gms | German Medical Science

Purpose: Ocular injuries from paintballs have been rising annually for the past decade. Impact-related eye trauma now constitutes a substantially increased proportion of active military injury, as well. Better understanding of the pathophysiology of paintball-related blunt ocular trauma may assist us in dealing with both of these important new sources of visual loss. Effects of direct hits to the globe have been discussed previously (ARVO 2006).

Methods: Paintball/ocular impact and dynamic mechanical responses were studied through finite element analysis for 60 porcine abattoir eyes struck under controlled conditions with paintballs in our ballistics laboratory. The physics-based numerical codes CTH and LS-DYNA were utilized while incorporating robust geometric and constitutive models of the eye and orbit. Following impact, the eyes were subjected to detailed histopathology. A comparative parametric 3-D finite element model of the human eye was developed using True-Gird (XYZ Scientific, Livermore, CA), defining physical characteristics of cornea, lens, aqueous, zonule, CB, vitreous, sclera, and ON.

Results: Ten eyes (17%) underwent total avulsion from the orbit, with complete disengagement of the optic nerve from its cannulated tonographic apparatus. In each instance the paintball had entered the orbit adjacent to the globe, axially compressing it, and forcefully propelling the globe anteriorly once the loculated paint mass proceeded posteriorly beyond the equator of the longitudinally distorted globe. The speed and force of this “ricochet repulsion” was dramatically evident on high-speed film images. Detailed modeling experiments with long-term supercomputer analysis produced comparable results when orbital constraints were in place, but not when these were removed.

Conclusions: The placement of the globe within the bony pyramidal orbit creates potential for progressive axial globe distortion, followed by anterograde displacement, producing sufficient expulsive force to result in severe neuropraxia (or even avulsion to the optic nerve) when intraocular soft-tissue dissection by an intruding blunt loculus extends posterior to the equator of the globe. This sequence of events might be reasonably anticipated in semi-tangential impact injuries that compress the globe superiorly without orbital rim breach or laminar blowout decompression of the orbit. Supercomputer imaging confirmed this combined rebound/rotational mechanism could account for nerve avulsion with midperipheral globe impact. But computer simulation also revealed another surprising mechanism, when impact is more tangential, in which high-speed globe rotation almost instantly shears the nerve near the disc, with minimal globe distortion or intra-orbital globe displacement. This latter mechanism is likely to explain the classic clinical scenario of a blind eye with clear media, normal motility, and a circle of heme surrounding a papillary void.

Keywords: 738 trauma, 634 pathology: experimental, 550 imaging/image analysis: non-clinical