Toric IOLs refer to astigmatism correcting intraocular lenses used at the time of cataract surgery to decrease post-operative astigmatism.
To learn more about astigmatism, log on to http://www.geteyesmart.org/eyesmart/diseases/astigmatism.cfm
Available Toric IOLs
AcrySof Toric IOL (Alcon Labs, Fort Worth, Tx) (Figure 1) is a one-piece SN60AT design, hydrophobic acrylic lens with a yellow chromophore. The optic is 6.0 mm. This lens can go through incision sizes as small as 2.2 mm corneal incision. The IQ Toric version of this lens is aspheric and comes in the cylindrical powers as on the table below. They are able to correct up to +4.5 D of corneal astigmatism. The spherical range available is 6 to 30 D. The lens has 3 axis marking dots on either side of the optic periphery which designate the direction of the steep axis of toric power. These are aligned with the haptics for easy alignment.
|Model||Cylinder Power (D)||Cylinder Power (D)|
|at IOL plane||at Corneal Plane|
Staar Toric IOL (Staar Surgical, Monrovia, Ca) (Figure 2) is a silicone plate haptic design IOL. It can be implanted through a 3.0 mm clear corneal incision. As with all plate lenses, it should not be implanted in a compromised capsular bag. It is available in 2 models with differing lengths. Model AA4203-TF (10.8 mm in length) comes in spherical powers of 21.5 to 28.5 D. The AA4203-TL (11.2 mm) comes in spherical powers of 9.5 to 23.5 D. This lens comes in 2 cylindrical corrective powers: +2.0 D corrects corneal astigmatism of +1.2 to +1.4 D and the +3.5 D toric corrects +2.2 to +2.4 D of astigmatism. The axis of toric power is marked with 2 hash marks at the optic periphery.
AMO Tecnis Toric IOL (Abbott Medical Optics, Inc, Santa Ana, CA) (Figure 3) is a one-piece design asheric, hydrophobic acrylic IOL which was FDA approved in April 2013. It can be implanted through a 2.2 mm incision with the AMO Platinum inserter using the wound assist technique. It comes in 4 powers which can correct astigmatism from 0.75 diopters to 3.62 Diopters.
|IOL Model||Cylinder Power||Correction ranges based on combined corneal astigmatism (preop Kcyl + SIA)|
|IOL Plane||Corneal Plane|
|ZCT150||1.50||1.03||0.75 - 1.50 D|
|ZCT225||2.25||1.55||1.50 - 2.00 D|
|ZCT300||3.00||2.06||2.00 - 2.75 D|
|ZCT400||4.00||2.74||2.75 - 3.62 D|
These are monofocal intraocular lenses so patients will still require spectacle correction for other distances.
Patient should have a visually significant cataract and astigmatism. A good candidate will be interested in spectacle independence even if only for one distance (usually far). It should be made clear to the patient that lenticular correction will be required for other distances (typically near and intermediate). Realistic expectations on the patient's part make for a successful outcome. The toric lenses currently available are designed to correct regular corneal astigmatism. Patients with irregular astigmatism will not fare as well.
Accurate determination of the amount of astigmatism is paramount. Astigmatism can be measured further defined as refractive, lenticular and corneal. A careful manual refraction is key to determining refractive astigmatism. Corneal astigmatism is usually measured with keratometry and/or topography. Cylinder and axis are confirmed on manual keratometry, K's on a optical biometer such as an IOL Master or Lenstar LS 900 and corneal topography. All measurements should be similar. It is crucial to differentiate the lenticular contribution to the total astigmatism. The patient will be undergoing cataract surgery so only the corneal astigmatism will need to be addressed.
The cataract corneal incision may induce astigmatism. Surgeons should consider the impact of their incision size and location on the cornea as it will determine the final residual corneal astigmatism. The Alcon online toric calculator (www.AcrySofToriccalculator.com) takes into account the effect of the main incision-induced astigmatism on the final recommended cylindrical power and the appropriate axis of IOL orientation.
Surgeons should perform IOL calculations as per routine. This will determine the spherical power needed. Each surgeon is advised to use their preferred IOL formula.
Determination of Toric power required
For the Alcon AcrySof models, the surgeon will log on to the online toric calculator (www.AcrySofToriccalculator.com) and inputs the required information. The calculator will then determine the best model and the axis of orientation of the lens. Surgeons will find that, depending on the amount of main corneal incision induced astigmatism and the location of the incision, the toric power may change. Surgeons are encouraged to input varying main incision sites to consider their effect. Those surgeons who are more flexible about where they place their main incision may find they are able to correct higher astigmatism powers.
