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Introduction

In the framework of cataract surgery, a comfortable postoperative uncorrected distance visual acuity (UDVA) may be successfully achieved due to the development of modern biometric methods and calculation formulas. However, patients also require to be able to see a normal reading text without further correction, which has led to the development of multifocal or accommodation intraocular lenses (IOLs). The challenge of these models is the number of contraindications, risk of asthenopic problems caused by higher-order aberrations, as well as the additional required payment by the patient.

For pseudophakic eyes, in addition to the optimal UDVA, the optimal uncorrected near visual acuity (UNVA) may also be seen, although a monofocal IOL has been implanted. In a previous study (1) it was proven that optimal near visual acuity may be achieved even without targeted postoperative myopisation in eyes with a short axial length (AL). For 30.33% of eyes with a UNVA of ≥0.6, the AL was ≤23.5 mm, whereas even for a UNVA of 0.8, the AL was up to 22.5 mm. Theoretically, a significant role of the postoperative pseudo-accommodation amplitude in such eyes may be expected, particularly the effect of an axial shift of the IOL causing a reduction of the anterior chamber depth (ACD), which occurs even when the direction of view of the eye is changed. It was proven that a change in the position of the eye affects the ACD, albeit to a lesser extent than predicted (2). The hydrodynamic status in the eye is practically indefinable with regard to several influencing factors. However, the validity of Pascal's law is hypothesised, and due to the hydrostatic pressure under the influence of a small gravitational force, the shift of the IOL causes a slight myopisation that likely confers an increase in near visual acuity.

The subject of the present study was a statistical evaluation of a change in the UNVA depending on the position of the text or a change of the direction of the eye, including an analysis of a mutual correlation of the individual eye parameters.

Materials and methods

Study parameters

In total, 121 eyes were evaluated following surgery in 65 patients. Patients who underwent cataract surgery with implantation of a monofocal IOL were included in the study. The patient selection was performed randomly, depending on the time the patients came for the check-up examinations between January and March 2017. The surgery was performed by a single surgeon using an identical technique (phacoemulsification using the initial incision of 2.2 mm), and the relation SRK/T was used to calculate the optical power of the IOL for emmetropy; the evaluation period was at least 1 month post-surgery. The following models of IOL were implanted: MA50BM (58.68% of eyes), SA60AT (23.14%), SN60WF (13.22%) and SN6ATx (4.96%). Only the postoperatively emmetropic eyes among the eyes examined were included, with 97.52% of the eyes achieving a vision of 1.0 or better (the remaining 2.48% of the eyes achieved a vision of 0.8, whereas no correction improved the vision). Input data for the study are summarised in Table I.

Table I Input values and parameters of the study.

Table I

Input values and parameters of the study.

A, AL <22.5 mm
Eye no.	AL (mm)	IOL	UDVA	ACD (mm)	KC (D)	KVA (D)	UNVAH	UNVAV
1	21.16	27.5 SA	0.80	3.90	44.68	44.95	0.40	0.50
2	21.28	26.5 SA	1.00	3.90	43.77	42.65	0.80	0.80
3	21.28	27.0 SA	1.00	4.00	46.05	45.33	0.50	0.60
4	21.43	23.0 SA	1.20	4.10	46.50	46.71	0.60	0.60
5	21.46	30.5 SA	1.00	3.40	42.15	41.68	0.80	0.80
6	21.47	23.0 SA	1.20	4.10	46.50	46.74	0.60	0.60
7	21.54	25.5 SA	1.20	3.80	44.86	45.46	1.00	1.00
8	21.64	24.5 SA	1.20	3.80	46.10	45.77	0.80	0.80
9	21.68	24.0 SA	1.20	3.80	46.10	45.84	0.80	0.80
10	21.75	26.0 SA	1.00	3.80	43.12	44.14	0.40	0.60
11	21.75	25.5 SA	1.00	3.80	42.00	43.00	0.40	0.60
12	21.80	24.5 SA	1.00	4.19	45.57	45.45	0.80	0.80
13	21.80	25.0 SA	1.00	4.19	44.67	45.06	0.80	0.80
14	21.80	27.0 SA	1.20	3.80	47.16	45.55	1.00	1.00
15	21.87	24.0 SA	1.20	3.50	45.20	45.67	0.50	0.50
16	21.87	24.5 SA	1.00	3.70	44.78	44.45	0.50	0.60
17	21.94	24.5 MA	1.00	3.30	45.91	44.76	0.30	0.40
18	21.95	24.0 SA	1.50	3.80	45.81	45.19	0.20	0.30
19	21.95	24.5 SA	1.00	3.50	45.00	44.81	0.40	0.40
20	21.96	24.0 SA	1.20	3.50	45.58	45.54	0.50	0.50
21	21.96	23.5 SA	1.50	3.90	45.64	45.93	0.20	0.30
22	21.99	25.0 SA	1.00	3.50	45.07	44.36	0.40	0.40
23	21.99	24.5 MA	1.00	3.50	45.70	44.47	0.30	0.40
24	22.12	26.5 SA	1.00	4.00	46.06	45.54	0.50	0.60
25	22.13	24.5 SA	1.00	3.50	44.84	44.53	0.50	0.60
26	22.14	22.0 SA	1.20	3.60	46.35	45.79	0.40	0.40
27	22.16	24.0 IQ	1.00	2.00	45.64	45.66	0.60	0.60
28	22.20	22.5 SA	1.20	3.80	45.93	46.25	0.50	0.60
29	22.22	25.0 IQ	1.20	4.20	44.00	44.01	0.30	0.30
30	22.28	22.0 SA	1.20	3.80	46.26	46.48	0.50	0.60
31	22.32	23.5 IQ	1.00	2.00	45.61	44.80	0.50	0.60
32	22.35	22.0 SA	1.00	3.60	45.94	46.52	0.60	0.60
33	22.38	24.5 SA	1.00	3.70	43.94	43.30	0.50	0.50
34	22.40	22.0 SA	1.00	3.70	45.28	45.11	0.60	0.60
35	22.42	24.5 IQ	1.20	3.60	44.27	44.38	0.30	0.30
36	22.46	24.0 SA	1.00	3.70	42.72	42.65	0.50	0.50
37	22.47	23.0 SA	1.20	4.20	44.45	44.80	0.30	0.40

