DIURNAL

CHANGES

AND

THEIR

P. DOLAN,

From

Diurnal

changes

We

have

in the loads

reviewed

the

SPINAL

CLINICAL

M. A. ADAMS,

discs.

IN

acting

effects

SIGNIFICANCE

W. C. HUTTON,

the University

on the spine of

these

MECHANICS

affect

changes

R. W. PORTER

of Bristol

the water on

spinal

content mechanics,

and height and

of the intervertebral their

possible

clinical

significance. Cadaveric lumbar spines subjected to periods of creep loading show a disc height change similar to the physiological change. As a result intervertebral discs bulge more, become stiffer in compression and more flexible in bending. Disc tissue becomes more elastic as its water content falls, and its affinity for water Disc prolapse becomes more difficult. The neural of the compressive and bending sfresses acting

increases. proportion

that these changes We conclude

are not fully compensated for by modified muscle activity. that different spinal structures are more heavily loaded

the time of onset of symptoms more about the pathophysiology

Therefore, understand

and signs, and any diurnal variation oflow back pain and sciatica.

During the recumbency of sleep, the loading on the intervertebral discs is reduced, and their relatively unopposed swelling pressure results in them absorbing fluid and increasing in volume (Urban and McMullin 1988). The absorbed fluid is expelled during the day when the loading of the spine is increased. There is, thus, a diurnal variation in the fluid content and height of the discs

which

properties We these

causes

different We then

a

variation

in

of the spine. review the experimental

changes,

and

spinal suggest

then

the changes

at different time of onset

variation

M. A. Adams, PhD, Research Fellow P. Dolan, PhD, Research Fellow Comparative Orthopaedic Research Row, Bristol BS1 SLS, England. W. C. Hutton, DSc, Professor Department of Orthopaedics, USA.

©

should

be sent

in their

in their

CADAVERIC

times severity,

of the may

day.

help

us

SPINES

Periods of creep loading of cadaveric lumbar spines cause a change in disc height similar to the diurnal change seen in vivo. Certain mechanical properties have been measured before and after the period of loading. Disc height. Constant loading at 1 000 N for six hours (simulating light manual labour: Nachemson 1981) causes disc height to decrease by 1 .53 ± 0.34 mm (Adams, Dolan and Hutton 1987). The height loss is rapid at first but much slower by the end of the six hours

concerning in loading

severity,

at different

of day. and

might

(Fig. 1). Similar results have been reported experiments (Adams and Hutton 1983 ; Koeller, and Hartmann 1984). Ifthe applied compressive increased and then

at hourly intervals to 3 000 N in order

in

other Funke force is

from 1 000 N to 2 000 N to simulate manual labour

of increasing severity, then the height loss shows no sign of slowing down, and the cumulative loss after three hours is 2.13 ± 0.35 mm (Adams et al 1987, see Fig. 1). Unit,

Emory

to Dr M.

1990 British Editorial Society ofBone 0301-620X/90/2027 $2.00 J Bone Joint Surg [Br] 1990 ; 72-B :266-70.

266

mechanical

times of the of symptoms

University

University,

R. W. Porter, MD, FRCS, Consultant Orthopaedic Doncaster Royal Infirmary, Armthorpe Road, Yorkshire DN2 5LT, England. Correspondence

the

evidence

discuss

structures that the

signs, and any diurnal be an aid to diagnosis.

arch and associated ligaments resist an increasing on the spine. Observations on living people show

of Bristol,

Atlanta, Surgeon Doncaster,

A. Adams. and

Joint

Surgery

Park

Georgia,

South

about mostly 1935). change

The average diurnal variation in human stature is 19 mm (Tyrrell, Reilly and Troup 1985) which is attributable to changes in disc height (De Puky A 19 mm change in stature corresponds to a ofabout 1 .5 mm in the height ofeach lumbar disc

(Adams et a! 1987), so the loading regimes above are sufficient to simulate physiological reduction in disc height. Changes in disc height are caused by fluid and creep deformation of the annulus fibrosus

THE

JOURNAL

OF BONE

AND

JOINT

discussed diurnal exchange (Koeller

SURGERY

DIURNAL

et a! 1984). probably loading

The

relative

CHANGES

importance

IN SPINAL

of each

depends upon the severity (Adams et a! 1987) and factors

the degree of disc The diurnal similar magnitude

MECHANICS

mechanism

and duration such as age

degeneration. disc height change of to the normal narrowing

1.5 mm ofthe

foramen averages and Goel 1983).

