Distribution of corneal spherical aberration in a Tanzanian population

Authors: Hiroki Asano aff001;  Takahiro Hiraoka aff002;  Yusuke Seki aff003;  Teppei Shibata aff003;  Hiromi Osada aff003;  Takanori Saruta aff001;  Natsuko Hatsusaka aff003;  Fukumi Fujikake aff005;  Yoshiaki Tabata aff006;  Cellina Mhina aff007;  Anna Sanyiwa aff007;  Tetsuro Oshika aff002;  Hiroshi Sasaki aff003
Authors place of work: Department of Ophthalmology, Tsuchiura Kyodo Hospital Namegata District Medical Center, Ibaraki, Japan aff001;  Department of Ophthalmology, Institute of Clinical Medicine, University of Tsukuba, Ibaraki, Japan aff002;  Department of Ophthalmology, Kanazawa Medical University, Ishikawa, Japan aff003;  Department of Ophthalmology, Nagano Matsushiro General Hospital, Nagano, Japan aff004;  Visual Science Course, Department of Rehabilitation, Faculty of Medical Science and Welfare, Tohoku Bunka Gakuen University, Miyagi, Japan aff005;  Kagoshima Minami Eye Clinic, Kagoshima, Japan aff006;  Department of Ophthalmology, Muhimbili University of Health and Allied Sciences, Dar es Salaam, Tanzania aff007
Published in the journal: PLoS ONE 14(9)
Category: Research Article
doi: 10.1371/journal.pone.0222297



To investigate the distribution of corneal spherical aberration (SA) in Tanzanian people of African descent, and to examine the correlation between corneal SA and ocular parameters.


Cross-sectional population-based study.


Residents aged 40 years and older in three villages in the Mkuranga district in Tanzania were enlisted as study participants. Corneal higher-order aberrations (HOAs) for the right eye were measured with a wavefront analyzer (KR-1W, Topcon) and calculated for the central 6.0-mm zone. Corneal curvature radius (CR), corneal astigmatism, and axial length (AL) were also measured and their correlation with corneal SA was assessed.


The right eyes of 657 participants (336 male, 321 female) were analyzed. The mean age of the subjects was 57.2 ± 10.3 years (mean ± SD). The mean corneal SA (Zernike spherical aberration coefficient C40) was 0.188 ± 0.095 μm (-0.242 to 0.613). The SAs in about three-quarters of all subjects were between 0.10 and 0.30 μm. The root mean squares of total corneal HOAs and the third- and fourth-order aberrations were 0.629 ± 0.250 μm, 0.539 ± 0.236 μm, and 0.269 ± 0.110 μm, respectively. Corneal SA showed weak significant correlations with CR (Spearman’s rank correlation coefficient, r = -0.177, p < 0.001), corneal astigmatism (r = -0.142, p < 0.001), AL (r = -0.168, p < 0.001), and age (r = -0.085, p < 0.05).


This finding may be beneficial for selecting aspheric intraocular lens in this population.


Biology and life sciences – Anatomy – Ocular system – Ocular anatomy – Cornea – Eyes – Head – Eyes – Neuroscience – Sensory perception – Vision – Visual acuity – Psychology – Sensory perception – Vision – Visual acuity – Medicine and health sciences – Anatomy – Ocular system – Ocular anatomy – Cornea – Eyes – Head – Eyes – Surgical and invasive medical procedures – Ophthalmic procedures – Cataract surgery – Ophthalmology – Social sciences – Psychology – Sensory perception – Vision – Visual acuity – People and places – Population groupings – Ethnicities – African people – Geographical locations – Africa – Tanzania – Physical sciences – Mathematics – Geometry – Radii


