Astigmatic Correction: Managing the Variables with Toric IOLs

Astigmatism is a common refractive ocular condition caused by unequal curvatures of the anterior or posterior cornea, crystalline lens, decentration or tilting of the lens, or unequal refractive indices across the crystalline lens.¹ Internal astigmatism can range from 0.30 to 0.85D with the majority of those affected having approximately 0.5D of internal astigmatism.² Uncorrected astigmatism causes an average loss of visual acuity of 1.5 lines per D at high contrast.⁴

Emmetropia could be better achieved with the AT TORBI IOL, whereas the Tecnis showed better positional stability

After age 40 years, there is an increase in corneal steepening (i.e. more in the horizontal meridian) and against-the-rule (ATR) astigmatism.^3,5 Indeed, corneal astigmatism is common in patients undergoing cataract surgery with more than 40% having 1D power or more of astigmatism.¹ If left untreated, this can cause significant visual impairment,¹ with patients requiring 0.50D or less of astigmatism after surgery for true independence from spectacles.⁶

ASTIGMATIC CORRECTION DURING CATARACT SURGERY: CURRENT CONCEPTS

Operative options for treating astigmatism during cataract surgery include primary corneal cataract incision on the steep corneal axis, limbal relaxing incisions, and implantation of toric intraocular lenses (IOLs).⁶ Selecting the most appropriate method depends on the magnitude of pre-existing corneal astigmatism, as determined by corneal keratometry and topography, in addition to a range of other patient factors.^6,7

Primary corneal cataract incisions are generally considered for astigmatism less than 1.0D ATR while limbal relaxing incisions are useful for treating 1.0D to 1.5–2.0D of regular corneal astigmatism.^2,6 Toric IOLs can correct 1.0 to 4.5D of corneal astigmatism and are now considered to be the preferred method for treating higher orders of regular corneal astigmatism in patients undergoing cataract surgery.^1,6

Table 1. Rotation of Vivinex IOLs over time.¹⁰

OPTIMISING ASTIGMATISM CORRECTION OUTCOMES WITH TORIC IOLS

Successful astigmatism correction with toric IOL implantation requires precise measurement and calculation of the power and axis of corneal astigmatism, together with correct primary alignment of the toric IOL axis.¹⁰ Furthermore, rotational stability and centration are critical as the corrective power of a toric IOL decreases in a linear manner according to rotational offset.¹⁰ Indeed, even smaller degrees of misalignment can undermine the refractive success of the procedure.¹¹ Rotation has been shown to occur in the early post-operative period, prior to the fusing of the anterior and posterior leaves of the capsule.^2,12 Inoue and colleagues observed that the greatest degree of misalignment was post-surgical rotation occurring within one hour of surgery and that IOL orientation was very stable one hour following surgery.¹³

PRECISE MEASUREMENT AND CALCULATION OF THE POWER AND AXIS OF CORNEAL ASTIGMATISM

Corneal astigmatism is a combination of posterior and anterior corneal astigmatism.2 Astigmatism contributed by the posterior cornea is approximately 0.30D (ranging from 0.01 to 1.10D), but the correlation between the posterior and anterior corneal astigmatism varies with the steep axis alignment.^2,14

Many patients with anterior with-the-rule (WTR) astigmatism, have posterior ATR astigmatism at the time of cataract surgery.¹⁴ Therefore, overall total corneal astigmatism is less than if the anterior corneal astigmatism is considered alone.^2,14 Koch and colleagues found that when anterior corneal measurements are considered alone, the average total corneal astigmatism is underestimated by 0.22D at 180°, and in 5% of cases the total astigmatism exceeded 0.5D.¹⁴ However, many toric calculators still use an approximation method based upon a constant for the ratio of the IOL toricity to corneal astigmatism, an approximation that can cause errors.¹⁵

Table 2. Percentage of eyes demonstrating rotational stability of Acrysof and Tecnis IOLs.11 CI = confidence interval; *P<0.05.

As a result, standard toric IOL calculations tend to result in under-correction of eyes with ATR astigmatism and overcorrection of eyes with WTR.¹⁵ The Abulafia-Koch Regression formula takes into account posterior corneal astigmatism, and when applied to a clinical patient cohort, has been shown to improve predictability of toric IOL refractive outcomes.¹⁵ Thus, calculators that provide an optional Abulafia-Koch Regression formula can account for added astigmatic effect of the posterior cornea when measured by standard keratometry of the anterior corneal surface.¹⁵

ADVANCED TORIC IOL CALCULATORS

There are a number of new, advanced toric IOL calculators which aim to improve the predictive value of toric IOL implantation surgery.¹⁶