The evaluated postoperative parameters included the typography values (Anterior Segment Analyser Orbscan II) for optical power in the central part of the cornea (KC), as well as in the visual axis (KVA), anterior chamber depth (ACD; OcuScan biometer) and eye axial length (AL; OcuScan biometer).

To determine the near vision values, each eye was examined separately using the Jäeger table ZEISS at a distance of 40 cm and its perpendicular position relative to the eye viewing axis. First, the value of the least read text was recorded at the horizontal position of the eye (UNVAH, viewing axis of the eye parallel with the floor) and subsequently at the vertical position (UNVAV, viewing axis of the eye perpendicular to the floor). Demonstration of the position of the mutual axes is shown on Fig. 1. The observed parameters were evaluated for the whole group of patients, but also following categorisation of the group into three cohorts according to the AL: The group of short eyes (AL<22.5) included eyes up to 22.5 mm, AL 22.5-23.5 mm was identified as the group of normal eyes (AL22.5-23.5) and the cohort of long eyes had an AL >23.5 mm (AL>23.5).

Figure 1. Demonstration of the position of the mutual axes. (A) The viewing axis of the eye is parallel with the ground-UNVAH; (B) the viewing axis of the eye is pointing towards the ground-UNVAV. UNVAH, decimal value of uncorrected near visual acuity in the horizontal position; UNVAV, decimal value of uncorrected near visual acuity in the vertical position.

Results

Mean parameters of the input group following categorization

The mean age of the group was 71 years. The mean parameter values of the eyes included in the study are presented in Table II.

Table II Mean parameters of the input group following categorization into the groups.

Table II

Mean parameters of the input group following categorization into the groups.

		Values based on the AL groups
Parameters	Values of the whole group	<22.5	≤22.5-23.5≥	>23.5
Count (eyes)	121	37	52	32
AL (mm)	22.96±0.85	21.93±0.36	23.09±0.31	23.96±0.43
ACD (mm)	3.83±0.37	3.68±0.47	3.84±0.35	3.68±0.47
KC (D)	44.19±1.73	45.11±1.21	44.04±1.65	43.36±1.91
KVA (D)	44.21±1.58	44.96±1.17	44.10±1.55	43.53±1.73

[i] Values are presented as mean ± standard deviation. AL, axial length of eye; ACD, anterior chamber depth; K_C, postoperative keratometry value-centre of the cornea; K_VA, postoperative keratometry value-visual axis of the cornea.

Mean values of UNVAH and UNVAV

The paired t-test was used to compare visual acuity at the horizontal and vertical position of the eye for the whole group of patients. The results revealed that, in the case of UNVAH for the horizontal position of the eye, the values were lower compared with UNVAV for the vertical position of the eye (mean 0.51 vs. 0.56, respectively; P<0.001). A higher or identical UNVAV value compared with UNVAH was always achieved in all groups based on the AL. Visual improvement in UNVAV was observed in 40.2% of the eyes. The lowest mean value for the UNVAH was recorded in eyes with an AL >23.5 mm. The highest mean value for the UNVAV was achieved in short eyes (mean 0.58; P<0.001). The complete values are summarised in Table III.

Table III Mean values of UNVAH and UNVAV.

Table III

Mean values of UNVAH and UNVAV.