In recent times, the goal of cataract surgery has been to not only restore visual acuity but also achieve better quality of vision (QOV), such as improvement in contrast sensitivity. Correcting higher-order aberrations (HOAs) as well as refractive power and astigmatism leads to an optimized retinal image quality [1,2]. Among all the components of HOAs derived from the cornea, only spherical aberration (SA) is correctable with commercially available aspherical intraocular lenses (IOLs). As the value of corneal SA is generally positive [25], aspherical IOLs designed to have a negative SA cancel the positive SA of the cornea and thus reduce the whole ocular SA, leading to an improvement in visual function, including contrast sensitivity [69]. The degree of compensation of corneal SA using the aspheric IOL varies with the type produced by each company. For example, aspherical IOLs with negative SA, such as Tecnis ZCB00 (Abbott Medical Optics, Santa Ana, CA, USA) and AcrySof IQ SN60WF (Alcon Laboratories Inc., Fort Worth, TX, USA) are designed to compensate for positive corneal SA by -0.27 and -0.20 μm, respectively.

Holladay et al. [2] reported that the mean value of corneal SA was 0.27 μm in Caucasian individuals with cataract. Several researchers also reported the distribution of corneal SA during middle and advanced ages in Caucasian (0.27–0.33 μm) [2,3,1012], Asian (0.20–0.31 μm) [4,5,13], and Middle-Eastern (0.28–0.32 μm) [14,15] individuals, indicating ethnic variations. However, to our knowledge, no study directly comparing the differences in corneal SA among different races in middle-aged and elderly people has been conducted. Besides, the distribution of corneal SA in sub-Saharan African people has not yet been reported.

Tanzania, which is located in east Africa, is one of the least developed countries in the world. In the past 25 years, the population has doubled to more than 50 million, and the average life expectancy has increased [16]. With the increase in the elderly population in the future, an increase in the occurrence of cataracts and the number of cataract operations can be expected. In Tanzania, approximately 70% of the people live in rural areas. Ideally, aspherical IOLs should be personalized for each patient for cataract surgery by measuring the corneal SA [1,17,18], but it is difficult for aberration measurement equipment to be available at each facility even in developed countries, and more so in African countries. Therefore, understanding the distribution of corneal aberrations is useful for selecting the type of the IOL to be inserted. In this study, we aimed to investigate the distribution of corneal SA in a Tanzanian population in order to assess whether the amount of negative SA in the currently available aspherical IOLs is suitable for this population. In addition, we examined the correlation between corneal SA and common preoperative ocular parameters such as corneal curvature radius (CR), corneal astigmatism, and axial length (AL).

Materials and methods

Study population

This study was performed as part of a cross-sectional population-based study conducted from August to September in 2014 in the Mkuranga district hospital in the Pwani region. The Mkuranga district is a rural area where approximately 85% of the people are engaged in agriculture. Out of the 18 wards of the Mkuranga district, 3 wards were selected, and one typical village within a driving distance of 1 hour by car from the district hospital was further chosen from each ward. A total of 1,276 adult residents, aged 40 years and over, in three villages (Tengelea, Kiziko, and Msfini-Kidete) were enlisted as study participants based on the records provided by village health workers.

Data collection

All ocular biometry measurements, including HOAs, were performed by ophthalmologists and orthoptists. CR, corneal astigmatism, and refraction were measured using an auto kerato-refractometer (KR-8900; Topcon Co., Tokyo, Japan). Uncorrected visual acuity (UCVA) and best-corrected visual acuity (BCVA) were assessed with a System Chart SC-1600 (NIDEK Co., Ltd, Aichi, Japan). After dilation with a combination of tropicamide and phenylephrine, ACD and AL were measured using a non-contact optical biometry device (AL-Scan, NIDEK). This device provides highly repeatable and reproducible estimates of ACD and AL that are in good agreement with the measurements obtained with IOLMaster (Carl Zeiss Meditec, Jena, Germany) [19]. Corneal and ocular HOAs including SA (Zernike spherical aberration coefficient C40) were measured three times for the right eye with a Hartmann-Shack wavefront analyzer with Placido disk topographer (KR-1W, Topcon Co.) and were calculated for the central 6.0-mm zone. Anterior segment and fundus examinations were performed using a slit-lamp (SL-D8Z, Topcon Co.) and indirect ophthalmoscope to check ocular health.

