Femtosecond laser useful in PK for keratoconus

Issue: January 10, 2009

ByAmar Agarwal, MS, FRCS, FRCOphth

Amar Agarwal

Keratoconus refers to bilateral noninflammatory corneal thinning and ectasia causing increasing myopia, irregular astigmatism and decreased quality of vision for the patient.

Keratoectasia or keratectasia after LASIK or other surface ablative procedures is similar to keratoconus in having progressive ectasia of the cornea. Management depends on the stage of progression. Penetrating keratoplasty was considered the most successful surgical option for patients who are contact lens intolerant or who are not getting useful levels of vision. With the advent of newer treatment options such as intracorneal rings, collagen cross-linking with riboflavin and development of deep anterior lamellar keratoplasty techniques, PK is being reserved for select indications in keratoconus and keratectasia.

In this column, I will cover the use of the IntraLase femtosecond laser (Advanced Medical Optics) for PK in keratoconus. Although PK provides good postoperative vision in patients with keratoconus and keratectasia, it involves replacement of healthy normal endothelium in young patients with grafted endothelium, thus exposing patients to the risk of graft rejection and graft failure.

It is also an intraocular procedure and therefore carries all the risks associated with such surgery, such as wound leak and endophthalmitis. Chances of intraoperative complications such as inadvertent damage to the crystalline lens or iris exist, as well as increased incidence of postoperative complications such as iris synechiae and graft host junction.

In an effort to overcome these disadvantages, there has been a resurging interest in lamellar procedures for keratoconus and ectasia.

Penetrating keratoplasty

Worsening of the disease and progressive ectasia with insufficient correction and intolerance to contact lenses is one of the indications for keratoplasty. Other indications include central scarring secondary to acute hydrops. PK for cases with severe thinning to prevent perforation is not justified, as these corneas almost never perforate.

The difference in performing a keratoplasty in ectatic disorders is that generally a same size graft is preferred to be sutured on rather than a slightly oversized graft, as is the norm in keratoplasties for other indications. This is to avoid postoperative increase in corneal curvature and myopic postop refractive results, which have been seen to occur with slightly oversized grafts.

Shaped keratoplasties

With shaped keratoplasties, the corneal donor button and the recipient bed are cut in geometrical shapes with mirror cuts, which make them fit together in a jigsaw puzzle manner. The lamellar surfaces increase the area of apposition between the two, improving wound healing, decreasing suture-induced astigmatism and therefore providing faster visual recovery than conventional PK offers. By increasing (top hat configuration) or decreasing (mushroom configuration) the posterior diameter as compared to the anterior diameter, one can get a larger or smaller area of endothelium transplanted. A mushroom keratoplasty is preferred in the eyes of keratoconic and keratectatic patients who generally have healthy endothelium.

Mushroom-shaped keratoplasty

The IntraLase-enabled keratoplasty software adds precision and programmability to shaped keratoplasties. It can be programmed to produce different configurations with computer precision. It also makes the procedure more reproducible and creates a better fit between the donor and the recipient. This is done using a combination of anterior side cut, ring lamellar cut and posterior side cut. An anterior side cut of larger diameter and a posterior side cut of smaller diameter give a mushroom-shaped resection on the graft and recipient, which fit together with extreme precision.

The technique involves programming the computer for anterior side cut, lamellar cut and the posterior side cut of desired dimensions. The laser procedure is then performed on the donor cornea, either on the corneoscleral rim mounted on an artificial chamber or on the whole eye. Once this is done, a mirror cut of the same dimensions is used for the keratoconic recipient eye, but in the recipient eye, a nonpenetrating posterior side cut is made, leaving behind about 60 µm of uncut tissue toward the anterior chamber. This aids in safe transfer of the patient to the operating theater and microscope.

This posterior part of the uncut tissue needs sharp dissection with either a blade or scissors. Alignment incisions function creates equally spaced, small incisions along the diameter of the anterior side cut in both the donor and recipient, which can be used for decreasing astigmatism while suturing. A point to be noted is that the femtosecond laser does not cut through any corneal opacity adequately dense to obscure visualization of iris details (Figures 1 a to c and 1 e to g).

A. Mushroom-shaped cut made with the IntraLase for performing IntraLase-enabled keratoplasty in the donor eye. B. Mirror image, mushroom-shaped cut made with the IntraLase for performing IntraLase-enabled keratoplasty in the recipient keratoconic eye. C. Donor graft dissected out with blunt dissection. E. Recipient cornea being dissected out. Terminal uncut portion near the anterior chamber needs to be cut with sharp dissection. F. Donor cornea being placed on the recipient bed. Both corneas are of the same diameter. G. Donor cornea sutured in place.

Images: Agarwal A, Jacobs S

Complications

Complications that may be seen after keratoplasty include rejection, post-PK astigmatism, or fixed or dilated pupil. Keratoconus has also been reported to recur in the donor graft.

Various mechanisms have been proposed for this phenomenon. Enzymatic degradation originating from the defective host epithelium, which replaces the donor epithelium within days to weeks, has been implicated in the ectatic changes seen in the graft. Abnormal host keratocytes gradually repopulating the donor graft, leading to abnormal collagen production and eventual thinning of the stroma, is another proposed mechanism. Another hypothesis is that if the entire extent of diseased tissue is not excised, it could lead to recurrence in the graft. Some studies have also postulated that graft keratoconus may occur because of transfer of the disease from the donor.

References:

Agarwal A. Handbook of Ophthalmology. Thorofare, NJ: SLACK Incorporated; 2005.

Agarwal A. Phaco Nightmares: Conquering Cataract Catastrophes. Thorofare, NJ: SLACK Incorporated; 2006.

Agarwal S, Agarwal A, Agarwal A. Phaco-emulsification – Two volume set. 3rd ed. Informa Healthcare; 2004.

Amar Agarwal, MS, FRCS, FRCOphth, is director of Dr. Agarwal’s Group of Eye Hospitals. Prof. Agarwal is the author of several books published by SLACK Incorporated, publisher of Ocular Surgery News, including Phaco Nightmares: Conquering Cataract Catastrophes, Bimanual Phaco: Mastering the Phakonit/MICS Technique, Dry Eye: A Practical Guide to Ocular Surface Disorders and Stem Cell Surgery and Presbyopia: A Surgical Textbook. He can be reached at 19 Cathedral Rd., Chennai 600 086, India; fax: 91-44-28115871; e-mail: dragarwal@vsnl.com; Web site: www.dragarwal.com.