Correlation of Angle Kappa and Angle Alpha on Visual Outcomes in Eyes Implanted with Three Types of Diffractive Trifocal Intraocular Lenses

doi:10.21203/rs.3.rs-4816017/v1

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Correlation of Angle Kappa and Angle Alpha on Visual Outcomes in Eyes Implanted with Three Types of Diffractive Trifocal Intraocular Lenses

https://doi.org/10.21203/rs.3.rs-4816017/v1

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Purpose: To evaluate the correlation between preoperative, postoperative, and change in angle kappa and alpha to visual outcomes following implantation of three diffractive trifocal intraocular lenses.

Methods: This is a retrospective study which included 122 eyes of 61 patients. There were 38 eyes in the Synergy (Johnson & Johnson, USA) group and 42 eyes each from both the Panoptix (Alcon Laboratories, USA) and FineVision (Physiol BVI, Belgium) groups. Visual outcome parameters (uncorrected distance, intermediate, near vision, halo and starburst) and angle kappa and alpha were recorded preoperatively and at three months postoperatively. The preoperative, postoperative and the amount of change in angle kappa and alpha were correlated to visual outcomes and photic phenomena among the three different diffractive trifocal IOLs.

Results: There were no significant differences in the preoperative and postoperative magnitude of angle kappa and alpha among the Synergy, Panoptix, and FineVision groups. There was no significant correlation between the preoperative and postoperative magnitude, and change in angle kappa and alpha with the postoperative uncorrected distance, intermediate and near visual acuity, and halo and starburst scores of the three different diffractive IOLs.

Conclusion: The frequency of eyes with greater than 0.5 mm angle kappa and alpha values were low. Visual outcomes can be good even in eyes in which the angle kappa and alpha were greater than 0.5 mm. The newer diffractive trifocal IOLs were observed to be more tolerant to low and intermediate levels of angle kappa and alpha.

Health sciences/Medical research/Outcomes research

Health sciences/Medical research/Translational research

Cataracts continue to be one of the leading causes of blindness and moderate to severe visual impairment. According to several large-scale population-based studies, the prevalence of cataracts increases with age, rising from 39% at age 55–64 to 92.6% at age 80 and older. In 2020, an estimated 15.2 million people aged 50 and above were blind, with another 78.8 million suffering from moderate to severe visual impairment due to cataracts. Therefore, cataract surgery is the most common ocular procedure performed. ^1–3

The use of multifocal intraocular lenses (IOLs) is one of the primary therapy choices for patients who wish to obtain spectacle-free vision. Better vision at a range of distances is possible with multifocal IOLs because they have numerous zones with varying refractive strengths that combine to create multiple focus points by splitting the light entering the eye into separate foci. It can be either bifocal, with two focus points to improve near and distant vision, or the more recent trifocal, with three focal points plus the ability to improve intermediate vision.⁴

The diffractive IOLs are engineered with microstructures in concentric zones that get progressively closer as the distance from periphery towards the center decreases, creating the so-called Fresnel zone plate that can produce optic foci. To achieve the optimal effect of this concentric ring design, the multifocal IOL should ideally be well-centered such that the light ray towards the fovea passes through it directly. Since diffractive multifocal IOLs have concentric rings, the central ring has to line up along the visual axis for best performance and least photic phenomena. ⁴

With improvement in diagnostic devices for cataract and refractive screening, the measurement of the visual axis in relation to ocular structures (angle kappa and alpha) as factors that affect postoperative outcomes are gaining attention. Angle kappa is the angle formed between the visual axis and pupillary center.⁵ Meng et al. found the average angle kappa within the range of 0.3 ± 0.18 mm.⁶ Several studies suggest that a large-angle kappa of more than 0.5 mm has been associated with decreased quality of vision and higher risk of postoperative photic phenomena. ^8–10 On the other hand, angle alpha is the angle formed between the visual axis and limbal center. Mahr et al. found the average angle alpha to be 0.42 mm, which is consistent with the findings of Meng et al., in which the average angle alpha is within the range of 0.45 ± 0.21 mm.^6,7 Only a few studies have been reported regarding the importance of angle alpha. An angle alpha of less than or equal to 0.3 mm is an acceptable criterion for implanting multifocal diffractive IOL, while an angle alpha of more than 0.5 mm increases the level of uncertainty for satisfactory results after implantation. ^11–13

Ideally, the center of a multifocal diffractive lens should coincide with the center of the visual axis so that the diffraction of light is symmetrical. However, IOLs have been designed to have symmetric, self-centering haptics that centers naturally within the capsular bag, decentered temporally to the visual axis and from the pupil center. Due to the misalignment of the center of the visual axis, pupil, and capsular bag center, light rays strike the diffractive edges of the IOL at the angles for which they weren’t designed, producing additional light scatter, creating halos, glare, and waxy vision. A large angle kappa and alpha will likely worsen the visual outcomes due to a wider distance between the limbal center and pupillary axis from the visual axis.¹⁴

There is no absolute consensus on the range of angle kappa and alpha values acceptable for implanting multifocal IOLs. This research aims to determine the importance of angle kappa and angle alpha during the screening of patients for cataract surgery. Furthermore, the visual outcomes of each multifocal IOL will be correlated to these measurements. This study will help cataract surgeons have a better grasp of the relevance of angle alpha and kappa and help them evaluate patients who will likely benefit from multifocal IOL implantation or in selecting the appropriate multifocal IOL for their patients. This may also further contribute to the advancement of multifocal IOL design to provide optimum visual performance to patients.

