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ORIGINAL ARTICLE
Flexor Tenorrhaphy Tensile Strength: Reduction
by Cyclic Loading
In Vitro and Ex Vivo Porcine Study
C. E. R. Gibbons &D. Thompson &M. J. Sandow
Received: 2 July 2008 / Accepted: 31 October 2008 / Published online: 17 December 2008
#American Association for Hand Surgery 2008
Abstract The integrity of the repair is critical to maintain
coaptation of the severed flexor tendon end until healing
has advanced sufficiently. In our hospital, we use a
modified Savage repair (four-strand Adelaide technique)
using 3–0 Ethibond (Ethicon, Somerville, NJ, USA) for
acute flexor tenorrhaphy and an active postrepair mobiliza-
tion protocol. To explain the apparent differences between
the theoretical and actual repair strength of a multistrand
repair in a single tension test and the reduced strength of a
repair subjected to cyclic loading, we compared single and
cyclical tensile loading with different suture in vitro
configurations of 3–0 Ethibond (Ethicon, Somerville, NJ,
USA; one, two, and four strands) and an ex vivo four-strand
repair of freshly divided porcine tendon to calculate the
ultimate tensile strength (UTS). Mechanical testing was
repeated 15 times with both single tensile and cyclical
loading for each suture configuration and porcine repair. In
the in vitro model, the presence of a knot in a single strand
reduced the UTS by 50%. The stiffness of a knotted strand
was substantially less than the unknotted strand but became
identical after cyclical loading. There was no statistical
significance of the UTS between single and cyclical loading
with different numbers of strands in this model. In the ex
vivo four-strand porcine repair model, there was a
significant reduction in UTS with cyclical loading, which
equated to the number of strands times the strength of the
knotted strand. This discrepancy can be explained by the
change in stiffness of the knotted strand after cyclical
loading and has important implications for previous studies
of suture tendon repair using single tensile loading where
the UTS may have been overestimated. We believe that
cyclical loading is more representative of physiological
loading after acute flexor tendon repair and should be the
testing model of choice in suture tenorrhaphy studies.
Keywords Flexor .Tenorrhaphy.Cyclic loading .Stiffness
Introduction
Maintaining approximation of the severed flexor tendon
ends is critical after repair to achieve healing and there have
been multiple techniques and extensive research to identify
the optimal tenorrhaphy method [3,5,6,13]. Successful
flexor tenorrhaphy can depend on a number of factors
including the tensile properties of suture material, integrity
of each suture grasp, the type of suture repair, and the
surgical expertise available [1,9,11,12]. On the basis of
previous research in our unit, we use a four-strand single
cross grasp suture repair (Fig. 1) modified from the Savage
technique [10], and active mobilization is started as early as
possible (either the same day or day after operation) under
the care of the physiotherapist.
While the in vitro and ex vivo testing of various suture
constructs is important to identify the optimal technique,
the differences between the theoretical and actual repair
HAND (2009) 4:113–118
DOI 10.1007/s11552-008-9151-x
M. J. Sandow
Department of Orthopaedics and Trauma,
Royal Adelaide Hospital, The University of Adelaide,
Adelaide, South Australia, Australia
D. Thompson
Department of Mechanical Engineering, University of Adelaide,
North Terrace,
Adelaide, 5000 South Australia, Australia
C. E. R. Gibbons (*)
Department of Orthopaedics and Trauma,
Chelsea and Westminster Hospital,
369 Fulham Road,
London SW10 9RH, UK
e-mail: cergibbons@talktalk.net
strength of a multistrand repair [10] and the apparent
reduced strength on a repair subjected to cyclic loading [4,
8] have not been explained by this previous work. Many
studies have investigated the mechanical properties of
suture repair with single tensile loading [1], but few have
compared single tensile and cyclical loading [8].
To investigate these discrepancies, we assessed the
mechanical performance of our preferred tenorrhaphy
technique during cyclic and static loading in an in vitro
and ex vivo situation. The conduct of this study was to
firstly analyze, in both single tension and cyclic loading
conditions, the mechanical properties of in vitro 3–0
braided polyester suture material (Ethicon, Somerville, NJ,
USA) with different strand configurations (single strand
without knot, single strand with knot, two strands with
knot, and four strands with knot) and then a four-strand ex
vivo porcine tendon repair.
