SHOULDER

Novel single-loop and double-loop knot stitch in comparisonwith the modified Mason–Allen stitch for rotator cuff repair

Stephan Frosch • Gottfried Buchhorn • Anja Hoffmann • Peter Balcarek •

Jan Philipp Schuttrumpf • Florian August • Klaus Michael Sturmer •

Hans Joachim Walde • Tim Alexander Walde

Received: 13 June 2013 / Accepted: 30 March 2014

� Springer-Verlag Berlin Heidelberg 2014

Abstract

Purpose In rotator cuff repair, strong and long-lasting

suturing techniques that do not require additional implants

are needed. This study examines the ultimate load to failure

and the Young’s modulus at the suture–tendon interface for

a novel single-loop knot stitch and double-loop knot stitch.

These values are compared to those of the modified

Mason–Allen stitch.

Methods Twenty-four infraspinatus muscles with tendons

were dissected from porcine shoulders (twelve Goettingen

minipigs). The preparations were randomly allocated to

three groups of eight samples. Load-to-failure testing of the

single-loop knot stitch, the double-loop knot stitch and the

mMAS were performed using a Zwick 1446 universal

testing machine (Zwick-Roell AG, Ulm, Germany).

Results The highest ultimate load to failure for the three

techniques occurred with the double-loop knot stitch with a

median value of 382.2 N (range 291.8–454.2 N). These

values were significantly higher than those of the single-

loop knot stitch, which had a median value of 259.5 N

(range 139.6–366.3 N) and the modified Mason–Allen

stitch, which had a median value of 309.3 N (range

84.55–382.9 N). The values of the single-loop knot stitch

and the modified Mason–Allen stitch did not differ sig-

nificantly. Regarding the Young’s modulus, no significant

differences were found between the double-loop knot stitch

with a median value of 496.02 N/mm2 (range

400.4–572.6 N/mm2) and the modified Mason–Allen stitch

with 498.5 N/mm2 (range 375.5–749.2 N/mm2) with

respect to the stiffness of the suture–tendon complex. The

median value for the Young’s modulus of the single-loop

knot stitch of 392.1 N/mm2 (range 285.7–510.6 N/mm2)

was significantly lower than those of the double-loop knot

stitch and modified Mason–Allen stitch.

Conclusion This in vitro animal study demonstrated that

both the single-loop knot stitch and the double-loop knot

stitch have excellent ultimate load-to-failure properties

when used for rotator cuff repair. The introduced single-

loop knot stitch and double-loop knot stitch offer an

alternative to other common used stitch techniques in

rotator cuff repair.

Keywords Rotator cuff repair � Modified Mason–Allen

stitch � Biomechanical study � Ultimate load to failure �Shoulder � Stitch strength

Introduction

Reduction in high failure rates in rotator cuff repair is of

outmost clinical relevance as failures of 39 % in cases of

isolated supraspinatus tears and up to 94 % in cases of

massive rotator cuff tears have been reported [8, 14, 15].

Rotator cuff failure after repair most often occurs during

the early post-operative period while load transmission

mainly depends on fixation of the suture–tendon interface

[2, 27]. In addition to rupture of the thread, anchor dislo-

cation and knot malfunction, failure of the suture–tendon

Hans Joachim Walde: deceased.

S. Frosch (&) � A. Hoffmann � P. Balcarek �J. P. Schuttrumpf � F. August � K. M. Sturmer �H. J. Walde � T. A. Walde

Department of Trauma Surgery, Plastic and Reconstructive

Surgery, University Medicine Gottingen, Robert Koch Straße 40,

37075 Gottingen, Germany

e-mail: Stephan.Frosch@med.uni-goettingen.de

G. Buchhorn

Department of Orthopedics, University Medicine Gottingen,

Robert Koch Straße 40, 37075 Gottingen, Germany

123

Knee Surg Sports Traumatol Arthrosc

DOI 10.1007/s00167-014-2976-7

interface causes early rotator cuff failure [3, 10, 12, 13, 16].