This is a retrospective cohort study of patients who underwent uncomplicated cataract surgery at the Asian Eye Institute (Makati City, Philippines). Data were obtained from existing medical records of patients who were recruited from previous prospective trifocal IOL studies which underwent Institutional Review Board approval. All patients gave their written informed consent for the management of their cases, and all participants have already exited their respective studies. A waiver of informed consent was given since there was no direct interaction and intervention between the investigators and the patients. The study adhered to the Declaration of Helsinki and the National Data Privacy Act of 2012.

All patients included were 50 years old and above who underwent conventional phacoemulsification or femtosecond laser-assisted cataract surgery with implantation of a posterior chamber non-toric multifocal intraocular lens performed by a single surgeon (RTA) from December 2017 to December 2021 at the Asian Eye Institute. In this study, we included three different diffractive trifocal IOLs, namely the Johnson & Johnson Synergy, Alcon Panoptix, and Physiol FineVision IOLs. Surgeries are performed within one month after preoperative evaluation. Patients diagnosed with cataract and with complete preoperative and postoperative data until three to six months were included in the study. Patients who had irregular corneal astigmatism, more than one dioptric preoperative corneal astigmatism, a history of previous ocular surgery or trauma, intraoperative or postoperative complications, and other ocular pathologies or neurological lesions that might reduce visual acuity (e.g. corneal disease, lens dislocation, uveitis, retinopathy, glaucoma, amblyopia), and eyes with zonular instability (e.g. pseudoexfoliation, Marfan’s syndrome) were excluded.

Examinations and Measurements

All patients underwent a comprehensive ophthalmic examination and cataract screening preoperatively. This included uncorrected and best corrected visual acuity (distance, intermediate, near), manifest refraction, pupillometry, glare testing, optical biometry (IOL Master, Zeiss) and wavefront aberrometry (iTrace, Tracey Technologies, Houston, Texas). All technicians who obtained the measurements were trained, and multiple exams were taken in each eye.

iTrace wavefront aberration data is calculated from ray-tracing aberrometry and corneal topography, which allows analysis of visual quality. An infrared camera captures an iris image that displays the center of the pupil, the center of the visual axis, and the center of the limbus. Angle kappa is defined by the radial distance between the center of the pupil and the visual axis, which is estimated by the center of the first Purkinje reflex.

The iTrace exam with the largest undilated pupil as the patients fixate on a distant target in a dark room was selected. Whenever available, acquisitions with automatic machine measurements were prioritized over manual measurements done by the technicians. Angle kappa and angle alpha measurements were obtained from the iTrace, prioritized in descending order of green/yellow/red color-coded Tracey refraction from among the available exams. Values of both angle kappa and angle alpha were divided into three groups based on the standard color-coded guide programmed into the iTrace machine. An angle measurement of less than 0.3 mm is green (low), 0.3 to 0.5 mm is yellow (intermediate), and any measurement more than 0.5 mm is red (high). For the purpose of this paper, the cut-off will be labeled as low, intermediate and high angle values. Patient demographics, uncorrected and corrected visual acuity, manifest refraction spherical equivalent (MRSE), glare testing, IOL Master optical biometry data, and iTrace angle kappa and angle alpha measurements were collected and input manually into an electronic spreadsheet file. This included all preoperative measurements and postoperative measurements obtained until three to six months.

Surgical Technique

All surgeries were performed by the same experienced surgeon (RTA). The Signature Pro (Johnson & Johnson, USA) or the Stellaris (Bausch + Lomb, USA) was used for phacoemulsification. A 2.75 mm transparent clear corneal incision (at 30 degrees for the left eye, 210 degrees for the right eye) and 1.2 mm lateral incisions (at 90 and 270 degrees) were created, followed by a 5.0-5.5 mm continuous curvilinear capsulorhexis. The IOL was implanted in the capsular bag. Postoperatively, patients received topical NSAIDs for 2 weeks, and topical corticosteroids and topical antibiotics for 4 weeks.

For femtosecond laser-assisted cataract surgeries, the Victus Femtosecond Laser Platform (Bausch + Lomb, USA) was used. A 5.0 mm capsulotomy was created, followed by nuclear fragmentation by the femtosecond laser. The patient would then be transferred from the laser suite to the operating room, and phacoemulsification would then proceed as described above.

