Optimising Toric IOL Outcomes
Toric intraocular lenses (IOLs) have come a long way since Kimiya Shimizu implanted the first three-piece PMMA toric IOL through a 5.7mm incision in 1991.
While those first-generation lenses delivered respectable distance visual acuity of 20/25 or better in 77 per cent of eyes, the data was less impressive for rotation, with about 20 per cent of the IOLs rotating 30 degrees or more and almost half more than 10 degrees off axis.
Almost 25 years later, improvements in IOL material and design, and refinements in surgical technique have greatly enhanced postoperative rotational stability and, as a result, improved visual outcomes.
Before toric IOLs entered the picture, patients with pre-existing astigmatism were either left uncorrected or required corneal curvature-altering procedures to correct this condition. The number of patients affected made the need to find a viable treatment even more pressing: some 40 per cent of patients presenting for cataract surgery have corneal astigmatism of 1.0D or higher and more than 20 per cent have 1.50D or higher.
Fortunately, today’s cataract surgeons have no shortage of choice when it comes to treatment options for their astigmatic patients, with at least 10 monofocal toric IOL models and four multifocal toric lenses currently available.
SCOPE FOR IMPROVEMENT
But greater choice and increasing demand for toric lenses does not necessarily mean that the scope for further improvement has been exhausted.
“Toric lenses are still the way to go if we want to treat the one-third of our patients in the cataract population with moderate to high astigmatism,” Oliver Findl MD, PhD, Vienna Institute for Research in Ocular Surgery, Austria, told EuroTimes. “A lot of toric lenses are now available and most of them are rotationally very stable, and that has been shown in numerous studies. However, the results are far from being perfect and there is still a lot of inaccuracy, particularly when we are dealing with low astigmatism,” he said.
This inaccuracy was borne out by a study carried out by Dr Findl et al looking at toric IOL implantation in 250 eyes of 200 patients, which found that the most unpredictable outcomes were found in patients with low astigmatism.
“The problem we have here is that we do not really have a good definition of low astigmatism. What is clear is that below 0.75D of astigmatism, especially when measured on the cornea, the measurements are so noisy and so variable from method to method and device to device that it is difficult to assess the magnitude and orientation of the astigmatism,” he said.
The reasons for poor astigmatic reduction with toric IOLs typically stem from factors such as inaccurate preoperative calculations of IOL power and posterior surface of the cornea, as well as intraoperative issues such as mislabelling of the IOL, surgically-induced astigmatism and IOL misalignment.
Of these factors, preoperative measurement errors, with large inter-device variability within and between different keratometric measurements and topographic measurements, represent the main source of error, said Dr Findl. Add in diurnal changes in corneal measurements over the course of the day and differences in postoperative refraction measures and it is easy to see why there is so much variability correcting low-level astigmatism, he said.
Yet Dr Findl is optimistic that some of the current deficiencies may be overcome in the near future thanks to advances in diagnostic technology, and in particular swept-source optical coherence tomography (OCT).
“Our accuracy in measuring low astigmatism should improve with swept-source OCT such as the Casia (Tomey) or the IOLMaster 700 (Carl Zeiss Meditec). Once we get this type of technology into our hands for routine patients we should become better at predicting the real corneal astigmatism and that will hopefully make things a lot better with our toric IOL outcomes,” he said.
The scan speed in swept-source instruments is twice that of SD-OCT devices and enables the user to image structures that were previously unattainable. The IOLMaster 700, for instance, gives a longitudinal cut through the entire eye, allowing surgeons to screen for macular disease, verify fixation and check irregular eye geometries, which is particularly useful in trying to predict IOL tilt, said Dr Findl.
“With the swept-source OCT we can simulate how a toric IOL will perform after implantation and adapt our calculations based on predicted tilt. While we still need a lot more data to confirm this, predicting lens tilt may actually enhance the function and the optical performance of IOLs, especially toric lenses and aspheric lenses, in patients who had a tilted crystalline lens to start with,” he said.
While preoperative diagnostic tools have certainly improved in recent years, the progress has been arguably even more impressive in the array of devices now available to deliver intraoperative guidance for surgical steps such as corneal marking, toric IOL alignment and centration, among others.
As well as intraoperative aberrometers such as ORA (WaveTec Vision) and HOLOS IntraOp (Clarity Medical Systems), surgical guidance tools such as TrueGuide (TrueVision 3D Surgical), CALLISTO eye (Carl Zeiss Meditec) and VERION Image Guided System (Alcon Laboratories) all aim to reduce errors, increase accuracy and deliver better refractive outcomes by automating key aspects of the surgery.
“These new tools certainly help us to tighten up our refractive outcomes with toric IOLs,” said Stephen Slade MD, in private practice in Houston, USA.
After years of performing arcuate corneal incisions and manually marking the eye, Dr Slade has made the transition to using both the ORA Verisyse and the VERION systems, and he believes the clinical outcomes justify the investment. “The reason I like the VERION is that it is a different sort of system which works more as a digital marker – it is not really reading the astigmatism but gives excellent precision for toric IOL alignment," he said.
Dr Slade also uses the ORA device and the new HOLOS device and considers them useful because they give both a phakic and an aphakic reading. "The VERION is faster than the aberrometers though and a little more robust, but there are times when it is useful to have them both,” he added.
