OVDs play a crucial role in phacoemulsification to protect intraocular structures from maneuvers during intraocular surgery. The main purpose of OVDs are to stabilize the anterior chamber and protect the corneal endothelium. Dispersive OVDs have lower molecular weight and shorter molecular chains that protect more from the turbulence of fluid and lens fragments during phacoemulsification and better adhere to corneal endothelial cells(5). Cohesive OVDs are viscoelastics with high molecular weight and long molecular chains that aid in field protection (6).
Short molecular chains make dispersive OVDs more prone to cleavage, resulting in increased postoperative IOP. On the contrary, removal of cohesive OVDs are faster but protection to the corneal endothelium is lower (3).
In this study, we aimed to compare the safety and performance in cataract surgery of two OVDs, each having high viscosity cohezive characteristics. Both groups were operated by the same surgeon and compared in terms of ultrasound energy and other characteristics used in surgery.
The primary goal of OVD is the ability to protect corneal endothelial cells because the corneal endothelium is actively present where optical transparency can be maintained (7,8). The efficiency of OVDs on the corneal endothelium can be evaluated by postoperative endothelial cell concentration (9). The reduction of endothelial cells is compensating by cell expansion, cell shift, rearrangement, and cell fusion (10).
Auffarth et al. (9) in their study they compared the performance of the two ophtalmic devices-Twinvisc and Duovisc which were contain both dispersive and a cohesive OVD in a single device. They were pointing that there were no significant difference in the mean ECD reduction between two grups at three months postoperatively.
Holzer et al. (11) compared five OVDs commonly used during phacoemulsification in terms of ECD loss in a prospective randomized study. They did not confirm that dispersive OVDs protect the endothelium better than cohesive OVDs. In fact, the lowest mean cell loss (6.2±6.5%) was in the Healon 5 (Abbott Medical Optics, Inc., Santa Ana, CA, USA) group, a cohesive agent, and the greatest cell loss was seen in the OcuCoat (Bausch & Lomb, Rochester, NY, USA) group (16.7±10.8%). OcuCoat contains hydroxylpropyl methylcellulose (2.0%) and has dispersive characteristics.
Papaconstantinou et al. (12) assessed and compared the safety and the efficacy of VisThesia (Carl Zeiss Meditec AG, Jena, Germany) and Viscoat in a prospective randomized clinical trial. The mean ECD decrease was 212 cells/mm2 (9.1%) in the Viscoat group and 272 cells/mm2 (11.8%) in the VisThesia group. It was not significant as the difference between the two groups (t-test = 0.18, p> 0.1). They also states that the result is from surgery commensurate with the standard ECD decrease after cataract surgery.Travmatic stress on the endothelium with cataract surgery causes a decrease in ECD and morphological changes in cells. Mechanical reasons such as heat, vibration, micro air bubbles and free radical formation produced by phaco tips and the contact of surgical instruments and nucleus units with the endothelium cause endothelial damage during phacoemulsification surgery (13,14).
Bourne et al. reported a loss of ECD of 16.1% after phacoemulsification surgery in their large series observed for one year (15).
In our study, there is a significant difference between the repeated measurements of the mean ECD in terms of time within the groups (p=0.000).But regardless of the time factor there was no statistically significant difference in the mean ECD reduction between the two groups (p=0,616). As a result, the decrease in ECD was within the physiological limits that should be compatible with the literature.
Dispersif and cohesive visoelastic substances Twinvisc and Duovisc were compared in a prospective and randomized multicenter study conducted in seven centers in Europe and they showed that mean IOP measurements and incidence of IOP spikes postoperatively the two OVD’s were equally effective. These findings with early postoperative IOP elevation and then returning to baseline values were found to be consistent with the literature (9).
Major reason for the postoperative IOP increase seems to be the amount of remaninig OVD at the end of the surgery(16). It is assumed that post-operative OVD is released into the aqueous humor and mechanically inhibits the trabecular outflow tract and reduces its mission(17). It is important to clean the OVD after the operation in order to prevent postoperative IOP increase. Since the OVDs we compared in our study are cohesive with a high viscosity, they are much easier and faster to clean from the forearm, thus enabling us to encounter postoperative IOP spikes. Also Rainer et al. showed that ın contrast to the dispersive Viscoat (16), the cohesive Healon caused lower IOP increases than Viscoat 6 hours postoperatively (18,19).
In our study we found statistically significant increase in the mean IOP at postoperative first day, first week and first month after surgery compared to preoperative (p=0.000). However, IOP was <27 mmHg in all patients. This increase in IOP was in the nature of a slight increase that may occur after routine cataract operation and reached preoperative values in the postoperative third month.
Corneal thickness is a parameter that directly reflects the functional status of the corneal endothelium. Postoperative CCT reflects the stress of endothelium caused by US energy and the turbulence of irrigation solution and nuclear fragments (20,21). Auffarth et al. (9) showed that there was an increase of 9.8% and 9.5% in pachymetry 24 hours after surgery in Group 1 and Group 2 when compared with the preoperative values (P! .001), the difference was statistically significant, but the difference between the two groups was not.
In their randomized patient research masked study, Neuyamer et al.(22) found an increase in CCT on the first postoperative day in the Neochrome Cohesive group ( a new cohesive OVD) and a discrepancy between low endothelial cell density loss at three months postoperatively. While the difference in endothelial loss between the two OVD groups was not significant, the increase in CCT was moderate. On the contrary, Cheng et al. (23) found a significant correlation between CCT in the immediate postoperative period and the percentage of endothelial cell loss at the 1st and 6th months postoperatively. First week after surgery, there was significant increase in CCT compare to preoperative values in Group 1 and Group 2 respectively (p=0.000). We could not find a relationship between the minimal increase in CCT after surgery and minimal ECD loss in our study. The postoperative clinical reflection was not significant. Cornea was clear in all cases.
In our study there is a significant decrease in the cell number (p=0,002) and hexagonality values (p=0.000) in both goups compared to the preoperative values. Also there is a significant increase in cell area and CV in cell size values compared to preoparative values (p=0.000). There was no significant difference between the groups in all parameters at postoperative third month. These results are the morphological reflections of the damage in the endothelium in routine cataract surgery and were found to be within routine limits .Regarding the limitations of our study, it does not include inflammation, flare and anterior chamber reaction was not examined. This is due to the retrospective nature of our study.As a result, we observed that these two high viscosity cohesive OVDs created a good anterior chamber depth and well preserved endothelial in cataract surgery. We did not encounter anyserious complications. Both OVDs can be used safely in the cataract surgery. There is a wide range of OVDs in cataract surgery and comparative prospective studies with other OVDs may be required.