OCT accurately measures flap, corneal bed thickness prior to LASIK enhancements
July 9, 2008
Optical coherence tomography (OCT) is a non-contact imaging system that uses low-energy laser light to create two-dimensional cross-sectional images of different structures within the eye.
OCT devices are available to create images of anterior eye structures, such as the cornea, and posterior eye structures (e.g. the retina).
Applications of OCT for LASIK surgery
Anterior segment OCT can provide high-resolution cross-sectional images and thickness measurements of the cornea.
This is especially beneficial in LASIK surgery when a patient may have already had LASIK several years ago and now needs a second surgery because their vision has changed. (Incidentally, this occurred twice to professional golfer and LASIK patient Tiger Woods.)
Better measurements reduce risks
To reduce the risk of ectasia after LASIK, it's important for the surgeon to know not only the overall thickness of the cornea, but also how much corneal tissue remains under the original LASIK flap – what's called the residual corneal bed.
Conventional instruments used by surgeons to measure corneal thickness prior to LASIK measure only the overall corneal thickness – they can't measure the thickness of the corneal bed under the flap once a primary LASIK procedure has been performed.
But OCT can.
The cross-sectional image of the cornea that the OCT device creates shows the corneal flap, the thin scar between the flap and the underlying corneal tissue, and the residual corneal bed under the flap. The OCT device also provides accurate thickness measurements of the flap and the residual corneal bed.
Why corneal bed thickness measurements are important
The corneal bed under the LASIK flap provides the structural rigidity that maintains the stability and shape of the cornea. The generally-accepted minimum corneal bed thickness for a healthy, stable cornea after LASIK is 250 microns.
(The average overall corneal thickness of a human cornea prior to LASIK is about 550 microns. Most LASIK flaps are 120 to 160 microns thick.)
Though no one knows for sure what the "100% safe" minimum corneal bed thickness is, it's generally accepting that maintaining a residual corneal bed thickness of at least 250 micron after LASIK essentially eliminates the risk of corneal ectasia for over 99% of patients.
For an added measure of safety, some LASIK surgeons prefer using a higher minimum corneal bed thickness value than the current 250-micron "standard."
OCT more accurate than traditional methods
Until OCT, LASIK surgeons have been unable to directly measure the thickness of the residual corneal bed in a non-invasive way.
To determine if there is an adequate corneal bed thickness for an enhancement LASIK procedure, most surgeons calculate the expected thickness under the flap by measuring the overall corneal thickness (with a non-contact instrument called called a pachymeter), then subtracting the thickness setting used on the laser or microkeratome that created the corneal flap during the primary LASIK surgery.
The difference between these two figures – the overall corneal thickness and the presumed flap thickness – should equal the residual corneal bed thickness.
The problem with calculating the corneal bed thickness this way is that the flap thickness is not always what it's supposed to be. Studies show that whether the LASIK flap is made with a microkeratome or with a femtosecond laser, the actual corneal flap thickness can vary from the setting used on the surgical instrument.
If the actual flap thickness is thicker than expected, the residual corneal bed will be thinner than expected.
To safeguard against using the excimer laser on a thinner-than-expected residual corneal bed, during the LASIK enhancement procedure many surgeons will lift the primary LASIK flap and then (prior to using the excimer laser) use an ultrasound device to directly measure the thickness of the corneal bed under the flap.
Though this technique can alert the surgeon to a thinner-than-expected corneal bed so they can decide whether or not to proceed with the enhancement based on this new data, it still requires a full preparation for surgery and lifting of the flap in the operating room.
If the corneal bed is found to be too thin to proceed with the enhancement, the flap is repositioned and the patient has to follow normal post-operative procedures to prevent infections and other problems, despite the treatment being aborted.
OCT eliminates this guesswork and the potential problems associated with it. By taking OCT measurements in a non-invasive way during the pre-operative exam, the surgeon can tell the patient with greater certainty whether they have enough residual corneal bed thickness for a safe enhancement procedure.
Additional fee may be required
The use of optical coherence tomography for measuring corneal thickness prior to LASIK enhancements is an elevated level of care. Most refractive surgeons do not yet have this higher-priced technology.
Therefore, if you are considering a LASIK enhancement and you or your surgeon feel OCT measurements are needed to determine if you should proceed, you may be referred to a specialist and/or have to pay an additional fee for this advanced imaging technology.