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. Author manuscript; available in PMC: 2023 Apr 1.
Published in final edited form as: Injury. 2022 Feb 5;53(4):1401–1415. doi: 10.1016/j.injury.2022.02.008

Ocular biomechanics during improvised explosive device blast: a computational study using eye-specific models

Alireza Karimi a,*, Reza Razaghi b, Christopher A Girkin a, J Crawford Downs a
PMCID: PMC8940691  NIHMSID: NIHMS1780292  PMID: 35144807

Abstract

Background:

Eye injuries comprise 10–13% of civilian improvised explosive device (IED) injuries. The bomb blast wave induces a normal and shear forces on the tissues, causing a large acute IOP elevation. This study calculated the biomechanical stresses and strains in the eye due to IED explosion via eye-specific fluid-structure interaction (FSI) models.

Methods:

Blast occurred at 2, 3, and 4 m from the front and side of the victim and the weights of the IED were 1 and 2 kg. The ground was covered with the deformable soil to mimic the realistic IED explosion condition and reflect the blast wave.

Results:

The IOP elevation of ~6,000–48,000 mmHg was observed in the eyes while the highest IOP was occurred with the IED weight and distance of 2 kg and 2 m (front) and the lowest was occurred with the IED weight and distance of 1 kg and 4 m (side). Our findings suggest the importance of the victim location and orientation concerning the blast wave when it comes to ocular injury assessment. IOP elevation of ~2900 and ~2700 mmHg were observed in ~1.6 ms after the blast for the IEDS weight of 2 kg and a victim distance of 2 m in front and side blasts, respectively, in consistence with the literature. Nonetheless, IOPs were considerably higher after ~1.6 ms due to the merging of the bomb blast wave and its reflection off the ground.

Conclusions:

The stresses and strains were highest for the frontal blast. Both side and frontal blasts caused higher stresses and strains at the rectus muscle insertions where the sclera is thinnest and prone to rupture. Blast angle has no considerable role in the resultant IOP. Front blast with a heavier IED resulted a higher stresses and deformations in the eye connective tissues compared to the side blast.

Keywords: Improvised Explosive Devices, Ground Reinforcement, Intraocular Pressure, Finite Element Method

Introduction

An improvised explosive device (IED) is a bomb made and positioned in ways other than in conventional military action. IEDs are typically hidden in the side roads and are detonated once troops or vehicles pass [1, 2], often causing ocular injury [3]. IED explosions cause a mix of blast, penetrating, and burn injuries, as well as blunt trauma [4, 5]. More than 24,000 injuries to American service members have been ascribed to the military action in Iraq as of March 2007 [6] and ocular injuries account for ~ 5–13% [7, 8], 29% [9], and 72% [10] of battlefield casualties, depending on the report.

Experiments to investigate ocular injuries due to IED blast are difficult, especially in measurement of the tissue deformation and intraocular pressure (IOP), and assessment of ocular injury. Some of this difficulty is inherent in the extremely short time course of the blast and its aftereffects. Numerical simulations based on the finite element (FE) method can be used to estimate the eye injury in lieu of conventional experimental techniques, and are particularly effective in short-duration loadings such as blast. Ocular blast injuries may be either primary (direct effect of the blast wave), secondary (effect of fragments/debris), or tertiary (rapid acceleration of the body propelled by the explosion) [11]. In vivo IOP measurement during blast exposure in rats caused IOP elevation as orders of magnitude higher than that of the normal physiologic levels (1739±307 mmHg from the normal IOP range of 12–22 mmHg) [12]. Blast overpressure experiments in animals also resulted in globe rupture, retinal damage, hyphema, corneal edema, and optic nerve degeneration [13, 14]. IOP elevation in humans due to blast injury can cause globe rupture, angle recession, retinal commotio and detachment, hyphema, cataract and, infrequently, optic nerve avulsion [15–17].

While some experimental studies in animals [13, 14, 18–22] have been carried out to estimate ocular injury due to detonation, these studies are limited to some extent. The experimental setup in these studies most often uses a pressurization pipe, which induces overpressure in front of the subject’s face, limiting the primary injury to frontal blast.

