Astigmatism arises from asymmetry in the cornea or lens (e.g., asymmetric curvature, decentration, or tilt), or from variations in refractive index within the lens. Clinically, corneal astigmatism is characterized by the meridian of greatest refractive power, whereas refractive astigmatism is specified by the axis perpendicular to that meridian.

Astigmatism can be categorized as regular or irregular. Regular astigmatism is defined as a refractive error in which the cornea or lens has two principal meridians that are orthogonal (90° apart), each with a uniform curvature, resulting in a symmetric toric shape. This produces a cylindrical refractive error that can be fully described by a single axis and magnitude, and is correctable with spherocylindrical lenses or toric IOLs. ^, Irregular astigmatism occurs when the principal meridians are not orthogonal or curvature varies along a meridian, resulting from corneal pathology, scarring, or ectasia. It cannot be corrected solely by toric IOLs, and surgical correction should only be considered in specific types of irregular corneal astigmatism. ^,, Corneal astigmatism is determined by measuring the curvature radii along the two orthogonal principal meridians.

Dioptric power is calculated by dividing the difference in refractive indices of the incident medium (e.g., air, 1.000) and refracted medium (e.g., cornea, ∼1.376) by the curvature radius of a refracting surface.

Anterior corneal astigmatism is typically assessed through keratometry or corneal topography.

Although both anterior corneal astigmatism and keratometric astigmatism are derived from measurements of anterior corneal curvature, they differ in their optical assumptions. Anterior corneal astigmatism represents the true refractive power of the anterior corneal surface calculated using the physical corneal refractive index (∼1.376), whereas keratometric astigmatism uses an effective refractive index (typically ∼1.3375) that empirically incorporates the average contribution of the posterior corneal surface.

It is also important to note that keratometry only measures the central 3 mm of the corneal curvature, and considers the anterior corneal surface only, assuming a consistent relationship with the posterior curvature. Total corneal astigmatism is determined via corneal tomography of both corneal surfaces. Internal astigmatism arises from the toric shapes and misalignments of the posterior cornea and lens. It is quantified as the vector difference between refractive astigmatism (adjusted to the anterior corneal plane) and anterior corneal astigmatism.

Astigmatism is among the most prevalent refractive errors. Its prevalence varies markedly across age groups and populations. ^, A meta-analysis by Hashemi et al. (163 studies) reported global prevalence of 14.9% in children, rising to 40.4% in adults, exceeding both hyperopia and myopia. ^, Clinically significant refractive astigmatism of ≥ 0.5 D prevalence increases markedly with age, with notably higher rates in individuals aged 70 years and older.

With-the-rule (WTR) astigmatism predominates in individuals under 40 years of age, while against-the-rule (ATR) and oblique astigmatism increase with age. Most individuals exhibit symmetry in axis orientation between eyes (i.e., both eyes with WTR, ATR, or oblique astigmatism). This symmetry is less pronounced in geriatric populations.

Overall, more than two-thirds of patients undergoing cataract surgery have a corneal astigmatism of at least 0.5 D, and a substantial proportion exhibit values exceeding 1.0 D. ^,,,, In these prevalence studies, clinically relevant astigmatism was typically defined as corneal astigmatism magnitude ≥ 0.5 D, with lower values considered clinically insignificant. In a large population study of 39,744 eyes implanted with non-toric monofocal IOLs, Day et al. reported that 90% had postoperative refractive astigmatism ≥ 0.5 D.

In recent years, toric IOLs have been widely adopted as the preferred method for managing corneal astigmatism while improving postoperative clinical and patient-reported outcomes.

Although a preoperative regular corneal astigmatism of ≥ 1.0 D is often cited as an indication for toric IOL implantation in patients seeking spectacle independence, this threshold remains debated. Moreover, clinical decision-making has increasingly shifted toward the use of predicted postoperative residual refractive astigmatism (PRA). ^, An ongoing clinical trial, the European Society of Cataract and Refractive Surgeons (ESCRS) TORIC Study, is investigating toric IOL effectiveness for patients with PRA of 0.75 to 1.50 D.

Contraindications for implanting toric IOLs include (iatrogenic) zonular instability or posterior capsular rupture, as these factors compromise the rotational stability of the IOL. Poor pupillary dilation may impede visualization and accurate alignment of toric IOLs. However, this can often be managed with surgical strategies such as iris hooks.

