Scleral mechanics: comparing whole globe inflation and uniaxial testing

Abstract

The purpose of this study was to assess fundamental differences between the mechanics of the posterior sclera in paired eyes using uniaxial and whole globe inflation testing, with an emphasis on the relationship between testing conditions and observed tissue behavior. Twenty porcine eyes, consisting of matched pairs from 10 pigs, were used in this study. Within pairs, one eye was tested with 10 cycles of globe pressurization to 150 mmHg (∼10× normal IOP) while biaxial strains were tracked via an optical system at the posterior sclera. An excised posterior strip from the second eye was subjected to traditional uniaxial testing in which mechanical hysteresis was recorded from 10 cycles to a peak stress of 0.13 MPa (roughly equivalent to the circumferential wall stress produced by an IOP of 150 mmHg under the thin-walled pressure vessel assumption). For approximately equivalent loads, peak strains were more than twice as high in uniaxial tests than in inflation tests. Different trends in the load-deformation plots were seen between the tests, including an extended "toe" region in the uniaxial test, a generally steeper curve in the inflation tests, and reduced variability in the inflation tests. The unique opportunity of being able to mechanically load a whole globe under near physiologic conditions alongside a standard uniaxially tested specimen reveals the effects of testing artifacts relevant to most uniaxially tested soft tissues. Whole globe inflation offers testing conditions that significantly alter load-deformation behavior relative to uniaxial testing; consequently, laboratory studies of interventions or conditions that alter scleral mechanics may greatly benefit from these findings.

Figure 1

(A) Rendering of the camera view above showing the anatomical position of the excised scleral tensile sample and strain targets. For both uniaxial and whole globe inflation testing, the strain target grid was centered 4 to 5 mm temporal to the edge of the optic nerve head (*), corresponding to the posterior pole of the globe. (B) Isometric rendering of the excised scleral strip location relative to the optic nerve head and cornea (c).

Figure 2

Uniaxial and whole globe inflation system setup. (A) Schematic diagram of uniaxial scleral strip held in place by gripping pins with stress being applied via dual motors on either side of sample and (B) photograph of a scleral tensile sample with local strain markers undergoing uniaxial tension. (C) Whole globe held in place by skewer and inflation needle with pressure being delivered via inflation needle in the anterior chamber; (a) oil bath (b) computer controlled linear actuator (c) clamp with force transducer (d) local strain targets (e) camera (f) gripping pins (g) optic nerve head (h) 22- gauge needle skewer (i) anterior chamber (j) 20-gauge needle to computer controlled pressure reservoir and (D) whole globe with local strain targets undergoing circumferential tension.

Figure 3

(A) Sample stress-strain hysteresis data from one uniaxial tested eye; arrows indicate loading and unloading of tissue. (B) Uniaxial composite exponential curve fit of stress-strain hysteresis data (n = 10 pairs) for each half-cycle. (C) Sample stress-strain hysteresis data from one whole globe inflation tested eye. (D) Whole globe inflation composite exponential curve fit of IOP-strain hysteresis data (n = 10 pairs) for each half-cycle. Composite curves were generated by averaging the sum of the individual functions for each fit along the strain axis (i.e. 1/n Σε_n(σ or IOP), n=10). A zone encompassing the standard error (SE) of the mean strain values for the loading portion of each of the three hysteresis loops is delineated by a thin dashed or solid line.