AlllOO TfilDSD NBS

PUBLICATIONS

NBSIR 80-2037

Impact on Human Eyes by

Propelled Objects

Robert E. Berger

James A. Huckeba

Product Safety Technology Division

Center for Consumer Product Technology

National Engineering Laboratory

National Bureau of Standards

U.S. Department of Commerce

Washington, D.C. 20234

May 1980

Report to: -QC-—

100

.U56

80-2037

1980

c. 2

Consumer Product Safety Commission

5401 Westbard Avenue

Bethesda, Maryland 20016

NBSIR 80-2037

< • /

National Bureau of Standards

Library, E-01 Admin. Bldg.

MAR 2 1981

i^O-CC

D' JO

IMPACT ON HUMAN EYES BY ^ PROPELLED OBJECTS '

/f oi

Robert E. Berger

James A. Huckeba

Product Safety Technology Division

Center for Consumer Product Technology

National Engineering Laboratory

National Bureau of Standards

U.S. Department of Commerce

Washington, D.C. 20234

May 1980

Report to: Consumer Product Safety Commission

5401 Westbard Avenue

Bethesda, Maryland 20016

U.S. DEPARTMENT OF COMMERCE, Philip M. Klutznick, Secretary

Luther H. Hodges, Jr., Deputy Secretary

Jordan J. Baruch, Assistant Secretary for Productivity. Technology, and Innovation

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director
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IMPACT ON HUMAN EYES BY PROPELLED OBJECTS

The identification of injury mechanisms and tolerance criteria for

non-penetrating ocular contusions from projectile impact has been of

continuing interest in the ophthalmologic community. Several experimental

simulations of the injury process have been examined with animal eyes

vivo and in vitro^”^. Most of this work has been concerned with low

mass/high speed projectiles, such as BB's. Recently, however, there has

been an increased awareness of the px)tential for such injuries from larger,

slower moving propelled objects as are associated with projectile toys.

The U.S. Consumer Product Safety Commission is studying this problem as part of their toy safety program.

In previous work^, a mathematical model, based on elastic, quasistatic theories of impact, was developed to describe eyeball deformation

during impact and to predict the likelihood of injury. Based on this work,

a test method was developed to evaluate the injury potential of propelled

objects^. The validity of the test method is traceable to the mathematical

model; that is, for a range of projectiles (whose characteristics represent

the range of masses, geometries, and velocities of toy projectiles), the

response of the test method correlated well with the predictions of the

mathematical model.

The present paper describes high speed motion picture analysis of

impacts on enucleated human eyeballs by missiles which are representative

of toy projectiles. The purpose of this work was to determine whether

measured deformations of enucleated eyeballs correlate with deformations

predicted by the math model. Suitable agreement would support the use of

the proposed test method for evaluating projectile toys.

High speed motion picture analysis was also used by Delori, Pomerantz,

and Cox^ to observe the deformation of enucleated pigs’ eyes when impacted

by BB's. They reported that "no attempt was made to normalize intraocular

pressures." (The pressure of the pigs' eyes was very low: 15 to 20 scale

readings on the Schiotz tonometer; nonnal is 5 scale readings^.) This

suggested a secondary purpose for the present work, namely, to evaluate the

experimental deformation of eyeballs at normal intraocular pressure.

Materials and Methods

Eyeballs and Molds

Enucleated human eyeballs (not usable for transplants) were

contributed by a local eyebank. The eyeballs were tested within 24 hours

of death, having been kept refrigerated from the time of enucleation until

shortly before testing.

Each eyeball was mounted in a separate gelatin mold as described by

Delori et. al.^.. The container was made with flat glass plates and had

dimensions of 5x5x8 cm. The procedure for mounting the eyeballs differed
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from that employed in the aforementioned study. First, the container was

filled with a liquid 10% gelatin mixture which was allowed to gel. Next, a cone-shaped crater was cut into the exposed surface of thegelatin, and the

eyeball was positioned in the crater (see sketch in figure 1). Lastly,

liquid gelatin was added to the container to fill the rest of the crater

and to adjust the gelatin level relative to the eye. Because the new

gelatin was constrained to a thin layer in immediate contact with the

already cooled and previously set gelatin in the container, the newly added

mixture gelled within about ten minutes. The boundary between the new and

old gelatin was visibly indistinguishable. Impact experiments began

immediately after the new gelatin had set.

Before mounting, the intraocular pressures of the enucleated eyeballs

were negligible compared to normal (15-20 scale readings on Schiotz

tonometer; normal is 3-5 scale readings, about 17-24 mm Hg). The pressure

of the eyeballs could be increased by injecting water through the optic

nerve. However, in preliminary impact experiments, the pressure was

observed to have decreased after every impact.

