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Massimo Antonio Fazio, Mark E. Clark, Christopher A Girkin; Optic Nerve Head (ONH) In Vivo Biomechanics Assessed by Quantification of the IOP Fluctuations caused by the Ocular Pulse Pressure (OPP). Invest. Ophthalmol. Vis. Sci. 2017;58(8):2456. doi: https://doi.org/.
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© ARVO (1962-2015); The Authors (2016-present)
To quantify in-vivo and non-invasive the Ocular Pulse Amplitude (OPA)-induced mechanical deformations of the ONH.
The ONH of 2 eyes from 2 normal brain-dead organ donors were imaged with Spectralis SD-OCT (Heidelberg Eng.) mounted vertically. The OCT was equipped with research software that allowed continuous recording of a single B-scan over a period of 15 seconds at 13Hz at physiologic IOP. A collection of about 200 vertical B-scans of the same cross-section was processed with a custom algorithm. The time-varying 2D displacement and strain fields were computed by means of a template-matching approach.
The average maximum shear strain in the peripapillary retinal nerve fiber layer (ppRNFL) was significantly lower than in the peripapillary sclera (ppScl) (p<0.01) and lamina cribrosa (LC) (p<0.01) at any time-point in both eyes. In the two eyes OPA was 3.1 and 4.2 mmHg. The small changes in IOP caused by OPA induced a sizable regional and time-varying fluctuation of the local (Fig. 1) and average strain (Fig. 2). In correspondence of the systolic and diastolic pressure average strain reached a local maxima in all the tissue, indicating a dependency between ONH deformations and the cyclical IOP fluctuations induced by OPA (Fig. 2).
We estimated in-vivo and non-invasively the mechanical response of the connective and axonal ONH tisseus to the small IOP fluctuations caused by OPP. Strain levels appeared to be particularly high in the ppScl and the LC, and relatively low in the ppRNFL. It is plausible to hypothesize that the ONH anatomy and mechanical structure it is such that IOP-induced deformations are sustained by the connective tissues and mitigated in the axonal regions.
This is an abstract that was submitted for the 2017 ARVO Annual Meeting, held in Baltimore, MD, May 7-11, 2017.
1st row) Morphology of the ONH sagittal section depicted in the vertical B-scan at a given time-point. 2nd) Displacement magnitude map. The laminar tissue is highly displaced during the IOP fluctuations caused by the ocular pulse (trend in Fig. 2). 3rd) Maximum shear strain regional distribution. It can be noticed that sharp shear strain gradients are present in the ONH.
Time-dependent variation of the average shear strain computed the RNFL, LC, and ppScl tissues over the acquisition time of 15 seconds. Strain variations over time can be finely quantified, indeed the occurance of aortic valve closure can often be identified in the temporal strain profiles.
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