The incidence of IOL malpositioning is rare in modern cataract surgery with current phacoemulsification techniques. If a patient does not have a predisposing condition and surgery is uneventful, then bag fixation of the lens should be predictable and secure. That is primarily because we have moved away from fixing the lens in the sulcus, anterior chamber, or the pupil. With the lens now fixed completely in the bag, the chances of malpositioning are reduced
We can look at the causes of lens malpositioning in two ways. The first is primary malpositioning, (ie, when the lens is positioned incorrectly during implantation). Most times, a primary malposition occurs because of surgeon error. In these instances, part of the lens may be sitting in the capsular bag and part in the sulcus. Alternately, the lens haptic may break during insertion, and rather than centering properly within the bag, the IOL sits lopsided. Secondary malpositioning occurs during the postoperative period. Although the IOL is inserted properly and in the correct position during surgery, pseudoexfoliation, zonular dehiscence, or zonular trauma can generate asymmetric forces that result in lens malpositioning.
ETIOLOGY OF MALPOSITIONED LENSES
The interplay of patient factors and IOL design will determine the symptoms and signs of IOL malpositioning. It is important for the surgeon to keep these factors in mind when managing malpositioned lenses.
Patient factors. A malpositioned IOL may be mistaken for posterior capsular opacification (PCO), mostly because glare and dazzle are common symptoms of both conditions. Therefore, if a patient presents with these symptoms, it is important to dilate the pupil and perform a thorough examination to determine if the cause of symptoms is indeed a PCO or a malpositioned IOL.
Patients with pseudoexfoliation are more prone to malpositioning because of a preexisting zonule condition, causing zonular laxity. Because the equatorial tension in the capsular bag is asymmetrical, zonular dehiscence can develop postoperatively. A few cases1-7 of well-situated IOLs spontaneously dislocating or subluxating into the vitreous because of weak zonules have been reported.
A capsular tension ring should also be used in the presence of pseudoexfoliation, especially if there is any sign of capsular instability at the time of surgery. Other options include using a capsular tension ring that has eyelets or capsular tension segments. Posttrauma patients who have had zonular trauma and those who have poor capsular support are also prone to malpositioning.
Patients with ocular conditions that induce the breakdown of the blood aqueous barrier (eg, uveitis, diabetes) are also more prone to malpositioning of the IOL due to increased inflammation and an exaggerated reaction of the epithelial cells. Some of these patients develop iridocapsular synechiae resulting in lens malpositioning.
IOL design. The tolerance of malpositioning in more complex optical designs is much less than with monofocal IOLs. Modern IOLs including accommodating, multifocal, and aspheric designs tend to tolerate malpositioning poorly. For example, if a Tecnis aspheric lens (Advanced Medical Optics, Inc., Santa Ana, California) is malpositioned by more than 0.4 mm, the beneficial affect of that asphericity is lost. Similarly, if a refractive multifocal IOL is malpositioned by 0.5 mm, then the affect of the multifocality for reading vision decreases.
Certain haptic designs are important in IOL centration, and many modern lenses with haptics must be correctly oriented. If an accommodating lens with a hinged haptic is turned upside down, for instance, the resulting malposition may be quite dramatic. This is even more critical with toric IOLs, which not only have to be stable in the x- and y-plane, but also rotationally.
If the haptic design has a wider angle of contact with the capsular bag equator—rather than a single-point contact—then the chance of malpositioning decreases, because the narrower the contact angle, the more prone it is to destabilizing forces. For this reason, a wide C- or J-loop haptic design fairs better than an old-style J-loop with a narrow angle of contact. Plate haptic lenses are also problematic, because it is difficult to adjust the IOL size to the capsular bag diameter.
A separate consideration in older IOLs is their silicone material. Some evidence suggests8 that older materials induce metaplasia off the lens epithelial cells and cause capsular fibrosis, which could cause malpositioning. If the anterior or posterior capsule fuse generate asymmetric forces on the IOL, the IOL optic can buttonhole out of the capsulorrhexis and induce tilt and decentration.
TRICKS FOR PROPER INSERTION
Grossly tilted lenses usually indicate that the lens is sitting partially in the bag and partially in the sulcus. There are several extra steps that a surgeon can perform to ensure proper IOL placement. First, aim to make the capsulorrhexis smaller than the lens optic so that it is easy to visualize the capsulorrhexis in front of the optic. This will guarantee that the lens is placed in the bag. Second, give the lens optic a push in the same axis as the haptics are. If the lens recenters on its own, then it should be positioned correctly. Third, lightly push on the iris with a Y-hook after lens insertion, checking that the rhexis covers the whole lens. This is particularly helpful with small pupils.
If a primary malposition is noticed early enough, the course of action is pretty straightforward because the capsular bag is still open. Therefore, the surgeon can go back into the eye and reposition the lens. Postoperative management of a malpositioned IOL after the capsular bag is closed, however, is more challenging. In most cases, a refractive change is induced, and patients notice a drop in their vision or quality of vision. They may also experience prismatic effects, induced astigmatism, glare, dazzle, diplopia, or a loss of optical affect. The effects depend on the degree and orientation of malpositioning. It also depends on the IOL, lens material, and the individual power of the lens. A 30.00 D lens will have a lower tolerance for malpositioning compared with a lens that is 15.00 D. If any symptom is clinically significant and affects the patient's lifestyle, the malpositioning must be corrected.
Two strategies exist for repositioning an IOL after the capsular bag is closed. The first method is to reposition and secure the lens, and the second is to perform a lens exchange. Of these, repositioning is less invasive and can be performed in most eyes. Once the IOL is repositioned, secure the haptic to the sclera or iris with an anchor suture. If it is not possible, then perform a lens exchange, assessing the best position for the replacement lens to sit (ie, capsular bag, sulcus, sutured to the sclera). The appropriate position will depend on the integrity of the zonular capsular complex.
As patients' expectations continue to rise, we need better haptic designs to improve IOL stability. In the future when customized lenses are available, being able to predict and maintain the IOL position postoperatively—in other words to avoid malpositioning altogether—will be critical to the success of those technologies.
Milind V. Pande, DO, FRCS FRCOphth, is Head of the Vision Surgery & Research Centre, in East Yorkshire, United Kingdom. Dr. Pande states that he has no financial interest in the products or companies mentioned. He may be reached at +44 01482 339515.
