Measurement of lamina cribrosa morphological parameters

After delineation and reconstruction, LC parameters were calculated for the portion of the LC underneath a circular region of radius 750 μm and centered on the BMO center (roughly 80% of average BMO area). This maintained consistency and ensured that measurements were made for similar portions of the LC across eyes. Since LC visibility varied across different eyes, particularly in the peripheral region, eyes with a poorly visible LC (where the LC covered less than 70% of the BMO area from en face visualization) were excluded from the analysis. Additionally, we excluded eyes with extensive peripapillary atrophy as this would result in difficulty in discerning the BMO boundaries. We defined and calculated in MATLAB the following LC morphological parameters¹⁸.

The LC-GSI is a number from the range of −1 (corresponding to a spherical cup) to + 1 (corresponding to a spherical cap) that describes the local surface shape of the anterior LC (Fig. 1). It is independent of the size of the LC, and is derived from the maximum (K₁) and minimum (K₂) principal arc curvatures of the LC, according to the equation^18,24 below:

Illustration of the lamina cribrosa global shape index (LC-GSI) and the lamina cribrosa curvedness (LC-C). (A) The local surface shape of the anterior lamina cribrosa is divided into nine categories, with the LC-GSI in parentheses: cup, trough, rut, saddle rut, saddle, saddle ridge, ridge, dome and cap. The insert figures are shapes of identical curvedness. (B) A symmetrical saddle of equal shape index is depicted at increasing LC-C values: lowest on the left (zero), and highest on the right.

In contrast to the LC-GSI which specifies the type of shape the LC corresponds to, and is scale-independent, the LC-Curvedness (LC-C) is a positive number that indicates the “intensity” of the surface curvature, and is scale-dependent (Fig. 1). The LC-C is also derived from the principal arc curvatures K₁ and K₂ according to the equation²⁴ below:

Considered together, the LG-GSI and the LC-C specify the local second order 3D geometry of the LC. For example, a LC with a LC-GSI of 0 indicates that it is best described by a “saddle” shape, and an accompanying LC-C of 1.2 describes how intensely “saddle”-shaped it is (the rightmost example in the bottom panel of Fig. 1 in this instance).

Additionally, further measures of the curvature of the LC as assessed in a 2D plane were also reported: curvature in the nasal-temporal plane (N-T Curvature), and in a superior-inferior plane (S-I Curvature)¹⁸. To derive these parameters, we first intersected the anterior LC surface with horizontal (N-T) and vertical (S-I) radial cross-sections that passed through the center of the BMO ellipse, and were perpendicular to the BMO plane. A circular arc was then fitted to each LC curve. A positive or negative (N-T or S-I) arc curvature indicated the degree to which it was curved anteriorly or posteriorly, respectively.

Finally, the depth of the anterior LC was defined as the distance from the BMO reference plane to anterior LC surface, reconstructed from each delineated LC point. The LCD was calculated from the mean of these measurements¹⁸. The anterior depth of the LC was also plotted along the vertical and horizontal axes, as illustrated in Fig. 2.

Anterior lamina cribrosa (LC) depth along the superior-inferior (S-I) and nasal-temporal (N-T) axes across the three groups: healthy controls, mild-moderate primary open angle glaucoma (POAG) and advanced POAG. (A) Along the S-I direction, the LC is more posterior (deeper) with increasing severity of glaucoma; the central ridge also becomes less prominent, with the general shape changing from a “W” to a flatter “U”. (B) Along the N-T direction, the LC is overall “U”-shaped; the LC is also more posterior with increasing severity of glaucoma.