Biocompatibility of Polyethylene Terephthalate Knee Ligament

Biomaterials 19 (1998) 189 — 196

Biocompatibility of polyethylene terephthalate (Trevira' hochfest)
augmentation device in repair of the anterior cruciate ligament
Helmut Seitz!,*, Stefan Marlovits!, Ilse Schwendenwein", Elisabeth Mu¨ller!, Vilmos Ve´csei!
! Department of Traumatology, Vienna University School of Medicine, Waehringer Guertel 18-20, A-1090 Vienna, Austria
" Department of Biomedical Research, Vienna University School of Medicine, Waehringer Guertel 18-20, A-1090 Vienna, Austria
Received 10 March 1997; accepted 10 October 1997

Abstract
The biocompatibility of a 3 mm band made of polyethylene terephthalate (Trevira' hochfest) has been tested in an experimental
study within right knee joints of 60 sheep. After transsecting the anterior cruciate ligament (ACL), two randomized groups were
formed. In group I, the ACL was repaired according to the Marshall technique whilst in group II an additional 3 mm polyethylene
terephthalate (PET) augmentation band was implanted using the through-the-condyle (TTC) procedure. To assess the biocompatibility of the augmentation device the knee joints of both groups were punctured and the synovial fluids were analyzed before, as well as
2, 6, 16, 26, and 52 weeks after the operation. In addition, the histologic appearance of excised suprapatellar pouches and ipsilateral
inguinal and popliteal lymphatic noduli were examined. Comparing both groups no significant differences were found neither before
nor after the augmented and non-augmented ACL repair. No pathological increase in the total protein concentration occurred after
operation and no significant differences versus the preoperative analysis were found. No synovitis signalling a decrease in the glucose
concentration was observed. The cytological examination revealed no increase of the leukocyte cell count results. Within the synovial
specimen neither free nor phagocytosed PET wearparticles could be detected. In groups I and II the histological appearance of excised
popliteal and inguinal lymphatic noduli showed a normal result. In 25% of the PET augmented ACL repairs, a slight concentration of
PET wearparticles and solitary, multinuclear giant foreign body cells could be seen in the histological preparations of suprapatellar
pouches. ( 1998 Published by Elsevier Science Ltd. All rights reserved.
Keywords: Polyethylene terephthalate augmentation device; Anterior cruciate ligament repair; Synovial fluid; Leukocytes

1. Introduction
The anterior cruciate ligament (ACL) together with the
posterior cruciate ligament forms the central pivot of
knee joint kinematics and is indispersable for the exact
knee joint movement [1]. An ACL rupture leads to
severe disintegration of the roll-glide mechanism of the
knee joint. Possible pathologic consequences of impaired
joint kinematics due to instability as secondary lesions of
the menisci and premature degeneration of the cartilage
ensue a general deterioration of the knee joint function
[2, 3]. Thus, the restitution of knee joint stability is
absolutely essential in order to achieve satisfactory longterm results [4—6].

* Corresponding author. Tel.: #43-1-40400-5619; fax: #431 40400-5639; e-mail: [email protected].

For years the question of a golden standard for treating an acute ACL rupture has been discussed controversially [7]. The spectrum of different therapy concepts
ranges from a mere functional therapy through muscle
strenghthening [8], extraarticular procedures [9], suture
techniques [10], to primary ACL replacement by autogenic [11, 12], allogenic [13], xenogenic [14] and synthetic [15] materials. In order to achieve permanent
stability of the knee and make early accelerated postoperative rehabilitation possible synthetic materials have
been used for many years to augment repaired cruciate
ligaments or autogenic grafts [16—19]. The advantages of
synthetic ligaments are their unlimited availability and
immediate possibility of loading capacity and postoperative rehabilitation, plus the fact that the risks and
side-effects that go along with using autogenic grafts no
longer become an issue [15, 20—23]. In vitro and in vivo,
the PET band showed good mechanical and biological

0142-9612/98/$19.00 ( 1998 Published by Elsevier Science Ltd. All rights reserved.
PII S 0 1 4 2 - 9 6 1 2 ( 9 7 ) 0 0 2 0 1 - 9