The Barrett Universal II formula is the most popular formula and calculates the toric IOL cylinder power by deriving the posterior corneal curvature based on a theoretical model and from the predicted effective lens position (ELP) using vector calculations for each eye.¹⁶ More recently, the Kane formula has been shown to be more accurate than all other known modern toric IOL calculators.¹⁶ The Kane formula uses regression, theoretical optics and artificial intelligence techniques to calculate the total corneal astigmatism and then applies this to the ELP to predict the spherical and toric power needed to give the best result.¹⁶

ACCURATE ALIGNMENT OF TORIC IOLS USING MANUAL AND DIGITAL MARKERS

Accurate intraoperative alignment of the toric IOL is critical to achieving the predicted refractive outcomes.¹ Alignment can be guided by a number of different techniques, including manual and digital markers.⁶

Traditional manual marking of the axis, including the one-step and three-step approaches may be prone to inaccuracies due to wide marks or loss of ink intraoperatively. ¹

Digital marking of the axis involves capturing a reference image of the eye preoperatively. The digital marking system then displays a positioning guide for IOL alignment which is visible via the microscope.⁶

Digital marking is always going to be more accurate than manual marking, so long as the digital measuring instruments are appropriately calibrated and operated.¹⁷ The reference images need to be of sufficient quality, as poor quality reference images will produce inaccurate measurements and poor outcomes. Digital imaging tools are expensive and not always available. In these circumstances, manual marking is an excellent, reliable alternative if done carefully. Manual marking can be done at the slit-lamp, using a gravity marker or free-hand, with decreasing accuracy of these tools in that order.

Other systems for accurate toric alignment and power selection include use of intraoperative aberrometry in an aphakic eye and after implantation of the toric IOL.¹⁷

CLINICAL TRIALS

Recent clinical trials assessing rotational stability of toric IOLs available in Australia and New Zealand are as follows:

Vivinex IOLs 

Schartmuller and colleagues (2018) evaluated rotation of Hoya’s Vivinex IOLs for six months following surgery in 122 eyes of 66 patients.¹⁰ Alignment was assessed at the end of surgery, one hour, one week, one month and six months after implantation.¹⁰ Confounding factors, such as axial length, were also evaluated.¹⁰

Results showed that 100% of the implanted lenses (n=103) had ≤5° rotation from the initial axis following surgery to six months postoperatively (Table 1).¹⁰ Furthermore, there was no correlation between axial length and rotation (Spearman’s r=0.048, p=0.63).¹⁰

Acrysof vs. Tecnis IOLs 

Lee & Chang (2019) performed a retrospective cohort study to compare the rotational stability of Alcon’s Acrysof toric IOLs and Johnson & Johnson Vision’s Tecnis toric IOL in 1,273 consecutive eyes, either later on the day of surgery (at least one hour postoperatively) or the next morning.¹¹

Results showed that 91.9% of the Acrysof toric IOLs rotated ≤5° at the first postoperative check compared with 81.8% of Tecnis toric IOLs (P<0.0001).¹¹ The mean rotation was 2.72° for Acrysof and 3.79° for Tecnis toric IOLs (P<0.05).¹¹ In contrast to Acrysof, the Tecnis toric IOL showed a strong predisposition to rotate counter-clockwise.¹¹ Furthermore, 3.1% of Tecnis toric IOL patients required repositioning compared with 1.6% with Acrysof (P=0.10).¹¹

Tecnis vs. AT TORBI IOLs 

A separate prospective, randomised study compared the visual performance after implantation of a Tecnis toric IOL (Abbott Medical Optics) in one eye and with an AT TORBI 709M IOL (Carl Zeiss Meditec AG) in the other eye in 59 subjects with corneal astigmatism greater than 1.25D.¹⁸ Results showed similar mean toric IOL axis rotation with each toric IOL at 12 months (3.27 } 2.37° vs 3.0 } 2.26°, respectively; p=0.5).¹⁸

KEY FINDINGS

The study by Schartmuller and colleagues showed good rotational stability of the Hoya Vivinex IOL with no IOL rotation >5° after initial surgery.¹⁰

Unfortunately, the study was quite small and there was no comparison to any other toric IOL.10 Nevertheless, the study clearly demonstrates that the Hoya Vivinex IOL is stable in the eye and is a predictable and good toric IOL option.¹⁰

Lee and Chang’s study was larger and provided a direct comparison between the two lenses.¹¹ Furthermore, variables such as surgeon factors and peri-operative technologies were similar in the two arms of the study.¹¹ The results clearly showed better rotational stability of the Acrysof toric IOL versus the Tecnis lens.¹¹ This finding is clinically significant in that fewer Acrysof IOLs required repositioning than the Tecnis toric IOLs.¹¹

THE PRE-OPERATIVE PHASE^†

To ensure good results with toric IOLs it is important to:

1. Ensure the tear-film is healthy andthe ocular surface is in good condition.Dry eye syndrome has been shown to adversely affect keratometry readings and biometry accuracy,
2. Check the corneal parameters with atleast two separate instruments to ensurethat the keratometry readings are similar in both axis and power,
3. Perform corneal topography to ensurethere is regular corneal astigmatism, and
4. Perform a dilated examination,looking at the crystalline lens, to makesure there is no unrecognised instability of the lens and that further ocular health is satisfactory.

