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Ultrasound biomicroscopy of ZSAL-4 anterior chamber phakic intraocular lens for high myopia

Jiménez-Alfaro, Ignacio MD1,∗,a; García-Feijoó, Julián MD1,b; Pérez-Santonja, Juan J MD1,c; Cuiña, Ricardo MD1,b

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Journal of Cataract & Refractive Surgery: October 2001 - Volume 27 - Issue 10 - p 1567-1573
doi: 10.1016/S0886-3350(01)00964-6
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Abstract

There are several potential advantages of using phakic intraocular lenses (IOLs) to correct high myopia. These include excellent refractive results, fast visual recovery, preservation of accommodation, familiar surgical technique, reversibility, and combination with corneal refractive procedures to treat severe myopia.1–5 However, the procedure raises questions about the long-term potential risks to the corneal endothelium, lens, anterior uvea, and other ocular structures.3–7

In 1987, Joly and coauthors8 modified the Kelman Multiflex IOL used in cataract surgery into a negative biconcave lens for the correction of high myopia. This first-generation phakic IOL, called the Baikoff ZB lens, was associated with high endothelial cell loss, probably caused by excessive contact between the edge of the IOL optic and adjacent endothelium.9,10 Because of the endothelial problems, Baikoff modified the design to reduce the IOL's proximity to the cornea. The new design, the ZB5M lens, gained 0.4 mm in lens-cornea spacing in comparison with the first model,5,9,11 and recent clinical studies12,13 have shown less long-term endothelial cell loss with this lens.

In 1991, to avoid the complications related to the Baikoff lenses (ZB and ZB5M), Pérez-Santonja and coauthors14 developed a model of angle-supported anterior chamber phakic IOLs, and in 1994, they developed their fourth generation lens, the ZSAL-4 refractive lens (Morcher GMBH). Less endothelial cell loss has been reported with the ZSAL-4 lens.

Ultrasound biomicroscopy (UBM) is a high-resolution technique that allows examination of the anterior segment and peripheral retina.15–17 Its resolution provides a unique method to test the exact IOL location as well as the IOL's relationship to adjacent intraocular structures including the cornea and iris.11,18–20 Moreover, as UBM provides reproducible measurements, it can be used to measure distances between the IOLs and these structures.21,22

This study analyzed the anatomic relationships between the ZSAL-4 anterior chamber phakic IOL and anterior segment structures using high-frequency UBM.

Patients and methods

Patients

This retrospective study examined 8 eyes of 6 patients in whom the ZSAL-4 angle-supported phakic IOL was implanted to correct severe myopia by the same surgeon (I.J.-A.) between 1995 and 1999 at the Fundación Jiménez Díaz in Madrid, Spain.

The patient selection criteria were age greater than 22 years, best spectacle-corrected visual acuity of at least 0.1 (20/200), stable myopia greater than −14.00 diopters (D), contact lens intolerance, normal anterior segment with an anterior chamber depth greater than 3.2 mm, endothelial cell density greater than 2500 cell/mm2, normal peripheral retina, and no general health problems. The risks of this operation were explained to the patients, and an informed written consent was obtained from each one.

The basic preoperative and postoperative examinations included visual acuity, manifest and cyclopegic refractions, slitlamp microscope examination, applanation tonometry, and indirect ophthalmoscopy. Postoperative examinations were conducted at 1, 3, 6, and 12 months and then once a year. All patients were followed for a minimum of 12 months.

ZSAL-4 Angle-Supported Anterior Chamber Phakic IOL

The planoconcave ZSAL-4 phakic refractive lens (Morcher GMBH) is made of single-piece poly(methyl methacrylate) (PMMA) with Z-shaped haptics derived from the Kelman Multiflex lens (Figure 1). The total optical zone is 5.50 mm with an effective optical zone of 5.00 mm to decrease halos and night-vision problems. The optic has a 3-sided edge design that creates a transitional zone to reduce refracted glare. The haptic angulation is 19 degrees, which provides a central clearance from the crystalline lens of approximately 0.80 mm. The lens also has long Z-shaped haptics to increase haptic flexibility and to decrease compression forces against angle structures.14 The overall length of the lens is 12.50 or 13.00 mm, and the lens power ranges from −6.00 to −20.00 D in 1.00 D steps.

