Simulation of Changes in Tensile Strain by Airbag Impact on Eyes After Trabeculectomy by Using Finite Element Analysis

Purpose

We studied the kinetic phenomenon of an airbag impact on eyes after trabeculectomy using finite element analysis (FEA), a computerized method for predicting how an object reacts to real-world physical effects and showing whether an object will break, to sequentially determine the responses at various airbag deployment velocities.

Methods

A human eye model was used in the simulations using the FEA program PAM-GENERISTM (Nihon ESI, Tokyo, Japan). A half-thickness incised scleral flap was created on the limbus and the strength of its adhesion to the outer sclera was set at 30%, 50%, and 100%. The airbag was set to hit the surface of the post-trabeculectomy eye at various velocities in two directions: perpendicular to the corneal center or perpendicular to the scleral flap (30° gaze-down position), at initial velocities of 20, 30, 40, 50, and 60 m/s.

Results

When the airbag impacted at 20 m/s or 30 m/s, the strain on the cornea and sclera did not reach the mechanical threshold and globe rupture was not observed. Scleral flap lacerations were observed at 40 m/s or more in any eye position, and scleral rupture extending posteriorly from the scleral flap edge and rupture of the scleral flap resulting from extension of the corneal laceration through limbal damage were observed. Even in the case of 100% scleral flap adhesion strength, scleral flap rupture occurred at 50 m/s impact velocity in the 30° gaze-down position, whereas in eyes with 30% or 50% scleral flap adhesion strength, scleral rupture was observed at an impact velocity of 40 m/s or more in both eye positions.

Conclusion

An airbag impact of ≥40 m/s might induce scleral flap rupture, indicating that current airbags may induce globe rupture in the eyes after trabeculectomy. The considerable damage caused by an airbag on the eyes of short-stature patients with glaucoma who have undergone trabeculectomy might indicate the necessity of ocular protection to avoid permanent eye damage.

Finite Element Analysis of Changes in Deformation of Intraocular Segments by Airbag Impact in Eyes of Various Axial Lengths

Background

We studied the kinetic phenomenon of an airbag impact on eyes with different axial lengths using finite element analysis (FEA) to sequentially determine the physical and mechanical responses of intraocular segments at various airbag deployment velocities.

Methods

The human eye model we created was used in simulations with the FEA program PAM-GENERISTM. The airbag was set to impact eyes with axial lengths of 21.85 mm (hyperopia), 23.85 mm (emmetropia) and 25.85 mm (myopia), at initial velocities of 20, 30, 40, 50 and 60 m/s. The deformation rate was calculated as the ratio of the length of three segments, anterior chamber, lens and vitreous, to that at the baseline from 0.2 ms to 2.0 ms after the airbag impact.

Results

Deformation rate of the anterior chamber was greater than that of other segments, especially in the early phase, 0.2-0.4 ms after the impact (P < 0.001), and it reached its peak, 80%, at 0.8 ms. A higher deformation rate in the anterior chamber was found in hyperopia compared with other axial length eyes in the first half period, 0.2-0.8 ms, followed by the rate in emmetropia (P < 0.001). The lens deformation rate was low, its peak ranging from 40% to 75%, and exceeded that of the anterior chamber at 1.4 ms and 1.6 ms after the impact (P < 0.01). The vitreous deformation rate was lower throughout the simulation period than that of the other segments and ranged from a negative value (elongation) in the later phase.

Conclusion

Airbag impact on the eyeball causes evident deformation, especially in the anterior chamber. The results obtained in this study, such as the time lag of the peak deformation between the anterior chamber and lens, suggest a clue to the pathophysiological mechanism of airbag ocular injury.

Finite element analysis of changes in tensile strain and deformation by airbag impact in eyes of various axial lengths

Purpose

Airbags have substantially reduced mortality and morbidity, while ocular injuries caused by airbags have been reported. We applied a three-dimensional finite element analysis (FEA) model we have established for evaluation of the deformation of an intact eyeball of various axial lengths induced by an airbag impact at various impact velocities.

Methods

A model human eye we have created was used in simulations with an FEA program, PAM-GENERIS™ (Nihon ESI, Tokyo, Japan). The airbag was set to impact eyes with various axial lengths of 21.85 mm (hyperopia), 23.85 mm (emmetropia) and 25.85 mm (myopia), at initial velocities of 30, 40, 50 and 60 m/s. Changes in the shape of the eye and the strain induced were calculated. Deformation of the eye in a cross-sectional view was displayed sequentially in slow motion.

