Purpose To characterize the trabeculated connective tissue microarchitecture of the lamina cribrosa (LC) in terms of total connective tissue volume (CTV), connective tissue volume fraction (CTVF), predominant beam orientation, and material anisotropy in monkeys with early experimental glaucoma (EG). Methods The optic nerve heads from three monkeys with unilateral EG and four bilaterally normal monkeys were three dimensionally reconstructed from tissues perfusion fixed at an intraocular pressure of 10 mm Hg. A three-dimensional segmentation algorithm was used to extract a binary, voxel-based representation of the porous LC connective tissue microstructure that was regionalized into 45 subvolumes, and the following quantities were calculated: total CTV within the LC, mean and regional CTVF, regional predominant beam orientation, and mean and regional material anisotropy. Results Regional variation within the laminar microstructure was considerable within the normal eyes of all monkeys. The laminar connective tissue was generally most dense in the central and superior regions for the paired normal eyes, and laminar beams were radially oriented at the periphery for all eyes considered. CTV increased substantially in EG eyes compared with contralateral normal eyes (82%, 44%, 45% increases; P < 0.05), but average CTVF changed little (−7%, 1%, and −2% in the EG eyes). There were more laminar beams through the thickness of the LC in the EG eyes than in the normal controls (46%, 18%, 17% increases). Conclusions The substantial increase in laminar CTV with little change in CTVF suggests that significant alterations in connective and nonconnective tissue components in the laminar region occur in the early stages of glaucomatous damage.

Purpose To investigate the biomechanical response to IOP elevation of normal monkey eyes using eye-specific 3D finite element (FE) models of the ONH that incorporate lamina cribrosa (LC) microarchitectural information. Methods A serial sectioning and episcopic imaging technique was used to reconstruct the ONH and peripapillary sclera of four pairs of eyes fixed at 10 mmHg. FE models were generated with local LC material properties representing the connective tissue volume fraction (CTVF) and predominant LC beam orientation, and used to simulate an increase in IOP from 10 to 45 mmHg. An LC material stiffness constant was varied to assess its influence on biomechanical response. Results Strains and stresses within contralateral eyes were remarkably similar in both magnitude and distribution. Strain was inversely, and nonlinearly, correlated to CTVF (median r2=0.73) with tensile strains largest in the temporal region. Stress linearly correlated to CTVF (median r2=0.63), with central and superior regions bearing the highest stresses. Net average LC displacement was either posterior or anterior depending on whether the laminar material properties were compliant or stiff. Conclusion Our results show that contralateral eyes exhibit similar mechanical behavior and suggest that local mechanical stress and strain within the LC are highly correlated with local laminar CTVF. These simulations emphasize the importance of developing both high resolution imaging of the LC microarchitecture and next-generation, deep-scanning OCT techniques to clarify the relationships between IOP-related LC displacement and CTVF–related stress and strain in the LC. Such imaging may predict sites of IOP-related damage in glaucoma.

The measured IOP was successfully measured continuously by using a new, fully implantable IOP telemetry system. IOP fluctuates as much as 10 mm Hg from day to day and hour to hour in unrestrained nonhuman primates, which indicates that snapshot IOP measurements may be inadequate to capture the true dynamic character of IOP. The distributions, magnitudes, and patterns of IOP are not reproducible from day to day within animals, but IOP tends to be slightly higher at night when IOP data are averaged across multiple 24-hour periods within animals.

Purpose To investigate spectral domain optical coherence tomography (SD-OCT) detected optic disc margin anatomy in the monkey eye by co-localizing disc photographs to SD-OCT scans acquired from the same eyes. Methods The neural canal opening (NCO) was delineated within 40 digital radial sections generated from SD-OCT volumes acquired from 33 normal monkey eyes (15°, 290 × 768 horizontal grid pattern, Heidelberg Spectralis). Each volume was co-localized to its disc photograph by matching the retinal vessels within each photograph to vessel outlines visible within en face SD-OCT images. Border Tissue was delineated where it extended internal to the NCO. A clinician (masked to delineated points) marked the disc margin onto each photograph whilst viewing the relevant stereophotograph pair. Alignment of the clinician-ascribed disc margin to the NCO and Border Tissue delineations was assessed. The process was repeated in a single myopic human eye. Results In 23 eyes, the NCO aligned to the disc margin. In 10 eyes, externally oblique Border Tissue was detectable in the temporal disc. In these regions of the disc, the termination of Border Tissue was the disc margin. An exaggerated form of this phenotype was observed in the myopic human eye. In this case, temporal Border Tissue terminated at the anterior scleral canal opening, which was detected as the disc margin. Conclusions The termination of Bruch’s Membrane, Border Tissue and the anterior scleral canal opening may constitute the disc margin within the same eye depending upon the Border Tissue architecture; this anatomy is consistently visualized by SD-OCT.