purpose. To measure the rigidity coefficient of a large number of subjects at clinically encountered intraocular pressures (IOPs) and to examine the possible correlation of ocular rigidity with other factors, such as the age of the patients, ocular parameters (axial length and corneal thickness), and pathologic conditions affecting the eye.

methods. The pressure–volume relationship and the ocular rigidity coefficient (*K*) were determined in 79 eyes undergoing cataract surgery, by injecting 200 μL of saline solution (in steps of 4.5 μL) through the limbus into the anterior chamber, while continually monitoring the IOP with a transducer, up to the limit of 60 mm Hg. Data within an IOP range of 10 to 35 mm Hg were used to calculate the scleral rigidity coefficient. All measurements were taken at the same time of day, to eliminate any possible diurnal variation.

results. The mean ocular rigidity coefficient was 0.0126 mm Hg/μL (95% confidence interval [CI], 0.0112–0.0149). A statistically significant positive correlation between the rigidity coefficient and age of the patient was found (*P* = 0.02), whereas similar findings were not observed for the examined ocular parameters (axial length, *P* = 0.09; and corneal thickness, *P* = 0.12). No correlation was found for patients with diabetes mellitus (*P* = 0.39), age-related macular degeneration (*P* = 0.55), and hypertension (*P* = 0.45).

conclusions. The present study provides quantitative data on the ocular rigidity coefficient based on measurements in a large series of living human eyes. A positive correlation between the ocular rigidity coefficient and the patient’s age was documented.

^{ 1 }described the coefficient of ocular rigidity as a “measure of the resistance, which the eye exerts to distending forces,” and he developed a formula for ocular rigidity.

^{ 2 }

^{ 3 }

^{ 4 }They determined that rigidity increases with increasing intraocular pressure (IOP) and developed alternative formulas to characterize this change as a function of pressure. Although these formulations are more accurate than Friedenwald’s equations, they are more complicated and present difficulties when applied in daily clinical practice.

^{ 5 }

^{ 6 }

^{ 7 }

*K*).

^{ 1 }Forty-two of the participants were men (53%).

^{2}; Lectro-Cath 1155.05; Vygon). The tubules were connected by the stopcock ramp and formed a closed system that included the pressure transducer, the syringe, the saline solution container, and the eye. Special care was taken to exclude the possibility of aqueous leakage from the system. The saline solution was injected into the anterior chamber of the eye by a 22-gauge intravenous catheter needle (Vygon).

_{2}O to mm Hg (76 mm Hg equals 10,600 mm H

_{2}O). Before each experiment, the pressure transducer was tested with closed output, to identify possible leaks in the tubule manifold.

^{ 8 }

*t*-tests were used to correlate the ocular rigidity coefficient with the corresponding clinical dichotomous parameters, such as the presence of diabetes mellitus (DM), age-related macular degeneration (ARMD), and hypertension, whereas linear regression analyses were used to test the influence of continuous variables such as patient age, ocular axial length, and corneal thickness. Multivariable linear regression analysis was performed to identify independent variables associated with ocular rigidity (all variables included simultaneously). The level of significance was set at 5%.

*R*

^{2}coefficient for all the regression procedures was 0.9203 ± 0.0049. This level of

*R*

^{2}justified the linear approximation for the pressure range used in the analysis.

*K*=

*dP*/

*dV*[in mm Hg/μL]) was calculated as the slope of the pressure (

*P*) versus volume curve (

*V*) for the IOP range (10–35 mm Hg) in the analysis. The mean rigidity coefficient (

*K*) was found to be 0.0126 mm Hg/μL (95% CI: 0.0112–0.0149). The coefficient of repeatability (CR; twice the standard deviation of the mean difference between the two measurements) was 0.0023 (Fig. 3) .

*r*= 0.27,

*P*= 0.02; Fig. 4 ). A trend for decreased scleral rigidity in correlation with increase in axial length of the eye (

*r*= −0.24,

*P*= 0.09) was observed (Fig. 5) , whereas no statistically significant correlation was found in central corneal thickness (

*r*= 0.22,

*P*= 0.12, type II error = 0.64; Fig. 6 ). In parallel, there was no statistically significant correlation between the rigidity coefficient and the presence of diabetes mellitus (

*P*= 0.39), age-related macular degeneration (

*P*= 0.55), and hypertension (

*P*= 0.45). In multivariate analyses, none of the examined variables was found to have statistically significant correlation with the ocular rigidity coefficient (

*P*> 0.05).

