The Strain Response to Intraocular Pressure Decrease in the Lamina Cribrosa of Patients with Glaucoma.

OBJECTIVE

To measure biomechanical strains in the lamina cribrosa (LC) of living human eyes with intraocular pressure (IOP) lowering.

DESIGN

Cohort study.

PARTICIPANTS

Patients with glaucoma underwent imaging before and after laser suturelysis after trabeculectomy surgery (29 image pairs; 26 persons).

INTERVENTION

Noninvasive imaging of the eye.

MAIN OUTCOME MEASURES

Strains in optic nerve head tissue and changes in depths of the anterior border of the LC.

RESULTS

Intraocular pressure decreases caused the LC to expand in thickness in the anterior-posterior strain (E = 0.94 ± 1.2%; P = 0.00020) and contract in radius in the radial strain (E = - 0.19 ± 0.33%; P = 0.0043). The mean LC depth did not significantly change with IOP lowering (1.33 ± 6.26 μm; P = 0.26). A larger IOP decrease produced a larger, more tensile E (P < 0.0001), greater maximum principal strain (E; P < 0.0001), and greater maximum shear strain (Γ; P < 0.0001). The average LC depth change was associated with the Γ and radial-circumferential shear strain (E; P < 0.02) but was not significantly related to tensile or compressive strains. An analysis by clock hour showed that in temporal clock hours 3 to 6, a more anterior LC movement was associated with a more positive E, and in clock hours 3, 5, and 6, it was associated with a more positive Γ. At 10 o'clock, a more posterior LC movement was related to a more positive E (P < 0.004). Greater compliance (strain/ΔIOP) of E (P = 0.044), Γ (P = 0.052), and E (P = 0.018) was associated with a thinner retinal nerve fiber layer. Greater compliance of E (P = 0.041), Γ (P = 0.021), E (P = 0.024), and in-plane shear strain (E; P = 0.0069) was associated with more negative mean deviations. Greater compliance of Γ (P = 0.055), E (P = 0.040), and E (P = 0.015) was associated with lower visual field indices.

CONCLUSIONS

With IOP lowering, the LC moves either into or out of the eye but, on average, expands in thickness and contracts in radius. Shear strains are nearly as substantial as in-plane strains. Biomechanical strains are more compliant in eyes with greater glaucoma damage. This work was registered at ClinicalTrials.gov as NCT03267849.