Numerical models have been used to simulate the effects of explosive blast on the eye. A fluid structure interaction (FSI) model of the eye was developed by our group to calculate the stresses and strains in the eye connective tissues due to high explosive detonation [23], glass shard trauma only [24], and a mixture of glass shards and blast wave blunt trauma [25]. Several prior studies have been done to using FE simulations to calculate ocular damage due to the blast overpressure [26–30]. However, prior FE studies [28–30] have generally concentrated on anterior segment injury and so the posterior segment has not been well studied. In addition, all studies to date have simulated explosive detonation using the Conwep blast formulation available in the TM-855 US army handbook [31]; however, this method only accounts for the primary blast wave and disregards the interaction of the blast wave with the ground that induces ground blast reinforcement [32]. Moreover, many prior studies disregarded the boundary effects of the skull in the FE models, which ignores the key contributions of the extraocular tissues and bony orbit [30, 33–36]. Also, when the skull was considered, it was generally modeled as a rigid, elastic body [37, 38] with a fixed boundary condition at the bottom, thereby ignoring the roles of blast-induced head motion and body dislocation that may affect the results. Numerical studies to date [26–30] have only simulated the eye for a time period of ~2 ms after the IED blast wave made contact with the victim’s face. However, the IED blast wave itself can merge with the ground blast reinforcement wave, resulting in a much higher IOPs that occur well after the initial incident pressure wave. Finally, prior studies placed the blast in front of the victim’s face and eye, even though IEDs most commonly explode to the side of the victim when he/she is driving or passing by [39]. Very recently, our group showed the importance of ground blast reinforcement in ocular blast simulations [40]. We showed that when the role of ground blast reinforcement is disregarded, the stresses and strains in the eye were significantly different when ground blast reinforcement was taken into account.

To further elucidate the role of the victim’s location to the blast, the height of the IED from the ground (including ground blast reinforcement), and the weight of the explosive material, this study developed estimates of IOP for both IED blasts from the front and side of the victim. We used an anatomically accurate, 3D FSI model of the eye, skull, air, soil, and IED, using a multi-material, arbitrary, coupled Eulerian-Lagrangian approach, and IOP variation in the globe and its resultant stresses and deformations in the ocular connective tissues were estimated for front and side blasts at victim distances of 2, 3, and 4 m and IED weights of 1 and 2 kg.

Materials and Methods

Finite Element Model

The FE models were constructed based on our recent publication [40, 41]. The human ONH model was constructed from two human donors [42] and a generic anterior segment based on the Virginia Tech-Wake Forest University (VT-WFU) eye model [43] was added to the model. The eye model was thereafter incorporated into the human skull [40, 44–46] and volume meshed with 10-noded tetrahedral elements [47–49]. A mass element with the weight of 80 kg was defined underneath of the skull to mimic the body weight of the victim. The acceleration of gravity was applied to the victim and the blast wave as 9.80 m/s2. The final FE models of donors #118 and #129 are presented in Fig. 1. Detailed information about the number of elements/nodes, element length, Jacobian, volume aspect ratio, volume skewness, and simulation time for the two different eye models are summarized in Table 1. The material properties of the ocular connective tissues are listed in Table 2.

Fig. 1.

Fig. 1.

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The FE models of the skull (a) 118 and (b) 129. The cross section of the skull models (c) 118 and (d) 129. The temporal-nasal cross section of the eye donors (e) 118 and (f) 129. The closer view from the temporal-nasal section of the ONH of the eye donors (g) 118 and (h) 129.

Table 1.

The number of elements/nodes, element length, Jacobian, volume aspect ratio, volume skewness, and simulation time for three different eye model of donors #118 and #129.

Eye donor # Number of elements Number of nodes Element length (mm) Min/Max Jacobian Volume aspect ratio Volume skewness Simulation time (hr)
118 409,503 548,032 0.11/0.25 1 1.709 0.205 20.12
129 390,412 519,912 0.11/0.25 1 1.527 0.183 19.02
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Table 2.

Mechanical properties and element types in the skull-air-IED-soil models of donors #118 and #129. The bulk modulus was set to κ=100μ for all tissues (Cneo= μ/2).

