Five Elements For A Successful Clear Cornea Refractive MicroIncision
Robert M. Kershner, M.D., F.A.C.S.
Director, Eye Laser Center
Clinical Professor of Ophthalmology, University of Utah College of Medicine
Salt Lake City, Utah
Member of the Board, American College of Eye Surgeons
There are five key elements to creating the proper clear corneal refractive micro incision. By utilizing the following strategy and approach, surgeons can maximize the benefits of the Becton Dickinson BEAVER Self-Guided Clear Cornea Incision System. The clear cornea incision system is made up of a fixation ring to stabilize the globe, a corneal marker to mark the proper incision length and width, a double bevel slit blade to construct the primary phaco incision, and an accurate depth blade to construct the arcuate refractive portion of the clear cornea incision.
I. Incision Size
Incision sizes of greater than 3.2 millimeters on the cornea will induce flattening and unwanted aberration in the refractive or focusing power of the central cornea. These incisions fail to be self-sealing and usually require suturing. By choosing an incision size of less than 3.2 millimeters, the clear corneal refractive micro incision becomes possible. The size of the incision should reflect the surgeon's required instrumentation for phacoemulsification and lens implantation.
If the surgeon is able to utilize the new techniques of micro phacoemulsification using a micro tip, then the BEAVER Double Bevel Slit Knife (DBS) selected will reflect the smaller incision size required for this instrumentation. For example, micro tips can generally be used with a 2.8 millimeter incision size, which also accommodates the intraocular lens injector. For incision sizes of 2.0 millimeters or smaller, specialized instrumentation is required. In that case, a smaller size DBS will be used. The advantage of the BEAVER DBS is that it is available in a variety of sizes, accommodating the particular instrumentation the surgeon employs for clear corneal cataract surgery.
Incisions can be made on the corneal dome in two shapes: As a straight line or as a curved arcuate incision. A curved incision in the corneal dome is, in essence, a straight line when viewed from the side. A curved incision has the most flattening effect on corneal architecture and integrity, and is the ideal incision for clear corneal refractive surgery when correcting preexisting astigmatism. The shape, style and design of the keratome selected determine the incisional architecture created.
The BEAVER DBS is specially designed with a double bevel edge, diamond tip configuration, that creates this ideal architecture. The engineering of this blade allows easy, reproducible construction of the ideal incision shape for clear corneal refractive surgery.
The location of the clear corneal incision is critical to the refractive outcome of the procedure. Location also impacts the surgeon's ability to mobilize the eye and insert instruments during the cataract removal process. Many surgeons have adopted a temporal clear corneal approach. Temporal surgery creates unimpeded access to the globe without the interference from the upper and lower eyelids. For many surgeons, however, the temporal clear corneal approach is problematic. Because of the bed on which the patient lays, the position of the surgical chair, and the instrumentation in the operating room, it may be difficult to sit alongside the patient's head. In such instances, clear corneal temporal surgery can still be attempted with the surgeon sitting at the head of the bed, moving his or her instruments and hands to the side of the patient's eye. Temporal clear corneal surgery has become increasingly popular because the incision is furthest away from the optical center of the eye, where it is least likely to flatten or disrupt the central corneal architecture. The corneoscleral limbus is widest temporally and affords the widest approach and angle to create a clear corneal incision. For surgeons who would rather sit at the head of the bed, clear corneal incisions of sizes less than 3.0 millimeters can still be safely attempted in the oblique position or the superior or lateral positions of either eye.
If the incision is to be astigmatically neutral, it should be placed furthest from the optical center of the eye, usually in a temporal or oblique position. If one wants to purposely flatten the steepest meridian of the cornea, then the incision is simply placed on the steepest plus cylinder of the cornea as determined preoperatively by refraction and corneal topography.
