Limits...
Evaluation of the shape symmetry of bilateral normal corneas in a Chinese population.

Bao F, Chen H, Yu Y, Yu J, Zhou S, Wang J, Wang Q, Elsheikh A - PLoS ONE (2013)

Bottom Line: The mean geometric center (x0, y0, z0) of both right and left corneas was located on the temporal side and inferior-temporal side of the apex in anterior and posterior topography map, respectively.Further, rotation angle γ along Z axis, which is related to corneal astigmatism, showed clear mirror symmetry.This characteristic could help in detection of pathological abnormalities, disease diagnosis, measurement validation and surgery planning.

View Article: PubMed Central - PubMed

Affiliation: The Affiliated Eye Hospital of WenZhou Medical College, Wenzhou, China.

ABSTRACT

Purpose: To investigate the bilateral symmetry of the global corneal topography in normal corneas with a wide range of curvature, astigmatism and thickness values.

Design: Cross-Sectional Study.

Methods: Topography images were recorded for the anterior and posterior surfaces of 342 participants using a Pentacam. Elevation data were fitted to a general quadratic model that considered both translational and rotational displacements. Comparisons between fellow corneas of estimates of corneal shape parameters (elevation, radius in two main directions, Rx and Ry, and corresponding shape factors, Qx and Qy) and corneal position parameters (translational displacements: x0, y0 and z0, and rotational displacements: α, β and γ) were statistically analyzed.

Results: The general quadratic model provided average RMS of fit errors with the topography data of 1.7±0.6 µm and 5.7±1.3 µm in anterior and posterior corneal surfaces. The comparisons showed highly significant bilateral correlations with the differences between fellow corneas in Rx, Ry, Qx and Qy of anterior and posterior surfaces remaining insignificantly different from zero. Bilateral differences in elevation measurements at randomly-selected points in both corneal central and peripheral areas indicated strong mirror symmetry between fellow corneas. The mean geometric center (x0, y0, z0) of both right and left corneas was located on the temporal side and inferior-temporal side of the apex in anterior and posterior topography map, respectively. Rotational displacement angle α along X axis had similar distributions in bilateral corneas, while rotation angle β along Y axis showed both eyes tilting towards the nasal side. Further, rotation angle γ along Z axis, which is related to corneal astigmatism, showed clear mirror symmetry.

Conclusions: Analysis of corneal topography demonstrated strong and statistically-significant mirror symmetry between bilateral corneas. This characteristic could help in detection of pathological abnormalities, disease diagnosis, measurement validation and surgery planning.

Show MeSH

Related in: MedlinePlus

Convergence procedure.Bilateral eyes fixed on a long-distance target (A). When eyes are fixed on a near-distance target during measurement, bilateral eyes turn inward (B). Corneal geometric axis turns towards the nasal side and corneal apex provided by the instrument is located on the nasal side of the corneal geometric center.
© Copyright Policy
Related In: Results  -  Collection


getmorefigures.php?uid=PMC3757004&req=5

pone-0073412-g007: Convergence procedure.Bilateral eyes fixed on a long-distance target (A). When eyes are fixed on a near-distance target during measurement, bilateral eyes turn inward (B). Corneal geometric axis turns towards the nasal side and corneal apex provided by the instrument is located on the nasal side of the corneal geometric center.

Mentions: The study also evaluated agreement of corneal geometric center location between fellow eyes, and high mirror symmetry has been found in both anterior and posterior surfaces. The mean geometric centers of anterior surfaces in right and left corneas were located on the temporal side of the apex and the corresponding mean geometric axes involved rotation towards the nasal side of the keratometric axis of cornea. On the other hand, the mean geometric centers of posterior surfaces were on the inferior-temporal side of apex and the axes rotated towards the superior-nasal side. These findings are likely to be related to the instruction given to participants to fix their gaze on the target lamp in near distance during topography measurement, which would cause the convergence that makes both eyes turn inward (Fig. 7). The apex in Pentacam topography maps, defined as the point at which the mathematical gradient reduces to zero, coincides with corneal vertex where the instrument axis intersects the cornea [22]. In this study corneal vertex (or apex), which has been found earlier to be nasally displaced [23], has been similarly located on the nasal side of the geometric center of the cornea as illustrated in Fig. 7. Further, statistically significant correlation was found between y coordinate (vertical direction) of MCT point and that of the geometric center (r = 0.463, P<0.000) and α rotational displacement (r = 0.094, P = 0.014, the coordinate convention has positive α toward the inferior side). While the convergence described above affects the locations of the geometric centers of both anterior and posterior surfaces, the posterior center was further affected by the fact that the thinnest point on the cornea is located away from the apex and in the inferior-temporal region [24], [25].


