Introduction

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Astigmatism is one major reason for reduced visual acuity and optical quality. Its correction may be performed with most of the known procedures of refractive surgery, such as laser in situ keratomileusis, toric phakic intraocular lenses, or the refractive lens exchange. Incisional surgical procedures based on corneal relaxation principles are limbal relaxing incisions (LRI) and astigmatic keratotomy (AK). Both aim for the correction of astigmatism. LRI is usually performed as an initial or re-treatment for correction of lower astigmatism after refractive laser surgery, phakic intraocular lens implantation, cataract surgery, or refractive lens exchange. As opposed to LRI, AK offers the option to correct higher astigmatism, i.e. after penetrating keratoplasty. Principles, indications, techniques, and results of both procedures will be explained below.

Epidemiologic consideration and terminology

As opposed to a spherical cornea, where refractive power of all meridians is identical, regular toric corneas are defined by two perpendicular meridians with different (highest and lowest) refractive powers. This results in two longitudinal displaced orthogonal focus lines and a circle of least confusion between them instead of one focal point. Thus retinal images are blurred and drawn-out (Fig. 29.1).

Fig. 29.1 Sturm’s conoid, showing the principle of toric optics and resulting regular astigmatism in an positive refractive power lens, represented by the principal planes HH’1,red and HH’2,blue of its meridians 1 and 2, which are located 90° to each other. y – image. F – focal points. f – focal lenghts. Δf – difference in focal lengths of the two meridians: astigmatism.

Astigmatism with more than two meridians, non-perpendicular meridians, or no regular pattern at all is defined as irregular astigmatism. Influence of astigmatism on visual acuity is approximately half of the influence of a defocus aberration of the same magnitude¹.

Anatomical considerations

Of the eye’s optical elements the anterior corneal surface has the largest impact on total refraction. This is why its toricity has the largest impact on ocular astigmatism and thus visual acuity. Toricity of the posterior corneal surface and the crystalline lens (inner astigmatism) have only very little impact on ocular astigmatism, though they should not be underrated! Eventually existing lens astigmatism (often against-the-rule) is detected by comparison of subjective refraction and corneal astigmatism. However, refractive corneal surgery should always correct the subjective, not only the corneal astigmatism.

Fundamental principles

The principle of incisional astigmatism correction is the same in both procedures, LRI and AK. One or two arcuate and more or less peripheral incisions of different depth (according to the corneal thickness at the incision location) are performed perpendicular to the steep meridian. The biomechanical properties of the cornea this way induce a steepening of the flat meridian and a flattening of the steep meridian. This so called coupling effect is the ratio of flattening to steepening (coupling ratio, i.e. for AK: 0.85 ± 0.052). The effect increases with increasing incision length and depth as well as with decreasing distance from the central cornea. The predictability of refractive outcome, however, decreases with decreasing treatment radius. This is why AK may correct for larger astigmatism, e.g. after penetrating keratoplasty, but is predictability worse compared with LRI.

Goals of surgery

The goal of both incisional procedures is to reduce the existing astigmatism to 0.75 D or less, without over-correcting and thereby switching the orientation of the astigmatism.

Indications for surgery

Indication for incisional surgery is always astigmatism with negative influence on visual acuity and optical quality. Usually an astigmatism larger than 0.75 D is considered to be noticed by patients. Sensitive patients’ optical quality, however, may be affected by smaller astigmatism as well.

Typical cases where the application of LRIs is useful are procedures like the correction of a residual astigmatism after cataract surgery, refractive lens exchange, or phakic intraocular lens implantation with non-toric implants. Looking at the common LRI nomograms3,4 the correction of astigmatism between 0.75 D and 2.00 D (Table 29.1) (Fig. 29.2) is proven to be safe and effective.

Fig. 29.2 Figure shows the principle of limbal relaxing incisions. (A) Preoperative astigmatic cornea. (B) Positioning of the incisions according to the Wang nomogram. (C) Ideal postoperative status.

