Ординатура / Офтальмология / Английские материалы / Mastering theTechniques of Lens Based Refractive Surgery (Phakic IOLs)_Garg, Alio, Dementiev_2005

Figure 7.7: Vision membrane lens

maintain a constant thinness in the range of 200 to 300 μm for all refractive powers. The diffractive optics of the Vision Membrane provides a portion of the bifocal correction by means of an increased depth of field. As a result, an eye implanted with the IOL should achieve distance and near visual function with high contrast at each distance.

2.Kelman Duet phakic IOL: It is a two-piece phakic IOL. The PMMA haptic is first snaked through a 1.5 mm incision.The silicon optic is then compressed and inserted. Once the optic unfolds in the anterior chamber the two tabs on either side of the optic are snapped into projections on the haptic. The main advantage of this lens is that the optic can be exchanged with a new one if a patient’s refraction changes.

Determination of Anterior Chamber and Posterior Chamber Lens Sizes

Selection of appropriately sized phakic IOL is very essential in case of angle-fixated and posterior chamber lenses. The size of the lens is determined based on the horizontal white-to-white diameter of the eye or Horizontal visible Iris diameter (HVID).

Complications of Angle Fixated Phakic IOLs

•Glare

•Sizing error

•Pupil ovalization due to pressure necrosis and fibrosis of the peripheral iris

•Pigment loss, rubeosis iridis

•Endothelial cell loss-Progressive loss of endothelium has been found in various studies

•Low-grade postoperative inflammation.

Looking to the future, the ideal anterior chamber phakic IOL is the one with following features: Foldable IOL thereby avoiding surgically induced astigmatism, made of biocompatible material, and without long-term potential for endothelial cell loss, pupil ovalization, or chronic inflammation.

IRIS FIXATED PHAKIC IOL-VERISYSE

PHAKIC IOL

Verisyse phakic IOL is by far the most commonly used phakic IOL. It has been marketed by AMO. It is also marketed by Ophtec as the Artisan phakic IOL.

•Construction: One-piece design

•Material: Perspex CQ UV (Polymethylmethacrylate)

•Refractive index: 1.49

•Overall length of the lens=8.5 mm

•Available in a wide range of powers

•Myopia

5.0mm = –3.0D to –23.5D

6.0mm = –3.0D to –15.5D

•Hyperopia

5.0mm = +1.0D to +12.0D

Preop Assessment for Verisyse Phakic IOL

Following procedures are required for phakic IOL implantation.

•Refraction-Objective and Subjective

•K-readings and Topography-Orbscan-II

•Anterior chamber depth-From epithelium

•Intraocular pressure

•Anterior and posterior segment examinations

•Specular microscopy

Verisyse Phakic IOL Power Calculation

The calculation of the Verisyse phakic IOL power is based on the Van Der Heijde’s formula. The requirements for Verisyse phakic IOL power calculation using this formula are 1. Preoperative refractive error (spherical equivalent) at spectacle plane 12.0 mm in front of the cornea.

2. Corrected AC depth. The anterior chamber depth in Van der Heijde’s formula is the distance between the anterior corneal surface and the cardinal plane of the IOL. The cardinal plane for all IOL powers lies within the center part of the IOL. This is always 0.13 mm thick and, therefore, for all practical purposes the cardinal plane is identical with the anterior surface of the IOL. This surface is approximately 0.8 mm in front of the natural lens in myopic eyes and 0.6 mm in front of the natural lens in hyperopic eyes. Hence, in myopic eyes the corrected AC depth is calculated by subtracting 0.8 mm from the patient’s AC depth from the epithelium and in hyperopic eyes by subtracting 0.6 mm from the patient’s AC depth from the epithelium. 3. Average K reading.

n =Refractive index of aqueous(1.336)

k =Keratometric value of the cornea(Diopters)

Ps = Equivalent spectacle power of the corneal plane (Diopters)

d = Distance between the IOL plane and corneal plane (meters)

A refractive surgeon may also use the Van der Heijde’s table which is based on the corneal curvature and anterior chamber depth to get the phakic IOL power needed to correct a specific myopia as defined by the correcting glass in the spectacle plane.

Instruments Required for Implanting Verisyse Phakic IOL

Following instruments are required for the implantation of Verisyse phakic IOL.