Therefore, strong and lasting suture configurations are

needed in rotator cuff repair. Various studies have mea-

sured forces of the supraspinatus muscle that range from

175 to 353 N [7, 20, 21]. The infraspinatus muscle can

even endure forces up to 909 N, according to the results of

a cadaveric study by Hughes and An [20]. The modified

Mason–Allen stitch technique (mMAS) (Fig. 1) is consid-

ered to be superior to simple or mattress stitch with respect

to the initial fixation strength and shows similar biome-

chanical and clinical results when compared to double-row

fixation [18, 29, 30]. Ma et al. [29] reported an ultimate

tensile load of 246 N for the mMAS in a bovine in vitro

test. Even though the mMAS has better biomechanical and

clinical results, there remains the issue of the ultimate load

resistance of the mMAS when compared to the forces that

act on the intact rotator cuff muscles. Evidently, it is not

only the suture itself that is the weakest link but rather a

combination of several variables at the suture–tendon

interface. These variables include the orientation of the

thread, the length of contact, the number of fibres

embraced, compression of the fibre bundles, the self-cin-

ching of the knot, in addition to other variables. Previous

studies have pointed to anatomical conditions as being a

major cause of suture fixation problems [23, 31]. Since the

direction of the tendon fibres is parallel to the direction of

strain, the delicate soft tissue sheaths, which are composed

of fibre groups, are too weak to withstand the strain applied

by the thread under tension. The significant complication of

the thread cutting through these sheaths leads to elongation

and reduction in pressure within the footprint. At the

moment, a solution to this is a strain-dependent increase in

traverse compression of tendon fibres. Therefore, improved

stitch techniques to strengthen the suture–tendon interfaces

are needed. Additionally, these techniques should allow

reliable and easy application in both open and arthroscopic

approaches and not require additional implants.

The present study introduces the single-loop knot stitch

(SLKS) (Fig. 2) and the double-loop knot stitch (DLKS)

(Fig. 3) for rotator cuff repair. The rationale for these new

stitches is comprised mainly of three aspects: the knot

should not prevent the thread from compressing the fibres

of the tendon to strengthen the grip as strain increases; the

stitch should only make use of as many fibres of the tendon

as needed for secure fixation; and non-compressed fibres

are thought to be better nourished, thus supporting better

healing of the footprint.

This study focuses on the biomechanical strength of three

stitch techniques. In a tensile test, the ultimate load to failure

and the Young’s modulus were evaluated for the SLKS and

DLKS and compared with the results of the mMAS.

The hypothesis of this study is that the double-loop knot

stitch and the single-loop knot stitch are better or at least equal

to the modified Mason–Allen stitch with respect to ultimate

load to failure and the cutting of the tendon by the thread.

Materials and methods

The aim of this study was to examine performance at the

suture–tendon interface. It was determined that, due to

variations in quality of bone or suture anchors, refasteningFig. 1 Modified Mason–Allen Stitch (mMAS)

Fig. 2 Single-loop knot stitch (SLKS)

Knee Surg Sports Traumatol Arthrosc

123

the tendon at the original footprint might adversely affect

results. Thus, an in vitro study would be performed.

Sample preparation

Twenty-four porcine shoulders were harvested from twelve

Goettingen minipigs (female adult pigs at a comparable

weight and age) and stored at about -38 �C. The animals

had been previously killed for a non-related experiment,

thus no animal protection documentation was needed. Ten

hours before preparation, the shoulders were thawed at

room temperature. The infraspinatus muscle was exposed

with care to the enclosing fascia and dissected from the

protruding scapula crista. The tendon was then sharply cut

directly from its bony insertion at the tuberculum of the

humerus. The latter was inspected for regular anatomy and

discarded. All tendons were roughly 25–30 mm long and

had a cross-section of approximately 15 by 6 mm. The

preparations were then randomly allocated to three groups

of eight samples each. In each group, one of the three

suture configurations was tested. The testing began

immediately after the preparations were finished.

Suture

A high-strength, multi-strand polyethylene suture, Fiber-

Wire No. 2 � (Arthrex, Karlsfeld/Munchen, Germany), was

taken from a reel and combined with a round sharpened

solitary needle. Care was always taken to embrace com-

parable portions and to maintain a distance of 1.5 cm to the

end of the tendon. The width and thickness of the tendon in

the plane of the sutures were measured. To avoid the need

for an additional knot to anchor the ends of the threads, a

custom-made compensator device was designed. By means

of adjustable deflection rollers, the branches of the thread

were oriented parallel to the direction of tension. The

thread ends were then each clamped to a branch of the

axially centred compensator to allow for length compen-

sation in the case of single-sided suture lengthening. This

allowed both ends of the suture to be equally loaded.

