Presbyopia / SPO - 3.2.1.8. Kappa Angle and refractive surgery

3.2.1.8. Kappa Angle and refractive surgery

Maria João Quadrado 1,2 e João Quadrado Gil 1,2,3

1 - Centro de Responsabilidade Integrado de Oftalmologia – Centro Hospitalar e Universitário de Coimbra, Portugal

2 - Faculdade de Medicina da Universidade de Coimbra, Portugal

3 - Associação para a Investigação Biomédica em Luz e Imagem (AIBILI), Coimbra, Portugal

INTRODUCTION

A considerable part of the important advances that refractive surgery has achieved in the last decades is due not only to a technical evolution, but above all, to a conceptual evolution in the approach to the refractive patient. The surgical approach has shifted from “one size fits all” to an almost "obsession" with characterizing the operating eye and customizing the surgery. One of these concerns, increasingly present in the refractive surgeon's mind, concerns the study of the Kappa angle. The Kappa angle is defined as the angle between the visual axis (the imaginary line connecting the fixation point and the fovea) and the pupillary axis (perpendicular line passing through the center of the pupil and the center of the cornea curvature).

It represents the quantification of the misalignment between the beam of light that crosses the refractive surface of the cornea and the beam of light formed by the passage through the pupil. Clinically, it is easily identified by the distance between the corneal light reflection (first Purkinje image) and the center of the pupil: nasally off centered if the Kappa angle is positive or temporarily offset if the Kappa angle is negative2. The importance of aligning the different axes within the visual system – and the definition of what constitutes the "center" – has been widely discussed in the literature and is a central point in the preoperative evaluation and in the planning of refractive surgery3.

Most LASER Excimer platforms guide ablation through the center of the pupil, in part because the center of the pupil is easily identifiable, unlike the center of the visual axis. However, this option is not always the right choice. The presence of a high Kappa angle means that the misalignment between the various possible "centers" is potentially too large and can lead to unsatisfactory results. In a patient undergoing kerato-refractive surgery this may result in an area of off-center ablation and, consequently, in the appearance of irregular astigmatism2 or undercorrection3.

In a patient undergoing intraocular lens (IOL) implantation, this may lead to IOL offsetting and an increase in dysphotic phenomena4.

Attention dedicated to the Kappa angle – pertinent in any refractive surgery – is particularly important when the goal is to treat the presbyopic or pre-presbyopic patient. A considerable proportion of refractive surgery candidates for presbyopia treatment will be hyperopic, a group of patients with higher Kappa angle values, in whom its impact will be predictably greater1,2. On the other hand, the currently available surgical solutions for the treatment of presbyopic patients – namely the use of multifocal IOLs – are incomparably more demanding in terms of the selection of the ideal patient, than the surgical options that do not aim to restore multifocality.

In this chapter some concepts about the Kappa angle will be stubbornly revised, like characterization and evaluation, while also looking at how this evaluation interferes with the surgical options currently available to treat the presbyopic patient.

KAPPA ANGLE

As noted earlier, the Kappa angle comprises the angular distance between the pupillary axis – an imaginary perpendicular line connecting the center of the cornea to the center of the pupil – and the visual axis – the line representing the path of a beam of light from the attachment point to the fovea. Although in the clinical literature the term "Kappa angle" is the most commonly used, some authors, especially in the area of physics and optics, also use the notation "Lambda angle". The difference between the two is conceptual and of historical origin5. In practice, the difference is clinically negligible and for clarity purposes only the designation "Kappa angle" will be used, as it is the most common.

The estimation of the value of the Kappa angle can be performed in several ways. The most intuitive, but least accurate, is the simple observation of the corneal reflex, or Purkinje's first image. The patient should be instructed to look directly and fix a light source positioned in front of him. If there is a decentration of the reflection of the light image in relation to the center of the pupil, this may mean that one eye (or both) does not present foveal fixation. In the presence of this suspicion the cover test must be performed to exclude the presence of a deviation.