Statistical Analysis

The mean and SD of the participant’s demographic data, preoperative findings, optical biometry and postoperative visual outcome parameters were recorded. The mean and SD of the magnitude of angle kappa and angle alpha preoperatively, postoperatively, and the amount of change were determined. The paired t-test was performed to compare the preoperative and postoperative angle kappa and alpha values. Analysis of variance was used to compare the preoperative and postoperative parameters of three diffractive IOLs. The preoperative, postoperative and the amount of change were correlated to the visual outcome and photic phenomena among the three different diffractive IOL via Pearson r correlation. The level of significance is at 5%. Medical Statistical software was used to carry out statistical calculations.

Demographics

This study included 122 eyes of 61 patients. There were 38 eyes of 19 patients in the Synergy group, 42 eyes of 21 patients each in the Panoptix and FineVision groups. Table 1 shows the patient demographics, preoperative findings and optical biometry. The mean age was 66.7 years in the Synergy group, 66.3 years in the Panoptix group, and 68.2 years in the FineVision group. There were more male patients in the Synergy group (10 males vs. 9 females), whereas there were more females in the Panoptix (7 males vs. 14 females) and FineVision (5 males vs. 16 females) groups.

Table 1

Demographics, preoperative findings and optical biometry data (Mean ± SD)
	Synergy	Panoptix	FineVision	P-value
Demographic data
Age, years	66.7 (7.05)	66.3 (6.63)	68.2 (6.24)	0.627
Sex, Male/Female	10/9	7/14	5/16	-
Laterality, Right/Left	19/19	21/21	21/21	-
Preoperative findings
Uncorrected Distance Visual Acuity (UDVA)	0.55 (0.38)	0.62 (0.52)	0.42 (0.36)	0.113
Uncorrected Intermediate Visual Acuity (UIVA)	0.48 (0.21)	0.46 (0.25)	0.40 (0.13)	0.195
Uncorrected Near Visual Acuity (UNVA)	0.54 (0.27)	0.61 (0.24)	0.62 (0.15)	0.272
Distance Corrected Visual Acuity (DCVA)	0.13 (0.16)	0.28 (0.23)	0.15 (0.14)	0.001
Distance Corrected Intermediate Visual Acuity (DCIVA)	0.36 (0.24)	0.35 (0.21)	0.30 (0.11)	0.360
Distance Corrected Near Visual Acuity with Add (DCNVA)	0.10 (0.15)	0.19 (0.21)	0.12 (0.11)	0.015
Spherical equivalent (MRSE)	-0.15 (2.07)	-0.26 (2.97)	0.63 (0.16)	0.191
Pupil (Photopic)	3.9 (0.52)	3.8 (0.72)	3.9 (0.65)	0.587
Pupil (Mesopic)	4.48 (0.75)	4.4 (0.83)	4.55 (0.76)	0.691
Pupil (Scotopic)	4.9 (0.8)	4.8 (0.87)	5.1 (0.77)	0.210
Optical biometry
Axial Length (AL)	24.0 (1)	23.5(0.77)	23.6 (0.72)	0.018
Anterior Chamber Depth (ACD)	3.2 (0.33)	3.0 (0.34)	3.2 (0.54)	0.086
Keratometric Value (K1)	43.6 (1.22)	43.9 (1.12)	43.6 (1.2)	0.602
Keratometric Value (K2)	44.0 (1.3)	44.3 (1.2)	44.2 (1.2)	0.540
White to White (WTW)	12.1 (0.34)	12.0 (0.38)	11.9 (0.39)	0.585
IOL power	19.6 (2.5)	20.9 (1.9)	21.2 (1.9)	0.002
Procedure
Phacoemulsification	28 (73.7%)	30 (71.4%)	14 (33.3%)	-
Femtosecond laser-assisted Phacoemulsification	10 (26.3%)	12 (28.6%)	28 (66.7%)	-
*p-value < 0.05 is significant

Preoperative Data

The mean Uncorrected Distance Visual Acuity (UDVA, Logarithm of the Minimum Angle of Resolution [logMAR]) was 0.55 in the Synergy group, 0.62 in the Panoptix group, and 0.42 in the FineVision group. The MRSE was − 0.15D in the Synergy group, -0.26D in the Panoptix group, and 0.63D in the FineVision group. No statistically significant difference in UDVA, UIVA, UNVA and MRSE was noted among the groups.

Optical biometry

The mean axial length was 24 mm in the Synergy group, 23.5 mm in the Panoptix group, and 23.6 mm in the FineVision group. The mean anterior chamber depth (ACD) was 3.2 mm in both FineVision and Synergy groups, while it was 3.0 mm in the Panoptix group. The mean white-to-white (WTW) was 12.1 mm in the Synergy group, 12.0 mm in the Panoptix group, and 11.9 mm in the FineVision group. The average IOL power was + 19.6D for the Synergy group, + 20.9D for the Panoptix group, and + 21.2D for the FineVision group.