Results

We found that considerable damage, such as corneal or scleral lacerations, was observed especially at higher impact velocities, such as 50 or 60 m/s, in eyes with any axial length. Deformation was most evident in the anterior segment. The decrease rate of axial length was greatest in the hyperopic eye, followed by the myopic eye, and the emmetropic eye.

Conclusions

It was shown that hyperopic eyes are most susceptible to deformation by an airbag impact in this simulation. The considerable deformation by an airbag impact on the eye during a traffic accident shown in this study might indicate the necessity of ocular protection to avoid permanent eye damage.

Driver's side curtain air bag-related globe rupture

A 35-year-old man presented after a high-speed motor vehicle accident and the driver's side curtain air bag causing blunt force trauma rupturing his right globe. The tangential force of the air bag resulted in an unusual arcuate horseshoe-shaped rupture through the lateral rectus insertion, causing avulsion of the muscle and tearing the sclera, with the apices of the tear extending anteriorly towards the limbus. Repair of the globe rupture was undertaken, and secondary vitrectomy revealed that the scleral tear had not involved the retina abutting the ora serrata. Silicone oil tamponade was used to fill the globe and the postoperative best corrected vision was 6/9. This is the first reported case of a ruptured globe caused by a side curtain air bag, resulting in a uniquely shaped arcuate scleral wound combined with lateral rectus avulsion, not associated with rhegmatogenous retinal damage, and is the first air bag-related globe rupture with scleral involvement to report a final best-corrected visual acuity better than 6/60.

Detection of airbag impact-induced cone photoreceptor damage by adaptive optics scanning laser ophthalmoscopy: a case report

Background

The purpose of this study was to report a case of traumatic maculopathy with para-central visual field defects following an impact by airbag deployment using adaptive optics scanning laser ophthalmoscopy (AO-SLO).

Case presentation

A 51-year-old man was involved in a motor vehicular accident and his left eye was struck by the deployed airbag, resulting in a para-central scotoma. The patient underwent a full ophthalmologic examination, spectral-domain optical coherence tomography (SD-OCT), and imaging with prototype AO-SLO systems (Canon Inc.) at 14 and 22 months after the injury. Images focused on the photoreceptor layer were recorded in the foveal area, and a montage of AO-SLO images was created. On AO-SLO, focal dark areas could be observed in the left eye at 14 months after the injury. The analysis showed that the cone mosaic (cone density, 16503/mm²; ratio of hexagonal Voronoi domain, 36.3 %; average nearest-neighbor distance (NND)/expected NND, 0.606) was disordered compared with the normal area of the same eye (cone density, 24821/mm²; ratio of hexagonal Voronoi domain, 44.1 %; average NND/expected NND, 0.739). The cone defect area corresponded to the area of the scotoma. A second AO-SLO was performed on the patient at 22 months after the injury and although there were still areas with reduced cone reflectivity, partial improvement of cone mosaic was detected by AO-SLO at this time point.

Conclusion

Partial recovery of damaged cone photoreceptors following closed globe blunt ocular trauma can be documented using AO-SLO longitudinal tracking.

Vehicle occupant restraint systems impact on eye injuries: a review

Vehicle occupant trauma to the eyes and associated facial structures has evolved rapidly in conjunction with safety-oriented vehicle design, including restraint systems. Trends vary worldwide with culture, personal factors, vehicle safety equipment, and the traffic environment-including physical, legislative, and enforcement. Wearing safety belts is essential to occupant protection. Airbags were designed as a supplement to protect the head from hard surfaces in frontal crashes, not as a primary countermeasure. Even where vehicle fleets are new with high airbag prevalence, but safety culture and knowledge of restraints is less than robust, injury attributable to not wearing seatbelts is frequent, especially in countries where high-powered vehicles are prevalent. Upper bodies of rapidly forward-moving unrestrained occupants collide with rearward-accelerating airbags. Airbag deployment produces injuries such as corneal abrasions, alkali burns, and the effects of globe compression.

