Anterior–posterior asymmetry in iris mechanics measured by indentation
Highlights
► This study quantifies the stiffness of the anterior and posterior layers of the iris. ► A significant difference was found between anterior and posterior surface. ► This suggests that the dilator and sphincter are stiffer than the stroma. ► Shorter relaxation time was found on the anterior surface compared to the posterior. ► Suggesting stromal permeability could be a significant factor on the anterior.
Introduction
The interest in understanding iris mechanics arises from the influence of abnormal iris morphology on specific ocular disorders. For example, in angle closure (Epstein et al., 1997), the iris bows anteriorly, and the abnormal shape and position of the iris impede aqueous humor outflow, resulting in increased intraocular pressure. In contrast, pigment dispersion syndrome (Ritch, 2004; Niyadurupola and Broadway, 2008) is associated with posterior bowing of the iris and liberation of pigment due to iris–lens and/or iris–zonule contact. A relatively new ophthalmic disorder, intraoperative floppy iris syndrome (IFIS) also involves iris mechanics. First reported in 2005 by Chang and Campbell (Chang and Campbell, 2005), IFIS has been observed in patients who are taking or have taken Tamsulosin (Flomax) for the urinary complications of benign prostatic hypertrophy. Tamsulosin, a selective α1A-adrenergic receptor antagonist, has the side effect of inhibiting the iris dilator, and its effects can be irreversible, suggesting that the muscle atrophies from the drug (Takmaz and Can, 2007a,b). This theory has recently been verified structurally by visible alterations of the iris in patients taking Tamsulosin, such that the dilator muscle region measured as half of the distance between the scleral spur and the pupillary margin began to significantly thin with long-term use of the drug (Prata et al., 2009).
The contour of the iris is determined by two factors: external stresses arising from the flow of the aqueous humor, and internal stresses due to the passive and active components defining the structures of the iris. The external stresses can be understood in terms of fluid mechanics of the aqueous humor (Huang and Barocas, 2004; Heys and Barocas, 2002; Silver and Quigley, 2004). The internal stresses are more complex because the iris is a composite tissue (Fig. 1), in which each individual component may have different mechanical properties that can affect the iris’s natural contour.
The anterior border layer (ABL), principally composed of melanocytes and fibroblasts, is the predominant factor for the visible external color of the iris. Directly beneath the ABL is the stroma, composed of a loosely organized collagenous network of fibroblasts and melanocytes. The stroma is an open mesh containing mucopolysaccharide and fluid with minimal additional tissue (Hogan et al., 1971). The sphincter muscle, lying within the stromal framework, is a circumferentially aligned smooth muscle located at the pupillary margin. The sphincter in the pig is slightly larger (Fig. 1) than in the human. The posterior surface of the iris is composed of the dilator muscle and a layer of modified pigment epithelial cells. The radially aligned dilator extends along the posterior stromal layer from the base of the iris root to the mid-portion of the sphincter muscle similarly found in the porcine model (Hogan et al., 1971).
Toward our goal of understanding the mechanisms determining the iris contour in vivo, it is imperative that we explore how the different constituents contribute to the overall mechanical behavior of the passive iris. We have recently shown (Whitcomb et al., 2009) that the global stiffness of the ex vivo porcine iris increases with pharmaceutical stimulation, but to our knowledge, no work has been done to determine the relative contributions of the compromising segments of the iris. Our motivation to use indentation to further mechanically characterize the iris was due to its ability to identify individual constituents within composites or heterogenous samples (Ebenstein and Pruitt, 2006) and its depth-sensitivity at very small length scales (Oyen and Cook, 2009). As a preliminary study, we used indentation on the anterior and posterior surfaces of the ex vivo porcine iris to examine mechanical differences. Since indentation is far more sensitive to the tissue properties near the indenter than to those far from the indenter (Costa and Yin, 1999), the asymmetry between the anterior and posterior surfaces was revealed.
Section snippets
Isolation and preparation of tissue
Eyes were harvested from animals sacrificed by Experimental Surgical Services and the Visible Heart Laboratory® at the University of Minnesota. The porcine model was used for convenience, rapid post-mortem availability, size, and similarity of anatomical structure to the human eye (Olsen et al., 2002). Enucleated eyes were isolated and refrigerated in a modified Krebs–Ringer (KRB) bicarbonate buffer and the irides were indented within 4–8 h post-mortem. The KRB buffer was composed of the
Histology
A total of 25 histological cross-sections per iris with a thickness of 20 μm each were used to determine the dimensions of the iris components. The 25 slides per iris were chosen randomly, but were located in the mid-section of the iris to ensure that accurate dimensions of the tissues were captured. The average thickness of the dilator (D) was 26.0 ± 1.2 μm (mean ± 95% CI, n = 100 cross-sectional locations), sphincter (S) was 133.7 ± 2.0 μm, central stroma plus sphincter (CS–S) was
Discussion
It must first be noted that the indentation test is not measuring a true modulus but rather an effective modulus. The iris in both human and pig is heterogeneous, viscoelastic, anisotropic (Heys and Barocas, 1999), and possibly poroelastic (discussed below), so combining the entire indentation mechanical response into a linear modulus is a simplification. The effective moduli E0 and E∞ reported here must be seen as measures of the tissue response as a whole and not as specific moduli with
Acknowledgments
We gratefully acknowledge the help of Heidi Roehrich at the University of Minnesota Histology Core Lab and Dr. Vincent Barnett for his extensive knowledge and input on the muscle tissue. The porcine eyes were provided by the Visible Heart® Laboratory and Experimental Surgical Services at the University of Minnesota. The Louise Dosdall Fellowship at the University of Minnesota (Whitcomb) and NIH Grant R01-EY15795 supported this work.
References (32)
- et al.
Intraoperative floppy iris syndrome associated with tamsulosin
Journal of Cataract and Refractive Surgery
(Apr 2005) - et al.
Nanoindentation of biological materials
Nanotoday
(Aug 2006) - et al.
Current concepts in pigmentary glaucoma
Survey of Ophthalmology
(Jan–Feb 1993) - et al.
A mathematical analysis for indentation tests of articular cartilage
Journal of Biomechanics
(Sep 1972) - et al.
Mechanical characterization of the bovine iris
Journal of Biomechanics
(Sep 1999) - et al.
Viscoelastic studies of human subscapularis tendon: relaxation test and a Wiechert model
Computer Methods and Programs in Biomedicine
(Jul 2006) - et al.
Biphasic indentation of articular cartilage-I. Theoretical analysis
Journal of Biomechanics
(Feb 1987) - et al.
A practical guide for analysis of nanoindentation data
Journal of the Mechanical Behavior of Biomedical Materials
(Aug 2009) - et al.
Iris morphologic changes related to alpha(1)-adrenergic receptor antagonists implications for intraoperative floppy iris syndrome
Ophthalmology
(May 2009) - et al.
Intraoperative floppy-iris syndrome: do we know everything about it?
Journal of Cataract and Refractive Surgery
(Jun 2007)
Correlation between compression, tensile and tearing tests on healthy and calcified aortic tissues
Medical Engineering & Physics
ex vivo porcine iris stiffening due to drug stimulation
Experimental Eye Research
Polarization-sensitive optical coherence tomography based on polarization-maintaining fibers and frequency multiplexing
Optics Express
Analysis of indentation: implications for measuring mechanical properties with atomic force microscopy
Journal of Biomechanical Engineering
Chandler and Grant’s Glaucoma
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