Structures Element type Density Kg/nT Material model Material properties
Retina [48] Lagrangian 1100 Hyperelastic Cneo=0.005
Sclera [48] Lagrangian 1243 Hyperelastic Cneo=0.113 (118), 0.108 (129)
Lamina Cribrosa [48] Lagrangian 1243 Hyperelastic Cneo=0.113×0.38 (CTVF)=0.042 (118), 0.108×0.27 (CTVF)=0.029 (129)
Pia [48] Lagrangian 1100 Hyperelastic Cneo=0.113 (118), 0.108 (129)
Optic nerve [48] Lagrangian 1100 Hyperelastic Cneo=0.005
Cornea [83] Lagrangian 1076 Hyperelastic Cneo=0.276
Zonules [34] Lagrangian 1000 Hyperelastic Cneo=0.0595
Lens [34] Lagrangian 1078 Hyperelastic Cneo=0.1394
Ciliary body [34] Lagrangian 1600 Hyperelastic Cneo=0.1394
Extra ocular tissue [84] Lagrangian 970 Hyperelastic Cneo=0.0017
Aqueous [27] Lagrangian 1000 Hyperelastic Cneo=0. 0000035
Vitreous [85] Lagrangian 1000 Viscoelastic G0=0.01 kPa, G∞=0.0003 kPa, β=14.26 1/s, K=2 GPa
Skull [23] Lagrangian 1009 Elastic E=13.7 GPa, ν=0.3
IED [28] Eulerian 1570 Jones-Wilkins-Lee equation Detonation velocity = 6930 m/s, Chapman-Jouget pressure‡ = 21 GPa, Internal energy density (E0) = 4.3 GPa
Air [28] Eulerian 1.22 Ideal-gas gamma-law Internal energy density (E0) = 0.258 MPa, γ = 1.4, Pressure cut off=1e7 GPa
Soil [86] Eulerian 1986 Federal Highway Administration (FHWA) soil model NPLOT (Pore Water Pressure) = 5, SPGRAV (Specific Gravity of Soil used to get porosity)= 2.65, ρwater (Density of water in model units - used to determine air void strain (saturation)) = 1, VN (Viscoplasticity parameter (strain-rate enhanced strength)) = 2, GAMMAR (Viscoplasticity parameter (strain-rate enhanced strength)) =1e-4, ITERMX (Maximum number of plasticity iterations) = 10, K (Bulk Modulus) = 5.19 GPa, G (Shear modulus) = 343 MPa, PHIMAX (Peak Shear Strength Angle (friction angle) (radians)) = 0.611, AHYP (Coefficient A for modified Drucker-Prager Surface) = 4.4e-9, COH (Cohesion ñ Shear Strength at zero confinement (overburden)) = 6.2e-8, ECCEN (Eccentricity parameter for third invariant effects) = 1, AN (Strain hardening percent of phi max where non-linear effects start) = 0.25, ET (Strain Hardening Amount of non-linear effects) = 0.01, MCONT (Moisture Content of Soil (Determines amount of air voids) (0.0 – 1.00)) = 0.252, PWD1 (Parameter for pore water effects on bulk modulus) = 463, PWKSK (Skeleton bulk modulus- Pore water parameter ñ set to zero to eliminate effects) = 5.199e-4, PHIRES (The minimum internal friction angle, radians (residual shear strength)) = 0.001, DINT (Volumetric Strain at Initial damage threshold) = 0.1, VDFM (Void formation energy (like fracture energy)) = 1, DAMLEV (Level of damage that will cause element deletion (0.0 – 1.00)) = 0, EPSMAX (Maximum principle failure strain) = 1
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‡

The Chapman-Jouguet pressure is reached if the sonic velocity of the reaction gases reaches the detonation velocity.

The average IOP of 15 mmHg [50] was applied to the model as an initial condition. A 10-core Intel® Xeon® CPU W-2155@3.30 GHz computer with 256GB RAM was used to run the simulations in explicit-dynamic LS-DYNA (Ansys/LST, AL, US). The simulations were conducted in three steps, including IOP elevation from 0 to 15 mmHg (0.10 ms), relaxation (0.40 ms), and then IED explosion (9.5 ms) with time steps of 0.10 μs (100 time steps).

We fully describe the method to validate the apical corneal displacement of the FE model in our recent paper [40], which was performed by comparing the current model to a prior experimental study performed in human cadaver eyes [51] (Fig. 2).

Fig. 2.

Fig. 2.

