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Anand Parthasarathy(1,2); Yong Ming Por(1,2); Donald TH Tan(1-3)

1.Singapore National Eye Centre; 2.Singapore Eye Research Institute; 3.National University of Singapore

Correspondence: Professor Donald Tan
Email: snecdt{at}pacific.net.sg Singapore National Eye Center, 11, Third Hospital Avenue, Singapore 168751. Tel: (65) 6322 4500; Fax: 65) 63231903.

Date of acceptance: January 1st 2008

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Introduction

Anterior lamellar keratoplasty involves heterotopic lamellar replacement of the anterior layers of the cornea, whilst retaining the posterior layers of the recipient, including posterior stroma, descemet's membrane (DM) and the endothelial layer. A major disadvantage of lamellar keratoplasty as opposed to penetrating keratoplasty (PKP) is the irregularity of the corneal stromal bed, which occurs in relation to manual lamellar dissection techniques, which limits the final best visual acuity. Microkeratomes1,2 have been used to achieve smoother lamellar dissections. To completely obviate this problem, a variety of surgical techniques have been described for performing 'maximum depth' deep anterior lamellar keratoplasty (DALK), in which complete stromal removal with baring of DM is attempted, at least in the central visual axis.3-6 These include techniques using viscoelastic,7 hydrodissection and Anwar’s big bubble technique using air to cleave stroma from DM.3 The Anwar's big bubble technique for DALK has been shown to have good visual outcomes for keratoconus3. This could be due to the presence of a smoother donor-to-recipient interface between the DM and the donor stroma reducing interface irregularities.

Although Anwar's air dissection is an established and efficient technique for reproducibly detaching DM from corneal stroma, injected air in the stroma, which precedes DM separation, often produces a dense white overlying opacification and if extensive often precludes visualization of the deeper stromal layer, and the anterior chamber. The classic circular silvery sheen of the "big bubble" is thus often masked and it is then not be possible confirm air separation of DM. We describe a simple and elegant technique which reliably delineates the extent of the big bubble without the use of ancillary instruments, which we aptly name the "small bubble technique", to aid in success of performing the "big bubble" technique.

Surgical Technique

After performing partial thickness trephination with a Hanna trephine (Moria, France) set at 60 % to 70% of the preoperative stromal pachymetry, manual dissection and excision of the anterior stroma is initiated with a Mini-Crescent blade (1.25mm,Sharpoint, UK) and completed by the crescent blade (2.25mm,BD Visitec, UK). The diameter of the trephine is based on the size of the cone in keratoconus. Anwar's "big bubble" technique is then initiated with the use of a 27 or 30-gauge needle attached to a 3 ml syringe. The needle tip, facing bevel down, is carefully inserted tangentially into paracentral cornea to an approximate depth of 90% stromal thickness. Under direct visual control, the needle is carefully advanced and a firm injection of approximately 0.5ml of air is performed. The achievement of a big bubble is usually confirmed by the rapid appearance of a silvery opaque circle delineating the size of the bubble achieved. In contrast to the appearance of a big bubble, intrastromal air appears white rather than silvery, with irregular, fibrillary edges. If this intrastromal air extends peripherally, it will obscure the underlying big bubble, and it may be impossible to ascertain if DM separation has truly occurred. It is in this situation that our technique will become helpful.

Our technique involves injecting a small amount of air into the anterior chamber (AC) at the limbus with a 27 or 30-gauge needle (Video). The small bubble cannot be injected prior to injecting the big bubble since the breach in the DM may result in escape of air into the AC during air injection, preventing the big bubble from forming. Care is taken to stay peripheral and posteriorly close to the chamber angle when advancing the needle, so as to avoid inadvertent perforation of an unseen big bubble. The air injected should achieve an AC air bubble of approximately 2 mm in diameter. If the small bubble remains visible in the periphery, this suggests that the AC periphery is the highest point of the AC, and thus confirms that a central big bubble is present. More clues to this may be the shape of the bubble, which may assume a "kidney bean" configuration, and which can be made to travel completely around the corneal periphery by circular tilting of the globe, demonstrating the annular or “doughnut” shaped configuration of the AC periphery. If, in contrast, the small bubble immediately disappears upon injection, this means it has moved centrally to the highest point of the AC, in this case, the central AC, and therefore no central big bubble has been achieved. In this situation, repeat air injection should be performed, and if a big bubble has now been achieved, the small bubble will migrate to the periphery and reappear. In the event that even after multiple air injections the big bubble is not achieved then a manual dissection of the remaining stroma is done.

Once confirmation of the big bubble has been achieved, the big bubble is released by carefully incising the posterior stroma centrally to gain direct excess to DM. Rapid collapse of the big bubble now causes migration of the small bubble centrally, and it will be seen to immediately appear beneath the posterior stroma when big bubble air is lost (Video). Careful removal of the posterior stroma up to the margins of the lamellar trephination is now relatively easily achieved with blunt-tipped corneal scissors due to prior and complete separation of DM. Since there is complete removal of the recipient stroma with this technique, there is no need to perform stromal lamellar dissection of the donor. Donor tissue is trephined to the same diameter as the recipient bed, and DM membrane is stripped away with Macpherson's forceps. This is then positioned onto the bare DM of the recipient bed for suturing.

Comment

The advantages of retaining DM and endothelium in anterior lamellar keratoplasty (less risk of allograft rejection and graft failure due to endothelial cell loss as compared to PK8) are often balanced against the poorer visual outcomes of conventional or manual lamellar surgery. However recent reports indicate visual outcomes for DALK comparable to that for penetrating keratoplasty, especially for keratoconus, stromal dystrophies and corneal scars9,10,11. The Anwar "big bubble" technique represents perhaps the most dramatic, elegant, and rapid form of achieving complete separation of DM from posterior stroma in DLKP surgery. The confirmation of achievement of the big bubble is important in DALK surgery since it determines the next step for the surgeon i.e to go ahead with stromal removal or consider reinjecting air into the stroma. Our "small bubble" technique of injecting air into the AC to observe its behaviour is an elegant and simple method to confirm success of the big bubble injection and is a useful surgical tool for surgeons attempting DALK.

Conflict of Interest Statement

The authors do not have a proprietary or financial interest in the products used in the video

References