only when refractive index values were taken into account, Dr. Patel said.

“By taking into consideration the [refractive index] of the individual cornea, we have the missing link that will help surgeons reduce the need for, and rate of, re-treatments,” Dr. Patel said. “This has the potential to impact on the economics, accuracy and predictability of corneal refractive procedures. This should lead to better satisfaction all around.”

Clinicians should be able to feed VCH-1 measurements into their laser platforms to enhance the accuracy of laser refractive treatment, Dr. Patel said.

“This would improve efficiency, resulting in improved patient confidence because it will get around the guesswork associated with individual surgeon correction factors,” he said. – by Matt Hasson

Jorge L. Alió, MD, PhD, can be reached at Vissum, Instituto Oftalmologico de Alicante, Avenida de Denia, s/n, 03016 Alicante, Spain; +34-965-150-025; fax: +34-965-151-501; e-mail: jlalio@vissum.com.
Sudhir Patel, PhD, FCOptom, FAAO, can be reached at Vissum, Instituto Oftalmologico de Alicante, Avenida de Denia, s/n, 03016 Alicante, Spain; +34-965-150-025; fax: +34-965-151-501. He can also be reached at Practitioner Services, NHS National Services, Edinburgh, Scotland, UK, EH12 9EB; e-mail: spatel9@nhs.net.
The Vissum-Institute of Ophthalmology, Alicante, Spain, has a financial interest in the instrument mentioned in the article.