Quantifying Corneal Topographic Astigmatism (CorT Total) in Keratoconic Eyes.

PURPOSE: To determine optimal corneal regions from which to derive corneal topographic astigmatism (CorT) in kerato-conic eyes. METHODS: In this retrospective study, potential measures of corneal astigmatism are calculated from raw total corneal power data (179 eyes from 124 patients) from a corneal tomographer. The measures are derived from annular corneal regions varying in both extent and center position, and evaluated according to the variability of the ocular residual astigmatism (ORA) in the cohort. This variability is quantified by the ORArms, which is the root-mean-squared distance of the ORAs from their summated vector mean in double angle space. The lower the ORArms, the better the corneal astigmatism measure corresponds to manifest refractive cylinder. RESULTS: Corneal astigmatism measures derived from regions centered on corneal vertex had ORArms values (mild: 1.07 diopters [D], moderate: 1.61 D, severe: 2.65 D) as low or lower than other measures derived from regions centered on thinnest point, corneal apex (front or back), or pupil center. Corneal astigmatism measures derived from a region centered 30% of the way toward thinnest point from corneal vertex appeared to have even lower ORArms values (mild: 1.05 D, moderate: 1.45 D, severe: 2.56 D). None of the corneal astigmatism measures corresponded closely with manifest refractive cylinder for severe keratoconus (ORArms > 2.50 D). CONCLUSIONS: For keratoconic eyes, the CorT should be derived from an annular region centered 30% of the way toward thinnest point from corneal vertex, although when the keratoconus is mild, a standard corneal-vertex-centered CorT performs just as well. [J Refract Surg. 2023;39(3):206-213.].

Hemidivisional vector planning to reduce and regularize irregular astigmatism by laser treatment.

PURPOSE: To demonstrate how hemidivisional vector planning of refractive laser treatments of astigmatism can be used to directly address idiopathic corneal irregular astigmatism that has an asymmetrical, non-orthogonal bow tie topography appearance. DESIGN: Case study. METHODS: The cornea is conceptually divided into two hemidivisions along the flat meridian of the corneal topographic astigmatism (CorT), which means that each hemidivision will approximately correspond to one lobe of the asymmetric, non-orthogonal topographic bow tie. An astigmatism reduction treatment can then be planned separately for each hemidivision using the vector planning technique, based on both its two hemidivisional CorT measures and common manifest refractive cylinder. The remaining irregularity is then regularized, and the junctional zone smoothed across the flat meridian. The final intended treatment thus combines hemidivisional astigmatism reduction and regularization of the corneal astigmatism and spherical refractive error in one treatment application. This could be applied to LASIK, PRK, SMILE, and Transepithelial PRK procedures using Designer Cornea® software. RESULTS: A theoretical treatment profile is derived from an actual example of a cornea with idiopathic asymmetric non-orthogonal astigmatism. The three steps of the derivation are as follows: (i) astigmatism reduction through the use of the vector planning technique; (ii) regularization, and (iii) smoothing across the hemidivisional midline. CONCLUSIONS: Hemidivisional vector planning treatments could potentially both reduce and regularize asymmetric non-orthogonal astigmatism. These treatments can be systematically customized to account for qualitative and quantitative differences between the two corneal hemidivisions at the same time as correction of coexistent myopia or hyperopia.

Assessing Total Keratometry Astigmatism, Simulated Keratometry, and Total Corneal Topographic Astigmatism Against Manifest Refractive Cylinder.

