MECHANICAL
FACTORS
OF
INFLUENCING
SCREWS
IN
HANS
K.
UHTHOFF,
From
the
Institut
COMPACT
VERDUN,
de
HOLDING
THE
BONE
QUEBEC,
Recherches
POWER
CANADA
en Orthopedic,
Verdiiz
Screws
are widely
used for the internal
fixation
of fractures,
either
alone
or with plates.
Constant
improvement
in design
and in operative
techniques
has led to increased
holding
power,
which
now exceeds
the shear strength
of bone.
In fact, if one tries to pull out a freshly
inserted
screw,
button
of compact
If one considers
around
would
the
have
bone
the
bone
threads
screws
would
no practical
healing
do
not
strip
bone (Bechtol
1971).
only the result of such
certainly
seem to be
importance.
is a matter
but
the
screw
an immediate
pulls
test,
out
a small
the question
cone-shaped
of bone
formation
of no interest,
as a further
increase
in holding
power
In cases
of accidental
stripping
of screws,
however,
of considerable
importance,
as shown
by
Bechtol
In fact,
exceeded
he found
that during
the first four weeks
after
insertion
of a screw the holding
the initial
stripping
torque,
though
it should
be carefully
noted
that these
in place, were not subjected
to any mechanical
forces.
Though
the enormous
holding
power
of screws
in compact
bone generally
persists
once
the course
offracture
avulsed
from
bone.
healing,
In such
careful
examination
radiological
occurrence
first
clinical
cases
reveals
of this complication.
weeks
after osteosynthesis,
ten
responsible
later,
some
experience
shows that a plate and
no bone
can be detected
around
bone
resorption
around
for this loss of holding
time after
consolidation
power
screws,
during
its screws are sometimes
the screws.
Moreover,
the
If loosening
of the screws
occurs
it will cause
failure
of fixation.
(1959).
threads
early,
that
Mechanical
before
the
is, during
factors
the
are
power
(Wagner
1963).
If loosening
of the screws
of the fracture,
it is due to corrosion
of the
happens
implants
(Wagner).
Few experimental
studies
Wagner
as well as Hagmann
have
(1966)
been done to elucidate
the
have described
the reaction
types of screw.
A thorough
search
of the literature
has not
correlation
between
mechanical
factors
and cell differentiation
fixation
of fractures
or osteotomies.
Nor has any study been
about
the
An
contact
between
the
contact
screw
and
with
the
surrounding
study
was therefore
threads
and compact
bone
by measuring
the
screws,
plates
plates
Lavigne
led to stable
was unstable
osteosynthesis
and permitted
bone
soon
the
drills
and
the
Contact
Therefore
to preserve
taken
VOL.
55 B,
after
their
taps;
and
fixation
insertion.
extent
of the
of contact
3) to observe
with
so-called
1971), whereas
the montage
the bone fragments
(Uhthoff
1971).
between
screw
and bone-Spaces
if a space
exists
between
screw
was
Dubuc
between
of screws.
to different
any studies
of a possible
screws
after the internal
in the current
literature
in order
1) to determine
the
2) to explain
any incompleteness
(Uhthoffand
movement
MATERIALS
order
compact
devised
bone;
revealed
around
published
cell differentiation
around
screws
subjected
to mechanical
forces.
In earlier
experimental
studies
in the dog we had noted
that internal
compression
with Eggers
and
of screws
experimental
between
screw
process
of loosening
ofcompact
bone
out
NO.
this
sideways
3, AUGUST
delicate
after
1973
tissue,
the
which
lateral
AND
METHODS
in bone
are invaded
and bone,
it should
is torn
bone
wall
during
the
had
been
by loose
be invaded
routine
removed
removal
with
connective
tissue.
by soft tissue.
In
of a screw,
a rongeur.
this
This
633
634
H.
technique
was
and the animals
Measurements
manufacturers
at different
minor
used
in four
adult
K.
mongrel
UHTHOFF
dogs.
were killed
at one, two, three
of screws,
drills
and taps-The
were examined.
