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Angle kappa is defined as the angle formed between the pupillary axis (the line perpendicular to the corneal front surface passing through the entrance pupil's center) and the visual axis (the line joining the fixation point and the fovea).1 With a small angle kappa, the lines joining the fixation target to the pupil center and the visual axis are nearly superimposed. This makes positioning of an IOL relatively straightforward when using these landmarks. When angle kappa is large, the pupil's center does not fall on the line connecting the fovea and the fixation point, resulting in a mismatch between the pupil center and the visual axis and line of sight. This poses a dilemma for optimal IOL centration. Multifocal intraocular lens (MIOL) centration is thought to be important in achieving good visual results.2–4 Some surgeons attempt centration on the visual axis, in keeping with the theory that perfect centration results in optimal outcomes.5 However, the practical implementation of this ideal centration is fraught with complexities, and maintaining this position postoperatively is more challenging than anticipated.6,7 Many surgeons therefore choose to center the IOL in the capsular bag, irrespective of where the pupil or visual axis is.
It has been suggested that a large preoperative angle kappa may contribute to suboptimal MIOL outcomes. This is thought to be due to the interplay between lens optics and incoming light resulting in a deterioration of postoperative quality of vision and residual refractive error.8 A degradation in vision quality can occur with any IOL decentration. Aberration neutral IOLs might be more forgiving of decentration than IOLs with negative aspheric shapes or with a diffractive pattern. When IOLs are diffractive, such as MIOLs, decentration can cause light to strike some of the diffractive rings at an abnormal angle. In addition, the pupil aperture may limit light to the most peripheral rings, resulting in asymmetrical diffractive patterns and unwanted visual effects. The tolerability of decentration is thought to vary according to the diffractive optics. This means that a large angle kappa can in theory result in a functional decentration with a resultant reduction in image quality, even if the lens is perfectly centered on either the pupil or the visual axis.
The impact of the magnitude...