PURPOSE: To evaluate how the Total Keratometry astigmatism measure from a swept-source optical biometer compares with simulated keratometry astigmatism from the same device, and total corneal topographic astigmatism (CorT Total) derived from a Scheimpflug tomographer. METHODS: For normal virgin eyes, the ocular residual astigmatism (ORA) magnitudes were determined based on Total Keratometry, simulated keratometry from the same optical biometer (SimKbiom), and CorT Total and simulated keratometry (SimKScheim) from a Scheimpflug tomographer. The ORA magnitudes for each type of measure were summarized into the standard deviations (ORAsd) and means (ORAmean). The lower the ORAsd, the less variability there is between corneal astigmatism and manifest refractive cylinder. The ORAmean indicates the amount of vectorial difference between the total corneal astigmatism and manifest refractive cylinder. RESULTS: The ORAsd for Total Keratometry was not significantly different from the ORAsd for CorT Total (P = .06) or SimKbiom (P = .41). The ORAmean for Total Keratometry was not significantly different from the ORAmean for CorT Total (P = .15), but was significantly lower than the ORAmean for SimKbiom (P < .001). CONCLUSIONS: Total Keratometry astigmatism correlates as well with manifest refractive cylinder as simulated keratometry astigmatism from the same device and CorT Total from a Scheimpflug tomographer. The average difference (as quantified by the ORAmean) between Total Keratometry astigmatism and manifest refractive cylinder was comparable to that of CorT Total, and less than that of simulated keratometry. Both of these results support the use of Total Keratometry over simulated keratometry in the planning of astigmatism surgery when corneal values are required. [J Refract Surg. 2021;37(3):198-201.].

Optimized CorT Total to compare Scheimpflug vs dual Scheimpflug/Placido imaging devices.

PURPOSE: To compare the manufacturer-provided measures of total corneal power (TCP) generated by Scheimpflug and dual Scheimpflug/Placido imaging compared with corneal topographic astigmatism calculated on the basis of measured TCP data (CorT Total). SETTING: Emory University, Atlanta, Georgia, USA. DESIGN: Retrospective case series. METHODS: TCP values were exported from virgin 209 eyes that underwent imaging with both the Scheimpflug (Pentacam HR) and dual Scheimpflug-Placido (Galilei G4) imaging devices to compute an optimized CorT Total. The standard deviation of the ocular residual astigmatism (ORAsd), which serves as a value describing the vectoral difference between the corneal astigmatism measure and manifest refractive cylinder at the corneal plane, was evaluated for all eyes to compare manufacturer-provided measurements vs the optimized CorT Total. RESULTS: The Scheimpflug CorT Total had the lowest ORAsd (0.306 diopter [D]; spherical equivalent [SE] 0.018) of all the parameters evaluated, although the difference was not statistically significant (P = .22) from the dual Scheimpflug/Placido CorT Total (0.32 2 D; SE 0.017). For the Scheimpflug device, the CorT Total had a statistically significant lower (P < .05) ORAsd in comparison to the best measure on the device (total corneal refractive power apex zone 2 mm: 0.324 D; SE 0.021). For dual Scheimpflug/Placido measurements, the CorT Total had the lowest ORAsd (0.322 D; SE 0.017), but the difference was not statistically significant (P = .43) from the lowest manufacturer-provided measure (TCP 2). CONCLUSIONS: CorT Total generated with the Scheimpflug device corresponded better with the manifest refractive cylinder than all measures of total corneal astigmatism calculated by the software from both the Scheimpflug and the dual Scheimpflug/Placido devices.

Corneal Topographic Astigmatism Based on Total Corneal Power Data (CorT Total): A Benchmark for Total Corneal Astigmatism.