Instruments for Scleral Tunnel and Side Ports

•No.15 BP blade

•Crescent blade

•Keratome

Instruments for Phakic IOL Implantation

•Implantation forceps

•Verisyse IOL manipulator

•Enclavation needle

•Enclavation forceps

Instruments for Verisyse Toric Phakic IOL

•Astigmatic axis marker

•Axis verification ring

Implantation Forceps

This instrument provides a firm grip of the phakic IOL by allowing the surgeon to hold the IOL beyond the edge and also prevents scratches and damage to the phakic IOL. This is used to hold the lens while insertion and also during enclavation.

Figure 7.15: Implantation forceps

Verisyse IOL Manipulator

Figure 7.17: Enclavation needle

Enclavation Forceps

This instrument can be used for enclavating the IOL when the surgeon finds it difficult to enclavate with the needle.

Figure 7.18: Enclavation forceps

Surgical Procedure for Verisyse Phakic IOL

Preoperative miosis is obtained with Pilocar 2 percent drops.

Anesthesia

Topical Xylocaine 4 percent drops and local infiltration of Xylocaine 2 percent along with Intracameral Xylocard 1 percent are used for anesthetizing the eye.

Note: Peribulbar and retrobulbar anesthesia are not recommended, as the risk of globe perforation is very high in these severely myopic eyes. Parabulbar/general anesthesia may be used as alternatives.

This instrument has a hook and a notch and is used for manipulating the lens into the horizontal position by pushing and pulling on the haptics.

Figure 7.16: Verisyse IOL manipulator

Enclavation Needle

This has a right and a left needle for enclavating on right and left sides of the phakic IOL respectively.

Postoperative Assessment

All the patients are to be assessed at first day, first week, 15 days, 1 month, 3 months, one yearly check ups. Following entities have to be observed and recorded.

•Slit lamp examination for IOL position and enclavation

•IOP

•UCVA, BCVA, Residual refractive error

•Contrast sensitivity

•Specular microscopy

Difficulties and Complications

Following complications are common to all phakic IOLs.

Figure 7.19A: Step 1: Making of a scleral tunnel incision (5.0-6.0 mm) and a side port

Figure 7.19C: Step 3: Intracameral injection of Xylocard 1 percent, Carbachol and Healon5

Figure 7.19B: Step 2: Making of the second side port incision

Figure 7.19D: Step 4: Anterior chamber entry with a keratome

Figure 7.19G: Step 7: Rotation and centration of the IOL

Figure 7.19I: Step 9: Enclavation on the other side

Intraoperative complications

•Hyphema

•Iridodialysis

•Difficult enclavation

•Haptic breakage/lens tear

•Lenticular damage.

Early postoperative period

•Wound leak and shallow AC

•Corneal edema

•Iridocyclitis

•Pupillary block and iris bombé

Figure 7.19H: Step 8: Enclavation of the IOL using the enclavating needle

Figure 7.19J: Step10: Snip iridotomy at 12 o’clock position

Figure 7.20: A well centered Verisyse phakic IOL

Figure 7.21: Mild, acceptable decentration of IOL

•Secondary glaucoma

•Decentration/Dislocation.

Late postoperative period

•Unacceptable residual refractive error

•Induced astigmatism

•Loss of BCVA

•Corneal edema, Bullous keratopathy

•Rubeosis iridis

•Cataract

•CME, CNVM,

•Retinal detachment.

Secondary Glaucoma

The IOP has to be measured at each visit of the patient. There are various causes for the raised IOP during the postoperative period. They are:

1.Inadequate removal of the viscoelastic: This is usually seen during the immediate postoperative period (1-3 days). The patient has to be put on antiglaucoma medications (Timolol maleate 0.5 percent drops and Acetazolamide tablets) for a short period.

2.Pupillary block: This can be prevented by an intraoperative snip iridotomy or a preoperative YAG Iridotomy.

3.Steroid induced glaucoma: Steroid responsiveness in these young, highly myopic patients may be seen as early as 4 to 5 days and hence judicious use of topical steroid drops is advocated.

4.Pigmentary glaucoma

5.Rubeotic glaucoma

Endothelial Cell Loss

A progressive loss of endothelial cells has been noted following the implantation of phakic IOLs. The various causes for the endothelial loss are:

1.Intraoperative: The use of a good Viscoelastic minimizes the intraoperative damage to the endothelium.

