5. Discussion

In conventional phacoemulsification, the CCC is performed manually by using a rhexis forceps and generating shearing and tearing forces centripetally, thus counterbalancing the centrifugal forces from the zonules [17, 18]. In general, the capsule flap is regrasped every 3 to 4 clock hours to further minimize the contribution of centrifugal forces. It is challenging to repeatedly achieve a well-centered CCC of 5/5.5 mm even by the most experienced cataract surgeon. FS cataract laser platforms are showing increased predictability to obtain capsulotomies that are round, well centered, and of the desired size in comparison with manual CCC [6–9]. After docking the patient interface, the capsulotomy size and position are set according to the pupillary aperture using real-time high-resolution anterior segment imaging (either OCT or Scheimpflug imaging). Focal photodisruption of the anterior lens capsule generates multiple overlapping craters at the capsulotomy edge. At the end of the FS laser assisted procedure, the capsulotomy leaf is gently pulled centripetally with a forceps.

The human lens capsule is considered a specialized basement membrane. The major structural component of the lens capsule is basement membrane type IV collagen, which is organized into a three-dimensional molecular meshwork [19–22]. Accordingly, the human lens capsule has intrinsic elasticity [23–25]. The tensional force that tears off the capsule during manual CCC is directed in the same direction tangentially to the edge. On the contrary, the FS laser pulses cut perpendicularly the anterior capsule. The force that induces the manual capsulorhexis is oriented in one unique direction while the laser capsulotomy is the resultant of a sequence of circular oriented multiple spots [26]. Photodisruption effects at the capsulotomy edge disrupt the normal collagen arrangement of the anterior lens capsule and induce irregularities, as shown in the present and previous work [12–15]. The microirregularities at the capsulotomy edge have been found in all the commercial FS laser platforms. Previous work [12] showed the structure of laser-cut capsules obtained using three FS laser platforms, which included the LenSx, the Lensar, and the Catalys. Here we also showed the morphology of capsulotomy edges obtained using the Victus.

In this study, we compared the microstructure and irregularity of the FS laser capsulotomy edges in comparison with manually torn capsular edges. The eSEM images have not the surface artifacts occuring during sample preparation in conventional SEM imaging, such as graded alcoholic dehydration followed by metal coating that masks surface features [12–15]. The FS laser capsulotomies and manual CCC were only fixed in 2.5% glutaraldehyde solution for 24 hours and then imaged at low temperature (≤4°C) and high humidity (virtually 100%), thus greatly minimizing tissue dehydration and avoiding masking the microirregularities at the capsular edge.

The edges of manually torn capsules were smoother than the FS laser capsulotomy edges; no measurable differences in irregularity were found between the LenSx and the Victus specimens. The FS capsulotomies edges showed similar patterns, despite the intrinsic differences in laser settings and proprietary technology. Authors in [13–15] have shown that, using the LenSx laser, smoother cutting edges could be obtained by reducing the spot energy and by placing a soft contact lens between the cornea and the curved rigid interface; however, microdiscontinuities (e.g., cracks and tags) have been still shown when using low pulse energy.

The microscopic features and irregularity of the FS capsulotomy edges can be directly related both to photodisruption and to eye movements [12–15]. The photodisruptive mechanical and thermal effects contribute to the corrugating and stretching of the capsular edge [27–29], offering a mechanical basis for weakness in capsular integrity. These irregularities have been postulated to either limit the distension of the capsule or act as focal points for the concentration of stress that would increase the risk of capsular tear [12–15]. Auffarth et al. [30] have found, in porcine eyes, that FS laser capsulotomies resulted in a stronger anterior capsular opening than manual CCC, offering a hypothesis that tears may originate by increased stress at the capsular edges when pulling the capsulotomy leaf. On the other hand, biomechanical data from porcine specimens cannot be translated to the human lens capsule due to intrinsic differences in elasticity between species [18, 23–25, 31, 32]. Capsular tears in FS laser assisted cataract surgery have been mainly reported to occur with hydrodissection and during lens manipulations [12], suggesting that reduced capsular distensibility may represent an additional factor to increased risk of capsular tears during FS laser assisted cataract surgery. On the other hand, there is still no evidence supporting this hypothesis [17–19, 23–25, 33–36].

Eye movements during surgery (that are in the range between 20 and 100 μm) have been considered to contribute to increased capsulotomy edges' irregularities, by creating multiple, random cavitations that could compromise the integrity of the capsular edge and represent a point for a tear to initiate with adequate force [12] during the capsulotomy pulling, hydrodissection, or nucleus manipulations. In this study we showed, for the first time, microdiscontinuities at the capsular edge (i.e., linear cracks; ≤3 μm width). These features may originate by imprecise impact of the laser pulses with the lens capsule, likely due to eye movements during laser surgery, and may represent the real risk to generate tears in the case of increased capsular stress during FS laser assisted phacoemulsification. Further studies on the biomechanics of the human lens capsule, in relation to FS laser parameters and capsulotomy size and centration [37], are needed to understand the influence of FS photodisruption on capsular tears and how to create the capsulotomy edges with quality and morphology comparable to manually torn capsules [13, 14, 38]. Some irregularities in ex vivo studies may have arisen by increased IR absorption and scattering of donor corneal tissues, thus enhancing the differences between manually torn capsules and FS laser capsulotomy cutting edges characteristics.

In this study, we confirmed that the LEC boundary is closer to the manual CCC edge than FS capsulotomy edge. The results were in agreement with previous work [13–15]. Increased LEC death and inhibition of LEC proliferation may be beneficial for preventing PCO.

In conclusion, the FS laser capsulotomy edges show distinct irregularities, independent of the laser platform, that may be at risk of increased capsular tears during phacoemulsification. During the learning curve, the cataract surgeons should be conservative when pulling the capsule, during hydrodissection and nucleus manipulations. The implementation of robust eye tracking system in the FS laser platforms would greatly improve the smoothness of capsulotomy edges.