6.1 Corneal Astigmatism and Postoperative Visual Acuity
6.1.1 Core Messages
The distribution of corneal astigmatism has been clarified.
The influences of corneal astigmatism on visual performance and optical characteristics are understood.
The toric IOL is useful, and coping with axis shift is the key point.
In the future, consideration should also be given to the pupil diameter and influences on pseudo-accommodation.
6.2 Introduction: Methods and History of the Correction of Corneal Astigmatism
Astigmatism markedly reduces the retinal image. Severe astigmatism induces monocular diplopia and can cause amblyopia in children. Astigmatism is caused by an irregularly shaped cornea or lens, the influences of the decentration or tilting of these lens systems, or an irregularly shaped retina. This chapter mainly focuses on corneal astigmatism as a common type of astigmatism and postoperative visual acuity.
The history of astigmatism correction started with eyeglasses and contact lenses, and, subsequently, corneal refractive surgery was developed. The first surgical techniques were corneal incisions in the steepest meridian during cataract surgery [1] and astigmatic keratotomy (AK) [2], followed by limbal relaxing incisions (LRIs) [3], photoastigmatic keratectomy (PAK) using an excimer laser [4], and laser in situ keratomileusis (LASIK) [5]. Recently, small incision lenticule extraction (SMILE) [6] was developed as accurate, small incision, laser refractive surgery requiring no corneal flap. Thus, refractive surgery techniques have been improved to achieve more favorable postoperative vision. With advances in refractive surgery using such corneal approaches, advances have also been made in intraoperative approaches for the correction of refraction. In particular, the toric intraocular lens (IOL) is useful due to its high correction accuracy and no changes in higher order aberration when the surgical procedure is correctly performed. In 1991, we performed toric IOL (NT-98B: Nidek Co., Ltd., Aichi, Japan) implantation in 47 eyes and reported favorable clinical results [7–9]. At present, the correction of high cylindrical powers is possible using a toric IOL or toric phakic IOL [10], and the indications of toric IOL have been expanding. In addition, approaches for disorders accompanied by high astigmatism such as keratoconus have been advanced.
6.3 Distribution of Corneal Astigmatism, Visual Performance, and Optical Characteristics
As a result of examining 12,428 eyes with corneal astigmatism, astigmatism >1.0 diopter (D) and >1.5 D, requiring correction, was observed in 36 and 15 %, respectively (Fig. 6.1) [10]. With age, with-the-rule astigmatic eyes decreased, while those with against-the-rule astigmatism increased (Fig. 6.2). On the other hand, the rate of oblique astigmatism was constant in the age groups ≥40 years (Fig. 6.2). Although relatively infrequently observed, oblique astigmatism tends to reduce the reading speed [11] and may be an indication for active correction. In addition, since corneal astigmatism, even when mild, markedly reduces optical characteristics particularly in the intermediate-high frequency bands, correction is important.
Fig. 6.1
Distribution of corneal astigmatism according to power (n = 12,428)
Fig. 6.2
Distribution of corneal astigmatism according to age (with-the-rule, against-the-rule, and oblique astigmatisms)
6.4 Toric IOL
6.4.1 Factors Affecting Postoperative Visual Acuity and Performance
The toric IOL, which corrects corneal astigmatism in the IOL plane, markedly improves the optical characteristics after surgery in cases of corneal astigmatism (Fig. 6.3), showing favorable clinical results (Fig. 6.4). As its problem, since a 3 % astigmatism correction effect is lost for every 1° axis shift, there is no correction when the axis shift is 30° [9], and new astigmatism develops on another meridian (Fig. 6.5). When the axis shift exceeds 30°, which rarely occurs in clinical practice, the optic characteristics of the toric IOL become lower than those of the conventional IOL. Since most studies have shown an axis shift of less than 10°, the toric IOL is more useful than conventional IOL. Due to intricately involved factors such as axis shift errors, residual astigmatism, and lens power steps, the correction of astigmatism to zero has not been achieved, but performance better than a logMAR value of 0.0 can be obtained. However, since we encountered a patient with the long axial length and with-the-rule astigmatism who showed a marked axis shift immediately after surgery, caution is necessary (Fig. 6.6). Its cause was unknown, but one possibility is a long capsular bag diameter in the eye with a long axial length, as Vass et al. reported [12].
Fig. 6.3
Characteristics of retinal images produced by toric IOLs. A modified Liou-Brennann model eye was used for the optical simulation. Optical ray tracing was carried out using ZEMAX optical design software (Radiant Zemax, Redmond, WA, USA). Comparison of MTF between a customized design toric IOL and conventional IOL (Conv. IOL) for corneal astigmatism of 2.0. D MTF represents the mean in the sagittal and tangential directions. MTF modulation transfer function, S & T sagittal and tangential directions
Fig. 6.4
Postoperative visual acuity after toric IOL implantation. BCVA best corrected visual acuity, UCVA uncorrected visual acuity, MAR minimum angle of resolution, POD postoperative day
Fig. 6.5
Relationship between the axis shift of the toric IOL and correction effects
Fig. 6.6
Relationship between the axis shift of the toric IOL and axial length (n = 312)
The influences of the IOL decenter or tilt or changes in the ocular axis are slight, and IOL power calculation and coping with axis shift errors may be the most important.
6.4.2 Axis Determination Methods
At present, the astigmatism axis determination methods include the straightforward horizontal marking method and methods in which the astigmatism axis is evaluated based on information on the images of the conjunctival blood vessels or iris pattern to increase accuracy. In the horizontal marking method, vertical deviation of the marking or running/disappearance of ink causes errors. In the method based on conjunctival blood vessel arrangement, the association with the astigmatism axis should be clarified, and errors occur in conjunctival injection or chemosis while the method based on the iris pattern has problems regarding accurate recognition and is time-consuming. There is another method in which images of preoperative marking on the keratoconjunctiva (axis registration) are obtained using corneal topography or optical coherence tomography of the anterior segment, the axis is clarified, and the IOL position is determined based on this axis during the operation. This method is rational, considering that eye intorsion occurs with a change from the sitting to supine position during examination. Clinically, the slight superiority of this method has been reported, but further evaluation is necessary.
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