Staar Surgical also offers an online toric calculator (www.STAARToric.com). Again, the surgeon determines the spherical power as per routine IOL calculations methods. Corneal astigmatism will determine which of the two powers should be used. For corneal astigmatism from 1.4 to 2.3 D, Staar Surgical recommends the +2.5 D power and the +3.5 D toric for asigmatism above +2.3D.
Abbot Medical Optics has its own online calculator (www.amoeasy.com). This calculator gives the user a variety of options, including those that induce residual astigmatism on a different axis. It is imperative to check the orientation of the axis as well as the power of the suggest toric when using this calculator.
Patients may elect to treat astigmatism with corrective lenses (spectacle or contact).
Prior to beginning the surgical procedure, the precise reference marking of the cornea is done with the patient upright and looking forward. It is important that the patient be sitting for this procedure as cyclotorsion may occur when the patient lies down. The cornea is marked at the 3-, 6- and 9-o'clock positions after instillation of a topical anesthetic. Some surgeons select to simply mark 3- and 9-o'clock. This can be done in the preoperative area or in the operating room. Corneal marking instruments are available which ease this step. This can also be marked with a skin marking pen.
After the patient is draped, the steep axis should be marked with a degree gauge so as to provide a guide for the orienting marks on the toric IOL later in the procedure. It is wise to double check the axis with preoperative notes.
Then, surgeon proceeds with the surgery as per routine.
At this step, an ophthalmic viscoelastic device (OVDs) is injected into the capsular bag. Cohesive OVDs e.g. Healon (AMO) or Provisc (Alcon) is preferred over the dispersive agents. Cohesive OVDs are less likely to coat the IOL surface and are easier to remove at the end of the case.
At this time, the AcrySof Toric should be grossly aligned about 10-15 degrees counterclockwise of the final desired lens position. Viscoelastic is then carefully removed from behind the lens followed by anterior to the lens. The IOL may be secured in position with a second instrument if desired. Occasionally, during this step the IOL may rotate the 10-15 degrees needed into position. If not, a second instrument is used to rotate it into the final position. When appropriately oriented, a gentle nudge posteriorly will hold it there securely. This step increase IOL contact with the posterior capsule, decreasing the incidence of postoperative rotation. If the IOL over-rotates, viscoelastic should be injected into the eye and the above steps repeated as it can only be rotated clockwise. The AcrySof acrylic material is 'tacky' and the single piece platform tends to stay aligned quite nicely.
The procedure for the implantation of the STAAR toric is similar. It should be grossly aligned upon insertion into the capsular bag. A couple pearls to prevent postoperative rotation are as follows. Thorough OVD removal is critical. Again the surgeon should pay particular attention to OVD trapped behind the IOL. This maximizes IOL-posterior capsule contact which minimizes undesired rotation. The manufacturer also recommends the eye should be left a little 'softer' than usual.
The procedure for the implantation of the AMO Tecnis toric is similiar to that of the Alcon AcySof toric.
A key point for all toric IOL models is that the OVDs must be completely removed from the eye at the end of the case to prevent rotation.
For higher amounts of astigmatism, a combination of a Toric IOL and astigmatic keratotomy may be the best means to address total amount of cylinder.
Surgical follow up
Postoperative exams should be as for routine cataract extraction/IOL implantation cases. Most surgeons customarily see their patients on postoperative day #1, #7, and #28. Postoperative drop regimen is as per routine for uncomplicated cataract surgeries. An antibiotic, NSAID and steroid drops are part of most postoperative regimens.
In addition to the usual complications associated with cataract extraction with intraocular lens implantation surgery, Toric IOL's have a few other potential complications. Astigmatism may be overcorrected or undercorrected.
The toric IOL may rotate off axis. There will be some partial affect if it rotates off axis. For example, the correction affect of the AcrySof toric is reduced by 10% for every 3o rotation off axis. For the lens to lose its full effect, it would have to be off axis by 30o. Prevention of IOL rotation include complete removal of OVDs, especially posterior to the IOL.
The vast majority of patients do extremely well when careful attention is paid to patient and lens selection and surgical technique. The technical surgical learning curve for the surgeon is fairly minimal for an outcome that is reliably predictable. Toric IOLs, exclusively or in conjunction to astigmatic keratotomy and/or strategic cataract incision placement, can result in excellent management of astigmatism and should be considered for astigmatic patients who desire spectacle freedom at distance