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The force-displacement experimental data of the human eye indentation test [51]. The upper and lower bands data is related to the range of the data. The force-displacement responses of the eye models of donors #118 and #129 were calculated under the loading rate of 50 mm/min and plotted.

Detonation Model

The detonation formulations were all described in our recent publication [23, 25, 40]. Briefly, the energy of the IED was abruptly released inside the front of the detonation wave. Detonation development was presumed to be upright to the temporal and frontal surfaces of the skull at the distances of 2, 3, and 4 m from the victim as displayed in Fig. 3. While the approach used in this study is similar to a load-blast function, we included enhancements for treating ground-reflected blast waves, different blast origin locations and multiple blast sources. A hemispherical surface blast was chosen as the blast source in this study and the charge is located on or near the surface of the ground (up to 1.70 m), to mimic an IED explosion [32, 40]. The federal highway administration (FHWA) soil material model was used to simulate the soil beneath the air, IED, and skull model [52].

Fig. 3.

Fig. 3.

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The FSI model of the skull, eye, IED, air, and soil. The IED was placed within 2, 3, and 4 m of the victim at the height of 1.70 m from the ground. After the explosion, the blast wave from the bomb directly and its reflection from the ground hit the frontal and temporal bones of the skull.

Results

The pressure contours from the front and side blasts of a 2 kg IED placed 2 m from the victim are shown in Fig. 4 for donor 118. When the blast occurred, an overpressure was first observed in the ground, which then expands and propagates symmetrically around the center of explosion until it reaches the skull. The pattern of the blast overpressure expansion in the ground was the same, regardless of the orientation of the victim.

Fig. 4.

Fig. 4.

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The contours of pressure distribution in the ground (soil) and skull model from the initiation of the explosion until the end of the simulation time for the eye models of donor #118 (a) front and (b) side blast

The variation of IOP versus simulation time in the eye models of donors 118 and 129 are estimated and plotted in Fig. 5 for the front and side blasts. Note that the negative IOP represents the direction of the pressure on the vitreous body. The 2 kg IED located 2 m from the victim triggered an IOP elevation of 2940 and 2680 mmHg within 1.6 ms after the blast in donor 118 for the front and side blasts, respectively. In the eye model of donor 129, similar conditions led to a 2880 and 2720 mmHg IOP elevation ~ 1.6 ms following the IED front and side blasts, respectively. The blast wave from the 2 kg IED reaches the eye faster and led to a greater IOP elevation compared to with the lighter, 1 kg IED.

Fig. 5.

Fig. 5.

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The variation of the IOP versus the simulation time for (a) the eye model of donor 118, front blast, (b) the eye model of donor 118, side blast, (c) the eye model of donor 129, front blast, and (d) the eye model of donor 129, side blast.

The contours of displacement and pressure in donor 118 are displayed for all modeled scenarios in Figs. 6 & 7, respectively. Videos of the pressure distribution in the eye and skull through time for all modeled scenarios are provided as supplementary materials.

Fig. 6.

Fig. 6.

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The contours of displacement distribution in the skull model 118 from the (a) front and (b) side blasts under the IED weights of 1 and 2 kg as well as the distances of 2, 3, and 4 m from the victim.

Fig. 7.

Fig. 7.

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The contours of pressure distribution in the skull model 118 from the (a) front and (b) side blasts under the IED weights of 1 and 2 kg as well as the distances of 2, 3, and 4 m from the victim.

The von Mises stress contours in the eye of donor 118 are presented for all modeled scenarios in Fig. 8. Higher stresses were predicted in the temporal sclera of the eye posterior to the equator, regardless of victim orientation to the blast. The peak von Mises stresses in the rectus insertion, limbus, and ONH for models of donors 118 and 129 were calculated and listed in Table 3.

Fig. 8.

Fig. 8.

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The contours of von Mises stress distribution in the skull donor 118 from the (a) front and (b) side blasts under the IED weights of 1 and 2 kg as well as the distances of 2, 3, and 4 m from the victim.

Table 3.

Peak von Mises stress (MPa) in the rectus insertion, limbus, and ONH for the eye models of donors #118 and #129 at different IED weights and distances from the victim.

