Beginning surgeons should learn the essential aspects of wound construction in cataract surgery, including wound type, location, and architecture. This chapter focuses on the 2 types of wounds commonly used in cataract surgery today: the scleral tunnel and clear corneal incisions. We will also discuss the significance of wound location and wound architecture for modern phacoemulsification.
II. WOUND TYPE
The 2 principal types of wounds in cataract surgery are the scleral tunnel and the clear corneal incisions. In the last decade, the clear cornea incision has eclipsed the scleral tunnel as the preferred wound type for most surgeons, with reported use of clear cornea incisions rising from 1.5% in 1992 to 87.1% in 2010.1,2 Nonetheless, the scleral tunnel incision is a versatile wound that may be particularly useful during the early stages of learning phacoemulsification.
A. Scleral Tunnel Incision
Although a scleral tunnel wound can be fashioned under topical anesthesia alone, a retrobulbar block provides akinesia and improved analgesia for facilitation of conjunctival and scleral dissection (Figure 6-1). There are several steps in scleral tunnel wound construction, including conjunctival peritomy, scleral groove, tunnel creation, formation of a paracentesis port, injection of viscoelastic, and keratome entry into the anterior chamber.
a. Step 1
Once the eye is anesthetized, the first step is the conjunctival peritomy. Although the wound can theoretically be placed anywhere, many surgeons choose the superotemporal sclera. The conjunctival incision should be slightly longer than the planned scleral tunnel length, with or without a radial conjunctival cut to facilitate adequate exposure. The incision is followed by blunt dissection through Tenon fascia, using electrocautery for hemostasis as necessary.
b. Step 2
The scleral groove incision site is marked 1 to 2 mm posterior to the limbus. After fixating the globe with toothed grasping forceps, a blade (commonly a #69 beaver blade) angled perpendicular to the scleral surface is used to create an approximately half-thickness scleral groove.
c. Step 3
The scleral tunnel is then extended anteriorly with a pocket or crescent blade. First, the blade is placed into the groove with the heel of the blade off of the sclera to ensure the plane of the tunnel is at the full depth of the groove. Once adequately within the groove, the heel of the blade is lowered to be flush with the scleral surface, and the blade is advanced with circular motions tunneling toward the cornea, stopping once the tip has reached the limbus. As the curvature of the globe changes at the cornea, the tip of the blade is then angled upward slightly in order to avoid an overly thin posterior corneal lip or premature entry into the anterior chamber.
d. Step 4
A limbal paracentesis port is then created approximately 2 to 3 clock hours from the planned location of the scleral tunnel. Viscoelastic is subsequently injected through the paracentesis to fill the anterior chamber.
e. Step 5
While globe fixation is maintained with grasping forceps, the keratome is gently placed into the scleral tunnel. Small side-to-side movements of the keratome are used to ensure that the blade remains within the plane of the tunnel, avoiding creation of a secondary scleral plane. When the keratome tip is visible in clear cornea at the most anterior aspect of the tunnel, the heel of the blade is then elevated off the sclera, directing the keratome toward the iris opposite the wound. This downward pressure at the tip of the keratome creates small, visible folds in the cornea, described by the term dimple-down. The keratome is then advanced into the anterior chamber such that the shoulders of the blade pass through the internal aspect of the wound, thus ensuring adequate internal wound width.
There are advantages to scleral tunnel wounds over clear corneal incisions.
- A scleral tunnel incision can be safely enlarged for purposes such as insertion of nonfoldable intraocular lenses (IOLs) or conversion to extracapsular cataract extraction.
- Once the scleral tunnel incision is closed, conjunctiva covers these wounds, which may play a role in the lower reported incidence of endophthalmitis.
- Scleral tunnels begin further posteriorly than clear corneal tunnels, and thus anterior chamber entry is also relatively more posterior. The resultant vertical distance between the phacoemulsification probe tip and the corneal endothelium is greater in the scleral tunnel than that of clear corneal wounds, leading to less endothelial damage by ultra- sound phacoemulsification power (Figure 6-2).
- As scleral tunnel wounds are created further from the optical center, a phaco-induced wound burn at this location would have less astigmatic consequence than a burn in the clear cornea.
- The magnitude of induced postoperative astigmatism increases with both incision length and proximity to the optical center.Generally, larger incisions in the sclera induce less postoperative astigmatism than similarsized clear corneal incisions, as they are further from the optical center.However, while multiple studies have suggested that smaller incisions may be astigmatically equivalent between wound types, there is some discrepancy regarding the wound size at which this occurs. While 2 prospective, randomized trials found no difference in keratometric astigmatism between 3- and 3.2-mm scleral tunnel and clear cornea incisions at 3 months, another trial found that induced astigmatism as measured by vector analysis and videokeratography was higher in 3-mm clear corneal incisions compared to 3-mm scleral tunnels at 8 weeks.
There are disadvantages to using a scleral tunnel incision.
- The topography of the patient’s face can restrict surgical exposure, making a scleral tunnel incision difficult. For example, a prominent brow, a narrow palpebral fissure, or a sunken globe in a deep orbit may obstruct access to the superior and superotemporal sclera, if chosen asthe incision location.
- When tunneling forward with the pocket blade, failure to tilt the blade downward when creating the lateral aspect of the tunnel may result in severing the edge, creating a loose scleral flap at the posterolateral aspect of the wound.
- If the scleral groove incision is too deep, damage or disinsertion of the ciliary body may result. If the roof of the scleral tunnel is too thin, a blade entering the wound may cause an anterior buttonhole or perforation, creating an undesirable scleral defect.
- Globe perforation, premature posterior entry to the anterior chamber, iris prolapse, and poor wound apposition are other potential complications of a deep scleral groove.
- The presence of a filtering bleb or conjunctival scarring preoperatively may complicate the creation of a conjunctival peritomy. Functionality of preexisting blebs may be affected; conjunctival manipulation may result in scar tissue that can decrease functionality of potential future filtering blebs.
- Dissection and manipulation of vascular tissues such as the conjunctiva and sclera can cause blood to track forward through the tunneled wound into the anterior chamber, resulting in a hyphema. In addition, subconjunctival hemorrhages are more common with scleral tunnel wounds and may result in an inferior cosmetic outcome to that of the clear cornea phacoemulsification.
- Finally, while the surgically induced astigmatism of clear corneal incisions has been employed advantageously for astigmatic correction, the use of scleral tunnel wounds for this purpose has not been described.
*Dikutip dari Buku Essentials of Cataract Surgery 2nd Ed, halaman 51-55