In this study, we first analysed the size of IOL-PC space with cross-sectional anterior segment images that were acquired with RT-Au 100 OCT. It was reported that IOL-PC spaces should be gradually decreased and closed in 2 weeks after IOL implantation [13, 14]. In our study, we reported the median value of the size of IOL-PC space was 0.93 (0.63,1.36) mm2 in all these PCO eyes. Besides, it was 1.26 (0.83,1.79) mm2 in 23 PCO eyes which still showed unoccupied residual IOL-PC space. In high myopia eyes, there was a larger IOL-PC space and more severe PCO. The existence of obvious residual IOL-PC should be closely correlated with PCO formation and severity.
We reported that IOL-PC space was positively correlated with axial length. As reported, capsule space was positively related to axial length, the longer eyes had a larger capsule [6] and the implanted IOL was thinner, which tended to have less convexity of the IOL back surface. In addition, IOL apposition to capsule was negatively correlated with axial length [15]. These factors resulted in a larger IOL-PC space and higher PCO occurrence rate which could be supported by our results as well. In addition, PCO area and thickness were positively correlated with axial length.
The existence of PCO resulted in deceasing of visual acuity. In our study, log MAR visual acuity was positively correlated with PCO area and thickness at the 3 mm and 5 mm IOL optic regions when visual acuity was ≤ 0.52 Log MAR. There was no significant correlation when visual acuity was ༞0.52 Log MAR. So we believed the cut off level of visual acuity was 0.52 Log MAR in these PCO eyes. These result suggested that the decrease of visual acuity could describe the PCO severity (PCO area and thickness) to some degree. However, with severe PCO more items maybe needed to describe the decreased visual function. Besides, In visual acuity ༞0.52 Log MAR group, pear type PCO comprised the major part, the high light scattering of pear type PCO cause more severe visual function impairment, which may influence the results [16]. In previous study, Javier Moreno- Montan˜e et al. [17] analysed PCO peak density and posterior capsule thickening (PCT) with OCT-1 and reported that log MAR visual acuity was positively correlated with PCT (Spearman correlation coefficient r = 0.66). This agreed with our results. However, Maartje et al. [18] reported a curvilinear correlation between log MAR and PCO severity (EPCO score), the curvilinear correlation indicated that Log MAR is not affected by low PCO severity. This was different from our result. The reason might be the varied PCO severity and different evaluation methods.
We believed that residual IOL-PC and PCO severity could be also attributed to the varied IOL designs and materials. As reported, acrylic, silicone, and poly-methyl methacrylate IOLs showed various capsule closure times, the delayed closure of capsule could facilitate the migration and accumulation of LECs (lens epithelia cells). [11]. As it was reported that the rapid proliferation speed of LECs for patients younger than 40 years was only 5.8 ± 0.6 days, for those who were older than 60 years was 7.2 ± 0.7 days. Hence, theoretically, LECs could migrate to and proliferate on the posterior capsule before capsular bend formation (1 month after surgery [11]) and IOL apposition to a capsule (within 2 weeks for normal eyes [13, 14] and en-longed for high myopia eyes [15]). In addition, Elgohary et al. [19] reported closer apposition between IOL optic and mid-peripheral posterior capsules when comparing hydrophobic acrylic IOLs and PH silicone IOLs. When this apposition was missing with a PH silicone IOL, there was high reflectance material that accumulated between the IOL and posterior capsule, which was considered as LECs and extracellular matrix. In our study, we not only observed these materials but also quantified these high reflectance materials (including PCO area, thickness and density) in the IOP-PC space and proved that there was a positive relation between the size of the IOL-PC space and PCO severity (PCO area and thickness) with RT-Au 100 OCT.
In addition, in these PCO eyes recruited in our study, we analysed the correlation between PCO severity and IOL designs and materials.
