Gerrit Melles first introduced the 'Posterior lamellar keratoplasty' technique in 1998.
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2 Mark Terry and Paula Ousley modified this technique to 'Deep lamellar endothelial keratoplasty' (DLEK).
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4 Previously, the original large incision technique was modified by both Melles and Terry to a small incision technique.
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6 The small incision technique has theoretical advantages, which may reduce corneal flattening and refractive astigmatism, thereby resulting in better visual outcome.
5 However, manual lamellar dissection is time consuming and technically difficult, and may result in corneal perforation and interface scarring.
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In this report, we describe 'Femtosecond laser-assisted small incision DLEK' for treating patients with corneal endothelial dysfunction. By applying the femtosecond laser, we may be able to perform lamellar keratoplasty more easily and rapidly while reducing corneal perforation and interface scarring risk.
Discussion
Posterior keratoplasty refers to removal of diseased posterior corneal layers (endothelium, Descemet's membrane, and posterior stroma) and replacement by partial-thickness donor tissue. This procedure can be an attractive alternative to penetrating keratoplasty (PKP) in treating corneal endothelial diseases such as Fuchs dystrophy and aphakic and pseudophakic bullous keratopathy.
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15 The endothelial replacement technique via scleral incision was first described by Melles and later modified by Terry and Ousley to 'Deep lamellar endothelial keratoplasty' (DLEK).
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4 Recent modification includes introducing the folded donor disc through the a small incision.
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One problem in performing DLEK is the technical difficulty associated with manual lamellar dissection. It is not only time consuming, but also has the potential risk of corneal perforation.
8 Moreover, even with uncomplicated dissection, the plane may not be smooth and uniform, which results in interface scarring and reduction in potential best vision.
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In a recent paper, Azar and coauthors described microkeratome-assisted posterior keratoplasty.
16 Given that Laser-Assisted in situ Keratomileusis (LASIK) shows almost no interface haze on the cornea,
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18 this technique's potential advantage is less interface scarring than manual dissection. However, microkeratome-assisted lamellar dissection does have the risk of corneal perforation in deeper dissection.
Recently, the femtosecond laser was introduced in refractive surgery fields for LASIK flap creation. According to recent papers, it demonstrated improved flap uniformity and better flap thickness predictability than mechanical microkeratomes.
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Similar to the microkeratome, femtosecond laser technology has evolved. After initial procedures with a 2 kHz laser in 1996, 15 kHz (2003), 30 kHz (2005), and soon after, 60 kHz (2006) engines were introduced.
The femtosecond laser creates corneal resection by delivering laser pulses tissue microphotodisruption. Pulses are scanned and placed in a vertical pattern for trephination (side) cuts or in a spiral or raster (zigzag) pattern to achieve lamellar cuts. The smoothness of optical surfaces is determined by programmable parameters: energy per pulse, separation of adjacent laser spots (spot separation), and raster pattern row spacing (line separation). The closer the laser spots, the less energy required.
20 The IntraLase system scans tissue at a repetition rate which depends on laser engine capacity. Higher repetition rates allow use of lower pulse energy and closer spot/line separation settings, which results in a smoother lamellar interface, an easier anterior corneal flap lift, and a faster procedure. The newer 60 kHz engine permits a line/spot separation down to 6×6 µm and energy less than 1 µJ per pulse, which may show superior efficiency for harvesting the posterior corneal disk.
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One concern during preoperative femtosecond laser-assisted donor eye posterior lamellar discs (PLDs) preparation is the possibility of endothelial cell loss (ECL). ECL has been reported at 4% after 150 to 200 µm thick endothelial side PLDs preparations, and did not appear to be caused by laser pulse energy.
22 Another in vitro study reported 4.3% versus 7.7% ECL after 30 kHz laser lamellar cutting compared to 15 kHz laser for horizontal lamellar cuts at corneal depth of 400 µm and 9.5 mm diameter.
23 Favorable outcomes are consistent with histological results showing adjacent thermal damage to be on the order of 1 µm.
24 Clinical results from Case 1 showed relatively intact postoperative endothelium status, which was comparable to previous reports. The recently released 60 kHz engine could minimize ECL risk by reducing delivered energy.
