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The Staar plate haptic AA4203 IOL became the first FDA approved toric
IOL in November 1998. There are two types of patients for whom the toric
IOL is particularly helpful as an alternative to incisional keratotomy
for astigmatism correction. One is the younger (less than or equal to 65
years) cataract patient with greater than or equal to 2.00 D of astigmatism,
but whose younger age significantly reduces the attainable effect from
incisional keratotomy. The second group consists of patients with the largest
degrees of astigmatism (e.g. greater than or equal to 3.00 D) in whom corneal
relaxing incisions are much less predictable.
Earlier and concurrent studies indicated that the plate haptic design was superior to loop haptic designs in terms of long term rotational stability.1, 2 Patel used serial digital photographs to document that late rotation was unusual with plate haptic lenses, but more frequent with 3-piece designs.2 However, the Patel study also confirmed that either lens design could rotate in the immediate postoperative period. The plate haptic design did so more frequently. Presumably, it is not until the capsular bag contracts that the corners of the plate haptic lens are able to resist natural rotational forces. It is important to note that this study used a 10.5 mm long plate haptic IOL, which is shorter than the two toric IOL sizes available from Staar.
Misalignment of the toric IOL axis reduces the amount of astigmatism corrected. With 10 degrees of axis deviation, 1/3 of the desired effect is lost. With 20 degrees of axis deviation, 2/3 of the effect is lost. Lens misalignment greater than 30 degrees will actually increase the net astigmatic error. In the US FDA study cohort, 24% of the toric IOLs ended up >10 degrees off axis. (12% were >20 degrees off, 8% were >30 degrees off, and 5% were > 45 degrees off axis). However, only the shorter 10.8 mm toric lens was used in the FDA study.
A consecutive series of 40 cases was performed. The Staar toric plate haptic IOL (AA4203) has the larger haptic fenestrations, and comes in two astigmatic powers and two sizes. The +2.00 toric power corrects approximately 1.50 diopters of keratometric astigmatism, while the +3.50 toric power corrects approximately 2.25 diopters. The �TF� lens was the original FDA studied design. It has an overall length of 10.8 mm. Subsequently, the longer 11.2 mm �TL� model was released by Staar for spherical powers of < 23.5 D. The haptics of the longer lens also have a matte finish to make them less slippery.
The spherical power of the toric IOL is calculated in the same way as for a conventional IOL. The astigmatic power is then selected without having to adjust the spherical power. Hashmarks on the peripheral optic surface are aligned with the steeper �plus� axis of astigmatism during surgery. To avoid any potential influence of lenticular astigmatism, the kerotometric reading, rather than the refraction, should guide the selection of the toric power and axis.
To achieve as astigmatically neutral an incision as possible, a temporal clear corneal incision is used. Under topical anesthesia, the 6 o�clock limbus is marked with a pen with the patient sitting upright on the operating table just prior to surgery. Later, the final orientation of the IOL axis marks is aligned with a Mendez-style degree gauge, and double-checked against the preoperative notes or chart.
So far, significant late rotation of the Staar toric IOLs has not been reported, consistent with the published Patel findings.3 Thus, the key to success is in avoiding early postoperative rotation of the lens. Having a plate haptic IOL of sufficient length for the capsular bag diameter is a critical factor. The experience of my first 40 toric IOL cases underscores this point.
In February 1999, I implanted my first six TF (10.8 mm) toric IOLs in four patients. The longer TL (11.2 mm) lens had not yet been announced or released. I used Viscoat in these uncomplicated cases under topical anesthesia, and proper IOL orientation was documented each time by the operative videos. In three eyes, the axis remained within +15 degrees of the target postoperatively. However, in one patient, the IOL rotated 70 degrees off axis by postoperative day one (POD 1). When her second eye was done, the IOL rotated 85 degrees off axis by POD 1. These lenses were each successfully repositioned on POD 1 because the doubling of the patient�s astigmatism was intolerable. They have remained properly aligned since then.
A third IOL was 30 degrees misaligned by POD 1. This was better tolerated and I waited the recommended two weeks to surgically realign the lens. At surgery, I discovered that the anterior capsule was already fibrotic and the anterior and posterior capsules were already tightly fused peripheral to the IOL. Use of a spatula and viscodissection succeeded in reopening the capsular bag so that the IOL could be rotated. Although the IOL did not rotate easily, proper and stable alignment was eventually achieved.
Because of this frustrating 50% repositioning rate, Staar suggested and allowed me to implant the first TL (11.2 mm) lens in the U.S. in March 1999. I used the longer TL model in the next 34 consecutive cases with none requiring repositioning. All have carried the higher +3.50 D toric power, with spherical powers ranging from 9.5D to 21D.
My current technique follows these guidelines:
1) Use the longer TL lens unless dealing with a small eye.
2) Use a cohesive viscoelastic, e.g. Healon (Pharmacia), Provisc (Alcon), Biolon (Allergan). Compared to dispersive viscoelastics, these are less likely to coat the IOL surface.
3) Remove viscoelastic trapped behind the IOL with irrigation-aspiration to maximize posterior capsule-IOL surface contact.
4) Do not overinflate the eye. Leaving the eye somewhat �softer� than usual probably allows the capsular bag to collapse around the IOL more immediately.
Excellent rotational stability was achieved with this technique in my series of 34 consecutive TL (11.2 mm) toric implants. 65% were within 5 degrees of target axis; 91% were within 10 degrees of target axis; 100% were within 15 degrees of target axis. I have not documented any major rotation subsequent to POD 1 in any of the 40 total patients. The repositioning rate was 50% with the shorter IOL and 0% with the larger IOL. The obvious implication is that until the capsular bag begins to contract within the first day or so, a plate haptic IOL that is too small for the bag can rotate. Based upon my experience with three badly misaligned cases, I would perform repositioning by one week or earlier. Once the capsular bag becomes fibrotic (e.g. at two weeks with my third case), more force is required to rotate the IOL. This might increase the chance of tearing the capsular bag.
Toric IOLs are an excellent complement to corneal astigmatic incisions. They are helpful in those cases where LRIs are not powerful or predictable enough. Proper IOL sizing is critical for initial rotational stability, as lenses that are too short for the capsular bag will rotate within the first 24 hours. Since the availability of the longer 11.2 mm model, toric IOLs have been a welcome addition with excellent anatomic and refractive results.
|1.||Shimizu K, Misawa A, Suzuki Y. Toric intraocular lenses: correcting astigmatism while controlling axis shift. J Cataract Refract Surg 1994;20:523-526.|
|2.||Patel CK, Ormonde S, Rosen PH, Bron AJ. Postoperative intraocular lens rotation: A randomized comparison of plate and loop haptic implants. Ophthalmology 1999; 106:2190-2196.|
|3.||Ruhswurm I, Scholz U, Zehetmayer M, et.al. Astigmatism correction with a foldable toric intraocular lens in cataract patients. J Cataract Refract Surg 2000;26:1022-1027.|
David F Chang, University of California, San Francisco, and private practice in Los Altos, California, USA.
The author has no financial interest in any of the products mentioned.
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