MECHANICAL

PROPERTIES

OF CRANIAL SUTURES

CAROLYNRENZULLI JASLOW* The University

of Chicago,

Department

of Anatomy.

1025 E. 57th St., Chicago,

IL 60637, U.S.A.

Abstract -Many bones in mammalian skulls are linked together by cranial sutures. connective tissue joints that are morpholodcally variable and show different levels of interdigitation among and within species. The goal of this invest&ion was to determine whether sections of skull with cranial sutures have different mechanical propertIes than adjacent sections without sutures. and if these properties are enhanced with increased interdipitation. To test these hypotheses. bending strength and impact energy absorption were measured for samples of goat (Capra hircus) cranial bone without sutures and with sutures of different degrees of interdigitation. Bending strength was measured under both dynamic (9.7 mm displacement s- ‘) and relatively static (0.8 mm s - ‘) conditions, and at either speed. increased sutural interdigitation provided increased strength during three-point bending. However,except for very highly interdigitated sutures loaded slowly. sutures were not as strong in bending as bone. In contrast, sutures absorbed from 16% to loO?G more energy pr unit volume during impact loading than did bone. This five-fold increase in energy absorption by the sutures was significantly correlated with increased sutural interdigitation.

ISTRODl_‘CTION

forces e.xerted on the skull and sutures (Massler Schour,

The frequency inspired

of head injuries among

numerous

investipations

properties of the cranium.

has

of the cranial

hypothesize

bone.

phologies

might contribute

of canccllous

tures with mechanical

investigators bone

(McElhaney focused

as a

layered

‘sandwich’

et ul.. 1970: liubbard,

on

compacta

some

have studied whole sections of cranial

modeled

the mechanical

or the dipI&

baenid

1971). others have

properties

of just

associating

highly

advantages

reptiles

mastication

the

1960.

no data

anics is necessary to improve

the mechanical

properties and functional

more information

only ofskull

role of the cranium, little attention has been directed at

of the cranial humans.

the skull. the cranial

variable feature of

sutures. Hubbard

observed that in humans, cranial compliant

than the surrounding

rr 01. (1971)

bone, and that bone

and suture had equal bending strengths. However, neither this, nor any other study. has an attempt made to assess the mechanical

mine

in

response

how

the

are

increased

with

conditions, to virtually

bending strength

together (for descriptions

displacement

of sutures and fiber orient-

ation see Moss, 1957; Koskincn rr ul., 1976; Johansen 1986). Generally,

sutures are relatively simple, straight-edged sutural morphology

as the bones overlap or interdigitate. believed

to be associated

at birth.

were

static

trauma

may change

in

were to deter-

properties

of bending

differ between pure

suturat

loading

during

was measured

rates. In addition.

measured

sutures

not

sutures, and in sutures

interdigitation.

under a range of loading

tween age and mechanical

structures,

mcch-

from rapid loading during an impact blow

the collagenous connective tissue fibers that link them

but during growth,

greater

Because sutures function

suture is a joint between the bones of the

1982; Kokich.

to mechanical

mechanical

skull, consisting of the adjacent edges of the bones plus

and Hall,

sutural

to test the hypothesis that these properties

consequences of vari-

there

our understanding

cranial bone and cranial bone containing

ation in sutural morphology. A cranial

concerning

strength and energy absorption

been

and

1972). and

these hypotheses.

The purposes of this investigation

sutures were more

in amphis-

rooting

function in many diverse species, but also

an important

and morphologically

1974).

to support

Clearly

in spite of these etTorts to

su-

when the skull is

in sheep (Jaslow. 1989). However,

are currently

Wood.

understand

on sutural

interdigitated

in pigs and peccaries (Herring,

head-butting

structural

(Cans.

elements (Evans and Lissner. 1957; Melvin ef 01.. 1969; 1971). However.

skull func-

loaded during activities such as burrowing

structure

as independent

to particular

tions. These studies have focused primarily interdigitation,

While

to

ways in which sutures with certain mor-

compact bone (compaccta) that enclose a middle layer the diplo?.

vari-

ation among sutures, have led some investigators

bone is composed of inner and outer layers of bone called

and

1982). This

assertion, and observations of the morphological

of the mechanical

and much of this work has

focused on the characteristics Cranial

humans

1951; Moss, 1957. 1961: Oudhof,

to account

independent

of

growth,

sutural

under two direrent the correlations

properties

be-

of the sutures

for age effects on the changes

in

sutural

interdigitation.

Such changes are

with extrinsic

mechanical

MATERIAIS AND METHODS

Sumple preparation

Rzwid it1 jirul /,m~ 5 July 1989. *Current Address: Biology Department. Rhodes College. 2000 N. Parkway, Memphis. TN 381 I$, U.S.A.

