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Corresponding author: Stefanie Hoelscher-Doht, MD, Department of Trauma, Hand, Plastic and Reconstructive Surgery, Würzburg University Hospital, Oberdürrbacher Str. 6, 97080 Würzburg, Germany.
To evaluate a knotless Bunnell suture in flexor tendon repair.
Methods
Eighty porcine flexor digitorum tendons were assigned to 4 different suture techniques. Group 1 was repaired using a modified 4-strand knotted Kessler suture and served as a control group. Group 2 was repaired using a 4-strand knotted Bunnell suture. Group 3 used a 4-strand knotless Bunnell suture. Group 4 used a 4-strand knotless Bunnell and knotless peripheral suture. Under static and cyclic testing we studied mode of failure, 2-mm gap formation force, displacement, and maximum load.
Results
The 4-strand knotless Bunnell suture did not show a notable difference with regard to 2-mm gap formation force, displacement, or maximum load in comparison to the modified 4-strand knotted Kessler suture. Adding a knotless peripheral suture improved the repair by a significant reduction of gap formation and displacement and an increase in maximum load. The 4-strand knotted Bunnell showed the highest maximum load but also a considerable lower resistance to gap formation and major displacement.
Conclusions
Flexor tendon repair using a 4-strand knotless Bunnell suture showed similar tensile strength to a modified 4-strand knotted Kessler suture. Adding a knotless peripheral suture further improved the repair.
Clinical relevance
A complete knotless 4-strand Bunnell suture including a barbed core- and peripheral suture might be an option for flexor tendon reconstruction.
has improved treatment. The shift towards multistrand core sutures, the introduction of peripheral sutures, and the use of stronger suture material have made failure of the repair less common. Now, multistrand core sutures with different locking techniques are generally preferred,
whereas the Bunnell technique, with its 2 core strands, has been abandoned. McCoy et al reported a biomechanical study where a 4-strand knotted Bunnell suture was used for Achilles tendon repair. They found their 4-strand Bunnell suture to be significantly stronger than a 2-strand Bunnell repair. Nevertheless, the main mode of final failure was at the knot.
Howard et al showed a 4-strand knotted Bunnell suture to be a repair option in extensor tendons, but they documented a high tendency for gap formation.
We hypothesized that a barbed suture material could help to bypass the mentioned problems making a 4-strand knotless Bunnell suture a valuable repair technique for flexor tendons. Possible advantages include multiple locking sites, evenly distributed tension, and reduced bulk at the repair site. The purpose of this study was to biomechanically analyze a modified 4-strand knotted Kessler suture compared with a 4-strand knotted Bunnell suture, a 4-strand knotless Bunnell suture, and a 4-strand knotless Bunnell with a knotless peripheral suture.
Methods
We used 80 fresh-frozen porcine flexor digitorum profundus tendons. Porcine flexor tendons have similar biomechanical properties to human flexor tendons and are frequently used in biomechanical studies.
Harvested tendons were stored inside saline-soaked swabs and deep-frozen at –20°C. After thawing the tendons for 12 hours, their diameters and lengths were measured using a micrometer to obtain equal sample sizes. To avoid desiccation during biomechanical testing, a saline spray was used constantly. The cross-sectional area at the repair site was measured after thawing, using the formula for the area of an ellipse (area = πab, where a equals one-half tendon height and b equals one-half tendon width). The mean cross-sectional diameter was 29 mm2 ± 7 mm2 at the repair site. Tendons with a deviating diameter ±20% were excluded. A scalpel was used to carefully create the defect in the middle of each tendon.
All tendons were randomly assigned to one of the 4 groups (Fig. 1): Group 1 tendons were repaired with a modified 4-strand knotted Kessler suture using a 3-0 PDS suture (Ethicon, Johnson & Johnson, Bridgewater, NJ). Group 2 tendons were repaired with a 4-strand knotted Bunnell technique with a 3-0 PDS suture. Group 3 tendons were repaired with a 4-strand knotless Bunnell technique (Fig. 2) using a 3-0 Stratafix suture (Ethicon, Johnson & Johnson,). Group 4 tendons were repaired with a 4-strand knotless Bunnell core suture using a 3-0 Stratafix and a peripheral knotless suture using a 4-0 Stratafix. The Stratafix Spiral PDO Device is a bidirectional suture and consists of polydioxanone (C4H6O3)x, which is a synthetic absorbable suture with barbs placed circumferentially on its surface. On all sutures, a core suture purchase of 10 mm was obtained.