Figure 1.
Figure 1.:
(Jiménez-Alfaro) The ZSAL-4 phakic refractive IOL.

Intraocular lens power was calculated using the van der Heijde's formula,23 and the lens length was selected by adding 1.0 mm to the horizontal white-to-white distance.

Ultrasound Biomicroscopy

Ultrasound biomicroscopy was performed with the Humphrey ultrasound biomicroscope model UBM 840 (Zeiss-Humphrey Instruments). The system uses a 50 MHz transducer, which has a 50 μm axial and lateral resolution. The examinations were performed under standard illumination and accommodation conditions using an articulated arm with a fixation artifact. The same examiner (J.G.F.) performed all the echographic examinations under topical anesthesia (tetracaine 0.5%) using an orbital cup and a lid speculum with the technique described by García-Feijoo and coauthors,24 which allows the patient significant freedom of eye movement.

Hourly radial and transverse sections were taken, providing 360-degree coverage of the anterior chamber. To perform the central measurements, a section of the anterior chamber through the corneal apex and centered on the pupil that included the IOL and reached the anterior lens surface was obtained. All the scans were digitally recorded for later analysis.

The distance between the corneal endothelium and the IOL at the center and the peripheral zone and between the iris and the IOL was measured in each eye. All measurements were done with the calipers provided by the manufacturer using a 2-dimensional image recorded by the ultrasound biomicroscope. The distance between the corneal endothelium and the IOL was measured with a perpendicular cursor (line) passed through the highest part of the cornea and the center of the IOL optic. The distance between the corneal endothelium and the edge of the IOL optic was obtained by placing the first tip of the cursor on the optic edge and the second tip on the endothelium to get the shortest distance (Figure 2). This measurement was repeated in all anterior chamber quadrants (superior, inferior, nasal, and temporal). In a similar way, the distance between the optic edge and the anterior surface of the iris was measured. All measurements were performed with a constant ultrasound propagation rate of 1530 m/s.25 The distance between the IOL optic and the crystalline lens was not measured because PMMA produces numerous artifacts in the interfaces, which makes it difficult to identify the anterior surface of the crystalline lens and could lead to erroneous measurements.

Figure 2.
Figure 2.:
(Jiménez-Alfaro) Reconstruction of 3 consecutive radial scans. Note the distance between the ZSAL and the endothelium (arrows). AC = anterior chamber.

At each examination, the IOL axis was verified and the location of the haptics and the anatomic relationship among the IOL footplates, the angle, and the iris examined.

Results

The mean age of the 6 patients was 35.8 years ± 4.4 (SD) (range 29 to 41 years). Five of the 8 eyes were in 4 men, and 3 were in 2 women. Four patients were implanted with the ZSAL-4 refractive lens in 1 eye and 2, in both eyes. The preoperative mean spherical equivalent was −18.93 ± 2.62 D (range −14.00 to −22.00 D).

The mean power of the phakic IOL was −17.50 ± 1.85 D (range −14.00 to −19.00 D). Three lenses had an overall length of 13.0 mm and 5, 12.5 mm. Postoperatively, the mean spherical equivalent was −1.31 ± 0.70 D (range plano to −2.00 D).

Table 1 shows the clinical data for all eyes. Tables 2 and 3 show the mean IOL-cornea and IOL-iris distances for each quadrant, respectively. The mean distance between the central cornea and the IOL was 2361.37 ± 109.62 μm; the mean distance between the peripheral cornea and the edge of the IOL optic, 1646.24 ± 27.06 μm; and the mean distance between the IOL and the iris, 354.46 ± 41.61 μm.