The fresh heads of female domestic hircus) were used to obtain 313

goats (Capra

test specimens of cranial

314

C. R.

JASLOW

bones and sutures. Two sutures were studied: (1) the internasal suture, which remains straight-edged. ing a butt-joint, frontoparietal

throughout among

tain samples of crania) nasal

form-

and (2) the

suture, which exhibits variable amounts

of interdigitation bones

containing

development,

and

most

parietal

individuals

(Fig. 1). To ob-

bone and these sutures. the of the

and frontal

posterior

braincase

bones were removed

from each goat head. These sections of the cranium were stripped

of periosteum.

and frozen

at

-5°C.

They were then cut with a band saw into test specimens while

still frozen

to minimize

heating

of the

bone. Samples with frontoparietal

sutures or samples

of just

about

frontal

bone

were

cut

x 4-S cm long. Samples with internasal about

1 cm wide sutures were

1 cm wide x 2.5 cm long. Specimen

ranged from about samples

were irregularly

specimen

curved,

were individually

the ends of each

embedded

resin and cured at room temperature 2 h. During

thickness

2 to 5 mm. Because most of the in fiberglass

for no more than

this time, the bone or suture

moist with physiological

was kept

saline. After curing, the resin

blocks on the ends of each sample were trimmed produce

relatively

uniform

samples

to

that approxim-

ated straight beams. Sample beam width (w) and mean thickness(h)

at the suture (Fig. 2) were measured with

calipers (kO.05

mm) and thcsc values wcrc used later

to standardize

for diffcrcnces

in cross-sectional

arca

among test specimens. The dcgrcc of intcrdigitation each suture, a dimension& tracing

of

value. was cstimatcd

the path of the external

Fig. 1. Dorsal view of ;I goat skull showing the simple internasal (inJ and inlcrdigilalcd fronioparictal (fp) sutures. Rectangular openings in the skull show thcgcncral shape and orientation of typical test samples that wcrc cut from lurgcr sections that had been removed from the skull (dashcd lines).

by

suture surface. and I

Y

dividing that length by the straight lint distance from start to end of the suture. By this method, the straightedged internasal

sutures always

-x-

had intcrdigitation

values of 1, and the more convoluted

Y

a l

frontoparietal

I

I

sutures ranged from about 2 to about 6.5. The samples were stored in plastic bags at - 5 ‘C for up to 6 weeks, then placed in physiological

saline at 5°C to hydrate

for at least 12 h prior to testing. Bcforc testing. samples were brought during

to room temperature

(within

all tests, the samples were kept

I h) and

moist

with

saline.

The test specimens were loaded as beams in symmetrical

three-point

someter

modified

bending using a Monsanto to permit

electronic

imposed loads and deformations. formed such that the internal

Fig. 2. (a) Diagram of a test sample showing width (w) and thickness (h) measurements. (b) Diagram of the technique used to load the test samples in symmetrical three-point bending and impact. In both cases ths force was directed onto the external surLce of the suture or bone (ex). The span length (x) was 19.42 mm during three-point bending and approximately 25 mm during impact loading. The diameter of the force application platen during bending was I.59 mm with a radius OF curvature of 3.17 mm. On average, the span of collagen between the two bones was about 0.2 mm.

bone surface was sup-

suture surface to roughly

ditions that might accompany

of

The tests were per-

ported, and the force was applied at mid-span external

ten-

recording

b

simulate

on the

the con-

an external load on the

acceptable.

Bending tests of the frontoparietal

skull and sutures (Fig. 2). The total span length was

were performed

19.42 mm, therefore

displacement

thickness

the ratio

of span length

to the

(= depth) of the samples ranged from about

s-r

‘fast’ displacement

(henceforth

regarded as ‘slow’ and

rates, respectively). The internasal

4: I to over 9: 1. Sample beams were loaded until the

sutures were only loaded at 9.7 mm s-

recorded

To

force returned

to zero (the starting

level);

standardize

failure was defined as the peak force applied. Only test

strength

specimens that broke

culated

at the suture were considered

(peak

sutures

at crosshead speeds of 0.8 and 9.7 mm

for

size

differences,

’ displacement. the

bending

stress) of each test sample was calelastic beam theory according to

Xlechanlcul

[equation

315

properues of cramal sutures

(I b]: MC um*.=I

(1)

absorption

before failure.

supported.

unfixed, at either end while a pendulum

The

test specimens were

head swung through the unsupported

middle, striking

the external surface of the suture as in the three-point bending tests (Fig. 2). The

where

pendulum

apparatus

was

calibrated for energy losses of 14%. caused mainly by

M=Fd

maximum

/=(Hh’);l2

second moment

bending moment

(Nm)

friction

of area (m”)

at the axis of rotation

potentiometer

when a single-turn

was attached to the axis to record the

c

l/2 mean sample thickness (ml

angular displacement of the pendulum.

a mar

bending strength (Pa)

the height of the pendulum

By measuring

before and after loading

F

peak force (N)

the sample. the amount of energy absorbed by the test

d

l/2 span length (ml

specimen was calculated as:

\1

sample width (m)

h

mean sample thickness (m).

