Ординатура / Офтальмология / Английские материалы / Keratoconus Surgery and Cross-linking_Pinelli, Leccisotti_2008

Keratoconus Surgery and Cross-linking

Introduction

Intrastromal ring segments (ISRS) were originally developed to treat low levels of myopia in healthy eyes. By adding material to the peripheral cornea, they alter the curvature of the anterior cornea inducing a central flattening and thereby reducing myopia.

With increasing safety and accuracy of laser refractive surgery for vision correction, the use of ISRS for this indication has reduced dramatically in the last 10 years.

However, a new role for these segments in the treatment of corneal ectatic disorders has now been firmly established with numerous published articles to support their use.1-13

The purpose of this chapter is to provide an in-depth review of these ring segments for the treatment of keratoconus.

Although there are several types of ISRS, IntacsTM (Addition Technology, Inc, Fremont, California, USA) are the most commonly studied and used although implantation techniques and complications are similar between different types.

The implanted ring segments are two, clear, arcs of variable thickness up to 450 microns made of polymethylmethacrylate (PMMA), each with an arc length of 150 degrees. The outer diameter of each segment is 8.1 mm and the inner diameter measures 6.8 mm (Figure 6-1).

The segments are implanted in the peripheral cornea at the 7 mm optical zone. The rationale for treatment of conditions such as keratoconus is that they regularize the corneal surface essentially by providing a scaffold for the cornea.

As the segments are inserted peripherally, they have the effect of flattening the central cornea with the effect proportional to the thickness of the segments. As such, they reduce the amount of astigmatism, regularize the anterior corneal surface and induce a hyperopic shift.

In many cases, they can delay or obviate the need for a corneal graft and can allow patients to resume contact lens wear where they were intolerant before.

Indications

Indications for the use of ICRS include keratoconus,1-13 pellucid marginal degeneration14-16 and other causes of corneal ectasia, e.g. refractive surgery induced.17-20

In keratoconus, ICRS are used in those patients intolerant to contact lenses, have a clear central cornea and who have a minimal central corneal thickness of at least 400 microns.

There are several algorithms available to determine where the initial incision should be and what thickness segment should be used. Ultimately, treatment is individualized to each patient based on their refraction, keratometry readings and their corneal topography.

Intrastromal Ring Segments: Manual Implantation

FIGURE 6-1: : Intrastromal ring segments

(Courtesy of Addition Technology, Inc, Fremont, California, USA.)

FIGURE 6-2: Marking of corneal centre

2.The geometric center of the cornea is marked with a Sinskey hook or marker (Figure 6-2).

3.The ICRS marker which has previously been inked is then centered on the cornea with the incision site (arrowed on the marker) placed at the steep axis (Figure 6-3).

This provides a visual guide for the incision as well as placement of the segments themselves in the 7 mm optical zone (Figure 6-4).

4.A calibrated diamond knife set to 70% corneal depth is then used to make a 1.2 mm incision at the marked entry site in the peripheral cornea (Figure 6-5).

5.A pocketing hook is then used to make a mini tunnel in either direction perpendicular to the main incision. It is important to ensure that the hook is inserted to the full depth of the incision

(Figure 6-6).

6.A modified corneal dissector is then used to extend the tunnel further on each side in preparation

Intrastromal Ring Segments: Manual Implantation

FIGURE 6-12: Segments pushed with a Sinskey hook

FIGURES 6-2 TO 6-12: (Courtesy of Ali Mearza)

7.A suction ring is then placed on the eye to stabilize the globe. Low suction is applied and centering is checked with the ICRS marker. If centering is incorrect, the vacuum is re-applied until accurate centering is achieved.

8.If centering is correct, high vacuum is then engaged and the stromal dissectors are used both clockwise and anticlockwise to create the stromal channels. The short lamellar dissector is used to facilitate entry of the stromal dissector channels (Figures 6-8 to 6-10).

9.Once the channels are created, the segments are inserted clockwise and counter-clockwise after application of antibiotic eyedrops (Figure 6-11).

10.The segments are pushed in beyond the initial incision with a Sinskey hook (Figure 6-12).

11.The initial incision is sutured with 10-0 nylon.

Postoperative Management Regime

Postoperatively, patients are prescribed an antibiotic and steroid combination drop typically four times a day for 1 week, then twice a day for 1 week although protocols may vary between institutions.

Topical anesthetics and oral analgesics can be used in addition if there is pain in the immediate postoperative period. Alternatively, a bandage contact lens can be applied for short-term relief especially in the presence of an epithelial defect or significant pain.

Complications and Their Management

Keratoconus Surgery and Cross-linking

Other complications occurring at the time of surgery are relatively mild and resolve very quickly. These include epithelial defects, conjunctival chemosis and hemorrhage.

Perforation either anteriorly or posteriorly as well as ring decentration can all be prevented by meticulous surgical technique paying particular attention to centration and tunnel depth.

Wound complications include wound dehiscence with extrusion of the implants, epithelial ingrowth and new vessel formation.

With wound dehiscence and extrusion, the implant can be repositioned and the wound re-sutured. If there is new vessel formation, this is likely to progress and the implants should be removed.

Epithelial ingrowth is usually self limiting but if progressive, the implants may to be removed and the ingrowing epithelium debrided.

Deposits around the segments themselves are common and typically develop within the first six months of surgery. Their etiology is unclear but they typically have no effect on vision. If the deposits are marked, then removal of the segments are indicated.22,23

Infectious keratitis has also been reported with ICRS implantation and should be managed in the usual manner with a low threshold for segment removal. In some recalcitrant cases, penetrating keratoplasty has been required as a measure to control the infection.24-28

Glare and haloes have been reported and are sometimes debilitating enough to require segment removal. In one study1 the ICRS had to removed in 12% of eyes because of visual symptoms.

Overall, complications are rare and as long as they are recognized and managed appropriately, there is minimal compromise to the patient’s vision.

Clinical Results

There is now a wealth of published material on ICRS use for keratoconus and other causes of corneal ectasia.

Colin1 in his landmark European clinical evaluation paper prospectively studied 57 keratoconic eyes who were contact lens intolerant and underwent ICRS implantation. At the 6 month time gate, 78% patients had an improvement in uncorrected visual acuity (UCVA) of 2 lines or more. Best corrected vision acuity (BCVA) improved from 53% of patients pre-operatively seeing 20/40 or better to 74% patients postoperatively. Keratometry readings were also reduced by a mean of 4.3 +/- 2.8 D. In 12% of eyes, the ICRS were removed because of visual complaints, typically glare and haloes but there were no complications associated with their removal.

Colin and Malet2 recently published their 2 year prospective results in 100 keratoconic eyes. They found that the UCVA and BCVA improved in 80.5% and 68.3% of eyes at the 2 year time gate as well as a significant reduction in keratometry values. Overall, they concluded that significant and sustained improvements in vision occurred in most cases.

Other authors have also found significant improvements in keratoconic patients.

Aliò et al3 looked at 3 year results in their small series of 13 eyes and found that mean BCVA increased from 20/50 preoperatively to 20/30 postoperatively at the 3 year time gate. They also noted decreased inferior-superior asymmetry by a mean of 2.81 Diopters which was sustained at the 3 year time gate.

Aliò’s group4 also published an interesting paper on potential factors that influenced whether the outcome was good or poor with ICRS implantation. They concluded that ICRS provided better 48 results in those eyes with relatively low mean K values (less than or equal to 53 D) compared to eyes

with advanced keratoconus with mean K’s greater than or equal to 55 D.