Methods
The study was conducted in two parts: Firstly, an in vitro
study to test the mechanical performance of the suture
material in various configurations and secondly, an ex-vivo
study to test the performance of the multistrand repair in an
animal tendon model. A Hounsfield mechanical testing
machine (Hounsfield H25KM Universal Testing Machine,
Hounsfield Testing Equipment Ltd, Surrey, UK; Material
Testing System (MTS)) was used to measure stiffness
during cyclic loading and ultimate tensile load both for the
different suture configurations and the porcine four-strand
tendon repairs for both single tensile and cyclical loading
tests, with each particular test being repeated 15 times. A
standardized technique was used for testing both the suture
configurations and tendon repairs, with 3–0 Ethibond
(Ethicon, Somerville, NJ) suture. Different configurations
of single strand with and without a knot, two strands with a
knot, and four strands with a knot were tested.
The suture was attached to the testing apparatus by
means of two smooth 1 cm bars, 60 mm apart (Fig. 2). In
the tests using a closed loop (i.e., knotted two- and four-
strand knotted repairs), the strands were passed around the
bars. Where the repairs were not a closed loop (i.e., single
strand knotted and unknotted configurations), the free ends
of the two end strands were secured to the bar by multiple
wrapping and then adhesive tape over the entire wrap
(Fig. 3). Preliminary studies showed this technique of
fixation achieved no appreciable creep under the testing
conditions. A minimum of five square throws were
performed to secure the knot where used.
For the single tensile load testing, the suture config-
urations were preloaded to one Newton and then a tensile
force applied until ultimate failure. Cyclical tensile tests
were applied loading at 10 N per strand for ten cycles prior
to loading to ultimate failure. This level of submaximal
cyclic loading was chosen to eliminate the slack within the
knot prior to testing to tensile failure.
It was assumed that at the rate of loading used, the
viscoelastic effect is negligible. The rate of loading used is
best defined as quasistatic and changes within this range
have negligible influence on ultimate tensile strength (UTS)
and construct stiffness. Testing at 500 mm/s would most
likely create different viscoelastic loading effects, but this
was not assessed in this study:
1. In vitro study: The initial study was to assess the in
vitro properties of the commonly used tenorrhaphy
suture, 3–0 Braided Polyester (Ethibond, Ethicon,
Somerville, NJ, USA), when subjected to testing in
both single tensile and cyclic loading conditions. A
single strand with and without a knot and then two- and
subsequently four-strand configurations were tested
2. Ex vivo study: Thirty porcine forefoot flexor tendons
were freshly prepared which were similar in size and
appearance to human flexor profundus tendon. The
tendons were divided with a sharp blade and a four-
strand Adelaide repair performed with 3–0 Ethibond
Figure 2 Photograph of a two-strand Ethibond (Ethicon, Somerville,
NJ, USA) suture configuration with knot looped around restraining
bars of Hounsfield tensile machine.
Figure 1 Illustration demonstrating four-strand Adelaide repair used
for flexor tenorrhaphy.
114 HAND (2009) 4:113–118
(Ethicon, Somerville, NJ, USA). The tendon ends were
attached to securing clamps of the MTS. Fifteen single
and 15 cyclical tests before loading to ultimate tensile
strength were then applied under standardized conditions.
Using computer software, the results were extrapolated to
produce a load displacement curve for each test
The mean of both the in vitro suture tension tests and ex
vivo four-strand tendon repairs were calculated. Statistical
analysis was applied using the student ttest to compare the
single and cyclical loading tension tests for each suture
configuration or four-strand repair. Using computer soft-
ware, a curve of best fit for each test of the suture
configurations was plotted. This allowed assessment of
stiffness of the particular strand configuration by visualiza-
tion of the slope of the curve at different points.
Quantifying the stiffness is achieved by dividing the load
by the displacement (N/mm).
Results
Each suture configuration was tested to failure following
either a single tension loading or a cyclic loading sequence.
In all knotted strands, failure occurred at the site of the
knot. Quantifying the stiffness is achieved by dividing the
load by the displacement (N/mm). This is simple for linear
materials like the unknotted and perfectly secured unknot-
ted strand (i.e., a 5-mm displacement for 10 N load=2 N/
mm); however, when the knot slips, there is a nonlinear and
unpredictable initial stiffness. The stiffness is initially low
and ramps up to the unknotted equivalent stiffness.