190

H. Seitz et al. / Biomaterials 19 (1998) 189—196

characteristics under steady stress [24, 25]. Thus, considering the physicochemical, biomechanical, biological
and technical demands that have to be met by a device
used in reconstructive knee surgery the PET band seems
to be the best choice [6, 26, 27]. The overstress-shielding
device can be implanted as an augmentation band using
either the through-the-condyle (TTC) technique parallel
to the repaired ACL [28, 29] or divergent to it by the
over-the-top (OTT) procedure [6, 30]. For several years
now both techniques have been topics of numerous
papers in the field of experimental biomechanics as well
as of clinical studies [31—36].
Generally, after implantation of a foreign material the
organism reacts with an inflammatory infiltration of the
foreign body type [37, 38]. In the knee joint, correspondingly, this inflammatory reaction caused by wearparticles
of a synthetic band leads to characteristic changes in the
synovial fluid [19, 27, 37]. The synovia is a protein-deficient, clear, thixotrophic plasmadialysate which alters its
compounds when particles of foreign bodies are present.
Therefore, biochemical and cytological analysis allows to
draw conclusions concerning inflammatory processes
due to a foreign body reaction [27].
In the experimental knee surgery the sheep seems to be
a very appropriate model [20, 39]. Compared to other
species the sheep with its anatomic properties comes
closest to human proportions. Furthermore, the intraand periarticular structures resemble very much those of
human knee joints [40]. So the dimensions of knee joints
in fully grown sheep make the insertion of implants used
in human orthopaedic surgery possible [41, 42].
The purpose of this study on sheep knee joints was
to evaluate the biocompatibility features of the PET
band.

2. Material and methods
2.1. Experimental animals
After the approval of the study design by the Austrian
Ministry of Science and Research (GZ 66.009/185-I/A/
2/93) 60 non-pregnant mountain sheep about 2 yrs old
and weighing approximately 55.3$5.2 kg were selected.
Right before the operation a randomized assignment of
30 sheep for femoral ACL repair (group I) and 30 sheep
for TTC-PET-augmented femoral ACL repair (group II),
respectively, took place.
2.2. Augmentation device
The 3 mm PET band made of polyethylene terephthalate is registered under the brand name Trevira' hochfest
type 730 (Hoechst AG, Frankfurt/Main, Germany). It is
250 mm long, 3.7 mm broad and has a thickness of 1 mm,
woven with 12 longitudinal threads (warp) and four

threads in a cross-direction (weft). One single thread
consists of 200 filaments each 23 lm in diameter.
2.3. Surgical technique
Under general anaesthesia, in a supine position, the
unilateral right side operation of the ACL was performed
under sterile conditions. After midline skin incision, a
medial parapatellar arthrotomy without the need for
lateral dislocation of the patella was carried out. The
ACL was exposed and finally transsected with a retrograde knife (Arthrex, Naples, FL, USA) at its femoral
origin under visual control. After that the resulting
anterior instability of the extended knee joint was tested
in comparison to the opposite limb according to the
Lachman procedure [43].
2.3.1. Group I: femoral ACL repair
After performing a holding lateral surpracondylar
approach to the distal femur 3 cm in longitudinal diameter and putting the knee in a 130° flexed position two
2 mm canals were drilled, one in the anterior region of
the femoral ACL attachment (inside-out) the other
through the medial tibial condyle (outside-in) using
a drill guide (Acufex, Mansfield, MA, USA). The tip of
the tibial K-wire finally was placed directly behind the
medial part of the tibial ACL insertion. Then a USP 1
(metric 4) Ethibond' thread (Ethicon, Norderstedt, Germany) was put in as a testprosthesis through the tibial as
well as the femoral drill canal. The extraarticular femoral
end of the Ethibond' thread was armed with an artery
forceps. A spring-loaded strain gauge (Acufex, Mansfield,
MA, USA) for measuring the excursion of the testprosthesis (tension isometer) was introduced into the tibial
drill canal. With the strain gauge in the preload position
the tibial end of the thread was anchored to the plunger.
The spring was then engaged and the device zeroed with
the knee at 45° of flexion. Subsequently, the knee joint
was moved to the full extent (0/0/165) and the excursion
was measured at 0, 45, 90, and 165° of flexion (isometric
measurement). Maximal excursions of more than 2 mm
throughout the range of motion were not accepted. If
necessary, the position of the femoral drilled hole had to
be changed to accomodate isometric placement. After
removal of the tension isometer and the testprosthesis the
femoral transsected ACL was repaired with 4 Ethibond'
sutures USP 0 (metric 3.5), the stitches inserted from
anteromedial to posterolateral. Then the posterolateral
bundle of threads was passed through the femoral drill
canal whereas the anteromedial bundle was passed overthe-top (Fig. 1). All the corresponding ends of the threads
were tied with the knee flexed at 45° [44].
2.3.2. Group II: TTC-PET-augmented femoral AC¸ repair
The ACL was repaired by the above-mentioned
method with the Marshall sutures only being laid and