The
levels,
two measurements
diameter
was
also
external
being
determined
Three
screws
and four
products
were
American
or major
diameter
made
at right angles
and
corresponding
drill bit.
External
and core
Mechanical
factors
and cell differentiation.
internal
fixation
of forty femoral
osteotomies
compared
diameters
with
Unilateral
external
were
stable
also
internal
adult
I I and
so-called
plates
00
00
::::
;
#{176}
0
0
O
oo
0
0
0
which
of the
0
N T
o; :
0000
00
I
weeks.
:: :
ooo
0
#{176}
.
)
:
:
I
B
0
::
o
-
OO
:
:
:
:
:
:
:
c
:.
D
,______
the
:
:
:
:
,
-..‘
Unilateral
unstable
any
plates,
the
adhesive
The
and
nine
examinations.
supplemented
tape for four weeks,
when
interval
between
osteotomy
months.
The
All
bones
by
specimens
were
*
the
decalcified
Richards
in EDTA
Manufacturing,
protected
few days.
after
fixation
that
on
upon
legs
the dogs
limb
the
both
and
internal
femora
.
.
All
.
(group
Fixation
plates
as
dogs
were
killed
4)-I
n order
to
screws
in osteotomies
held by
held in a flexed
position
with
2 varied
histological
Memphis,
hindlegs
on three
time in three dogs.
same compression
1 and
radiological,
(group
noted
had
the
on
for group
1
four weeks.
the
was
at a time
To force
immediately
immobilisation
around
upon
side
fi.vation
by walking
.
dogs were killed.
and death
in groups
underwent
used.
fixation
2 we
dogs
at the same
was
by the
external
site and
operated
internal
I and
thus
put
more
stress
fixation
we operated
used
after
at the fracture
dogs
the hindleg
one
upon
first
walk
to
only
stable
operated
I
fixation
was
internal
upon.
groups
that
diameter.
movement
in three
the
of
the hindlegs
at the same
at least
two
splintage
Again,
Bilateral
A diagram
showing
how contact
between
screw and bone (B)
is limited
to a part ofthe
horizontal
thread
surface
(H). The
oblique
surface
(0) is separated
by a space shown as a dark
strip.
When
a tap has not been
used
(NT), the crest of the
thread
(C) touches
the bone,
but does not do so when
a
full-sized
tap
has
been
used
(T).
CD=core
diameter;
decrease
Eggers
was
well to the round
surface
of
and the fixation
is therefore
operated
for the
EDextemal
bone
the
In
compression
unstable
3)-In
E D
FIG
of
firm
increases
fixation.
greatly
internal
external
unstable.
was
______
4
No
adapt
bone
flot
0:000,,
:
the
(group
2)-The
fragments
of thirteen
osteotomies
were
held
with
Eggers
plates
S centimetres
in length. * Because
the under-surface
is fiat, these plates
do
) 0
d
the
Unilateral
o:
o.ooooooooc,oooooo
:o
-
time,
of
to give
In this group
operated
upon
interval
being
never
0
Swiss
under-surface
dogs
screws.
#{176}
two
was measured
The core or
and in twenty
osteotomies
were tapped
before
insertion
were
::
0
the
0
0
#{176} #{176}
::
-
H
0
0
#{176}
OO
“-
applied
holes
‘
:
)
o
#{176}
,,
The
‘K
twenty-nine
.
and
(group
1)-The
weighing
between
was achieved
with
plates
5 centi-
is designed
contact,
stability
T
femur
diameter
fixation
long.
these
each
recorded.
mongrel
dogs
19 kilograms
compression
metres
into
of the screws
to each other.
the
of taps
in twenty
inserted
weeks.
of two
between
and
in 10 per cent
five
days
histochemical
neutral
formalin.
Tennessee.
THE
JOURNAL
OF
BONE
AND
JOINT
SURGERY
MECHANICAL
FACTORS
INFLUENCING
THE
HOLDING
POWER
OF
IN
SCREWS
COMPACT
635
BONE
RESULTS
Contact
between
between
screw
intimate
surface
contact.
between
up to 150
even touch
screw
threads
ji in
the
2-A
fixation
bone-Our
and
bone.
histological
Indeed,
thickness
bone.
FIG. 2
of a screw
operation,
Measurements
bone
revealed
of the
a limited
horizontal
area
thread
of contact
surface
facing
toward
the tip of the screw,
and
separated
from the bone by interposed
In no instance
did bone come
cells with a spindle-like
nucleus
to
part
was
in
the vertical
soft tissue
(Fig.
1). When
a tap had been used,
the crest of the thread
did not
All spaces
were larger
on the side away
from compression
when
this
radiograph
leads
studies
only
Both the oblique
surface,
neighbouring
threads
were
had been applied.
layer of flattened
Figure
and
of screws,
how
drills
contact
with
interposed.
the
FIG.
formation
showing
into direct
was always
hole
taken
around
a
unstable
and
nine
months
screw.
internal
taps-The
after
Figure
fixation
leads
external
operation,
3-A
showing
radiograph
to bone
diameter
a single
3
how
taken
resorption
metal;
around
of different
five
stable
internal
months
after
a screw.
screws
from
each
of the four manufacturers
varied
within
a range
of 180 Ii. The screws
made
by one company
showed
an inconstant
external
diameter,
this measurement
varying
within
a range
of 70 ji for
a given
screw.
The screws
of two other
companies
were not exactly
round,
the range
of the
difference
being
30 .t.
The external
diameter
of the drill bits of every
manufacturer
was
consistently
larger
than the core diameter
of the screws,
the difference
being
in the range
of
t.
The external
diameter
of the taps from
each
manufacturer
than the mean external
diameter
of the screws,
the difference
being
Mechanical
factors
and cell differentiation.
Unilateral
stable
internal
forty
osteotomies
consolidated
without
showing
any radiological
200
loosening
of screws
compression
were seen
and those
not.
Migrating
cells
close to the periosteal
and endosteal
VOL.
55B,
No.
3,
AUGUST
(Fig.
2).
1973
No
difference
was
noted
between
the
was
in the
consistently
larger
range
of 50 j.t.
fixation
(group
1 )-All
or histological
sign of
osteotomies
subjected
to
in the spaces
between
the screws
and the bone
surfaces
as early as one week after operation.
636
H.
At this
time
connective
no cells
tissue
After
four
weeks
4).
Little
bone
(Fig.
were
filled
found
these
in the
spaces,
dense
deep
in some
filled
or
UHTHOFF
spaces
and
callus
resorption
K.
the
within
cases
spaces
bone
the
cortex.
osteoid
between
remodelling
After
was
the
could
compact
be
two
weeks
loose
the
screws
seen.
bone
observed
and
in
the
pre-existing
cortex
(Fig. 5). During
the next four weeks
the callus underwent
conversion
to Haversian
At eight weeks
bone
with concentric
lamellae
around
a central
canal
was observed
right
angle
to the long axis of the shaft
(Fig.
6).
During
the subsequent
months
remodelling
nine
in the
pre-existing
cortex
took
place.
Some
necrotic
bone
still
bone.
at a
bone
persisted
after
months.
4
FIG.
Figure
4-Histological
the crest
and
over
the
(Toluidine
blue,
45.)
fixation,
showing
new
.
remodelling.
Routine
FIG.
removal
of a screw
between
In the
necrosis
medullary
around
Unilateral
screw
canal
the screws
unstable
Of the other
ofthe
screws
the
was
j.t.
No
weeks
loose connective
activity
was noted;
the
level
horizontal
Collagen
distance
of the
four
fibres
between
two
weeks,
were
(group
After
two
osteoblastic
weeks
previously
thread
however,
also appeared
screws
and
surrounded
or
thirteen
no
no important
increased.
less
resorption
activity
callus
always
The
influence
time
was
was
elsewhere
depth
only
the
bone
(Fig.
dogs
of this
one
of
bone
necrosis.
consolidated.
and in four loosening
3). Three
osteotomies
were
killed.
between
the screws
and
seen in the bone surfaces.
bone.
At
observed.
present
formation
THE
least
the extent
the
spaces
were
present
bone
at
surface.
internal
or bone
25.)
osteotomies,
at the
filled the
osteoclasts
#{149}
of cells
eosin,
by callus.
did not
2)-Of
tissue
instead,
surface
layer
and
become
avulsed
from
as well as histologically
osteoclastic
but
bone
the single
(Haematoxylin
Compression
in five the plates
had
evident
radiologically
been
away
bone.
threads
200
had
After
performed
takes
and
internalfixation
twelve,
became
5
section
made
four
weeks
after
stable
internal
fixation,
showing
no new
horizontal
thread
surface,
but callus
formation
(C) next the oblique
Figure
5-Another
histological
section
made
four
weeks
after
stable
bone
formation
on necrotic
cortex
without
previous
bone
resorption
also
could
JOURNAL
in this
area
be detected.
OF
BONE
AND
JOINT
(Fig.
7).
Thus
the
SURGERY
MECHANICAL
FACTORS
INFLUENCING
THE
HOLDING
POWER
OF
SCREWS
After
twelve
weeks
the screws
layers of which were less cellular
than
in the area.
In some specimens
the
were
embedded
in dense
the deeper
layers (Fig. 8).
presence
of fibrocartilaginous
Furthermore,
the
in the
medullary
canal
FIG.
Figure
6-Histological
seen
at right
angles
nitrate,
#{149}
45.)
Figure
bone
resorption
threads
were
and
surrounded
FIG.
;:
tissue formation
(Haematoxylin
*T
637
tissue.
7
Four
Haversian
is incomplete.
internal
fixation.
over an oblique
45.)
surface.
systems
can be
(Holmes
silver
Osteoclastic
No callus is seen.
,
:
f:
are evident
and eosin,
BONE
by collagenous
6
fibrous
‘:
COMPACT
fibrous
tissue,
the superficial
Osteoclastic
activity
continued
tissue
was noted
(Fig. 9).
section
made
eight
weeks
after
stable
internal
fixation.
to the long
axis
of the shaft.
Intracortical
remodelling
7-Histological
section
made
four
weeks
after
unstable
(arrows)
IN
.
-.
‘
1
,.
.
.
‘It.
-
,
.r..
#{149}
;..#{149}.
,;
- #{149}
...
.
1.\
FIG.
8
FIG.
9
Figure
8-Histological
section
made
eight
months
after
unstable
internal
fixation.
The diaphysis
(D) is being
remodelled.
The threads
of the screws
(5) are completely
surrounded
by collagenous
tissue
(C). (Haematoxylin
and eosin,
: 16.) Figure
9-Histological
section
made twelve weeks after unstable
internal
fixation
showing
the formation
of cartilage
around
the screw
threads.
(Toluidine
blue.
‘
50.)
Bilateral
stable
internal
showed
some
bone
showed
less callus
osteoclastic
activity
VOL.
55 B,
NO.
3,
fixation
(group
3)-Radiographs
of the
femora
of these
three
resorption
around
the screws
after
four
weeks.
Histological
than
in dogs
with
unilateral
stable
internal
fixation.
Moreover,
and the presence
of fibrous
tissue were evident.
AUGUST
1973
dogs
sections
some
638
H.
Unilateral
unstable
resorption
Histological
was seen
sections
internal
fixation
around
did not
K.
UHTHOFF
supplemented
the screws
reveal
the
delicate
bone trabeculae
intermingled
less than in the dogs with unilateral
by
external
splintage
on radiographs
taken
presence
of osteoclasts;
with
stable
fibrous
internal
tissue.
The
fixation.
(group
four
the
4)-No
weeks
spaces
amount
after
were
of callus
bone
operation.
filled with
was
definitely
DISCUSSION
Immediately
after
the surrounding
with
orientated
the
the
towards
its insertion,
bone.
Only
the
head
a screw
at the
of the
screw,
of the type being studied
level of the horizontal
do the
threads
horizontal
plate.
thread
shape
used
surface
ln doing
is lifted
away
so, this
from
; indeed
it.
some
permit
The
on the one
are
the
microscopic
against
at
the
of uniform
partly
the bone
other
the presence
screw
cells
differentiate
into
confirms
the theory
of
absence
of any mechanical
on forces
acting
directly
not
considered
On the
cartilaginous
subjected
Dogs
the
Moveover,
core diameter
of
contact
between
the
screws
oblique
is increased
of spaces
If undue
stress
move
in its bed.
can
between
screw
between
is put
threads
osteogenic
cells
and
screw
on
and
the
bone
and
plated
screws
are
produce
a solid
vary
an oversize
bone
or
invaded
with
limited
if the
callus
is
bone.
area
can
internal
by migrating
of these
of stable
in
tap
on the other
bone,
differentiation
in all cases
to the
under-surface
when
When
contact
which
is
cells.
cells growing
in
internal
fixation
in four
weeks.
This
Pauwels
(1960)
that direct
bone
formation
takes
place
only in the
forces
acting
directly
on the cells.
Because
our interest
is centred
on cells, the effect of mechanical
forces
acting
on bone,
as a tissue,
here.
other
tissue
hand,
and
unstable
to bone
internal
resorption.
to stretch
and
with unilateral
hydrostatic
internal
pressure
differentiate
fixation
protect
their
Elimination
operation.
bone.
by tightening
throughout.
The presence
or absence
of movement
influences
the
from
the external
or internal
bone
surfaces.
Indeed,
these
the
than the
intimate
whereas
areas
diameter
oppose
larger
The
in perfect
surface,
even smaller
and the crest ofthe thread
loses contact
of screws
exceeds
the shear
strength
of bone,
the
and
spaces
to be caused
contact
not
becomes
power
hand
seems
is squeezed
movements.
is unstable,
The
bone
limited
screws
microscopic
fixation
and
surface
the area of contact
Even if the holding
of contact
is
firmly
use of a drill bit, the diameter
of which
is considerably
screw,
leads
to a decrease
in depth
of the bone threads.
is not
thread
of this
natural
fixation
This
protection
leads
to the
again
confirms
into
hindlegs
formation
Pauwel’s
of fibrous
theory
that
and
cells
fibroblasts
and chondroblasts.
during
the first few days after
by operating
on both
hindlegs
at the
segments.
of bone
screws,
as well
the other hand,
and so favoured
held by compression
plates.
On
fixation,
we increased
the stability
with less fibrous
tissue.
These
tion.
this
experiments
The
osteosynthesis
stress until
process
as decreased
callus formation
in osteotomies
by splinting
the hindleg
after unstable
internal
callus formation
around
the screw threads
show
logical
clearly
conclusion
is
that
mechanical
that
fractured
forces
or
are
We were
resorption
same
time leads
to an increase
of stress
exerted
on the plated
observe
the formation
of fibrous
tissue
and the appearance
responsible
for
osteotomised
thus able to
around
the
cell
with screws
alone,
or with screws
and plates,
should
be protected
the spaces
around
the threads
are filled with callus.
Judging
from our
takes
The
fracture.
formation
In fact,
observed
an increase
four
treated
by
from undue
experiments,
weeks.
of
the
differentia-
extremities
this callus
unused
path
in the holding
support”
during
the first few
Bone
formation
around
osteoblasts
change
into cells
between
bone and metal.
proceeds
of a tap
power
much
faster
is filled with
of stripped
weeks.
screws
proceeds
like fibrocytes
than
solid
screws,
centripetally.
and remain
it does
in the healing
callus
by five weeks.
Bechtol
also
Having
as a single
THE
JOURNAL
of the
Having
recommends
“extra
reached
the screw,
layer
of flattened
OF
BONE
AND
JOINT
the
cells
SURGERY
MECHANICAL
FACTORS
INFLUENCING
THE
HOLDING
POWER
It is also noteworthy
that the new bone formation
of bone remodelling.
Cohen
(1961) has also observed
bone.
which
The newly
formed
provides
evidence
remodelling
of bone
Haversian
systems
of the transmission
around
screws
OF SCREWS
COMPACT
639
BONE
around
screws
takes place independently
deposition
of new bone on dead cortical
are at a right
of mechanical
is still
IN
incomplete
angle to the long axis
forces
by screws.
after
nine
of the shaft,
Finally,
the
months.
SUMMARY
1.
Cell differentiation
and
inserted
into
around
seventy
screws
femora
manufactured
by two
in forty-one
adult
periods
varying
between
two weeks
and nine months.
2. This study reveals
that, despite
their excellent
holding
in firm contact
thread
with
facing
surface
the surrounding
bone
the head of the screw
at the
touches
power,
differentiate
into osteogenic
and osteoclasts,
and
by osteogenic
bone
5.
cells
fragments
Extremities
I am
anchors
screws
has been established.
should
be protected
osteosynthesis,
6. This study
their
firmly
cells;
failure
power
at all stages
to five
undue
stresses
to Dr C. A.
Plante,
of fracture
Dr
has
such
screws
two
Swiss
been
companies
observed
over
are not everywhere
employed
and
weeks.
from
the
In the absence
weeks,
well
during
oftesting
those
screws
inaccurate
into
callus
before
fibroblasts,
formation
callus
first
uniting
few
in living
of the
being
of movement,
leads to differentiation
ensues.
In contrast,
in four
whatever
the technique.
clearly
demonstrates
the importance
holding
indebted
movement
of fixation
from
and
dogs
time of insertion.
Indeed,
only part
the compact
bone,
all other
surfaces
separated
by a space up to 1 50 t in thickness.
3. These
spaces
result
both from
the surgical
technique
measurements
of drills,
screws
and taps.
4. M igrating
cells invade
these spaces
during
the first two
these cells
chondroblasts
American
mongrel
weeks
bone
the
after
to ascertain
healing.
P. Lavigne,
Dr
D. Soucy
and
Dr
L. Colliou
for
their
financial
assistance.
REFERENCES
C. 0. (1959):
Internal
Fixation
Surgery,
p. 152. C. 0. Bechtol,
A.
with Plates
B. Ferguson,
EIE(HTOL,
Wilkins
( I 97 1 ) : Personal
J. (1961):
COHEN,
In Metals
and Engineering
P. G. Laing.
Baltimore:
in Boize
The
a,zd Joint
Williams
and
Company.
C. 0.
BECHTOL,
and Screws.
Jun., and
Tissue
communication.
Reactions
to
43-A, 687.
HAGMANN,
S. (1966):
Vergleichende
Implantation
von Metallschrauben
Metals-The
Influence
of
Surface
Finish.
Journal
of
Boize
cizd
Joint
Surger;’,
PAUWELS,
F.
(1960):
StUtzgewebe.
UHTHOFF,
H.
Clinical
UHTHOFF,
K.,
and
H.
K.,
H.
(1963):
Chirurgie
55 B,
NO.
3,
F.
Uber
P.
mit
AUGUST
Deutsche
1973
Bone
von
Zeitschrift
Acta
Einfluss
anatomica,
Structure
Knochenbildung
64, 3 1 1.
mechanischer
Reize
121
Changes
in the
,
auf
nach
die
intrafemoraler
Differenzierung
der
478.
Dog
under
Rigid
Internal
Fixation.
81, 165.
(1971):
Einfluss
#{252}ber
die reaktive
Entwicklungsgeschichte,
Research,
37, 654.
Die Einbettung
dem
den
iiid
L. (1971):
Related
LAVIGNE,
unter
‘ereinigt
Theorie
f#{252}rAnatomic
a,zd
and
Belgica,
Knochengewebes
VOL.
neue
DUBUC,
Orthopaedics
orthopaedica
WAGNER,
Eine
Zeitschrift
Untersuchungen
bei der Ratte.
Influence
de
Metallschrauben
der
stabilen
f#{252}rChirurgie,
Ia
plaque
im
rigide
Knochen
Osteosynthese.
305,
sur
Ia structure
und
Langenhecks
28.
die
osseuse.
Acta
Heilungsvorg#{228}nge
Archiv
f#{252}rklinische
des