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Cataract Surgery | Nov 2010

Big Bubble Versus Manual Dissection

Visual outcomes after deep anterior lamellar keratoplasty are best when the surgical goal is to reach the plane of Descemet's membrane.

Deep anterior lamellar keratoplasty (DALK) is a lamellar transplantation surgical technique indicated for the treatment of corneal diseases that do not affect the endothelium.1,2 The donor lamella is positioned directly on Descemet's membrane, preserving the recipient's endothelium and decreasing the risk of immunologic rejection.3

Despite the development of new surgical techniques and instruments, the success of DALK has been limited by its technical difficulty, protracted operating times, and the risk of intraoperative corneal penetration. This article reviews the history of DALK and assesses a variety of techniques, with special attention given to the differences between manual deep dissection and big-bubble techniques.

BACKGROUND
In 1974, Mohammed Anwar, DO, FRCS, was the first to describe baring Descemet's membrane in corneal dissection, reporting excellent smoothness of the stromal bed for the donor graft button.4 Since then, a number of surgical approaches to DALK have been proposed.5-9 The bigbubble technique, introduced by Anwar and Teichmann in 2002, is probably currently the most common approach.3 In the big-bubble technique, air is injected into the deep stroma to achieve a large air bubble between Descemet's membrane and the stroma. This facilitates dissection at the pre-Descemet's plane (Figures 1 and 2).

Depending on corneal pathology and the surgeon's experience, a direct open-dissection approach during DALK can be used freehand in several layers with the use of a crescent blade10 in anticipation of injection of air or secondary to the partial formation of a big bubble. The dissection, which can be carried out in several steps, is facilitated by stretching and lifting the superficial stromal layers and sweeping the blade in a flat plane over the stroma, which develops a frothy appearance due to traction as air enters between the collagen fibrils. The dissection becomes progressively more difficult the closer the blade is to Descemet's membrane. Sometimes the last residual stroma can be peeled off the membrane.8

Studying the histopathology of DALK with the big-bubble technique, Lim et al observed that pneumatic dissection in DALK produces diffuse intrastromal air bubbles and that the air dissection clearly occurs above the level of Descemet's membrane.11

Abdelkader et al used confocal microscopy to compare wound healing and morphologic characteristics of keratocytes and the donor-recipient interface after near- and maximum-depth DALK in rabbit corneas.12 In dissections to Descemet's membrane, the smooth interface showed less scarring and decreased intensity and reflectivity of activated keratocytes. In the near-Descemet's dissections, stromal healing stimulated more activated keratocytes and more haze. Reaching the level of Descemet's membrane minimizes the healing process and thus the production of haze, thereby providing good corneal clarity.

Several studies have compared visual outcomes after bigbubble and layer-by-layer manual dissection techniques for DALK (Figure 3).13-15 Some authors reported faster visual recovery in true Descemet's plane DALK compared with big-bubble formation, but no difference in visual outcomes between the techniques was seen at 12 months.13,14 Fontana et al15 observed better and faster visual recovery that was still statistically significant at 2 years in patients in whom big-bubble formation was achieved. This variation in findings could be due to a difference in the depth of the pre-Descemet's plane reached by the surgeons. In my experience, the deeper the plane and the smoother and more regular the surface is, the better the recovery of visual acuity.

COMPLICATIONS WITH DISSECTION
All lamellar dissection techniques increase the risk of inadvertent penetration when deep dissections are performed, particularly in opaque corneas. In these eyes, the depth of stromal dissection cannot be visualized during surgery, and the risk of penetration increases as the recipient bed is created by layer-by-layer removal of corneal stroma. Melles proposed a three-step surgical technique for DALK to allow visualization of the depth of the dissection relative to the corneal thickness.7 The posterior corneal surface is visualized by injecting air into the anterior chamber to create an air-to-endothelium interface. Then a deep stromal pocket is created and filled with an ophthalmic viscosurgical device (OVD), and finally the anterior lamella is excised. Although this method increases the precision of localizing the depth of the surgical dissection plane in closed dissection, it does not allow reliable baring of Descemet's membrane, even if OVD is injected.8

FEMTOSECOND-ASSISTED DALK
Solid-state femtosecond lasers have been used successfully in several corneal surgical procedures.16 The femtosecond laser facilitates flap creation in LASIK; creates arcuate incisions and channels for intrastromal corneal rings; and prepares donor and host tissue for anterior, posterior, and penetrating keratoplasty (PKP). Use of a femtosecond laser has been shown to improve reproducibility and outcomes of lamellar surgery.17-19

We have proposed a variant of the big-bubble technique for DALK using the IntraLase femtosecond laser (Abbott Medical Optics Inc., Santa Ana, California).20

We subsequently devised two variants of what we named the IntraBubble technique. In the first, after the recipient lamella created by the IntraLase laser is removed, a 27-gauge needle attached to a 5-mL syringe filled with air is directly inserted into the stroma to forcefully inject air to achieve the formation of the big bubble.21 Keratectomy is completed layer by layer with a crescent knife up to Descemet's membrane.