(I) overestimates

Equation

the absolute

strength of bone by as much as 50-100% plastic yielding

E=[tflK(i--j)]-f., bending

1970; Burstein cat(II., 1972). However,

(Currey,

this was not a

problem here since values of bone and sutural strength were compared

relative to one another.

values of absolute culated according

bending strength,

In contrast to the values cal-

(I) correspond to what

to equation

Currcy calls ‘the modulus of rupture’

(1970, p. 222).

Least squares regressions (LSR) were used to dctcrmint

if

the strength

of the

where

because of

in the bone before failure

Irontoparietal

samples varied with the dcgroc of sutural

sulurc

intcrdigit-

energy absorbed (J)

E m

mass of the pendulum (kg)

R

gravitational initial

L

Values of energy absorption

were divided by the cross-

sectional area of the beam (mean thickness x width) to normalize for size differences among the test samples. Cross-sectional

gun content and orientation

same for

in the suture may occur

height (m) height (m)

energy loss (J).

frictional

ation

regressions were performed initially

acceleration (9.81 m sd2)

pendulum

final pendulum

;

ation for each bending rate. Because changes in collawith age, multi@

(2)

arca was sullicicnt

bccausc the third all

for this standardiz-

dimension

the test spccimcns.

bctwocn energy

absorption

(Icngth) was the The

relationship

and degree of sutural

to assess the potential elTccts on sutural strength of the

interdigitation

goat’s age alone or with the changes in interdigitation.

regression.

The age of each goat was estimated

using the tooth-

using age and sutural

wear criteria

(19X2). A more

test for the potential e!Tccts of age on energy absorp-

of Deniz

and

Payne

general test of the elTect of age on sutural obtained

by regressing

age against

strength was the values

of

tion.

was dctormincd

using

Again, a preliminary

Only

samples

intcrdigitation that

least-squares

multiple

broke

regression

was executed to

at the suture

were

included in these analyses.

strength for the internasal sutures, which lacked interdigitation.

These

test

animals ranging from

samples

were

taken

from

RESUI.TS

I to 9 years of age. a range that

Test results of 88 cranial samples from 33 goats were

covers most of the lifespan of a domestic goat.

used in the different approximately Samples of cranial

bone or frontoparietal

suture

tested (Table

65%

regressions;

these represented

of the total number

I). Age had no significant

of samples

etTect on the

were subjected to impact loading using a pendulum

bending strength

impact

sutures across the age range used in this study. The

testing

apparatus

to measure

their

energy

of the cranial

samples containing

T;~blc I. Descriptive summary ol Ihc numhcr (,I) and percentage (%) of goal cranial suture samples that failed succcss~ully in the ditTerent mechanical

” (%I used Goals Range

tesls

(1) Fast bending strength (FP suture)

(2) Slow bending strength (FP sulurc)

(3) Fast bending strength (IN sulure)

(4) Energy absorption (FP suture)

22 (63%) IO IA

21 (70”/,) 6 3-I

27 (68%) II 2-4

18 (60%) 9 l-4

‘Success’ was defined as failure across the SUIUTC. The tests include: (I) fast bending strcnglh and (2) slow bending strength of the fronlopariclal suture (FP). (3) fast bending strength ol the internasal sulurc (IN), and (4) energy absorption of the frontoparictal sulurc. For each test. additional values show the number ol ditTcrent goat skulls from which the successful test specimens were obtained (goats), and the range of successlid lest specimens used per goat skull (range).

316

C. R. 40.

a 3

JASLOW

Table 2. Multiple regression significance tests of age and interdigitation as predictor variables for rapid bending strength(R). slow bending strength(S), and energy absorption of frontoparietal sutures

30.

.

Regression coefficient

: 20. P ::

.

Bending strength (R)

z

Interdigitation Age

.o

‘O-

I

0

10

4

AGE (yearm)

Bending strength (S) Interdigitation Age

Fig. 3. Mean bending strengfh of internasal sutures for samples of diRerent ages. Lines indicate _+ I SE. R =0.288. p z= 0.05. Slope =0.9. intercept = 17.5 (n = 27).

regression of age against (which

lack

strength

8.23 0.56

2.37’ 0.09

33.69

4.26t

12.90

1.34

1.01 1.76

2.62’ 1.64

Energy absorption Interdigitation Age

lp