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Bowman layer transplantation another option for advanced keratoconus
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Irregular astigmatism and decreased visual acuity secondary to corneal thinning and ectasia associated with keratoconus can deleteriously affect visual quality and hence quality of life in these patients.
Keratoconus prevalence varies based on location and race, affecting 57 in 100,000 Caucasian patients and 229 in 100,000 Asian patients. Clinical and surgical options decrease as this condition progresses to advanced stages of keratoconus. Glasses and contact lenses (rigid gas permeable, soft or hybrid) are usually not the best option in advanced keratoconus because the quality of vision is often suboptimal and may not meet the requirements for the patient’s daily activities. Corneal cross-linking and intracorneal ring segment implantation may not be an option in a subset of advanced keratoconus patients with thin corneas. In these cases, anterior lamellar keratoplasty or penetrating keratoplasty may be a surgical choice. PK often becomes the choice in cases with full-thickness corneal scars that are in the central or paracentral regions of the cornea.
In this column, Drs. Parker, Birbal, van Dijk and Melles describe a new surgical technique of Bowman layer transplantation for advanced keratoconus with early promising results. However, this technique is in its infancy and needs to be evaluated with a larger patient pool, looking at safety and reproducibility of the procedure and the overall success of this procedure for patients with advanced keratoconus.
Thomas “TJ” John, MD
OSN Surgical Maneuvers Editor
Recently, we have developed a novel surgical approach for the treatment of advanced progressive keratoconus. Bowman layer transplantation entails implantation of an isolated Bowman layer graft into a mid-stromal pocket in order to remodel (ie, flatten) the corneal curvature. Meanwhile, stabilization of corneal ectasia may be obtained by the Bowman layer graft and the wound-healing effect within the stroma. Although UV cross-linking has previously been reported to safely and effectively halt progression of keratoconus, employment of the procedure requires a minimal corneal thickness of 400 µm. While implementation of hypo-osmolar riboflavin has been described to treat thinner corneas, more advanced keratoconus may prohibit the use of UV cross-linking. Hence, Bowman layer transplantation may present an alternative or complementary treatment option to halt or delay progression of disease in order to defer, or possibly avoid, more invasive surgical treatment such as corneal grafting.
Bowman layer graft preparation
Bowman layer grafts are typically prepared in an eye bank in the days before surgery. A corneoscleral rim is excised from a whole globe and then mounted, epithelial side up, on an artificial anterior chamber. After removing all corneal epithelium, a superficial incision is made just within the limbal area (Figure 1a), after which the peripheral edge of the Bowman layer can be lifted from the underlying anterior stroma, using the tip of McPherson forceps. To free the Bowman layer tissue from its underlying attachments, the entire Bowman layer can be carefully peeled away via gentle slow movements in a circular method by grasping the edge with McPherson forceps (Figure 1b). After preparation, the Bowman layer graft rolls up into a single or double roll, owing to the elasticity of the tissue (Figure 1c).
Images: Melles GRJ and colleagues
Surgical technique
The operation is performed under local anesthesia. After a retrobulbar block is administered, digital ocular massage is performed followed by the placement of a Honan balloon to achieve a soft eye. Patient position is also attended to; an anti-Trendelenburg posture is preferred, and before the surgery commences, the eyelid speculum is checked for excessive tightness, which can generate unwanted posterior pressure.
The first step in the operation is to perform a superior peritomy. A 5 mm long half-thickness scleral incision is made 1.5 mm posterior to the limbus and then tunneled up into the clear cornea using a crescent blade (Figure 2a). Subsequently, a paracentesis is created, and the anterior chamber is filled with air (Figures 2b and 2c). A series of curved spatulas is used to dissect through the recipient cornea, guided by the “air-endothelial reflex”(Figures 3a to 3c). Specifically, when the anterior chamber is filled with air and instruments are placed into the peripheral cornea, a reflection of the tip of those instruments appears, and the deeper (ie, the more posterior) the instrument is pressed into the cornea, the closer it appears to its reflection (and when the two appear to just meet, a 99% depth has been achieved) (Figure 4). In this way, a dissection to an intended 50% corneal depth can be achieved, continuously guided by this reference image. This dissection is carried out throughout the entire cornea, from limbus to limbus, 360° around (Figures 3a to 3c). This produces a pocket within the mid-cornea (Figure 5).
Once complete, air is withdrawn from the anterior chamber, and the isolated Bowman layer graft is removed from its storage solution, submerged for 30 seconds in 70% alcohol to remove any lingering epithelial cells, rinsed with balanced salt solution and stained with trypan blue. Then, a surgical glide is placed through the mouth of the corneoscleral tunnel and advanced into the corneal pocket (Figure 6a). The graft is placed on top and then pushed inside the recipient cornea using a blunt spatula (Figure 6a). After this, the glide is removed and the graft is unfolded using gentle, direct touching with the same cannula (Figure 6b). Once the graft is fully unfolded within the pocket (Figure 6c), the anterior chamber is reformed and the conjunctiva is reapproximated to its original superior location at the limbus, and the operation is concluded.
Results
The average amount of corneal flattening achieved by Bowman layer transplantation is about 8 D, which is often achieved by the first postoperative month (Figure 7). Best spectacle vision typically improves by one or two lines, while contact lens vision is usually unchanged. Ninety percent of patients may experience a cessation of their disease progression, at least throughout the longest follow-up of about 5 years today. Because the Bowman layer graft is acellular, topical steroids may be tapered early after surgery; consequently, the rate of cataract formation and glaucoma development may be low. Suture-related problems are eliminated because the operation does not involve sutures. Most operated eyes are indistinguishable from unoperated eyes unless the slit beam is used to view the cornea in profile (Figure 8).
- References:
- Fernández Meijide N. Keratoconus epidemiology. In: Albertazzi R, ed. Keratoconus: Guidelines for Diagnosis and Treatment. Buenos Aires, Argentina: Ediciones Científicas Argentinas; 2010:11-15.
- Groeneveld-van Beek EA, et al. Cornea. 2016;doi:10.1097/ICO.0000000000000905.
- Melles GR, et al. Br J Ophthalmol. 1999;doi:10.1136/bjo.83.3.327.
- Raiskup F, et al. Am J Ophthalmol. 2011;doi:10.1016/j.ajo.2011.01.016.
- van Dijk K, et al. JAMA Ophthalmol. 2014;doi:10.1001/jamaophthalmol.2013.5841.
- van Dijk K, et al. Ophthalmology. 2015;doi:10.1016/j.ophtha.2014.12.005.
- Wollensak G. Curr Opin Ophthalmol. 2006;doi:10.1097/01.icu.0000233954.86723.25.
- For more information:
- Gerrit R.J. Melles, MD, PhD, is founder of the Netherlands Institute for Innovative Ocular Surgery. He can be reached at NIIOS, Laan Op Zuid 88, 3071 AA Rotterdam, Netherlands; email: melles@niios.nl.
- Thomas “TJ” John, MD, is a clinical associate professor at Loyola University at Chicago, and is in private practice in Oak Brook, Tinley Park and Oak Lawn, Illinois. He can be reached at email: tjcornea@gmail.com.
Disclosures: Melles reports he is a consultant for DORC International, SurgiCube International and Hippocratech. John reports no relevant financial disclosures.