PURPOSE: To evaluate how closely manufacturer-provided measures of total corneal astigmatism correspond with the manifest refractive cylinder, as compared to a benchmark of corneal topographic astigmatism calculated on the basis of measured total corneal power (TCP) data (CorT Total). METHODS: The SD of the ocular residual astigmatism magnitude (ORAsd) was evaluated for normal virgin eyes based on an optimized benchmark CorT Total and the various measures of total corneal astigmatism provided by 3 different Scheimpflug tomographers. RESULTS: The CorT Total corresponded with the manifest refractive cylinder at least as well as all the measures of total corneal astigmatism provided by the tomographers [Sirius CorT Total ORAsd: 0.320D (standard error [SE] 0.017D), Sirius TCP 4 mm ORAsd: 0.324D (SE 0.017D); Pentacam CorT Total ORAsd: 0.338D (SE 0.027D), Pentacam total corneal refractive power apex zone 4 mm ORAsd: 0.337D (SE 0.029D); Galilei CorT Total ORAsd: 0.472D (SE 0.068D), and Galilei TCP2 ORAsd: 0.536D (SE 0.124D)]. The difference between CorT Total and best measure on each tomographer was not statistically significant (Sirius TCP 4 mm: P = 0.24, Pentacam total corneal refractive power apex zone 4 mm: P = 0.64, Galilei TCP2: P = 0.24). Most of the manufacturer-provided measures did not correspond closely with the manifest refractive cylinder. When there were multiple measures of total corneal astigmatism, those derived from a zone with a diameter of 4.0 mm corresponded best with the manifest refractive cylinder. CONCLUSIONS: The CorT Total is a reliable benchmark measure that can be used to assess how well other measures of total corneal astigmatism correspond with the manifest refractive cylinder.

Role of Hemidivisional Corneal Topographic Astigmatisms (CorTs) in the Regularization and Reduction of Irregular Astigmatism.

PURPOSE: To demonstrate how the concept of hemidivisional corneal topographic astigmatism (hemiCorT) enables the planning of hemidivisional corneal treatments to reduce irregularity and overall astigmatism. METHODS: Whole-of-cornea corneal topographic astigmatism (CorT) is calculated from topography data derived from a corneal topographer or tomographer. The cornea is conceptually divided into 2 hemidivisions along the flat meridian of the CorT. For each hemidivision, hemiCorTs are calculated. The regularization treatment for each hemidivision is the treatment required to target the whole-of-cornea CorT, which is a symmetrical orthogonal corneal astigmatism. The regularization is then combined with astigmatism reduction treatment, which could be a conventional refractive treatment or a vector-planned treatment. For each hemidivision, the combined astigmatic effect of the regularization treatment and reduction treatment can be determined through double-angle vector summation. The 2 hemidivisional treatments together regularize and reduce corneal astigmatism. RESULTS: A theoretical pair of hemidivisional treatments is derived from an actual example of a cornea displaying idiopathic asymmetric nonorthogonal astigmatism. CONCLUSIONS: HemiCorTs allow for the design of hemidivisional corneal treatments of asymmetric nonorthogonal astigmatism. Such treatments should be suitable in the routine treatment of commonly occurring irregular astigmatism, while also allowing the spherical refractive error to be treated concurrently.

Planning for Coupling Effects in Bitoric Mixed Astigmatism Ablative Treatments.

PURPOSE: To demonstrate how to determine the historical coupling adjustments of bitoric mixed astigmatism ablative treatments and how to use these historical coupling adjustments to adjust future bitoric treatments. METHODS: The individual coupling adjustments of the myopic and hyperopic cylindrical components of a bitoric treatment were derived empirically from a retrospective study where the theoretical combined treatment effect on spherical equivalent was compared to the actual change in refractive spherical equivalent. The coupling adjustments that provided the best fit in both mean and standard deviation were determined to be the historical coupling adjustments. Theoretical treatments that incorporated the historical coupling adjustments were then calculated. The actual distribution of postoperative spherical equivalent errors was compared to the theoretically adjusted distribution. RESULTS: The study group comprised 242 eyes and included 118 virgin right eyes and 124 virgin left eyes of 155 individuals. For the laser used, the myopic coupling adjustment was -0.02 and the hyperopic coupling adjustment was 0.30, as derived by global nonlinear optimization. This implies that almost no adjustment of the myopic component of the bitoric treatment is necessary, but that the hyperopic component of the bitoric treatment generates a large amount of unintended spherical shift. The theoretically adjusted treatments targeted zero mean spherical equivalent error, as intended, and the distribution of the theoretical spherical equivalent errors had the same spread as the distribution of actual postoperative spherical equivalent errors. CONCLUSIONS: Bitoric mixed astigmatism ablative treatments may display non-trivial coupling effects. Historical coupling adjustments should be taken into consideration when planning mixed astigmatism treatments to improve surgical outcomes. [J Refract Surg. 2017;33(8):545-551.].

