Use of intraoperative anterior segment optical coherence tomography for Bowman layer transplantation

Bowman layer (BL) transplantation is a relatively new surgical procedure for treatment of advanced keratoconus in corneas that are too thin or steep for intracorneal ring segment implantation or ultraviolet cross-linking (van Dijk et al. 2015). An isolated, donor BL is inserted into a manually dissected, mid-stromal pocket to serve as a mechanical splint, flattening the recipient cornea by approximately 8–9 diopters (D) (van Dijk et al. 2018). Disease progression may be halted, and comfortable contact lens wear may be preserved or restored. Potentially riskier procedures, including penetrating keratoplasty (PK) and deep anterior lamellar keratoplasty (DALK), may be delayed or avoided entirely (van Dijk et al. 2018). One obstacle to the popularization of BL transplantation is the perceived difficulty of the manual stromal dissection in thin, ectatic corneas. In the technique's original description, dissection depth is monitored by the ‘air-endothelial-reflex’: when the anterior chamber is filled with air, the distance between an instrument and its reflection represents the ongoing dissection depth (Melles et al. 1999). This reflex may be subtle and difficult to perceive, such as in corneas with stromal scarring or haze. Intraoperative anterior segment optical coherence tomography (iAS-OCT) has recently been described as a useful tool to better visualize the anterior segment, and ultimately alter surgical decision making for DALK and Descemet membrane endothelial keratoplasty (Saad et al. 2015). Here, we describe the novel use of iAS-OCT to facilitate visualization of BL transplantation in 21 patients with keratoconus. The surgical technique was performed as described by van Dijk et al. (2014). A superior conjunctival peritomy is made, followed by a 5 mm long, partial-thickness scleral incision 1–2 mm posterior to the limbus that is tunneled up into the clear cornea with a crescent blade. The anterior chamber is filled with air through a paracentesis. A dedicated set of curved spatulas (Melles spatula set; DORC International) is used to dissect a mid-stromal pocket (aiming at 50% stromal depth) from limbus-to-limbus, 360 degrees around within the recipient cornea, into which the donor BL graft will be placed. Using an operating microscope (Lumera 700; Carl ZeissMeditec, Inc) fitted with iAS-OCT (Callisto; Carl Zeiss Meditec, Inc) the dissection plane may be visualized even if blood, edema, or scarring otherwise obscures the surgeon's view of the air-endothelial reflex (Figure 1A). This may allow dissections proceeding too anteriorly or posteriorly to be course-corrected before inadvertent perforation occurs. Further, it may enable the detection, or prevention, of multiple-plane dissections, which may otherwise be difficult to evaluate (Figure 1B), and the confirmation of proper placement and total unfolding of the donor graft (Figure 1C). In our initial series, 21 consecutive eyes of 21 patients (16 male, mean age 30 ± 12(SD) years (range 19 to 64 years) underwent iAS-OCT assisted BL transplantation. In 2 eyes, intraoperative perforation during stromal dissection resulted in the operation being aborted. In the remaining 19 eyes, BL transplantation was successfully completed. Mean thinnest point corneal thickness measured 380 (±43) μm preoperatively, 423 (±87) μm 1 day postoperatively, 389 (±63) μm 1 month postoperatively, and 379 (±52) μm at 6 months postoperatively. 89% of patients (16/18) demonstrated a reduction in Kmax 1 day postoperatively, which was maintained in 86% of patients (12/14) at 6 months postoperatively. These topographic results are similar to those achieved by van Dijk et al. (2018). This suggests that while iAS-OCT assisted BL transplantation helps to facilitate surgery, it may still be a difficult procedure. While early detection and crosslinking is preferable for keratoconus, some patients still present with advanced disease. For these patients, adequate visualization is perhaps the most important element to successful outcomes with challenging ophthalmic surgery. Particularly during the surgical learning curve, or for surgeons not experienced with manual dissection DALK, iAS-OCT may enable corneal surgeons to be more confident and comfortable with their stromal dissections and to achieve better results with BL transplantation.

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