Ординатура / Офтальмология / Английские материалы / Modern Cataract Surgery_Kohnen_2002

Table 3. Parameters for OVD removal

fantastic visibility of the capsule. When the Vision Blue is injected below the viscoadaptive, it is merely painted along the capsular surface in a very thin layer, sufficient to dye the capsule. Since only a very thin layer of the Vision Blue® is injected with the ultimate soft shell technique, there is no need to wash it out before performing the capsulorhexis, and the procedure is very easy.

Removal

Viscoadaptives are indeed different, and they are designed to fracture. Therefore, if insufficient care is taken, it is easy to leave OVD behind at the end of the case, which may result in a postoperative spike in intraocular pressure. Pharmacia has sponsored a lot of work to determine optimal removal techniques. As often turns out, our understanding of removal of all OVDs has improved.

The first point to make is that in order to remove any OVD, turbulence must be created in the AC to fracture the molecular mass. Only once the mass is fractured are pieces of the OVD free to move around and be aspirated by the irrigation/aspiration device. Therefore a high flow rate and high vacuum are necessary.

The desirable parameters of I/A were worked on by the initial Healon®5 investigators, and the consensus is shown in table 3. High flow rates (25–30 cm3 min) and vacuum (350–500 mm Hg) require an adequate bottle height (60–70 cm above the patient’s eye) to prevent AC collapse. The 0.3-mm aspiration port size was found experimentally to be most effective [K. Solomon, MD, pers. commun.].

The ‘rock ‘n’ roll’ method of OVD removal has hitherto been demonstrated to be the most efficient and effective technique for OVD removal (before the advent of viscoadaptives) [6–8]. This has been challenged by Tetz et al. [9], specifically with respect to viscoadaptive removal. Tetz et al. have found their two-compartment technique to be quicker and more effective in removing Healon®5. The studies performed for the US FDA Healon®5 submission showed that both of these techniques are superior to attempts to remove Healon®5 without using any formalized technique.

In summary, viscoadaptives are new fracturable higher viscosity cohesive OVDs that can behave in a pseudodispersive fashion in an environment of high shear stress. This allows them to emulate the behavior of either cohesive or dispersive OVDs when the BSS flow rate is altered in phacoemulsification, considerably enhancing the viscoelastic maneuvers that can be performed by the surgeon, and enabling new surgical techniques. Because of their unique behavior, specific attention must be paid to removal techniques when viscoadaptives are used.

References

1Arshinoff SA: Dispersive and cohesive viscoelastic materials in phacoemulsification revisited 1998. Ophthalmic Pract 1998;16:24–32.

2Arshinoff SA: Dispersive-cohesive viscoelastic soft shell technique. J Cataract Refract Surg 1999; 25:167–173.

3Arshinoff SA: The unusual rheology of viscoadaptives, and why it matters (video). American Society of Cataract and Refractive Surgery Film Festival, Seattle, Wash. April 1999 (video available from Pharmacia & Upjohn AB).

4Arshinoff SA: Why Healon5. The meaning of viscoadaptive. Ophthalmic Pract 1999;17:332–334.

5Arshinoff SA: The ultimate soft shell technique. Ophthalmic Pract 2000;18:289–290.

6Arshinoff SA: Rock ‘n’ roll removal of Healon GV; in Arshinoff SA (ed): Proceedings of the 7th Annual National Ophthalmic Speakers Program, Ottawa, June 1996. Medicopea 1997.

7Arshinoff SA: Rock ‘n’ roll removal of Healon GV (video). American Society of Cataract and Refractive Surgery Film Festival, Seattle, Wash. June 1996.

8Auffarth GU, Wesendahl TA, Solomon KD et al: Evaluation of different removal techniques of a high-viscosity viscoelastic. Best papers of sessions 1994 Symposium on Cataract, IOL and Refractive Surgery. J Cataract Refract Surg 1994(suppl):30–32.

9Tetz MR, Holzer MP: Two-compartment technique to remove ophthalmic viscosurgical devices. J Cataract Refract Surg 2000;26:641–643.