Three suture techniques were tested:

1. The modified Mason–Allen stitch technique (mMAS)

(Fig. 1).

2. The SLKS (Figs. 2, 4). This stitch requires two

horizontal passes through the tissue to form a sling

with a knot that continuously tightens as tension on the

thread increases. Care was taken to ensure that the

needle did not penetrate the tendon fully, but rather

encompassed the upper, bursal portion of the cross-

section.

3. The self-cinching double-loop knot stitch (DLKS)

(Fig. 3). Two consecutive, horizontal, single-loop

stitches with knots are performed using a single thread.

This stitch is performed using the same techniques as

the SLKS, but with a mirrored second stitch; both

stitches are only half the width of a SLKS.

In contrast to the SLKS and DLKS, the mMAS enters at

the upper (bursal) side and exits at the lower side of the

Fig. 3 Double-loop knot stitch (DLKS)

Fig. 4 Macroscopic view of a single-loop knot stitch applied to the

end of a tendon; both strings of the thread enter and leave on the

bursal surface

Knee Surg Sports Traumatol Arthrosc

123

tendon, encompassing the cross-section rectangular to the

long axis of the cross-section.

Biomechanical testing

The specimens were subjected to unidirectional, continu-

ous tension to failure using a Zwick 1446 universal testing

machine (UTM) (Zwick-Roell AG, Ulm, Germany).

The protocol described by Baums et al. [4] was utilized.

In short, the infraspinatus muscle was clamped in com-

pression by the two metal brackets of a cryo-jaw [33]. Each

bracket had three traverse recesses, 5 mm deep, to be filled

with muscle tissue under compression. To achieve reliable

fixation and to prevent a slippage of the muscle, the metal

bags of the brackets were filled with pellets of dry ice

freezing the protuberances and preventing slippage. Care

was taken to only freeze the clamped part of the muscle,

while the downwards protruding tendon and suture

remained unaffected. The cryo-jaw was attached using a

cardan joint to the load cell and crossbar of the UTM; the

compensator device was mounted on the UTM base.

The data were recorded using testing software (textX-

pert V 112.1, Zwick-Roell AG, Ulm, Germany). The

elongation (precision = 0.5 mm) and load (pre-

cision = 0.1 N) were measured and displayed in a load/

elongation curve. The Young’s modulus was calculated

using a tangent to the linear portion (elastic deformation)

and was registered with the ultimate load. The maximum

possible error of traverse movement was 0.05 %. The

calibrated force transducer (maximum load 500 N) had an

accuracy of 1 % with values above 200 N.

After pre-tension to 2 N, the prepared suture complex was

axially stressed at a displacement rate of 1 mm/s until failure.

Failure was defined as an 80 % loss of the ultimate tensile

strength independent of either failure mode (suture thread

cutting through the tendon or breaking of the suture thread).

Statistical Analysis

From quantile–quantile plots the assumption of normal dis-

tribution was not satisfied for our data. Therefore, the indi-

vidual study parameters were compared between the three

techniques using the nonparametric Kruskal–Wallis test.

Subsequent pairwise comparisons were performed using the

Mann–Whitney U test. A value of a = 5 % was determined

to indicate statistical significance. For all statistical analyses,

the software R (version 3.0, www.r-project.org) was used.

Results

The branches of the suture increased in length as the knot

tightened under tension. During pretension (2 N), the

compensator device showed minimal multidirectional

deflection. During the load-to-failure test, no further

deflections were observed, indicating concordant tension of

both branches of the thread.

Ultimate load

The highest ultimate loads were achieved with DLSK,

whereas the SLKS were inferior to the mMAS results. A

significant difference could be shown for the values of the

ultimate load comparing the DLKS to the SLKS

(p = 0.003) as well as to the mMAS (p = 0.038) (Table 1;

Fig. 5).

No significant difference (p = 0.23) was found between

ultimate load of the SLKS and the mMAS (Table 1;

Fig. 5).

