INTRODUCTION

The 21^st century will be characterized by the development of sophisticated intraocular lenses (IOLs). Such lenses will be developed with the basic aim of allowing patients to achieve both close and distance vision without a requirement for corrective glasses⁽¹⁾. Although these lenses reduce internal aberrations and compensate for corneal astigmatism and aberrations, these benefits are minimized in the context of an uncorrected astigmatism exceeding 0.75 diopters (D). In addition, image quality is significantly affected in the presence of other higher-order corneal aberrations (HOCAs)⁽²⁾. The associated surgical procedure remains the primary cause of induced corneal changes, especially astigmatism and aberrations⁽³⁾.

Induced refractive corneal changes after cataract surgery depend mainly on the incision location and width^(3,4). However, various undefined factors might also affect refractive outcomes. In some cases, surgically-induced corneal changes are more extensive than expected. Such changes impair visual outcomes and patient satisfaction, especially when multifocal or toric IOLs (premium IOLs) are used. Hence, the identification of other factors that might influence the impact of a surgical procedure on corneal optical properties is crucial to achieve perfect refractive results from premium cataract surgery⁽⁵⁾.

The present study aimed to investigate the effects of biomechanical properties of the cornea on refractive changes after cataract surgery in patients with low-level astigmatism.

METHODS

RESULTS

Twenty-eight eyes of 28 patients were analyzed in this study. The demographic characteristics of the participants are summarized in table 1. The preoperative astigmatism level was 0.23 ± 0.12 D. Thirteen patients had no astigmatism. Astigmatism with the rule was observed in eight eyes (flat axis: 0-30º/150-180º), astigmatism against the rule was observed in two eyes (flat axis: 60-120º), and oblique astigmatism (flat axis: 30-60º/120-150º) was observed in 5 eyes. The preoperative corneal hysteresis (CH) was 8.68 ± 1.86 mmHg, and the corneal resistance factor (CRF) was 8.66 ± 1.61 mm Hg. The preoperative and postoperative HOCA results, as well as the 1- and 3-month postoperative SIAs, are listed in table 2.

The 1-month measurements differed significantly from the preoperative measurements in terms of the root mean square (RMS; p=0.05), higher-order RMS (HORMS; p=0.023), astigmatism (p=0.02), and trefoil (p=0.033). No differences were observed in coma (p=0.386) and SA (p=0.947). When the 3-month results were evaluated, only the total RMS (p=0.020) and HORMS (p=0.012) differed significantly. No statistically significant differences were observed between the preoperative and 3-month postoperative measurements of coma (p=0.847), SA (p=0.630), trefoil (p=0.847), and astigmatism (p=0.102). Similarly, the 1- and 3-month SIA results did not differ significantly (p=0.854).

The correlations between the CH and CRF values and postoperative HOCAs and SIAs are presented in table 3.

Except for a positive correlation between the 3-month postoperative CH and SA, no relationships were observed between the preoperative and 1- and 3-month postoperative values of CH/CRF, total RMS, HORMS, astigmatism, coma, trefoil, SA, and SIA. Figures 1 and 2 demonstrate the relationship between CH/CRF and SIA, and figure 3 presents the relationship between CH and SA at 3-months postoperatively.

Figure 1 Relationship between corneal hysteresis and surgically-induced astigmatism at 3 months. 

Figure 2 Relationship between the corneal resistance factor and surgically-induced astigmatism at 3 months. 

Figure 3 Relationship between corneal hysteresis and spherical aberration at 3 months. 

DISCUSSION

The achievement of optimum visual outcomes from premium cataract surgery relies on accurate and reliable predictions of the refractive results⁽⁵⁾. Residual ametropia, especially astigmatism, is among the main causes (64%) of dissatisfaction after implantation of a multifocal IOL⁽⁹⁾. Therefore, recommendations suggest the correction of astigmatism >1.0 D before implanting a multifocal IOL⁽⁷⁾. Previous findings have indicated that both astigmatism and aberrations might affect the visual quality of patients with multifocal IOLs. For instance, one study found that anterior coma values >0.32 μm might result in intolerable dysphotopsia in the presence of a diffractive multifocal IOL (A). Implantation of a premium IOL necessitates high anticipated postoperative refractive results; however, even under ideal circumstances, unexpected refractive deviations may be observed. Accordingly, with this study we aimed to investigate postoperative refractive changes and the relationships between these changes and corneal biomechanical features in a cohort of patients with low-level astigmatism, as these are favorable candidates for multifocal IOL implantation.