Table 1
Table 1:
Clinical data and distance between the ZSAL-4 IOL and the peripheral and central endothelium, measured by UBM, in all eyes (N = 8).
Table 2
Table 2:
Distance (microns) between the ZSAL-4 optic edge and the peripheral and central endothelium (N = 8).
Table 3
Table 3:
Distance (microns) between the ZSAL-4 lens optic edge and the anterior surface of the iris (N = 8).

The IOL footplates were correctly positioned in the angle in all eyes. No goniosynechias were observed; however, there was a discrete posterior indentation of the iris in an area of contact with the haptics in all eyes (Figure 3). Although the phakic IOL was always placed along the horizontal meridian (±10 degrees), after surgery the IOL axis remained in the horizontal meridian (±10 degrees) in only 4 eyes; it rotated 30 degrees in 2 eyes, 45 degrees in 1 eye, and 60 degrees in 1 eye. A discrete, nonprogressive pupillary ovalization was observed in 2 eyes.

Figure 3.
Figure 3.:
(Jiménez-Alfaro) Indentation of the iris (arrows) produced by the IOL haptic. AC = anterior chamber.

Discussion

Renewed interest in the implantation of phakic anterior chamber IOLs in highly myopic eyes has been generated in recent years because this method obtains better optical results in the correction of high myopia than other procedures.1–5 However, the procedure raises questions concerning the long-term potential risks to the corneal endothelium and other anterior segment structures.6,7,9,26

Ultrasound biomicroscopy is an ideal method to visualize structures that are impossible or difficult to study in living eyes. Its resolution and ability to produce images of the anterior and posterior chambers, the angle, and the sulcus provide a unique method for testing the exact location of a phakic IOL and its relationship to adjacent intraocular structures.11,18–20 Ultrasound biomicroscopy also provides reproducible intraocular measurements, so it can also be used to measure distances between the IOL and ocular structures.21,22 Therefore, UBM constitutes an essential exploratory technique for evaluating aspects related to the safety of phakic IOLs.

One of the main aspects of phakic anterior chamber IOL safety is corneal endothelial damage. Endothelial cell loss after implantation of the Baikoff ZB angle-supported IOL range from 16.0% to 18.8% at 1 year and from 20.0% to 28.0% at 2 years.5,9,27–29 This lens has a 4.50 mm diameter biconcave optic and 25-degree angulated haptics. The ZB lens is placed 1.12 mm anterior to the crystalline lens; the distance between the edge of the IOL optic and the endothelium is approximately 1.16 mm.9 The high endothelial cell loss with the ZB IOL is related to intermittent contact between the optic edge and the endothelium when the patient rubs his or her eyes.5,10 Because of these endothelial problems, Baikoff modified the lens design to move the optic edge away from the endothelium and reduce the possibility of peripheral contact between them. In the new design, the ZB5M lens, the optic edge was thinned, the effective optical diameter was reduced from 4.50 mm to 4.00 mm (total optical diameter of 5.00 mm), and the haptic angulation was decreased from 25 degrees to 20 degrees.12 Thus, this lens is placed 0.59 mm anterior to the natural lens, and the distance between the edge of the IOL optic and the peripheral endothelium is 1.56 mm.11 Therefore, the ZB5M lens gains 0.40 mm in lens-cornea space compared with the ZB model and the temporary IOL-endothelial contact induced by eye rubbing may be less probable.

Recent clinical studies report endothelial cell loss ranging from 4.5% to 5.5% at 1 year, 5.6% to 6.8% at 2 years, and 5.5% to 7.5% at 3 years in eyes with the ZB5M phakic IOL.12,13 Most of the reduction in endothelial cells was not from the postoperative effects of the ZB5M lens but could be attributed to the acute effects of surgery.12 When the various rates of endothelial cell loss after ZB and ZB5M lens implantation and the various distances between the optic edge and the endothelium are taken into account, it appears that the optic−endothelium distance plays an important role in corneal endothelial damage in eyes with angle-supported phakic lenses, as previously reported.9,10,28 This parameter should be provided by the manufacturer for all available lenses.