Eye Models Model #118 Model #129
Eye connective tissue IED weight - distance Front Side Front Side
1 kg – 2 m 4.3 3.9 4.3 3.9
1 kg – 3 m 2.8 1.0 2.8 1.0
Rectus insertion 1 kg – 4 m 0.9 0.7 0.9 0.7
2 kg – 2 m 7.0 5.0 7.0 5.0
2 kg – 3 m 2.8 2.1 2.9 2.2
2 kg – 4 m 1.7 1.4 1.7 1.4
1 kg – 2 m 0.5 0.5 0.5 0.5
1 kg – 3 m 0.3 0.4 0.4 0.4
Limbus 1 kg – 4 m 0.3 0.3 0.3 0.3
2 kg – 2 m 0.7 0.6 0.7 0.6
2 kg – 3 m 0.6 0.6 0.6 0.6
2 kg – 4 m 0.5 0.4 0.5 0.4
1 kg – 2 m 0.08 0.5 0.08 0.5
1 kg – 3 m 0.07 0.3 0.07 0.3
ONH 1 kg – 4 m 0.06 0.1 0.06 0.1
2 kg – 2 m 0.5 0.6 0.5 0.6
2 kg – 3 m 0.3 0.5 0.2 0.5
2 kg – 4 m 0.1 0.2 0.1 0.2
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The contours of first (maximum) principal stress in donor 118 for all modeled scenarios are presented in Fig. 9. The average engineering strain in donors 118 and 129 is illustrated in Fig. 10. The peak strains in the rectus insertion, limbus, and ONH were calculated for both donors and listed in Table 4.

Fig. 9.

Fig. 9.

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The contours of first (maximum) principal stress distribution in the skull donor 118 from the (a) front and (b) side blasts under the IED weights of 1 and 2 kg as well as the distances of 2, 3, and 4 m from the victim.

Fig. 10.

Fig. 10.

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The contours of average engineering strain distribution in the skull donor 118 from the (a) front and (b) side blasts under the IED weights of 1 and 2 kg as well as the distances of 2, 3, and 4 m from the victim.

Table 4.

Peak strain (%) in the rectus insertion, limbus, and ONH for the eye models of donors #118 and #129 at different IED weights and distances from the victim.

Eye Models Model #118 Model #129
Eye connective tissue IED weight - distance Front Side Front Side
1 kg – 2 m 47.3 34.2 47.3 34.1
1 kg – 3 m 41.3 32.1 41.3 29.1
Rectus insertion 1 kg – 4 m 35.4 25.0 35.4 25.1
2 kg – 2 m 59.2 35.4 59.2 35.4
2 kg – 3 m 47.3 32.9 47.3 30.2
2 kg – 4 m 37.1 25.1 37.1 25.1
1 kg – 2 m 20.5 21.3 20.5 21.3
1 kg – 3 m 17.5 18.2 17.5 18.2
Limbus 1 kg – 4 m 11.6 13.1 11.6 13.1
2 kg – 2 m 23.5 23.5 23.5 23.5
2 kg – 3 m 18.2 19.1 18.2 19.1
2 kg – 4 m 15.1 16.0 15.1 16.0
1 kg – 2 m 23.5 17.5 23.6 17.8
1 kg – 3 m 17.5 11.6 17.5 11.7
ONH 1 kg – 4 m 11.6 5.6 11.7 5.7
2 kg – 2 m 35.4 23.5 35.5 23.6
2 kg – 3 m 23.5 17.5 23.5 17.6
2 kg – 4 m 17.5 11.6 17.4 11.6
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Discussion

In recent years, terrorist activities worldwide have increased and IEDs are the main tool in such activities [53]. Ocular injuries from IEDs often require extensive surgical repair or evisceration/enucleation [54–57]. However, the impacts of IED blasts on the eyes are among the most complicated to study and experimentally cannot be simply analyzed. The position of the victim in relation to the bomb, including the angle and height of the victim in relation to the center of the explosive device, affect the extent of injury [58]. In addition, injuries to the eye as a result of an IED explosion can range from simple corneal abrasion to globe rupture [59] and complete avulsion of the optic nerve [3, 60], due to the abrupt rise in IOP [20, 61]. Numerical simulation using FEM can estimate pressures and deformations in an eye at different blast angles, intensities, and distances that are not possible to determine experimentally. In this study, the simulated IED was located at distances of 2, 3, and 4 m from the victim at a height of 1.70 m from the ground. The main objective of this work was to estimate the stresses and deformations in the human eye as a result of the IOP elevation due to the IED blast using a set of two eye-specific FSI models (Figs. 1 & 2).