Angle Kappa

The highest mean preoperative angle kappa was seen in the FineVision (0.323 mm) group, followed by the Panoptix (0.272 mm) and Synergy (0.245 mm) groups. The highest mean postoperative angle kappa was noted in the Panoptix (0.313 mm) group, followed by the FineVision (0.241 mm) and Synergy (0.212 mm) groups. (Table 2). There was no significant difference between Synergy, Panoptix, and FineVision groups in preoperative, postoperative and angle kappa change (P = 0.09, 0.06, and 0.71). There was also no significant difference from preoperative to postoperative angle kappa in all 3 groups. (P = 0.36, 0.3 and 0.06).

Table 2

Means and Distribution of Preoperative, Postoperative and Change in Angle Kappa (Mean ± SD)
	Preoperative	Postoperative	Change
Synergy
Mean	0.245 ± 0.13	0.212 ± 0.18	0.139 ± 0.12
<0.3 mm	24 (63.2%)	31 (81.6%)	36 (94.7%)
0.3 to 0.5 mm	13 (34.2%)	5 (13.2%)	1 (2.6%)
>0.5 mm	1 (2.6%)	2 (5.2%)	1 (2.6%)
Panoptix
Mean	0.272 ± 0.15	0.313 ± 0.21	0.141 ± 0.15
<0.3 mm	22 (52.4%)	24 (57.1%)	37 (88%)
0.3 to 0.5 mm	18 (42.9%)	10 (23.8%)	3 (7.1%)
>0.5 mm	2 (4.8%)	8 (19.0%)	2 (4.8%)
FineVision
Mean	0.323 ± 0.19	0.241 ± 0.20	0.163 ± 0.17
<0.3 mm	23 (54.8%)	32 (76.1%)	35 (83.3%)
0.3 to 0.5 mm	12 (28.6%)	6 (14.3%)	5 (11.9%)
>0.5 mm	7 (16.7%)	4 (9.5%)	2 (4.8%)

Angle Alpha

The highest mean preoperative angle alpha was obtained from the Panoptix (0.382 mm) group, followed by the FineVision (0.352 mm) and Synergy (0.314 mm) groups. The highest mean postoperative angle alpha was obtained from the FineVision (0.362 mm) group, followed by the Panoptix (0.335 mm) and Synergy (0.306 mm) groups (Table 3). There was a significant difference between Synergy, Panoptix, and FineVision groups in preoperative and change in angle alpha (P = 0.04 & 0.01). No significant difference from preoperative to postoperative angle alpha in Synergy and FineVision groups was seen, however there was a significant difference observed in the Panoptix group (P = 0.05) (Table 3).

Table 3

Means and Distribution of Preoperative, Postoperative, and Change in Angle Alpha (Mean ± SD)
	Preoperative	Postoperative	Change
Synergy
Mean	0.314 ± 0.13	0.306 ± 0.18	0.045 ± 0.06
<0.3 mm	20 (52.6%)	22 (57.9%)	37 (97.3%)
0.3–0.5 mm	14 (36.8)	12 (31.6%)	1 (2.6%)
>0.5 mm	4 (10.5%)	4 (10.5%)	-
Panoptix
Mean	0.382 ± 0.15	0.335 ± 0.21	0.091 ± 0.07
<0.3 mm	10 (23.8%)	17 (40.5%)	42 (100%)
0.3–0.5 mm	27 (64.3)	23 (54.8%)	-
>0.5 mm	5 (11.9%)	2 (4.8%)	-
FineVision
Mean	0.352 ± 0.19	0.362 ± 0.20	0.051 ± 0.04
<0.3 mm	15 (35.7%)	12 (28.6%)	41 (97.6%)
0.3–0.5 mm	22 (52.4)	26 (61.9%)	1 (2.4%)
>0.5 mm	5 (11.9%)	4 (9.5%)	-

Postoperative Visual Outcomes

The mean UDVA (logMAR) of Panoptix and FineVision groups are the same at 0.05, while the Synergy group was 0.04. The mean UIVA (logMAR) is 0.04 in the Synergy group, 0.08 in the Panoptix group, and 0.09 in the FineVision group. The mean UNVA (logMAR) in the Synergy, Panoptix and FineVision groups is 0.04, 0.06, and 0.12 respectively. The mean scores of halo in Synergy, Panoptix and Finevision groups were 2.14, 1.4 and 1.4 while the mean scores of starburst were 2.19, 1.9, and 1.75 respectively. (Table 4)