In the second variant, IntraLase Enabled Keratoplasty (IEK) computer software is used to create a pre-Descemet's plane lamellar dissection at a predefined corneal depth. A channel in the posterior stroma of the recipient is created 50 µm above the endothelium,22 and a smooth cannula is introduced into this channel for air injection. To date, Descemet's membrane was consistently reached in all eyes (20 patients) treated with this version of the IntraBubble technique (Figure 4).

For more information, see The IntraBubble: A Variant of the Big Bubble in the September 2010 issue of CRST Europe.20

RESULTS
In eyes in which dissection was completed with a knife after the IntraBubble technique (n=13), mean BCVA was 0.25 ±2.10 at 1 month, 0.35 ±1.00 at 3 months, 0.41 ±1.80 at 6 months, and 0.52±1.20 at 12 months. Mean spherical equivalent was -1.20 ±2.90 D at 1 month, -0.50 ±3.40 D at 3 months, -1.90 ±3.10 D at 6 months, and -2.50 ±2.70 D at 12 months. Mean keratometric astigmatism values were 6.50 ±2.70 D at 1 month, 4.40 ±1.70 D at 3 months, 3.90 ±1.50 D at 6 months, and 2.90 ±1.60 D at 12 months.

In eyes completed with the big-bubble technique after IntraBubble (n=20), BCVA was 0.50 ±3.00, mean spherical equivalent was -1.50 ±3.10 D, and mean keratometric astigmatism was 2.90 ±3.90 D at 1 month. At 3 months postoperative, mean BCVA was 0.60 ±1.20, mean spherical equivalent was -2.25 ±2.50 D, and mean keratometric astigmatism was 3.20 ±2.90 D.

At 1 and 3 months postoperative, patients who underwent the big-bubble variant achieved statistically better visual acuity compared with patients who underwent the dissection variant. In both groups, all sutures remained in place at 3 months. We believe that the access channel created by the IntraLase laser, which allows the maintenance of a predefined corneal depth very close to the endothelium, is responsible for successfully achieving the bubble more than 80% of cases.

Like conventional big-bubble DALK, the IntraBubble technique decreases the risk of immune rejection. However, the creation of a channel for smooth air injection significantly reduces the risk of inadvertent penetration, which is the principal complication of DALK. Although further long-term study is needed, we believe that the IntraBubble technique could help standardize big-bubble DALK by reducing the learning curve for surgeons and facilitating good refractive outcomes.

CONCLUSION
The big-bubble technique appears to be the best surgical approach to DALK. Dissection can be considered as an option to avoid conversion to PKP in the event of intraoperative complications or as an alternative to the big-bubble technique. When the surgical goal is to reach the plane of Descemet's membrane, visual outcomes are better. Ardjomand et al demonstrated that the quality of vision in the postgraft keratoconic cornea is correlated to the thickness of the residual recipient stromal bed; an eye with an anterior lamellar keratoplasty and a residual bed of less than 20 µm can achieve a visual result similar to that of PKP.23

Our preliminary results indicate that use of the IntraBubble technique may improve the standardization and reproducibility of the big-bubble technique for DALK. Compared with other lamellar procedures, IntraBubble has a high success rate and produces good refractive outcomes.

Luca Buzzonetti, MD, practices in the Ophthalmology Department, Bambino Gesù Children's Hospital, Rome. Dr. Buzzonetti states that he has no financial interest in the products or companies mentioned. He may be reached at tel: +39 3386899038; e-mail: lucabuzzonetti@yahoo.it.

Gianni Petrocelli, MD, practices in the Ophthalmology Department, Bambino Gesù Children's Hospital, Rome. Dr. Petrocelli states that he has no financial interest in the products or companies mentioned. He may be reached at e-mail: gpetrocelli@tin.it.

Paola Valente, MD, practices in the Ophthalmology Department, Bambino Gesù Children's Hospital, Rome. Dr. Valente states that she has no financial interest in the products or companies mentioned. She may be reached at e-mail: valentepaola@gmail.com.

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