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Corneal thickness and visual acuity after phacoemulsification with 3 viscoelastic materials

Davis, Elizabeth A. MDa; Lindstrom, Richard L. MDa,*

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Journal of Cataract & Refractive Surgery: October 2000 - Volume 26 - Issue 10 - p 1505-1509
doi: 10.1016/S0886-3350(00)00436-3
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Abstract

Corneal clarity is largely dependent on the function of the corneal endothelial pump. The endothelium can be damaged during phacoemulsification by contact with surgical instruments, lens fragments, air bubbles, or the intraocular lens (IOL).1–8 Viscoelastic materials provide a barrier that prevents contact between these agents and the endothelium. Several studies have shown that these substances can effectively reduce endothelial cell loss during cataract surgery.9–16

Many viscoelastic substances are available to the cataract surgeon. The intraoperative behavior of a particular viscoelastic agent depends on its physical and chemical properties. An experimental study in rabbits17 found that Viscoat® (chondroitin sulfate 4%–sodium hyaluronate 3%) had greater retentiveness adjacent to the corneal endothelium after phacoemulsification than Amvisc Plus® (sodium hyaluronate 1.6%) or Healon GV® (sodium hyaluronate 1.4%). However, comparative studies of viscoelastic materials have not shown statistically significant differences in postoperative endothelial cell loss.9–16 Whether these findings translate to differences in clinical outcomes is of interest.

This study sought to determine whether differences exist in visual acuity and corneal thickness after phacoemulsification using various viscoelastic substances.

Patients and Methods

Patients having routine phacoemulsification cataract extraction and IOL implantation were randomly assigned to receive one of 3 viscoelastics intraoperatively: Amvisc Plus, OcuCoat® (hydroxypropyl methylcellulose 2%), or Viscoat. The trial was single masked in that the surgeon was aware of the viscoelastic material used but the technician performing postoperative measurements was not.

Patients were included in the study if they met the following criteria: The cataract was visually significant with a preoperative best corrected visual acuity (BCVA) of 20/50 or worse using manifest refraction with a Baylor Visual Acuity Tester (BVAT) or a BVAT and medium brightness acuity test (BAT). Visual potential was 20/40 or worse as determined by a potential acuity meter (PAM), laser interferometry, or surgeon judgment. The cornea of the affected eye was without scarring, edema, guttata, or other pathology of potential visual significance. Cataracts were nuclear sclerotic with a density between +1 and +3 as determined by the surgeon. All patients signed an informed consent to participate in the study.

Patients were excluded if the affected eye had glaucoma, corneal guttata, age-related macular degeneration, nuclear sclerosis greater than +3, or other pathology thought to limit potential visual acuity to worse than 20/40.

All operations were performed by a single surgeon (R.L.L.) using the previously described tilt and tumble modified supracapsular technique.18 Phacoemulsification was performed using a Storz Millennium machine. The IOL was a Chiron C31UB, an Allergan SI-40, or an Alcon MA60 inserted with an injector. Viscoelastic material was removed by automated irrigation/aspiration. Next, intracameral carbachol diluted 1:5 in a balanced salt solution was instilled into the anterior chamber. At the end of the case, 1 drop each of tobramycin and dexamethasone (TobraDex®), diclofenac sodium (Voltaren®), and levobunalol (Betagan®) were administered.

Patients were examined preoperatively and 1 day postoperatively. At each visit, BCVA and central ultrasonic pachymetry measurements were obtained by a technician. A careful slitlamp examination of the anterior and posterior segments was performed by the surgeon.

Statistical analyses comparing pachymetry measurements within a group were performed using a paired Student t test. An unpaired t test was used when comparing BCVA and pachymetry measurements among groups. The BCVA values were first converted to logMAR equivalent values19 before statistical tests were performed. A P value less than 0.05 was considered significant.

Results

Of the 50 patients enrolled in the study, 17 received Amvisc Plus, 17 OcuCoat, and 16 Viscoat. All patients had grade +2 or +3 nuclear sclerosis. Mean effective phacoemulsification time was 30 seconds and fewer than 60 seconds in all cases. There were no intraoperative or postoperative complications.

Table 1 shows the mean preoperative, postoperative, and percentage increase in central pachymetry measurements. Postoperative pachymetry measurements were significantly greater than preoperative values within each group (P < .05). There was no significant difference among groups for any value shown in Table 1.