While head motion resulting from the blast has been neglected in prior studies [62], our models predict a rigid body translation of 0.15 and 0.3 cm for 1 and 2 kg IEDs, respectively, when the explosion occurred within the distance of 2 m from the victim. Rigid body motion can influence the force vectors applying on the victim during the blast wave, and so it is an important variable to consider.

Most studies to date [23, 25, 27, 28, 30, 33, 37] have used Conwep formulations [31] that does not consider the interaction of the blast wave with the ground. Nevertheless, it has been documented that blast wave can be reinforced by the ground and can considerably change injury intensity [32], and results in an up to eightfold amplification of the blast energy [63]. Herein, the ground blast reinforcement was taken into account (Fig. 4). It has been shown that the detonation Mach front that is formed by the reflected pressure waves depends on the angle between the ground and incident wave [32]. Merging of the ground blast reinforcement overpressure and the bomb blast pressure itself caused a larger overpressure in the skull in our simulations, specifically around the orbital bone (Fig. 4). In addition, it has been shown that the energy transmitted to the victim is greater when ground blast reinforcement is taken into account [40, 64, 65]. Furthermore, the Mach front arrives before the incident shock wave as computed for surface burst formulations [32]. It may seem counterintuitive that the reflected ground blast wave would reach the victim before the IED blast wave itself, yet experimental studies have demonstrated this rapid overpressure from the ground blast reflection [66]. Herein, we showed that the blast wave reflected off the ground hit the victim before the direct bomb blast pressure, which is in good agreement with our previous study [40]. The defined origins for the enhanced blast pressure in the skull were located on the frontal and temporal bones for front and side blasts, respectively (Fig. 4).

Similarly, merging the two blast waves invoked an abrupt IOP elevation to ~ 48,600 mmHg, occurring ~ 3 ms after the blast (Fig. 5). This is in contrast with most prior studies, which ignored ground blast reinforcement and reported IOP only up to 1.6 ms after the blast [26–30]. When comparing our results to prior studies for similar time courses, our models estimate an IOP elevation of ~2900 and ~2700 mmHg for the front and side blasts, respectively that is in the ranges of 2625 mmHg [38], 3150 mmHg [67], 1739±307 mmHg [12], and 3000 mmHg [30] reported previously. These results are also in agreement with the porcine eyes under blast loading [68, 69], wherein the highest IOPs measured ranged from 3000 to 5850 mmHg at a highest pressure of 1575 mmHg.

It has been reported that maximum rupture stress of the human eye is 11.2 MPa under quasi-static loading [70], and 20.4 MPa [30] to 30.2 MPa [70] for dynamic loading. Globe rupture pressures of 1.6 MPa [30], and 9.4 MPa [70] were also reported. These values are both higher than the maximum stress of 7 MPa and pressure of 0.8 MPa in our simulations (Figs. 7 & 8). Eye ruptures in the limbus are frequently orientated circumferentially while at the equator they are mostly coronally orientated, presumably due to the circumferential collagen fiber orientation at the limbus and meridional orientation at the equator [71]. Esposito et al., simulated 1.5 kg TNT equivalent at 1 m, which resulted in pressures of 5, 4, 9, and 15 MPa at the corneal apex, vitreous base, equator area, and macula, respectively [72]. In our simulations, the IED distance of 2 m from the victim led to peak pressures of ~ 5, 5, 7.50, 12.10 MPa at the corneal apex, vitreous base, equator area, and macula, respectively (Fig. 7). In the limbus area the stress was found to be ~ 1 MPa, which is lower than the 6.4 MPa reported previously [28]. The difference between our results and prior studies could be related to the distance of the victim and the IED, the ground reinforcement of the blast pressure, and the material properties of the connective tissues, which can significantly affect the stresses and pressures in the ocular connective tissues. Results revealed higher stresses at the temporal side of the eye, specifically in the scleral wall, regardless of the orientation of the IED to the victim (Fig. 7); this can be attributed to the thin sclera at the equator [73]. Furthermore, the temporal sclera is less protected from the blast wave by the nose. The temporal sclera may also be more prone to injury or rupture due to its different embryologic origin, wherein the temporal wedge of sclera forms first from neural crest cell migration after formation of the anterior segment [74]. The front explosion led to a high compressive stress at the nasal, inferior, and part of the temporal-inferior of the globe, while a high tensile stress of 1 MPa was observed at the temporal side of the eye. In side blast, the compressive stress of 1 MPa was primarily observed on the temporal side. Due the nature of the side blast, wherein the blast wave impacts the nasal part of the globe, the temporal aspect of the eye was compressed toward the extra ocular tissue and orbital bone (Fig. 9). The highest stresses and strains were around the rectus muscle insertions into the sclera (Table 3 and 4), that are the most vulnerable site of globe rupture in traumatic eye injury [75–78]. The resultant stresses and deformation increased when the blast size was larger and located nearer to the eye, which is in agreement with Weaver et al., [30]. The average engineering strain [79] resulting from front and side blasts with an IED weight of 2 kg and a victim distance of 2 m exhibited the highest strains, while a lighter, more distant IED resulted in significantly lower strain and the associated risk of globe rupture (Fig. 10). Our results revealed the highest average engineering strain of 25–59% in the sclera at the rectus muscle insertion (Table 4 & Fig. 10), which is within the range of scleral rupture test results of ~38–75% reported by Kinon et al., [80]. There is significant eye-specific variability in the rupture test data available, so it’s difficult to ascribe a precise strain level at which rupture is likely to occur. However, it is reasonable to assume that higher scleral strains are associated with increased rupture probability.