Steve A. Arshinoff, MD, York Finch Eye Associates, 2115 Finch Ave. W. #316, Toronto, Ont M3N 2V6 (Canada)

Tel. 1 416 745 6969, Fax 1 416 745 6724, E-Mail saaeyes@idirect.com

Table 1. Physical and chemical properties of the investigated viscoelastics

Patients and Methods

We performed a prospective, randomized and intraindividually comparing trial in patients with bilateral preexisting compromised endothelium (cornea guttata) in 90 patients (180 eyes). Thirty patients each were randomly assigned to Provisc®, Ocucoat®, and Healon®5. The reference viscoelastic applicated in the fellow eye was Healon®GV in all patients. The main physical and chemical properties of the four viscoelastics are summarized in table 1.

Patients included in this study had to fulfill the following criteria: (1) Comparable cataract formation in both eyes. Exclusion of advanced nuclear sclerosis. (2) Bilateral cornea guttata with 10% difference of corneal endothelial cell density. (3) Intraoperative ultrasonic time 1 min, use of 150 ml BSS irrigating fluid. (4) Bilateral surgery within 4 weeks, performed by the same surgeon. (5) In the bag phacoemulsification after capsulorhexis with IOL implantation into the capsular bag without complication (e.g. damage of the posterior capsule, vitreous loss, zonulolysis). (6) No postoperative raise of intraocular pressure beyond 24 mmHg. (7) Control examinations preoperatively, at day 1, and 1–2 weeks postoperatively including assessment of best corrected visual acuity, intraocular pressure, biomicroscopy of

Table 2. Central endothelial cell density (cells/mm2) (30 eyes, mean values of three measurements, significance level: p 0.01)

the anterior and posterior segment, mean endothelial cell density (Endothelial specular microscope EM-1100, Tomey), anterior chamber flare (Laser flare meter, KOWA FM-500), and central corneal thickness (Teknar pachymeter). Mean values based on 3 (endothelial cell density), 5 (flare) and 10 (pachymetry) measurements.

The statistical evaluation was performed using the Wilcoxon and t tests with a level of significance of 0.01. Out of the 30 patients of the Healon®GV vs. Healon®5 group, 21 were re-examined after 5 months.

Results

The results of central endothelial cell density assessment in the three groups (Healon®GV vs. Provisc®; Healon®GV vs. Ocucoat®; Healon®GV vs. Healon®5) are shown in table 2. Table 3 demonstrates the pachymetric data of central corneal thickness in the three groups.

The results did not show a statistically significant difference between both eyes concerning improvement of visual acuity, intraocular pressure, and anterior chamber flare. Corneal endothelial cell density revealed a postoperative reduction compared to the preoperative findings, but without statistically significant difference between both eyes for all viscoelastics investigated.

The only highly significant difference was found concerning the pachymetric values: The eyes which had received Provisc® demonstrated a marked increase of central corneal thickness compared to the fellow eyes with Healon®GV at day 1 postoperatively (p 0.0002) and 1–2 weeks after surgery (p 0.007).

Table 3. Central corneal thickness ( m) (30 eyes, mean values of ten measurements, significance level p 0.01)

Additionally, two eyes with transient corneal decompensation belonged to the Provisc® group.

Discussion

Viscoelastics have been proved to protect the corneal endothelium from surgical trauma: Fry et al. [5] compared Healon®GV with Healon® in extracapsular cataract surgery, Arshinoff et al. [6] performed a randomized study using Microvisc® and Healon®. Alpar et al. [7] compared Viscoat® and Healon®. Further comparative studies investigated the corneal protection of Ocucoat®, Viscoat®, and Healon® [2] and the effect of Provisc® compared to Healon® [3]. More recent studies investigated the application of Healon®5 [8–10]. While Viscoat® proved to be superior to Healon® [11, 13], most of the studies were unable to demonstrate a statistically significant difference in postoperative endothelial cell loss or corneal thickness.

Furthermore, some clinical studies showed inconsistent results: Lane et al. [2] did not find a difference in endothelial protection between an HPMC product (Ocucoat®) and a hyaluronate (Healon®). In contrast, the assessment of postoperative corneal thickness by Pederson [12] revealed a superior corneal protection by Healon® compared to a HPMC product. Ravalico et al. [13] found less increased corneal thickness and endothelial permeability when using Healon®GV or Viscoat® compared to Healon® and Hymecel®.

