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Biomechanical Analysis of Flexor Tendon Repair Using Knotted Kessler and Bunnell Techniques and the Knotless Bunnell Technique

Published:October 22, 2014DOI:https://doi.org/10.1016/j.jhsa.2014.09.015

      Purpose

      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.

      Key words

      Achieving sufficient flexor tendon repair strength is a challenge in hand surgery.
      • Strickland J.W.
      Development of flexor tendon surgery: twenty-five years of progress.
      Bunnell introduced a 2-strand suture technique allowing primary repair.
      • Bunnell S.
      Repair of tendons in the fingers.
      The modification of the suture technique and development of postoperative mobilization protocols
      • Strickland J.W.
      Development of flexor tendon surgery: twenty-five years of progress.
      • Kleinert H.E.
      • Kutz J.E.
      • Atasoy E.
      • Stormo A.
      Primary repair of flexor tendons.
      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,
      • Strickland J.W.
      Development of flexor tendon surgery: twenty-five years of progress.
      • Savage R.
      The search for the ideal tendon repair in zone 2: strand number, anchor points and suture thickness.
      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.
      • McCoy B.W.
      • Haddad S.L.
      The strength of achilles tendon repair: a comparison of three suture techniques in human cadaver tendons.
      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.
      • Howard R.F.
      • Ondrovic L.
      • Greenwald D.P.
      Biomechanical analysis of four-strand extensor tendon repair techniques.
      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.
      • Mao W.F.
      • Wu Y.F.
      • Zhou Y.L.
      • Tang J.B.
      A study of the anatomy and repair strengths of porcine flexor and extensor tendons: are they appropriate experimental models?.
      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 thumbnail gr1
      Figure 1Illustration of 4 different suture techniques. A, B Repair performed with an unbarbed monofilament suture. C, D Repair performed with a barbed suture.
      Figure thumbnail gr2
      Figure 2Successive steps of the 4-strand knotless Bunnell suture technique.
      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.
      • Peltz T.S.
      • Haddad R.
      • Scougall P.J.
      • et al.
      Performance of a knotless four-strand flexor tendon repair with a unidirectional barbed suture device: a dynamic ex vivo comparison.
      • Haddad R.
      • Peltz T.S.
      • Lau A.
      • et al.
      The relationship between gap formation and grip-to-grip displacement during cyclic testing of repaired flexor tendons.
      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 thumbnail gr3
      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 thumbnail gr4
      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.
      • Wu Y.F.
      • Tang J.B.
      Recent developments in flexor tendon repair techniques and factors influencing strength of the tendon repair.
      • Tang J.B.
      • Amadio P.C.
      • Boyer M.I.
      • et al.
      Current practice of primary flexor tendon repair: a global view.
      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.
      • Strickland J.W.
      The scientific basis for advances in flexor tendon surgery.
      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.
      • Strickland J.W.
      Development of flexor tendon surgery: twenty-five years of progress.
      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
      • McCoy B.W.
      • Haddad S.L.
      The strength of achilles tendon repair: a comparison of three suture techniques in human cadaver tendons.
      and Howard et al.
      • Howard R.F.
      • Ondrovic L.
      • Greenwald D.P.
      Biomechanical analysis of four-strand extensor tendon repair techniques.
      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.
      • Sebastin S.J.
      • Ho A.
      • Karjalainen T.
      • Chung K.C.
      History and evolution of the Kessler repair.
      The modified 4-strand Kessler suture has several locking-loops,
      • Wu Y.F.
      • Tang J.B.
      Recent developments in flexor tendon repair techniques and factors influencing strength of the tendon repair.
      holding tendon fibrils within the loop and anchoring the suture to the tendon.
      • Sebastin S.J.
      • Ho A.
      • Karjalainen T.
      • Chung K.C.
      History and evolution of the Kessler repair.
      The absent anchor points in the 4-strand knotted Bunnell repair allow sliding.
      • Howard R.F.
      • Ondrovic L.
      • Greenwald D.P.
      Biomechanical analysis of four-strand extensor tendon repair techniques.
      • Sebastin S.J.
      • Ho A.
      • Karjalainen T.
      • Chung K.C.
      History and evolution of the Kessler repair.