In this study, the mean distance between the center of the ZSAL-4 lens and the central endothelium was 2.361 mm and the mean distance between the optic edge and the peripheral endothelium, 1.646 mm. The mean distance between the IOL edge and the peripheral endothelium was greater than 1.50 mm in all quadrants. The lens-cornea space with the ZSAL-4 lens was greater than that reported for the ZB5M lens despite its larger optical zone. The possibility of peripheral contact between the IOL and the endothelium from eye rubbing would be less with the ZSAL-4 phakic lens than with the ZB5M lens.

Pérez-Santonja and coauthors14 report an endothelial cell loss of 3.50% at 1 year and 4.18% at 2 years after implantation of ZSAL-4 phakic IOLs. In addition, endothelial cell density was stable 12 months after surgery. The authors suggest that the surgical trauma itself appeared to be the main cause of endothelial cell loss after the implantation of these lenses, since most of the reduction observed in endothelial density took place during the first year.30–32 Thus, the possibility that there was intermittent contact between the IOL edge and the midperipheral cornea, principally when the patient rubbed his or her eyes, would play a minor role, if any, in the mechanism of endothelial damage with the ZSAL-4 phakic lens. The results from our study support this statement.

We did not evaluate the distance between the IOL posterior surface and the crystalline lens because of ultrasound artifacts behind the IOL. Although it is possible to differentiate between these echographic artifacts and the anterior surface of the crystalline lens, we do not think the position of the latter would be correctly represented; because of the artifacts, it might appear to be closer than it really is (Figures 4 and 5). In fact, in many eyes the ultrasound image from the anterior surface of the crystalline lens is seen behind the IOL but anterior to the iris surface. We think the only way to determine the real distance between the IOL and the anterior surface of the crystalline lens is by subtracting the anterior chamber depth UBM measurements before and after surgery.

Figure 4.
Figure 4.:
(Jiménez-Alfaro) Reconstruction of 2 consecutive radial scans. The artifactual elevation of the anterior surface of the crystalline lens (arrows) can be seen. AC = anterior chamber.
Figure 5.
Figure 5.:
(Jiménez-Alfaro) The artifactual elevation of the lens can be seen better in this scan of a ZB5M IOL. Direct measure of the IOL−crystalline lens distance in these kinds of scans produces inaccurate measurements. IR = iris; AC = anterior chamber.

In all eyes, the IOL footplates were correctly positioned in the angle, but UBM images showed a discrete posterior indentation of the iris in areas of contact with the haptics. Moreover, a discrete and nonprogressive pupillary ovalization was observed in 2 eyes. Although eyes with IOLs that exhibit zones of iris indentation may be more predisposed to pupillary ovalization, this statement requires further investigation.

High-frequency UBM of the anterior segment appears to be a valuable tool for evaluating the anatomic relationships between phakic IOLs and anterior segment structures. The long-term potential risks to the corneal endothelium after phakic IOL implantation have been a concern for a long time. Our study showed that the ZSAL-4 angle-supported phakic IOL was 1.64 mm away from the peripheral endothelium and that this distance appeared to be sufficient to prevent corneal endothelial damage caused by the contact between the optic edge and the endothelium from eye rubbing. Nevertheless, other causes of endothelial cell loss, such as subclinical inflammation or endothelial damage due to the angle fixation, could exist.

To prevent endothelial, iris, and pupillary complications after phakic IOL implantation, the best IOL profile and correct positioning and sizing of the lens must be obtained and the anatomic relationships between the phakic IOL and anterior segment structures must be taken into account. With these goals in mind, the ZSAL-4 phakic lens has been redesigned and a fifth- generation lens (ZSAL-4/Plus phakic lens) with a larger optic diameter and a new haptic geometry to increase haptic flexibility and disperse compression forces against angle structures has been created.14

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