Category	Mean (mm)	SD	t	P-value
All AL
UNVAH	0.51	0.15	-8.18	<0.001
UNVAV	0.56	0.15
AL<22.5
UNVAH	0.53	0.20	-4.62	<0.001
UNVAV	0.58	0.18
AL22.5-23.5
UNVAH	0.52	0.12	-5.25	<0.001
UNVAV	0.56	0.12
AL>23.5
UNVAH	0.47	0.13	-4.19	<0.001
UNVAV	0.52	0.16

[i] AL, axial length of eye (different groups); UNVA_H, decimal value of uncorrected near visual acuity in the horizontal position; UNVA_V, decimal value of uncorrected near visual acuity in the vertical position; SD, standard deviation.

Correlation coefficients

Evaluation of the association of the eye parameters for UNVAH and UNVAH was performed using correlation coefficients (Table IV). We did not identify a more significant than weak correlation value for the whole group. For the AL<22.5 group, we observed a weak negative correlation of the UNVAH with AL (-0.39), but a moderate negative correlation was observed for AL and UNVAV (-0.45). In the AL22.5-23.5 group of eyes, the positive correlation of the UNVAH with KC (0.45) and KVA (0.34), and of UNVAV with KC (0.42) and KVA (0.33), was found to be more significant. For eyes with an AL >23.5 mm there was only a weak negative correlation of UNVAV with ACD (-0.28).

Table IV Correlation coefficients for the whole group and different ALs.

Table IV

Correlation coefficients for the whole group and different ALs.

	Variables
AL (mm)	ACD	KC	KVA	AL
Whole group
UNVAH	-0.06	0.24	0.18	-0.21
UNVAV	-0.04	0.22	0.17	-0.22
<22.5
UNVAH	0.06	0.11	0.06	-0.39
UNVAV	0.07	0.06	0.04	-0.45
22.5-23.5
UNVAH	-0.09	0.45	0.34	-0.21
UNVAV	0.01	0.42	0.33	-0.29
>23.5
UNVAH	-0.11	0.03	-0.01	0.12
UNVAV	-0.28	0.07	0.02	0.05

[i] AL, axial length of eye; ACD, anterior chamber depth; K_C, postoperative keratometry value-centre of the cornea; K_VA, postoperative keratometry value-visual axis of the cornea; UNVA_H, decimal value of uncorrected near visual acuity in the horizontal position; UNVA_V, decimal value of uncorrected near visual acuity in the vertical position.

Discussion

During the postoperative evaluation of patients with implanted monofocal IOL following standard cataract surgery, an unexpectedly high postoperative near visual acuity was observed. To predict this effect, scientific studies have gradually attempted to identify a correlation between eye parameters and this phenomenon. The pupil size and AL were not conclusively found to be correlated with near vision in 84 patients. However, a pupil diameter <2.6 mm along with AL <23 mm demonstrated better near visual acuity (3). Our previous study partially supports these conclusions, as our data revealed a moderate negative correlation of the postoperative UNVA with a decreasing AL (<22.5 mm) (1).

Association of age with UNVA was not proven in the present study, whereas Hayashi et al (4) confirmed that patient age is a negative factor affecting the postoperative amplitude of pseudo-accommodation (correlation coefficient of -0.49); however, that study also included patients aged <40 years, while only 3 patients were <60 years of age in the present study. A relevant assessment of the dependence on age would require a higher age range. According to Nanavatyet al (5), corneal astigmatism (against the rule) is a significant factor that increases the possibility of pseudo-accommodation up to 10-fold.

A more statistically significant dependence on preoperative ACD, KC or KVA for the whole group of patients was not observed. When comparing different positions of the read text and the position of the eyes, there was a probability of increasing the value of the near vision for the vertical position (UNVAV). The mean values show an increase of near visual acuity in all patients, particularly those with an AL <22.5 mm. It is believed that, in short eyes with an implanted IOL of higher optical power, the same value of its displacement towards the cornea will cause a higher myopia compared with an IOL that of lower optical power.

Acknowledgements

The authors would like to thank the entire staff of the Eye Clinic JL FBMI CTU in Prague for their cooperation in this study.

Funding

No funding was received.

Availability of data and materials

The input data for the study are summarised in Table I and do not include any direct or indirect identifiers. The datasets generated and analysed in the present study are available from the corresponding author on reasonable request.

Authors' contributions

MZ conceived, designed and performed the data analysis of this study. MF and JL coordinated preoperative and postoperative examinations of the eyes and made general revisions. SP performed cataract surgery in all the patients and made general revisions of study. All the authors have read and approved the final version of this manuscript for publication.

Ethics approval and consent to participate

All patients included in the present study provided written informed to participate, and the study protocol conformed to the principles outlined in the Declaration of Helsinki under approval from the ethic committee of JL Clinic FBME CTU in Prague.

Patient consent to publication

All patients consented to disclose their postoperative condition, visual acuity and eye parameters to be published in the present study.

Competing interests

The authors state that there are no conflicts of interest regarding the publication of this article.

References