Table 4

Postoperative Visual Outcomes (Mean ± SD)
Visual Outcomes	Synergy	Panoptix	FineVision	p-value
Uncorrected Distance Visual Acuity (UDVA)	0.04 (0.07)	0.05 (0.06)	0.05 (0.07)	0.541
Uncorrected Intermediate Visual Acuity (UIVA)	0.04 (0.07)	0.08 (0.08)	0.09 (0.11)	0.051
Uncorrected Near Visual Acuity (UNVA)	0.04 (0.05)	0.06 (0.07)	0.12 (0.13)	0.001
Halo	2.14 (0.93)	1.4 (0.59)	1.4 (0.59)	0.001
Starburst	2.19 (0.74)	1.9 (0.55)	1.75 (0.49)	0.006

Correlation between Preoperative, Postoperative, and Change in Angle Kappa with Visual Outcomes

In the Synergy group, there was no significant correlation between preoperative, postoperative and change in angle kappa with UDVA, UIVA and UNVA. There was no correlation between preoperative angle kappa with halo and starburst, however there is a moderate positive correlation between postoperative and change in angle kappa with halo and starburst. In the Panoptix and Finevision groups, there was no correlation between preoperative, postoperative and change in angle kappa with UDVA, UIVA, UNVA, halo and starburst. (Table 5)

Table 5

Correlation between Preoperative, Postoperative and Change in Angle Kappa with Visual Outcomes
	Preoperative Angle Kappa		Postoperative Angle Kappa		Change in Angle Kappa
	Correlation coefficient r	p-value	Correlation coefficient r	p-value	Correlation coefficient r	p-value
Synergy
UDVA	-0.054	0.749	0.098	0.558	-0.004	0.981
UIVA	-0.148	0.375	-0.049	0.767	-0.059	0.726
UNVA	-0.121	0.471	-0.034	0.839	-0.135	0.418
Halo	0.226	0.185	0.404	0.015	0.434	0.008
Starburst	0.066	0.700	0.366	0.028	0.380	0.025
Panoptix
UDVA	0.126	0.426	0.025	0.873	0.080	0.613
UIVA	-0.077	0.627	-0.259	0.098	-0.054	0.736
UNVA	-0.044	0.779	-0.029	0.855	0.114	0.472
Halo	-0.177	0.274	-0.243	0.131	-0.270	0.092
Starburst	-0.031	0.850	0.037	0.822	0.178	0.273
FineVision
UDVA	-0.043	0.786	0.065	0.682	-0.193	0.220
UIVA	-0.172	0.276	0.084	0.595	-0.112	0.480
UNVA	-0.214	0.174	-0.143	0.365	-0.171	0.277
Halo	-0.096	0.555	-0.149	0.358	-0.082	0.613
Starburst	0.074	0.651	-0.012	0.941	-0.028	0.863
UDVA-Uncorrected Distance Visual Acuity; UIVA-Uncorrected Intermediate Visual Acuity; UNVA-Uncorrected Near Visual Acuity

Correlation between Preoperative, Postoperative, and Change in Angle Alpha with Visual Outcomes

In Synergy, Panoptix and Fine Vision there was no correlation between preoperative, postoperative and change in angle alpha with UDVA, UIVA, UNVA, halo and starburst, except for the Panoptix group where there was a moderate correlation observed between UIVA and change in angle alpha. (Table 6)

Table 6

Correlation between Preoperative, Postoperative and Change in Angle Alpha with Visual Outcomes
	Preoperative Angle Alpha		Postoperative Angle Alpha		Change in Angle Alpha
	Correlation coefficient r	p-value	Correlation coefficient r	p-value	Correlation coefficient r	p-value
Synergy
UDVA	-0.016	0.923	-0.089	0.596	0.032	0.847
UIVA	-0.098	0.560	-0.133	0.425	0.028	0.869
UNVA	-0.129	0.441	-0.167	0.316	0.072	0.666
Halo	-0.098	0.569	-0.123	0.475	0.001	0.993
Starburst	-0.179	0.297	-0.169	0.325	-0.056	0.747
Panoptix
UDVA	0.261	0.095	0.060	0.704	0.270	0.084
UIVA	0.018	0.909	-0.170	0.279	0.455	0.002
UNVA	-0.234	0.136	-0.258	0.098	0.265	0.089
Halo	-0.081	0.619	-0.179	0.267	-0.169	0.298
Starburst	0.106	0.515	0.179	0.269	0.194	0.231
FineVision
UDVA	0.072	0.651	0.139	0.377	0.009	0.950
UIVA	-0.204	0.196	-0.197	0.210	-0.154	0.328
UNVA	-0.087	0.582	-0.073	0.641	-0.105	0.509
Halo	-0.253	0.114	-0.208	0.196	0.021	0.896
Starburst	0.251	0.118	0.194	0.229	-0.125	0.444
UDVA-Uncorrected Distance Visual Acuity; UIVA-Uncorrected Intermediate Visual Acuity; UNVA-Uncorrected Near Visual Acuity