OPTIMISING ALIGNMENT OF THE TORIC IOL^†

When choosing a toric IOL, surgeons should consider the following:

Platform – Surgeons should be familiar with the IOL platform they choose when using a toric IOL. Familiarity allows for more confident use of the lens, easier lens insertion and lens rotation.

Design – Toric IOLs come in both plate and three-piece IOL designs. The plate haptic IOLs can be rotated both clockwise and anti-clockwise while three-piece IOLs need to be rotated clockwise. This can lead to over-manipulation and put stress on the zonules if the IOL is unstable in the bag. It is essential to remove viscoelastic from behind the IOL after rotation is complete. This can be easier with three-piece IOLs and failure to do so can lead to a myopic shift and post-operative IOL rotation.

IOL material – Hydrophilic IOLs are easier to manipulate in the eye. However, these can develop significant early posterior capsular opacity and capsular phimosis, which can lead to late rotation of the IOL. Hydrophobic IOLs tend to be less malleable in the eye and can take longer to ‘settle’ into position during surgery, which can also lead to rotation post-surgery.

MANAGEMENT OF IOL ROTATION^†

The sooner IOL misalignment is recognised the better, so it’s good practice to dilate the pupil and check the IOL rotation if there is a refractive surprise within the first week after surgery. Basic steps to follow if this is the case are:

1. Check the visual acuity and refraction(the patient may be happy with their visualoutcome so no action may be required even if the IOL is misaligned),
2. Check and note the new IOLorientation,

iii. Check the operation notes; i.e. was the IOL implanted as planned?,

1. Check the topography, and
2. Anterior segment OCT/UBM to assessfor retained visco-elastic behind the IOL.

Surgical Options for Realignment 

If indeed there is a significant refractive surprise and the IOL is misaligned, then the following surgical options should be considered:

Rotation and Reorientation of Toric IOL 

The sooner toric IOL rotation is performed the better. Capsular fibrosis occurs quite quickly and adhesions between the capsule and lens can result in difficult rotation, zonular damage and poor outcomes.

Toric IOL rotation is best performed under sterile conditions in theatre. Some surgeons have demonstrated toric IOL rotation at the slit-lamp, which is time and cost effective, but inherently riskier than performing this procedure in theatre.

The pre-surgical IOL axis is not necessarily the optimal axis after initial IOL insertion. There are a number of online calculators, which can be used prior to re-orientation surgery in order to determine the ‘new’ angle required to give the best outcome.

Piggy-back Toric IOL 

A sulcus fixated ‘piggy-back’ toric IOL can be used to correct the residual astigmatism if IOL rotation is considered too risky. The Sulcoflex IOL is easy to use and a good option in this circumstance.

Corneal Refractive Surgery 

Corneal refractive enhancement is an accurate and relatively simple surgical solution for most post-cataract surgery refractive errors. SMILE, LASIK and Photorefractive keratectomy (PRK) surgery techniques can all be used in this situation so long as the cornea is healthy and there are no contraindications for corneal laser surgery.

Refraction should be stable for six to12 weeks and the corneal surface needs to have fully recovered from initial surgery before performing laser surgery enhancement.

IOL Exchange 

IOL exchange may be valuable if there is a sphero-cylindrical refractive surprise that cannot be corrected with any of the other options. IOL exchange can be difficult surgery and can lead to loss of capsular support and the need to perform anterior vitrectomy. Scleral sutured/fixated toric IOLs are not available so good visual and refractive outcomes can be very hard to achieve in this instance.

This article was sponsored by an educational grant from Hoya Surgical Optics. 

The views and opinions expressed in this article are those of the author and do not necessarily reflect the views and opinions of Hoya Surgical Optics. 

† Based on Dr Anton van Heerden’s clinical experience. 

Dr Anton van Heerden FRANZCO is a cataract and refractive surgeon. He is Head-of-Unit, Surgical Ophthalmology Services at the Royal Victorian Eye and Ear Hospital in Melbourne. He also consults at Armadale Eye Clinic, Mornington Peninsula Eye Clinic and Eye Laser Specialists. 

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