The flattening effect of the clear corneal incision can be used to maximize the correction of preexisting astigmatism. The location of the clear corneal refractive micro incision becomes important when considering preexisting astigmatism of greater than 0.5 diopters. Patients with preexisting astigmatism should have the clear corneal incision placed on the steepest meridian of the eye undergoing surgery. In this fashion, any flattening effect induced by the corneal incision will be neutralized by the steepness of the meridian in which it's placed. Conversely, placing the clear corneal refractive micro incision on any meridian other than the steep meridian will tend to exacerbate the existing astigmatism or even add astigmatism where it is not wanted. Placing the incision elsewhere than the steepest meridian can allow the astigmatism to shift towards the incision. This creates a new axis of astigmatism, which may not be well tolerated by the patient.
If the incision is placed on a steep meridian, preexisting astigmatism can be predictably reduced and the patient can have better uncorrected visual acuity at the conclusion of the procedure. In addition, such an approach is amenable to the use of toric intraocular lenses. With a specialized power placed on the longitudinal axis of the intraocular lens, these lenses can correct astigmatism.
To construct a clear corneal incision with the best refractive result, the best surgical access, and self-sealing capability, the surgeon must keep several concepts in mind. First, the incision should not exceed a ratio of 3:2. That is to say, if the incision width is 3.0 millimeters wide and a 3.0 keratome is selected, then the incisional length through the cornea before penetration into the eye should be approximately 2 millimeters. Similarly, a 2.5 millimeter incision will have a less than 2 millimeter interior tunnel to correspond and maintain the 3:2 ratio.
This 3 to 2 incision width-to-length ratio is automatically considered in the design of the BEAVER Clear Cornea Incision System. Using the fixation ring the eye is maintained in the proper orientation. For two-plane incisions, pressing the corneal marker onto the surface of the cornea at the correct location creates two marks- groove location and length, and tunnel length. Once the marks are made on the cornea, the partial depth groove is created by incising along the larger arcuate mark using the Accurate Depth blade. Then the primary incision can be completed by placing the tip of the DBS at the base of the groove in the central location and advancing the blade through the stroma. Once the blade tip aligns with the second mark, the appropriate corneal tunnel length has been created, and the anterior chamber can be entered at the proper plane. Alternatively, the laser mark, located on the anterior surface of the DBS, can be used to guide the surgeon as to the incision length. While the DBS is advanced through the stroma, once the laser mark aligns with the lip of the external incision, the appropriate tunnel length has been created, and the anterior chamber can be entered at the proper plane. Whether the corneal marker or the laser mark on the DBS is employed, an incision width to length ratio of 3:2 is automatically created, and assures that the incision will be correctly constructed, be self-sealing, and have the optimum refractive benefit for the patient.
V. Incision Construction. Once the surgeon appreciates the variables of size, shape, location, and architecture, he or she can incorporate these concepts in creating the ideal clear corneal incision. If the eye, prior to surgery, has no measurable astigmatism and is spherical by refraction and topography, then an astigmatically neutral incision should be employed—a planar, single-stab, paracentesis style incision in the temporal or oblique location. If, the surgeon wishes to correct preexisting astigmatism of less than one diopter, then a single planar stab incision can be placed on the steepest meridian. If more than one diopter of astigmatism is present, the Accurate Depth BEAVER blade can be used to create a two-step clear cornea incision and maximize incisional flattening. At a preset depth of 550 to 600 microns, selecting the appropriate Accurate Depth BEAVER blade will create an arcuate astigmatic keratotomy to flatten the cornea in the meridian in which it's placed. Following this, the 3.0 millimeter DBS blade is inserted into the base of the vertical incision and used to enter the eye in the same manner as for a planar stab incision. In this fashion, one can create a two-step incision with the same internal architecture as the planar stab incision, but with the maximal flattening effect of an arcuate keratotomy.
The Becton Dickinson BEAVER Clear Cornea Incision System makes it possible to reproducibly create the ideal clear cornea incision architecture each and every time, with an economical, disposable, incision system. By using the fixation ring, the cornea marker and the DBS, the surgeon can master the construction of the ideal corneal cataract incision. The astigmatic benefits of the Beaver accurate depth blade further makes it possible to correct preexisting astigmatism with the corneal cataract incision. Beaver can put clear cornea cataract surgery in your hands, with the complete self-guided incision system at your fingertips.
Copyright. Robert M. Kershner, MD, FACS