5.3 Cornea Refractive Surgery

5.3.1Laser In Situ Keratomileusis (LASIK)

LASIK is currently the most popular refractive surgery technique. Since the introduction of LASIK in 1992, the goal of all refractive surgeons performing LASIK is to achieve predictable and accurate results while at the same time minimising the amount of tissue ablation and improving safety of the procedure. The LASIK procedure consists of two key parts, corneal flap creation and laser ablation of the stromal bed. Hence, the above-mentioned goals of refractive surgeons can be accomplished by technological advancements in microkeratome/flap creation technology and improvements in LASIK delivery systems. The trends in LASIK are:

1.Decreasing the amount of tissue ablation per dioptre treated

2.Increasing the predictability of LASIK

3.Reducing the incidence of complications associated with LASIK

4.Decreasing the time required for the procedure

5.Increasing the optical quality post-operatively

5.3.1.1 Advances in Flap Creation Technology

The creation of a corneal flap prior to excimer laser ablation of the stromal bed is a key feature of LASIK. This is also perhaps the riskiest step during LASIK as complications during flap creation like buttonholes, partial or free flaps can be devastating. The thickness of the flap is also an important factor. A thinner corneal flap will result in a thicker stromal bed prior to excimer laser ablation. This means that more corneal tissue can be ablated while at the same time maintaining a safe residual stromal bed after ablation. Hence, higher degrees of refractive error may be treated. Furthermore, the risk of corneal ectasia may be reduced with a thicker residual stromal bed.

Currently, corneal flaps can be created by using a microkeratome or by femtosecond laser. Microkeratomes use blades to cut the corneal stroma while femtosecond laser utilises laser to create bubbles which are delivered in a raster pattern across the cornea to create an interface cut and then a side cut. The flap is

then lifted with blunt dissection using a spatula or similar instrument. In both methods, technologies have been developed with the goals of creating safer, thinner and more precise corneal flaps.

Microkeratomes

The first commercially available microkeratome was the Automated Corneal Shaper (ACS), manufactured by Bausch & Lomb. It was a good unit in the hands of an experienced surgeon but had inherent problems that increased the risk of complications. The ACS has different parts which must be taken apart during the sterilisation process and subsequently reassembled. Errors in reassembly may result in major complications during flap creation. Second-generation microkeratomes include Draeger Lamellar Keratome (Storz Instrument GmbH, Heidelberg, Germany) and Microprecision test model (Microprecision Instrument Company, Inc, Phoenix, Az). Hoffman et al. compared the three second-gener- ation microkeratomes and showed that the three systems produced irregular surfaces with chatter lines and the variability in flap thickness was over 20 mm in all three systems [25].

Third-generation microkeratomes include the Hansatome microkeratome (Bausch and Lomb Surgical) which was commercially available in 1997. The Hansatome microkeratome incorporated many design and functional improvements over the ACS microkeratome which aims to improve the safety of the instrument to minimise intraoperative flap complications. Studies by Walker et al. and Jacobs et al. confirmed that intraoperative flap complications are less likely to occur with the Hansatome microkeratome than with the ACS microkeratome [29, 64]. The Hansatome microkeratome is available with the 200, 180 or 160 mm head. Hence, theoretically, the minimum flap thickness possible with the Hansatome microkeratome was 160 mm. But in one study, the mean flap thickness created with the Hansatome microkeratome using a 160 mm head resulted in a mean flap thickness of 97 mm ± 18 (SD) (range 65–163 mm). Thus, while the third-generation microkeratomes afford greater safety compared to the earlier microkeratomes, the flaps created are not as precise as is demanded by refractive surgeons today.

Today, a wide range of new microkeratomes are available to the refractive surgeons, each promising safer

and more precise corneal flap creation. These newer microkeratomes include the Amadeus II microkeratome (Advanced Medical Optics), Zyoptix XP microkeratome (Bausch & Lomb), Moria’s One Use-Plus and M2 microkeratomes, BD K-4000 microkeratome (Becton Dickinson), Nidek’s MK-2000 microkeratome and the Carriazo-Pendular microkeratome (SCHWIND eye-tech solutions). Most of these microkeratomes are available with heads designed to create either 120 or 130 mm flaps. Some of the microkeratomes, for example the Carriazo-Pendular microkeratome, is available with a 90-mm cutting head (Fig. 5.1). Comparison of the Hansatome and Zyoptix XP microkeratome in one study showed that although the two microkeratomes produced flaps of similar mean thickness, the Zyoptix

Fig. 5.1 The Carriazo-Pendular microkeratome

XP showed significantly less variation in flap thickness than the Hansatome. The Zyoptix XP microkeratome was less affected by measurable pre-operative variables such as pre-operative spherical equivalent of the eye and was closer to nominal labelling [52]. While current available microkeratomes produce corneal flaps which are more precise than third-generation microkeratomes, variations in flap thickness are still present [59].