Young’s Modulus

No significant differences (p = 0.83) were found between

the median value for the Young’s modulus of the DLKS

and the mMAS. The Young’s modulus for the SLKS was

Table 1 Median values of the ultimate load and the Young’s mod-

ulus of the double-loop knot stitch (DLKS), the single-loop knot stitch

(SLKS) and the modified Mason–Allen stitch (mMAS)

Ultimate load Young’s Modulus

DLKS 382.2 N (range

291.8–454.2 N)

496.0 N/mm2 (range

400.4–572.6 N/mm2)

SLKS 259.5 N (range

139.6–366.28 N)

392.1 N/mm2 (range

285.7–510.6 N/mm2)

mMAS 309.3 N (range

84.55–382.9 N)

498.5 N/mm2 (range

375.5–749.2 N/mm2)

Fig. 5 Box-whisker-plot for results of the ultimate load (DLKS

double-loop knot stitch, SLKS single-loop knot stitch, mMAS modified

Mason–Allen stitch)

Knee Surg Sports Traumatol Arthrosc

123

significantly lower when compared to the DLKS

(p = 0.03) and the mMAS (p = 0.049) (Table 1; Fig. 6).

Failure analysis

Of the 24 performed tests, 21 failed due to suture pull-out

at the suture–tendon interface. Three failures were caused

by rupture of the suture. Two ruptures from the mMAS

group and one from the DLKS group were located at the

enlacement close to the surface of the tendon. Reasons for

rupture of the suture remain unclear. Though the thread

was markedly compressed in the compensating device, no

rupture (partial or complete) was observed at this location.

No other inspected characteristic (deficiencies of the ten-

don, slippage of the thread in the clamp, failure of the cryo-

jaw) raised attention.

Discussion

The most important finding of this study is that the double-

loop knot stitch shows superior initial fixation strength

when compared to the common modified Mason–Allen

stitch and the single-loop knot stitch. The results for the

single-loop as well as the double-loop knot stitch confirm

the hypothesis of this study.

Regardless of the technique used for rotator cuff repair,

high failure rates are described in the literature [19]. Bio-

mechanical studies have demonstrated a significant differ-

ence in ultimate load to failure depending on the stitch

technique. More complex single-row stitches such as the

modified Mason–Allen stitch (mMAS) or the massive cuff

stitch (MAC) provide superior initial strength relative to

other simple stitches [26, 31]. Ponce et al. described—in a

biomechanical in vitro animal study—a correlation

between the ultimate load to failure of a stitch and the

number and configuration of sutures passing through the

tissue [31]. Two stitches through the tendon, as with the

mattress stitch, increased the ultimate load failure by 20 N

over a single stitch, such as the simple stitch. When a third

pass was introduced, as with the mMAS and MAC stitches,

an increase in 50 N was observed.

In order to further decrease the failure rate, the double-

row (DR) repair was introduced, which showed promising

biomechanical properties relative to single-row (SR) repair

[5, 22, 28]. However, Lorbach and Tompkins point out the

fact that in literature most DR repairs were compared to SR

repairs using simple sutures [27]. Taking into account that

SR repair using modified suture configurations shows

comparable biomechanical results to DR repair, the authors

concluded that modified SR repair is a good alternative to

DR repair techniques [27]. Moreover, literature fails to

show significant differences in the clinical results and re-

rupture rates compared with DR to SR fixation [6]. Finally,

DR fixation is more time consuming and leads to higher

implant costs [32].

Most re-ruptures occur in the early post-operative per-

iod, during the time that load is primarily transmitted via

the suture–tendon interface [24]. Since the suture–tendon

interface is the most common failure point of the repair, a

modified SR repair technique providing high initial ulti-

mate failure load without ignoring biological milieu is

desirable.

The new SLKS and DLKS introduced in this study were

created in order to meet the aforementioned criteria. The

current study was designed to examine the initial fixation

strength of the SLKS and DLKS and compare the results to

the modified mMAS as the gold standard in modified SR

repair. In a biomechanical in vitro study comparing the

ultimate load to failure of SR repair techniques (simple and

mattress stitch, massive cuff stitch, modified Mason–Allen

stitch), Ponce et al. [31] illustrated the typical failure

mechanisms for SR repair techniques. In 189 out of 192

specimens (infraspinatus tendons of sheep shoulders),

suture pullout through the tendon was the failure mode.

