Abstract
Purpose::
To quantify how the elastic modulus (i.e., the stiffness) of the iris changes following stimulation by pilocarpine, phenylephrine, and tropicamide.
Methods::
Irides (n = 20) were dissected from porcine eyes within 4 hr post mortem and mounted on a mechanical testing system. The samples were stretched up to 30% strain in the radial direction, and the modulus was calculated from the linear portion of the stress-strain curve. One of the three drugs (n = 6 or 7) of interest was then added (80 ug/ml) to the bath surrounding the tissue, and the experiment was repeated. Experiments were performed on intact irides and on strips cut from the iris.
Results::
Changes in pupil diameter of free-floating samples and in isometric force of mounted samples confirmed that the tissue was responsive to the drugs. For the strips, the untreated iris modulus was 4.0±0.9 kPa (mean ± s.d., n = 20), and the treated iris modulus was 7.7±2.0 (pilocarpine, n = 7), 6.9±2.2 (phenylephrine, n = 6), and 8.4±1.7 (tropicamide, n = 7). Whole irides (n = 10 total) gave similar trends but values approximately 25% higher, presumably because the shape of the intact iris makes the stress and strain fields inhomogeneous within the sample.
Conclusions::
Although pilocarpine, phenylephrine, and tropicamide work by different mechanisms, all three gave similar results - an increase in modulus of roughly two times. We conclude that in most normal situations, the iris stiffness varies within about a factor of two range, remaining quite compliant at all pupil diameters.
Keywords: iris • anterior segment • stress response