only

about

15 to 20 mm

THEIR

Fluid of and

is of a lumbar

discs expected with age (Koeller et a! 1986). It could have a significant effect when there is pathology in the nerve root canal since the total height of the lumbar intervertebral Takata

AND

CLINICAL

loss

dissipation

267

SIGNIFICANCE

is accompanied

during

by a reduction

a loading/unloading

in energy

cycle

(Koeller

et

a! 1984). This means that the dehydrated disc behaves more like an elastic solid and less like a viscous fluid. Disc swelling pressure. Disc swelling pressure can be defined

as that

physical

pressure

which

must

be applied

to the disc in order to prevent it from swelling up in saline (Urban and McMullin 1988). It is a measure of the tissue’s affinity for water. Swelling pressure can be measured by adjusting the

(Panjabi,

compressive

force

acting

on a motion

segment

until

there

Disc water content. Creep loading reduces the water content of the discs. After four hours loading at about 700 N, when disc height is reduced by about 1.5 mm, the average fluid loss is 12% from the annulus and 5% from the nucleus (Adams and Hutton 1983). Discs from people under the age of 35 years lose almost twice this amount.

is no detectable change in disc height. This force is then divided by the cross-sectional area of the disc. Swelling pressure increases rapidly during creep loading as shown in Figure 1, and its rise can be accelerated by more intense loading (unpublished results from our laboratory, 1988). The clinical significance of swelling pressure is

Most hours

that it determines the height (and mechanical of high loading.

of the fluid loss probably occurs of loading because longer-term

in the first few creep tests cause

only a little more fluid loss : 24 hours loading at about 1 000 N reduces the fluid content of annulus and nucleus by 1 1% and Gowin 1985).

8%

respectively

(Kraemer,

Kolditz

Compressive stiffness. Creep loading increases the disc’s compressive stiffness. The increase is about 50% after two to three hours ofphysiological cyclic loading (Koeller et al 1984) and can rise to 100% after 28 hours (Smeathers 1984). Motion segment stiffness is ofclinical significance because it determines how much the disc and surrounding soft tissues deform during physiological dynamic loading

and

/

2

,

Height loss

ofthe spine. Disc bulging.

(mm)

100

it may

//

ofthe

disc

has been

observed

after creep loading (Koeller et a! 1984). increase has not been measured directly,

be inferred

from

the

This suggests that the diurnal 1 .5 mm should be accompanied

C 140

results

The but

of Brinckmann

and

reduction by an

in disc height of increased radial

bulge of about 0.5 mm. For comparison, the increased radial bulge caused by increasing the compressive force on the spine from 300 N (lying in bed) to 1 000 N (light manual work), is only about 0.2 mm (Brinckmann and Horst 1985).

Compressive stiffness

(%) 1 fl(l

Diurnal disc bulging will have clinical implications when the central or root canal is stenotic. The width of the intervertebral foramen is normally about 8 to 10 mm (Panjabi et a! 1983) but it can be much less in the root

100 Resistance to flexion

(%) 2

L.J

3

Hours Fig.

of creep

No. 2, MARCH

1990

4

5

6

loading

I

The effect ofcompressive creep loading on of lumbar motion segments. The solid compressive force of 1 000 N applied for refer to a compressive force of 1 000 N in second, and 3 000 N in the third. Resistance the physiological limit determined before pressure’ of an intervertebral disc is defined

72-B,

bulging

Horst (1985). They altered the volume of the disc, either by injecting fluid into it or by fracturing the vertebral body end-plate, and found that the change in radial bulging was about one-third of the change in disc height.

/ /

(N/cm2)

VOL.

Radial

to increase size of this

C

Swelling pressure

rate at which a disc recovers lost properties) at the end of a period

some mechanical properties lines refer to a constant six hours. The broken lines the first hour, 2 000 N in the to flexion was measured at creep loading. The ‘swelling in the text.

canal and the lateral recess. Loading of the apophysial joints.