Modeling thermal burns due to airbag deployment

Automotive airbags are now a widely accepted safety measure designed to reduce morbidity associated with motor vehicle accidents. Their usage is increasing with multiple airbags (driver, passenger and side curtain) being fitted to many vehicles. However the deployment of airbags has been identified as causing injuries in some instances including minor burns. There are three mechanisms for thermal burns due to an airbag; contact with the hot expelled gases from the airbag, contact with the hot airbag itself and melting of clothing from either of these contacts. A mathematical model is used here to predict the likelihood and severity of the first two types of burns. It is shown that direct contact with high temperature exhaust gases venting from the airbag can indeed lead to burns and that burns from contacting the hot airbag material are possible but far less likely to occur.

Simulation of air-bag impact on an eye with transsclerally fixated posterior chamber intraocular lens using finite element analysis

PURPOSE

To determine the physical and mechanical conditions of an impacting air bag that would rupture an eye with a transsclerally fixated posterior chamber intraocular lens (IOL).

SETTING

Numerical simulation study on a computer.

METHODS

Simulations in a model human eye were performed with a computer using the finite element analysis program PAM-CRASH (Nihon ESI). The air bag was set to impact the surface of an eye with a transsclerally fixated posterior chamber IOL at various velocities. The tensile force limit of a 10-1 polypropylene suture was assumed to be 0.16 N, which is specified in the U.S. Pharmacopeia XXII.

RESULTS

At the lowest velocity of 20.0 m/s, 10-0 polypropylene sutures were not likely to break. Sutures fixating the IOL might break and a corneoscleral incision was likely to open after 0.3 second at the medium impacting velocity (30 m/s). Suture rupture was very likely at the highest velocity (40 m/s) since the tensile force on the sutures continuously exceeded the breaking force after the impact.

CONCLUSIONS

In an eye with a transsclerally fixated posterior chamber IOL, severe ocular trauma can be caused by an air bag at high velocity. Small individuals such as elderly women are at greater risk for air-bag ocular injury. Further research on modifying air-bag design and deployment is important to minimize the risk for ocular injury.

A Case of Air-bag Associated Severe Ocular Injury

Air-bags have received widespread support as an effective means of enhancing automotive safety, and they are becoming more common as standard automobile equipment on many cars. Although air-bag induced ocular injuries are rare, they present a serious concern because of the possibility of permanent damage or visual impairment. To date, most reports have investigated ocular injury from high velocity motor vehicle accidents and reports of ocular injury from low speed motor vehicle accidents have been rare. We describe a patient who sustained severe ocular injury, including periorbital fracture, hyphema, vitreous hemorrhage, and choroidal rupture of the macular area, due to an inflated air-bag in a low speed motor vehicle accident.

Do motor vehicle airbags increase risk of ocular injuries in adults?

PURPOSE

This study was undertaken to evaluate the risk of eye injury in motor vehicle accidents in which airbags deploy. An attempt was made to assess the possible associations between eye injuries and eyewear in these accidents.

DESIGN

Retrospective observational case series and literature review with analysis.

PARTICIPANTS/METHODS

We conducted a literature review of 62 case reports and articles describing 110 adult cases of eye injury after deployment of an airbag and examined two Finnish accident cohorts. The fatal accident series (FAS; fatal injuries with one or more cars involved) included 121 individuals sitting behind an airbag that deployed (65 survivors), and the Airbag study (AB; nonfatal, relatively serious accidents) included 210 individuals (survivors).

MAIN OUTCOME MEASURES

The type of eye injury, eyewear, and crash dynamics were studied in each of the reviewed case reports. The fatal accident series and AB studies were analyzed to disclose the eye injuries and use of eyewear and to estimate their possible relation to deployment of airbags.

RESULTS

Analysis of the published reports revealed that airbag-induced eye injuries were not more frequently reported among wearers of eyeglasses than among nonwearers. However, open-eye injuries were reported three times more often among eyeglass wearers (P = 0.04), whereas all injuries from airbag chemicals occurred among nonwearers. With the exception of one orbital fracture with hyphema, all eye traumas (n = 7) in the FAS and AB cohorts were mild (eyebrow laceration, lid contusion, bruising). The risk of airbag-related eye injury was 2.5% for any eye injury and 0.4% for severe eye injury. In single accidents when seat belts were used, the risks were 2.0% and 0.5%, respectively. In the accidents from the FAS data no difference was observed in the risk for eye injury between survivors in incidents involving airbag deployment and incidents not involving airbags. This risk was not found to be greater among eyeglass wearers.