If the cover test is negative, it is also important to evaluate visual acuity and biomicroscopy. An off-center corneal reflex may occur in cases of corectopia, iris colobomas, or other anomalies of the anterior segment. In addition, the presence of significant macular pathology can condition an eccentric fixation and alter the position of the corneal reflex. In the absence of strabismus, pupillary anomalies or low visual acuity, the decentration between the corneal reflex and the center of the pupil corresponds to the physiological Kappa angle.

Under normal conditions this decentration is nasal, corresponding to a positive Kappa angle. This means that the fovea is located slightly temporal to the point at which the pupillary axis intersects the posterior pole. A temporal offset of the corneal reflex relative to the center of the pupil is designated as a negative Kappa angle and is highly unusual in the normal population2,6. Evaluation through objective examination, while useful, does not allow precise quantification of the value of the Kappa angle and is frankly insufficient as a preoperative study of the refractive patient.

There are several methods available for objective evaluation of the Kappa angle value. All of the most commercially available anterior segment study devices – Orbscan II (Bausch & Lomb Surgical, Inc), Pentacam (Oculus, Inc) and Galilei (Ziemer Ophthalmic Systems AG), automatically provide the Kappa angle value. In the case of Orbscan II the Kappa angle is automatically measured as the distance between the center of the pupil and the center of the reflex of the Placido ring on the cornea, with a resolution of 0.01 mm. In Galilei the Kappa angle is calculated from the distance between the center of the pupil and the center of the first 4 Purkinje reflexes created by 4 light points included in the apparatus. The resolution is 0.01 mm.

The Pentacam does not directly calculate the Kappa angle but provides the distance between the Cartesian values X and Y, corresponding to the corneal vertex and center of the pupil, respectively. The corneal vertex corresponds to the point of maximum corneal elevation at the moment of fixation on the target. If the eye functioned as a perfect coaxial system, the corneal vertex would correspond perfectly to the intersection of the optic axis in the cornea. The angle formed between the optical axis and the visual axis is called the Alpha angle and does not correspond to the Kappa angle but is positively correlated with it, providing a clinically useful estimate of its value.

POPULATION DATA

There are not many descriptive studies on the distribution of the Kappa angle in the normal population. Those that exist have variable differences in both the population studied and the measurement methods used, which make their comparison problematic. There are however some trends that are consistent and clinically relevant throughout the literature. The value of the Kappa angle appears to vary significantly according to the subject's refractive state and almost all studies present the data divided by this criterion. In the 19th century, Donders described mean Kappa angle values of 2.0º in myopes, 5.082º in emmetropes and 7.55º in hyperopes7.

Smaller values were described by Giovanni et al but maintaining the same tendency: 1.9º in myopes, 2.5º in emmetropes and 2.8º in hyperopes (excluding the negative values)8. In 2006, Basmak et al analyzed 300 healthy individuals (150 men and 150 women, mean age 28.74 ± 1.63 [20-40] years), calculating the value of the Kappa angle through the synoptophore and Orbscan II2. Mean Kappa angle of 5.65 ± 0.108 in the right eyes and 5.73 ± 0.108 in the left eyes of hyperopes, higher than the 5.55 ± 0.138 in the right eyes and 5.62 ± 0.108 in the left eyes of emmetropes, and even more than the 4.51 ± 0.118 in right eyes and 4.73 ± 0.118 in the left eyes of myopes.

The correlation with the refractive state also occurred within each subgroup: in myopic eyes the Kappa angle decreased in the direction of more negative refractive errors; in hyperopic eyes the Kappa angle increased with the increase of positive refractive error. Note that the mean value of the Kappa angle was consistently higher in the left eye2.

In another study using Orbscan II, with 442 participants (mean age 40.6 ± 15.8 [14-81] years), the mean Kappa angle was 5.46 ± 1.33° considering all individuals analyzed1. In myopic, emmetropic or hyperopic individuals, the mean Kappa angle was 5.13 ± 1.50°, 5.72 ± 1.10°, and 5.52 ± 1.19° respectively. There was also a significant decrease (p <0.001) in the Kappa angle with age: 0.015°/year.