Several surveys have shown that toric IOL adoption has increased in recent years. The ESCRS Clinical Trends Survey (2016-2021) revealed a 7.4% to 15.5% increase during this period ( P <.01). Cost was identified as a limiting factor, suggesting that usage would rise even further if financial barriers were eliminated. In a separate UK-based survey 30% of respondents considered toric IOLs for corneal astigmatism under 1.0 D, while 40% recommended them for 1.0 D or greater. Notably, years of surgical experience did not significantly influence these thresholds. These findings align with existing evidence supporting the benefits of toric IOLs in patients with as little as 0.75 D of corneal astigmatism. A Japanese Society of Cataract and Refractive Surgery survey (2013 to 2022) showed the proportion of surgeons using toric IOLs rose from 28% (2010) to 87% (2023). The most common indication for toric IOL implantation remained stable, typically for eyes with corneal astigmatism of ≥ 1.5 D.

Accurate preoperative assessment of both corneal and refractive astigmatism is essential for toric IOL implantation. ^,, Advances in diagnostic technology have greatly improved the precision of corneal measurements. Tools such as manual and automated keratometers, Placido-disc topographers, Scheimpflug imaging, anterior segment optical coherence tomography (AS-OCT) are commonly used to measure anterior corneal astigmatism. However, readings can be influenced by tear film quality and ocular surface conditions, which are crucial for obtaining reliable measurements. Tear film instability can distort keratometric readings, leading to inaccurate IOL calculations and suboptimal visual outcomes. Treating ocular surface disease preoperatively has been shown to modify corneal astigmatism measurements in most cases, often altering surgical plans. Contact lens discontinuation prior to measurement is advised (soft lenses: 24 hours to 2 weeks; rigid gas permeable lenses: 2 to 4 weeks).

Refractive astigmatism can vary with ocular surface changes, accommodation, cycloplegia, and pupil size, all of which should be considered during assessment. In irregular corneas, measurements become more variable; however, good outcomes are achievable in selected cases when corneal astigmatism is consistent across devices. Validation involves comparing multiple measurements with clinical indicators such as prior refractive surgery or asymmetry between eyes necessitating further verification. Many surgeons recommend using at least three different measurement techniques, and if there is poor agreement between them, toric IOL use should be reconsidered.

Beyond objective measures, uncorrected refractive astigmatism has a substantial impact on both visual function and quality of life. It was shown to contribute to a range of debilitating symptoms that interfere with everyday activities. Patients frequently report decreased visual quality, increased glare (53%-77%), haloes (28%-80%), difficulty with night driving (66%), a higher risk of falls, and substantial spectacle dependence (45%-85%). These subjective experiences are supported by objective data: vision-dependent task performance is significantly impaired, with individuals having 1.0 D of uncorrected refractive astigmatism performing tasks 9% more slowly and making 38% more errors than fully corrected individuals. At 2.0 D, task completion slows by 29%, and error rates increase by 370%. Such findings emphasize the importance of identifying and correcting clinically significant refractive astigmatism. ^,

Patient-reported outcome measures (PROMs), such as the NEI-RQL 42 questionnaire, consistently demonstrate that toric IOL implantation provides significant improvements in both visual performance and quality of life compared with non-toric IOLs. These subjective benefits are corroborated by objective aberrometric data. ^,, Buscacio et al. demonstrated that toric IOL implantation in patients with low corneal astigmatism (0.75 to 1.50 D) improved visual acuity and quality-of-life scores, confirming meaningful benefits even at low astigmatism levels.

The outcomes of toric IOLs have been frequently compared with those of limbal relaxing incisions (LRIs) and femtosecond laser-assisted astigmatic keratotomy (FSAK), which represent alternative surgical options for corneal astigmatism management. ^,,,,,,,, Overall, toric IOLs appear to provide superior correction of regular corneal astigmatism, offering more predictable and stable refractive results, even at low levels of astigmatism. A systematic review and meta-analysis of 13 RCTs analyzed the results of 707 eyes receiving toric IOLs and 706 eyes receiving non-toric IOLs (225 of which underwent LRIs). Postoperative UDVA was significantly better in the toric IOL group, and these patients experienced a higher rate of distance spectacle independence. Residual refractive astigmatism was also significantly lower with toric IOLs compared to non-toric IOLs plus LRIs. Importantly, there was no significant increase in surgical complications. A Cochrane review of 10 RCTs (626 eyes) found toric IOLs more effective than LRIs in reducing residual astigmatism, with more patients achieving < 0.5 D residual refractive astigmatism and greater spectacle independence. Overall, these findings underscore the advantages of toric IOLs over LRIs for astigmatism correction, offering superior visual outcomes without increasing surgical risk.

