B. Clear Corneal Incision
The steps in the construction of clear corneal incisions include anesthesia, paracentesis creation, corneal groove incision, and keratome entry into the anterior chamber (Figure 6-3). Although various modes and combinations of anesthesia may be employed, many surgeons use topical anesthesia for clear corneal incisions.
a. Step 1
The paracentesis is created in the same manner as previously described for a scleral tunnel.
b. Step 2
The next step for some surgeons is the corneal groove incision, which is usually placed temporally at the limbus. A Thornton fixation ring is used to stabilize the globe, and a guarded knife is angled perpendicular to the cornea in order to achieve a consistent fixed groove depth. A groove can be chosen to be at the same depth as the tunnel, or deeper, with the latter resulting in a hinge at the base of the tunnel, which has been found to improve self-sealability of the wound upon application of external pressure.Some surgeons skip this step completely, creating a single plane incision.
c. Step 3
The keratome is then placed in the corneal groove with the heel down flush with the ocular surface and advanced approximately 2 mm anteriorly, dissecting a plane through the corneal stroma. The heel of the blade is subsequently elevated off of the globe so that the tip of the keratome is directed toward the iris opposite the wound. Depending on the sharpness of the blade being used, this can create visual folds in the cornea as described above for scleral tunnel incisions. When using diamond blades or ultra-sharp metallic keratomes, some surgeons simply create a single plane incision without a limbal groove or a “dimple-down” maneuver. The keratome is advanced into the anterior chamber such that the shoulders of the blade penetrate the inter- nal aspect of the wound, ensuring adequate internal wound width.
While some surgeons prefer the tactile tissue resistance during creation of the corneal tunnel afforded by metallic keratomes, diamond keratomes have been shown to cause less tissue disruption in corneal stroma. Furthermore, diamond keratomes remain sharp and resist wear longer than their metal counterparts.
Clear corneal incisions have some advantages over scleral tunnel wounds.
- As clear corneal incisions may be performed with topical anesthesia alone, a retrobulbar block is avoided. This is an important consideration in patients who have a bleeding diathesis or who are highly myopic, as there is a higher risk of retrobulbar hemorrhage or globe perforation,respectively, from a retrobulbar block. A retrobulbar block can also cause complications inducing retinal vascular occlusion, optic nerve injury, strabismus, and brainstem anesthesia.Topical anesthesia allows for rapid visual rehabilitation following the surgical procedure while avoiding the risks of retrobulbar anesthesia.
- A clear corneal incision is more time efficient and often has a better immediate cosmetic result.
- Since the conjunctiva is relatively untouched, it is left naïve for future filtering surgery if necessary. Similarly, a clear corneal approach leaves preexisting filtering blebs undisturbed.
- With regard to refractive outcomes, preoperative astigmatism can be corrected at the time of cataract surgery through modifications in incision length, clock hour position, and proximity to the optical center. For example, placement of the incision on the steep axis of astigmatism can be used to correct astigmatic errors of <1 D.Furthermore, relatively simple modifications to the clear cornea incision such as limbal relaxing incisions can be employed.
There are disadvantages to clear corneal incisions.
- Preexisting corneal problems such as Fuchs’ endothelial dystrophy,peripheral corneal degeneration, previous penetrating keratoplasty, or radial keratotomy scars are relative contraindications to corneal wounds.In addition, in patients with impaired blinking, as seen in Parkinson’s disease, exposed incisions in the cornea should be avoided due to the risk of corneal melt. The incision in these patients should generally be protected by conjunctiva.
- While conjunctiva covers the wound in scleral tunnel incisions, a clear corneal wound is exposed, and a postoperative wound leak allows for potential ingress of contaminated ocular surface fluid into the anterior chamber. Several reviews in the literature have reported a 3- to 4-fold higher rate of endophthalmitis in sutureless clear corneal incisions compared to scleral tunnel incisions, and yet other series have found no significant difference. Stromal hydration and postoperative hypotony are thought to contribute to poor wound closure.
- A corneal groove that is placed too posterior can cause an inadvertent incision in the conjunctiva and lead to ballooning as irrigation fluid collects in the subconjunctival space.
- Instrument manipulation can cause corneal striae, which may result in death of corneal endothelial cells.
- Incisions longer than 4 mm are preferentially made in the sclera, as unsutured corneal incisions of this size can gape and fail to self-seal. Subsequent suture closure of such an incision may result in greater induced postoperative astigmatism. In addition, the recent use of optical coherence tomography to evaluate the postoperative structure of clear corneal incisions less than 4 mm in length has revealed a high incidence of early Descemet detachments and posterior wound gape. Posterior wound retraction also occurs in the majority of clear corneal incisions after 3 years. The clinical relevance of these findings is still unknown, but it is hypothesized that they could lead to long- term refractive changes. Clear corneal incisions with a length of 3.2 mm have been shown to induce 0.5 D of astigmatism, and this amount has not been shown to decrease with smaller wound sizes. As mentioned previously, this can be advantageous when used to correct preexisting astigmatism but may be undesirable in other circumstances.
*Dikutip dari Buku Essentials of Cataract Surgery 2nd Ed, halaman 55-59