Dr Slade explained that the VERION works by capturing a preoperative high-resolution reference image of a patient’s eye in order to determine the radii and corneal curvature of steep and flat axes, limbal position and diameter, pupil position and diameter, and corneal reflex position.
“I use it systematically now for every patient that has astigmatism and the results have been excellent. I can’t think of even one patient where we have had to make a lens adjustment postoperatively because the VERION got the axis alignment wrong. It just hasn’t happened. We have even used it to reposition toric lenses that were implanted elsewhere and where the alignment was off – it is just so precise compared to any other tools we had and this is reflected in patient satisfaction,” he said.
Patient satisfaction, of course, remains the perennial dynamo driving development in all IOLs, toric lenses included. While rigorous surgical technique, precise measurements and quality implants are key to delivering the best possible refractive outcome, the importance of chair time should not be underestimated in ensuring the subjective happiness of patients after surgery.
The tried-and-trusted formula of under-promising and over-delivering applies to toric IOLs too, Dr Slade told EuroTimes.
“The main thing is to make sure that patients’ expectations are appropriate, but otherwise it is fairly easy to explain that they have astigmatism and that a toric lens will address that problem and give them very good postoperative vision. It’s a lot easier to explain than presbyopia, for instance, which requires more chair time to explain the compromises involved,” he said.
Nevertheless, deciding which patients will benefit from a toric lens is not always based solely on the extent of their preoperative astigmatism – economic factors and the patient's willingness to pay for a premium IOL may also come into play, points out Jesper Hjortdal MD, PhD, Aarhus University Hospital, Denmark.
“In the Danish public health system we can offer free toric IOLs to patients with more than 2.0 dioptres of corneal astigmatism. While patients with more than 0.75D astigmatism may possibly benefit, this is a trade-off considering best value for money within a fixed budget,” he said.
Dr Hjortdal said that he uses a variety of toric IOLs, with alignment based on preoperative keratometry and manual marking of the patient’s cornea in the upright position. In terms of improving outcomes from toric IOLs, Dr Hjortdal said that his clinic is planning to purchase a perioperative alignment system in the near future.
“Perioperative keratometry, or even better perioperative evaluation of the refractive properties of the anterior and posterior surface, with a direct coupling of overlay video in the microscope, should be useful and help to improve our results,” he said.
For exclusion criteria, the Danish guidelines for cataract surgery are helpful in selecting which patients may not benefit from a toric lens, said Dr Hjortdal.
“Patients who may not derive benefit from a toric lens include those who have not used correction for astigmatism in spectacles. Some degree of multifocality can be achieved by some patients with regular astigmatism, which may reduce the need for reading glasses. Furthermore, patients who wish to continue to use spectacles for distance vision after surgery or those who cannot expect good central vision postoperatively are usually not good choices for a toric lens,” he said.
While toric lenses offer a high degree of predictability in patients with regular astigmatism, the picture becomes more complicated in cases of irregular astigmatism and ocular co-morbidities. One possible solution for such complex eyes is a spherotoric bag-in-the-lens (BIL) IOL (Morcher GmbH), which allows surgeon-controlled IOL centration along the patient’s line of sight.
Marie-José Tassignon MD, PhD, FEBO, of the University of Antwerp, Belgium, the inventor of the BIL technique, said that a toric version was a logical evolution given that the original lens has shown excellent long-term protection against posterior capsule opacification and very good rotational stability. The toric lens used by Prof Tassignon has the cylindrical correction located on the anterior side of the lens optic and the calculation method has been adapted for one-sided implantation anteriorly. The IOL is centred based on the patient’s pupillary entrance using Purkinje reflexes of the surgical microscope light.
A recent prospective study of 52 eyes with corneal astigmatism ranging from 0.90 to 6.19 dioptres found that the lens delivered excellent refractive outcomes with minimal rotation of the lens six months after surgery.
“Implantation of the spherotoric IOL using the BIL technique gave excellent clinical results and was beneficial in eyes with up to 15 degrees of irregular astigmatism,” said Prof Tassignon, noting that 5.2 per cent of eyes in the study had irregular astigmatism of that degree.
“Implantation of the BIL IOL in eyes with irregular corneal astigmatism up to 10 degrees gave excellent results, but patients with a higher degree of astigmatism irregularity should be informed that the outcomes may be less predictable. However, the BIL IOL can be easily rotated in a secondary surgery without the surgeon having to deal with haptics embedded in fibrotic proliferative capsule tissue,” she added.
While much progress has been made in recent years in IOL design and materials, Dr Findl sees further developments in the pipeline that will help to improve toric outcomes even more.
“To come in the future, we will see corneal back-surface curvature measurements which is especially relevant for toric lenses. Also predicting IOL position according to the lens shape is something we are working on, because this may help us much more than the current central scan with two peaks. If we know the entire shape of the crystalline lens that may give a better estimate of where the equator is and where the IOL will be positioned. Finally, we will see our diagnostic devices incorporating ray tracing to enable us to get away from all these power calculation formulae that are based on old-generation ultrasound techniques,” he said.
Oliver Findl: firstname.lastname@example.org
Jesper Hjortdal: email@example.com
Stephen Slade: firstname.lastname@example.org