Data analysis

We excluded cases with BCVA worse than 0.1 (20/200), corneal opacity, ocular phthisis, moderate to advanced pterygium, less dilatation, poor fixation, and history of ocular surgery or trauma. For wavefront aberration data, three measurements were averaged in each eye.

Statistical analysis

All statistical analyses were performed using IBM SPSS statistics version 25 (IBM Japan, Tokyo, Japan). Correlations between corneal C40 and each parameter such as CR, corneal astigmatism, and AL were assessed using the Spearman’s rank correlation analysis. P-value < 0.05 was considered to indicate statistical significance.


Written informed consent was obtained from each participant. This study was conducted according to the Declaration of Helsinki and was approved by the ethics committees of Muhimbili University of Health and Allied Sciences (approval number: MU/DRP/AEC/Vol.XVIII/136), Kanazawa Medical University (2014–206), and University of Tsukuba (2015–989).


A total of 937 individuals participated in the entire survey. The participation rate was 73.4% and all participants were of African descent. After applying exclusion criteria, 657 right eyes (336 male, 321 female) were included in the analysis. The age of the subjects was 57.2 ± 10.3 (mean ± standard deviation) years. CR was 7.67 ± 0.29 mm; corneal astigmatism was 0.90 ± 0.60 diopters; and ACD and AL were 3.07 ± 0.33 mm and 23.03 ± 0.84 mm, respectively (Table 1). The mean corneal SA was 0.188 ± 0.095 μm (range -0.242 to 0.613), and the median value was 0.195 μm (Fig 1). When SA was divided at every 0.05 μm, the mode value was between 0.20–0.25 μm. The distribution of corneal SA did not exhibit a normal Gaussian curve (Kolmogorov-Smirnov normality test, p = 0.001). The root mean square (RMS) of the corneal total HOAs and the third- and fourth-order aberrations were 0.629 ± 0.250 μm (range 0.204 to 1.896), 0.539 ± 0.236 μm (0.123 to 1.774), and 0.269 ± 0.110 μm (0.052 to 0.920), respectively.

Tab. 1.

Demographic and clinical characteristics of subjects (n = 657).

<h2>Demographic and clinical characteristics of subjects (n = 657).</h2>
<h2>Distribution of corneal spherical aberration (SA; C<sub>4</sub><sup>0</sup>).</h2>
Fig. 1.

Distribution of corneal spherical aberration (SA; C40).

The mean corneal SA was 0.188 ± 0.095 μm, and is represented by the thick solid line. The median value was 0.195 μm, and is represented by the dotted line.

In correlation analyses, corneal SA showed significant correlations with CR (Spearman’s correlation coefficient, r = -0.177, p < 0.001), corneal astigmatism (r = -0.142, p < 0.001), and AL (r = -0.168, p < 0.001). A weak correlation was also found between SA and age (r = -0.085, p < 0.05) (Fig 2).

<h2>Relationships between corneal spherical aberration (C<sub>4</sub><sup>0</sup>) and ocular parameters, and age.</h2>
Fig. 2.

Relationships between corneal spherical aberration (C40) and ocular parameters, and age.

(a) Scatterplots demonstrating corneal curvature radius and C40. A significant correlation was observed between the two (Spearman’s correlation coefficient, r = -0.177, p < 0.001). (b) Scatterplots demonstrating corneal astigmatism and C40. A significant correlation was observed between the two (r = -0.142, p < 0.001). (c) Scatterplots demonstrating axial length and C40. A significant correlation was observed between the two (r = -0.168, p < 0.001) (d) Scatterplots demonstrating age and C40. A significant correlation was observed between the two (r = -0.085, p < 0.05).


We conducted a population-based study in Tanzania and, to the best of our knowledge, reported the distribution of corneal SA in an African population in the sub-Saharan area for the first time. The mean corneal SA was 0.188 ± 0.095 μm at the mean age of 57.2 ± 10.3 years. The corneal SAs in 38.4% of eyes (251 / 657) were between 0.20 and 0.30 μm and those in 35.3% (232 eyes) were between 0.10 and 0.20 μm; therefore, the SAs of about three-quarters of all the subjects were between 0.10 and 0.30 μm. When the SAs were divided at every 0.05 μm, both the mean and median values were between 0.15 and 0.20 μm, and the mode value was between 0.20 and 0.25 μm. The SAs in 3.0% (20 eyes) were negative values, which could be attributed to the smaller mean and median values compared to the mode value.

