Ординатура / Офтальмология / Английские материалы / LASIK and Beyond LASIK Wavefront Analysis and Customized Ablation_Boyd_2001

Figure 8-5: An uncut section of tissue is pre-programmed to create the hinge for the flap.

Marking the Cornea

-Mark the cornea with appropriate alignment markers.

-Center the eye between the lids and focus the laser.

Preparation

Step 1. Outer Tray.

Position tray with Pulsion PI label facing up, firmly grasp lower left corner and peel lid from left to right to reveal inner tray. Discard lid.

Step 2. Inner Tray.

To preserve sterility, remove inner tray while wearing sterile, powderless surgical gloves and complete steps 2 through 6.

Position inner tray with Pulsion PI label facing up, grasp lower right corner and peel lid from right to left to remove.

Step 3. Inspection.

Remove applanation lens and suction ring assembly from tray, and place onto sterile field. Inspect all parts for damage or disconnection. Do not attempt to use any damaged product.

Figure 8-6: Lid speculum is applied to open the lids to a comfortable position.

Application

Step 4. Applanation Lens.

Grasp applanation lens by the upper rim with the contact lens facing downwards. Slide the applanation lens into the guides located at the bottom of the objective lens assembly on the Pulsion FS Laser. Lock the lens assembly by turning the locking mechanism upward. When the applanation lens is properly seated, a slight click should be heard. (Figure 8-7)

Remove protective cap on applanation lens and inspect through the operating microscope for scratches. A scratched applanation lens should not be used.

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Figure 8-11. Surgeon slowly lowers the delivery device using the z joystick control and gently guiding the applanation lens through the cylinder while holding the suction ring.

Releasing the Cornea

When the resection is complete, depress the syringe plunger to release the cornea from suction. Utilize the z joystick on the control panel to raise the delivery device from the eye. Lift the suction ring assembly from the eye, release lens assembly lock and remove the applanation lens by grasping the cone and sliding it away from the objective. Depress the “Home” button on the control panel to raise the delivery device and safely remove the patient from the surgical field. Discard the Pulsion PI components in an appropriate receptacle.

Flap elevation and Excimer Laser Ablation

The patient may be safely switch to the Excimer Laser for Refractive ablation

The corneal flap is lifted superiorly with curved forceps (Figure 8-14); the laser focus is achieved over the pupillary center. At this point, the surgeon can proceed with the ablation of stromal bed (Figures 8-15 and 8-16).

Figure 8-12. Intrastromal resection.

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Figure 8-13. Cornea is fully applanated and the lens is well Subjects Index centered in the suction ring

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Figure 8-14. The corneal flap is lifted with curved forceps.

Figure 8-15. The laser focus is achieved over the pupillary center.

Reposition of the Flap

When the ablation is complete, the corneal flap is replaced onto the stromal bed using the cannula starting superiorly (Figures 8-17 and 8-18). The

Figure 8-16. Surgeon can proceed with the ablation of stromal bed.

cannula is placed underneath the flap and irrigation is completed to clear any remaining debris from the interface as well as allowing BSS under the flap to facilitate “floating” back into its original position. (Figures 8-19 and 8-20)

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Figure 8-21. The Merocel sponge is moistened and squeezed dry and then used to “paint the flap” in the direction of the hinge.

The Merocel sponge is moistened and squeezed dry and then used to “paint the flap” in the direction of the hinge (Figure 8-21).

The flap is inspected to reassure that there are no wrinkles and for proper position by making sure an identical distance between the gutter and keratectomy edge is present all over the flap circumference. Depressing the peripheral “non flap” cornea with closed blunt 0.12 forceps completes a Slade’s striae test. When striae test is positive around the flap edge appropriate apposition has been achieved. During this phase it is recommended to keep a BSS drop over the central corneal epithelium to keep it wet. There is no specific waiting time with this technique, but we recommend waiting 3-5 minutes before removing the speculum.

Figure 8-22. One day Postoperative.

The case is completed by carefully removing the speculum. When doing this step, make sure to lift and close the speculum at the same time to avoid displacement of the flap.

Patient is then instructed to blink normally, and is observed through the microscope. The flap should remain in the same position and appear adhered to the cornea bed.

Postoperative Care

Immediately postoperatively, several drops of an antibiotic are instilled. The eye is not taped or shielded. The patient is asked to follow the home care instructions (Figure 8-22).

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The belief that any one refractive procedure is best for all ranges of refractive error places patients at risk for a poor outcome. Laser in situ keratomileusis (LASIK), like any surgical procedure, has its limitations. It is essential for the refractive surgeon to understand the limitations of LASIK when performing preoperative exams and counseling patients. During the preoperative assessment the surgeon can identify specific measurements that disqualify patients from LASIK, thus preventing suboptimal postoperative results.