Asymmetric Corneal Flattening Effect After Small Incision Cataract Surgery.

PURPOSE: To determine whether the flattening effect of corneal incisions differs between the right and left eye. METHODS: A retrospective study of preoperative and postoperative corneal astigmatism was performed for patients who had bilateral cataract surgery by a right-handed surgeon. The change in corneal astigmatism was attributed to the 2.2-mm phacoemulsification incision, and the incisional flattening effect was calculated. The incisions were grouped by position on the eye and whether they were performed on the preoperative steep corneal meridian. RESULTS: A total of 1,298 eyes of 649 patients were evaluated. The flattening effect of temporal 2.2-mm incisions performed on the preoperative corneal steep meridian was different for right eyes (0.53 diopters [D]) and left eyes (0.34 D) (P = .017). The flattening effect of superior 2.2-mm incisions performed on the preoperative corneal steep meridian was equivalent in the two eyes. CONCLUSIONS: The flattening effect of a corneal incision may depend on whether it has been performed on the right or the left eye. [J Refract Surg. 2016;32(9):598-603.].

Corneal topographic astigmatism (CorT) to quantify total corneal astigmatism.

PURPOSE: To evaluate the performance of corneal topographic astigmatism (CorT) based on total corneal power measurements. METHODS: Anterior, posterior, and total corneal power measurements of 526 virgin eyes obtained using the CSO Sirius tomographer (Costuzione Strumenti Oftalmici, Scandicci, Florence, Italy) were analyzed. Individual CorTs were created from each set of data. These CorTs were assessed using ocular residual astigmatism (ORA), which quantifies corneo-refractive differences. A low standard deviation of the ocular residual astigmatism (ORAsd) indicates a low variability between corneal astigmatism and refractive cylinder. A low mean of the ORA magnitude indicates a close correlation of refractive cylinder and corneal astigmatism. RESULTS: The CorT based on total corneal power measurements had an ORAsd of 0.30 diopters (D) and a mean ORA magnitude of 0.53 D. The CorT candidates based on anterior corneal power measurements all had an ORAsd of at least 0.32 D, and the mean ORA magnitudes were all 0.64 D or greater. Both the ORAsd and mean ORA magnitude of the CorT based on total corneal power measurements were significantly less than those of the CorT based on anterior corneal power measurements (both P < .001, as estimated via bootstrapping). CONCLUSIONS: The CorT based on total corneal power measurements corresponds better, both in variability and closeness, with manifest refractive cylinder than the CorT based on anterior corneal power measurements. This total CorT would be fundamental when planning toric intraocular lenses or limbal relaxing incisions or other corneal astigmatic surgery.

Corneal coupling of astigmatism applied to incisional and ablative surgery.

PURPOSE: To redefine measures of corneal coupling for use with incisional and ablation procedures for astigmatism. SETTING: Private clinics, Melbourne, Victoria, Australia. DESIGN: Retrospective nonrandomized study. METHODS: The measures known as the coupling ratio (CR) and coupling constant (CC) were redefined to ensure validity in most cases of incisional procedures and laser vision correction procedures. In addition, a new measure--the coupling adjustment (CAdj)--was developed to quantify the amount of spherical adjustment that must be applied to compensate for coupling that occurs as a result of astigmatism treatment. These quantitative measures of coupling were applied to retrospective data to show their applicability. RESULTS: Pure myopic, compound myopic, and compound hyperopic astigmatism excimer laser treatments showed a CR close to zero, a CC close to 0.5, and a CAdj close to zero. Incision LRIs showed a CR close to 1.0 and a CC close to zero. In all cases, the coupling measures were consistent for treatments with a larger astigmatic component (>1.0 diopter) but variable when the astigmatic component of the treatment was smaller. CONCLUSIONS: The revised definitions of CR and CC can be used with incisional and ablative surgery. Incorporating the CAdj into the planning of spherocylindrical treatments allows one to factor in the effect of the astigmatic treatment on the spherical component and thus to more accurately target the desired spherical equivalent. FINANCIAL DISCLOSURE: Dr. Alpins and Mr. Stamatelatos have a financial interest in the Assort software program. Dr. Ong is an employee of Assort.