Implantation of multifocal IOLs during cataract surgery is a popular and effective method for providing better vision at different distances leading to spectacle independence and improving the patient’s quality of life. However, in some cases, visual outcomes do not meet the patient’s expectations. For many years, reasons for patient dissatisfaction with multifocal IOLs have been discussed. Alio reported the following factors that contribute to patient dissatisfaction: lens decentration or tilt, inadequate pupil size, residual refractive error, posterior capsule opacification, photic phenomena and contrast sensitivity, and dry-eye syndrome. Another factor being considered is the role of the visual axis and IOL position, represented by measurements of angle kappa and alpha. ⁴

Angle Kappa

Angle kappa is the angle formed between the visual axis (line passing the fixation point with the fovea) and the pupillary axis (line that perpendicularly passes through the entrance pupil and the center of the cornea). Since the fovea lies temporal to the point at which the pupillary axis intersects with the optical axis, positive angle kappa is formed. Angle kappa values vary among studies depending on the machine that is used. In a study done by Meng et al., the average angle kappa was determined to be within the range of 0.3 ± 0.18 mm. Angle kappa is considered large if the distance between the visual axis and pupillary axis is > 0.5 mm. Angle kappa values are likely to be higher in hyperopic eyes compared to myopic eyes as well as in exotropia compared to esotropia. ^5–7

Angle Kappa in Refractive Surgery

Angle kappa is important in laser refractive surgery because the aiming beam, which represents the center of the laser treatment zone, can be arbitrarily positioned by the surgeon either on the visual axis (represented by the corneal light reflex) or on the center of the pupil. Hyperopic patients tend to have a large angle kappa and therefore have a higher likelihood of treatment decentration if the ablation is centered on the pupil, which can be far from the visual axis, leading to undesirable visual outcomes. Therefore, the recommendation for hyperopic LASIK treatments is to center the ablation over the corneal light reflex (as near to the visual axis) and not on the pupil center. For myopic treatments, the pupil center and visual axis are very close to each other, therefore the pupil center is a good landmark to follow. ¹⁵

Angle Kappa in Cataract Surgery

According to Holladay, one of the requirements of diffractive IOL implantation is that the lens should be concentric with the opening of the pupil and at the same time should also be centered at the visual axis to lessen the scattering of light rays. However, this is not possible because the visual axis will never be centered on the pupil. Therefore, they advised positioning the lens between the visual axis and pupillary axis to minimize postoperative visual problems. ¹⁴ According to Berdahl, if the angle kappa is larger than half of the diameter of the central optical zone of multifocal IOLs, they should not be used.¹⁶ Several studies concluded that large-angle kappa of more than 0.5 mm had been associated with decreased quality of vision and higher risk of postoperative photic phenomena. ^8–10

Distribution and Change in Angle Kappa

In our study, the majority of the patients’ angle kappa measurements belong to the < 0.3 mm category, with a mean preoperative angle kappa of 0.245 mm (± 0.13) in the Synergy group, 0.272 mm (± 0.15) in the Panoptix group, and 0.323 (± 0.19) in the FineVision group. The mean postoperative angle kappa was 0.212 mm (± 0.18) in the Synergy group, 0.313 mm (± 0.21) in the Panoptix group, and 0.241 mm (± 0.20) in the FineVision group. Similar findings were also observed by Meng et al., wherein the average angle kappa was 0.3 mm ± 0.18. The angle kappa of Synergy and FineVision groups decreased postoperatively, while it increased in the Panoptix group. However, there was no significant change between preoperative to postoperative angle kappa. This is in contrast to the findings of Wang where they observed that the magnitude of angle kappa significantly decreased postoperatively making it an unstable and unreliable predictor of postoperative visual outcomes. They attribute these observations to changes in the diameter or position of the pupil or uneven lens opacity.¹³

Relationship to Visual Acuity

In the Synergy group, two eyes had an angle kappa of > 0.5 mm preoperatively and achieved a postoperative uncorrected visual acuity in all distances of 20/20 (logMAR 0.0). However, they reported high scores of haloes and starbursts. In the Panoptix group, eight eyes had an angle kappa > 0.5 mm preoperatively. Among the eight eyes, four eyes achieved a visual acuity of 20/20 (logMAR 0.0) in all distances and reported a low score in haloes and starburst. In the FineVision group, four eyes had an angle kappa > 0.5 mm preoperatively and achieved a visual acuity of at least 20/25 (logMAR 0.1) in all distances, with a low score in haloes and starburst.