The influence of the surgical technique on endothelial impairment during cataract surgery was shown in a clinical investigation comparing Healon® vs. Viscoat® by Koch et al. [11]: In eyes receiving Viscoat®, iris-plane phacoemulsification resulted in significantly less corneal endothelial damage compared to the eyes which received Healon®. Eyes with posterior-chamber phacoemulsification, conceivably with less surgical trauma, did not reveal different results, regardless the viscoelastic used.

Ravalico et al. [13] investigated four viscoelastics (Healon®, Healon®GV, Viscoat®, Hymecel®) in a prospective, randomized study. One major result was that, despite the lack of postoperative differences in endothelial cell density, significant differences in functional endothelial impairment could be detected by the assessment of central corneal thickness and the endothelial permeability coefficient. These findings indicate that comparative clinical investigations of viscoelastics should be addressed more to the functional impairment of the corneal endothelium than to postoperative endothelial cell loss.

To improve the sensitivity of a clinical study investigating the endothelial protection of different viscoelastics during cataract surgery, we chose an intraindividual comparative study design in patients with bilateral cornea guttata, that means eyes prone to endothelial decompensation.

Additionally, strong entrance criteria and comparable surgical procedures should minimize the risk of bias. Despite the very sensitive study design most of the parameters investigated did not reveal statistically significant differences between the four viscoelastics tested, though the hyaluronates (Provisc®, Healon®GV, Healon®5) have markedly different molecular weights and viscosity, and Ocucoat® as an HPCM product has completely different chemical and physical properties.

The only statistically highly significant difference in our study was found comparing Provisc® with Healon®GV: The central corneal thickness in eyes which received Provisc® was increased not only at day 1 (p 0.0002) but also 1–2 weeks after surgery (p 0.007). Two transient clinical corneal decompensations also occurred in the Provisc® group only.

Our results confirm the observation of Ravalico et al. [13] that a transient functional impairment of the corneal endothelium may be present without a corresponding reduction of endothelial cell density.

Arshinoff and Hoffman [6] demonstrated that the corneal thickness reached the preoperative values at about 1 week after surgery. In our patients the corneal thickness was still increased in all patients 1–2 weeks after surgery. Out of the 30 patients in the Healon®GV vs. Healon®5 group, 21 were re-examined after 5 months. The central corneal thickness had reached the preoperative value. The endothelial cell density showed a reduction of 2% (Healon®) and 4% (Healon®GV). Similar results were reported by Schwenn et al. [10] comparing

Healon®5 with Viscoat®. Our results are not easy to explain: There is a general assumption that viscoelastics with high molecular weight and great viscosity (cohesive) have better space-maintaining properties than substances with low molecular weight and viscosity (dispersive). Conversely, low molecular viscoelastics are more likely to protect the corneal endothelium due to a better surface retention on the intraocular structures [7].

The identical endothelial protection by Ocucoat® and Healon®GV may be the result of the specific binding affinity of the hyaluronic acid to the endothelial cells [14] and its superior protection against compressive and drag forces on the corneal endothelium [13, 15], respectively the better surface retention on intraocular structures by the HPMC product Ocucoat®.

If the superior space-maintaining properties of the hyaluronates are causative for the endothelial protection during surgery, one should expect different results using hyaluronates of different viscosity and molecular weight. Our results demonstrate this effect comparing Healon®GV with Provisc® only, not in the comparison between Healon®GV and Healon®5 despite the much higher viscosity of Healon®5. That may be due to several factors: Provisc® and Healon®GV as well as Healon®5 are manufactured by different companies using different techniques, which may lead to different chemical and physical properties (e.g. osmolality, galenic composition, pH value) [14]. Furthermore, compromised eyes with cornea guttata are particularly susceptible to pH value change. Dick and Schwenn [16] measured pH values of 7.11 0.09 for Provisc® and 7.5 0.07 for Healon®GV.

The identical findings in endothelial protection of Healon®GV and Healon®5 despite the markedly higher viscosity of the latter may also be caused by the somewhat more complicated removal maneuver for Healon® 5 [9].

Conclusions

The results of our study may permit the following conclusions:

(1)The intraindividually comparative study design, phacoemulsification in eyes with compromised corneal endothelium, strong entrance criteria, and identical surgical procedures are able to reveal differences between viscoelastics which are not detected using less sensitive study designs.