H. Seitz et al. / Biomaterials 19 (1998) 189—196

191

Fig. 1. Schematic drawing of the surgical technique used for femoral
ACL repair in group I (medial aspect after osteotomy of the medial
femoral condyle).

Fig. 2. Schematic drawing of the surgical technique used for TTCPET-augmented femoral ACL repair in group II (medial aspect after
osteotomy of the medial femoral condyle).

not knotted. After exact positioning of isometric femoral
and tibial drill canals, they were overdrilled with a holding cannulated reamer 4.5 mm in diameter. The drill
canal entry and outlet sites were carefully smoothened
with a rasp. The PET band was pulled into the joint
through the tibial drill canal, and together with the
posterolateral bundle of threads passed through the
femoral drill canal (through-the-condyle). The femoral
end of the PET band was fixed approximately 1 cm away
from the extraarticular femoral drill canal entry site with
a 2.7 mm AO-cortical screw and a washer (Mathys,
Bettlach, Switzerland) and a 4 mm staple (Telos, Hungen,
Germany). The proximal end of the PET band was
shortened to a length of 1.5 cm. After fastening a spring
balance to the tibial end of the PET band the knee joint
was fully extended. Under a preload of 60 N the augmentation band was fixed with two 4 mm staples to the
medial tibial head using the ‘buckle-belt’ technique
[45, 46]. The distal end of the PET band was also
shortened to a length of 1.5 cm. Finally, the anteromedial
bundle of the Marshall sutures was passed through the
dorsal capsule over-the-top and the corresponding
threads were tied, the knee being flexed in a 45° position
(Fig. 2).

In neither group were the knee joints immobilized after
operation. Ten days after the operation the skin sutures
were removed and all the animals were transferred to
a farm and let free.
2.4. Study times
Before implantation, as well as 2, 6, 16, 26, and 52
weeks afterwards, the synovial fluid was aspirated from
the knee joints of all the animals of either group and
analyzed.
After arthrotomy and macroscopic examination of
each knee joint, excision of the suprapatellar pouch,
ipsilateral inguinal and politeal lymphatic noduli was
performed under general anaesthesia. Specifications were
carried out in 56 cases (Table 1) as the amount of
synovial specimen within four sheep was not sufficient
for cytological analyses.
2.5. Biochemical study
The synovial fluid was examined for total protein and
glucose concentration right after aspiration using dry,

192

H. Seitz et al. / Biomaterials 19 (1998) 189—196

Table 1
Number of sheep per group and the time of follow up examination
(n"28)
Weeks after operation

2

6

16

26

52

Group I!
Group II"

6
6

5
5

5
6

6
6

6
5

! Femoral ACL repair.
" TTC-PET-augmented femoral ACL repair.

multilayered films in plastic supports (Kodak Ektachem'
DT Slides, Rochester, NY, USA) [47, 48].
2.6. Morphological study
Additionally synovia smears were made (Boehringer
Hemacolor' Rapid Staining Set, Mannheim, Germany)
and evaluated under light microscopy (original magnification]100). Also a total cell count and a differential cell
count of leukocytes were performed (High Power Field[HPF-] technique).
The suprapatellar pouch and ipsilateral inguinal and
popliteal lymphatic noduli of each animal belonging to
either group were fixed in 8% formalin after excision and
dehydrated in an alcohol series with increasing concentration, finally embedded in paraffin, and longitudinally
cut into halves. One micrometer single sections were
investigated under light microscopy using polarized light
(original magnification ]40, ]100) after being stained in
hematoxylin and eosin. Ten fields per tissue type were
examined.
2.7. Statistical study
The statistic evaluation of synovial glucose concentration results was done by the analysis of variance and covariance components (p)0.05). The results of total protein concentration was evaluated by the signed rank test.
3. Results
3.1. Macroscopic findings
All animals survived their designated periods and
walked normally before being sacrificed. Clinically, all
PET bands were tight and showed no signs of abrasion
or broken strands. Two weeks postoperatively the whole
PET augmentation devices were surrounded by connective tissue. The artificial ligaments were not incorporated
within the healing ACLs.
3.2. Total protein concentration
No pathological increase in the total protein concentration (normal range: (3.0 g dl~1) [49] occurred after

