Retinal strain values peaked at 20% to 25% along different axes (
Fig. 7), for both the macula and vitreous base, with strain rates up to 30,000 seconds
−1 for the macula and over 50,000 seconds
−1 for the vitreous base (
Fig. 9) and a displacement velocity between 5,000 and 10,000 mm/s. The question of whether those values are capable of ripping the retina is a difficult one. Wollensak et al.
35,36 reported a much higher strain at failure of 51% at 1.6 mm/s, whereas Wu et al.
37 measured extension ratios of isolated animal retina strips between 1.7 and 1.9, calculated at extremely low displacement velocity varying between 0.36 and 3.60 mm/sec. Jones et al.
23 calculated a Young's modulus of 20 kPa for the isolated retina. Because of inconsistency in the methods, these data also are hardly comparable and may be not be representative of the eye response. Extrapolating data from isolated tissues, in fact, warrants extreme caution, especially because the retina, choroid and sclera all show nonlinear, anisotropic, and inhomogeneous mechanical characteristics
25,29 and living, perfused organs can behave in a significantly different way. It must be noted, however, that the strain rates and displacement velocity that we measured along multiple axes (
Figs. 9,
10) are more than 10
3 times higher than most reported experimental data. Several classes of materials show a strain rate effect on the constitutive response (i.e., an increased stress value at the same strain level for higher strain rates), but the same cannot be inferred for strain-to-failure values that may either increase or decrease as a function of the strain rate. In any case, we were able to measure a high level of stress multiaxiality (measured by the ratio of the hydrostatic and the deviatoric part of the stress tensor), which is known to reduce drastically the material strain to failure.
38 We therefore propose that multiaxial strain also participates in the pathogenesis of anterior and posterior retinal lesions.