Clear corneal incisions have become the default entry point for routine phacoemulsification due to their simplicity, rapid healing and low induced astigmatism in healthy eyes. However, they rely heavily on the structural and optical integrity of the cornea.
When the cornea is thinned, scarred, unstable or otherwise fragile, creating a standard corneal incision can compromise safety and hinder postoperative recovery. For this reason, the scleral tunnel incision remains a useful technique, offering structural advantages in eyes in which a corneal approach could worsen existing pathology.
Eyes with compromised corneas often present with irregular astigmatism, unstable stromal architecture or reduced endothelial reserve. A clear corneal incision in such tissue can inadvertently exacerbate these problems. In corneas affected by post-RK ectasia, such as the case presented in this column, even a small incision can intersect the prior RK cuts, leading to rupture or unpredictable biomechanical changes. By shifting the surgical entry to the sclera, we can avoid touching the RK cuts and further weakening the cornea.
A scleral tunnel incision offers several advantages that make it particularly suitable for cases involving compromised corneal tissue. The sclera, being thicker and biomechanically more robust than the cornea, provides a dependable substrate for wound construction even when corneal tissue cannot be safely manipulated. A well-constructed scleral tunnel affords excellent structural stability without relying on the elasticity of diseased corneal stroma. Furthermore, scleral incisions tend to induce minimal corneal astigmatism and do not distort the anterior corneal surface, an effect that is especially beneficial in patients already struggling with significant irregularity or instability of corneal shape. And because the conjunctiva can be reposited over the wound, healing occurs in a protected environment, reducing the exposure-related complications that are more likely in eyes with fragile corneas.
Constructing a scleral tunnel starts with creating a small conjunctival peritomy and achieving hemostasis with cautery. The incision is made approximately 1.5 mm to 2 mm posterior to the limbus, where the sclera is thick enough to support a stable tunnel yet close enough to the limbus to allow a smooth transition into the cornea. The crescent blade is used to create a half-scleral depth groove, and then a tunnel is dissected. The tunnel itself must be of sufficient length, usually around 3 mm, to form an effective valve that maintains chamber stability during surgery. Once the tip of the crescent blade enters the edge of the peripheral cornea, a keratome is used in the tunnel to make the entry into the anterior chamber. The internal corneal lip should taper smoothly into the anterior chamber to permit safe introduction of instruments and to prevent wound leakage (Figure 1).
Because the incision sits farther from the visual axis than a typical clear corneal incision, instrument manipulation angles differ slightly, and surgeons must be mindful of ergonomics to ensure that instruments move freely within the tunnel. With the typical corneal incision, the angle of approach for instruments is steeper and shorter, whereas for the scleral tunnel, it is a flatter angle and longer (Figure 2). This means that when creating a capsulorrhexis, surgeons may find access and pivoting more challenging.
Operating through a scleral tunnel incision alters instrument dynamics in subtle but important ways compared with a clear corneal incision. Because the entry point is more posterior, the phaco probe and irrigation/aspiration handpiece pass through a longer, more rigid tunnel, which changes the angle of approach to the anterior chamber. This geometry can slightly restrict lateral maneuverability, requiring more deliberate wrist rotation and careful tip positioning, particularly during sculpting or chopping maneuvers. The tunnel’s increased resistance also stabilizes the instruments, reducing wound leakage but demanding precise centration to avoid distorting the roof of the tunnel. In contrast, clear corneal incisions offer a more direct, shallow trajectory with greater freedom of movement but less inherent wound stability.
Although scleral tunnel wounds are generally self-sealing, some cases warrant reinforcement with a 10-0 nylon suture, especially in eyes with thin sclera, high myopia or elevated IOP. The knot of this permanent suture is buried, and then the conjunctiva is closed over it and secured with an 8-0 Vicryl dissolving suture (Figure 3).
In corneal ectatic disorders, like in this patient with prior RK, but also in patients with keratoconus, pellucid marginal degeneration or corneal disorders, the stability provided by a scleral approach prevents exacerbation of irregular astigmatism. Also, patients often experience improved postoperative comfort and healing when the incision is hidden beneath healthy conjunctiva rather than exposed on a compromised corneal surface.
Although the scleral tunnel incision demands greater technical precision and may slightly increase operative time, its advantages often outweigh these modest drawbacks in the setting of compromised corneas. The scleral tunnel incision is a vital technique in the cataract surgeon’s armamentarium, particularly for patients whose corneas lack the structural resilience to tolerate a standard corneal incision. By shifting the wound posteriorly, the scleral approach preserves corneal integrity, protects endothelial function, mitigates induced astigmatism and fosters more stable healing.