In previous studies, a 3.2-mm incision had a significant impact on astigmatism and trefoil, but no significant effect on SA and coma during a 45-day postoperative follow-up^(10,11). However, these aberrations normalized after 3 months⁽¹²⁾. After micro-incisional surgery (≤2-mm incision), no significant changes were observed in any of the aberrations at 3 months of follow-up^(3,13). Therefore, our postoperative follow-up period was limited to 3 months. Similarly, we observed that astigmatism, trefoil, total RMS, and HORMS increased significantly within the first month, although no significant changes were observed in coma and SA. However, at 3 months, the incidence of all of these aberrations decreased, whereas the total RMS and HORMS increased significantly relative to the preoperative values.

The incision size is one factor that affects the extent of SIA after cataract surgery. In our study, which implemented 2.8-mm incisions, we observed SIAs of 0.85 D in the first month and 0.76 D in the third month. In other words, these SIA results did not differ significantly. Previous studies that used similar incision sizes reported different SIA values. A study by Hayashi et al., in which a 2.65-mm incision size was used for cataract surgery, reported a final incision size as 2.84 mm and 2-month SIA of 0.74 D⁽¹⁴⁾. Another study that used a 2.75-mm incision reported a 2-month postoperative SIA of 0.31 D⁽¹⁵⁾. In summary, both studies found that the SIA decreased with the incision size.

The incision location also affects the level of SIA. Compared with superior incisions, temporal incisions are associated with a lower level of SIA and are accordingly used for patients with low-level astigmatism⁽¹⁶⁾. In our study, we used superior incisions even though our patients had a low-level astigmatism. This might explain the high level of SIA identified in our study. The use of temporal incisions might have reduced the SIA.

Similar to SIA, corneal aberrations that developed after cataract surgery were found to be closely related to the incision size. Specifically, the aberrations decreased with decreasing incision size (a) should be replaced with⁽¹⁴⁾. According to our findings following the creation of 2.8-mm incisions, in the third month the total RMS and HORMS increased significantly from the preoperative values. Although trefoil and astigmatism increased at 1 month postoperatively; these values decreased at 3 months and approached the preoperative values. Given these decreases in trefoil and astigmatism, the aberration-induced dissatisfaction experienced by multifocal IOL-implanted patients might decrease by 3 months postoperatively.

Numerous studies have investigated the effects of incision location and size on postoperative changes in refraction and aberrations; however, few studies have investigated the effects of corneal biomechanical features on SIA and HOCAs. Denoyer et al. published the first such unique study and observed positive correlations among the preoperative CH, CRF, and SIA in the first month⁽⁵⁾. This correlation is represented by the following formula: SIA (D)= 2.08 + 0.24 × Incision Size (mm)-0.19 × CH (mmHg). Denoyer et al. did not observe a correlation between the preoperative CH and CRF and the postoperative HOCAs. In our study, we did not find a relationship between the preoperative CH/CRF and postoperative SIA/HOCAs. We merely noted a positive correlation between the CH and SA at 3 months. According to this result, the formation of SA increases as the CH increases or the corneal hardness decreases after surgery. Because the corneal biomechanical resistance is lower in patients with a high CH, greater postoperative disruption of corneal sphericity may be observed. Awareness of this potential outcome may be needed when an aspheric IOL is selected. These two studies differ in terms of characteristics of the patient cohort; notably, patients with a low level of astigmatism (<0.50 D) are more suitable candidates for multifocal IOL implantation. In addition, we extended the follow-up duration to 3 months.

Our study had some limitations. First, the incision size was not measured at the end of surgery. However, the incision size may expand during surgery, especially when an IOL is implanted. Second, aberrations were assessed using a Pentacam and not an aberrometer. However, analyses of the repeatability of the Pentacam for the measurement of aberrations have yielded conflicting data⁽¹⁷⁾. Third, because of the low number of patients, we were unable to evaluate whether the patients abided by the astigmatism rule, and could not investigate correlations with surgical results or the effects of posterior surface astigmatism.

In conclusion, our results regarding the relationship between corneal biomechanical properties and SIA differ from those of the original study published on this topic. Accordingly, further prospective studies involving larger patient groups are needed. This relationship might help us to understand the incidence of unexpected astigmatism after cataract surgery and explain the unexpected refractive aberrations and ectasias that may occur after corneal refractive procedures.