Understanding Eye Injuries

More than 60% of all eye injuries are caused by blunt impact, i.e., impacts with objects of various kinds that do not cause a perforation of the globe. The clinical manifestations of such injuries are quite different and include retinal rupture, choroid rupture (the tissue between the retina and the sclera), retinal tear and retinal detachment, macular holes, and dialysis (see Figure 1 for the primary structures of the human eye).

Although the clinical phenomenology of such injuries has been accurately described already, the mechanisms responsible for the lesions of the retina and choroid associated with blunt impacts have not yet been fully understood. Among the theories proposed to explain the mechanism of damaging internal structures of the eye during a blunt impact, the most widely accepted one is called vitreous chord pulling-traction. According to this theory, during the compression of the eyeball in the direction of the impact, the expansion of the sclera in the orthogonal direction generates a critical stress in the internal structures favored by the viscous action of the vitreous, the jelly-like fluid contained in the main eye socket. This mechanism would explain why the retinal tears occur mainly at the vitreous base, i.e., near the circumferential junction between retina and choroid and in the vicinity of the macula.

Based on evidence of a patient who, despite having undergone the removal of the vitreous, had a clear macular hole resulting from a blunt impact, it was decided to investigate this phenomenon in more detail in order to validate the various hypotheses of damaging mechanism with the help of a MSC Dytran simulation. In particular, the behavior of a human eye subjected to blunt impact as a result of impact with the ball of steel has been simulated. The model, validated through a comparison of the simulation results against measured data available in literature, was then used to study the intensity of the dynamic stress waves produced during the impact, in order to assess the primary source of retina failures.

The Simulation Model
The computational model of the ocular globe was generated starting from an average size human eye represented with the help of the code MSC Dytran. The code was selected between the different explicit solvers available on the market mainly because of its advanced fluid-structure interaction capabilities. Assuming as symmetry plane the meridian section of the globe that contains the longitudinal axis, a half-eye model has been created that includes all substructures and tissues that could potentially affect its dynamic behavior: cornea, sclera, aqueous and vitreous humor, crystalline, ciliary body, and zonules. The retina was modeled as a thin layer connected to the sclera with constant thickness of 0.2mm. The related 3D mesh, shown in Figure 2, consisted of 6,912 brick elements. The spherical projectile with a diameter of 4.5mm was modeled as a rigid body. The configuration used for the simulations is that of a normal impact, in which the bullet hits the apex of the cornea in longitudinal direction at a speed of 62.5 m/s (Figure 3).

The Case in Exam
Given the complexity and variability of the physical and mechanical properties of all biological materials that are strongly dependent, i.e., by the hydration of the tissues and on how they are stimulated during the characterization tests, the related identification has been performed using the experimental results. These were obtained by Deloria et. al., 1967, through a non-penetrating impact test of the human eye. In the present case, the experimental values of corneal apex displacement caused by the penetration of the projectile as a function of time have been used as reference.

Additionally, the methodological principle of Occam’s Razor has been adopted as constitutive model to describe the behavior of all tissues. This principle minimizes the number of parameters necessary to describe the phenomenon with the requested accuracy.

The description of the tissues is based on a linear elastic material model (that neglects all visco-elasto-plastic effects) and on linear state equations. The vitreous has been modeled with viscoelastic fluid with damping. Crystalline and choroid have been described through linear state equations. The model parameters have been identified then through a reverse calibration process that uses as objective function the experimental measurements produced by Deloria et. al. Figures 4 and 5 show the experimental values compared with the optimized response of the numerical model.

The Results
A careful examination of the results obtained through the simulation show that most of the retinal ruptures, caused by blunt impact, are located in the macular and in the vitreous area, but very rarely affect the equatorial area. In order to understand the simulation results fully, the pressure values have been extracted in three points of interest. Figure 6 shows that the pressure waves, generated by the impact, propagate in the eye and are reflected as traction waves, in turn, affecting mainly the ocular fundus and the macula. The speed of propagation of the waves in the eye is much higher than the speed of a bullet, therefore, the peak values of tensile pressure (about 0.6MPa) were observed within 0.05 milliseconds after the impact when the ocular globe is not yet affected by large deformations (Figure 7). Shortly after the impact (at time = 0.1 milliseconds), the sclera starts to be affected by large deformations due to the penetration of the projectile. This causes the macular area to be subjected to compression, while the vitreous area is mainly subjected to traction. The pressure in the equatorial area is much lower than in other areas, and this confirms that there is a lower risk of rupture of that portion of the retina.

Future Developments
The preliminary results of the project indicate that the laceration of the retina mainly occurs due to the tension resulting from the reflection of compression waves in the moments immediately following the impact, and not necessarily due to the deformation of the whole eye. The availability of a reliable and validated model for the simulation enabled the research team to understand in detail the pathogenesis of the blunt impact phenomenon, which is particularly difficult to reproduce in a controlled laboratory test. Practical applications of this study are to be found especially in the military industry, for example, in the design of advanced security systems for personnel and for helicopter pilots in the event of a crash landing.

MSC Software Corp.
Santa Ana, Calif.
www.mscsoftware.com

University of Cassino, Italy
The University of Cassino was founded in 1979 and – due to its geographical location – it occupies a strategic location between all main cities in Central Italy. The university is composed by five faculties: Economics, Engineering, Humanities, Law, and Physical Education, with approximately 12,000 students and 336 professors/researchers distributed in seven departments with 47 laboratories. The students can choose between 18 undergraduate programs, 14 master degree programs, and eight doctoral programs. The university has a strong international presence in terms of research and educational activities.