Patient History

At our center we offer LASIK to patients who are at least 18 years of age and have two or more years of refractive stability. The range of correction varies depending on the patient’s corneal thickness and pupil size; but we generally correct between – 12.00 to +4.00 diopters of ametropia with LASIK. The ocular history should include details regarding medications, allergies, and any previous ocular surgery or infection (e.g. Herpes Simplex Virus). Ocular Herpes Simplex is a contraindication to LASIK because the surgery may induce reactivation. Questions regarding recurrent corneal erosion should be addressed, as these patients are more likely to have epithelial adherence problems during surgery. Patients with significant glaucoma should avoid LASIK due to the increased intraocular pressure resulting from the microkeratome. Patients with a history of

Figure 9-1: Nidek MK 2000 Microkeratome

operative exposure problems. Smaller footprint microkeratomes, such as the Nidek MK 2000 may be helpful in such cases (Fig 9-1). Otherwise, PRK or a possible lateral canthotomy can be discussed in advance so the patient is aware of the possible alternatives. Patients with incomplete eyelid closure are at risk of flap dislocation, and should not undergo refractive surgery. Keratitis Sicca should be diagnosed and treated preoperatively with adequate pre and postoperative artificial tears. Punctal plugs should also be considered preoperatively. Blepharitis should be treated prior to surgery because it may predispose patients to an increased risk for infections or infiltrates.

Computerized corneal topography is performed on all of our refractive surgery candidates to screen for keratoconus, pellucid marginal degeneration, and other corneal disorders (Figs 9-2 & 9-3). It is also helpful to obtain ORBSCAN II topographies in high ametropes because it provides measurements of the anterior chamber depth for phakic intraocular lens implantation. (Fig 9-4)

After completing corneal topography, manual keratometry is measured. The results are compared to the manifest refraction and topographical simulated keratometry to identify lenticular cylinder. It is also important to calculate the theoretical postoperative keratometry prior to surgery in order to prevent the cornea from becoming excessively flat or steep. In myopic treatments we try to avoid creating postoperative corneal curvatures less than 34 diopt-

Figure 9-2: Keratoconus

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Figure 9-3: Pellucid Marginal Degeneration

Note the characteristic loop cylinder pattern on this patient with pellucid marginal degeneration.

ers. For hyperopic treatments we do not steepen the cornea beyond 48 diopters. Iatrogenic keratoconus with apical stromal scarring has occurred after hyperopic LASIK when the postoperative curvature exceeds 48 diopters.1

Figure 9-4: ORBSCAN II Corneal Topographer

When reviewing the preoperative keratometry measurements, it is also important to consider that there is an increased likelihood of a buttonhole when performing LASIK on corneas with a preoperative curvature greater than 46 diopters (Fig 9-5). Additionally, there is an increased likelihood of free caps in large flat corneas (corneal curvature is less than 41 diopters) due to the smaller than usual area of applanation as the microkeratome pass is performed. This may be due to the interaction of the suction ring and microkeratome with the globe.2 . Although this data was predominantly determined with the Chiron ACS, it should be kept in mind regardless of the microkeratome used.

Pupil size in dim and room light is carefully measured and recorded. It is helpful to document the nocturnal pupil size with the Colvard Infrared Pupillometer. Patients with large pupils should be counseled regarding pupillary induced optical aberrations such as nocturnal halos, glare, and decreased contrast sensitivity. In our center we pay particular attention to pupil size in myopic patients with greater than 5 mm pupils in room light or 7 mm in dim illumination. This is even more important in those with

Figure 9-5: Buttonhole following LASIK Retroillumination

Figure 9-6: Toric IOL

higher myopic refractive errors (greater than 5 diopters). In hyperopic patients we avoid treating patients with greater than 6 mm pupils in dim illumination because the effective optical zone is smaller.

On slit lamp exam the surgeon should check for corneal dystrophies. For example, patients with anterior basement membrane dystrophy are likely to have shifted corneal epithelium intraoperatively. They are also more likely to suffer from recurrent corneal erosions exacerbated postoperatively. The slit lamp exam should also include an assessment of the lens clarity. In our practice any patient over 50 with lenticular changes is counseled about refractive lensectomy. Significant lenticular changes include more than 2+ nuclear sclerosis, any posterior subcapsular cataract, or cortical changes. A toric IOL is considered in patients with greater than 1.5 diopters of astigmatism (Fig 9-6). Astigmatic keratotomy alone can be considered in patients with more than 2

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LIMITATIONS AND CONTRAINDICATIONS OF LASIK

Table 1-2

Calculation of Ablation Depth Using Munnerlyn Formula

Optical Zone Diameter

Formula: Ablation Depth = (Diopters*((Optical Zone)^2))/3

This chart gives an approximate ablation depth for a given correction and optical zone. Surgeons should consider these calculations a rough estimate of ablation depth. The laser software calculations of ablation depth should also be checked in cases where the calculation results in a residual stromal bed thickness close to 250um.

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