      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,
      • Komatsu F.
      • Mori R.
      • Uchio Y.
      Optimum surgical suture material and methods to obtain high tensile strength at knots: problems of conventional knots and the reinforcement effect of adhesive agent.
      as unraveling of the knot is a likely source of failure
      • Waitayawinyu T.
      • Martineau P.A.
      • Luria S.
      • Hanel D.P.
      • Trumble T.E.
      Comparative biomechanic study of flexor tendon repair using FiberWire.
      • Zhao C.
      • Hsu C.C.
      • Moriya T.
      • et al.
      Beyond the square knot: a novel knotting technique for surgical use.
      and the reduced bulk at the repair site can improve the gliding quality inside the tendon sheath.
      • Papandrea R.
      • Seitz Jr., W.H.
      • Shapiro P.
      • Borden B.
      Biomechanical and clinical evaluation of the epitenon-first technique of flexor tendon repair.
      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.
      • Schuind F.
      • Garcia-Elias M.
      • Cooney 3rd, W.P.
      • An K.N.
      Flexor tendon forces: in vivo measurements.
      It is not known, however, whether the knotless Bunnell repair would allow early postoperative mobilization based on the results of our ex vivo study.
      Bunnell mentioned the idea of flexor tendon repair with a barbed suture in 1954.
      • Bunnell S.
      Gig pull-out suture for tendons.
      The limiting factor has long been the inferior quality of the barbed suture material. McKenzie performed an in vivo study in 1967,
      • McKenzie A.R.
      An experimental multiple barbed suture for the long flexor tendons of the palm and fingers. Preliminary report.
      and in 2009 a new generation of barbed suture material allowed the re-exploration of knotless repairs. Trocchia
      • Trocchia A.M.
      • Aho H.N.
      • Sobol G.
      A re-exploration of the use of barbed sutures in flexor tendon repairs.
      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.
      • Peltz T.S.
      • Haddad R.
      • Scougall P.J.
      • et al.
      Performance of a knotless four-strand flexor tendon repair with a unidirectional barbed suture device: a dynamic ex vivo comparison.
      • Trocchia A.M.
      • Aho H.N.
      • Sobol G.
      A re-exploration of the use of barbed sutures in flexor tendon repairs.
      • Haddad R.
      • Peltz T.S.
      • Walsh W.R.
      Biomechanical evaluation of flexor tendon repair using barbed suture material: a comparative ex vivo study.
      • Joyce C.W.
      • Whately K.E.
      • Chan J.C.
      • et al.
      Flexor tendon repair: a comparative study between a knotless barbed suture repair and a traditional four-strand monofilament suture repair.
      • Marrero-Amadeo I.C.
      • Chauhan A.
      • Warden S.J.
      • Merrell G.A.
      Flexor tendon repair with a knotless barbed suture: a comparative biomechanical study.
      • McClellan W.T.
      • Schessler M.J.
      • Ruch D.S.
      • Levin L.S.
      • Goldner R.D.
      A knotless flexor tendon repair technique using a bidirectional barbed suture: an ex vivo comparison of three methods.
      • Parikh P.M.
      • Davison S.P.
      • Higgins J.P.
      Barbed suture tenorrhaphy: an ex vivo biomechanical analysis.
      • Sato M.
      • Matsumura H.
      • Gondo M.
      • Shimada K.
      • Watanabe K.
      Flexor tendon repair with barbed suture: an experimental study.
      • Zeplin P.H.
      • Henle M.
      • Zahn R.K.
      • Meffert R.H.
      • Schmidt K.
      Tensile strength of flexor tendon repair using barbed suture material in a dynamic ex vivo model.
      • Zeplin P.H.
      • Zahn R.K.
      • Meffert R.H.
      • Schmidt K.
      Biomechanical evaluation of flexor tendon repair using barbed suture material: a comparative ex vivo study.
      The ideal suture technique has not been found, and in vivo repairs may or may not show similar findings.
      • Peltz T.S.
      • Scougall P.
      • Oliver R.S.
      • Mark P.
      The knotless tendon repair with a resorbable unidirectional barbed suture device: an in vivo comparison in the turkey foot: not a clinical study.
      Our results, with regard to maximum load and gap formation, are comparable to work presented by McCellan et al
      • McClellan W.T.
      • Schessler M.J.
      • Ruch D.S.
      • Levin L.S.
      • Goldner R.D.
      A knotless flexor tendon repair technique using a bidirectional barbed suture: an ex vivo comparison of three methods.
      and Peltz et al.
      • Peltz T.S.
      • Haddad R.
      • Scougall P.J.
      • et al.
      Performance of a knotless four-strand flexor tendon repair with a unidirectional barbed suture device: a dynamic ex vivo comparison.
      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.

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