AK is especially indicated in large astigmatisms, i.e. after penetrating keratoplasty5,6. Incisions are placed on the transplant in this case.

Preoperative assessment

Preoperative assessment includes keratometry, subjective refraction, and topography. The last two are mainly used to exclude secondary pathologies or irregular astigmatism. Depending on the corneal astigmatism (K values) and the initial pathology either LRI (0.75 D–2.00 D, post-refractive or cataract surgery) or AK (>2.00 D, post-penetrating keratoplasty) is performed. Nomograms given by Wang et al.4 and Nichamin et al.3 (Table 29.1) show options for localization, length, and depth of LRI. Hoffart et al.5 present results for AK applicability; however, there is still a lack of reliable nomograms for AK. Therefore, and for the inter-individual difference in surgical approaches, it is important for surgeons to record their procedures and results, to modify their individual nomograms, for both LRI and AK. Over-correction of astigmatism is the optical worst-case-scenario and should be avoided by conservative planning of surgery. Under-correction of astigmatism is tolerated much better by patients than over-correction and resulting 90° switch of axes. Eventually existing, residual postoperative astigmatism can be corrected with other procedures of refractive surgery like toric intraocular lenses or corneal refractive surgery. Glasses and contact lenses are an option as well.

Due to the lack of predictability, AK is hardly ever used in refractive surgery today.

Anesthesia

Topical anesthesia is sufficient for incisional refractive surgery.

Operation techniques

The classical instrument for incisional refractive surgery is a diamond knife. Compared with the femtosecond laser, which might be used as well, it is an instrument that enables fast surgery and may be used even in blurred corneas. However, femtosecond lasers have some other advantages that should be taken into consideration. Localization, length, and depth of incisions are of higher precision with the laser, resulting in a better predictability of refractive outcomes5.

During LRI one or two approximately 600 µm deep arcuate incisions are placed parallel to the limbus and perpendicular to the steep corneal meridian4. Incisions are usually performed with a diamond knife. The applicability of a femtosecond laser in LRI is very limited, if indicated at all. The reason for this is the localization of the incisions near the limbus. Limbal vascularizations or other pathologic artifacts decrease the cornea’s transparency, which is a basic requirement for the femtosecond laser to perform cutting.

In the case of AK, incisions are placed more centrally compared with LRI. Often a radius of 3.0–3.5 mm around the center of the cornea is used.

Intraoperative complications

Very few instruments are needed and the surgery can be performed fast and easily. Over-corrections are, when treatment is planned carefully, rare. Complications may be the loss of bulbus integrity and the limited predictability. Intraoperative complications of incisional corneal surgery are rare and patients feel only a little pain.

Postoperative care

Postoperative medication may include a continuous wear contact lens for the first night, to provide better comfort for patients. Medication should consist of steroid and antibiotic eye drops for the first postoperative week, and tapering steroid eyedrops for 3 more weeks. Artificial tears are recommended for the first 4 weeks.

Postoperative complications

Postoperative complications of incisional surgery are often optical ones, over-correction and under-correction being the most frequent. Further on, the formation of irregular astigmatism can be a complication. AK is especially known for inducing irregular astigmatism and, for reasons of low treatment diameters, increased postoperative glare disability. In general, predictability of AK is not as good as in LRI7; further on fluctuations in postoperative refraction may occur8.

Assessment of surgery: self-evaluation; result of surgery

Results of incisional astigmatic surgery are most often reported as postoperative corrected and uncorrected high contrast visual acuity, induced and absolute subjective or keratometric astigmatism, and vectorial analyses of astigmatism9. Surgically induced astigmatism is considered to be the main outcome measure, as this shows the results of surgery in the easiest way. From these metrics, several other parameters may be calculated in order to have a closer look at single effects of incisional astigmatic surgery. These metrics vary a lot, depending on the publication and the purpose of the trials.

The use of AK in cataract and primary refractive surgery should be avoided. Predictability of the results is not sufficient. LRI provides much better results.