In order to obtain multiple impact data with each eyeball, a method

was devised to adjust the intraocular pressure prior to each impact. A

small needle was inserted through the optic nerve stub and secured in place

by suturing the stub around the needle. Thin flexible tubing, attached to

the needle, ran through the gelatin and back out the exposed surface to a syringe filled with water. (During the eyeball mounting process, a narrow

trench was cut into the set gelatin to accommodate the needle and tubing.)

Liquid could thus be added to the eye prior to each impact to readjust the

intraocular pressure. The pressures were adjusted so that the tonometer

would register in the normal range. After impact, pressures dropped to 6-

10 scale readings.

Projectiles

To maintain accurate impacts, a system was designed^ in which a cylindrical nylon guide travels in a linear ball bushing on a projectile

launcher (described below) . The leading edge of the guide was drilled and

tapped to accommodate a set of hemispherical caps (see figure 2). For this

experiment, two caps, with radii 0.635 and 1.27 cm, were used. To increase

the mass of the projectiles, brass weights could be inserted between th^

guide and the caps.

The projectile launcher is shown in figure 3. A plunger rides in an aluminum support tube and is attached to a spring which may be compressed

to three alternate stop positions. When released from the stop, the spring

uncoils and causes the plunger to impact the nylon guide end of the

projectile. The nylon guide then advances in a linear ball bushing mounted

within the aluminum support tube and remains in the bearing during impact.

The velocity of the projectile is measured by a meter which records

the time interval required for edges A and B of the guide tube (figure 2)

to interrupt a light beam. A window in the aluminum support tube allows
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the beam to penetrate the window in the projectile guide tube. The

velocities achieved by the projectiles depend on the stop position to which

the spring is compressed and the mass of the projectile. . Velocities can be

further varied by adjusting the tension on the spring with the screw at the rear of the aluminum support tube.

In this experiment, the masses of the projectiles varied between 1.5

and 8 g; the velocities varied between 2.5 and 15 m/sec. It was felt that

the range of parameters observed on toys^ would be well represented by the

experimental range of tip geometries, masses, and velocities.

Cinematography

Motion pictures at 8,000 frames per second recorded each impact event.

The firing of the projectiles was synchronized to insure that the film in

the camera was moving full speed at the time of impact. Enlargements of

the motion pictures were analyzed frame by frame, carefully measuring the

deformations of the eyeball, especially the displacement of the projectile

toward the eyeball. (This displacement is thus the decrease in the

anterior-posterior diameter of the eyeball.)

Results

For eyeballs with normal intraocular pressure, the observed

deformations were restricted to the region of the eyeball immediately

surrounding the tip of the projectile. A typical example is shown in

figure 4, where the eyeball is seen both before impact and at the moment of

maximum projectile displacement. The increase in the diameter at the

equator is on the order of 1 to 2%. In preliminary experiments with

unpressurized eyeballs, equatorial expansions on the order of 10 to ^ 2% were observed.

The displacement of an eyeball at normal pressure is shown as a function of time in figure 5. The characteristics of the projectiles which

impacted the eyeball are also shown. In general, the displacement, y,

increases with the projectile kinetic energy, T , and the duration of

impact. At, increases as the velocity, V, decreases. These general trends

were predicted by the elastic, quasi-static mathematical model discussed

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where D. and Dp are the diameters of the projectile and eyeball,

respectively, ihe coefficients of proportionality depend upon the material

properties of the eye and projectile.

In figure 6, thej.measured maximum projectile displacements are plotted

as a function of T ^ /Xy'^ for the impacts on three normally pressurized

enucleated human eyeballs. Ceach represented by a different symbol). To

vary the value of T

^ ^ /Xy ^ ^ projectile configurations were changed for

each impact. For e§ch eyeball a straight line was fit to the data by the

method of least squares. A reasonable correlation exists between the

predictions of the mathematical model and the results of the experiment.

The figure also suggests that non-negligible differences exist in impact

response for .different donors. For fixed projectile characteristics (fixed

value of T ^ /Xy'^) ^ differences on the order of 1 mm exist in the maximum

displacement.

Discussion

Large gross deformations in unpressurized pigs' eyes were also

observed in the earlier study of Delori et al-^ (the projectile of that

study was quite different: BB (.345 g) travelling at 62 m/sec). These

authors (and others) have propounded a mechanism of contusive injury:

namely, the stretching of the anterior sclera perpendicular to the impact

direction (sometimes referred to as "eyeball expansion"); in combination

with the posterior displacement of the lens, this expansion causes high

stresses in the chamber angle region and the vitreous base. Although the

gross expansion is not nearly so exaggerated when the eyeball is at normal

pressures, there is no evidence to suggest that the proposed mechanism of

contusive injury needs to be modified. It appears as though the eyeball

expansion in pressurized eyes may only be significant in the immediate

vicinity of the corneo- scleral junction.

n The elastic model is, of course, a gross simplification of the impact

behavior of a system as complex as an eye. As explained in the earlier

report, its purpose is to suggest the relative importance of projectile

characteristics (mass, velocity, geometry) to the likelihood of eye injury.