CONCLUSIONS

Despite reported cases in the literature, we found that the risk of severe eye injury from airbags was very low (0.4%) in fatal or relatively serious accidents. Eyewear did not seem to increase this risk but might interfere with the injury pattern.

Incidence of ocular injuries in motor vehicle crash victims with concomitant air bag deployment

OBJECTIVE

To report the incidence of ocular and nonocular trauma in patients admitted to the Henry Ford Hospital via the emergency room since 1994 after a motor vehicle crash (MVC) with and without air bag deployment.

DESIGN

A retrospective cohort study.

METHODS

The Henry Ford Hospital Trauma Registry database was reviewed for patients involved in MVCs with and without air bag deployment since 1994.

RESULTS

From 1994 to 1999, there were only seven air bag-related ocular injuries, representing 4.4% of all MVC-related ocular injuries. From 1997 to 1999, MVC-related ocular injuries with and without air bags represented 5.0% and 12.7%, respectively, of all MVC-related injuries. For that same period, the death rate and average Injury Severity Score for MVCs with air bag deployment were 3.4% and 10.75, compared with 8% and 14.5, respectively, for MVCs without air bag deployment.

CONCLUSIONS

MVC-related ocular injuries associated with air bag deployment are rare, and the incidence of ocular injuries associated with MVCs was lower when air bags were deployed.

Simulation of air-bag impact on post-radial keratotomy eye using finite element analysis

PURPOSE

To determine the physical and mechanical conditions of an impacting air bag that causes corneal rupture in a post-radial keratotomy (RK) eye using a simulation model of the human eye.

SETTING

Numerical simulation study on a computer.

METHODS

The simulations were performed by a computer using the finite element analysis program PAM-CRASH (Nihon ESI). The air bag was set to impact the surface of a post-RK eye with 4, 6, or 8 corneal incisions at various velocities. Strain on the corneal tissue including scarred incisions exceeding 9.0% was assumed to indicate the possibility of corneal rupture.

RESULTS

At a medium velocity of 30 m/s, corneal rupture was likely to occur. At an air-bag impact velocity of 40 m/s, 3 of 4, 5 of 6, and 8 of 8 incisions were likely to rupture in the case of 4-, 6-, and 8-incision procedures, respectively, leading to likely globe rupture in all situations. Lacerations extended beyond the incisions and involved the intact cornea at a velocity of 40 m/s. If the corneal tissue strength reduction was increased to 90%, most incisions were likely to rupture at impact velocities greater than 35 m/s in all incision procedures.

CONCLUSIONS

The results could partly reflect a reported case of globe rupture after RK and suggest that severe ocular trauma can be caused in the post-RK eye by air bags at ordinary impact velocities.

Airbags and eye injuries: epidemiology, spectrum of injury, and analysis of risk factors

Although airbags measurably reduce the overall risk of injury to adults (including eye injury), and death from motor vehicle accidents, injuries attributed to airbag deployment have been reported. To identify reported cases of ocular trauma related to airbag deployment, a MEDLINE search from 1991 to 2000 was performed. A total of 263 injuries in 101 patients were identified. Patient demographics, details of the accident, specific ocular structures injured, and visual outcomes when available where tabulated and analyzed. The most common of these affect the eyes. Damage to the orbit and virtually every ocular and adnexal structure has been seen. Although most injuries are self-limited and do not significantly compromise vision, some result in severe, permanent visual loss. Most common is damage to anterior structures due to either blunt, contusive forces and/or chemical injury. Posterior segment trauma is less common but generally more visually devastating because of the involvement of the retina or optic nerve. Data are not available to determine whether the wearing of eyeglasses or previous intraocular surgery affects the nature, severity, or outcome of these injuries. Awareness of the spectrum of airbag-associated ocular trauma will help physicians recognize these problems early and optimize their management. Data derived from analyses of these injuries will be critical to the development of safer, more effective devices.

Ocular morbidity associated with airbag deployment: a report of seven cases and a review of the literature

PURPOSE

To review ocular injuries secondary to airbag deployment that were seen in our institution and were reported in the literature.

