Technological advances mean that eyes with cataract and astigmatism, as well as those with myopia and hyperopia, and eyes that have had previous laser refractive surgery, can now be successfully implanted with toric intraocular lenses (IOL).
At a webinar hosted by the Asia Pacific Association of Cataract and Refractive Surgeons (APACRS) in May, four global leaders in refractive cataract surgery shared their essential pearls for successfully doing so.
The esteemed presenters were Dr Graham Barrett (Australia), Dr Tetsuro Oshika (Japan), Dr Ronald Yeoh (Singapore), and Dr Hungwon Tchah (South Korea) and the focus was on the Tecnis Toric II intraocular lens.
The Importance of Lens Calculation
Calculating the correct toric IOL for patients requires careful consideration of the magnitude and axis of the required toric cylinder. Measuring the cornea, interpreting measurements, predicting the required cylinder, and accurately aligning the toric IOL axis are all essential to avoid unexpected astigmatic outcomes.
Dr Graham Barrett, best known for founding the Barrett Toric Calculator, spoke about how he has evolved his much used formula to enable practitioners to accurately choose toric IOLs that meet each patient’s needs.
His Barrett Toric Calculator, based on the widely used Barrett Universal II formula (BUII), provides a spherical power prediction as well as the predicted toric cylinder value. Available on the APACRS website, the calculator predicts the posterior corneal power for each eye based on a theoretical model.
Importantly it has an integrated keratometry (K) calculator and provides measured posterior corneal surgery options once the data has been entered and calculated. Determining a vector mean or median K is helpful when measuring corneal astigmatism for toric IOL calculations because corneal astigmatism measurements are not always repeatable when measuring the axial length from different devices.
Evolution of a Formula
Dr Barrett explained that he developed the True K formula to enable greater accuracy when managing patients who have undergone myopic LASIK or laser correction for hyperopia. More recently, he developed the True K TK formula, which allows for the incorporation of the measured posterior cornea. He did this because his studies had confirmed that this combination provides the most accurate and repeatable option in both myopic and hyperopic patients undertaking cataract surgery when there is no prior refractive information available.
But that’s not where the story ends. Having determined that a custom toric calculator needs to be used when selecting a toric IOL for eyes that have undergone previous refractive surgery, Dr Barrett went on to further evolve his formula.
His latest iteration, known as the True K Toric Calculator, can be used with the predicted posterior cornea or a measured option for posterior corneal astigmatism, and it includes the K Calculator, allowing users to enter up to three different values for the anterior cornea. The True K Toric Calculator calculates a new integrated K or median vector value which is used for the final lens recommendation.
Dr Barrett advised that when using the formula, the patient’s surgically induced astigmatism (SIA) should be listed as the centroid value, which is typically in the range of 0.1-0.12 for a 2.4 mm temporal incision. Additionally, he recommended a lens factor of 2.09, although a personal constant can be entered if preferred.
Optimising Surgical Outcomes
Choosing the best toric lens to avoid astigmatic outcomes is one thing – then comes the surgery itself.
Dr Tetsuro Oshika from the University of Tsukuba in Japan, with input from Drs Barrett, Yeoh and Tchah, provided tips on optimising patient outcomes of toric IOLs.
In a nod to Dr Barrett’s True K Toric Calculator, Dr Oshika said a proper toric calculator is essential to accurately measure the K value by taking into consideration both the anterior and posterior corneal surface. This is because the anterior surface of younger eyes will change over time, moving from with-the-rule astigmatism (WTR) to against-the-rule (ATR) astigmatism with age. Conversely, the posterior cornea surface will primarily be ATR in eyes of all ages. An accurate calculation will avoid under correcting for ATR or overcorrecting for WTR astigmatism.
Can it Be Corrected?
However, he said it is important to note that toric IOLs will only correct regular astigmatism. A Fourier map, which separates topography into four different components (spherical, regular astigmatism, asymmetry, and higher-order irregularity) is the most useful way to differentiate between regular and irregular astigmatism (and hence appropriate candidates). Eyes with spherical and regular astigmatism can be successfully corrected with toric lenses.
Mark Up with Precision
Whether undertaken manually or digitally, precise reference marking before surgery and axis marking during surgery is essential to avoid postoperative axis misalignment. Digital marking, using devices such as the Verion, Callisto Eye or Ora systems is quick and convenient, however Dr Oshika said a comparative study he conducted found no significant difference in outcomes compared with manual markups. Accuracy is key.
Dr Oshika highlighted advances in toric IOLs that have significantly improved rotational stability and hence, patient outcomes.
For example, the frosted haptics of Johnson and Johnson Vision’s (JJV) new Tecnis Toric II IOL offers more surface texture and increased friction within the capsule than the polished haptics of the original Tecnis Toric IOL.
This enables the Toric II to unfold faster than the original IOL, as the frosted haptic does not stick to the optic while in the cartridge during insertion.
He said results of his studies have shown that the amount of axis misalignment was significantly reduced in the Toric II IOL compared to the Toric IOL, clearly indicating that the rotational stability is very much improved with the TECNIS Toric II IOL.
To further improve rotational stability Dr Oshika recommended securely fitting the lens in the capsular bag. Additionally, having examined eyes six months post-surgery, he said eyes with complete anterior coverage of the continuous curvilinear capsulorhexis (CCC) edge achieve significantly more rotational stability than eyes with partial cover.
Dr Yeoh advised surgeons to be sure to inject the IOL and align the axis 10º shy of the desired position, remove ophthalmic viscosurgical devices behind the IOL (to maintain stability of the lens), and then dial the IOL to the intended position. Additionally, while being careful not to over-rotate the IOL, they should press the IOL onto the posterior capsule using the side port.
He said an advantage of the Tecnis Toric IOL is that the A-constant is the same for all single piece Tecnis lens implants. Once a lens has been selected using the Barrett Toric Calculator, the lens factor and A-constant will automatically populate for the calculation.
A study conducted by Dr Oshika found that if repositioning surgery is performed immediately after cataract surgery, there is a significantly higher degree of final misalignment, and some IOLs will even rotate in the same direction following the repositioning surgery. This is because the capsular bag has not completely undergone fibrosis and the lens itself may stick to the capsular bag.
Thus the timing of repositioning surgery is critical to achieving sound rotational stability. Waiting one week or longer postoperatively, significantly decreased the degree of final misalignment of the IOL.
Patients should be instructed to rest and avoid any sudden movements in the first hour after surgery as the largest rotation of the IOL was found to occur during one hour post-surgery. Eye rubbing and high impact exercising (like jogging) should be avoided for two to three weeks.
TECNIS Toric II is currently available as TECNIS Synergy TORIC II in Australia and New Zealand to provide high-quality continuous vision and engineered for rotational stability.