Figure 1Illustration of 4 different suture techniques. A, B Repair performed with an unbarbed monofilament suture. C, D Repair performed with a barbed suture.
Biomechanical tests were conducted with a mechanical testing machine (Z020, Zwick, Ulm, Germany) and the testXpert II software (Version 3.0, Zwick, Ulm Germany). The testing gauge length (distance between 2 clamps) was standardized at 3 cm. Pretesting was performed up to 300 N to prove sufficient gripping of the tendon ends without slipping. The uniaxial testing was performed using a 100-N load cell (Gassmann Theiss Messtechnik, Germany) and 2 stainless steel clamps. The 2 suture materials (Stratafix and PDS) had been measured for their viscoelasticity before testing to prove similar quality. Static and dynamic testing (N = 10) was performed with a 3-N preload. The static test was a load to failure test with an advancement rate of 20 mm/min. We recorded the 2-mm gap formation force and the maximum load in the load displacement curve controlled by observation. For final failure, we distinguished between suture rupture and suture pullout. The dynamic test started with a setting cycle between 5 and 15 N for 15 cycles. Thereafter we applied 500 cycles between 5 and 20 N followed by a load to failure test. The advancement rate was 20 mm/min. We measured the displacement caused by cyclic testing for 500 cycles.
Our pre-tests with 500 and 2,000 cycles showed that most of the gap had occurred by 200 cycles. The displacement seems to be a reliable parameter to compare different suture materials. All testing was performed under monotonic uniaxial load. Load, displacement, and time were continuously recorded by the test-Xpert software to generate a load displacement curve for each tendon (Fig. 3). The displacement presented here corresponds to the waveform in the load-displacement curve.
Figure 3Load displacement curves for A static and B cyclic testing. A During static testing, 2-mm gap formation force and maximum load were measured. B The displacement was measured after 500 cyclic loads.
We measured 2-mm gap formation force (N), maximum load (N), displacement (mm), and mechanism of failure (pullout vs rupture). A power assessment using a significance level of 5% and a power of 80% indicated a sample size of 7. The Shapiro-Wilk test was performed to analyze the distribution, and analysis of variance was used for comparison of the different groups. A P value less than .05 was considered statistically significant.
Results
When measuring the 2-mm gap formation force, there was no difference between the 4-strand knotless Bunnell and the modified 4-strand knotted Kessler suture. The 4-strand knotted Bunnell suture showed a significantly (P < .05) lower resistance to gap formation (Fig. 4), whereas the 4-strand knotless Bunnell and knotless peripheral suture showed the significantly highest resistance against gap formation (P < .05).
Figure 4Results for the 4 tendon repair types for A gap formation force, B maximum load to failure, and C displacement. Asterisks mark a significant difference (P < .05).
Displacement was similar for the modified 4-strand knotted Kessler and the 4-strand knotless Bunnell suture. In the 4-strand knotted Bunnell suture group, the displacement was significantly higher (P < .05). The 4-strand knotless Bunnell and knotless peripheral suture showed a significantly lower displacement (P > .05) in comparison to all other groups.
The difference between the modified 4-strand knotted Kessler suture and the 4-strand knotless Bunnell suture was not significant, but the 4-strand knotted Bunnell suture withstood significantly more tension (P > .05). The 4-strand knotless Bunnell and knotless peripheral suture showed a significantly higher maximum load (P > .05) in comparison with the 4-strand knotless Bunnell suture without peripheral repair.
Out of 60 repaired tendons, 56 failed by suture rupture and 4 by suture pullout. Two pullouts occurred in the modified 4-strand Kessler group and 2 in the knotted 4-strand Bunnell group.