Qazi et al also described a statistically significant difference (p <0.01) in mean Kappa angle values in myopic (5.0 ± 1.2) and hyperopic (6.9 ± 1.3) patients who underwent LASIK9. With aging, the human eye undergoes multiple changes.

Some of them, such as changes in pupil position and changes in the anterior corneal radius of curvature or in the radius of curvature of the anterior and posterior surfaces of the lens, could potentially imply consistent changes of the Kappa angle with age.

However, it is not certain that a possible reduction of the Kappa angle with age occurs linearly over time, as its values are relatively stable over the various age groups10.

An important reference should be made in relation to the study by Peace et al, who demonstrated that the Kappa angle values as measured by Pentacam might vary, depending on the position of the patient. By moving from the sitting position to a supine position (during surgery, for example) the horizontal component increases and the vertical component decreases. This potential discrepancy between values should be taken into account, for example, in patients with values close to the upper limits of normality11.

Other results reported in the literature are summarized in

Table 1.

CLINICAL IMPORTANCE IN PRESBYOPIA SURGERY

Presbyopia is manifested as the progressive loss of amplitude of accommodation and is the most common refractive error after 40 years of age.

The causes of presbyopia, probably of multifactorial origin, remain a source of controversy and beyond the scope of this chapter.

There are multiple surgical options for presbyopia correction and in almost all of them, Kappa angle evaluation can be an important factor in clinical success. Among the most widespread, one can select those that involve corneal surgery and those that involve the implantation of intraocular lenses (IOLs).

Figure 1.

CORNEAL SURGERY

INLAYS

The basic concept of corneal inlays is not new and was proposed by Barraquer in the 1940's17. He proposed the use of intra-stromal implants that allow to alter the characteristics of the cornea, increasing the depth of focus or increasing the refractive power of the central cornea. They can be divided into three types: refractive, like Invue® or Flexivue® (central implants with a refractive index different from the cornea), remodeling devices, such as Raindrop® (creating a hyperprolated central cornea), and pinhole, like Kamra® (increasing the depth of focus through the pinhole effect).

With the emergence of femtosecond technology there was a resurgence of interest in this solution. In theory, being an additive surgery (no tissue removal required), it has as main advantage its reversibility or possibility of being combined with other techniques, like excimer ablation or cataract surgery. Some doubts remain however, regarding the actual reversibility of the surgery, with reports of permanent changes in the cornea18.

There are also reports of loss of better uncorrected vision, decreased contrast sensitivity, and increased high-order aberrations19. A significant percentage of patients have also reported complaints of halos and glare after the implantation of these products20. Some of these problems may potentially result from incorrect centering of the implants, as this could mean an irregular central cornea shape (in the case of Raindrop®) or a misalignment between the central opening of a Kamra® type implant and the visual axis. Most authors suggest that the implants be centered on the corneal reflex with coaxial illumination (CRCI) or first Purkinje reflex21. In practice this allows a reasonable approximation to what will be the point of theoretical interception of the visual axis in the plane of the cornea3. A study using theoretical optical models reinforced the choice of the first corneal reflex of Purkinje as position for the centering of the implants, adding also that small deviations in relation to what would be the optimal centering do not lead to great losses of visual quality22. In a similar way, the same optical model showed that the visual results deteriorated significantly with any degree of residual hyperopia and from cylinder values greater than 1.0 diopter. However, there are authors who defend the use of the pupil center as a target for implant centering, because the pupillary area defines the area where the light responsible for the formation of the image in the plane of the retina passes23,24. A study of 992 patients treated with Kamra® implants, analyzed the results by dividing the cases based on the distance between the center of the pupil and the Purkinje reflex (Pp-Pk). There were no statistically significant differences between patients where this distance was greater than 300 μm and those where that distance was less than 300 μm25. There were also no differences depending on the final position of the implant in relation to the Purkinje reflex or in relation to the intermediate point between the Purkinje reflex and the center of the pupil25. This reinforces the idea that, in most cases, placing the implants near the first Purkinje reflex allows satisfactory results. However, it is important to note that, of the 992 patients analyzed, only 17 had Pp-Pk distances higher than 500 μm and that in none of the cases analyzed was the final position of the implant more than 400 μm from the first Purkinje reflex. There is at least one publication describing two cases of patients with visual complaints and low postoperative visual acuity, whose complaints disappeared with re-centration of the implants26. Both patients were hyperopic and in both the re-centration approach involved placement of the implant at the midpoint between the center of the pupil and the corneal vertex.

