Hyperguides

Ophthalmology / Refractive surgery

Phakic Intraocular Lenses

Stephen D. McLeod, MD
Article


Introduction

The treatment of high refractive error presents unique challenges to refractive surgeons. In keratorefractive surgery, the anterior corneal curvature is altered to change the conjugation point of the eye. However, a high refractive error requires substantial curvature change that can be difficult to achieve in a stable, predictable fashion that preserves optical quality. Photorefractive keratectomy for the treatment of high myopia is associated with an increased risk of scarring and regression of effect,1 whereas laser in situ keratomileusis (LASIK) for high error necessitates the ablation of a significant thickness of corneal tissue that can threaten corneal integrity and lead to progressive keratectasia.2 Phakic intraocular lenses (IOLs) present an opportunity to correct high degrees of refractive error while preserving corneal integrity and avoiding the instability of a corneal wound healing effect. Since the optical correction is located closer to the entrance pupil, in theory, less object minification occurs in the correction of high myopia, and larger optic zones more effectively reduce the risk for postoperative halos compared to corneal corrections. Given these advantages, numerous IOL designs are under investigation.

Lens Designs and Surgical Considerations

IOLs were first introduced in the 1950s for the correction of aphakia. Soon thereafter, Strampelli3 designed a minus power IOL for phakic patients with extreme myopia. Barraquer also designed a one-piece PMMA plate anterior chamber angle fixated IOL for use in high myopia.4 However, a substantial complication rate was observed and 60% of these IOLs were removed as a result of endothelial damage and iritis. In the 1980s, a number of modifications were applied to earlier designs to address specific complications observed. Current designs included a porcine collagen/HAMA copolymer plate-haptic posterior chamber IOL (Staar Surgical, Monrovia, Calif.), a variety of rigid PMMA and flexible acrylic hydrophilic angle-supported anterior chamber lenses, and an iris-fixated lens (Ophtec BV, Groningen, The Netherlands). These IOLs differ by way of location and fixation within the confines of the anterior segment. Therefore, although the optical properties are similar, the technique of implantation and the incidence and nature of complications differ among these lenses.

The posterior chamber design allows a relatively simple implantation technique. The IOL is folded in an injector system and guided through a self-sealing corneal incision into the anterior chamber that has been filled with viscoelastic to protect the crystalline lens and endothelium. Both haptics are then placed beneath the iris with an intraocular hook, and the viscoelastic removed (Slide 1). The IOL resides between the anterior surface of the crystalline lens and the posterior aspect of the iris. Therefore, inadequate anterior chamber depth or a poorly sized IOL can lead to crowding and attendant complications. Early experience reported episodes of pupillary block for which routine preoperative prophylactic peripheral iridotomy is now performed. Other complications included elevated intraocular pressure (IOP), IOL decentration, pigment dispersion, and increased aqueous flare measurements indicative of chronic low grade inflammation.5 Endothelial cell loss and cataract formation are concerns with the posterior chamber design. A recent study has suggested 12% endothelial cell loss at 4 years compared to preoperative counts,6 and earlier studies have documented a progressive decrease in crystalline lens transmittance,5 as well as frank anterior subcapsular cataract.7

Slide 1

To reduce the risk of crystalline lens trauma, implants have been designed for angle support. However, potential complications of this design include pupil ovalization, chronic subclinical inflammation, elevated IOP, and pupillary block,8,9 in addition to IOL rotation and endothelial trauma with endothelial cell loss. A study of angle-supported anterior chamber phakic IOLs reported that 80% of eyes showed higher than 15° of rotation by 2 years, and 60% had rotated between years 1 and 2. By the end of the second year, a 12% endothelial cell loss had been documented.8 These results raise questions with regard to the long-term stability and safety of this design.

An alternative to angle support for stability in the anterior chamber is iris fixation as seen in the Artisan design (Ophtec BV, Groningen, The Netherlands). The Artisan has two pincer-like haptics through which a small knuckle of peripheral iris is drawn during implantation (Slide 2). These knuckles of iris secure the lens so that the optic lies immediately anterior to the plane of the iris. The implantation technique requires some degree of expertise. The pupil is constricted pharmacologically, and viscoelastic material introduced to the anterior chamber. A scleral tunnel, limbal incision or corneal incision is made that is approximately equal to the lens optic diameter. Two stab incisions are made on either side of the main incision. The lens is inserted into the anterior chamber and rotated 90° so that the axis of the lens lies perpendicular to the direction of insertion. The optic of the lens is then grasped with a forceps, and a small knuckle of iris is drawn through the pincer of each haptic with a disposable needle. A peripheral iridectomy or iridotomy is performed, the viscoelastic is removed, and the wound is closed with sutures.

Slide 2

Early postoperative complications with the iris-fixated lens design and procedure include anterior chamber cells and flare, corneal edema, hyphema, and pupil irregularity. Late complications include glare and halos associated with the optic edge.10 Nonprogressive cataractous change associated with intraoperative trauma has also been observed, and 4 years after surgery endothelial cell counts suggest a progressive loss of up to 13% that is most likely related to intraoperative trauma.11 These events suggest that long-term data will be necessary to fully evaluate the relative merits of this design.

Despite the surgical challenges, the optical advantages of phakic lens correction in the setting of high ametropia warrant further investigation and development. Whereas LASIK correction of high refractive error is associated with decreased optical quality and an elevated risk for loss of best spectacle corrected visual acuity and other measures of visual function,12,13 the optical quality of the refractive lens is similar to that of the normal pseudophakic lens. Also, the refractive lens is less likely to produce corneal irregularity that increases aberration. Recently, a study was performed that involved a prospective, randomized comparison of LASIK and refractive lens implantation in which one eye of each patient received LASIK and the other eye a phakic IOL. The investigators found a lower incidence of reduced best spectacle corrected visual acuity and superior best-corrected visual acuity and subjective evaluation of quality of vision in the eye implanted with a phakic IOL.14 Long-term data will allow better evaluation of the risk:benefit profile of these devices.

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