A Review of Refractive Surgery
Incisional Management of Astigmatism Incisional keratotomy procedures to address naturally occurring astigmatism have become more limited, due to the availability of improved toric phakic intraocular lenses (pIOLs) and the development of laser refractive surgery. These offer better predictability compared with incisional techniques. Nevertheless, astigmatic keratotomy (AK) is the procedure of choice to correct high post-keratoplasty astigmatism and small degrees of pre-existing astigmatism at the time of cataract surgery.
Astigmatic Keratotomy
Different patterns for AK have been developed over the years but all of them share the principle of placing an incision at the steep axis of astigmatism. Currently, transverse incisions and arcuate incisions are most commonly used.17
the advantage of being equidistant from the optical zone.18
Arcuate incisions have AK is a
relatively safe and easy procedure involving placement one or two incisions perpendicular to the steep axis of astigmatism. This flattens the given corneal meridian with reciprocal steepening of the meridian, which is 90º away. The ratio of the flattening of the steepest meridian to the steepening of the flattest meridian is known as ‘coupling ratio’.19 Incisions can be performed by freehand techniques,15,19 using Hanna arcitome20
and, recently, by femtosecond laser.21,22
mechanically by Several
factors affect the refractive outcome of AK; the number of incisions, incision length, incision depth, gender and age of the patient.23
Wilkins et al. reported a significant reduction of the mean of the astigmatism from 10.99 to 3.33D after performing a pair of standardised arcuate incisions.19
AK is an established method of the management of post-keratoplasty astigmatism.14–16
cornea inducing steepening of the central cornea thus correcting mild to moderate hyperopia and can also address presbyopia.29,30
Laser Thermal Keratoplasty
The non-contact holmium:yttrium aluminium garnet (YAG) laser is used to place radial spots outside the visual axis. This heats the corneal surface, resulting in a cone-shaped zone of collagen shrinkage. The apex of the cone extends up to approximately 60% of the stroma.31 There are few well-controlled LTK studies in the literature. LTK has been used to correct hyperopia up to 4.00D and has shown some initial promising results in the correction of low hyperopia.31
However,
it has been associated with induced irregular astigmatism and a significant regression rate.31–33
Alió et al.33
31% after six months. Conductive Keratoplasty
CK is a non-invasive procedure that delivers radiofrequency current (350kHz) directly into the corneal stroma. CK uses the electrical properties of corneal tissue to generate heat in the cornea. The resistance of stromal tissue to the current flow generates gentle and controlled collagen heating and causes optimal collagen shrinkage when temperature reaches 65ºC. This produces a cylindrical footprint that extends approximately to 80% of the depth of the peripheral cornea.31,34
Deep penetration is desirable and necessary to minimise regression, because permanent collagen contraction is dependent on achieving a consistent deep zone of collagen shrinkage.35
A probe
is used to create eight to 32 points in a ring pattern at 6, 7 or 8mm optical zones. The number of treatment spots is determined by the level of hyperopia.34
Similarly, Nubile et al. showed a mean reduction of astigmatism of 5.00D after arcuate incisions by femtosecond laser.22 Arcuate incisions may be combined with compression sutures placed 90º away from them to reduce large degrees of astigmatism after keratoplasties. The compression suture is often placed across the graft–host junction in the flattest meridian to increase the curvature of the cornea in that meridian.24,25
Relaxing Incisions
Limbal relaxing incisions (LRIs) and peripheral corneal relaxing incisions (PCRIs) are incisional procedures used to correct small degrees of astigmatism. They are commonly used to correct pre-existing astigmatism in patients undergoing cataract surgery. They have the advantage of sparing the optical zone, thus minimising night vision problems.
Different nomograms for relaxing incisions are available in the literature. In general, the number, depth, length and placement of incision(s) are dependent on the age of patient, the degree and the type of pre-operative astigmatism.26
be safe and effective for correcting astigmatism up to 2.50D.17
Relaxing incisions are believed to However,
Amesbury and Miller suggested that patients with more than 1.50D are better treated by toric IOLs.27
Thermal Procedures
The most common types of this class are laser thermal keratoplasty (LTK) and conductive keratoplasty (CK). They promote collagen fibre shrinkage within the mid-peripheral and/or peripheral
EUROPEAN OPHTHALMIC REVIEW
Thermal procedures are non-invasive and non-excimer-based modalities. The principle of using thermal energy to treat hyperopia via stromal collagen shrinkage within the cornea was used over 40 years ago.28
CK offers several advantages over LTK. As previously discussed, the footprints of CK are deeper, homogeneous and cylindrical than those created by LTK. Therefore, CK shows mild to moderate regression rates compared with LTK.36
A low regression rate has been estimated
Furthermore, it has a more controlled delivery system and causes less thermal damage to the surrounding collagen lamellae compared with LTK.
to be +0.024D per month between the first and the second year after CK.37
reported a regression rate of
Furthermore, thermal procedures have the advantage over excimer laser ablation techniques in the following domains: they take place outside the optical zone, avoid flap-related complications, preserve the integrity of the cornea and are cheaper and easier to perform.36
Low to moderate hyperopic patients who are not suitable for excimer laser surgery may be candidates for thermal techniques, CK in particular.29
In comparison with pIOLs, CK has the advantage that it can be used to treat hyperopia and astigmatism of less than 1.00D whereas pIOLs are available only from +1.00D for sphere and astigmatism.38
Nevertheless, thermal procedures currently have a small place in the refractive surgery market due to their limitations in the effective treatment of high levels of hyperopia. Moreover, recent advances in laser ablation surgery including femtosecond flap and wavefront-guided (WG) treatments have enhanced their potential of achieving greater accuracy and predictability. Similarly, the development of multifocal and accommodative IOLs provides a wider spectrum of addressing hyperopia and/or presbyopia.
Most studies reported that 100% of treated eyes had best corrected visual acuity (BCVA) of 20/40 or better with no loss of more than two
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