The

on the apophysia!joints segments were loaded

measured a typical

(lumbar posture

spine (spine

slightly in slight

apophysialjoints either there simulated

has been to simulate

resist

posture is little

(Adams resistance

standing

flexed) and extension). little,

if any,

compressive when sitting

an erect standing Before creep, the compressive

force

and Hutton 1980). After in the flexed spine, but posture

the

force motion posture

apophysial

joints

in

creep, in the resist

268

M. A. ADAMS,

an average some cases, more result

of 16% of the the proportion

extended in high

of the Hutton Forward

applied can

compressive be as high

postures, compressive stress concentrations

apophysial 1984) and bending

force. as 70%.

In In

performed stresses

Adams 1984). increases

separately,

and a

and

in the early morning in the osteoligamentous

generate lumbar

performed proportion

much higher spine than do

later in the day. ofthese increased

45%

Backward bending properties. Creep loading reduces the disc’s resistance to backward bending by about 40% (Adams, Dolan and Hutton 1988). This is balanced by increased resistance from the apophysial joints and spinous processes, so that the resistance to backward bending

of the

movement, Prolapsed be induced combined

whole

motion

are unaltered intervertebral to prolapse bending, shear

segment,

and

Table I. Diurnal the lumbar spine

variation

Intervertebral disc Posterior longitudinal ligament Vertebral body end-plate Segmental nerve root

Apophysial joint Articular surface Capsule and ligaments Supra/interspinous

of maximal

stress

Period

Comment

AM AM AM AM PM

Especially Especially

in bending in flexion

Increased Increased

tension compression

PM AM PM AM PM

ligaments

on

the

Flexion Extension Increased Increased extension

various

diurnal

changes

structures

in

in spinal

mechanics

segments prolapse repeated cadavers

reduce content lowered

fluid

content

is artificially (by injecting

can

probably

of the

nucleus

raised (by chymopapain)

(see

Table

I).

loaded in this way, 26 failed by posterior (Adams and Hutton 1982). The experiment on a further group of 19 motion segments of a similar age range after they had been

loaded (Adams prolapsed, and

et they

a! 1987). were both

Only from

two of the same

disc was from creep

these spine.

Creep-loaded discs, in vivo, may also susceptible to prolapse, perhaps because of the fluid content of the nucleus pulposus and the flexion stresses in the posterior annulus.

discs

be less reduced reduced

tension in flexion compression in

CLINICAL posture

the muscles effects

of

2

Spinal These

range

PM

Fig. showing

the

by creep loading. disc. Some cadaveric discs can posteriorly by loading them in and compression. Of61 motion

AM

Diagrams

Also, the stresses

in the morning.

before creep to bending is for the disc

is 85%

R. W. PORTER

similar movements discs resist a higher

by 2#{176} or 3#{176} (Adams et to about 12.5#{176}extra lumbar spine. At the

physiological limit of flexion (as determined loading) the motion segment’s resistance reduced by about 70% (Fig. 1). The reduction measured

W. C. HUTFON,

creep loading can on the lower margins

joint surfaces (Dunlop, capsule (Yang and King properties. Creep loading

motion segment’s range of flexion al 1987) which is equivalent movement for the whole of the

and ligaments, respectively.

P. DOLAN,

be

attributed

pulposus. injecting then

to

the

posture

If the

fluid

saline) there

or is a

changes results

corresponding increase or decrease in the disc’s resistance to bending (Andersson and Schultz 1979 ; Dolan, Adams and Hutton 1987). These results indicate that, inlife, flexion movements

DIURNAL

and of the

in order

mobifity. back

VARIATION It is possible

and

abdomen

to compensate

in the underlying from our laboratory

for

spine. show

may some

that, modify of the

in vivo, spinal diurnal

However, unpublished that the lumbar lordosis

increases by about 3#{176} during the day. This would increase the loading of the apophysial joints and compound the effects due to loss of disc height. It could be thought that the higher bending stiffness ofthe osteoligamentous spine in the early morning would

THE JOURNAL

OF BONE

AND

JOINT

SURGERY

DIURNAL

be offset bending,

CHANGES

IN SPINAL

MECHANICS

by the trunk muscles restricting the range of so that the bending stresses on the spine remain

AND

THEIR

We anics

CLINICAL

suggest

SIGNIFICANCE

that

are ofclinical

diurnal

269

variations

significance.