A very recent meta-analysis of 9 studies (590 participants) compared FSAK with toric IOLs for astigmatism correction in cataract patients. Toric IOLs provided significantly better UDVA than FSAK, and toric IOLs were also more likely to achieve a residual refractive cylinder of 1.0 D or less. Trial sequential analysis confirmed robust evidence favoring toric IOLs for reducing postoperative refractive astigmatism. FSAK was associated with significantly lower target-induced astigmatism (TIA) and smaller correction index and SIA. Both approaches were safe, with no severe complications reported. Similar findings were reported by Zheng et al., with toric IOLs demonstrating superior efficacy in reducing refractive astigmatism compared to FSAK, particularly in higher preoperative astigmatism.

Toric IOL rotational stability depends primarily on haptic design. C-loop haptics are most common; frosted loop haptics (e.g., Tecnis Toric II) have been shown to reduce postoperative rotation through increased friction. Plate-haptic designs show less rotation in myopic eyes, while double C-loop (quad-haptics) haptics demonstrate the greatest rotational stability. ^,,,, For spectacle independence, remaining refractive astigmatism defeats the purpose of this type of IOL. The magnitude of residual postoperative astigmatism has been shown to correlate negatively with visual acuity in eyes implanted with multifocal IOLs. Multifocal toric IOLs can be either diffractive or refractive. Diffractive designs can provide better distance vision, while refractive types offer better near vision and fewer photic phenomena. Additionally, diffractive IOLs appear to be particularly sensitive to residual postoperative astigmatism, adversely affecting visual performance.

The main materials used in IOLs are polymethylmethacrylate (PMMA), silicone, and hydrophilic and hydrophobic acrylate. Silicone is currently still used for toric IOLs and is associated with lower rates of PCO, while PMMA is associated with higher rates. Silicone poses a higher risk for postoperative infection due to bacterial adhesion, and its quick unfolding can increase the risk of capsular bag injury. Acrylic IOLs are subdivided into hydrophilic and hydrophobic IOLs. Their foldable nature results in a smaller incision size, resulting in less SIA. Hydrophobic acrylic is associated with less postoperative rotation, this is attributed to the stronger adhesion of the lens to the capsular bag. This stronger adhesion is also associated with a lower rate of PCO in hydrophobic IOLs. ^, More rigid IOLs, like hydrophobic IOLs, also have slower unfolding, leading to a more controlled implantation.

Rotational stability is critical for toric IOLs, since any off-axis rotation reduces the magnitude of astigmatism correction. For every degree of misalignment from the intended axis, approximately 3.3% of the intended astigmatic correction is lost. Thus, a 10° misalignment results in about a 33% reduction in astigmatic magnitude correction, and a 30° misalignment negates the corrective effect entirely and may even induce astigmatism in a different axis. Clinical studies confirm that even small rotational deviations (≥ 10°) can lead to significant residual refractive astigmatism and suboptimal uncorrected visual acuity, often necessitating surgical realignment. Furthermore, higher cylinder power IOLs are more sensitive to misalignment, with greater loss of correction per degree of rotation. ^,

Studies consistently report that significant IOL rotation is infrequent where over 90 to 95% of toric IOLs stay within 5° to 10° of intended axis without need for reoperation. ^,,, In a large multicentre series of 6431 eyes, only 42 eyes (0.65%) required surgical repositioning of a toric IOL due to clinically relevant misalignment. The mean rotation prompting reoperation in that study was about 33° off-axis, and repositioning was typically performed around 1 to 2 weeks post op (average of 10 days). Other published series show toric IOL repositioning rates on the order of 0.5% to 2% of cases, varying with sample size and lens models.

Evidence suggests that waiting roughly 1 week post op before a repositioning surgery yields better stability; lenses repositioned too early (e.g., within 1 to 2 days) have a higher tendency to re-rotate, whereas waiting approx. 7 days allows capsular fibrosis to begin, helping “lock in” the lens after adjustment.

Complications of IOL repositioning are generally infrequent, with most being mild or transient. In a large retrospective, multicenter case series evaluating the incidence and outcomes of secondary repositioning surgery for misaligned toric IOLs, no sight-threatening complications were documented. The authors note that late repositioning may carry a theoretical risk of zonular stress due to capsular bag contraction. Other potential adverse events include temporary intraocular pressure (IOP) elevation, cystoid macular edema, IOL repeated dislocation, intraocular hemorrhage or inflammation, and endothelial cell loss.

There is a lack of evidence guiding postoperative behavioral instructions specifically for toric IOL patients. A recent study by Jandewerth et al. saw no significant postoperative IOL rotation for a non-toric IOL in lying down vs walking. No reviews were found on this subject. Most guidelines focus on general cataract surgery recovery rather than addressing the specific rotational risks associated with toric IOLs.