Moreover, the femtosecond laser is expected to be applied in full-thickness PKP. Cuts are customizable to achieve different graft geometric configurations, potentially allowing for sutureless, self-adhesive, or shaped keratoplasty. Two or more side/lamellar cut segments can be combined to create patterns for shaped keratoplasty, including top-hat (larger diameter cut posteriorly), mushroom (larger diameter cut anteriorly), zigzag, and Christmas tree patterns.
25 Peripheral wound edge shaping can provide stronger healing by increasing surface area, reduce corneal astigmatism by decreasing suture number, reduce donor-host topographic distortion disparity, and quicken visual recovery.
Following we describe small incision DLEK by femtosecond laser for donor corneal lamellar dissection.
Before performing femtosecond laser-assisted donor lamellar dissection, we measured donor corneal thickness by ultrasound pachymetry. If corneal thickness was thicker than 565 µm, glycerin was applied to dehydrate the cornea, and avoid a thinner graft than intended, as well as to ascertain graft thickness of 100 to 165 µm for transplantation. The laser was then used to create a corneal flap of 9.5 mm diameter and 400 µm thickness. Afterwards, two possible methods to obtain the posterior donor disc were used. In the first method, stromal adhesions are released without fully lifting the flap. This bi-hinged donor corneoscleral button is then placed in medium, transported to the operating room, and punched out with an 8.0 mm trephine. Separation of the anterior and posterior donor corneal disc follows. In the second method, stromal and side cut adhesions are fully released, and the anterior corneal flap is fully lifted. This donor corneoscleral button is placed in medium and transported; following, the anterior flap is excised using scissors and the posterior part is punched out with an 8.0 mm trephine. In our cases we used the second method for posterior donor disc preparation. Although the disadvantage is using scissors for anterior cap removal, there are other advantages. By removing the anterior cap, it is easier to visualize the posterior donor button edge, which may reduce a decentered donor cut. Furthermore, anterior and posterior donor disc separation is much easier, since the anterior flap is fully lifted and removed before trephination.
Case 1 clinical results showed a clear and compact graft without interface haze at two months postoperation. These findings were further confirmed in OCT, which showed a clear and uniform graft with minimal interface reflection and good graft attachment. Despite good clinical findings, postop visual acuity was not as expected. However, the patient was satisfied and his vision fluctuation (previously worse in the morning) was eliminated. Considering findings and short-term follow-up, we may expect further improvement with time. Case 2 clinical results also showed clear and compact graft with minimal interface haze. However, postop visual acuity did not improve because of low visual potential from dry ARMD and glaucoma. Unfortunately, this patient had an IOP increase at one month postoperation and was unable to follow-up. This increase may be due to either frequent steroid use or glaucoma progression.
We can assume several possible factors for the unexpected postoperative visual acuity. Visual acuity after either PKP or DLEK is mainly a function of retinal macular potential and cornea optical quality. In studies of PKP in young patients with keratoconus, postoperative visual results are uniformly better compared to results in older Fuchs' dystrophy patients, despite similar graft problems of high or irregular astigmatism.
26 This suggests that older maculas, may play a role in visual loss after any form of corneal transplantation, including DLEK.
In contrast to PKP where the cornea contributes to refractive visual loss, often resulting from high or irregular astigmatism, in DLEK, topography is usually normal and corneal visual loss is most likely attributed to the donor-recipient stromal interface. The interface in DLEK may account, on average, for 1 line of visual loss compared to the individual macular potential vision.
27 Other potential corneal factors include increased corneal thickness and posterior corneal curvature. We observed irregular posterior floating map corneal curvature in Case 1's postoperative topography. How this irregularity influences visual acuity requires further investigation.
Due to lack of long term follow-up and small case number, we can not fully determine potential advantages of 'femtosecond laser-assisted small incision DLEK'. However, these cases demonstrate graft uniformity, technical feasibility, and potential application of femtosecond laser in lamellar keratoplasty. Further evaluation with longer follow-up and more cases would reveal potential advantages. Unfortunately, the use of femtosecond laser in recipient corneas is currently limited by the Institutional Review Board. Also the femtosecond laser is not approved for cuts deeper than 400 µm; therefore recipient cornea posterior lip creation requires scissors. Hopefully in the near future, we can use the femtosecond laser and perform both donor and recipient corneal lamellar dissection.