Discussion
Since Bunnell introduced the 2-strand repair, research has raised the level of knowledge for the treatment of injured flexor tendons. To improve the biomechanical conditions, a shift towards 4-strand core sutures has been widely accepted.
It has not been proven whether a 4-strand Bunnell suture will allow reliable repair and postoperative mobilization, as this is required to promote the intrinsic healing response and avoid the development of adhesions.
We found no statistically significant difference between the modified 4-strand knotted Kessler and the 4-strand knotless Bunnell suture, as both sutures revealed comparable tensile strengths. The addition of a knotless peripheral suture improved the tensile strength and reduces gapping, making it a complete knotless repair. Therefore, knotless peripheral suture show comparable benefit to the use of knotted peripheral sutures.
The 4-strand knotted Bunnell suture was a less reliable repair. Despite a high load to failure, we observed early gapping and high displacement of the 4-strand knotted Bunnell suture, as recently reported by McCoy et al
Missing anchor points of the 4-strand knotted Bunnell suture might explain the low resistance against gap formation and the increased displacement. For instance, anchor points such as locking-loops allow a secure suture technique.
That might explain the low resistance against gap formation.
To approach the problem of missing anchor points in the 4-strand knotted Bunnell repair, we decided to use a bidirectional barbed suture instead of the monofilament PDS. The barbs can lock inside the tendon and anchor the suture. The barbs may substitute for missing anchor points and may distribute the fixation evenly through the tendon. The absent knot is an advantage,
Optimum surgical suture material and methods to obtain high tensile strength at knots: problems of conventional knots and the reinforcement effect of adhesive agent.
While suturing with a barbed thread, barbs lock inside the tendon and keep the suture under continuous tension, which helps to avoid primary loosening.
The 4-strand knotless Bunnell suture presented here meets major requirements for a knotless repair because the suture courses several times through the tendon, which places a high number of barbs inside the tendon with only a few of them on the tendon surface. The mean 2-mm gap formation force in the 4-strand knotless Bunnell and knotless peripheral suture was 54 N, and the mean maximum load was 82 N. Schuind et al tested in vivo forces of the hand and reported about 9 N during passive mobilization and 34 N during active mobilization of the fingers. During prehensile pinch and grasp forces up to 118 N were present along the flexor tendons.
et al substantiated the possible role of modern barbed sutures in flexor tendon repair and triggered a new discussion. Since that time barbed sutures have been the source of ongoing controversy. The majority of studies are ex vivo and many authors recommend a knotless repair.
Different study designs and parameters preclude direct comparison, however.
The technique presented here has certain disadvantages. A high amount of foreign material is placed inside the tendon and might interfere with perfusion. It is inevitable that some barbs will be found on the tendon surface, especially with a barbed peripheral suture. Desiccation of the tendon during testing cannot be completely avoided by using a saline spray, and it is unclear if our repaired tendons behaved equally to in vivo tendon repair. Hence, in vivo studies will be essential to gather more information about knotless repair including consideration of absorption of the barbs.
References
Strickland J.W.
Development of flexor tendon surgery: twenty-five years of progress.
Optimum surgical suture material and methods to obtain high tensile strength at knots: problems of conventional knots and the reinforcement effect of adhesive agent.
No benefits in any form have been received or will be received related directly or indirectly to the subject of this article.
The authors thank Johannes Hain from the Mathematical Institute for statistical support and the IZKF (Interdisciplinary Center for Clinical Research, University Clinics of Wuerzburg) for support of our biomechanical studies.
Concerning the article of Jordan et al, “Biomechanical analysis of flexor tendon repair using knotted Kessler and Bunnell techniques and the knotless Bunnell technique,”1 I would like to make some remarks.
We congratulate the authors on their work and publication in the field of barbed suture repair of digital flexor tendons.1 Indeed, hand caregivers continue to debate the ideal flexor tendon surgical technique, suture material, rehabilitation, and other aspects of this challenging injury. We have also studied the use of barbed suture and found the concept appealing, among other reasons because of the potential for developing a knotless flexor tendon repair that could remove that known weak link in repair technique.
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