PET augmented (group II) nor after the non-augmented
femoral ACL repair (group I), and no significant differences with respect to the preoperative analysis were
found. In 13 sheep out of group I (46%) and seven out of
group II (25%), changes of the protein concentration
within the normal range were seen. In group I, eight cases
(29%) showed an increase, five cases (18%) a decrease
(Table 2), in group II, five cases (18%) showed an increase
and two cases (7%) a decrease of total protein concentration in comparison to the preoperative results
(Table 3).

3.3. Glucose concentration
At all times of follow up examination, pre- and postoperative glucose concentration in either group was
within the normal range of glucose concentration of the
synovial fluid in sheep ()40 mg dl~1) [49]. Before
the operation a mean glucose concentration of 86$
2 mg dl~1 in group I and 86.3$13.1 mg dl~1 in group II
was found. Amounting postoperatively to a concentration of 87$24 mg dl~1 in group I and 94.1$
13.9 mg dl~1 in group II, which does not indicate a significant change. In four animals (14%) belonging to
group I, and 10 animals belonging to group II (36%),
differences between pre- and postoperative results in glucose concentration within a range of 10 mg dl~1 were
noticed. More than the 10 mg dl~1 alteration was observed in three cases of group I (11%) showing a decrease, one case in group I showed an increase (Table 2),
while within group II nine cases (32%) showed an increase and in one case a decrease in comparison to the
results obtained before the operation could be registered
(Table 3).

3.4. Total and differential cell count
A leukocyte count of 0—3/HPF in the synovial fluid,
differentiated into polymorphonuclear (PM) and mononuclear (MN) leukocytes, represents a normal physiologic range [49]. However, in none of the synovia smears
of the sheep of either group, neither pre- nor postoperatively, at no time of follow up examination were more
than 3 leukocytes/HPF counted. In 14 animals (50%) of
group I and in 19 animals (68%) of group II no leukocytes at all could be detected under light microscopy
neither pre- nor postoperatively. In three animals of both
groups (11%), the leukocyte counts remained completely
the same, be it pre-, postoperatively or at any time of
follow up (Fig. 3). In nine sheep (32%) of group I and in
six sheep (21%) of group II alterations within normal
range could be seen, that is to say three animals of
group I and two animals of group II showed only preoperative changes, and in six animals of group I and four

H. Seitz et al. / Biomaterials 19 (1998) 189—196

193

Table 2
Concentration of total protein (TP) and glucose (G), total leukocyte counts and differential counts (Zyto) of the synovial fluid before (prOP) as well as
2, 6, 16, 26 and 52 weeks after (pOP) femoral ACL repair in sheep (group I)
Sheep

Follow up
(weeks)

prOPTP!
(g dl~1)

pOPTP!
(g dl~1)

prOPG"
(mg dl~1)

pOPG"
(mg dl~1)

prOPZyto#
(n HPF~1)

pOPZyto#
(n HPF~1)

2
2
2
2
2
6
6
6
6
6
26
16
52
16
52
52
26
26
52
52
26
26
16
52
26
16
16
2

3.0
(2.0
(2.0
(2.0
(2.0
2.5
(2.0
(2.0
2.8
(2.0
(2.0
3.0
(2.0
2.5
(2.0
(2.0
(2.0
(2.0
(2.0
(2.0
(2.0
(2.0
2.5
(2.0
2.2
(2.0
(2.0
(2.0

(2.0
3.0
(2.0
(2.0
(2.0
2.7
(2.0
2.8
2.7
3.0
(2.0
(2.0
2.9
2.6
(2.0
(2.0
(2.0
3.0
(2.0
(2.0
(2.0
(2.0
2.4
2.9
2.3
(2.0
(2.0
(2.0

68
65
70
45
60
90
85
110
70
60
115
51
95
75
95
90
80
125
114
95
75
105
111
104
60
88
92
108

55
60
73
43
57
95
90
81
71
68
110
135
44
73
98
95
85
133
112
100
73
110
105
108
67
84
95
111