2.Early postoperative: Wound leak and a shallow AC in the early postoperative period leads to IOLendothelial touch and endothelial cell loss.

3.Late postoperative period: Endothelial cell loss in the late postoperative period is due to the IOL-endothelial touch and is caused by a shallow AC, thicker IOLs of higher powers, rubbing of eyes and contact specular microscopy.

Difficult Enclavation

A refractive surgeon may face a slight difficulty in enclavating the phakic IOL in eyes with small HVID.

Advantages of Verisyse Phakic IOL Over

Other Phakic IOLs

•Lower rate of endothelial cell loss when compared to angle-fixated phakic IOLs

•Lower incidence of cataract when compared to posterior chamber phakic IOLs

Figure 7.22: Dislocated phakic IOL due to detached enclavation

Verisyse Toric Phakic IOL

The aim of Verisyse toric phakic IOL is the correction of total refractive error: Spherical and cylindrical (corneal and lenticular). It has the following features.

•Total diameter: 8.5 mm

•Optic diameter: 5.0 mm

•Spherical anterior and toric posterior surface

•Power: –3 to –20D, +2 to+12.0D

•Cylindrical correction: 2-7 D.

Indications for Toric Phakic IOL

•Presence of a high spherocylinder or a pure cylindrical power

•Stable keratoconus

•Postkeratoplasty cases.

Equipment Needed

Astigmatic axis marker and axis verification ring are the additional equipments required.

Figure 7.23: Axis marker (Rhein medical)

Procedure

The horizontal meridian (0 to 180 axis) is first marked using the axis marker dipped in Methylene blue in the sitting position of the patient. The axis marker has a weight in the lower portion, which always aligns the marker in the horizontal meridian irrespective of the way

Figure 7.24: Axis verification ring (Geuder, Heidelberg, Germany)

in which it is held (Due to gravity). The patient is then made to lie down and the axis of the cylinder is marked using the axis verification ring. The Verisyse toric phakic IOL is then enclavated in this axis.

Veriflex (Artiflex)

It is a foldable iris claw lens. It is a modification of Verisyse (Artisan) phakic IOL. It has the following features.

•Total diameter: 8.5 mm

•Optic diameter: 6.0 mm

•Material: Optic—silicone

Haptics—PMMA

•Power: –2.0 to –12.0D.Hyperopic and toric foldable lenses are under development.

Posterior Chamber Lenses

These phakic IOLs are placed in the posterior chamber between the iris and the crystalline lens. These are 1.Staar ICL 2 Cibavision PRL.

STAAR ICL

The STAAR Collamer ICL™ and the TORIC ICL are posterior chamber phakic intraocular lenses. Made of Collamer™, STAAR’s proprietary collagen copolymer (collagen/HEMA), the lens rests behind the iris in the ciliary sulcus.

Figure 7.25: Features of Staar ICL

The ICL is currently available in three models. – ICM for myopia, ICH for hyperopia and TICM for Toric ICL for myopia.

Refractive index-1.453

Procedure

The lens is gently folded and injected into the anterior chamber through a 3.0 mm, temporal, clear corneal incision. The ICL is then carefully positioned by manipulating the footplates of the lens posterior to the iris plane and into the sulcus. Preoperative YAG iridotomy is essential.

Complications

•ICL decentration

•Pupillary block

•Pigment dispersion

•Subcapsular cataract

PRL™

CIBA Vision’s PRL (Phakic Refractive Lens) is a foldable, posterior chamber phakic refractive lens designed to float

on the eye’s natural lens. An injector system, designed to improve consistency and ease of delivery is now available for use to facilitate the implantation of the PRL.

Advantages of Phakic IOLs Over Laser

Corrective Procedures

•A higher range of refractive errors can be corrected

•Reversible: Phakic IOL implantation is a potentially reversible procedure.

•Safe: No structural changes are induced. Hence it is safe in any eye with high error and also thin corneas.

•Better quality of vision: Quality of vision (contrast sensitivity) is better than the laser refractive

Figure 7.26: PRL myopic arched lens

Figure 7.27: PRL myopic flat lens