As seen in figure 6, the measured displacements of the anterior surface of

the eye are related to a parameter which characterizes the projectile.

This is not' to suggest that the elastic model is appropriate (actually, the

model is being asked to represent data in a range where deformations are

finite - the ratio of displacement to the length of the anterior/posterior

axis approaches 1/3); rather, that the model is able to represent some gross aspects of eyeball deformation.
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EYEBALL

FIGURE 1

NEW GELATIN

CONTAINER

PREVIOUSLY

SET GELATIN

. Sketch illustrating procedure used to mount eyeballs in gelatin

molds.

- 6 -

FIGURE 2. Projectile guide tube with hemispherical caps. Distance AB used

in velocity measurements.

I

FIGURE 3 . Projectile launching system.

Figure 4b.

FIGURE 4. Example of eyeball deformation (a) before impact, (b) at moment

of maximum projectile displacement (shadow in (b) is wire used in

velocity measurement)

Figure 4a,

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DISPLACEMENT

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IMPACT

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FIGURE 5. Projectile displacement versus time for the impacts

of several projectiles against the same eyeball.
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MAXIMUM

DISPLACEMENT

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EYE

BALL

BY

PROJECTILE

IMPACT

|cm|

DISPLACEMENT PARAMETER, Tn^^V O'^’

V sec^ /

FIGURE 6. Maximum projectile displacement

versus displacement parameter,

which was predicted

by mathematical model. Each symbol

represents a different eyeball.

The straight lines were fit to the

data for each eyeball.
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References

1. WEIDENTHAL, D.T. , Experimental Ocular Contusion, Archives of Ophthal.,

71, 77-81 (1964).

2. WEIDENTHAL, D.T. and SCHEPENS, C.L., Peripheral Fundus Changes

Associated with Ocular Contusion, Am. J. Ophthal. 465-477 (1965).

3. DELORI, F., POMERANTZ, 0., and COX, M.S., Deformation of the Globe

Under High Speed Impact: Its Relation to Contusion Injuries,

Investigative Ophthal., 290-301 (1969).

4. BERGER, R.E., A Model for Evaluating the Ocular Contusion Injury

Potential of Propelled Objects, J. Bioengineering, 2, 345-358 (1978).

5. BERGER, R.E., Systan for Assessing Eye Injury Potential of Propelled

Objects, J. Research of the National Bureau of Standards, 84 (1), 9-19

(1979).
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NBS-114A (REV. s- 78 )

U.S, DEPT. OF COMM. 3. Acfi«$5ioa Wo. BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO.

NBSIR80-2037

4. TITLE AND SUBTITLE

Impact on Human Eyes by Propelled Objects

5. Publication Date MAY 1 980

7, AUTHOR(S)

Robert E. Berger, James A. Huckeba

8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, DC 20234

%9, Unit No.

11. Contract/Grant No.

12. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS fsrreer. City, st^te, ZIP)

National Bureau of Standards

Department of Commerce

Washington, D.C. 20234

13. Type of Report & Period Covered

14. Spomiwing Agency Coda

15. SUPPLEMENTARY NOTES

I I Document describes a computer program; SF-185, FIPS Software Summary, is attached.

16. ABSTRACT {A 20Q-v/OTd or leaa {actual aummary of moat aignificant information. If document includes a significant bibliography or

literature survey, mention it here.)

High speed motion pictures were employed to observe experimental impacts on enucleated human eyeballs by low speed missiles which are representative

of toy projectiles. A procedure was devised to adjust the intraocular

pressure prior to each impact. For similar impact conditions, the gross

deformation of eyeballs with normal pressure was not as severe as had been

previously reported for eyeballs with near zero pressure. The maximum

displacement was measured for several eyeballs with normal pressure. These

displacements were shown to correlate with the predictions of a mathematical

model which had been used to develop a test method for toy projectiles.

17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the firat letter of the first key word unleaa a proper name;

separated by aemicolona)

Contusion; eyeball deformation; impact; ocular injury; projectiles;

safety

18. AVAILABILITY [X3(Unlimited

MI I For Official Distribution. Do Not Release to NTIS

I I Order From Sup. of Doc., U.S. Government Printing Office, Washington, DC

20402, SD Stock No. SN003-003-

pt] Order From National Technical Information Service (NTIS), Springfield,

VA. 22161

19. SECURITY CLASS

(THIS REPORT)

UNCLASSIFIED

20. SECURITY CLASS

(THIS PAGE)

UNCLASSIFIED

21. NO. OF

PRINTED PAGES

13

22. Price

$ 5.00

USCOMM-OC
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