Use of a handheld circular polarizing filter to intercept afferent and efferent light from a slit lamp or operating microscope highlights corneal structures that are otherwise invisible or barely seen.
Circular polarization is a simple and effective technique for corneal imaging that provides valuable clinical information and new insights, according to Julian D. Stevens, MD.
“Ophthalmologists are all familiar with the benefits of cross polarization for improving imaging at the slit lamp, but conventionally, that would require an instrument with a built-in linear polarizer and use of another handheld polarizer,” said Dr. Stevens, consultant ophthalmic surgeon, Moorfields Eye Hospital, London.
“Use of a handheld circular polarizing filter gives the desired effect in a much simpler approach which can be used in by any practitioner, and I highly recommend it to corneal and refractive surgeons,” Dr. Stevens said.
Features within the cornea that are otherwise hidden are rendered visible, and the imaging technique highlights stress and biomechanical effects within the cornea, he noted.
“Through the ability to see these changes, we are better able to measure and understand them,” he said.
The photoelastic effect or stress birefringence was first described in 1816 by Scottish physicist, Sir David Brewster, according to Dr. Stevens.
“Brewster noted that optically isotropic or anisotropic transparent substances become birefringent when subjected to mechanical stress,” he said. “Cross polarization takes advantage of the strong polarizing effect of the cornea that arises from the arrangement of the collagen fibrils throughout its different layers.”
The technique can be performed in the office at the slit lamp using a handheld circular polarizing filter which intercepts the afferent and efferent light. By rotating the polarization planes, the filter provides effective cross polarization imaging of the cornea, Dr. Stevens explained.
He has used the technique to visualize changes in eyes after intrastromal femtosecond laser astigmatic keratotomy, keratoplasty, radial keratotomy, and corneal crosslinking.
“Cross polarization reveals the optical and biomechanical effects that arise when the cornea is stressed by these surgical incisions or after there is contraction post-crosslinking,” he said. “With cross polarization, stress within the cornea, the corneal layers, and orientation of the collagen lamellae within the cornea are all visible.”
Practical applications include improved intraoperative assessment of corneal suture tension and improved visualization of the LASIK flap edge in eyes that had excimer laser surgery many years earlier.
“If LASIK was performed many years ago, I struggle to see the edge because the flap is essentially invisible in white light,” Dr. Stevens said. “Even with optical coherence tomography (OCT), it can be very difficult to image the flap, but the flap really ‘lights up’ with circular polarization.”
The ability to see how sutures are distorting the cornea is very helpful for ophthalmologists in training, he noted.
“When trainees are in their learning phase, we simply hold the polarizing filter under the operating microscope so they can see the tension that has been created and adjust the sutures as needed,” he said.
Dr. Stevens acknowledged the contributions of Gary Mission, MD, PhD, optical engineer and consultant ophthalmic surgeon, Warwickshire Hospital, Leamington Spa, England, for developing the imaging technique.
Julian D. Stevens, MD
This article was adapted from Dr. Stevens’ presentation at the 2016 meeting of the American Society of Cataract and Refractive Surgery. Dr. Stevens has no relevant financial interests to disclose.