11 Managing Residual Astigmatism after Toric IOLs
11.1 Introduction
Replacing the crystalline lens with a toric intraocular lens (IOL) during cataract surgery allows the significant number of patients with significant corneal astigmatism to pursue emmetropia and spectacle independence. 1 Some of the earlier toric studies showed IOL rotation of more than 10° in 50% of cases and more than 30° in 20% of cases. 2 Many advances have since been made in lens designs that promote stability (Table 11-1). 3 , 4 , 5 , 6 However, some patients experience an unsatisfying amount of postoperative astigmatism due to residual or induced astigmatism and aren’t satisfied with their results. Understanding the causes of residual astigmatism can help surgeons take the proper steps to reduce it, and by analyzing the residual astigmatism, the proper management can be carried out to get patients back on the road to spectacle independence.
Study | Study population (eyes) | Misalignment (°± SD) | > 10° (%) | IOL Model | Follow-up (months) |
Holland et al 2010 3 | 256 | 2 ± 2 | 7 | AcrySof T3–T5 | 12 |
Ahmed et al 2010s. Literatur | 234 | 3 | 0 | AcrySof T3–T5 | 6 |
Chang 2008s. Literatur | 100 | 3.4 ± 3.4 | 1 | AcrySof T3–T5 | 1 |
Visser et al 2011 6 | 67 | 3.2 ± 2.8 | 1 | AcrySof T6–T9 | 6 |
Abbreviation: SD, standard deviation. | |||||
11.2 Causes of Residual Astigmatism
Residual astigmatism occurs when an implanted toric IOL isn’t in the ideal position or doesn’t have the correct power and therefore doesn’t neutralize the corneal astigmatism. There are many factors to consider when determining the cause of residual astigmatism (Table 11-2).
1. Was the patient not a good candidate for a toric intraocular lens (IOL) implant (e.g., irregular astigmatism, anterior basement membrane corneal dystrophy)? |
2. Was there a measurement, calculation, or transcription error? |
3. Was there an error made during preoperative marking? |
4. Was lens stability ensured before finishing the procedure (e.g., was viscoelastic removed from behind the lens; Were haptics completely unfolded)? |
5. Was surgically induced astigmatism (SIA) considered or was there a surprising amount of SIA? |
6. Was there a significant amount of posterior corneal astigmatism that wasn’t considered? |
7. Was there postoperative toric IOL rotation? |
11.2.1 Candidacy
Who Makes a Good Candidate for Toric IOLs?
Patients Who Desire Spectacle Independence
The highest priority for determining candidacy should always be the visual goals of the patient. Toric IOLs are generally intended for patients with regular astigmatism who desire greater spectacle independence for distance vision. Some patients may have the goal of monovision with near in one eye and also would benefit from reduced astigmatism. In Ahmed et al’s 2010 study, 54 of 78 patients (69%) with bilateral toric implants were spectacle independent for distance vision.s. Literatur
Patients with a Significant Amount of Regular Astigmatism
Toric IOLs currently available in the United States allow correction of regular anterior corneal astigmatism between 0.75 diopters (D) and 4.1 D (after factoring in astigmatism induced by surgical incisions). Although astigmatism > 4.1 D can be reduced with a single implanted toric IOL, it can’t be eliminated, and residual astigmatism should be expected.
Who Doesn’t Make a Good Candidate for Toric IOLs?
Patients Who Do Not Desire Spectacle Independence
Although this has already been mentioned, it is important to understand the importance of the patient’s postoperative visual goals. In pondering the option of a toric IOL implant, patients should consider their desire for greater spectacle independence for distance vision, the amount of astigmatism they currently have, the realistic expected outcome of toric IOL implants versus other options, and the increase in cost for this procedure versus the cost of prescription glasses over time.