0
0
1 PM, 2 MN
0
0
1 MN
0
0
2 PM
0
3 PM
1 PM, 2 MN
0
0
0
1 MN
0
0
0
0
0
0
2 MN
0
0
1 MN
0
0

0
0
0
0
0
1 MN
1 MN
1 PM
2 PM
1 MN
2 PM, 1 MN
0
2 MN
0
0
0
0
0
0
0
0
0
1 PM, 1 MN
1 MN
0
1 MN
0
1 PM, 1 MN

nPET2-1
nPET2-2
nPET2-3
nPET2-4
nPET2-5
nPET6-6
nPET6-7
nPET6-8
nPET6-9
nPET6-10
nPET26-11
nPET16-12
nPET52-13
nPET16-14
nPET52-15
nPET52-16
nPET26-17
nPET26-18
nPET52-19
nPET52-20
nPET26-21
nPET26-22
nPET16-23
nPET52-24
nPET26-25
nPET16-26
nPET16-27
nPET2-28

Standard values:
! )3.0 g dl~1.
" *40 mg dl~1.
# 0—3 polymorphonuclear (PM) or mononuclear (MN) leukocytes/high power field (HPF).

animals of group II, only postoperative changes were
recognizable (Table 2 and Table 3).
3.5. Wear particles
In the synovial specimens of both groups examined
after the operation, no synthetic wearparticles could be
detected, neither in a free nor in a phagocytosed
form. The histologic picture of the excised popliteal
and inguinal lymphatic noduli revealed normal results,
showing follicular lymphatic tissue with inactive
centrocytes and centroblasts at the germinative centre.
No wearparticles could be observed. Giant foreign
body cells and PET wearparticles were constantly but
only scantily detectable in the histologic appearance
of excised suprapatellar pouches of 25% of the PET
augmented ACL repairs (Fig. 4). These results could not
be seen in the knee joints of non-PET augmented animals. No partial or total rupture of any PET band was
evident.

4. Discussion
Besides the technical properties of a synthetic device its
local and systemic biocompatibility seems to be the most
important requirement for its suitability as an alloplastic
material in modern knee surgery. In principle, the organism reacts equally to every implantation of foreign material. This reaction is an inflammatory infiltration of the
foreign body type [37, 38] caused by inert, non-biodegradable material and leads to characteristic alterations in the vicinity of the implant [50]. In the knee joint
this inflammatory response can be caused by wearparticles of a synthetic band [19, 27]. When wearparticles
are present, the synovial fluid representing a proteindeficient plasmadialysate changes its compounds. Biochemical and cytological examinations of the synovial
fluid allows to draw conclusions with respect to inflammatory processes [51].
The amount of wearparticles coming into existence is
not only due to the duration and level of the mechanic

194

H. Seitz et al. / Biomaterials 19 (1998) 189—196

Table 3
Concentration of total protein (TP) and glucose (G), total leukocyte counts and differential counts (Zyto) of the synovial fluid before (prOP) as well as
2, 6, 16, 26 and 52 weeks after (pOP) TTC-PET-augmented femoral ACL repair in sheep (group II)
Sheep

PET2-1
PET2-2
PET2-3
PET2-4
PET6-5
PET6-6
PET2-7
PET6-8
PET6-9
PET6-10
PET26-11
PET52-12
PET52-13
PET26-14
PET26-15
PET16-16
PET26-17
PET52-18
PET52-19
PET26-20
PET16-21
PET16-22
PET16-23
PET16-24
PET2-25
PET16-26
PET26-27
PET52-28

Follow up
(weeks)

prOPTP!
(g dl~1)

pOPTP!
(g dl~1)

prOPG"
(mg dl~1)

pOPG"
(mg dl~1)

prOPZyto#
(n HPF~1)

pOPZyto#
(n HPF)

2
2
2
2
6
6
2
6
6
6
26
52
52
26
26
16
26
52
52
26
16
16
16
16
2
16
26
52

2.3
(2.0
(2.0
(2.0
2.8
(2.0
2.5
2.2
(2.0
(2.0
(2.0
(2.0
(2.0
(2.0
(2.0
(2.0
(2.0
(2.0
2.7
2.2
(2.0
(2.0
(2.0
(2.0
(2.0
(2.0
(2.0
(2.0

