A biomechanical comparison of different tying techniques of a double-stranded looped suture

Abstract

Purpose:

This study aims to assess the biomechanical performance of different tying techniques of a double-stranded looped suture (DSLS).

Methods:

Loop and knot security of DSLS tying techniques (nice knot (NK), modified nice knot (MNK), double-twist knot (DTK), and double-barrel knot (DBK)) were compared. The square knot of DSLS (SKD) and the square knot of single-stranded suture (SKS) had been used as references. Twenty-four loops of each configuration were created using No. 2 Fiberwire (Arthrex, Naples, Florida, USA) and tested with a material testing machine. Samples were loaded with 10 N preloads for loop security assessment. Knot security was subsequently evaluated. Twelve loops of each knot were loaded to failure. The rest were subjected to cyclic load testing and the elongation at the 50th and 1000th cycles were measured. Knot bulkiness was determined by measuring knot height before testing. Data were compared with analysis of variance and post hoc tests. Statistical significance was p < 0.05.

Results:

All knots showed no statistically significant difference in displacement with preload. The load-to-failure was highest in NK, followed by MNK, DTK, DBK, SKD, and SKS. The cyclic loading test at the 50th cycle and the 1000th cycle demonstrated that NK has significantly less displacement than the others except MNK. DTK provided a minimal average knot height followed by NK, SKS, DBK, MNK, and SKD.

Conclusion:

The different tying techniques in DSLS provided the similar loop security but different knot security and knot bulkiness. NK and MNK are biomechanically superior to the other knots, whereas DTK is the least bulky. The findings in the present study may help set the guide for the surgeons to select the tying technique of DSLS to best suit their requirement.

Keywords

bulkiness double-barrel knot double-stranded looped suture double-twist knot knot security loop security modified nice knot nice knot

Introduction

High-tensile-strength sutures are widely used in the shoulder surgery such as tuberosity fixation in the treatment of proximal humeral fracture, rotator cuff repair, and bony or soft tissue reattachment during shoulder arthroplasty.^1

–4 Sufficient stability owing to the suture fixation is essential to a good clinical outcome.⁴ A double-stranded suture has been proposed to increase the fixation strength.⁵ Single passing of a double-stranded looped suture (DSLS) has been described to reduce the number of suture passing and save time during the operations (Figure 1).^6,7 A loop connecting each strand can be utilized to form the sliding knots including nice knot (NK), modified nice knot (MNK), double-barrel knot (DBK), and double-twist knot (DTK). Biomechanical studies have shown that DSLS knots have a significantly higher mean peak to failure compared with the single-stranded suture (SSS) knots.^7

–12 However, the information regarding the biomechanical comparison among these knots is quite limited.

Figure 1.

Schematic drawings of a DSLS. DSLS: double-stranded looped suture.

Loop security and knot security are the key characteristics of the knot to maintain bone or soft tissue approximation. Loop security has been defined as the tightness of the initial loop after the knot has been tied. Knot security refers to a knot’s ability to maintain its integrity, without breaking or loosening, in the aspect of significant loads. These loads can be promptly increasing linear loads or low-level repetitive loads.¹³

The purpose of this study was to biomechanically evaluate the loop security and knot security of the previously described DSLS knots.

Methods

The biomechanical performance of previously described sliding DSLS knots including NK, MNK, DBK, and DTK was evaluated. The tying technique of each DSLS knots in the present study was based on its original description.^7,9,11,12 The formation of these knots is shown in Figure 2. All knots were backed up with three alternated half-hitches. The five-throw square knots of SSS (SKS) and DSLS (SKD) were also tested and used as the standard references of nonsliding knots. FiberWire was selected to be used in the present study because it achieved superior results in biomechanical testing compared with other sutures and monofilament wire.^9,14,15 The study employed only one orthopedic surgeon to tie all the knots to eliminate surgeon-to-surgeon variation. The order of tying the knots was randomized. Twenty-four samples of each knot type were tested in a closed-loop system on a material testing machine ((ElectroPuls™ E1000, Materials Testing System, Instron, England). Each knot was securely tied using No. 2 FiberWire (Arthrex, Naples, Florida, USA) over two rods to form a closed-loop system of 65 mm circumference. All knots were hand-tied without instruments or cannulae to minimize suture abrasion. Each knot was tied with as much initial tension as possible and located between the two rods to maximize the distraction forces across the knot. The distance between the two rods was measured (crosshead displacement), representing the zero reference point.

Figure 2.

The formation of the knots being tested. (a) NK, (b) MNK, (c) DTK, (d) DBK, (e) SKD, and (f) SKS. NK: nice knot; MNK: modified nice knot; DTK: double-twist knot; DBK: double-barrel knot; SKD: square knot of double-stranded suture; SKS: square knot of single-stranded suture.

Each loop was loaded with a 10-N preload. This preload was chosen based on the data from similar studies.^16
–18 The initial slippage was recorded to represent the loop security.