Patients Who Have Significant Irregular Astigmatism
Toric IOLs are not designed to eliminate irregular corneal astigmatism. Ectatic and nonectatic irregular astigmatisms as well as irregular astigmatisms caused by refractive surgery, keratoplasty, corneal scars, and corneal degeneration are all relative contraindications for toric IOL implantation. Although there have been case studies that demonstrate the benefit of toric IOLs in eyes with diseases, such as keratoconus 7 , 8 and pellucid marginal degeneration, 9 it is important to note the likelihood of residual astigmatism. In ectatic irregular astigmatism cases it is important to assess the risk of disease progression, ensure minimal astigmatic variability, and understand that specialty contact lens fitting, if necessary, could be more difficult due to the presence of lenticular astigmatism.
Patients Who Have Poor Visual Potential
When using a toric calculator to determine the correct toric IOL, it is important to consider the estimated residual astigmatism given with the calculations. Because this is the amount of astigmatism that can be expected if the IOL is in the optimal position and corneal astigmatism is unchanged after surgery, it should be used as a baseline when predicting the potential amount of residual astigmatism.
Also, patients with advanced macular degeneration, glaucoma, or amblyopia may not have enough improvement in vision to justify the cost of a toric IOL.
Patients Who Have Weak Zonules
Pseudoexfoliation syndrome, phacodonesis, and other indications of weak zonules can be relative contraindications of toric IOL implantation. Abnormal capsular integrity decreases the IOL stability and therefore increases the probability of a misaligned toric IOL.
11.2.2 Preoperative Measurement and Calculation Error
Discontinuation of contact lens use and upright head positioning during testing and marking will allow measurements to be more accurate. Using two or more confirmatory keratometry sources, such as topography, optical biometry, or manual keratometry, is important because there can be variability in measurements 10 (Table 11-3). It is also important to note the opportunity for human error when transcribing measurements and using toric IOL calculators.
Topography: Atlas 9000 (Carl Zeiss Meditec), RT-7000 (TOMEY), OPD Scan-III (NIDEK, Inc.) |
Manual keratometry |
Optical keratometry: LENSTAR LS 900 (Haag-Streit Diagnostics), IOLmaster 500 (Carl Zeiss Meditec) |
Tomography: Cassini (i-Optics), Galilei (Ziemer, Inc.), Orbscan IIz (B&L), Pentacam (Oculus) |
11.2.3 Marking
Preoperative Manual Ink Marking
Marking the desired axis of the toric IOL is an important step in toric IOL implantation. Manual marking of the cornea with ink is typically carried out in a three-step process: (1) Mark the horizontal and/or vertical axis with a marking device, (2) align a secondary device with angular graduations, and (3) mark the intended IOL axis on the limbus. Other methods using a slit lamp have also been described. 11 Manual ink marking should be done with the patient sitting up to prevent improper positioning that would be caused by cyclotorsion of the eyes when the patient is in the supine position. 12
Preoperative Photographically Assisted Marking
There are also increasing numbers of devices that can photographically assist the surgeon in marking the cornea, through identifying limbal landmarks and then allowing the topographic axis to be aligned with these marks. Comparison intraoperatively can then be performed to assist in aligning the IOL with the correct astigmatic axis.
Intraoperative Surgical Guidance and Aberrometry
Intraoperative surgical guidance systems use features of the scleral vessels, limbus, and iris to create a reference image of the eye prior to surgery, eliminating the need for preoperative manual ink marking. Incision location and toric IOL axis marks can then be digitally projected onto the eye during surgery to reduce the human error of manual marking (Fig. 11.1).
Aphakic wavefront aberrometry measurements taken after the lens has been removed allow for the true net refraction of the entire eye to be known before implanting the toric IOL (Fig. 11.2). Factoring in variables that are difficult or impossible to measure preoperatively, such as posterior corneal astigmatism and surgically induced astigmatism, may yield a more accurate determination of the toric IOL power and axis. Wiley and Bafna’s 2011 study reported that using intraoperative aberrometry via ORange (WaveTec Vision Systems, Inc.) resulted in a mean 0.16 ± 0.22 D of residual refractive cylinder, and 96% of eyes were within 0.5 D of their target. This is compared to the control group without intraoperative aberrometry, which resulted in a mean 0.61 ± 0.54 D of residual refractive cylinder, and only 56.5% of eyes were within 0.5 D of their target.s. Literatur
Stay updated, free articles. Join our Telegram channel
Full access? Get Clinical Tree