In our study, the 1 piece IOL had a larger IOL-PC space and PCO area and thickness than the 3 piece IOL, this might be due to poor axial stability and efficiency of IOL adherence, which was agreed with a previous study [20]. Further more, haptic junction points interrupted the 360-degree sharp edge that hampered capsule bend formation, weakened the mechanical barrier effect of the IOL optic edge [21], and the high multi-crease occurrences [22] which may be the channel for LEC migrations [8, 12].
We also found more severe PCOs with hydrophilic IOLs than hydrophobic IOLs. This was agreed with the higher PCO occurrences, PCO scores and Nd:YAG laser surgery rates in previous hydrophilic IOLs related studies [1, 15]. This might be due to the surface of hydrophilic IOLs, which was more suitable for migration and proliferation of LECs [23]. While, the surface of hydrophobic IOLs showed stiffer adherence with the capsule, and the fibronectin and laminin strengthened the adherence to eliminate the IOL-PC space [10]. In addition, there was higher capsule crease occurrence in hydrophilic IOL eyes which facilitated the LEC migration.
It was believed that IOLs suited to the capsule could prevent a PCO, ACO, and IOL tile and decentration [24]. In our study, when the implanted IOL diameter was ≤ 11.00 mm, the IOL-PC space and PCO area and thickness increased compared with the IOL diameter ≥ 12.5 mm group. This might be due to poor IOL adherence and multi-capsule creases. It was reported that the diameter of the empty capsule for emmetropia eyes was 10 mm-10.8 mm [6], when the IOL implanted capsule diameter was 10 mm or could be stretched to 12.0 mm [25]. With an IOL diameter ≥ 12.5 mm, the capsule could be stretched to eliminate creases and IOL-PC space. Meanwhile, the powerful mechanical stress of the IOL optic edge prevented the migration of LECs. However, with an IOL diameter increase, PCO may increase. As Javier Moreno-Montan˜e´s et al. [4] reported, when the IOL diameter was ≤ 12.0 mm, 12.5 mm, or ≥ 13.0 mm, PCTs were 0 µm (0–37), 56 µm (17.33–78.66), and 78.66 µm (12-133.33), respectively. A thicker PCO in the ≥ 13.0 mm group might be due to over stretching and could have caused more creases on the haptic axial, which induced the migration of LECs. However, a study also reported that there was no effect of the IOL diameter on PCO formation [6, 26]. Georgios [26] compared Corneal A501D IOLs and Corneal J501D IOLs with different IOL diameters (11.0 mm and 10.5 ± 0.1 mm), and the PCO AQUA scores were 2.13 ± 1.64 and 2.0 ± 1.74, respectively. Efstathios Vounotrypidis [6] reported no significant difference in PCO EPCO scores between Quatrix ®IOL (11.0 mm) and Quatrix Evolutive ®IOL (10.47 ± 0.13 mm, 10.3–10.8 mm). The difference between these results and ours might be due to the smaller IOL diameter differences in Georgios’ and Efstathios Vounotrypidis’s studies.
We also found that PCO area and thickness were increased along with the increases in IOL haptic angulation. This agreed with Javier Moreno-Montan˜e´s [4] results, which were IOL haptic angulation of 0, 5, and 10 degrees when the PCTs were 0 mm (0-37.33), 58.67 mm (17.33-92.00), and 59.33 mm (0.67–110.50), respectively, which were significantly different. It was reported that AcrySof ® MA60BM with 10-degree haptic angulation showed axial movement at 3 months after surgery. The poor axial stability hampered the closing of the capsule, which may promote PCO formation. However, it was believed that haptic angulation could increase the contact between the IOL and capsule and that 10-degree haptic angulation was preferable [27]. Some others also suggested that there was no influence of IOL haptic angulation on PCO formation. Schmidbauer [28] implanted Centerflex® IOLs (Rayner) with 0, 5, and 10 degree haptic angulations in rabbit eyes and reported no significant differences in the central PCO, peripheral PCO and Soemmering’s ring scores. The author believed that the mechanical stress of the IOL edge may overwhelm the effect of haptic angulation.