Knot security was evaluated by determining the response to load-to-failure and to cyclic loading. Twelve loops of each configuration were loaded to failure at a crosshead speed of 1 mm/s, and force and displacement data were (measured from the distance between the two rods) recorded every 50 ms. Mode of failure and maximum force to failure were recorded. Failure was classified as either absolute or clinical failure. Absolute failure was defined as the opening of a loop due to knot unraveling or material failure (within or outside of the knot). Clinical failure was defined as 3 mm displacement or more from 10 N initial load. In a sample, which had clinical failure, increased loading was continued until reaching absolute failure.

The other 12 samples of each knot were subjected to cyclic load testing. The cyclic loads from 10 N to 45 N were applied at a frequency of 1.8 Hz for 1000 cycles. Elongation of the suture was calculated using the difference between the loop circumference of the first cycle and the loop circumference of the final cycle (50th cycle and 1000th cycle). The failure was defined as used in load-to-failure test.

Prior to testing, the knot height, which was also its maximum diameter, was measured with digital calipers for each knot to get a sense of knot bulk.

An analysis of variance was performed to determine differences of the elongation after preload, load-to-failure test, cyclic loading test, and knot bulkiness. Contrasts were specified to compare each knot to the other knots with multiple comparisons post hoc test method. The modes of failure were described descriptively. All statistical analyses were performed with SPSS version 23.0 (SPSS Inc., Chicago, Illinois, USA). Significant differences were determined at p < 0.05.

Results

Preload

The elongation at a 10 N load is presented in Table 1. All knot types exhibited less than 0.01 mm of elongation (range 0.005–0.009 mm). The results did not significantly differ for any of the knots (p = 0.299).

Table 1.

Results of preload, load-to-failure, cyclic loading and knot bulkiness.

Knot type	Elongation at a 10 N load (mm), mean ± SD (range)	Load at failure (N), mean ± SD (range)	Elongation at 50th cycle (mm), mean ± SD (range)	Elongation at 1000th cycle (mm), mean ± SD (range)	Knot height (mm), mean ± SD (range)
NK	0.006 ± 0.00341 (0.001–0.013)	221.24 ± 30.08 (160.92–268.26)	0.978 ± 0.204 (0.671 –1.298)	1.099 ± 0.195 (0.812 –1.367)	4.90 ± 0.48 (4.13–5.90)
MNK	0.007 ± 0.00397 (0.001–0.013)	188.78 ± 34.59 (115.31–240.81)	1.034 ± 0.204 (0.715 –1.298)	1.168 ± 0.178 (0.843 –1.411)	6.68 ± 0.27 (6.32–7.18)
DTK	0.005 ± 0.00481 (0.001–0.014)	176.77 ± 22.55 (140.34–222.14)	1.429 ± 0.279 (1.054–2.042)	1.696 ± 0.402 (1.298–2.699)	4.15 ± 0.16 (3.88–4.42)
DBK	0.007 ± 0.00259 (0.003–0.013)	128.16 ± 27.89 (82.76–180.32)	1.490 ± 0.314 (1.061–2.347)	1.689 ± 0.301 (1.335–2.531)	6.39 ± 0.27 (6.19–6.89)
SKD	0.009 ± 0.00845 (0.001–0.027)	117.85 ± 23.47 (50.76–133.95)	1.467 ± 0.323 (0.961 –1.978)	1.688 ± 0.320 (1.172–2.155)	7.85 ± 0.17 (7.56–8.09)
SKS	0.009 ± 0.00685 (0.002–0.023)	106.65 ± 22.14 (64.34–132.65)	1.815 ± 0.342 (1.412–2.455)	2.031 ± 0.350 (1.552–2.627)	5.83 ± 0.39 (4.92–6.34)

NK: nice knot; MNK: modified Nice knot; DTK: double-twist knot; DBK: double-barrel knot; SKD: square knot of double-stranded looped suture; SKS: square knot of single-stranded suture.

Load-to-failure

The load-to-failure of each knot is displayed in Table 1.

There was a statistical difference detected among the knot configurations (p < 0.001). All loops failed by elongation of 3 mm (clinical failure). The maximal force to failure was highest in NK, followed by MNK, DTK, DBK, SKD, and SKS. According to the post hoc analysis (Table 2), there was no significant difference when comparing NK with MNK, DTK with MNK, and DBK with SKD and SKS. The mode of failure after the samples reaching to absolute failure is presented in Table 3.

Table 2.

Post hoc tests for load-to-failure and elongation from cyclic loading at the 50th cycle and the 1000th cycle.

Suture knot		p Value
Suture knot		Load-to- failure	Cyclic load (50th)	Cyclic load (1000th)
NK	MNK	0.051	0.997	0.993
	DTK	0.002^a	0.003^a	<0.001^a
	DBK	<0.001^a	0.001^a	<0.001^a
	SKD	<0.001^a	0.001^a	<0.001^a
	SKS	<0.001^a	<0.001^a	<0.001^a
MNK	DTK	0.887	0.013^a	0.001^a
	DBK	<0.001^a	0.003^a	0.001^a
	SKD	<0.001^a	0.005^a	0.001^a
	SKS	<0.001^a	<0.001^a	<0.001^a
DTK	DBK	0.001^a	0.995	1.000
	SKD	<0.001^a	0.999	1.000
	SKS	<0.001^a	0.019^a	0.085
DBK	SKD	0.938	1.000	1.000
DBK	SKS	0.389	0.074	0.075
SKD	SKS	0.914	0.045^a	0.073

NK: nice knot; MNK: modified nice knot; DTK: double-twist knot; DBK: double-barrel knot; SKD: square knot of double-stranded looped suture; SKS: square knot of single-stranded suture.

