Non-Orthogonal Corneal Astigmatism among Normal and Keratoconic Brazilian and Chinese populations

Abass, Ahmed; Clamp, John; Bao, Fangjun; Ambrósio, Renato; Elsheikh, Ahmed

doi:10.1080/02713683.2018.1433858

Cited by 12 publications

(11 citation statements)

References 35 publications

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“…Forthcoming studies including spectacle-simulation or comparing the performance of toric IOLs and cIOLs might lead to additional insights. In many normal corneas, the steep axis and the flat axis are not perpendicular to each other 34 , thus the aberrations can not fully be compensated with standard corrections (toric IOLs, spectacles, or contact lenses). Custom IOLs (such as the cIOL) might become an option for eyes with such an irregular astigmatism including keratoconic eyes with stable corneal tomography (e. g. after cross-linking) and eyes after corneal transplantation (penetrating keratoplasty, deep anterior lamellar keratoplasty).…”

Section: Discussionmentioning

confidence: 99%

Keratoconic eyes with stable corneal tomography could benefit more from custom intraocular lens design than normal eyes

Schröder

Eppig

Liu

et al. 2019

Sci Rep

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We investigated whether eyes with keratoconic corneal tomography pattern could benefit more from aberration correction with custom intraocular lenses (IOLs) than normal cataractous eyes despite the effect of misalignment on the correction of aberrations. Custom IOLs (cIOLs) were calculated for twelve normal and twelve keratoconic eyes using personalized numerical ray tracing models. The Stiles-Crawford weighted root-mean-square spot-size (wRMS) at the virtual fovea was evaluated for cIOLs and aberration-neutral IOLs (nIOLs) in a simulated clinical study with 500 virtual IOL implantations per eye and per IOL. IOL misalignment (decentration, tilt, rotation) and pupillary ectopia (4.5 mm iris aperture) were varied upon each virtual implantation. The nIOLs achieved average wRMS of 16.4 ± 4.3 μm for normal, and 92.7 ± 34.4 μm for keratoconic eyes (mean ± standard deviation). The cIOLs reduced the average wRMS to 10.3 ± 5.8 μm for normal, and 28.5 ± 18.6 μm for keratoconic eyes. The cIOLs produced smaller wRMS than nIOLs in most virtual implantations (86.7% for normal and 99.4% for keratoconic eyes). IOL misalignment resulted in larger wRMS variations in the keratoconus group than in the normal group. Custom freeform IOL-optics-design may become a promising option for the correction of advanced aberrations in eyes with non-progressive keratoconic corneal tomography pattern.

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Section: Discussionmentioning

confidence: 99%

Keratoconic eyes with stable corneal tomography could benefit more from custom intraocular lens design than normal eyes

Schröder

Eppig

Liu

et al. 2019

Sci Rep

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“…Nevertheless, axial or sagittal topography maps are very sensitive to tilt and are not fully representative of the actual eye shape, especially in patients with keratoconus. Therefore, tangential or meridional maps are introduced in which they are much less sensitive to tilt and more sensitive in detecting local curvature changes 34 . The tangential map is defined by the curvature of the corneal meridian at a point on a surface.…”

Section: Methodsmentioning

confidence: 99%

Effects of intracorneal ring segments implementation technique and design on corneal biomechanics and keratometry in a personalized computational analysis