For battlefield injuries, prevention is better than cure. A theoretical analysis found that soldiers who wore the standard current US Army 2-mm-thick defensive goggle experienced 52% less eye injuries [81]. Projecting this figure to the Vietnam War, 5,000 eye injuries in allied forces would have been prevented if goggles had been worn [82]. Future simulations using this modeling framework will include greater distances from the IED to the victim, multiple detonations, barriers such as vehicle shields/windows, and personal protective gear such as helmets and goggles. Such work could aid in the design optimization of protective gear to reduce blast-induced injuries to the eye and head.

Limitations

First, validation of the ocular blast FE simulation is a challenging task because experimental results are not generally available. However, we attempted to validate our model using indentation testing of the cornea, although the experimental deformation velocity is much lower than that associated with blast. Future experimental studies that capture both IOP and strain data would be helpful for full validation. Second, we simulated only 1 and 2 kg IEDs placed 2, 3, and 4 m to the side and front of the victim; prior studies have shown that various distances result in different levels of eye injury [28] and we plan to incorporate a wider range of distances in future studies. Third, the skull and IED in our simulations were placed at the same height from the ground, although an IED could be located at many heights, even below the ground surface, which would affect the resultant ocular stresses and deformations; these conditions will also be addressed in future studies. Fourth, only two eye-specific models from two donors were used, and both were incorporated into the same human skull model. This is logical since we focused our results on ocular injury, and so we kept other aspects of the simulation constant (skull geometry and material properties) even though they would obviously be individual-specific. CT/MRI data of the skull and eyes are not available for the donor eyes we collect, so this is unavoidable, although we plan to conduct parameter sensitivity studies of the effects of variations in ocular, skull and orbital anatomy and material properties in the future. Finally, even considering the aforementioned limitations and simplifications inherent in our approach, this study should provide valuable information for ophthalmologists and army medical experts treating blast trauma.

Conclusion

In summary, in this study we used a computational FE model to calculate the stresses, strains, and IOP in the eye due to 1 kg and 2 kg IED blast at side and frnt orientations and different distances from the victim. The results revealed an abrupt, large IOP elevation, which led to large stresses and deformations within the eye. In particular, stresses and strains were highest around the scleral rectus muscle insertions where the sclera is thinnest and prone to rupture.

Supplementary Material

1
Download video file (56MB, avi)
2
Download video file (57.9MB, avi)

Acknowledgments and Fundings

This work was supported in part by the National Institutes of Health Grants R01-EY027924, R01-EY018926, and P30-EY003039 (Bethesda, Maryland); EyeSight Foundation of Alabama (Birmingham, Alabama); and Research to Prevent Blindness (New York, New York).

Footnotes

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Declaration of Competing Interest

None

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