Table 1
Table 1:
Central ultrasonic pachymetry (μm).

Table 2 shows BCVA on the first postoperative day. There was a slight trend toward better BCVA at day 1 with the more viscous cohesive viscoelastics, but the differences were not significant.

Table 2
Table 2:
Best corrected visual acuity 1 day postoperatively.

The cumulative BCVA 1 day postoperatively is shown in Table 3. No patient had a BCVA of worse than 20/60 BCVA on day 1, and 88% or more had a BCVA of 20/40 or better in each group.

Table 3
Table 3:
Cumulative BCVA 1 day postoperatively.

Four patients, 2 in the OcuCoat group and 2 in the Viscoat group, had a BCVA of worse than 20/40 1 day postoperatively. They had a significantly greater percentage increase in corneal thickness than those with a BCVA of 20/40 or better (Table 4).

Table 4
Table 4:
Central pachymetry in eyes with a BCVA of 20/40 or worse versus those with 20/40 or better.

Discussion

The development and use of viscoelastic materials was a tremendous advance in cataract surgery. These agents serve multiple purposes intraoperatively. They create and maintain space, facilitate IOL insertion, and protect the corneal endothelium from trauma.

Many viscoelastic materials have been produced, differing in chemical and physical properties. Three viscoelastic compounds are available: sodium hyaluronate (Healon, Healon GV, Amvisc, Amvisc Plus, Vitrax®, and Provisc®), a combination of sodium hyaluronate and chondroitin sulfate (Viscoat), and hydroxypropyl methylcellulose (OcuCoat).

Amvisc Plus, sodium hyaluronate 1.6%, has a high viscosity. This characteristic results in good space maintenance, tissue manipulation, and tissue immobilization. Because of the large molecular weight of sodium hyaluronate, Amvisc Plus exhibits high zero-shear viscosity and cohesive behavior.20 It is therefore readily removed from the eye.

Viscoat, a 1:3 mixture of chondroitin sulfate 4% and sodium hyaluronate 3%, combines the higher viscosity and chamber-maintaining properties of sodium hyaluronate with the improved coating and cell-protection properties of chondroitin sulfate.21

OcuCoat, hydroxypropyl methylcellulose 2%, is a relatively inelastic material. Because of its significant coating properties, it has been marketed as a viscoadherent rather than as a viscoelastic material.

The lower molecular weight polymers, such as OcuCoat and Viscoat, have low zero-shear viscosity and dispersive characteristics.20 These viscoelastic agents are more likely to be retained in the eye.

Our study used pachymetry as an outcome measure. Future studies are needed to examine long-term outcome, particularly regarding endothelial cell loss. Some suggest that early pachymetry may not correlate well with ultimate endothelial cell loss (S.A. Arshinoff, MD, D. Calogero, MD, M. Eydelman, MD, “The Problems Associated with Viscoelastic Use in Cataract Surgery,” presented at the Symposium on Cataract, IOL and Refractive Surgery, Seattle, Washington, USA, April 1999). However, that analysis was a review of the literature and not a prospective randomized study. Nevertheless, the findings warrant consideration and additional investigations.

It has been of great interest to surgeons and pharmaceutical companies whether one viscoelastic substance affords superior endothelial protection during cataract surgery. Every viscoelastic material has advantages and disadvantages; however, there has been no general consensus within the ophthalmic community that one product outperforms the rest.

In our study, patients with a BCVA of 20/40 or better 1 day postoperatively had significantly thinner corneas (596 μm) than those with a BCVA worse than 20/40 (639 μm). However, we found no differences in postoperative BCVA or percentage increase in central corneal pachymetry among the 3 viscoelastic agents used. Although the group sizes were limited and hence very small differences may have been missed, no single viscoelastic material was overwhelmingly advantageous or disadvantageous for the outcomes measured. Thus, in a randomized study with a single surgeon using the same technique of phacoemulsification, all 3 viscoelastic agents were comparable in their ability to produce clear corneas and good vision. This suggests that the phacoemulsification surgeon may select a viscoelastic material based on a preference for its handling characteristics without compromising the clinical outcome.

References

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© 2000 by Lippincott Williams & Wilkins, Inc.