Braeu, F. A. | Search bioRxiv

bioRxiv2024

Iris Morphological and Biomechanical Factors Influencing Angle Closure During Pupil Dilation

PurposeTo use finite element (FE) analysis to assess what morphological and biomechanical factors of the iris and of the anterior chamber are more likely to influence angle narrowing during pupil dilation. MethodsThe study consisted of 1,344 FE models comprising of the cornea, sclera, lens and iris (stroma, sphincter and dilator tissues) to simulate pupil dilation and to assess changes in angle. For each model, we varied the following parameters: anterior chamber depth (ACD = 2 -4 mm) and width (ACW = 10-12 mm), iris convexity (IC = 0-0.3 mm), thickness (IT = 0.3-0.5 mm), stiffness (E = 4-24 kPa) and Poissons ratio (v = 0-0.3), and simulated pupil dilation. We evaluated for the change in anterior chamber angle ({bigtriangleup}{angle}) and the final dilated anterior chamber angles ({angle}f) from baseline to dilation for each parameter. ResultsThe final dilated AC angles decreased with a smaller ACD ({angle}f = 53.4{degrees}{+/-}12.3{degrees} to 21.3{degrees}{+/-}14.9{degrees}), smaller ACW ({angle}f = 48.2{degrees}{+/-}13.5{degrees} to 26.2{degrees}{+/-}18.2{degrees}), larger IT ({angle}f = 52.6{degrees}{+/-}12.3{degrees} to 24.4{degrees}{+/-}15.1{degrees}), larger IC ({angle}f = 45.0{degrees}{+/-}19.2{degrees} to 33.9{degrees}{+/-}16.5{degrees}), larger E ({angle}f = 40.3{degrees}{+/-}17.3{degrees} to 37.4{degrees}{+/-}19.2{degrees}) and larger v ({angle}f = 42.7{degrees}{+/-}17.7{degrees} to 34.2{degrees}{+/-}18.1{degrees}). The change in AC angle increased with larger ACD ({bigtriangleup}{angle} = 9.37{degrees}{+/-}11.1{degrees} to 15.4{degrees}{+/-}9.3{degrees}), smaller ACW ({bigtriangleup}{angle} = 7.4{degrees}{+/-}6.8{degrees} to 16.4{degrees}{+/-}11.5{degrees}), larger IT ({bigtriangleup}{angle} = 5.3{degrees}{+/-}7.1{degrees} to 19.3{degrees}{+/-}10.2{degrees}), smaller IC ({bigtriangleup}{angle} = 5.4{degrees}{+/-}8.2{degrees} to 19.5{degrees}{+/-}10.2{degrees}), larger E ({bigtriangleup}{angle} = 10.9{degrees}{+/-}12.2{degrees} to 13.1{degrees}{+/-}8.8{degrees}) and larger v ({bigtriangleup}{angle} = 8.1{degrees}{+/-}9.4{degrees} to 16.6{degrees}{+/-}10.4{degrees}). ConclusionsThis parametric study offered valuable insights into the factors that could influence angle closure. The morphology of the iris (IT and IC) and its innate biomechanical behavior (E and v) were crucial in influencing the way the iris deformed during dilation, and angle closure was further exacerbated by decreased AC biometry (ACD and ACW).

bioengineering↗

bioRxiv2022

The Structural Layers of the Porcine Iris Exhibit Inherently Different Biomechanical Properties

PurposeTo isolate the structural components of the ex vivo porcine iris tissue and to determine their biomechanical properties. MethodsThe porcine stroma and dilator tissues were separated, and their dimensions were assessed using optical coherence tomography (OCT). The stroma underwent flow test (n = 32) to evaluate for permeability using Darcys Law ({Delta}P = 2000 Pa, A = 0.0391 mm2), and both tissues underwent stress relaxation experiments ({varepsilon} = 0.5 with initial ramp of {delta}{varepsilon} = 0.1) to evaluate for their viscoelastic behaviours (n = 28). Viscoelasticity was characterised by the parameters {beta} (half width of the Gaussian distribution), {tau}m(mean relaxation time constant), E0 (instantaneous modulus) and E{infty} (equilibrium modulus). ResultsFor the stroma, the hydraulic permeability was 9.49 {+/-} 3.05 x 10-6 mm2/Pa{middle dot}s, and the viscoelastic parameters were {beta} = 2.50 {+/-} 1.40, and {tau}m = 7.43 {+/-} 4.96 s, with the two moduli calculated to be E0= 14.14 {+/-} 6.44 kPa and E{infty} = 6.08 {+/-} 2.74 kPa. For the dilator tissue, the viscoelastic parameters were {beta} = 2.06 {+/-} 1.33 and {tau}m = 1.28 {+/-} 1.27 s, with the two moduli calculated to be E0 = 9.16 {+/-} 3.03 kPa and E{infty} = 5.54 {+/-} 1.98 kPa. ConclusionWe have established a new protocol to evaluate the biomechanical properties of the structural layers of the iris. Overall, the stroma was permeable and exhibited smaller moduli than those of the dilator muscle. An improved characterisation of iris biomechanics may form the basis to further our understanding of angle closure glaucoma.

bioengineering↗