Fig 3 shows the comparisons of corneal SAs in the central 6-mm zone between the current study and previous studies in subjects over a mean age of 40 years, and each study analyzing over 50 eyes. The mean value of 0.19 μm in Tanzanian individuals tended to be small compared with the value of 0.27–0.33 μm in Caucasian [2,3,1012], Middle-Eastern [14,15], and most Asian individuals [5,13] (S1 Table).

<h2>Comparison of corneal spherical aberrations (SA) at the 6-mm optical zone between this study and previous reports in individuals over the mean age of 40 years [<em class="ref">2</em>–<em class="ref">5</em>,<em class="ref">10</em>–<em class="ref">15</em>].</h2>
Fig. 3.

Comparison of corneal spherical aberrations (SA) at the 6-mm optical zone between this study and previous reports in individuals over the mean age of 40 years [25,1015].

The mean value of corneal SA in Tanzanian individuals was 0.19 μm, which tended to be small among the previously reported values.

As for the dispersion of corneal SA, the standard deviation was 0.10 μm in this study, and the minimum and maximum were -0.242 and 0.613 μm, respectively, showing a big difference between individuals. As shown in Fig 3, the standard deviations of SA were reported to be more than 0.08 μm, except in one study. This indicates that considerable variation of corneal SA is observed in Tanzanian individuals as well as in other ethnicities.

In this research, KR-1W (Topcon) was used for the measurement of HOAs. Since the devices used for measuring HOAs were different among the studies shown in S1 Table, the reported aberration values could not be compared directly. With regard to the reliability of KR-1W in comparison with other equipment, Hao et al. [20] reported that the corneal HOAs measured by the KR-1W and iTrace showed no statistically significant differences (p > 0.05). In addition, López-Miguel et al. [21] and Xu et al. [22] reported that the KR-1W device provides excellent repeatability and intersession reproducibility in the measurement of corneal SA. Hence, our results seemed sufficiently accurate and reliable, and it is unlikely that corneal SA of Tanzanian individuals was underestimated in this study. For reference, the mean value of the total corneal HOAs was 0.63 ± 0.25 μm in this study, which is similar to the value of 0.65 ± 0.46 μm in a previous study by Guirao et al. [12] that used videokeratoscopy (Eyemap EH-290; Alcon, Fort Worth, TX). However, the mean value of corneal SA was 0.19 ± 0.10 μm in this study, which is much smaller than the value of 0.32 ± 0.12 μm in the study by Guirao et al. [12]. Thus, the corneal SAs seemed to be relatively small in the Tanzanian individuals in this study.

We examined the correlations between ocular parameters and corneal SA. A weak negative correlation was found between CR and corneal SA (r = -0.177, p < 0.001) in this study, which was compatible with that in a previous study [3]. Although corneal astigmatism showed a significant correlation with corneal SA in this study (r = -0.142, p < 0.001), Zhao et al. [22] reported that corneal SA of astigmatic corneas was similar to those of nonastigmatic corneas at a mean age of 42.6 ± 11 years. AL also showed a weak negative correlation with corneal SA (r = -0.168, p < 0.001) in our study, similar to a previous report [4]. In this study the correlation of corneal SA with age was very weak (r = -0.085, p < 0.05). The correlation between corneal SA and age remains somewhat controversial. Some researchers reported that there was no correlation [23,24] or poor correlation [3]; however, in a study conducted in China by Kemraz et al. [25] it was reported that corneal SA increased non-linearly with age and became more positive after 39 years of age. The correlations may be different among different ethnicities.

The limitation in this study was a low participation rate for a population-based study (participation rate of only 73.8% [937 participants]). In addition, the subjects available for current data analysis with regard to HOAs decreased to 657 eyes, owing to the exclusion criteria. However, compared with other previous studies on corneal SA, the population in this study was large, and the obtained results can be considered to be meaningful.