Refractive surprise after toric intraocular lens implantation: graph analysis.

PURPOSE: To determine the refractive cylinder effect of rotating a toric intraocular lens (IOL) and identify the sources of refractive astigmatic surprise after toric IOL implantation. SETTING: Private practice, Melbourne, Australia. DESIGN: Experimental study. METHODS: Vergence formulas using a standard reduced eye model were used to bring all lens powers to the corneal plane. Double-angle vector diagrams were then used to (1) determine the refractive cylinder effect of rotating a toric IOL and (2) show how the prevailing astigmatism and the various planning and surgical steps involved in implanting a toric IOL contribute to the postoperative manifest refractive cylinder. RESULTS: An example calculation is given to illustrate the method. CONCLUSIONS: Refractive cylinder surprises can occur after toric IOL implantation. Understanding the causes enables surgeons to address contributory factors and choose an appropriate surgical method for managing individual cases of refractive cylinder surprise. FINANCIAL DISCLOSURE: Dr. Alpins and Mr. Stamatelatos have a financial interest in the Assort software program used to support the planning and analysis of astigmatic correction. Dr. Ong is an employee of Assort.

New method of quantifying corneal topographic astigmatism that corresponds with manifest refractive cylinder.

PURPOSE: To derive a method of quantifying corneal topographic astigmatism (CorT) that accurately represents manifest refractive cylinder. SETTING: Private practice, Melbourne, Australia. DESIGN: Retrospective study. METHOD: Axial power measurements obtained using topography in right eyes and left eyes were analyzed. For each Placido ring, an astigmatism value was calculated. The ring astigmatism values were combined via vector summation to create a new measure termed CorT. This parameter was assessed against other commonly used measures of corneal astigmatism using the ocular residual astigmatism (ORA) and its standard deviation (SD) on how closely each measure matched manifest refractive cylinder. The flat meridian of the CorT can also be used to conceptually divide the cornea into 2 hemidivisions and a CorT value subsequently calculated for each hemidivision of the cornea. RESULTS: The CorT was assessed against other commonly used measures of corneal astigmatism using the ORA (0.62 diopters [D] ± 0.33 [SD]) and had better correlation with manifest refractive cylinder than manual keratometry (K) (ORA 0.68 ± 0.38 D), simulated K (ORA 0.70 ± 0.35 D), corneal wavefront (ORA 0.74 ± 0.36 D), and paraxial curvature matching (ORA 0.85 ± 0.48 D). The SD of the ORA for CorT was significantly less than the other measures of astigmatism (P<.001). CONCLUSIONS: An alternative measure of corneal astigmatism, known as CorT, corresponded better to manifest refractive cylinder than other commonly used measures. A hemidivisional CorT can also represent the nonorthogonal asymmetrical astigmatism in irregular corneas. FINANCIAL DISCLOSURE: Dr. Alpins and Mr. Stamatelatos have a financial interest in the ASSORT software program used to support the planning and analysis of astigmatic correction. Dr. Ong is an employee of ASSORT.

Measuring torsional eye movements by tracking stable iris features.

We propose a new method to measure torsional eye movements from videos taken of the eye. In this method, we track iris features that have been identified as Maximally Stable Volumes. These features, which are stable over time, are dark regions with bright borders that are steep in intensity. The advantage of Maximally Stable Volumes is that they are robust to nonuniform illumination and to large changes in eye and camera position. The method performs well even when the iris is partially occluded by reflections or eyelids, and is faster than cross-correlation. In addition, it is possible to use the method on videos of macaque eyes taken in the infrared, where the iris appears almost featureless.