Other authors report similar findings of good overall vision with different models of diffractive IOLs despite the changes in angle kappa values. Garzon et al. found that a large pupil offset (> 0.30mm) did not negatively impact visual outcomes after implantation of multifocal FineVision POD F hydrophilic trifocal IOL. Fu et al. likewise had excellent visual acuity in all distances following implantation of Tecnis Symfony EDOF IOL. Qi et al. reported that postoperative far, intermediate, and near vision were not affected by angle kappa after implantation of AT LISA trifocal IOL. Sandoval et al., with the Panoptix trifocal IOL, noted no statistically significant difference between the small and large kappa groups in the reported ability to see for any distance or light condition, as well as in their overall satisfaction. ^9,10,17,18

Photic Phenomenon

In our study, only two eyes were reported to have significant halo and starburst in patients with preoperative angle kappa of more than 0.5 mm, with both belonging to the Synergy group. Qi et al. (AT Lisa trifocal IOL) also reported that halo and glare are increased when the angle kappa is greater than 0.4mm.¹⁸ During their evaluation of the ReZoom multifocal IOL (Abbott Medical Optics), Prakash et al. theorized that larger angle kappa values may increase the incidence of haloes and glare due to the light rays hitting the edge of the innermost ring of IOL. While they similarly noted that there were significant correlations between angle kappa and haloes and glare, many patients with high angle kappa values were also asymptomatic. ⁸

Angle Alpha

Angle alpha on the other hand, is the angle formed between the visual axis and limbal center Theoretically, the angle alpha should be more important in the implantation of multifocal IOLs because the IOL centers itself inside the capsular bag which is presumably centered on the limbal center. Same with angle kappa, if the patient has a large angle alpha, the center of the IOL will be displaced far from the visual axis hence the central light rays will hit and pass through the diffractive rings of the multifocal IOL leading to visual problems. ^5,14

Angle Alpha in Cataract Surgery

Upon implantation of a multifocal IOL, the center of a multifocal diffractive lens should coincide with the center of the visual axis so that the diffraction of light is symmetrical. However, IOLs have been designed to have symmetric, self-centering haptics that center naturally within the capsular bag, which is usually not the same as the center of the pupil or on the corneal light reflex. During slit lamp examination, the center of the diffractive rings is typically decentered temporally to the visual axis and from the pupil center. When the distance of the capsular bag from the visual axis is bigger, the central light rays may miss the central optical zone of the lens and may pass through one of the multifocal rings, leading to visual problems. An angle alpha less than or equal to 0.3 mm is an acceptable criterion for implanting multifocal diffractive IOL. On the other hand, it is not advisable to pursue multifocal IOL implantation if the angle alpha is more than 0.5 mm. Only a few studies have been reported regarding the importance of angle alpha. Wang et al., reported that the angle alpha value is relatively stable from pre- to postoperative compared to angle kappa, thus could be used as a possible preoperative factor when selecting potential patients for multifocal IOL implantation.^12–14

Distribution and Change in Angle Alpha

The angle alpha in the majority of patients in the Panoptix and FineVision groups fall under the 0.3 to 0.5 mm category, while the Synergy group all belonged to the < 0.3 mm category. The mean preoperative angle alpha in Synergy, Panoptix and FineVision groups were 0.314 mm (± 0.13), 0.382 mm (± 0.15) and 0.352 mm (± 0.19), respectively. The mean postoperative angle alpha in Synergy, Panoptix and FineVision groups were 0.306 mm (± 0.18), 0.335 mm (± 0.21) and 0.362 mm (± 0.20), respectively. The mean angle alpha values were observed to be lower in our study compared to the study of Mahr et al. (0.42 mm) and Meng et al. (0.45 ± 0.21 mm). ^6,7

Relationship to Visual Acuity and Photic Phenomenon

Piracha reported that it is not advisable to implant a multifocal IOL if the angle alpha is more than 0.5 mm because it may worsen the postoperative visual outcomes. However, in our study, four eyes in the Synergy group, two eyes in the Panoptix group, and four eyes in the FineVision group that had more than 0.5 mm preoperative angle alpha measurements achieved a visual acuity of at least 20/25 (logMAR 0.1) in all distances and reported a low score in haloes & starbursts. Fu et al. (TECNIS Symfony) and Qin et al. (Tecnis EDOF IOL) had similar findings that postoperative visual acuity was excellent despite different angle alpha values. ^10,12,19

Summary

In summary, there are three important reference points in the optical system – the visual axis, pupil center and limbal center. These three do not coincide and are not in the same location. Eyes see along the visual axis, but the IOL centers in the limbal center and the IOL is seen through the pupil. With the visual axis as the main reference point, angle kappa is the distance from visual axis to the pupil center whereas angle alpha is the distance between the visual axis to limbal center. In the current study, we investigated the possible correlation between preoperative, postoperative, and change in angle kappa and alpha to visual outcomes following implantation of three diffractive trifocal multifocal intraocular lens. The purpose was to determine if there was an acceptable tolerance level to these measures and to determine whether it is important to use angle kappa and angle alpha preoperatively as a screening tool or predictor of good visual outcomes with diffractive trifocal IOLs.