In summary, this study was the first to document the corneal SA in sub-Saharan African individuals. The mean value of the corneal SA in a large number of Tanzanian individuals tended to be small among the previously reported values. We observed that corneal SA varied widely between different subjects. If an aspherical IOL with a SA of -0.27 μm such as Tecnis ZCB00 is selected, the ocular SA may show overcorrection. IOLs with milder asphericity may be suitable for Tanzanian individuals.

Supporting information

S1 Dataset [xlsx]
Detailed information for all subjects.

S1 Table [docx]
Comparison of corneal spherical aberrations between this study and previous reports.


1. Guo H, Goncharov AV, Dainty C. Comparison of retinal image quality with spherical and customized aspheric intraocular lenses. Biomed Opt Express. 2012;3:681–691. doi: 10.1364/BOE.3.000681 22574257

2. Holladay JT, Piers PA, Koranyi G, van der Mooren M, Norrby NES. A new intraocular lens design to reduce spherical aberration of pseudophakic eyes. J Refract Surg. 2002;18:683–691. 12458861

3. Beiko GH, Haigis W, Steinmueller A. Distribution of corneal spherical aberration in a comprehensive ophthalmology practice and whether keratometry can predict aberration values. J Cataract Refract Surg. 2007;33:848–858. doi: 10.1016/j.jcrs.2007.01.035 17466860

4. Shimozono M, Uemura A, Hirami Y, Ishida K, Kurimoto Y. Corneal spherical aberration of eyes with cataract in a Japanese population. J Refract Surg. 2010;26:457–459. doi: 10.3928/1081597X-20100212-03 20166626

5. Lai YJ, Yeh SI, Cheng HC. Distribution of corneal and ocular spherical aberrations in eyes with cataract in the Taiwanese population. Taiwan J Ophthalmol. 2015;5:72–75. doi: 10.1016/j.tjo.2015.03.003 29018671

6. Ohtani S, Miyata K, Samejima T, Honbou M, Oshika T. Intraindividual comparison of aspherical and spherical intraocular lenses of same material and platform. Ophthalmology. 2009;116:896–901. doi: 10.1016/j.ophtha.2008.11.022 19410948

7. Caporossi A, Martone G, Casprini F, Rapisarda L. Prospective randomized study of clinical performance of 3 aspheric and 2 spherical intraocular lenses in 250 eyes. J Refract Surg. 2007;23:639–648. 17912933

8. Montés-Micó R, Ferrer-Blasco T, Cerviño A. Analysis of the possible benefits of aspheric intraocular lenses: Review of the literature. J Cataract Refract Surg. 2009;35:172–181. doi: 10.1016/j.jcrs.2008.09.017 19101441

9. Kohnen T, Klaproth OK, Bühren J. Effect of intraocular lens asphericity on quality of vision after cataract removal. Ophthalmology. 2009;116:1697–1706. doi: 10.1016/j.ophtha.2009.03.052 19643497

10. Wang L, Dai E, Koch DD, Nathoo A. Optical aberrations of the human anterior cornea. J Cataract Refract Surg. 2003;29:1514–1521. doi: 10.1016/s0886-3350(03)00467-x 12954298

11. de Sanctis U, Vinai L, Bartoli E, Donna P, Grignolo F. Total spherical aberration of the cornea in patients with cataract. Optom Vis Sci. 2014;91:1251–1258. doi: 10.1097/OPX.0000000000000380 25192433

12. Guirao A, Tejedor J, Artal P. Corneal aberrations before and after small-incision cataract surgery. Invest Ophthalmol Vis Sci. 2004;45:4312–4319. doi: 10.1167/iovs.04-0693 15557437

13. Li ZH, Jia LX, Huang YF. Analysis of corneal spherical aberration in patients before and after phacoemulsification. Eye Sci. 2012;27:165–168. 23225835

14. Al-Sayyari TM, Fawzy SM, Al-Saleh AA. Corneal spherical aberration in Saudi population. Saudi J Ophthalmol. 2014;28:207–213. 25278799

15. Assaf A, Kotb A. Ocular aberrations and visual performance with an aspheric single-piece intraocular lens: Contralateral comparative study. J Cataract Refract Surg. 2010;36:1536–1542. doi: 10.1016/j.jcrs.2010.03.046 20692567

16. The United Nations Development Programme, The Government of the United Republic of Tanzania. Tanzania human development report 2014: Economic transformation for human development. Dar es Salaam, Tanzania, Economic and Social Research Foundation, 2015.

