Cataracts develop in the crystalline lens of the eye and occur for a number of reasons include diabetes, hypertension, and advanced age. In patients with this disease, a clouding of the crystalline lens of the eye is caused by protein present in the lens clumping together. In patients where this disease is pronounced, intraocular surgery is used to replace the eye's natural lens with an intraocular lens (IOL).
Several types of IOLs can be used as natural lens replacements to allow near and far objects to be viewed without the use of eyeglasses or contact lenses. In patients with astigmatism, a specialized, more expensive IOL, known as a toric IOL, is used to correct for both cataract problems and astigmatism. Because these lenses are not symmetrical, toric or cylindrical IOLs must be inserted in the eye at a specific angle or the astigmatism will not be fully corrected.
Since astigmatism results from a difference in the degree of the curvature refraction of light in horizontal and vertical planes, these planes or reference meridians must be marked before surgery. Since the eye rotates when the patient is supine, these meridians are first marked using reference markers from companies such as Duckworth & Kent (Baldock, UK;www.duckworth-and-kent.com).
These meridians are then used to identify other meridians for surgical incisions and IOL placement. As toric lenses are marked with tiny striations, these striations are used with the marked reference meridians to position the lens correctly.
To view a rather graphic video of both this eye-marking procedure and IOL surgery, readers are directed towww.duckworth-and-kent.com/feature/toricmarkers.asp.
To mark these meridians before and during surgery, a marking pen is used. Unfortunately, this ink may run and disperse and has been called “as insensitive as diagnosing pregnancy at eight and a half months,” by Dr. Robert Osher, professor of ophthalmology at the University of Cincinnati (Cincinnati, OH, USA;www.uc.edu) and Medical Director Emeritus of the Cincinnati Eye Institute (see “The Best Way to Straighten a Bow Tie,” Review of Opthalmology, 17:1, Jan. 1, 2010).
To overcome these limitations, a number of different methods are now being proposed to mark these meridians digitally by analyzing specific features within the eye. Rather than use features of the iris to compute this meridian, engineers at Ikona (Westwood, MA, USA;www.ikonacorp.com) have opted to reference the anatomical features, specifically the blood vessels found in the sclera or white part of the eye (see figure).
According to Ilias Levis, president of Ikona, after Osher helped define the requirements of the software, Ikona filed two preliminary patents and, with funding from a large medical company, hopes to realize a final product in the near future.
“Since every degree of misalignment can result in a 3.3% loss of correction when placing an IOL,” says Levis, “it is vitally important that the lens is placed as accurately as possible.” Ikona's first patent relates to registering the anatomical features of the sclera; the second relates to centration placement of the IOL.
“Since the IOL must be centered on the visual axis (corneal vertex) as opposed to the geographical center or optical axis of the eye (pupil center),” he says, “our software will also incorporate data from an eye topographer device to achieve increased accuracy in calculating the center of the coordinate system utilized to place the IOL.” Such methods, Levis hopes, will eliminate the need for manual eye marking to be performed both pre-operatively and during surgery.
After the software has been developed, it will undergo clinical trials at the offices of Dr. Jonathan Talamo of Talamo Laser Eye Consultants (Cambridge, MA, USA;www.lasikofboston.com).