2.4
2.9
(2.0
(2.0
(2.0
(2.0
2.7
2.3
(2.0
(2.0
(2.0
(2.0
(2.0
(2.0
(2.0
(2.0
2.9
(2.0
(2.0
2.2
(2.0
(2.0
(2.0
(2.0
(2.0
(2.0
(2.0
(2.0

80
90
84
81
90
93
105
94
70
77
74
70
72
96
61
68
100
85
89
90
108
92
97
100
105
79
102
64

85
77
89
90
114
95
106
91
98
80
110
105
104
117
65
101
66
88
90
105
110
95
100
105
100
80
100
67

0
0
0
0
0
0
0
1 PM, 1 MN
0
1 PM
1 PM
0
0
0
0
0
0
1 PM, 1 MN
1 MN
0
0
0
0
0
0
0
0
0

0
0
0
0
0
0
0
1 PM, 1 MN
0
1 PM
1 PM
0
1 PM, 1 MN
0
0
0
1 MN
0
0
0
0
2 MN
0
0
1 MN
0
0
0

Standard values:
! )3.0 g dl~1.
" *40 mg dl~1.
# 0—3 polymorphonuclear (PM) or mononuclear (MN) leukocytes/high power field (HPF).

Fig. 3. Polymorphonuclear leukocyte in a synovial smear 26 weeks
after TTC-PET-augmented femoral ACL repair (group II). Light
micrograph, original magnification ]100.

Fig. 4. Multinuclear giant foreign body cells in a suprapatellar pouch
52 weeks after TTC-PET-augmented femoral ACL repair (group II).
Light micrograph, original magnification ]100.

H. Seitz et al. / Biomaterials 19 (1998) 189—196

stress but also correlates with the surgical technique
applied. Thus, absolute emphasis must be placed on the
importance of proper surgical technique, especially on
how the synthetic band is passed through the joint. The
main disadvantage of the TTC procedure is the angulation of the band at its intraarticular entrance out of the
femoral drill canal connected with intensified stress on
the synthetic band and the higher amount of wearparticles coming into existence [52].
When synovitis signalling reactions occur, the protein
concentration within the synovial fluid increases due to
enhanced permeability of the synovial membrane [49].
In this bioexperimental investigation no pathologic increase in protein concentration in any of the sheep knee
joints could be detected at any time of follow up examination. Furthermore, a decrease in glucose concentration
within the synovial fluid indicates an inflammatory
reaction as the intensive intracellular metabolism of
numerous media uses up more glucose than normal [49].
Nevertheless, a significant decrease in glucose concentration was not recognizable in any of the probes aspirated
after operation. These results were confirmed indirectly
by the leukocyte counts under light microscopy where no
increase could be seen. In addition to that in none of the
synovial specimens examined after the operation were
wearparticles present, neither in a free nor in a phagocytosed form. Although, this fact does not totally exclude
the existence of wearparticles. Eventhough an insubstantial concentration of wearparticles in the suprapatellar
pouch [53, 54] can lead to a singular foreign body cell
formation, but they cannot be detected with absolute
security. One-fourth of the suprapatellar pouches of knee
joints after PET augmented femoral ACL repair showed
a constant but slight concentration of wearparticles
chiefly responsible for the appearance of solitary giant
foreign body cells. Whereas in all the examined popliteal
and inguinal lymphatic noduli wearparticles were always
absent.
Summarizing, it can be stated that an improper surgical technique, non-isometric implantation of the synthetic augmentation device in particular as well as the
careless smoothening of drillhole sites, multiplies shearing forces and promotes the development of wearparticles and ruptures of the synthetic band. Consequently,
inflammatory infiltration of the foreign body type is very
likely to be induced and synovitis and effusion may
be derived [54, 55]. These synthetic ligaments, whose
implantation is technically incorrect, also show poor
clinical results according to knee stability [56, 57].
Whilst very good midterm results, not only regarding to
knee stability but also the clinically non-manifestation of
inflammatory infiltration of the foreign body type, can be
achieved by the PET augmented ACL repair using the
TTC procedure when isometry as well as careful smoothening of intraarticular drill hole sites are strictly adhered
to [26, 55, 56, 58].

195

Acknowledgements
We thank Telos Company for supplying the augmentation devices. This work was supported by the Austrian
Scientific Research Funds (Grant No. P09827-MED)
and by the Medical Scientific Funds of the Mayor of
Vienna.
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