in spinal

Since

different

mech-

structures

the same. However, the experimental evidence suggests that this does not happen to any significant extent. In vivo, the range of lumbar flexion is reduced by only 5#{176} in

are more heavily loaded at different times of the day, the time of onset of a patient’s symptoms, and any diurnal changes in their degree of severity, might help us to

the early morning (Adams et a! 1987) whereas the range of flexion of the underlying spine is reduced by about 12.5#{176}before creep loading (see above). Calculations comparing the in vivo and in vitro evidence suggest that, in life, bending stresses on the lumbar discs and ligaments

understand

can be increased the early morning The movement

by about (Adams

slight diurnal in vivo may

‘warm-up’

(Baxter

observed

in

hip

300% and 80% et a! 1987). variation be partly

1987). movements

though there is unlikely mechanical properties of normnal articular like the disc.

cartilage

has

(Adams

et a!

1987)

to be any variation the underlying joints, swell

up

been even in

the since

overnight

spinal

injury

little published information symptoms and signs, nor in pain. Varma (1987) recorded of back pain occurred in the

day. This who found more

agrees with that mine

commonly

pathophysiology

of different

back

pain

I lists some of the structures thought to be for low back pain and sciatica, and indicates are most heavily loaded. The disc resists all of

the compressive force on the spine in the morning, and in addition, is much more highly stressed during flexion and extension movements. A herniated disc, however, may behave The posterior the morning

a study workers

differently from one with an intact annulus. longitudinal ligament is stretched more in because of the increased height of the disc,

although reduced radial bulging ofthe disc will counteract this effect to some extent. Vertebral body end-plates can be ruptured by increasing the fluid content of the nucleus

pulposus

of the working et a! (1980)

sustained

effect

not

in

of spinal to muscle

similar

does

Low back pain. There is about diurnal variations in the time ofonset oflow back that 47% of ‘first episodes’ early part by Evans

A

Table responsible when they

respectively

in the range attributable

the

syndromes.

(Jayson,

Herbert

The morning

segmental because

they are compressed bulging of the

intervertebral

in the morning.

and

Barks

1973)

that the end-plates are more highly morning when the discs are swollen with

so it is likely

stressed fluid.

nerve roots are stretched the spine is about 19 mm more disc,

in the afternoon the reduced

by

foramen,

and

by

more in the longer; but

by extra radial height of the

the

buckling

this timejoint the capsular

variation

nous ligaments will all be most stretched during forward bending movements performed in the early morning because the increased disc height allows them less slack.

injuries.

DISCUSSION The

experimental

evidence

can

be

summarised

as

follows : with creep loading, the intervertebral discs lose height, bulge more, become stiffer in compression and more flexible in bending. Disc tissue becomes more elastic as its water content is reduced, and its affinity for water increases. Disc prolapse becomes less likely. The neural arch proportion acting Figure

and associated ligaments resist an increasing of the compressive and bending stresses

on the spine. These results are summarised 2. In life, these changes will occur mostly in the

few hours of the

ofthe

changes

day, will

but the time depend

upon

scale the

and

throughout the spinal mechanics

changes. VOL. 72-B.

No. 2, MARCH

1990

day

spinal

in at

pain may be expected to increase. However, ligaments and the supraspinous and interspi-

we

attempt clinical

symptoms

to understand studies into the

and

signs

could

more diurnal

about spinal changes of

be rewarding.

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minor diurnal

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apophysial Bone

of loading

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are

especially bending, and

This work was supported by the Arthritis and Rheumatism Council, Action Research for the Crippled Child and the Back Pain Association. No benefits in any form have been received or will be received from a commercial party related directly or indirectly to the subject of this article.

the magnitude

severity

in the afternoon, during backward

surfaces

in

on the spine : heavy labour will have a greater effect, and in less time, than sedentary activity. The swelling pressure results suggest that the effects of intense loading will be reversed more rapidly than the effects of less intense activity of longer duration. Alternating periods of rest and activity changes in

As pathology,

joint

the

trends due to changes in spinal mechanics and they must be taken into consideration in any surveys of diurnal

closer together postures and

apophysial

of

ligamentum pressed lordotic

The

the

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flavum.

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OF BONE

AND

JOINT

for as

SURGERY

a