Effect of Knot—Single Strand In Vitro Repair
The mean ultimate tensile load for a single strand 3–0
Ethibond (Ethicon, Somerville, NJ, USA) was 34 N under
single tensile loading and 36 N under cyclical loading
conditions (n.s. p>0.05). The presence of a knot with a
single strand reduced the ultimate tensile load by approx-
imately 50% (p<0.05) in both cyclic loading and single
tension testing groups (Fig. 4).
Number of Suture Strands
In both single and cyclical loading tests, increasing the
number of strands increased the ultimate tensile load
(Table 1). Doubling the suture strand from single strand to
double strand with knot and from double strand to four
strands with knot slightly more than doubled the ultimate
tensile load.
Single Versus Cyclical Loading
All groups were compared with single tensile and cyclical
loading. There was no statistical difference found with a
single strand, single strand with knot, and double strand
with knot with different loading tests. There was a slight
decrease in ultimate tensile load in the four-strand group
with cyclical loading (Fig. 4), but this was not significant
(p>0.05). With the four-strand tendon (ex vivo) repair,
Figure 4 Bar chart demonstrating effect of cyclic loading on in vitro
strand configurations and four-strand ex vivo porcine repair.
Figure 3 Illustration demon-
strating unknotted and knotted
single suture strand between
restraining bars.
HAND (2009) 4:113–118 115115
there was a decrease from 80 N in the single tensile loading
group to 70 N in the cyclical loading group (p<0.05).
Stiffness of Single Strand Group
For the single strand configurations (single strand with and
without knot), all 15 tests were expressed as a linear line of
best fit for a load displacement curve. The relative stiffness
of the suture material was represented by the slope of the
load displacement curve. It was found that the knotted
single strand was less stiff than a single strand (mean
stiffness=stiffness value) under tensile loading (p< 0.05;
Fig. 5); however, after cyclical loading, the single strand
and the single strand with a knot had the same stiffness
represented by the same slope on the load displacement
graph (Fig. 6).
Variance in Mean Ultimate Tensile Strength
There was found to be less variance in ultimate tensile load
with cyclical loading in all groups (Table 1).
Discussion
An active postrepair mobilization protocol places increasing
stresses on the suture construct as it is the suture material
itself which maintains the integrity of the tendon repair
until the healing is sufficiently advanced. In an effort to
improve the ultimate mechanical strength of repairs, multi-
strand techniques have been introduced. To identify the best
suture materials for tendon repair, in vitro and ex vivo
studies of suture ultimate tensile strength of different
techniques of acute flexor tenorrhaphy have previously
been described [1,2,7,9,11,12].
In the first part of this study, the mechanical strengths of
different numbers of strands of suture material was
reviewed. Savage [9] has previously shown the weakest
area being at the site of a knot. This was confirmed in our
study which showed the ultimate tensile strength of a single
suture strand to be reduced by approximately 50% with the
presence of a knot (Fig. 6). Savage and Rositano [10] also
suggested that in a multistrand repair in a frictionless
environment, each strand should share the load and the
construct should fail once the weakest strand (i.e., the
knotted strand) has exceeded its ultimate tensile. Previous
studies [8,10] have documented a discrepancy of this
simple arithmetic formula (strength of the weakest (knotted)
strand times the number of strands) with a higher measured
UTS on single tension testing being attributed to some form
of friction in the experimental system. Aoki et al. identified
that the tensile strength of a tenorrhaphy following cyclic
loading was less than the same repair under single tension
testing loads. Again, the difference is attributed to the
effects of friction in the testing system.
Our study, however, suggests that the discrepancy in the
UTS of the multistrand repair between single tension and
COMPARISON BETWEEN UNKNOTTED AND KNOTTED
CYCLIC TENSILE TEST BEHAVIOUR
0
5
10
15
20
25
30
35
40
0 2 4 6 8 10 12 14 16
EXTENSION, (mm)
LOAD, (N)
Single Strand
Single Strand with Knot
Figure 6 Load displacement curve demonstrating effect of cyclic
loading on unknotted and knotted single suture strand.
SINGLE TENSION TEST
0
5
10
15
20
25
30
35
40
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
EXTENSION, (mm)
LOAD, (N)
Single Strand with Knot
Single Strand
Figure 5 Load displacement curve demonstrating effect of single
tensile loading on unknotted and knotted single suture strand.
