A 71 year old man underwent toric implantation with uneventful cataract surgery. It was a pretty straight forward case, with a preoperative with-the-rule (WTR) astigmatism on the anterior cornea, an average axial length and a simple non tricky Grade II nuclear sclerosis. The online renowned calculator available at the ASCRS website, as well as the company sponsored Toric calculator had promised for a minimal residual astigmaitsm of .1 and .2 respectively, without flipping the axis. The patient however landed with a plano -.75@90. Despite a perfectly aligned Toric IOL to its planned axis, a well planned biometry involving IOL Master 700 that accounted for posterior corneal astigmatism (PCA), and post operative subjective refraction done after a month of surgery to account for a stable refraction, the doctor was at his wit's end of what could have been the problem.
I am sure, as industry professionals, all of us have come across such an unfortunate result, if not multiple, atleast once. There are fortunately post operative online toric calculators that would help re align the lens to a desired position based on refractive outcomes. But knowing the reason, could equally help to avoid such ocassions in future, and spare the patient as well as the doctor from another round of operation room hassle. Assuming that biometry was perfectly done, the patient had a normal, non deceiving cornea, the toric lens is obediently sitting at its designated place, time has come to acknowledge the fact that lens tilt and IOL decentration may contribute to residual astigmatism. In this article, we will address this important aspect.
IOL tilt induced post operative astigmatism was found to be significantly more with increasing degree of tilt and higher IOL power in a study by Wiekert et al(1). Tilt around 90 degrees meridian with the nasal border of the IOL displaced anteriorly and the temporal border displaced posteriorly induced against-the-rule (ATR) astigmatism.
IOL tilt induced post operative residual astigmatism was found to be significantly more with increasing degree of tilt and higher IOL power in a study by Wiekert et al(1).
For average IOL powers (eg, 22.0 D) with an average tilt of 5 to 6 degrees, the induced astigmatism was approximately 0.1 to 0.2 D. When the tilt of these IOLs increased to 10 degrees, the resultant astigmatism increased significantly to 0.4 to 0.5 D. In the model used, aspheric IOLs and toric IOLs were tilted horizontally around a vertical meridian with the nasal
border tilted anteriorly and the temporal border tilted posteriorly. Interestingly, residual refractive astigmatism was more with higher diopter IOL power.
Most practices will however have no means to measure pseudophakic tilt in the eye unless equipped with additional devices like ultrasound biomicroscopy. However a featue in most optical biometry machines may help us to have an understanding of the ocular tilt and decentration of the eye preoperatively. The Iris barycenter coordinates available in many optical biometry machines may give an approximate idea of the angle alpha, which in turn is the difference between the optical axis and visual axis of the eye.
The Iris barycenter coordinates available in many optical biometry machines may give an approximate idea of the angle alpha
Therefore, when selecting toric IOLs in eyes with greater crystalline lens tilt or in eyes
with high IOL powers, consideration of the effect of tilt on ocular astigmatism might improve refractive accuracy as per Weikert et al.
Thus total keratometric astigmatism (anterior and posterior corneal astigmatism) may not be able to explain total ocular astigmatism or refractive astigmatism. Javal recognized this aspect two centuries back when he noticed significant difference in values between measured anterior cornea and refractive astigmatism. He proposed that refractive astigmatism could be calculated by multiplying keratometric astigmatism with a factor of 1.25 and then add the result with an additional .5D of against the rule (ATR) astigmatism. Koch et al in 2012 (2) further helped us in understanding total ocular astigmatism when he discovered the role played by posterior corneal astigmatism(PCA). Ignoring the PCA would lead to over correction in WTR and undercorrection in ATR astigmatism. However, even after accounting for PCA, total refractive astigmatism may remain unexplained without the consideration of lens tilt and decentration.
Kansara et al (2021) investigated the relationship between measured anterior and posterior corneal astigmatism and postoperative refractive astigmatism in pseudophakes implanted with a monofocal IOL (3). They studied the relationship between corneal astigmatism and refractive astigmatism on monofocal eyes to rule out any confounding factors that may result from a misaligned Toric IOL that may inturn add to post refractive astigmatism. They found out that eyes with significant anterior with-the-rule astigmatism (WTR) have a higher post operative ocular residual refractive astigmatism. They also noticed that the meridian or axis of this ocular residual refractive astigmatism is against the rule, that is opposite to the pre operative (WTR) anterior corneal astigmatism. The study postulated that the post surgery ocular residual refractive astigmatism could only be attributed to - 1) IOL lens tilt and decentration, 2) neuroadaptation.
They found out that eyes with significant anterior with-the-rule astigmatism (WTR) have a higher post operative ocular residual refractive astigmatism. They also noticed that the meridian or axis of this ocular residual refractive astigmatism is against the rule, that is opposite to the pre operative (WTR) anterior corneal astigmatism
Another aspect that may play a role in residual refractive astigmatism post Toric IOls is the fact that we have failed to recognize the importance of total surgicaly induced astigmatism(SIAtotal). Traditionally, surgically induced astigmatism(SIA) has been calculated only on the basis of anterior corneal curvature changes postoperativey minus preoperative anterior corneal curvature. Total SIA is the vector difference between the postoperative refractive astigmatism at the corneal plane and the pre operative Keratometry reading. The Total SIA, that is postoperative refractive astigmatism minus preoperative keratometric astigmatism , according to Holladay, will include post operative posterior corneal astigmatism changes, astigmatism resulting from IOL tilt and decentration, etc. However, Holladay cautions that the postoperative refractive astigmatism should be vertexed to the corneal plane to determine the Total SIA.
Total SIA is the vector difference between the postoperative refractive astigmatism at the corneal plane and the pre operative Keratometry reading
Holladay and Pettit(4) proposed to consider totalSIA that would account for IOL decentration and tilt that may have an impact on residual refractive astigmatism post Toric. They noted that the result of the physiologic tilt of the eye (angle alpha) of 5.2 degrees and mean temporal decentration of 0.2 mm, results in approximately 0.20 D of refractive ATR astigmatism.
result of the physiologic tilt of the eye (angle alpha) of 5.2 degrees and mean temporal decentration of 0.2 mm, results in approximately 0.20 D of refractive ATR astigmatism
In conclusion, we have discussed here a cause of post operative refractive surprise with Toric IOLs that has not been considered commonly in a clinic before, that is the impact of ocular tilt and decentration that may cause residual refractive astigmatism. Consideration of Total SIA, a concept first coined by Jack Holladay, may help to reduce the impact of tilt and decentration in an average eye, since unlike traditional corneal SIA, Total SIA accounts for lenticular astigmatism that may arise from such decentrations.
Reference:
1. Weikert MP, Golla A, Wang L. Astigmatism induced by intraocular lens tilt
evaluated via ray tracing. J Cataract Refract Surg 2018;44:745–749.
2.och DD, Ali SF, Weikert MP, Shirayama M, Jenkins R, Wang L. Contribution of posterior corneal astigmatism to total corneal astigmatism. J Cataract Refract Surg 2012;38:2080–20
3. Anterior, posterior, and nonkeratometric contributions to refractive astigmatism
in pseudophakes Neal Kansara, MD, David Cui, BA, Kalins Banerjee, MS, Zachary Landis, MD, Ingrid U. Scott, MD, MPH, Seth M. Pantanelli, MD, MS;J Cataract Refract Surg 2021; 47:93–99
4. Improving toric intraocular lens calculations using total surgically induced astigmatism for a 2.5 mm temporal incision Jack T. Holladay, MD, MSEE, George Pettit, MD, PhD;JCRS 2018