Bagheri

Kadkhodaei

Pirhadi

et al. 2021

Sci Rep

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The implementation of intracorneal ring segments (ICRS) is one of the successfully applied refractive operations for the treatment of keratoconus (kc) progression. The different selection of ICRS types along with the surgical implementation techniques can significantly affect surgical outcomes. Thus, this study aimed to investigate the influence of ICRS implementation techniques and design on the postoperative biomechanical state and keratometry results. The clinical data of three patients with different stages and patterns of keratoconus were assessed to develop a three-dimensional (3D) patient-specific finite-element model (FEM) of the keratoconic cornea. For each patient, the exact surgery procedure definitions were interpreted in the step-by-step FEM. Then, seven surgical scenarios, including different ICRS designs (complete and incomplete segment), with two surgical implementation methods (tunnel incision and lamellar pocket cut), were simulated. The pre- and postoperative predicted results of FEM were validated with the corresponding clinical data. For the pre- and postoperative results, the average error of 0.4% and 3.7% for the mean keratometry value ($$\text {K}_{\text{mean}}$$ K mean ) were predicted. Furthermore, the difference in induced flattening effects was negligible for three ICRS types (KeraRing segment with arc-length of 355, 320, and two separate 160) of equal thickness. In contrast, the single and double progressive thickness of KeraRing 160 caused a significantly lower flattening effect compared to the same type with constant thickness. The observations indicated that the greater the segment thickness and arc-length, the lower the induced mean keratometry values. While the application of the tunnel incision method resulted in a lower $$\text {K}_{\text{mean}}$$ K mean value for moderate and advanced KC, the induced maximum Von Mises stress on the postoperative cornea exceeded the induced maximum stress on the cornea more than two to five times compared to the pocket incision and the preoperative state of the cornea. In particular, an asymmetric regional Von Mises stress on the corneal surface was generated with a progressive ICRS thickness. These findings could be an early biomechanical sign for a later corneal instability and ICRS migration. The developed methodology provided a platform to personalize ICRS refractive surgery with regard to the patient’s keratoconus stage in order to facilitate the efficiency and biomechanical stability of the surgery.

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“…The topography maps enabled calculation of the height of the cornea relative to a best-fit spherical (BFS) surface, with the highest point assumed to represent the centre of the keratoconic cone [33,34]. The refractive power of healthy corneas P was calculated using the Gaussian optics formula [35][36][37]…”

Section: Identification Of Cone Area In Keratoconus Corneasmentioning

confidence: 99%

“…where α is the tangent angle at the calculation point and x is the distance from the apex. The mean power map was constructed based on the average of the two principal curvatures at each scanned point [37].…”

Section: Identification Of Cone Area In Keratoconus Corneasmentioning

confidence: 99%

Fibril density reduction in keratoconic corneas

Zhou

Abass

Lopes

et al. 2021

J. R. Soc. Interface.

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This study aims to estimate the reduction in collagen fibril density within the central 6 mm radius of keratoconic corneas through the processing of microstructure and videokeratography data. Collagen fibril distribution maps and topography maps were obtained for seven keratoconic and six healthy corneas, and topographic features were assessed to detect and calculate the area of the cone in each keratoconic eye. The reduction in collagen fibril density within the cone area was estimated with reference to the same region in the characteristic collagen fibril maps of healthy corneas. Together with minimum thickness and mean central corneal refractive power, the cone area was correlated with the reduction in the cone collagen fibrils. For the corneas considered, the mean area of keratoconic cones was 3.30 ± 1.90 mm 2 . Compared with healthy corneas, fibril density in the cones of keratoconic corneas was lower by as much as 35%, and the mean reduction was 17 ± 10%. A linear approximation was developed to relate the magnitude of reduction to the refractive power, minimum corneal thickness and cone area ( R 2 = 0.95, p < 0.001). Outside the cone area, there was no significant difference between fibril arrangement in healthy and keratoconic corneas. The presented method can predict the mean fibril density in the keratoconic eye's cone area. The technique can be applied in microstructure-based finite-element models of the eye to regulate its stiffness level and the stiffness distribution within the areas affected by keratoconus.

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Non-Orthogonal Corneal Astigmatism among Normal and Keratoconic Brazilian and Chinese populations

Cited by 12 publications

References 35 publications

Keratoconic eyes with stable corneal tomography could benefit more from custom intraocular lens design than normal eyes

Keratoconic eyes with stable corneal tomography could benefit more from custom intraocular lens design than normal eyes

Effects of intracorneal ring segments implementation technique and design on corneal biomechanics and keratometry in a personalized computational analysis

Fibril density reduction in keratoconic corneas

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