(2)Corneal protection may also be influenced by other factors than the basic chemical structure, viscosity and molecular weight of the viscoelastic. Therefore, each viscoelastic substance has to be investigated before clinical application.

(3)The surgeon’s choice of agents must balance several factors: The endothelial protection of HPMC products is basically not worse than by

hyaluronates. Higher viscosity of hyaluronates does not inevitably lead to better corneal protection. On the other hand stand the superior space-maintaining properties of hyaluronate products, depending on molecular weight and viscosity [4].

References

1Dick B, Pakula T, Hirschmann T, Pfeiffer N: Viskoelastica: Ein aktueller Vergleich und praktische Konsequenzen; in Dunker G, Ohrloff C, Wilhelm F (eds): 12. Kongress der Deutschsprachigen Gesellschaft für Intraokularlinsen-Implantation und refraktive Chirurgie. Berlin, Springer, 1999, pp 267–277.

2Lane S, Naylor D, Kullerstand L, Knauth K, Lindstrom R: Prospective comparison of the effects of Ocucoat, Viscoat and Healon on intraocular pressure and endothelial cell loss. J Cataract Refract Surg 1991;17:21–26.

3Lehmann R, Brint S, Stewart R, White G, McCarty G, Taylor R, Disbrow D, Defaller J: Clinical comparison of Provisc and Healon in cataract surgery. J Cataract Refract Surg 1995;21:543–547.

4Hütz W, Eckhardt B, Kohnen T: Comparison of viscoelastic substances used in phacoemulsification. J Cataract Refract Surg 1996;22:955–959.

6Arshinoff S, Hofmann I: Prospective, randomized trial of Microvisc and Healon in routine phacoemulsification. J Cataract Refract Surg 1997;23:761–765.

7Alpar J, Alpar A, Baca J, Chapman D: Comparison of Healon and Viscoat in cataract extraction and intraocular lens implantation. Ophthalmic Surg 1988;9:636–642.

8Rainer G, Menapace R, Findl O, Georgopoulos M, Kiss B, Petternel V: Intraocular pressure after small incision cataract surgery with Healon 5 and Viscoat. J Cataract Refract Surg 2000;26:271–276.

9Auffarth G, Dick B, Vissesook N, Völcker H: Entfernungstechniken eines neuen viskoadaptiven Viskoelastikums (Healon®5); in Dunker G, Ohrloff C, Wilhelm F (eds): 12. Kongress der Deutschsprachigen Gesellschaft für Intraokularlinsen-Implantation und refraktive Chirurgie. Berlin, Springer, 1999, pp 478–481.

10Schwenn O, Dick B, Krummenauer F, Christmann S, Vogel A, Pfeiffer N: Healon 5 versus Viscoat during cataract surgery: Intraocular pressure, laser flare and corneal changes. Graefes Arch Clin Exp Ophthalmol 2000;238:861–867.

11Koch D, Liu J, Glasser D, Merlin L, Haft E: A comparison of corneal endothelial changes after use of Healon or Viscoat during phacoemulsification. Am J Ophthalmol 1993;115:188–201.

12Pederson O: Comparsion of the protective effects of methylcellulose and sodium hyaluronate on corneal swelling following phacoemulsification of senile cataracts. J Cataract Refract Surg 1990; 16:594–596.

13Ravalico G, Tognetto D, Baccara F, Lovisato A: Corneal endothelial protection by different viscoelastics during phacoemulsification. J Cataract Refract Surg 1997;23:433– 439.

14Hessemer V, Dick B: Viskoelastische Substanzen in der Kataraktchirurgie, Grundlagen und aktuelle Übersicht. Klin Monatsbl Augenheilkd 1996;209:55–61.

15Stenevi U, Gwin T, Härfstrand A, Apple D: Demonstration of hyaluronic acid binding to corneal endothelial cells in human eye-bank eyes. Eur J Implant Refract Surg 1993;5:228–232.

16Dick B, Schwenn O: Viskoelastika: Eine Übersicht. Berlin, Springer, 1998.

Prof. Dr. Ulrich Mester, Department of Ophthalmology, Bundesknappschaft’s Hospital Sulzbach, D–66280 Sulzbach (Germany) E-Mail augen@kk-sulzbach.de