To the best of our knowledge, there is no study evaluating the impact of the orientation of decentration (nasal or temporal, superior or inferior) but only its magnitude. In short, the placement of inlays at a minimum distance (preferably less than 400 μm) from the first Purkinje reflex should offer no problems. Hyperopic or large-Kappa angle patients are predictably worse candidates for this type of implant. If inlays are used in these cases, their positioning should be centered at the intermediate point between the Purkinje reflex and the center of the pupil.

EXCIMER LASER SURGERY

With the introduction of flying spot technology in the excimer laser, a multiplicity of ablative patterns to reshape the cornea became possible. In addition to the correction of astigmatism, this allowed the introduction of "multifocal" ablations that use the presence of spherical aberrations to increase the depth of focus in detriment of some visual quality. There are multiple ablation profiles published in clinical trials over the years but the two most widespread techniques involve the creation of one of two ablative profiles: peripheral presbyopic excimer (a central oblate zone for distance vision and a prolated peripheral ring for near vision) or central presbyopic excimer (peripheral myopic ablation for distance vision followed by a steepening of the center for near vision).

As mentioned earlier, Uozato and Guyton suggest the center of the pupil as a reference for refractive treatment focus23, whereas Pande and Hillman recommend the use of the first Purkinje reflex (as a marker of the corneal intersection of the visual axis) as the target for ablative treatments3. However, most excimer laser devices target the center of the pupil – with good results for the overwhelming majority of patients – presumably because of the low frequency of individuals with abnormally high Kappa angle values in the general population. Despite this, some authors have satisfactorily tested the hypothesis of centering or adjusting the ablations according to the CRCI, especially in hyperopic eyes that present higher Kappa angles27,28. In addition, hyperopic LASIKs require larger optical zones and, as such, have less tolerance to eventual decentration.

Predictably, it will also be hyperopic patients who present themselves disproportionately as candidates for ablative corneal surgery for presbyopia treatment. As far as the authors are aware, there are yet no studies comparing the impact of the Kappa angle or of different centering axes specifically on the results of multifocal ablation. Chan et al achieved better topographic centering using the CRCI (mean 0.06 ± 0.18 mm), compared to using the center of the pupil as a marker (mean deviation 0.40 ± 0.24 mm).

Reinstein et al also obtained good results with hyperopic treatment focused on CRCI30. These good results extended to eyes with large Kappa angles, where the center of treatment was more than 0.55 mm from the center of the pupil, reinforcing the notion that the CRCI may be a preferable marker in many cases and that the ablations should not be systematically centered by the center of the pupil (CP)30. The use of CRCI as a target is not without limitations, since the corneal reflex can vary according to the surgeon's ocular dominance and the angle of stereopsis of the microscope used31.

To overcome this, it was proposed to use corneal vertex as a target for centering hyperopic treatments with excimer laser, since it is a reproducible and fixed target32,33. In the study by Soler et al, the results of treatments with hyperopic LASIK were compared using the corneal vertex or CP target. Interestingly, the results were globally similar, with the exception of patients with greater Kappa angle, where centering by the pupillary center seems to provide better results32. Thus, it is likely that the corneal vertex is also not the recommended target for multifocal ablations in patients with high Kappa angles.