Table 1 In vitro 3–0 Ethibond (Ethicon, Somerville, NJ, USA) suture
and ex vivo four-strand tendon repair mean ultimate tensile strength
(N).
Single tensile
loading
(variance)
Cyclical
loading
(variance)
pvalue
1 strand (no knot) 34 (8) 36 (6) n.s.
1 strand with knot 16 (9) 18 (4) n.s.
2 strand with knot 39 (13) 41 (10) n.s.
4 strand with knot 90 (10) 87 (9) n.s.
4 strand tendon
repair
80 (9) 70 (6) <0.05
116 HAND (2009) 4:113–118
cyclic loading can be explained by the behavior of the knot
during initial and subsequent loading. The load displace-
ment curves for a knotted and unknotted strand for both
single tensile and cyclical loading (Figs. 5and 6) were
compared. It was clear that the stiffness of the knotted
strand was substantially less than the unknotted strand
(represented by the slope of the curve) for a single tensile
test. After cyclical loading, the stiffness of the knotted
strand was identical to that of the unknotted strand, but the
UTS remained the same being 50% weaker in the presence
of a knot.
This difference in the stiffness can explain the discrep-
ancy in the UTS of the four-strand tendon repair in this
study. With the ex vivo model, there was a mean decrease
from 80 N for single tensile to 70 N for cyclical loading (p<
0.05). The reduction in the stiffness gradient of the knotted
strand effectively delays the ultimate maximal loading of the
knotted strand. In a multistrand repair where there is friction
which prevents immediate equilibration of the strand tension,
the unknotted strand which has a higher stiffness strands
(i.e., the unknotted strands) will take a proportionally greater
load across the tenorrhaphy resulting in higher value of UTS
with single tensile testing. The slight redundancy within the
knot causing the reduced stiffness in that strand can be
corrected by cyclical loading such that the stiffness of a
knotted strand becomes identical to the stiffness of an
unknotted strand (Fig. 6).
In this study, cyclical loading of the ex vivo tendon
repair does reduce the mean UTS (80 N for single tensile to
70 N for cyclical) to a value that is very close to the
arithmetic formula of the knotted strand times the number
of strands (17 N×4=68 N). In a physiological situation
where cyclic loading will occur in the course of active and
passive mobilization, the stiffness of the unknotted strand
will maximize, the loading between the various strands will
be even, and the UTS will approach the weakest strand
strength times the number of strands.
This study has not investigated the presence of grasp
migration or the viscose elastic deformation of 3–0
Ethibond (Ethicon, Somerville, NJ, USA) which contribute
to differences in the mean UTS between the in vitro and ex
vivo four-strand models. The main difference can be
attributed to the effect of friction combined with the
variation in stiffness of the knotted and unknotted strands
in the testing model.
We conclude that the UTS of a given knotted length of
suture will be constant, regardless of single or cyclic
loading. However, the knotted strand is both the weakest
and has a lower stiffness (during initial loading) than other
strands in a multistrand repair. During single tension
testing, if the strands of a repair cannot undergo immediate
equilibration due to friction, a lower knotted strand stiffness
is evident and this leads to delayed rate of loading until
maximum loading and ultimate failure when compared to
the loading of the stiffer unknotted strands. This in turn
leads to an incorrectly elevated estimate of the likely UTS.
This discrepancy can be addressed by cyclic loading of the
construct which maximizes (or normalizes) the stiffness of
the knotted strand. This change in the knotted systems
stiffness occurs because slack within the knot cannot be
completely removed by hand during construction and is
removed during initial tensile loading. This removal of
slack within the knot during the initial loading phase
produces a reduction in system stiffness until a point where
the slack is completely removed. At this point, the knotted
system will exhibit identical stiffness to an unknotted
system; however, the knotted system will fail at signifi-
cantly lower UTS due to the knot acting as a ‘stress raiser’.
These findings are clinically important because slack
within the knot could tighten causing a gap between tendons.
The magnitude of the gap would be minimized with
increasing numbers of strands. Cyclical mechanical loading
is a more exacting technique and more closely reproduces
physiological loading of a flexor tendon repair which has
important clinical implications for previous studies
concerning strength of tendon repair. We therefore recom-
mend the use of cyclical mechanical testing techniques in the
future study of tendon repair techniques as single tensile
testing of tendon repairs may give falsely generous results
with regards to UTS. Removing the slack from the knot within
the knotted system allows more accurate measurement of the
ultimate system strength and stiffness.
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