Evaluation of the shape symmetry of bilateral normal corneas in a Chinese population.

Bao F, Chen H, Yu Y, Yu J, Zhou S, Wang J, Wang Q, Elsheikh A - PLoS ONE (2013)

Convergence procedure.Bilateral eyes fixed on a long-distance target (A). When eyes are fixed on a near-distance target during measurement, bilateral eyes turn inward (B). Corneal geometric axis turns towards the nasal side and corneal apex provided by the instrument is located on the nasal side of the corneal geometric center.
© Copyright Policy
Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC3757004&req=5

pone-0073412-g007: Convergence procedure.Bilateral eyes fixed on a long-distance target (A). When eyes are fixed on a near-distance target during measurement, bilateral eyes turn inward (B). Corneal geometric axis turns towards the nasal side and corneal apex provided by the instrument is located on the nasal side of the corneal geometric center.
Mentions: The study also evaluated agreement of corneal geometric center location between fellow eyes, and high mirror symmetry has been found in both anterior and posterior surfaces. The mean geometric centers of anterior surfaces in right and left corneas were located on the temporal side of the apex and the corresponding mean geometric axes involved rotation towards the nasal side of the keratometric axis of cornea. On the other hand, the mean geometric centers of posterior surfaces were on the inferior-temporal side of apex and the axes rotated towards the superior-nasal side. These findings are likely to be related to the instruction given to participants to fix their gaze on the target lamp in near distance during topography measurement, which would cause the convergence that makes both eyes turn inward (Fig. 7). The apex in Pentacam topography maps, defined as the point at which the mathematical gradient reduces to zero, coincides with corneal vertex where the instrument axis intersects the cornea [22]. In this study corneal vertex (or apex), which has been found earlier to be nasally displaced [23], has been similarly located on the nasal side of the geometric center of the cornea as illustrated in Fig. 7. Further, statistically significant correlation was found between y coordinate (vertical direction) of MCT point and that of the geometric center (r = 0.463, P<0.000) and α rotational displacement (r = 0.094, P = 0.014, the coordinate convention has positive α toward the inferior side). While the convergence described above affects the locations of the geometric centers of both anterior and posterior surfaces, the posterior center was further affected by the fact that the thinnest point on the cornea is located away from the apex and in the inferior-temporal region [24], [25].

Bottom Line: The mean geometric center (x0, y0, z0) of both right and left corneas was located on the temporal side and inferior-temporal side of the apex in anterior and posterior topography map, respectively.Further, rotation angle γ along Z axis, which is related to corneal astigmatism, showed clear mirror symmetry.This characteristic could help in detection of pathological abnormalities, disease diagnosis, measurement validation and surgery planning.

View Article: PubMed Central - PubMed

Affiliation: The Affiliated Eye Hospital of WenZhou Medical College, Wenzhou, China.

ABSTRACT

Purpose: To investigate the bilateral symmetry of the global corneal topography in normal corneas with a wide range of curvature, astigmatism and thickness values.

Design: Cross-Sectional Study.

Methods: Topography images were recorded for the anterior and posterior surfaces of 342 participants using a Pentacam. Elevation data were fitted to a general quadratic model that considered both translational and rotational displacements. Comparisons between fellow corneas of estimates of corneal shape parameters (elevation, radius in two main directions, Rx and Ry, and corresponding shape factors, Qx and Qy) and corneal position parameters (translational displacements: x0, y0 and z0, and rotational displacements: α, β and γ) were statistically analyzed.

Results: The general quadratic model provided average RMS of fit errors with the topography data of 1.7±0.6 µm and 5.7±1.3 µm in anterior and posterior corneal surfaces. The comparisons showed highly significant bilateral correlations with the differences between fellow corneas in Rx, Ry, Qx and Qy of anterior and posterior surfaces remaining insignificantly different from zero. Bilateral differences in elevation measurements at randomly-selected points in both corneal central and peripheral areas indicated strong mirror symmetry between fellow corneas. The mean geometric center (x0, y0, z0) of both right and left corneas was located on the temporal side and inferior-temporal side of the apex in anterior and posterior topography map, respectively. Rotational displacement angle α along X axis had similar distributions in bilateral corneas, while rotation angle β along Y axis showed both eyes tilting towards the nasal side. Further, rotation angle γ along Z axis, which is related to corneal astigmatism, showed clear mirror symmetry.

Conclusions: Analysis of corneal topography demonstrated strong and statistically-significant mirror symmetry between bilateral corneas. This characteristic could help in detection of pathological abnormalities, disease diagnosis, measurement validation and surgery planning.

Show MeSH
Related in: MedlinePlus