^a p < 0.05.

Table 3.

Mode of absolute failure.

Knot type	Material failure within the knot	Material failure outside of the knot	Knot unraveling
NK	10	0	2
MNK	12	0	0
DTK	9	1	2
DBK	9	1	2
SKD	7	1	4
SKS	7	2	3

NK: nice knot; MNK: modified nice knot; DTK: double-twist knot; DBK: double-barrel knot; SKD: square knot of double-stranded looped suture; SKS: square knot of single-stranded suture.

Cyclic loading

The loop elongation of both groups at 50th cycle and 1000th cycle is presented in Table 1.

There was a statistical difference among the knot configurations at 50th cycle and 1000th cycle (p < 0.001). Maximum elongation at 50th cycle and 1000th cycle was less than 3 mm in all samples. No knots unraveled or ruptured with the cyclic loading test. NK demonstrated the best performance, but this was not a significant difference compared with the MNK (Table 2). There was no statistical difference in elongation of DTK, DBK, SKD, and SKS at 50th cycle and 1000th cycle.

Knot bulkiness

DTK provided a minimal average knot height followed by NK, SKS, DBK, MNK, and SKD, respectively (Table 1). The average knot height of DTK and NK was lower than that of SKS, whereas the average of DBK, MKN, and SKD was greater. There was the statistical difference in comparison with each DSLS knot to the others (p < 0.001).

Discussion

This investigation focuses on the biomechanical performance including the loop security and knot security of the various types of sliding DSLS knots, while the nonsliding DSLS and SSS knots had been used as the references. From the present study, it was found that the different tying techniques in DSLS provided similar loop security but different knot security and knot bulkiness.

Loop security refers to as the integrity of the knot construct at time zero after it has been tied. Although biomechanical evaluation of each DSLS knot had been reported previously, the information regarding the loop security has never been revealed.^{8,9

–12,14} The current study demonstrated that the loop security of sliding DSLS knots is not significantly different from the nonsliding DSLS and SSS knots.

Knot security is the ability of the knot to resist slippage in response to load-to-failure and to cyclic loading. The single load-to-failure test of NK and DTK compared with the knots in SSS has been described.^12,14 However, the evidence in the other DSLS knots is lacking. In the current study, we found that NK provided the maximal load-to-failure, followed by MNK, DTK, DBK, and SKD. In contrast, a previous in vitro biomechanical study of the DSLS knots using the similar suture material (No. 2 FiberWire) demonstrated the higher load-to-failure of DTK (Cow hitch or Larks head) than that of NK.¹⁰ We hypothesized that the relatively decreased stiffness of DTK in the present study may be attributed to the different tying technique with more number of twists and less number of additional half-hitches.

In the present study, all loops reached clinical failure prior to the absolute failure. The clinical failure was defined as the crosshead displacement of 3 mm. This definition has also been used in previous studies, although the clinical relevance of this measurement is unverified.^19

–23 When reaching to absolute failure, suture material breakage within the knot was most commonly found in all configurations. Opening of the loop due to knot unraveling was observed in some samples of all knot configurations except MNK.

The cyclic loading test in the present study demonstrated that NK provided the best performance, but this was not a significant difference compared with the MNK. A similar finding was observed in a recent biomechanical analysis reported by Westberg et al.⁸. To our knowledge, the biomechanical testing during cyclic loading of other DSLS knots has not been reported in the literature. The results of the cyclic testing in the present study showed that the knot security was altered significantly by the choice of knot. DTK, DBK, and SKD had the inferior performance to NK and MNK.

A bulky knot which contains a large quantity of suture material may cause the soft tissue irritation and sometimes necessitate the surgical intervention for knot removal.^24
–26 The previous biomechanical studies evaluated on the bulkiness of the knot with various methods. In the present study, the bulkiness was determined by measuring the knot height as described by Ilahi et al. Among DSLS, the different tying techniques provided the significant difference of knot height. DTK and NK provided less bulkiness than SKS, which corresponds to the study by Meyer et al.¹⁰

There are some limitations in the present study. Only one suture material in one size had been tested. We did not perform in vivo clinical comparison. Body temperature, aqueous environment, and cadaveric bone were not used to reproduce the in vivo conditions to which these materials would be exposed. The impact of these limitations on our study parameters has not been demonstrated.

In conclusion, different tying techniques in DSLS provided similar loop security but different knot security and knot bulkiness. NK and MNK are biomechanically superior to the other knots, whereas DTK is the least bulky. The findings of the present study may guide surgeons to select the appropriate tying technique of DSLS based on their requirement.

Footnotes

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

ORCID iD

Suriya Luenam

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