17. Beiko GH. Personalized correction of spherical aberration in cataract surgery. J Cataract Refract Surg. 2007;33:1455–1460. doi: 10.1016/j.jcrs.2007.04.019 17662441

18. Packer M, Fine IH, Hoffman RS. Aspheric intraocular lens selection based on corneal wavefront. J Refract Surg. 2009;25:12–20. 19244948

19. Huang J, Savini G, Li J, Lu W, Wu F, Wang J, et al. Evaluation of a new optical biometry device for measurements of ocular components and its comparison with IOLMaster. Br J Ophthalmol. 2014;98:1277–1281. doi: 10.1136/bjophthalmol-2014-305150 24795336

20. Hao J, Li L, Tian F, Zhang H. Comparison of two types of visual quality analyzer for the measurement of high order aberrations. Int J Ophthalmol. 2016; 9:292–297. doi: 10.18240/ijo.2016.02.22 26949654

21. López-Miguel A, Martínez-Almeida L, González-García MJ, Coco-Martín MB, Sobrado-Calvo P, Maldonado MJ. Precision of higher-order aberration measurements with a new Placido-disk topographer and Hartmann-Shack wavefront sensor. J Cataract Refract Surg. 2013;39:242–249. doi: 10.1016/j.jcrs.2012.08.061 23142546

22. Xu Z, Hua Y, Qiu W, Li G, Wu Q. Precision and agreement of higher order aberrations measured with ray tracing and Hartmann-Shack aberrometers. BMC Ophthalmology 2018;18:18 doi: 10.1186/s12886-018-0683-8 29374460

23. Zhao H, Dai GM, Chen L, Weeber HA, Piers PA. Spherical aberrations of human astigmatic corneas. J Refract Surg. 2011;27:846–848. doi: 10.3928/1081597X-20111005-05 22045577

24. Oshika T, Klyce SD, Applegate RA, Howland HC. Changes in corneal wavefront aberrations with aging. Invest Ophthalmol Vis Sci. 1999;40:1351–1355. 10359316

25. Kemraz D, Cheng XY, Shao X, Zhou KJ, Pan AP, Lu F, et al. Age-related changes in corneal spherical aberration. J Refract Surg. 2018;34:760–767. doi: 10.3928/1081597X-20181011-01 30428096

Článek vyšel v časopise


2019 Číslo 9

Nejčtenější v tomto čísle

Tomuto tématu se dále věnují…


Zvyšte si kvalifikaci online z pohodlí domova

Léčba bolesti v ordinaci praktického lékaře
nový kurz
Autoři: MUDr. PhDr. Zdeňka Nováková, Ph.D.

Revmatoidní artritida: včas a k cíli
Autoři: MUDr. Heřman Mann

Jistoty a nástrahy antikoagulační léčby aneb kardiolog - neurolog - farmakolog - nefrolog - právník diskutují
Autoři: doc. MUDr. Štěpán Havránek, Ph.D., prof. MUDr. Roman Herzig, Ph.D., doc. MUDr. Karel Urbánek, Ph.D., prim. MUDr. Jan Vachek, MUDr. et Mgr. Jolana Těšínová, Ph.D.

Léčba akutní pooperační bolesti
Autoři: doc. MUDr. Jiří Málek, CSc.

Nové antipsychotikum kariprazin v léčbě schizofrenie
Autoři: prof. MUDr. Cyril Höschl, DrSc., FRCPsych.

Všechny kurzy
Kurzy Doporučená témata