Recently, Moria has launched its One Use-Plus SBK (Sub Bowman’s Keratomileusis) microkeratome (Fig. 5.2) which is designed to create ultra thin flaps which are comparable to flaps created by femtosecond laser. While conventional LASIK flap has a thickness of 120–180 mm, the One Use-Plus SBK is designed to create thinner flaps of between 90 and 110 mm. The cutting of thinner flaps will preserve more of the integrity of the cornea, thereby lowering the risk of ectasia while at the same time provide greater comfort to the patient post-LASIK. This microkeratome is also extremely fast compared to existing microkeratomes, thereby reducing suction time. The microkeratome is also reported by the company to produce smoother stromal beds (Fig. 5.3). This may lead to a reduction in the induction of post-operative corneal aberrations. But more studies will be needed to assess its performance.

Femtosecond Laser

The introduction of the femtosecond laser to create a corneal flap, the first and most popular being the Intralase® femtosecond laser (Advanced Medical Optics), was a significant advancement in LASIK technology. This “bladeless” procedure involves the creation of cavitation bubbles by laser in a raster pattern across the cornea to create an interface which can then be lifted by a blunt dissection instrument. While

Fig. 5.3 Scanning electron microscope image of a stromal bed after creation of a flap by the Moria One Use-Plus SBK microkeratome

the femtosecond laser is a relatively new technology, improvements in technology can be observed over this short time period. The trends in femtosecond laser technology include:

1.Ability to create thinner flaps

2.Decreasing the amount of energy delivered to the cornea

3.Decreasing the time required for the procedure with increased precision

4.Decreasing the biomechanical impact of the flap creation

5.Reducing trauma (including changes in intraocular pressures during surgery)

The main advantage of using a femtosecond laser for flap creation is the greater degree of control and monitoring before and during flap creation compared to microkeratomes. The size, position of the flap and position of the hinge can be determined pre-operatively. Furthermore, since the corneal flap is only created after the flap is mechanically lifted, should a complication occur during the raster pattern delivery, the process can technically be “reversed” by waiting a few weeks before performing the procedure again. This affords greater safety and almost eliminates the risk of flap complications like partial flaps or buttonholes, especially for high-risk eyes, e.g. eyes with steep corneas.

Intralase® is also able to create thinner flaps of up to 90 mm compared to conventional microkeratomes. Several studies have showed that the flaps created by

Intralase® are more precise and predictable compared to conventional microkeratomes [13, 31]. The precision and predictability of the Intralase® machine allows the refractive surgeon to better plan and invidualise the LASIK surgery for their patients, especially patients with borderline corneal thickness. At the same time, ability to create thinner flaps will also help preserve the structural integrity of the cornea and lower the risk of corneal ectasia.

We compared the flaps created by the Intralase® FS laser, the Moria M2 microkeratome (Moria, Antony, France) and the Carriazo-Pendular microkeratome (Schwind eye-tech solutions, Kleinostheim, Germany). Several mechanical microkeratomes create flaps that are thinner at the centre than the periphery (meniscusshaped configuration). Analysis of the flaps created by the Moria M2 microkeratome, which is an applanation microkeratome, using the very high-frequency (VHF) eye scanner, Artemis 2 (Ultralink LLC, St Petersburg, Fla) with a 50-MHz probe confirmed this. This is because the plate induces an excessive compression of the peripheral tissue of the cornea with posterior indentation of the central area (Fig. 5.4a). The CarriazoPendularmicrokeratome,anindentationmicrokeratome, produced an almost planar configuration with slight thickening in the inferior area (Fig. 5.4b). The flaps created by the Intralase® FS laser were also more homogeneous, showing a planar or almost-planar thickness flap profile in most cases, compared to the M2 microkeratome (Fig. 5.4c) A meniscus-shaped flap behaves as an additional optical element of the ocular system, introducing new higher-order aberrations and modifying the predictability of the second-order corrections. In our study, the corneal spherical-like RMS was higher in the M2 group compared to the other two groups.

Studies have also compared the visual quality and amount of corneal aberrations after LASIK with Intralase® and conventional microkeratomes. Durrie et al. compared eyes which have undergone LASIK with Intralase® and the Hansatome microkeratome. His study showed statistically better UCVA and manifest refractive outcomes after LASIK with the IntraLase femtosecond laser may be the result of reduced postoperative astigmatism and trefoil induced by the machine [22]. Studies have also demonstrated that the Intralase® induces less post-operative aberrations compared to conventional microkeratomes [39, 61].

The main disadvantage of the femtosecond laser is the longer time required to create the raster pattern.