Our study shows that the incidence of preoperative and postoperative angle kappa and angle alpha values higher than the suggested cut-off value of 0.5 mm is quite low in a normal population of cataract surgery patients. The change in angle kappa and angle alpha values from preoperative to postoperative measurements were not statistically significant. Overall visual outcomes in the three groups of trifocal IOLs are good and there were low scores of haloes and starbursts despite some patients having > 0.5 mm values. Overall outcomes of the three trifocal IOL groups did not seem to be affected by the preoperative and postoperative as well as the change in angle kappa and alpha measurements.

While caution is advised in implanting diffractive multifocal IOLs in eyes with greater than 0.5 mm angle kappa and angle alpha, our study results suggest visual outcomes can still be good in these eyes. There is no finding yet of an absolute cut-off value in the literature. Angle kappa and angle alpha are measured in patients about to undergo cataract surgery, but our findings do not support it as an absolute necessity in cataract screening and inclusion as standard of care when considering multifocal IOL implantation.

Limitations

IOL decentration would have been another important metric to analyze but it could not be measured because the iTrace device is not able to automatically detect the diffractive rings. Angle kappa and alpha measurements are comprised of displacement values and axis of orientation. This study, similar to other studies, analyzed only the magnitude of displacement. Lastly, only very few patients had a large angle kappa and angle alpha values in any of the groups, with most belonging to the low to intermediate levels of < 0.5 mm. This may be reflective of the real-world situation wherein there are really a minority in the population with large angle kappa and alpha. Therefore, a vast majority of patients have been implanted with diffractive multifocal IOLs with good results despite angle kappa and alpha not being measured in most practices.

Recommendations

Further studies may be performed that considers the measurement of the innermost diffractive ring in relation to the tolerance for multifocal IOL decentration or large angle kappa or angle values. Measurement of IOL decentration may also be done. Objective visual quality metrics such as MTF, Strehl ratio and OSI may be included to further evaluate the effects of angle kappa and alpha. Larger sample sizes would also be ideal for a more accurate determination of the distribution and frequency of angle kappa and angle alpha values in the general population.

Intraocular Lenses IOL

Uncorrected Distance Visual Acuity UDVA

Best Corrected Distance Visual Acuity BCDVA

Uncorrected Intermediate Visual Acuity UIVA

Best Corrected Intermediate Visual Acuity BCIVA

Uncorrected Near Visual Acuity UNVA

Best Corrected Near Visual Acuity BCNVA

Manifest Refraction Spherical Equivalent MRSE

Ethics approval and consent to participate- In compliance with institutional guidelines for research involving human subjects, the St. Frances Cabrini Medical Center-Asian Eye Institute (SCMC-AEI) Ethics Review Committee has officially approved the study protocol and waived the need to obtain participants' informed consent. The decision was made on the basis of the study's retrospective design, low participant risk, and use of anonymized data. The decision was made after a careful evaluation of the research protocol and ethical issues, with a strong emphasis on protecting participant rights and welfare. All procedures were carried out in complete conformity to ethical norms and guidelines.
Consent for publication-NOT APPLICABLE
Availability of data and material-The datasets generated and/or analyzed during the current study are not publicly available due to compliance with the National Data Privacy Law. However, they are available from the corresponding author upon reasonable request.
Competing interests

a. Funding/Support:
- RTA (none), ZMD (none), JAY (none), EMC (none)
b. Financial Disclosures:
- Consultant / advisory positions:
  - RTA: Staar Surgical, Acevision, Acufocus, Bausch and Lomb, Johnson & Johnson, Physiol BVI
  - ZMD, JAY, EMC none
Authors' contributions:
- Design and conduct of the study (RTA); collection (ZMD, JAY), management (RTA), analysis (RTA, ZMD, JAY, EMC), interpretation of the data (RTA, ZMD, JAY, EMC); manuscript preparation (RTA, ZMD, JAY), manuscript review (RTA, ZMD, JAY, EMC), manuscript approval (RTA, EMC).

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No competing interests reported.

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Correlation of Angle Kappa and Angle Alpha on Visual Outcomes in Eyes Implanted with Three Types of Diffractive Trifocal Intraocular Lenses

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Abstract

INTRODUCTION

PATIENTS AND METHODOLOGY

Examinations and Measurements

Surgical Technique

Statistical Analysis

RESULTS

Demographics

Preoperative Data

Optical biometry

Angle Kappa

Angle Alpha

Postoperative Visual Outcomes

Correlation between Preoperative, Postoperative, and Change in Angle Kappa with Visual Outcomes

Correlation between Preoperative, Postoperative, and Change in Angle Alpha with Visual Outcomes

DISCUSSION

Angle Kappa

Angle Kappa in Refractive Surgery

Angle Kappa in Cataract Surgery

Distribution and Change in Angle Kappa

Relationship to Visual Acuity

Photic Phenomenon

Angle Alpha

Angle Alpha in Cataract Surgery

Distribution and Change in Angle Alpha

Relationship to Visual Acuity and Photic Phenomenon

Summary

Conclusion

Limitations

Recommendations

Abbreviations

Declarations

References

Additional Declarations

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