This failure mechanism is not surprising, given that the

tendon fibres are in line with the direction of strain. An

increase in transverse compression of the bundle fibres

decreases the splitting effect of the thread. Transverse

compression is utilized in the massive cuff stitch as well as

the mMAS, which results in superior biomechanical

properties when compared to simple stitches without

transverse compression [11, 25, 31]. The rationale for the

SLKS and DLKS is to not only provide transverse com-

pression of the tendon fibres, but to also adjust the com-

pression depending on the applied strain forces. The loop

knot, with its self-tightening property, produces increasing

Fig. 6 Box-whisker-plot for results of the Young’s Modulus (DLKS

double-loop knot stitch, SLKS single-loop knot stitch, mMAS modified

Mason–Allen stitch)

Knee Surg Sports Traumatol Arthrosc

123

compression forces that enhance tissue grip as axial stain

increases.

In the current study, the SLKS had comparable results to

the mMAS with respect to ultimate load to failure (283.5 vs

309.3 N). Furthermore, the evaluation of the DLKS showed

statistically significant superior ultimate load to failure

(382.2 N) when compared to the SLKS and the mMAS. The

Young’s moduli did not differ significantly between the

DLKS and the mMAS, thus showing inferior stiffness at the

suture–tendon interface for the SLKS when compared to the

DLKS and the mMAS. Literature shows varying ultimate

tensile strengths for the mMAS using similar biomechanical

models [4, 28, 29, 31, 34]. However, the results are not

comparable with each other, since the absolute values are

affected by the amount of encompassed tissue, different

tissue-penetrating instruments and size of the bite [31].

Enhanced transverse compression forces act on the part

of the tendon encompassed by the suture. This raises the

concern of local tendon necrosis. However, Gerber et al.

[17] demonstrated that with the mMAS these forces do not

cause long-term histological changes within the tendon and

that they are biologically tolerated.

Christoforetti et al. [9] described a significant decline of

blood flow at the tendon repair site when suture-related

compression acts on the tendon during repair. A strangu-

lation of the tissue obviously leads to reduced blood per-

fusion possibly compromising biological healing of the

defect. However, the tendon is a region of hypovascularity.

The highest vascularity of the rotator cuff region after

repair comes from the bursal tissue and from the bone-

tendon interface promoting fibrovascular scar formation [1,

35]. With these findings in mind, the SLKS and DLKS

encompass, in contrast to the mMAS, the bursal side of the

tendon only, without penetrating the articular side (Figs. 2,

3, 4). Therefore, the tendon fibres in direct contact with the

foot print do not experience suture compression forces.

Under these conditions, normal blood flow should be

retained, promoting a suitable environment for biological

healing at the tendon-footprint interface.

One of the limitations of this study is the use of an

in vitro animal model to investigate suture techniques for

rotator cuff repair in human beings. However, mechanical

properties of pig tendons are comparable to human tendons

and sheep infraspinatus tendons are considered to be sim-

ilar to human rotator cuff tendons in size, shape, histo-

logical parameters and mechanical properties [16, 36].

Another limitation of the present study is that the ten-

don-suture interface was tested exclusively. Other causes

of rotator cuff repair failure were not tested in this setting.

Since failure at the tendon-suture interface is the most

common reason for malfunction, the present study uses a

testing configuration that exclusively focuses on the main

failure mode and excludes other factors.

The potential clinical relevance of the results is that the

introduced single-loop and double-loop knot stitch might

be an alternative to other common used stitch techniques in

rotator cuff repair, because of its self-cinching abilities, the

small amount of tendon fibres encompassed by the suture

promoting biological healing and providing high initial

fixation strength without the need of further implants.

Conclusion

This in vitro animal study demonstrated that secure fixation

of a suture can be achieved with a transverse, self-tight-

ening knot that only engages the bursal half of the tendon.

Both the single-loop knot stitch and the double-loop knot

stitch have excellent ultimate load-to-failure properties and

offer an opportunity for better healing in rotator cuff repair.

Acknowledgments This work is published in memory of Hans

Joachim Walde, MD, who passed unexpectedly. He was an ardent

physician, a great supporter of younger fellows and a wonderful

inspiration in many other aspects of his life. His basic ideas on tendon

restoration, suture and knot characteristics, as well as training in

manual skills assisted the study described here. We owe thanks to

Klaus Jung, PhD, Department of Clinical Statistics at the University

Medical Center Gottingen, as well as the Department of Mouth—Jaw

and Facial Surgery at the University Medical Center Gottingen for the

assistance and provision of the porcine specimens.

Conflict of interest The authors declare that they have no conflict

of interest.

Ethical standards The manuscript submitted provides no conflict

with animal ethical standards. All authors listed contributed to the

study.

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