An important point to note is the difficulty in performing wavefront-guided treatments in patients with a high degree of division between axes. The wavefront is currently calculated centered on the pupil, and may not perfectly represent the patient's view, if the Kappa angle is too high and consequently it is not looking through the center of the pupil. In some cases, this may lead to unsatisfactory results after wavefront-guided treatments34.

In summary, the ideal target for focusing ablative treatments remains a source of controversy. Most studies using laser Excimer for the treatment of presbyopia used the pupillary center as the central target for ablation. However, some evidence suggests that in patients with high Kappa angles, there may be an advantage in using the CRCI as a target, so a thorough study of this parameter should be part of the preoperative period.

IMPLANTO-REFRACTIVE SURGERY

The increased popularity of multifocal intraocular lenses (M-IOLs) and the increasing trend of refractive IOLs have increased symmetrically the requirement of patients and the concern of ophthalmologists to respond to this requirement. Despite the good results, there are some complaints particularly associated with the implantation of M-IOLs to treat presbyopia such as halos, glare or decreased contrast sensitivity35,36. The causes behind these phenomena include, but are not limited, IOL decentration, anterior capsule opacification, dry eye or residual refractive error37.As the perception of these factors increased, so did the interest in preoperative metrics that aid in the selection of the best candidates. One of these metrics that has deserved greater prominence is the Kappa angle12,38. Horizontal decentration seems to be one of the factors related to worse visual results39 but it is not always easy to define where the M-IOL implant should be centered, and, like other refractive treatments, doubts remain about the advantages between centering by the visual axis or by the pupillary axis. There are not many studies available on the role of misalignment between the two axes, that is, the magnitude of the Kappa angle, as a predictor of complaints after M-IOL implantation. In a study by Prakash et al, although the largest predictor of overall dissatisfaction was the best uncorrected visual acuity, the magnitude of the Kappa angle was positively correlated with the presence of halos (R2 = 0.26, p = 0.029) and of glare (R2 = 0.26, p = 0.033). On the other hand, the same study described the presence of many patients with elevated Kappa angles who remained asymptomatic, reinforcing the notion that these phenomena are multifactorial. Another study, using an optical model, concluded not only that the high Kappa angle is a predictive factor of photic phenomena but also that this impact is greater when the anterior chamber depth is lower38. Thus, eyes with a high Kappa angle and a narrow anterior chamber are predictably worse candidates for M-IOL implantation. Another study using ray-tracing aberrometry reported measurably the same results: the Kappa angle value was positively correlated with the total/internal high order Root Mean Square (RMS) and negatively with the Modulation Transfer Function and Strehl-ratio. Equally important, in these studies, IOL centering was performed by the first Purkinje image, that is, by the visual axis, and at the end of the 1st postoperative month the IOL position was maintained, reinforcing the importance of intraoperative centering. It should be noted that in all the mentioned studies, diffractive IOLs were used. In these cases, and in the presence of a high Kappa angle, there is a potential risk that the central rays of light intended for the fovea will cross the margin of the concentric diffractive rings and not the central zone of the IOL, leading to possible undesirable photic effects. As such, the potential impact of the shift induced by the Kappa angle is naturally dependent on the diameter of the central optic zone in the M-IOL in question. Some authors recommend that the central optic zone should be higher than the Kappa angle in at least half of its diameter in order to obtain optimal results41.

In conclusion, in a context in which the expectation regarding the results of the M-IOL implant is increasingly high, it is fundamental that the preoperative study includes the evaluation of the Kappa angle. The presence of a greater Kappa angle appears to be correlated with increased postoperative photic phenomena, and although many patients with elevated Kappa angles remain asymptomatic, this possibility must be taken into account and discussed with the candidate for surgery. The concomitant presence of a narrow anterior chamber seems to further potentiate this association. Placement of IOLs centered on the visual axis rather than the pupillary axis may be a way of minimizing this effect in patients with elevated Kappa angles and of optimizing visual outcomes in all patients.

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