Toric Intraocular Lens Alignment Technologies
Jonathan M. Pargament
Robert H. Osher
Astigmatism is a type of refractive error that is most commonly due to an increased radius of curvature along one meridian of the corneal surface. The axes of maximum and minimum curvature are referred to as the principle meridians. The difference between the steepest and the flattest curvature, expressed in diopters (D), is equal to the magnitude of the corneal astigmatism. When the principle meridians are perpendicular to each other, the astigmatism is considered regular and can be corrected with spherocylindrical or toric lenses. Although astigmatism may also occur due to abnormalities of the surface or orientation of the crystalline lens, only those patients who have regular astigmatism due to corneal asphericity are ideal candidate for toric lenses.
Astigmatism is a common refractive error among prospective cataract patients. It has been estimated that greater than 34% of potential cataract surgery patients have more than 1 D of astigmatism.1,2 Surgical techniques, including corneal and Limbal Relaxing Incisions (LRIs), have been used to reduce corneal astigmatism.3,4 However, these methods are limited by the magnitude of astigmatism to be corrected, postoperative corneal structural stability, and the variability of the individual healing response.
In 1994, Shimizu et al.5 introduced the toric intraocular lens (IOL). An expanded range of powers and designs offered by the IOL industry has fueled a steady increase in their use (Market Scope 2011 Survey Report, December 31, 2011). However, efficacy and predictability are dependent upon precise alignment of the toric IOL axis with the principle meridian of greatest curvature. The cylinder power effect is reduced approximately 3.3% by each degree of error in alignment.6
IOL misalignment may be caused by imprecise lens placement or by rotational instability within the capsular bag. The earlier generation of toric IOLs, made of silicone, had significant problems with postoperative IOL rotation.7 Modification in IOL material and design has resulted in improved stability.8 Thus, the accurate preoperative measure of astigmatism and intraoperative placement of the toric IOL along the steepest meridian of corneal astigmatism are the major remaining barriers to neutralizing cylinder.
In this chapter, we focus on recent advances in IOL alignment techniques. The traditional marking technique follows a three-step procedure. First, the patient is seated in an upright position to correct for cyclotorsion and the eye is marked with a reference marker at the 3, 6, and 9 o’clock positions to designate the horizontal and vertical meridians. In the operating room, the desired target axis is marked at the cornea, limbus, or conjunctiva using an angular graduation instrument and a reference marker. The IOL is implanted into the capsular bag and rotated clockwise until its axis markings are in line with the cornea, limbal, or conjunctival markings. Unfortunately, it is estimated that errors due to cyclotorsion, inaccurate reference axis markings, inaccurate alignment axis markings, and imprecise placement of the IOL along the alignment meridian account for an average total error of approximately 5° in alignment or approximately 17% of residual cylinder.9 In addition, problems related to the fading, diffusing, or washing out of the ink marks further degrade the precision of the IOL placement.
As the higher cylinder power IOLs become available, it is even more critical to develop techniques that improve the precision of IOL alignment. The use of limbal vessels to aid orientation has been attempted but topical drops, like Neo-Synephrine, may cause vessel blanching and topical antibiotics may cause hyperemia. Subconjunctival hemorrhage or chemosis can be associated with subconjunctival anesthesia, obscuring limbal anatomy.10 Other techniques involve the use of a slit-lamp reticule and unreliable ink markings to identify the target axis. At least three novel solutions have been developed and proven to be very useful. The first approach, termed iris fingerprinting, involves the use of a captured iris image and a software-aided superimposed protractor that takes advantage of unique iris anatomy.10 Iris registration is a related two-stage approach that utilizes a preoperative technology for capturing the anterior segment, then allowing precise intraoperative guidance by superimposing a protractor over the image of the eye. Another solution uses intraoperative wavefront aberrometry to analyze refractive error and provide feedback when orienting the toric IOL.