METHODS

Patients examined at our institution between 1997 and 2000 were evaluated for ocular injuries caused by airbags. A review of the medical literature using Medline was performed. All reports involving ocular injuries secondary to airbags were included in this study.

RESULTS

Seven cases from our medical center were identified to involve airbag-related eye injuries. The ages of the patients ranged from 4 to 73 years. Ocular injuries included corneal abrasion, corneal decompensation, corneal alkali injury, hyphema, iris sphincter tears, vitreous hemorrhage, macular retinal pigment epithelium disruption, dislocated posterior chamber intraocular lens, and commotio retinae. A review of the medical literature showed 74 cases involving 80 eyes. The ages of the patients ranged from 2 to 81 years. Males slightly outnumbered females by a ratio of 1.1 to 1.0. The speed of the vehicles ranged from 0 to 65 miles per hour, with an average reported speed of 31 miles per hour. Reported injuries ranged from mild corneal abrasions to open globes.

CONCLUSIONS

Ocular morbidity secondary to airbag deployment must be recognized as a significant risk for motor vehicle drivers and passengers. Improvements in airbag safety will include increased consumer awareness and manufacturer design modification.

Airbags and bilateral eye injury: five case reports and a review of the literature

We report five cases of bilateral eye injuries from airbag deployment in motor vehicle crashes and review the world's literature on ocular injuries associated with airbags. The cases in the literature were identified by cross-referencing Medline searches from airbags and ocular injuries. Additional cases were identified after review of references from each article in the search. An additional 89 cases from the literature were identified and are included for discussion. Patients were treated individually in a noncontrolled, nonrandomized fashion according to the nature of each injury with regular follow-up examinations in clinic. Of the 94 cases studied, 24 (27%) were bilateral eye injuries, and 15 (16%) patients were wearing spectacles at the time of the accident. The most common injuries included corneal abrasions, eyelid trauma, and hyphemas. Outcomes ranged from complete resolution of symptoms and return of normal visual acuity to primary enucleation. This report describes the wide spectrum of eye injuries that may occur after airbag deployment. We suggest a management plan for the evaluation and treatment of the ocular complications of airbag-related trauma.

Partial Dislocation of Laser in situ Keratomileusis Flap by Air Bag Injury

PURPOSE

A patient developed significant corneal complications from air bag deployment, 17 months after laser in situ keratomileusis (LASIK).

METHODS

Case report, slit-lamp microscopy, and review of the medical literature.

RESULTS

A 37-year-old woman underwent bilateral LASIK with resultant 20/20 uncorrected visual acuity. Seventeen months later, she sustained facial and ocular injuries from air bag deployment during a motor vehicle accident. Examination revealed bilateral corneal abrasions, partial dislocation of the right corneal LASIK flap, and a hyphema in the right eye. The LASIK flap was realigned, but recovery was complicated by a slowly healing epithelial defect and flap edema. One month following the injury, epithelial ingrowth beneath the LASIK flap was noted. Surgical elevation of the flap and removal of the epithelial ingrowth was performed. Eight months later, epithelial ingrowth was absent and the visual acuity was 20/40. Residual irregular astigmatism necessitated rigid gas permeable contact lens fitting to achieve 20/20 visual acuity.

CONCLUSIONS

Air bags may cause significant ocular trauma. The wound healing response of LASIK allows corneal flap separation from its stromal bed for an indeterminate time after surgery. Discussion of the possible risk of corneal trauma as part of informed consent prior to LASIK may be appropriate.

Driver distance from the steering wheel: perception and objective measurement

OBJECTIVES

This study assessed the accuracy of driver perceptions of the distance between the driver's nose and the steering wheel of the vehicle as a factor in considering driver disconnection of an airbag contained in the steering wheel for preventing injury to the driver in an accident.

METHODS

A cross-sectional survey of 1000 drivers was done to obtain perceived and objective measurements of the distance between the driver's nose and the steering wheel of the vehicle.

RESULTS

Of 234 drivers who believed that they sat within 12 inches of the steering wheel, only 8 (3%) actually did so, whereas of 658 drivers who did not believe that they sat within 12 inches of the wheel, 14 (2%) did so. Shorter drivers were more likely than taller ones to both underestimate and overestimate their seating distance.

CONCLUSIONS

Considerable misperception of drivers' distance from the wheel indicates that drivers should objectively measure this distance.