Introduction

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The aim of refractive surgery is to modify the refractive power of the eye in a permanent and stable way. As we all know there are two main refractive structures in the eye: the cornea and the natural lens. Changing the shape and thickness of the cornea as in laser corneal surgery, or exchanging the natural lens by an intraocular lens (IOL) as in refractive clear lens or cataract surgery, allows us to achieve the goal of refractive surgery. However there is also another possibility: the introduction into the eye of a new refractive surface without touching the cornea or the natural lens. This is the concept of a phakic IOL. Phakic IOLs have been used for more than 20 years and have become an indispensable tool in refractive surgery.

Fundamental principles

Phakic IOLs have several advantages over other types of refractive surgery in high ametropias. Unlike laser corneal surgery, they are not associated with regression (no wound healing) and do not change the normal shape of the cornea (better quality of vision)1. Also phakic IOLs do not interfere with accommodation, presenting in young patients important advantages over lens surgery.

Indications for surgery, anatomical considerations, and preoperative assessment

The general indications for phakic IOLs are:

Although these are general indications for phakic IOLs, in some special circumstances these criteria can be slightly adapted to a particular case. In myopia, phakic IOLs may be the procedure of choice in lower degrees if the cornea is too thin (less than 480 µm) or some form of ectacic disorder (even subclinical) is present. Very steep corneas are not suitable for laser treatment in hyperopia and may be more correctly addressed with intraocular surgery. Concerning age, phakic IOLs may be also eventually used in anisometropia in children2. Above the age of 50, as accommodation is waning, the use of phakic IOLs is generally discontinued. Intraocular pathology like diabetic retinopathy, uncontrolled glaucoma, and any form of uveitis are considered contraindications to the implantation of phakic IOLs. On the other hand, corneal ectactic disorders may be (if stable) a good indication.

Besides these general indications, to implant a phakic IOL safely and successfully it is very important to study extensively the anatomy of the eye. The following parameters must be determined before surgery:

Anterior chamber depth is very important for the safety of implantation of all types of phakic IOLs. It must be measured from the endothelium to the anterior surface of the natural lens and the minimum is 2.80 mm. Recent studies even show that a 3.00 mm depth improves the safety3. The anterior chamber depth may be determined with different devices. The most common are the Orbscan (B&L), Pentacam (Oculus), IOL Master (Zeiss), and the OCT (Zeiss or Heidelberg Engineering). Ultrasonic biometers are not suitable as they include the thickness of the cornea in the AC depth measurement. If implantation of a phakic IOL is done in a shallow anterior chamber, the risk of endothelial cell loss and eventually corneal decompensation is too high.

Anterior and posterior chamber sizes are very important for the implantation of some phakic IOLs. Angle-supported anterior chamber phakic IOLs need to match exactly with anterior chamber dimensions. If they do not, important complications like rotation and decentration of IOL (IOL too small) or pupil distortion and iris atrophy (IOL too large) may occur. Likewise the sulcus fixated posterior chamber phakic IOLs need adequate size to avoid touching the natural lens, causing cataracts (small IOL), or pushing the iris forward closing the angle (IOL too large). Concerning this problem, iris-fixated phakic IOLs have a great advantage as ‘one size fits all’. Classically to determine the angle-to-angle and sulcus-to-sulcus distances the white-to-white measurement (W/W) was done with a calliper or with instruments such as Orbscan, Pentacam, or IOL Master. However, this measurement does not always correlate with the intraocular measurements leading to errors in IOL size4. Nowadays, at least for angle-supported anterior chamber phakic IOLs, the OCT does a precise ‘in vivo’ measurement of the angle-to-angle distance. Unfortunately, there is still no device allowing us a perfect measurement of sulcus-to-sulcus distance.

Most irises are flat. However, in some eyes (mostly hyperopic) the iris is somewhat convex. These eyes are not suitable for implantation with iris-fixated phakic IOLs, and in these cases other types of phakic IOLs must be used. Again the OCT is the most reliable method to assess iris configuration.

Pupil size is another important criterion for implantation of phakic IOLs. The rule is that the mesopic pupil diameter should not be larger than 1 mm over the optic of the phakic IOL.

A healthy endothelium (low index of pleomorphism and polymegathism) with a cell density of at least 2200 cells/mm is also a prerequisite for phakic IOL implantation.

Once all the above criteria are met, to implant a phakic IOL, we need to determine its power. To do that, the most used formula is the one developed by Van der Hejde5, which takes into account the spherical equivalent (cycloplegic for hyperopia), anterior chamber depth, and keratometry.

Goals of surgery

The goal of surgery is to correct the ametropia in a permanent and stable way.

To achieve this goal there are different phakic IOLs available. According to their location inside the eye the phakic IOLs can be divided into three main groups: