University of Groningen Biodegradable plates and screws in ... · BIODEGRADABLE AND TITANIUM SCREW SYSTEMS: A COMPARISON ... controlled by the construction of the screwdriver han-dles

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Biodegradable plates and screws in oral and maxillofacial surgeryBuijs, Gerrit Jacob

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CHAPTER 3.1

TORSION STRENGTH OF

BIODEGRADABLE AND

TITANIUM SCREW SYSTEMS:

A COMPARISON

G.J. BUIJS

E.B. VAN DER HOUWEN

B. STEGENGA

R.R.M. BOS

G.J. VERKERKE

Published in: J Oral Maxillofac Surg. 2007 Nov;65(11):2142-7.

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Abstract:

Objectives- To determine: (1) the differences in maximum torque between 7 biodegradable

and 2 titanium screw systems, and (2) the differences of maximum torque between ‘hand

tight’ and break of the biodegradable and the titanium osteofixation screw systems.

Materials & Methods- Four oral and maxillofacial surgeons inserted 8 specimens of all 9

screw systems in polymethylmethacrylate (PMMA) plates. The surgeons were instructed to

insert the screws as they would do in the clinic (‘hand tight’). The data were recorded by a

torque measurement meter. A PhD resident inserted 8 specimens of the same set of 9 screw

systems until fracture occurred. The maximum applied torque was recorded likewise.

Results- (1) the mean maximum torque of the 2 titanium screw systems was significantly

higher than that of the 7 biodegradable screw systems, and (2) the mean maximum

torque for ‘hand tight’ was significantly lower than for break regarding 2 biodegradable,

and both titanium screw systems.

Conclusion & discussion- Based on the results, we conclude that the 1.5- and 2.0

mm titanium screw systems still present the highest torque strength compared to the

biodegradable screw systems. When there is an intention to use biodegradable screws,

we recommend the use of 2.0 mm BioSorb FX, 2.0 mm LactoSorb or the larger 2.5 mm

Inion CPS screws.

Keywords: screw; osteofixation; biodegradable; titanium; torsion strength; properties.

INTRODUCTION

Background

Fast, anatomical and pain-free re-union of bone fragments are the essential goals in

orthognathic and trauma surgery (84). Adequate reposition, stabilization and fixation of

fractured or osteotomized bone segments are essential preconditions (7;121). Plates and

screws are generally used for the internal stabilization and fixation of the bone segments

(35;36). Screws are used to fix osteofixation plates or to position bone segments (e.g. sag-

ittal split osteotomies) (3). During insertion, the screws occasionally break (4). Fracture of

a screw occurs when the applied torque is higher than the maximum allowable torque of

the screw. Removal of broken screws and re-application of screws is expensive and time-

consuming. Besides, additional operations may result in complications and subsequent

compromised bone healing.

It is generally acknowledged that biodegradable screws have different torsion character-

istics than titanium screws. Some clinical studies reported a higher number of broken bio-

degradable screws compared to titanium screws (2;4). Several authors have reported this

experience as a considerable disadvantage (40;111;112). The maximum torque strength

differs for the various screws mainly because of the use of different materials for manu-

facturing (biodegradable) screws, and different geometry of those screws.

The manufacturers do not specify the torque for inserting the screws. The torque to be

applied for adequate tightening the screws can be defined as ‘hand tight’. The maximally

applied torque is, to some extent, controlled by the construction of the screwdriver han-

dles (diameter, hand posture, geometry, and texture). But with most handles, the maxi-

mum torque that can be applied exceeds the torque strength of the screws, so fracture of

the screws might occur. An estimate of a safe torque for screws of different diameter and

length is difficult, especially for biodegradable screws (82). Moreover, many surgeons are

not that experienced in using polymeric screws. To guide decisions regarding the selec-

tion and application of different osteofixation screws, clarification of the differences in

torque strength of biodegradable as well as titanium osteofixation screw systems could

be valuable (122).

Objectives

The objectives of this study were to determine: (1) the differences in maximum torque

between 7 biodegradable and 2 titanium screw systems, and (2) the differences in maxi-

mum torque between ‘hand tight’ and break of the biodegradable as well as the titanium

screw systems.

MATERIALS AND METHODS

Seven (5 x 2.0-mm, 1 x 2.1-mm, and 1 x 2.5-mm) commercially available biodegradable

as well as two (1.5- and 2.0-mm) commercially available titanium screw systems were

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investigated. The biodegradable and titanium implants were gratuitously supplied by the

manufacturers. The manufacturers, with one exception (MacroPore BioSurgery Inc.), sup-

plied sterile implants. The Macropore implants exceeded the expiry date by 6-12 months.

Nevertheless, we decided to include these implants in the tests. The general characteris-

tics of the investigated screw systems are summarized in table I.

Four oral and maxillofacial surgeons were requested to insert 8 specimens of all 9 screw

systems in polymethylmethacrylate (PMMA) plates. The holes were predrilled for both the

titanium as for the biodegradable screws and subsequently pre-tapped (as prescribed) for

the biodegradable screws according to the prescriptions of the individual manufactur-

ers (with prescribed burs and taps). The surgeons were instructed to insert the screws

as they would do in the clinic (‘hand tight’). A PhD resident inserted 8 specimens of the

same set of 9 screw systems until fracture occurred. The screws were inserted at room

temperature, as this is the regular operating room temperature. Before insertion of the

screws, the holes were irrigated with physiological fluid to simulate the in situ lubrication.

The maximally applied torque was recorded by a torque measurement meter (Nemesis

Howards Torque Gauge, Smart MT-TH 50 sensor; accuracy 2.5 mNm, range 0-500 mNm;

supplied by Hartech, Wormerveer, The Netherlands).

Statistical analysis

The data were analyzed using the Statistical Package of Social Sciences (SPSS), version

14.0. Descriptive statistics was used to calculate means and standard deviation. The meas-

ured maximum torque of the 32 different specimens (8 specimen times four surgeons)

of each screw system were averaged. To determine whether there were significant dif-

ferences between the biodegradable and the titanium osteofixation screw systems, the

mean maximum torques were subjected to a One-Way ANalysis Of VAriance (ANOVA).

A correction for multiple testing was performed according to Dunnet T3 (equal variances

not assumed). The differences between maximum torque of ‘hand tight’ and break of the

various screw systems were statistically compared with Student’s t-tests. Differences were

considered to be significant when p < 0.05 for all tests.

RESULTS

The mean maximum torque and standard deviation of the 9 osteofixation screws systems

for ‘hand tight’ are graphically plotted in figure 1. The mean maximum torque of the bio-

degradable systems was significantly lower compared to the mean maximum torque of

both titanium systems (table II). The standard deviations of the titanium screw systems were

considerable larger than those of the biodegradable screw systems. Figure 2 represents the

mean maximum torque of the 9 osteofixation screw systems at break. The standard devia-

tions of the titanium systems showed in figure 2, were lower than those of the biodegrad-

able systems, especially when compared to the results showed in figure 1. The plot of the

2.0-mm titanium screw system did not show a standard deviation because the torque for

Tab

le I

. C

hara

cter

istic

s of

incl

uded

ost

eofix

atio

n sc

rew

s

Bra

nd

nam

eM

anu

fact

ure

r (c

ity

and

sta

te)

Co

mp

osi

tio

nSt

eril

ity

Scre

w #

ØSc

rew

*

Bio

de

gra

dab

le s

crew

s

Bio

Sorb

FX

Linv

atec

Bio

mat

eria

ls L

td. (

Tam

per

e, F

inla

nd)

SR 7

0L/

30

DL

PLA

Ster

ile2.

0 m

m6

.0 m

m

Res

orb

XG

ebrü

der

Mar

tin

Gm

bH

& C

o. (

Tutt

ling

en, G

erm

any

)10

0 D

L-La

ctid

eSt

erile

2.1

mm

7.0

mm

Inio

n C

PS

2.0

Inio

n Lt

d. (

Tam

per

e, F

inla

nd)

LDL

Lact

ide/

TMC

*St

erile

2.0

mm

7.0

mm

Inio

n C

PS

2.5

Inio

n Lt

d. (

Tam

per

e, F

inla

nd)

LDL

Lact

ide/

TMC

*St

erile

2.5

mm

7.0

mm

Lac

toSo

rbW

alte

r Lo

renz

Sur

gica

l Inc

. (Ja

ckso

nvill

e, F

lori

da)

82

PLLA

/18

PGA

Ster

ile2.

0 m

m7.

0 m

m

Pol

ymax

Mat

hys

Med

ical

Ltd

. (B

ettl

ach

Swit

zerl

and

)70

L/3

0D

L PL

ASt

erile

2.0

mm

6.0

mm

Mac

roP

ore

Mac

roPo

re B

ioSu

rger

y In

c. (M

emp

his,

USA

)70

L/3

0D

L PL

AEx

pire

d2.

0 m

m6

.0 m

m

Tit

aniu

m s

crew

s

KL

S M

artin

Geb

rüd

er M

arti

n G

mb

H &

Co.

(Tu

ttlin

gen

, Ger

man

y)Ti

tani

um (

pure

)St

erile

1.5

mm

6.0

mm

KL

S M

artin

Geb

rüd

er M

arti

n G

mb

H &

Co.

(Tu

ttlin

gen

, Ger

man

y)Ti

tani

um (

pure

)St

erile

2.0

mm

6.0

mm

*

= L

eng

th o

f sc

rew

s (a

cco

rdin

g t

he

spec

ifica

tio

ns

of

the

man

ufa

ctu

rers

)#Ø

= D

iam

eter

of

scre

ws

(acc

ord

ing

th

e sp

ecifi

cati

on

s o

f th

e m

anu

fact

ure

rs)

* =

Po

lym

er c

om

po

siti

on

no

t sp

ecifi

ed t

hro

ug

h t

he

man

ufa

ctu

rer

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Figure 2. Mean maximum torque regarding method ‘Break’ organized by screw system

Legend:X-axis = brand names of the investigated osteofixation systemsY-axis = maximum torque measured during insertionPoints in figure: represents mean maximum torqueBars: represents the standard deviation of the mean maximum torque

Figure 3. Mean maximum torque of four surgeons organized by method and surgeon

Legend:X-axis = brand names of the investigated osteofixation systemsY-axis = maximum torque measured during insertionPoints in figure: represents mean maximum torque Surgeons: represents the four surgeons who inserted the screws

breaking the screws exceeded the maximum limit of the torque measurement apparatus.

The mean maximum torque was set at 680 mNm (as measured by the torque measurement

apparatus, however not with the accuracy of 2.5 mNm). The mean maximum torque of

both titanium screw systems were significantly higher than the 7 different biodegradable

screw systems. With respect to the 7 biodegradable screw system, the Inion CPS 2.5 screw

system represented a significantly higher torque than the other biodegradable systems for

the method ‘handtight’. Regarding the method break, the mean maximum torque of the

BioSorb FX, Inion CPS 2.5 and LactoSorb screw systems were significantly higher than the

four remaining biodegradable screw systems. Different comparisons regarding significant

differences of the various screw systems for ‘hand tight’ and break are outlined in table II.

Figure 3 represents the mean maximum torque of the screw systems organized by surgeon

and screw system. The surgeons showed a wider distribution of the mean maximum torque

of the titanium screw systems compared to the biodegradable screw systems. This corre-

sponds to the large standard deviations for ‘hand tight’ presented in figure 1.

Table III presents a summary of the descriptive statistics. The mean, standard deviation,

95% confidence interval, and the range are presented and organized by method. Table III

revealed that for each screw system, the mean maximum torque at break was above the

mean maximum torque at ‘hand tight’. A statistical comparison of the mean maximum

torque of ‘hand tight’ and break for the LactoSorb, Inion CPS 2.5, titanium 1.5 mm, and

titanium 2.0 mm screw systems revealed that the mean maximum torques for break were

significantly higher than the mean maximum torque for ‘hand tight’ (diagonal of Table II).

Figure 1. Mean maximum torque regarding method ‘Handtight’ organized by screw system

Legend:X-axis = brand names of the investigated osteofixation systemsY-axis = maximum torque measured during insertionPoints in figure: represents mean maximum torque Bars: represents the standard deviation of the mean maximum torque

Degradability

Degradable

Non degradable

Method: Hand tight

System

BioSorb FX 2.0 mm

500.0

400.0

300.0

200.0

100.0

0.0

Macropore 2.0 m

m

Polymax 2.0 m

m

Resorb X 2.1 mm

Titanium 1.5 m

m

Titanium 2.0 m

m

Inion CPS 2.0 mm

Inion CPS 2.5 mm

LactoSorb 2.0 mm

Mea

n m

axi

mu

m t

orq

ue

(mN

m)

Degradability

Degradable

Non degradable

Method: Break

System

BioSorb FX 2.0 mm

700.0

600.0

500.0

400.0

300.0

200.0

100.0

0.0

Macropore 2.0 m

m

Polymax 2.0 m

m

Resorb X 2.1 mm

Titanium 1.5 m

m

Titanium 2.0 m

m

Inion CPS 2.5 mm

Inion CPS 2.5 mm

LactoSorb 2.0 mm

Mea

n m

axi

mu

m t

orq

ue

(mN

m)

Chirurg

Surgeon 1

Surgeon 2

Surgeon 3

Surgeon 4

System

BioSorb FX 2.0 mm

600.0

500.0

400.0

300.0

200.0

100.0

0.0

Macropore 2.0 m

m

Polymax 2.0 m

m

Resorb X 2.1 mm

Titanium 1.5 m

m

Titanium 2.0 m

m

Inion CPS 2.0 mm

Inion CPS 2.5 mm

LactoSorb 2.0 mm

Mea

n m

axi

mu

m t

orq

ue

(mN

m)

Mean maximum torque of four surgeons

42 43

Tab

le I

I. S

tatis

tical

dif

fere

nces

bet

wee

n os

teofi

xatio

n sc

rew

s

Syst

emB

ioSo

rb F

X

2.0

mm

Inio

n C

PS

2.0

mm

Inio

n C

PS

2.5

mm

Lac

toSo

rb

2.0

mm

Mac

ropo

re

2.0

mm

Pol

ymax

2.

0 m

mR

esor

b X

2.1

mm

Tita

nium

1.

5 m

mT

itani

um

2.0

mm

Bio

Sorb

FX

2.

0 m

mN

SS

NS

NS

SS

SS

S

Inio

n C

PS

2.0

mm

NS

NS

SS

NS

NS

NS

SS

Inio

n C

PS

2.5

mm

SS

SN

SS

SS

SS

Lac

toSo

rb

2.0

mm

NS

SS

SS

SS

SS

Mac

ropo

re

2.0

mm

SS

SS

NS

NS

NS

SS

Pol

ymax

2.0

mm

SS

SS

NS

NS

NS

SS

Res

orb

X2.

1 m

mS

S S

SN

SN

SN

SS

S

Tita

nium

1.5

mm

SS

SS

SS

SS

S

Tita

nium

2.0

mm

SS

SS

SS

SS

S

Met

ho

d =

‘H

and

tig

ht’

Met

ho

d =

Bre

akD

iag

on

al =

‘H

and

tig

ht’

ver

sus

Bre

akS

= S

ign

ifica

ntN

S =

No

n Si

gn

ifica

nt

Tab

le I

II.

Sum

mar

y of

des

crip

tive

stat

istic

s

Met

ho

d =

‘H

and

tig

ht’

Sys

tem

Mea

n*

SD*

95%

Co

nfi

den

ce I

nte

rval

Ran

ge

Low

er B

ou

nd

*U

pp

er B

ou

nd

*Lo

we

st v

alu

e*H

igh

est

val

ue*

Bio

Sorb

FX

2.0

mm

80.

2323

.41

69.6

49

0.81

38

.10

132.

40

Inio

n C

PS

2.0

mm

73.4

212

.22

62.8

48

4.0

137

.30

94

.20

Inio

n C

PS

2.5

mm

156

.85

17.9

814

6.2

716

7.4

410

5.0

018

2.5

Lac

toSo

rb 2

.0 m

m9

6.9

023

.51

86

.31

107.

48

62.8

013

9.3

0

Mac

ropo

re 2

.0 m

m61

.60

10.2

351

.06

72.2

335

.70

83.

40

Pol

ymax

2.0

mm

56

.70

14.3

04

6.0

867

.26

30.

108

9.3

0

Res

orb

X 2

.1 m

m55

.40

11.4

74

4.8

165

.98

27.8

069

.80

Tita

nium

1.5

mm

246

.90

89.

1023

6.3

027

.48

94

.40

379.

70

Tita

nium

2.0

mm

36

6.6

012

2.11

356

.01

377.

1819

4.2

061

1.0

0

Met

ho

d =

Bre

ak

Syst

emM

ean

*SD

*95

% C

on

fid

ence

In

terv

alR

ang

e

Low

er B

ou

nd

*U

pp

er B

ou

nd

*Lo

we

st v

alu

e*H

igh

est

val

ue*

Bio

Sorb

FX

2.0

mm

192.

2014

.18

184

.92

199.

48

175.

40

210.

50

Inio

n C

PS

2.0

mm

85.0

812

.29

77.7

992

.36

63.0

010

4.2

0

Inio

n C

PS

2.5

mm

181.

34

5.49

174

.09

188

.66

173.

818

9.2

Lac

toSo

rb 2

.0 m

m18

8.8

015

.74

181.

4719

6.0

316

0.10

216

.00

Mac

ropo

re 2

.0 m

m77

.19

5.0

569

.90

84

.47

69.6

08

3.8

0

Pol

ymax

2.0

mm

89.

48

8.9

28

2.19

96

.76

71.8

09

8.9

0

Res

orb

X 2

.1 m

m72

.86

11.8

565

.58

80.

155

8.0

09

6.8

0

Tita

nium

1.5

mm

396

.48

9.01

38

9.19

403

.76

38

8.2

041

6.3

0

Tita

nium

2.0

mm

68

0.0

00.

00

672.

726

87.2

86

80.

00

68

0.0

0

SD =

Sta

nd

ard

Dev

iati

on

*in

mN

m

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DISCUSSION

The differences in maximum torque found for the studied systems can be explained

by the different screw diameters (1.5-, 2.0-, 2.1- and 2.5 mm), different (co-polymer)

compositions, different geometry (pitch and shaft) of the screws, different tools used

to insert the screws, different ages of the screws, and different methods to sterilize

the screws. As expected, the use of titanium for manufacturing osteofixation screws

revealed a high maximum torque strength whereas the use of polymers revealed a sig-

nificantly lower torque strength. A surprising finding was the significant mean maximum

torque difference of the BioSorb FX, Inion CPS 2.5 and LactoSorb screw systems com-

pared to the remaining four biodegradable screw systems for the method break. The

self-reinforced polymers of the BioSorb FX screw system, the larger dimensions of the

2.5 mm Inion CPS screws, and the ponderous geometry of the LactoSorb screws are

probably responsible for the high maximum torque. The large standard deviations of

the 2 titanium screw systems presented in figure 1 are probably caused by the higher

maximum torque. After all, when surgeons apply higher torque forces, this inevitably

implies loss of accuracy.

The comparison of the maximum torque of ‘hand tight’ and break for the individual

screw systems revealed statistically significant differences for 4 (LactoSorb, Inion CPS

2.5, titanium 1.5 mm, and titanium 2.0 mm) of the 9 osteofixation screw systems (diago-

nal Table II). In the case of individual biodegradable screws (Inion CPS 2.0 mm, Inion CPS

2.5 mm, Macropore 2.0 mm, and Resorb X 2.1 mm), the lowest torque at break was not

always above the highest torque of ‘hand tight’. Besides, the 95% confidence intervals

of the maximum torque with respect to break and ‘hand tight’ of biodegradable screws

did overlap (Table III). These two aspects indicate that the torsion characteristics of bio-

degradable screws are not always that repeatable.

For analyzing the results, the data of the four surgeons have been combined in order to

reduce the influence of outliers and to determine statistical significant differences. The

results of the independent surgeons are graphically presented in figure 3. Note the large

differences in mean maximum torque regarding the 2 titanium systems compared to the

7 biodegradable systems. Statistical analysis yielded no significant differences between

most surgeons except for two surgeons. This is largely due to the statistical influence of

the large differences in mean maximum torque for titanium screws. Despite the signifi-

cant difference between the two surgeons, the data were combined. After all, combin-

ing the results of the four surgeons should be allowed because the insertion torque of

screws of maxillofacial surgeons should be approximately equal.

Investigating 7 different biodegradable screws theoretically implies 7 learning curves, as

is the case with every new technique (64;123;124). These learning curves could influence

the results and consequent statistically significant differences. To find out whether the

learning curves affected the results, the screw 1- and 2- data have been deleted for every

surgeon and system. The raw data were then analyzed (6 instead of 8 screws) again.

Eliminating the first 2 screws did not reveal different statistically (significant) results be-

tween the osteofixation screw systems.

Statistically significant differences do not necessarily imply differences to be clinically rel-

evant. With respect to the investigated osteosynthesis screws in this study, it is ques-

tionable whether the statistically significant differences are clinically relevant. The large

significant differences between titanium screws and biodegradable screws in mean maxi-

mum torque are clinically relevant, although the field of application may be different. In

contrast, the statistically significant differences between some of the 7 biodegradable

devices regarding the method ‘hand tight’ are not clinically relevant, because they are

considered to be too small. Moreover, it has been reported that biodegradable devices

physically relax under constant force (a process called creep). In this case, the applied

torque is ‘counteracted’ by the reorganizing polymer chains (111). Titanium screws do not

undergo this material relaxation. The significant differences between some of the 7 bio-

degradable devices for the method break are of clinical importance, because biodegrad-

able screws can fracture easily during insertion. The significant differences of maximum

torque for ‘hand tight’ and break of 2 biodegradable (Inion CPS 2.5, and LactoSorb) as

well as both titanium screw systems presented in the current study are clinically relevant.

After all, screws will break easily during insertion, when the differences between ‘hand

tight’ and break are small.

The objectives of this investigation were to determine: (1) the differences in mean maxi-

mum torque between 7 biodegradable and 2 titanium screw systems, and (2) the differ-

ences of mean maximum torque between ‘hand tight’ and break of the biodegradable

as well as the titanium osteofixation screw systems. This study has presented that: (1) the

mean maximum torque of titanium screw systems was significantly higher than of the

biodegradable screw systems, and (2) the mean maximum torque of all 9 screw systems

at break was (significantly) higher than at ‘hand tight’. Based on the results and discus-

sion points mentioned above, we can conclude that the 1.5- and 2.0 mm titanium screw

systems still present the highest torque strength compared to the biodegradable screw

systems. When there is an intention to use biodegradable screws, we would recommend

the use of 2.0 mm BioSorb FX, 2.0 mm LactoSorb or the larger 2.5 mm Inion CPS screws.

Acknowledgements

We would like to thank, prof. dr. G.M. Raghoebar, dr. F.K.L. Spijkervet and dr. J. Jansma

for inserting the osteofixation screws. The authors also would like to thank dr. H. Groen

and dr. M.M. Span for their statistical assistance. The gratuitously supply of biodegrad-

able screws through the manufacturers (Linvatec Biomaterials Ltd., KLS Martin, Inion Ltd.,

Walter Lorenz Surgical Inc., Synthes, and Macropore Inc.) was gratefully appreciated.

CHAPTER 3.2.1

MECHANICAL STRENGTH

AND STIFFNESS OF

BIODEGRADABLE AND

TITANIUM OSTEOFIXATION

SYSTEMS

G.J. BUIJS

E.B. VAN DER HOUWEN

B. STEGENGA

R.R.M. BOS

G.J. VERKERKE

Published in: J Oral Maxillofac Surg. 2007 Nov;65(11):2148-58.

48 49

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Abstract:

Objective - The objective of this study was to present relevant mechanical data in order

to simplify the selection of an osteofixation system for situations requiring immobilization

in oral and maxillofacial surgery.

Materials & Methods - 7 biodegradable and 2 titanium osteofixation systems were in-

vestigated. The plates and screws were fixed to 2 polymethylmethacrylate (PMMA) blocks

to simulate bone segments. The plates and screws were subjected to tensile, side bend-

ing, and torsion tests. During tensile tests, the strength of the osteofixation system was

monitored. The stiffness was calculated for the tensile, side bending, and torsion tests.

Results - The two titanium systems (1.5 mm and 2.0 mm) presented significantly higher

tensile strength and stiffness compared to the 7 biodegradable systems (2.0 mm, 2.1 mm,

and 2.5 mm). The 2.0 mm titanium system revealed significantly higher side bending and

torsion stiffness than the other 7 systems.

Conclusion & discussion - Based on the results of the current study, it can be concluded

that the titanium osteofixation systems were (significantly) stronger and stiffer than the

biodegradable systems. The BioSorb FX, LactoSorb, and Inion CPS 2.5 mm systems have

high mechanical device strength and stiffness compared to the investigated biodegrad-

able osteofixation systems. With the cross-sectional surface taken into account, the Bio-

Sorb FX system (with its subtle design), proves to be the far more superior system. The

Resorb X and MacroPore systems present to be, at least from a mechanical point of view,

the least strong and stiff systems in the test.

Key words: osteofixation system; biodegradable; titanium; mechanical; strength; stiff-

ness; properties.

INTRODUCTION

Background

Sufficient revascularization, anatomical reduction, and proper immobilization of bone

segments are essential aspects of the healing of fractures and osteotomies (7;10). Im-

mobilization of bone fragments is currently obtained by the use of osteofixation plates

and screws (125;126). The plates and screws are applied subperiostally in order to secure

sufficient revascularization (7). These fixation devices must withstand the local deforming

forces that are exerted through the maxillofacial muscles.

Currently, titanium fixation systems are successfully used to realize adequate immobiliza-

tion (39). These systems, however, have several disadvantages: (1) the need for a second

intervention to remove the devices, if indicated (46-48), (2) interference with imaging or

radio-therapeutic techniques (37;41;127), (3) possible growth disturbance or mutagenic

effects (37;41;43-45), (4) brain damage (44;128), (5) and thermal sensitivity (129).

Biodegradable ‘dissolving’ fixation systems could reduce the problems associated with

titanium systems (74). However, these systems are mechanically weaker than titanium

systems due to the use of biodegradable polymers. Moreover, adverse reactions to the

degradation products have been reported (66;67;100;114). Despite these disadvantages,

there is a continuous drive to explore fixation devices which will ‘dissolve’ when bone

healing has been occurred (4). In order to investigate whether biodegradable systems

are proper alternatives for titanium systems, they have been the subject of research for

decades (58). Nevertheless, the mechanical properties of biodegradable systems have

hardly been objectively compared in the scientific literature. In addition, many biodegrad-

able fixation systems with a great variety in dimensions and co-polymer compositions

are commercially available. As a result, the mechanical characteristics differ substantially,

which consequently hampers surgeons to select an adequate fixation system for a specif-

ic situation (82). Determining the different mechanical properties of titanium and biode-

gradable osteofixation systems could support the procedure of finding the right fixation

system for the right situation (122).

Objectives

The objective of this study was to present relevant mechanical data in order to simplify

the selection of an osteofixation system for situations requiring immobilization in oral and

maxillofacial surgery.


The specimens to be investigated were 7 commercially available biodegradable (5 x 2.0

mm, 1 x 2.1 mm, and 1 x 2.5 mm) and 2 commonly used commercially available titanium

(1.5 mm and 2.0 mm) osteofixation systems. The general characteristics of the included

plates and screws are summarized in table I.

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Tab

le I

. C

hara

cter

istic

s of

incl

uded

ost

eofix

atio

n sy

stem

s

Bra

nd

na

me

Ma

nu

fact

ure

r (c

ity

an

d s

tate

)C

om

po

siti

on

Ste

rili

tyS

crew

D

iam

ete

r*S

crew

Le

ng

th*

Plat

eLe

ng

th*

Plat

eW

idth

*Pl

ate

Th

ick

ne

ss*

Bio

de

gra

dab

le s

yste

ms

Bio

Sorb

FX

Linv

atec

Bio

mat

eria

ls L

td.

(Tam

per

e, F

inla

nd)

SR 7

0L/

30

DL

PLA

Ster

ile2.

0 m

m6

.0 m

m25

.5 m

m5.

5 m

m1.

3 m

m

Res

orb

XG

ebrü

der

Mar

tin

Gm

bH

& C

o.

(Tu

ttlin

gen

, Ger

man

y )

100

DL-

Lact

ide

Ster

ile2.

1 m

m7.

0 m

m26

.0 m

m6

.0 m

m1.

1 m

m

Inio

n C

PS

2.0

mm

Inio

n Lt

d. (

Tam

per

e, F

inla

nd)

LDL

Lact

ide/

TMC

/PG

ASt

erile

2.0

mm

7.0

mm

28.0

mm

7.0

mm

1.3

mm

Inio

n C

PS

2.5

mm

Inio

n Lt

d. (

Tam

per

e, F

inla

nd)

LDL

Lact

ide/

TMC

/PG

ASt

erile

2.5

mm

6.0

mm

32.0

mm

8.5

mm

1.6

mm

Lac

toSo

rbW

alte

r Lo

renz

Sur

gica

l Inc

.

(Jac

kso

nvill

e, F

lori

da)

82

PLLA

18

PGA

Ster

ile2.

0 m

m7.

0 m

m28

.5 m

m7.

0 m

m1.

3 m

m

Pol

ymax

Mat

hys

Med

ical

Ltd

.

(Bet

tlac

h Sw

itze

rlan

d)

70L/

30

DL

PLA

Ster

ile2.

0 m

m6

.0 m

m28

.0 m

m6

.0 m

m1.

3 m

m

Mac

roP

ore

Mac

roPo

re B

ioSu

rger

y In

c.

(Mem

phi

s, U

SA)

70L/

30

DL

PLA

Exp

ired

2.0

mm

6.0

mm

25.0

mm

6.7

mm

1.2

mm

Tit

aniu

m s

yste

ms

KL

S M

artin

Geb

rüd

er M

arti

n G

mb

H &

Co.

(Tu

ttlin

gen

, Ger

man

y)Ti

tani

um (

pure

)St

erile

1.5

mm

6.0

mm

18.5

mm

3.5

mm

0.6

mm

KL

S M

artin

Geb

rüd

er M

arti

n G

mb

H &

Co.

(Tu

ttlin

gen

, Ger

man

y)Ti

tani

um (

pure

)St

erile

2.0

mm

6.0

mm

25.5

mm

5.0

mm

1.0

mm

* =

acc

ord

ing

th

e sp

ecifi

cati

on

s o

f th

e m

anu

fact

ure

rs.

The non-sterile titanium plates and screws were sterilized in our department in the usual

manner. The manufacturers of the biodegradable systems supplied sterile implants, with

the exception of the MacroPore implants of which the expiry date was passed (aver-

age 6-12 months). The plates under investigation were 4-hole extended plates. Eighteen

plates and 72 screws of each system were subjected to three different mechanical tests.

The osteofixation plates and screws were fixed to 2 polymethylmethacrylate (PMMA)

blocks that simulated bone segments. There was no interfragmentary contact to simulate

the most unfavourable clinical situation. Two screws were inserted in both PMMA blocks

according to the prescriptions of the individual manufacturer (with prescribed burs and

taps). The applied torque for inserting the screws was measured to check whether it

was comparable to the clinically applied torque (‘hand tight’) defined in a previous study

(130). The holes were irrigated with saline before insertion of the screws, to simulate the

in situ lubrication. The two PMMA blocks, linked by the osteofixation device (1 plate and

4 screws) were restored in a water tank containing water of 37.2 degrees Celsius for 24

hours to simulate the relaxation of biodegradable screws at body temperature (111). The

tests were performed in another tank containing water at the same temperature to simu-

late body temperature. Saline was not used because of possible corrosion of the test- and

environment set-up. Omitting the use of saline was expected not to be of influence to

the test results.

Figure 1. Tensile test set-up

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52 53

The plates and screws were subjected to tensile, side bending, and torsion tests. The

tensile test was performed as a standard loading test (figure 1). Side bending tests were

performed to simulate an in vivo bi-lateral sagittal split osteotomy (BSSO) situation (figure

2). Torsion tests were performed to subject the osteofixation devices to high torque in

order to simulate the most unfavourable situation (figure 3). The 2 PMMA blocks, linked

by the osteofixation device, were mounted in a test machine (Zwick/Roell TC-FR2, 5TS.

D09, 2.5kN Test machine. Force accuracy 0.2%, positioning accuracy 0.0001mm; Zwick/

Roell Nederland, Venlo, The Netherlands). Regarding the tensile tests, the 2 PMMA blocks

and thus the osteofixation plate were subjected to a tensile force with a constant speed

of 5 mm/min until fracture occurred (according to the standard ASTM D638M). For the

side bending test the 2 PMMA blocks were supported at their ends whereas the plates

were loaded in the centre of the construction with a constant speed of 30 mm/min (with

this speed the outer fibers were loaded as fast as the fibers of the osteofixation system in

the tensile test) until the plate was bended 30 degrees. For the torsion test the 2 PMMA

blocks were twisted along the long axis of the osteofixation system with a constant speed

of 90 degrees/min (with this speed the outer fibers were loaded as fast as the fibers of the

osteofixation system in the tensile test) until the plate was turned 160 degrees.

During testing the applied force was recorded by the load cell of the test machine. Both

force and displacement were measured with a sample frequency of 500 hertz and graphi-

cally presented in force-displacement diagrams. During tensile tests, the strength of the

osteofixation system was monitored. The stiffness was calculated for the tensile, side

bending and torsion tests by linking the 25% and 75% points (to exclude inaccuracies of

the start and end of the curves) of the maximum force on the force-displacement curves

and determining the direction-coefficients of the curves.


Statistical Package of Social Sciences (SPSS, version 12.0) was used to analyze the data.

Mean and standard deviation were calculated to describe the data. To determine whether

there were significant differences between the biodegradable and the titanium osteofixa-

tion systems in (1) tensile strength and stiffness, (2) side bending stiffness, and (3) tor-

sion stiffness, the maximum values were subjected to a One-Way ANalysis Of VAriance

(ANOVA). A correction for multiple testing was performed according to Dunnet T3 (equal

variances not assumed). Differences were considered to be significant when p < 0.05 for

all tests.

RESULTS

The torques used to insert the screws of the 9 osteofixation systems regarding the tensile,

side bending, and torsion tests are outlined in table II. The mean torques as well as the

standard deviations for each system in all three tests were nearly similar.

The mean tensile strength and stiffness of the 9 osteofixation systems are graphically

Figure 2. Side bending test set-up

Figure 3. Torsion test set-up

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54 55

Table II. Applied torque of inserted osteofixation screws

Test System Mean* SD*

Tensile BioSorb FX 81.23 0.41

Inion CPS 2.0 74.29 0.31

Inion CPS 2.5 156.81 0.76

LactoSorb 97.96 0.48

MacroPore 62.42 0.47

Polymax 57.05 0.58

ResorbX 56.13 0.23

KLS 1.5 251.21 1.54

KLS 2.0 369.84 1.09

Side Bending BioSorb FX 81.50 0.57

Inion CPS 2.0 74.40 0.54

Inion CPS 2.5 157.24 0.35



Polymax 56.83 0.23

ResorbX 55.90 0.26

KLS 1.5 248.23 0.70

KLS 2.0 370.20 1.02

Torsion BioSorb FX 80.93 0.43

Inion CPS 2.0 74.50 0.83

Inion CPS 2.5 156.80 0.76



Polymax 57.46 0.41

ResorbX 55.91 0.30

KLS 1.5 248.53 1.36

KLS 2.0 367.96 1.97

*in mNm SD = Standard Deviation

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Figure 5. Mean tensile stiffness organized by system

Legend:X-axis = brand names of the investigated osteofixation systemsY-axis = mean stiffness in Newton/mmPoints in figure: represents mean stiffnessBars: represents the standard deviation of the mean stiffness

Figure 4. Mean tensile strength organized by system

Legend:X-axis = brand names of the investigated osteofixation systemsY-axis = mean strength in Newton’sPoints in figure: represents mean strengthBars: represents the standard deviation of the mean strength

Degradability

Degradable

Non degradable

Method: Strength Tensile Test

System

BioSorb FX 2.0 mm

800.0

600.0

400.0

200.0

0.0

Macropore 2.0 m

m

Polymax 2.0 m

m

Resorb X 2.0 mm

Titanium 1.5 m

m

Titanium 2.0 m

m

Inion CPS 2.0 mm

Inion CPS 2.5 mm

LactoSorb 2.0 mm

Mea

n s

tren

gth

(N

)

Degradability

Degradable

Non degradable

Method: Stiffness Tensile Test

System

BioSorb FX 2.0 mm

600.0

500.0

400.0

300.0

200.0

100.0

0.0

Macropore 2.0 m

m

Polymax 2.0 m

m

Resorb X 2.0 mm

Titanium 1.5 m

m

Titanium 2.0 m

m

Inion CPS 2.0 mm

Inion CPS 2.5 mm

LactoSorb 2.0 mm

Mea

n S

tiff

ne

s (N

/mm

)

56 57

Tab

le I

II.

Sign

ifica

nce

bet

wee

n os

teofi

xatio

n sy

stem

s

Syst

emB

ioSo

rb F

X

2.0

mm

Inio

n C

PS

2.0

mm

Inio

n C

PS

2.5

mm

Lac

toSo

rb

2.0

mm

Mac

roP

ore

2.0

mm

Pol

ymax

2.

0 m

mR

esor

b X

2.

1 m

mT

itani

um

1.5

mm

Tita

nium

2.

0 m

m

Bio

Sorb

FX

2.

0 m

mX

XX

XS

SS

SS

SS

S

Inio

n C

PS

2.0

mm

SX

XX

XS

SS

NS

SS

S

Inio

n C

PS

2.5

mm

SN

SX

XX

XS

SS

SS

S

Lac

toSo

rb

2.0

mm

NS

SS

XX

XX

SS

SS

S

Mac

roP

ore

2.0

mm

SN

SN

SS

XX

XX

NS

NS

SS

Pol

ymax

2.

0 m

mS

NS

NS

SN

SX

XX

XS

SS

Res

orb

X

2.1

mm

SS

SS

NS

SX

XX

XS

S

Tita

nium

1.

5 m

mS

SS

SS

SS

XX

XX

S

Tita

nium

2.0

mm

SS

SS

SS

SS

XX

XX

Un

der

line

= T

ensi

le s

tren

gth

Ital

ic =

Ten

sile

sti

ffn

ess

S =

Sig

nifi

cant

NS

= N

on

Sig

nifi

cant

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Table IV. Summary of descriptive statistics tensile test

Tensile strength

System Mean^ SD^ 95% Confidence Interval

Lower Bound^ Upper Bound^

BioSorb FX 2.0 mm 162.00 3.18 155.16 168.85

Inion CPS 2.0 mm 101.98 5.11 95.13 108.82

Inion CPS 2.5 mm 219.82 13.43 212.98 226.67

LactoSorb 2.0 mm 175.17 2.40 168.33 182.02

MacroPore 2.0 mm 65.07 16.92 58.23 71.92

Polymax 2.0 mm 89.68 5.52 82.84 96.53

Resorb X 2.1 mm 59.87 4.73 53.02 66.71

Titanium 1.5 mm 266.71 6.74 259.86 273.55

Titanium 2.0 mm 741.21 4.08 734.36 748.05

Tensile stiffness

System Mean* SD* 95% Confidence Interval

Lower Bound* Upper Bound*

BioSorb FX 2.0 mm 248.00 24.28 235.57 260.43

Inion CPS 2.0 mm 87.56 11.66 75.12 99.99

Inion CPS 2.5 mm 79.52 3.74 67.09 91.95

LactoSorb 2.0 mm 203.78 4.82 191.34 216.21

MacroPore 2.0 mm 52.87 16.57 40.44 65.31

Polymax 2.0 mm 80.08 5.74 67.65 92.51

Resorb X 2.1 mm 42.86 5.82 30.44 55.30

Titanium 1.5 mm 448.56 24.68 436.12 460.99

Titanium 2.0 mm 521.27 18.56 508.84 533.70

^ in N *in N/mm SD = Standard Deviation

presented in figure 4 and 5, respectively. The two titanium systems (1.5 mm and 2.0 mm)

presented significantly higher tensile strength and stiffness compared to the biodegrad-

able systems (2.0 mm, 2.1 mm, and 2.5 mm). Regarding the biodegradable systems, the

BioSorb FX, Inion CPS 2.5 mm, and LactoSorb systems presented a significantly higher

tensile strength whereas the BioSorb FX and LactoSorb systems presented a significantly

higher tensile stiffness compared to the other biodegradable systems. The differences

between the systems are outlined in table III. The standard deviations for the systems

regarding the tensile strength and stiffness were small. A summary of the descriptive

statistics is presented in table V.

58 59

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Figure 6. Mean side bending stiffness organized by system

Legend:X-axis = brand names of the investigated osteofixation systemsY-axis = mean stiffness in Newton/mm (deducted unit)Points in figure: represents mean stiffnessBars: represents the standard deviation of the mean stiffness

Figure 7. Mean torsion stiffness organized by system

Legend:X-axis = brand names of the investigated osteofixation systemsY-axis = mean stiffness in Newton/mm (deducted unit)Points in figure: represents mean stiffnessBars: represents the standard deviation of the mean stiffness. Ta

ble

V.

Sign

ifica

nce

bet

wee

n os

teofi

xatio

n sy

stem

s

Syst

emB

ioSo

rb F

X

2.0

mm

Inio

n C

PS

2.0

mm

Inio

n C

PS

2.5

mm

Lac

toSo

rb

2.0

mm

Mac

roP

ore

2.0

mm

Pol

ymax

2.

0 m

mR

esor

b X

2.

1 m

mT

itani

um

1.5

mm

Tita

nium

2.

0 m

m

Bio

Sorb

FX

2.

0 m

mX

XX

XS

SS

SS

SN

SS

Inio

n C

PS

2.0

mm

SX

XX

XS

SS

SS

NS

S

Inio

n C

PS

2.5

mm

SS

XX

XX

NS

SS

SN

SS

Lac

toSo

rb

2.0

mm

SN

SS

XX

XX

SS

SN

SS

Mac

roP

ore

2.0

mm

SS

SS

XX

XX

SN

SN

SS

Pol

ymax

2.

0 m

mN

SS

SS

SX

XX

XS

NS

S

Res

orb

X

2.1

mm

SS

SS

SS

XX

XX

NS

S

Tita

nium

1.

5 m

mS

SS

SN

SS

SX

XX

XS

Tita

nium

2.

0 m

mS

SS

SS

SS

SX

XX

X

Un

der

line

= S

ide

ben

din

g st

iffn

ess

Ital

ic =

To

rsio

n s

tiff

ne

ssS

= S

ign

ifica

ntN

S =

No

n Si

gn

ifica

nt

Degradability

Degradable

Non degradable

Method: Stiffness Side Bending Test

System

BioSorb FX 2.0 mm

4.00

2.00

0.0

Macropore 2.0 m

m

Polymax 2.0 m

m

Resorb X 2.0 mm

Titanium 1.5 m

m

Titanium 2.0 m

m

Inion CPS 2.5 mm

Inion CPS 2.5 mm

LactoSorb 2.0 mm

Mea

n S

tiff

ne

s (N

/mm

)

System

Degradability

Degradable

Non degradable

Mean: Stiffness Torsion Test

BioSorb FX 2.0 mm

4.00

2.00

0.0

Macropore 2.0 m

m

Polymax 2.0 m

m

Resorb X 2.0 mm

Titanium 1.5 m

m

Titanium 2.0 m

m

Inion CPS 2.0 mm

Inion CPS 2.5 mm

LactoSorb 2.0 mm

Mea

n S

tiff

ne

s (N

/mm

)

60 61

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Table VI. Summary of descriptive statistics torsion and bending test

Side bending stiffness



BioSorb FX 2.0 mm 1.55 0.13 1.28 1.81

Inion CPS 2.0 mm 0.57 0.06 0.31 0.84

Inion CPS 2.5 mm 0.82 0.08 0.55 1.08

LactoSorb 2.0 mm 0.75 0.06 0.48 1.01

MacroPore 2.0 mm 0.24 0.02 -.03 0.50

Polymax 2.0 mm 0.37 0.04 0.11 0.64

Resorb X 2.1 mm 0.25 0.03 -0.02 0.52

Titanium 1.5 mm 1.64 0.81 1.37 1.90

Titanium 2.0 mm 4.33 0.50 4.07 4.60

Torsion stiffness



BioSorb FX 2.0 mm 0.96 0.10 0.80 1.12

Inion CPS 2.0 mm 0.67 0.05 0.52 0.84

Inion CPS 2.5 mm 2.36 0.12 2.20 2.53

LactoSorb 2.0 mm 0.56 0.04 0.40 0.73

MacroPore 2.0 mm 1.27 0.14 1.10 1.43

Polymax 2.0 mm 0.86 0.08 0.70 1.02

Resorb X 2.1 mm 0.32 0.04 0.16 0.48

Titanium 1.5 mm 1.34 0.08 1.18 1.50

Titanium 2.0 mm 4.17 0.54 4.00 4.33

*in N/mmSD = Standard Deviation

The mean side bending stiffness of the 9 osteofixation systems is plotted in figure 6. The

2.0 mm titanium system revealed significantly higher side bending stiffness compared

to the other 8 systems. The 1.5 mm titanium and the BioSorb FX system presented a

nearly similar mean side bending stiffness. The side bending stiffness of the BioSorb FX

system was significantly higher compared to the other 6 biodegradable systems, whereas

significance was not reached for the 1.5 mm titanium system mainly because of the large

standard deviation of the mean of the 1.5 mm titanium system (see table IV). The non-

significant results were additionally illustrated by the 95% confidence interval of the 1.5

mm titanium system which overlaps the interval of the BioSorb FX system. The standard

deviations of the biodegradable systems were small, while the 2.0 mm titanium system

revealed a higher standard deviation too (in table VI).

The mean torsion stiffness of the 9 osteofixation systems is graphically plotted in figure 7.

As presented with the side bending stiffness, the torsion stiffness of the 2.0 mm titanium

system was significantly higher compared to the remaining systems. The standard devia-

tions of the biodegradable and 1.5 mm titanium systems were small, particularly com-

pared to the standard deviation of the 2.0 mm titanium system. The mean torsion stiff-

ness for the 1.5 mm titanium and 2.0 mm MacroPore system were nearly equal revealing

non significance between these two systems. The Inion CPS 2.5 mm system presented

by far the highest torsion stiffness of the biodegradable systems. Comparisons of the dif-

ferences between the 9 osteofixation systems are outlined in table IV. Table VI presents a

summary of the descriptive statistics of the side bending and torsion tests.

DISCUSSION

The differences in strength and stiffness can be explained by many different factors, in-

cluding dimension (1.5 mm, 2.0 mm, 2.1 mm, and 2.5 mm), (co-polymer) compositions,

geometry of the plates and screws, ageing of the plates and screws, and methods to

sterilize and manufacture the plates and screws. Due to the fact that the differences be-

tween the osteofixation systems are multi-factorial, it remains difficult to pose (a) specific

reason(s).

The maxillofacial muscles exert high forces in different directions (7). Consequently, it is

difficult to simulate the in situ conditions in in vitro situations. To obtain clinical valuable

information regarding the selection of an osteofixation system, the tensile strength and

stiffness, side bending stiffness, and torsion stiffness were investigated as mentioned

above. Adequate tensile strength and stiffness of an osteofixation system is essential for

fixation of fractures and osteotomies. The osteofixation system is inevitably exposed to

tensile forces when adequately repositioned bone segments are exposed to local deform-

ing forces (22;23;44;131). The side bending test has been performed in order to simulate

the bi-lateral sagittal split osteotomies (BSSO) of the mandible (132). The BSSO procedure

is often performed in oral and maxillofacial surgery (35). The torsion test was used to

simulate the torsion forces that are developed in the area between the two canine teeth

when a median fracture of the mandible is present. These torsion forces, however, are

predominantly counteracted by the interfragmentary fracture segments (133). A second

argument to subject the osteofixation system to the torsion test, is that torsion forces are

extraordinary destructive for osteofixation systems. During torsion of the PMMA blocks,

they were prevented to move along the long axis of the system in order to additionally

load the system to tensile forces. This simulates the most unfavourable in situ situation

imaginable. Another important aspect of simulating the in situ situation was to test the

system as it is used and applied in the clinic. The plates and screws were fixed with

prescribed burs and taps. Fixing the plates with corresponding screws will provide more

62 63

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the biodegradable systems, whereas the differences between the biodegradable systems

also revealed significance in most cases with regard to tensile strength as well as stiffness.

Moreover, it showed that the side bending stiffness of the titanium 2.0 mm was signifi-

cantly higher than the 8 remaining systems. The BioSorb FX revealed high side bending

stiffness too in comparison to the other biodegradable systems, with both Resorb X and

MacroPore at the lower side. Finally, this study has shown that the torsion stiffness of the

titanium 2.0 mm system was high compared to the other systems. Based on the results

of the current study, it can be concluded the BioSorb FX, Inion CPS 2.5 and LactoSorb

systems represent the highest strength and stiffness’s amongst the investigated biode-

gradable osteofixation systems. With the cross-sectional surface taken into account, the

BioSorb FX system (with its subtle design), proves to be the far more strong and stiff sys-

tem. The Resorb X and MacroPore systems are, at least from a mechanical point of view,

the least strong and stiff systems in the test.

Acknowledgements

The gratuitously supply of titanium as well as biodegradable plates and screws through

the manufacturers (Linvatec Biomaterials Ltd., Gebrüder Martin GmbH & Co., Inion Ltd.,

Walter Lorenz Surgical Inc., Mathys Medical Ltd., and MacroPore BioSurgery Inc.) was

gratefully appreciated. The authors also would like to thank dr. H. Groen for his statistical

assistance. Mr. J. de Jonge is acknowledged for the fabrication of the test set-ups.

clinical relevant information rather than fix the plates with metal screws (122). In this way,

information on the entire system’s (device) mechanical characteristics was obtained.

The stiffness was calculated in all three tests (tensile, side bending, and torsion), while the

strength is reported in just one case (tensile test). The stiffness of an osteofixation system

is a more clinically applicable characteristic (134). Contrary to the stiffness, the maximum

strength will ‘only’ become relevant when the bone segments are separated more than a

few millimeters which inherently results in compromised bone healing. Enlargement of the

healing period is the result, and loosening of the screws and plates, or infection is possible

(134). The stiffness was calculated from the raw data as described in the materials and meth-

ods section. Determining the 25% Fmax and 75% Fmax point as well as the corresponding

displacement implies loss of accuracy due to the limited sample frequency (500 Hz.). This

results in higher relative standard deviations when comparing the tensile strength.

The small standard deviations regarding the tensile strength (predominantly the titanium

systems), elucidate that the method of testing and the test hardware were properly de-

signed regarding reproducibility. The high standard deviations concerning the stiffness of

the titanium systems, however, in both the torsion (titanium 2.0 mm) and side bending

(titanium 1.5 en 2.0 mm) tests, did not support that obviously the assumption of proper

method and hardware design. The explanation for these phenomena could be the meas-

urement imprecision mentioned above or the variety in mechanical properties of the

specimens of each system.

Conspicuous are the torsion and side bending stiffness of the 1.5 mm titanium system

and 4 (BioSorb FX, Inion CPS 2.0, Inion CPS 2.5, and LactoSorb) of the biodegradable

systems which were nearly in the same range of stiffness. This is most probably a result

of the smaller dimensions of the 1.5 mm titanium system. Table IV reveals significant dif-

ferences between the side bending stiffness of the biodegradable systems (caused by the

small standard deviations) while the differences between the 1.5 mm titanium and the

biodegradable systems were non significant.

Titanium osteofixation systems were (significantly) stronger and stiffer than biodegrad-

able systems. Despite the favourable mechanical properties of these systems compared

to the biodegradable systems, the question arises whether the biodegradable systems

pose adequate resistance to the local deforming forces in order to achieve adequate

bone healing in patients (83). After all, the disappearance of a fixation system when

bone union of the bone segments has been obtained, is still very appealing. The question

mentioned above, can only be answered through well-designed randomized clinical trials

which compare biodegradable and titanium osteofixation systems. The present study,

however, provides well-funded information to help surgeons to select a mechanically po-

tent bone fixation system for restoring, fixing, and stabilizing bone segments in specific

situations in the maxillofacial area. The objective of this study was to present relevant

mechanical data in order to simplify the selection of an osteofixation system for situations

requiring immobilization in oral and maxillofacial surgery. This study has presented that

the tensile strength and stiffness of both titanium systems were significantly higher than

CHAPTER 3.2.2

MECHANICAL STRENGTH

AND STIFFNESS OF THE

BIODEGRADABLE SONICWELD

RX OSTEOFIXATION

SYSTEM

G.J. BUIJS

E.B. VAN DER HOUWEN

B. STEGENGA

R.R.M. BOS

G.J. VERKERKE

Published in: J Oral Maxillofac Surg. 2009 Apr;67(4):782-7.

66 67

Abstract:

Objective - To determine the mechanical strength and stiffness of the new 2.1 mm biode-

gradable ultra-sound activated SonicWeld Rx (Gebrüder Martin GmbH & Co., Tuttlingen,

Germany) osteofixation system in comparison with the conventional 2.1 mm biodegrada-

ble Resorb X (Gebrüder Martin GmbH & Co., Tuttlingen, Germany) osteofixation system.

Materials & Methods - The plates and screws were fixed to 2 polymethylmethacrylate

(PMMA) blocks to simulate bone segments and were subjected to tensile, side bending,

and torsion tests. During testing, force and displacement were recorded and graphically

presented in force-displacement diagrams. For the tensile tests, the strength of the oste-

ofixation system was measured. The stiffness was calculated for the tensile, side bending,

and torsion tests.

Results - The tensile strength and stiffness as well as the side bending stiffness of the

SonicWeld Rx system presented up to 11.5 times higher mean values than the conven-

tional Resorb X system. The torsion stiffness of both systems presents similar mean values

and standard deviations.

Conclusion & discussion - The SonicWeld Rx system is an improvement in the search

for a mechanically strong and stiff as well as a biodegradable osteofixation system. Future

research should be done in order to find out whether the promising in vitro results can be

transferred to the in situ clinical situation.

Key words: plate; screw; biodegradable; titanium; mechanical; strength; stiffness; prop-

erties; SonicWeld Rx.

Abbreviations: PMMA, PolyMethylMethAcrylate; SPSS, Statistical Package of Social Sci-

ences; BSSO, Bi-lateral Sagittal Split Osteotomy;

INTRODUCTION

Background

Biodegradable plates and screws are used increasingly in today’s oral and maxillofacial

practice. These biodegradable plates and screws have several advantages over conventional

titanium plates and screws. There is (1) no need for a second intervention to remove

the devices (46-48), (2) no interference with imaging or radio-therapeutic techniques

(37;41;127), (3) no possible growth disturbance or mutagenic effects (37;41;43-45), (4) no

potential brain damage (44;128), (5) and no thermal sensitivity (129). However, the use

of biodegradable plates and screws also has introduced several disadvantages. First, the

boreholes need to be tapped before the screws can be inserted which is time-consuming.

A second disadvantage could be that the biodegradable plates and screws represent

inferior mechanical strength and stiffness compared with conventional titanium plates and

screws (135). In order to resolve these disadvantages, a new biodegradable osteofixation

system, SonicWeld Rx, has been developed. In contrast to conventional biodegradable

osteofixation systems, tapping of the cortical bone layer is not necessary before inserting

the SonicWeld Rx biodegradable pins. A biodegradable pin is simply placed onto an ultra-

sound activated sonic electrode, called a sonotrode, and inserted into the borehole. As

a result of the added ultra-sound energy, the thermoplastic biodegradable pin will melt,

resulting in a flow of biodegradable polymers into the cortical bone layer and the cavities

of the cancellous bone. There is no cellular reaction due to thermal stress during insertion

(136). At the same time the biodegradable plate and pinhead fuse. Theoretically, the

fusion of plate and pinhead will result into superior mechanical device characteristics

in comparison with conventional biodegradable osteofixation systems. This has been

claimed as a second advantage.

The mechanical strength and stiffness of 7 biodegradable as well as 2 titanium

osteofixation systems have recently been investigated (135). One of these investigated

biodegradable systems is the Resorb X biodegradable osteofixation system. The

SonicWeld Rx and the Resorb X biodegradable osteofixation systems are made of the

same co-polymer compositions and have the same device dimensions. These systems are

supplied by the same manufacturer (Gebrüder Martin GmbH & Co. (Tuttlingen, Germany

)). The question arises to what extent the biodegradable ultra-sound activated SonicWeld

Rx osteofixation system presents superior mechanical strength and stiffness as compared

with the conventional biodegradable Resorb X osteofixation system.

Objectives

The objective of this study was to determine the mechanical strength and stiffness of the

biodegradable ultra-sound activated SonicWeld Rx osteofixation system in comparison

with the conventional biodegradable Resorb X osteofixation system.

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68 69


The specimens to be investigated were 2 commercially available biodegradable

osteofixation systems (i.e. 2.1 mm Resorb X and 2.1 mm ultra-sound activated SonicWeld

Rx). All the specimens consisted of biodegradable amorphous poly-(50%D, 50%L) -

Lactide. The plates under investigation were 4-hole extended plates. The manufacturer

(Gebrüder Martin GmbH & Co., Tuttlingen, Germany) supplied sterile implants. The

general characteristics of the included plates and screws are summarized in table II.

Eighteen plates and 72 screws/pins of each system were available to perform three

different mechanical tests. The osteofixation plates and screws were fixed in 2 different

ways to 2 polymethylmethacrylate (PMMA) blocks (with polished surface) that simulated

bone segments. For the Resorb X osteofixation system, the screws were inserted in both

PMMA blocks according to the prescriptions of the manufacturer (using prescribed burs

and taps). The applied torque for inserting the screws was measured to check whether

it was comparable to the clinically applied torque (‘hand tight’) defined in a previous

study (130). For the SonicWeld Rx system, the biodegradable pins were inserted into

the boreholes (after the use of prescribed burs) with the sonotrode. The biodegradable

polymers melted due to the ultra-sound vibrations of the sonotrode. Subsequently,

the biodegradable material flowed into the borehole and the pinhead fused with the

biodegradable plate. In both situations, the boreholes were irrigated with saline before

insertion of the screws/pins to simulate the in situ lubrication.

The two PMMA blocks, linked by the osteofixation device (1 plate and 4 screws/pins)

were stored in a water tank containing water of 37.2 degrees Celsius for 24 hours to

simulate the relaxation of biodegradable screws/pins at body temperature (111). The tests

were performed in another tank containing water at the same temperature to simulate

physiological conditions. The use of saline was omitted because of the associated

corrosion problems of the test set-up. Omitting the use of saline was expected not to be

of influence to the test results.

The plates and screws/pins were subjected to tensile, side bending, and torsion tests. The

tensile test was performed as a standard loading test (figure 1). Side bending tests were

performed to simulate an in vivo bi-lateral sagittal split osteotomy (BSSO) situation (figure

2). Torsion tests were performed to subject the osteofixation devices to high torque in

order to simulate the most unfavourable situation (figure 3). The 2 PMMA blocks, linked

by the osteofixation device, were mounted in a test machine (Zwick/Roell TC-FR2, 5TS.

D09, 2.5kN test machine. Force accuracy 0.2%, positioning accuracy 0.0001mm; Zwick/

Roell Nederland, Venlo, The Netherlands). Regarding the tensile tests, the 2 PMMA blocks,

and thus the osteofixation plate, were subjected to a tensile force with a constant speed

of 5 mm/min until fracture occurred (according to the standard ASTM D638M). For the

side bending test the 2 PMMA blocks were supported at their ends whereas the plates

were loaded in the centre of the construction with a constant speed of 30 mm/min (with

this speed the outer fibers were loaded as fast as the fibers of the osteofixation system in

Figure 1. Tensile test set-up

Figure 2. Side bending test set-up

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70 71

the tensile test) until the plate was bended 30 degrees. For the torsion test the 2 PMMA

blocks were rotated along the long axis of the osteofixation system with a constant speed

of 90 degrees/min (with this speed the outer fibers were loaded as fast as the fibers of the

osteofixation system in the tensile test) until the plate was turned 160 degrees.

During testing the applied force was monitored by the load cell of the test machine.

Both force and displacement were recorded with a sample frequency of 500 hertz and

graphically presented in force-displacement diagrams. During tensile tests, the strength

of the osteofixation system was measured. The stiffness was calculated for the tensile,

side bending and torsion tests by determining the slope of the curve between 25% and

75% of Fmax on the force-displacement curves.


Statistical Package of Social Sciences (SPSS, version 14.0) was used to analyze the data.

Means and standard deviations were calculated to describe the data. To determine whether

there were significant differences between the 2 biodegradable osteofixation systems

in (1) tensile strength and stiffness, (2) side bending stiffness, and (3) torsion stiffness,

the maximum values were subjected to Independent-Samples T-Tests. Differences were

considered to be statistically significant when p < 0.05 for all tests.

Figure 3. Torsion test set-up RESULTS

The mean tensile strength and stiffness of the Resorb X as well as the SonicWeld Rx

biodegradable osteofixation systems are graphically presented in figures 4 and 5,

respectively. Tensile strength and stiffness of the SonicWeld Rx system were significantly

higher than those of the Resorb X system. The tensile strength of the SonicWeld Rx system

was approximately 2 times the tensile strength of the Resorb X system, while the tensile

stiffness of the SonicWeld Rx system was about 11.5 time that of the Resorb X system.

The significant differences between the 2 systems are outlined in table III. The standard

deviations for the systems regarding the tensile strength and stiffness were small.

The mean side bending stiffness of the 2 biodegradable osteofixation systems is plotted

in figure 6. The SonicWeld Rx system revealed significantly higher side bending stiffness

than with the Resorb X system. The standard deviations of the 2 systems were small (table

I). The significant results were additionally illustrated by the 95% confidence interval of

the difference, which did not include zero.

There was no significant difference between the mean torsion stiffness of the SonicWeld

Rx and the Resorb X osteofixation system (Table III), as is graphically displayed in figure

7. Table I presents a summary of the descriptive statistics of the tensile strength and

stiffness, side bending stiffness as well as torsion stiffness.

Regarding the side bending test, no fracture at all of neither the plate nor the screws/

pins has been observed for both systems. For the tensile as well as the torsion test, shear

of the screw-heads was observed regarding the Resorb X system whereas fracture of the

plates was observed regarding the SonicWeld Rx system.

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Table I. Summary of descriptive statistics tensile, side bending and torsion test

System Mean* SD*

Tensile strength

Resorb X 2.1 mm 59.87 4.73

SonicWeld Rx 114.55 8.69

Tensile stiffness

Resorb X 2.1 mm 42.86 5.82


Side Bending stiffness

Resorb X 2.1 mm 0.25 0.03


Torsion stiffness

Resorb X 2.1 mm 0.32 0.04


*in N/mmSD = Standard Deviation

72 73

Figure 6. Mean side bending stiffness organized by system

Legend:X-axis = brand names of the investigated osteofixation systemsY-axis = mean stiffness in Newton/mm (deducted unit)Points in figure: represents mean stiffnessBars: represents the standard deviation of the mean stiffness

Figure 7. Mean torsion stiffness organized by system

Legend:X-axis = brand names of the investigated osteofixation systemsY-axis = mean stiffness in Newton/mm (deducted unit)Points in figure: represents mean stiffnessBars: represents the standard deviation of the mean stiffness.

Figure 4. Mean tensile strength organized by system

Legend:X-axis = brand names of the investigated osteofixation systemsY-axis = mean strength in Newton’sPoints in figure: represents mean strengthBars: represents the standard deviation of the mean strength

Figure 5. Mean tensile stiffness organized by system

Legend:X-axis = brand names of the investigated osteofixation systemsY-axis = mean stiffness in Newton/mmPoints in figure: represents mean stiffnessBars: represents the standard deviation of the mean stiffness

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System

120,00

100,00

80,00

60,00

Resorb X 2.1 mm SonicWeld Rx 2.1 mm

Mea

n S

tren

gth

(N

)Test: Tensile Strength

System

600,00

500,00

400,00

300,00

200,00

100,00

0,00

Resorb X 2.1 mm SonicWeld Rx 2.1 mm

Mea

n S

tiff

ne

s (N

/mm

)

Test: Tensile Stiffness

System

1,20

1,00

0,80

0,60

0,40

0,20

SonicWeld Rx 2.1 mm

Mea

n S

tiff

ne

s (N

/mm

)

Test: Side Bending Stiffness

Resorb X 2.1 mm

System

0,38

0,36

0,34

0,32

0,30

0,28

SonicWeld Rx 2.1 mm

Mea

n S

tiff

ne

s (N

/mm

)

Test: Torsion Stiffness

Resorb X 2.1 mm

74 75

Tab

le I

II.

Com

par

ison

bet

wee

n os

teofi

xatio

n sy

stem

s

Syst

ems

Test

Pro

per

ty95

% C

on

fid

ence

In

terv

alLo

wer

Bo

un

dU

pp

er B

ou

nd

Res

orb

X 2

.1 m

m v

s. So

nicW

eld

Rx

2.1

mm

*Te

nsile

Stre

ngth

45.3

16

4.0

5

Res

orb

X 2

.1 m

m v

s. So

nicW

eld

Rx

2.1

mm

*Te

nsile

Stif

fnes

s42

0.5

44

83.

20

Res

orb

X 2

.1 m

m v

s. So

nicW

eld

Rx

2.1

mm

*Si

de

Ben

ding

Stif

fnes

s0.

760.

95

Res

orb

X 2

.1 m

m v

s. So

nicW

eld

Rx

2.1

mm

Tors

ion

Stif

fnes

s-0

.06

0.0

5

* =

Sig

nifi

can

t

Tab

le I

I. C

hara

cter

istic

s of

incl

uded

bio

degr

adab

le o

steo

fixat

ion

syst

ems

Bra

nd

nam

eM

anu

fact

ure

r (c

ity

and

sta

te)

Co

mp

osi

tio

nSt

eril

ity

Scre

w/p

inD

iam

eter

*Sc

rew

/pin

Len

gth

*Pl

ate

Len

gth

*Pl

ate

Wid

th*

Plat

eT

hic

kne

ss*

Res

orb

XG

ebrü

der

Mar

tin

Gm

bH

& C

o. (

Tutt

ling

en, G

erm

any

)

100

D(5

0%)L

(50%

)

-Lac

tid

eSt

erile

2.1

mm

7.0

mm

26.0

mm

6.0

mm

1.1

mm

Soni

cWel

d R

xG

ebrü

der

Mar

tin

Gm

bH

& C

o. (

Tutt

ling

en, G

erm

any

)

100

D(5

0%)L

(50%

)

-Lac

tid

eSt

erile

2.1

mm

7.0

mm

26.0

mm

6.0

mm

1.1

mm

* =

acc

ord

ing

th

e sp

ecifi

cati

on

s o

f th

e m

anu

fact

ure

rs.

DISCUSSION

The differences in strength and stiffness between the SonicWeld Rx and the Resorb X

biodegradable osteofixation systems can partly be explained by the difference in geometry

of the screws and pins, but predominantly by the 2 different methods of application.

Using a sonotrode to bring the plate and pin in a thermoplastic state fusing the plate and

pin, results in a firm and stable fixation. The tensile strength and stiffness as well as the

side bending stiffness of the SonicWeld Rx system presented significantly higher mean

values compared with the conventional Resorb X system (table II). In contrast, the torsion

stiffness of both systems presents remarkably similar means and standard deviations. The

torsion test was used to simulate the torsion forces that exist in the area between the

two canine teeth when a median fracture of the mandible is present. In various clinical

cases however, these torsion forces are neutralized by the interdigitation of the fracture

segments (133). The torsion forces exerted on the fixation devices are subsequently

transferred to tensile forces in these cases.

The biodegradable polymers used to manufacture the SonicWeld Rx plates and pins are

melted through an ultra-sound activated sonotrode resulting in a fusion of the plate

and screwhead/pinhead. As mentioned before, fusion results in a firm and stable device

especially where shear strength and stiffness of the device are concerned. This is supported

by the authors’ experience that in all test samples of the SonicWeld Rx system for both the

tensile and side bending test, fracture of the plate occurred away from the pin, and not

near the pin or of the pin or pin-head itself. Regarding the conventionally screwed Resorb

X system, the authors experienced shear of the screw-heads in all test samples. These in

vitro observations support the hypothesis that the principle of fusion of the plate and the

pinheads results in better mechanical biodegradable device strength and stiffness. For

orthopaedic and maxillofacial metallic plates and screws, this principle is well-known as

locking plates. These locking plates present increased in vitro strength and stiffness of the

device characteristics (137-139) as well as good clinical performances (137).

As described in the Materials & Methods section, the Resorb X screws were applied with a

specific torque defined in a previous study (130), resulting in a pressure of the plates to the

PMMA blocks. For the SonicWeld RX pins this pressure was not specified; the pins were

applied as the surgeon would do in clinical practice. This difference could theoretically

confound the test results of especially the SonicWeld RX system. When looking to the test

results, however, the authors conclude that the lack of pressure of the plates to the PMMA

blocks for the SonicWeld RX system could not confound the test results, since, after all,

fracture of the plates (instead of shear of the screws) occurred in all specimens.

The use of PMMA instead of real bone was a conscious decision of the authors. Real bone

could have different calcification levels which could result in different fracture patterns of

the plates and screws. Subsequently, this could influence the results. PMMA blocks have

the same mechanical characteristics as real bone and each block does have the same

‘quality’ level. Moreover, the difference between cancellous/cortical bone and PMMA

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76 77

was not a major concern. Theoretically, the flow of polymers of the ultra-sound activated

SonicWeld Rx pin into the cavities of the cancellous bone would enhance the pull out

strength of the screws. However, none of the screws were pulled out during testing.

Regarding the thermoplastic state of the biodegradable pin, we were concerned about the

fusion or sticking of the biodegradable pin to the PMMA blocks. This could theoretically

affect the test results. To prevent this, the boreholes were irrigated with saline before

insertion of the pins. To check whether fusion or sticking had occurred, we checked

whether the pin could be pulled out the PMMA blocks after the test. Despite not actually

measuring the pull out strength of the pins, the authors noted that high forces were not

required to do so.

The SonicWeld Rx system is obviously an improvement in the search for a mechanically

strong and stiff as well as a biodegradable osteofixation system. Moreover, usage of the

device is relatively easy and comfortable. The application of SonicWeld Rx plates and

pins is fast and easy. Nevertheless, the plates and screws are still bulky compared to the

conventional titanium plates and screws. The question, though, is whether the promising

in vitro results can be transferred to the in situ clinical situation. Future research about

biodegradable osteofixation devices should therefore include the SonicWeld Rx system

in randomized clinical trials in which a conventional titanium fixation device serves as the

´golden´ standard fixation device.

Acknowledgements

The gratuitously supply of the biodegradable plates and screws/pins through the

manufacturer (Gebrüder Martin GmbH & Co.) was gratefully appreciated. The authors

also would like to thank dr. H. Groen for his statistical assistance. Mr. J. de Jonge is

acknowledged for the fabrication of the test set-ups.

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