INTRODUCTION

Bowman’s membrane or layer has been classically described as an acellular condensation of the anterior corneal stroma. It is a sheet of smooth surface, acellular and without regenerative capacity, from 8 to 12 μm thick. It is located between the basal membrane of the epithelium and the anterior stroma. Since it can be dissected from a donor cornea, we propose the possibility of carrying out an isolated transplant of said layer in patients with typical alterations at that level.

To date, we have used Bowman's membrane transplantation (BMT) in two indications and techniques: (a) as an inclusion (inlay), to stabilize keratoconus (KC) in progression, not a candidate for other treatments due to its advanced stage; and (b) as a superimposition or patch (onlay), for the treatment of cicatricial opacities (haze) secondary to surface ablation with excimer laser.

OBTAINING THE BOWMAN’S MEMBRANE GRAFT

We fix a corneoscleral segment in an artificial anterior chamber (Katena, distributed by Rockmed BV) and carefully remove the epithelium by mechanical scraping. With a 30 G needle we make a superficial incision in the 360° of the corneal periphery. Then, using a self-made separator (DORC International BV), we carefully isolate and separate the Bowman's membrane from the corneal stroma, from the periphery at 360° to the center. Once completely peeled, we trepan it to obtain a Bowman’s membrane disc of 9.0 to 11.0 mm in diameter. Due to its elastic properties, it spontaneously forms a roll (Bowman's roll), which is immersed in 70% ethanol to eliminate the possible remains of epithelial cells. After irrigating with balanced saline solution (BSS, Bausch and Lomb), we store the Bowman's roll in culture medium (CorneaMax, Eurobio) at 31°C until the moment of use (Figure 1).

INCLUSION (INLAY) OF BOWMAN’S MEMBRANE FOR ADVANCED KERATOCONUS TREATMENT

Since 2003, crosslinking by ultraviolet UV light and riboflavin (CXL) has become an alternative for the treatment of KC in corneas with a minimum of 400 μm in thickness and a maximum keratometry of ≤58 diopters (D)¹. Although technical improvements have subsequently been made to treat even thinner corneas with higher curvatures², in the more advanced cases of KC, treatment options are still limited to keratoplasty, either penetrating (PK) or deep anterior lamellar (DALK)^3,4.

At the Netherlands Institute for Innovative Ocular Surgery (Rotterdam, The Netherlands) we have developed a new alternative for the treatment of advanced KC (K ≥70 D) and progressive for corneas not suitable for CXL. Since the fragmentation of the Bowman’s layer is one of the pathognomonic features of the corneas that have KC^4,5, we developed the hypothesis that, by implanting the stroma of a BMT, the corneal anatomy and its curvature could be partially restored. In this way, the progression of KC could be slowed down thanks to the support and intrastromal scarring effect induced by the graft, which would stabilize the corneal structure. In addition, the need for keratoplasty would be delayed by allowing the wearing of contact lenses in the long term and avoiding much of the complications associated with this disease.

Surgical technique

To implant the BMT, we first create a stromal pocket. To do this, we perform a lamellar dissection in the stroma with spatulas in a similar way to the technique previously described by us for DALK (Figure 2A-F)^6,7. Once the stromal pocket is completed 360° to the limbus (Figures 2E and F), we introduce a surgical slider [BD Visitec (Fichman); BeaverVisitec International] in the space created and on itself, we delicately slide the roll-shaped graft into the stromal pocket, unfold and center it, using a 30G cannula connected to a syringe with BSS to manipulate the tissue (Figures 2G and H). To finish, we pressurize the eye with BSS⁸. The postoperative treatment includes antibiotic eye drops 6 times a day and corticosteroids 4 times a day.

Results

We performed a BMT inclusion study in 22 eyes with progressive KC and in the final stage, for an average of 21 ± 7 months⁸. Two patients (2 eyes) underwent an intraoperative perforation of Descemet's membrane. The rest of the surgeries were carried out without complications (Figure 3). After surgery, an effect of corneal flattening and a decrease in mean maximum keratometry (Kmax) was observed. This went from 77.2 ± 6.2 D before to 69.2 ± 3.7 D one month after surgery (p <0.001) and remained stable afterwards (p ≥0.072). One year after surgery, an improvement in best-corrected visual acuity (BCVA) with glasses was observed (p <0.001) and visual acuity with contact lenses remained stable (p = 0.105). No change in endothelial cell density was observed (p = 0.355)⁸.

Advantages of the inclusion (inlay) of Bowman's membrane

The BMT by inclusion produces a corneal flattening and, consequently, a more homogeneous surface and with greater stability in the long term, thanks to the increase of the tensional force. At the same time, and unlike the intracorneal rings, the BMT has a rigidity similar to that of the surrounding tissues, so that the tissue reaction and graft migration are practically nil. Another important aspect of BMT is the low number of complications associated with the technique. Although the goal is not to improve visual acuity, the BMT allows patients to better adapt their contact lenses and their long-term wearing without complications, because both the anterior and posterior surfaces of the cornea remain intact after surgery. In this way, BMT is an alternative in the treatment of advanced KC, delaying a DALK or PK in these patients.

BOWMAN’S MEMBRANE PATCH (ONLAY) FOR THE TREATMENT OF CICATRICIAL OPACITY AFTER ABLATION WITH LASER

As far as we know, the development of a cicatricial opacity (haze) is rare or practically non-existent after traumatic or iatrogenic corneal abrasions. In this sense, it has been hypothesized that the presence of the Bowman's membrane would condition the interaction between the epithelium and the stroma in the wound repair process, in such a way that a subepithelial scar is not generated^9-11. If this layer is absent, the interaction between the epithelium and the stroma would induce an abnormal repair response of the tissue, clinically observed as said scar.

In a patient with low vision (BCVA of finger count) due to a severe haze-like opacity, secondary to superficial laser-assisted keratectomy (LASEK), we evaluated the effectiveness of scar tissue excision together with the application of a BMT superimposed as a patch to restore corneal transparency and improve visual acuity.

Surgical technique

In the receiving eye, we dissect an anterior stromal flap of 9.0 mm in diameter and 60 μm thick, using a femtosecond laser (IntraLase, Abbott Medical Optics), with a hinge at the 12 o'clock position. After lifting the flap, we irrigate the stromal bed and then deploy and position the BMT on it. We reposition the flap over the BMT, but we purposely leave its lower part folded inward (the epithelium towards the graft), in order to fix the BMT and allow a rapid re-epithelialization of it by migration of the epithelial cells from the flap. We place a rigid scleral support lens on the surface of the eye to avoid BMT dislocation and promote adhesion to the flap. The postoperative medication includes antibiotic eye drops 6 times a day, and corticosteroid 4 times a day. After 2 weeks, we dissect and resect the flap completely, under topical anesthesia (Figure 4)¹²

Results

Two months after the intervention, the patient achieved a visual acuity with a rigid scleral support lens of 1.2. After more than 5 years of follow-up, the vision has remained stable and no recurrence of the scar has been observed¹².

Advantages of Bowman's membrane (onlay) patch

The superimposed graft or patch of BMT could be an effective treatment for persistent opacities (haze) after laser corneal ablation. This extraocular technique offers important advantages over other more aggressive treatments such as PK or DALK, and avoids the risks derived from the use of antiproliferative agents. The use of a BMT could be an effective and safe treatment to restore visual acuity in cases of anterior stromal opacity and alteration of epithelial wound repair processes.