^{ 1 }This equation has received criticism because the data that were used for its computations were obtained from enucleated eyes. Because of postmortem changes (such as edema and consequent thickening, active flow of blood, effect of extraocular muscles, and vascular rigidity in the intact living eye), marked differences were noted when the ocular rigidity was measured in a live eye and compared with rigidity in the same eye obtained after enucleation.

^{ 4 }

^{ 9 }

^{ 10 }

^{ 11 }

^{ 2 }

^{ 3 }

^{ 4 }The number of eyes measured, however, was always small, and in all cases these eyes had serious diseases and were scheduled for enucleation. Recently, Silver and Geyer,

^{ 6 }in an attempt to derive a uniform formula for the calculation of the ocular rigidity coefficient, collected all the data available in the literature that were obtained with direct manometric measurements from living human eyes. Based on these data, they described a new equation that provides the best fit for the collected data.

^{ 6 }They also found a larger volume increment for a given increment of pressure than was provided by the Friedenwald equation. The main limitation of this work is that it is based on collected data of different experiments in a relatively small number of pathologic eyes (21 eyes).

^{ 4 }we used the 10 mm Hg as the common set point for the initiation of measurements in all eyes. In parallel, an automated process adding or removing fluid from the anterior chamber until the desired IOP pressure was achieved at the beginning of our measurements was used. Therefore, ocular rigidity was always measured within the range of interest.

^{ 7 }The time between retrobulbar anesthetic injection and the measurement (15 minutes), as well as the fact that before measurements the IOP was regulated to 10 mm Hg by appropriate irrigation or aspiration of saline solution, minimized these possible effects. In parallel, the alternative anesthesia with topical drops could also affect the ocular rigidity measurements because of eye movements and the patient’s refusal to cooperate, increasing the possibility of intraoperative complications.

^{ 1 }reported an average scleral rigidity coefficient of 0.021 mm Hg/μL. The difference in these results may be due to differences in rigidity coefficient calculation and measurement methodology, as well as to sample size and composition. Investigators who used direct manometric measurements reported ocular rigidity coefficients similar to ours.

^{ 2 }

^{ 3 }

^{ 4 }

^{ 6 }

^{ 5 }the sclera becomes increasingly more rigid and noncompliant with age, because of the aging process or other causes. A rigid sclera limits the filling of the vortex veins and thereby increases the resistance to venous outflow. This relative obstruction ultimately leads to dilatation and decompensation of the choroidal venous system at the posterior pole, compromising Bruch’s membrane, the choriocapillaris, and the retinal pigment epithelium of the macular area. This aging process could be an explanation for the development of ARMD.

^{ 12 }In our study, we did not observe increased ocular rigidity in patients with ARMD (12 patients). Because of the small number of these patients, however, it is difficult to draw reliable conclusions about ARMD. Future studies enrolling more patients with ARMD are needed.

^{ 13 }

^{ 14 }

^{ 15 }In our study, we did not find any statistically significant correlation between the ocular rigidity coefficient and central corneal thickness. It seems that differences in corneal thickness over the applanation area (3.06 mm in diameter for a Goldmann instrument) may have an increased effect in IOP measurement through alterations in topical corneal rigidity and corneal elastic properties, but may have less impact in ocular rigidity (total response of the eye) measured in the present study. However, this finding cannot be considered conclusive, since the power to detect such a correlation was low (type II error = 0.64). Further studies (including more patients in whom corneal thickness is estimated prospectively) are needed, to elucidate the possible correlation of corneal thickness and ocular rigidity.

^{ 7 }found that changes in the shape and stress distribution of the scleral shell are the main factors of the observed reduction of ocular rigidity after scleral buckling. In our study, the measured ocular rigidity coefficient described the total response of the eye without separate evaluation of the function of the two major contributory components: morphologic and material.

^{ 16 }

^{ 17 }Although a complex approach that would take into consideration these parameters may be more accurate, it requires complex calculations that make it less functional.

^{ 18 }

^{ 19 }Future studies are needed to elucidate the clinical impact of ocular rigidity. Our device for direct manometric measurement of ocular rigidity could be a uniform instrument for use in the future to calibrate instruments for noninvasive estimation of ocular rigidity.

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