A Re-exploration of the Use of Barbed Sutures in Flexor Tendon Repairs

Abstract

Flexor tendon repairs continue to improve thanks to advancements in suture material and technique. The role of barbed sutures in flexor tendon repairs has been previously investigated, but with the advent of a new material, interest in their use has been rekindled. We hypothesized that the use of modern barbed sutures will have comparable maximum tensile strength and 2-mm gapping strength to that of conventional sutures, allowing their use to theoretically decrease adhesions and tissue damage in flexor tendon repairs.

Flexor tendon repairs were performed on a cadaver model using either 3-0 Ethibond (Ethicon, Inc, Somerville, New Jersey) (Kessler repair) or 2-0 Quill sutures (Angiotech, Vancouver, British Columbia, Canada ) (Kessler-Bunnell repair) and were biomechanically tested. The mode of failure for the Ethibond sutures was suture pullout 2 times and knot failure 18 of 20 times, while the Quill sutures failed entirely by pullout. Maximum load to failure was 34.7±5.4 N and 29.6±3.6 N for Ethibond and Quill, respectively. This was found to be statistically significant (P=.001). Tensile load at 2-mm gapping was 22.8±6.3 N and 22.2±4.0 N for Ethibond and Quill, respectively. No statistical significance was found (P=.723).

This study helps substantiate the possible role of modern barbed sutures in flexor tendon repair. Additional biomechanical studies will need to be performed to further assess the use of barbed sutures in flexor tendon repair.

With the advent of modern suture material and improvements in suture technique, the results for flexor tendon repair continue to improve. As knowledge of the variables that contribute to proper tendon repairs has advanced, once-troubling consequences of a repair such as gapping, adhesions, and strangulation of tissue have been minimized. New techniques have provided surgeons with a means of repairing tendons that allow quick passive or early active motion to avoid further postoperative complications.

The use of barbed sutures for flexor tendon repair in a cadaveric model was reported as early as the 1960s, but interest in their use soon fell out of favor due to poor biomaterials and poorly constructed barb configurations.^1,2 With further development in technology and biomaterials, a resurgence of interest in barbed sutures in soft tissue repair has occurred.^3-5 However, no studies have been reported exploring the use of newly designed barbed sutures for flexor tendon repair.

This study compared the biomechanical stability of a flexor tendon repair using 2-0 barbed Quill sutures (Angiotech, Vancouver, British Columbia, Canada) (Figure 1) in a Modified Kessler-Bunnell stitch configuration with 3-0 Ethibond sutures (Ethicon, Inc, Somerville, New Jersey) in a Kessler stitch. We hypothesized that modern barbed sutures would have comparable maximum tensile strength and 2-mm gapping strength to that of conventional sutures, thus allowing their use to theoretically decrease adhesions and tissue damage in flexor tendon repairs.

Figure 1: 2-0 polypropylene Quill suture.

Materials and Methods

Forty flexor tendons were harvested from adult human cadavers of both genders without evidence of pathological abnormality. Tendons were harvested from both the right and left forearms and consisted of flexor digitorum superficialis, flexor digitorum profundus, and flexor policis longus tendons. Tendons were harvested from the proximal portion of the carpal tunnel to the proximal interphalangeal joint. Sheaths were excised and tendons were stored with refrigeration and normal saline. The flexor tendon repairs were randomized between hand and digits, limiting bias by controlling specimen variation. A single surgeon (A.M.T.) harvested all tendons and performed all repairs.

After harvest, each tendon was randomly assigned to 1 of the 2 repair groups, such that there were 20 tendon specimens for each repair. Each tendon was transected at the midpoint and then repaired using either a 3-0 Ethibond suture with a 2-strand Kessler technique (Figure 2A) or a 2-0 polypropylene Quill barbed suture with a modified 2-strand Kessler-Bunnell technique (Figure 2B). The 3-0 Ethibond suture was chosen because it closely mimics many in vitro tendon repairs. Due to sizing limitations by the manufacturer, the 2-0 Quill was used because it offered the most reasonable comparison to the 3-0 Ethibond suture.

Figure 2: Modified Kessler stitch used with 3-0 Ethibond suture (A). Modified Kessler-Bunnell stitch used with 2-0 Quill suture with no knot. Two cut ends of suture are present on the side of the Bunnell configuration (B).

Because of the nature of the barbed suture, a knotless repair was devised taking advantage of the direction of the barbs for tensioning on a single side of the repair. This side was repaired similarly to the pattern of a Bunnell technique. The length of the longitudinal arms in the Bunnell configuration was 2.5 cm. On the other side of the repair, a locking modified Kessler configuration, which was independent of engagement of the barbs, was used. No suturing was performed in the epitenon.

After repair, the tendons were mounted into custom soft tissue clamps to prevent slippage during tensile testing. Each clamp was tightened enough to hold the tendon in place during testing, but enough that failure would occur at the interface with the clamp and tendon. A differential variable reluctance transducer (M-DVRT; MicroStrain, Inc, Williston, Vermont) was attached to the tendon, bridging the suture site to measure gapping during the test (Figure 3). The soft tissue clamps were mounted to a servohydraulic materials testing machine (858 Bionix; MTS, Eden Prairie, Minnesota). The bottom clamp was attached to the immobile platform of the materials testing machine, while the top clamp was attached to the mobile actuator of the machine.

Figure 3: Flexor tendon in tension on MTS with DVRT bridging the suture site.

Each tendon was first preloaded to 2 N by slightly raising the actuator.⁶ The preload was chosen to be small enough so that the tendon would be properly tensioned to aid in correct placement of the transducer measuring the gap, without placing significant tension on the repair. The tendons were then stretched until failure in displacement-controlled uniaxial tension, at a rate of 5 cm/min, as previously performed by Trail et al.⁷ The test was performed at room temperature and the tendons were kept moist throughout with normal saline solution. As the repair site gapped, the voltage across the transducer changed. This voltage change was later converted to a displacement using the calibration curve specified by the manufacturer. Data on load and gapping was collected at a sampling rate of 10 Hz. The load at 2 mm of gap, the load to failure, and the amount of gapping at the suture site were noted. The mode of failure for each repair was also noted (suture rupture, knot rupture, suture pullout).

A two-sample Student t test was performed to determine if a significant difference existed between the maximum load to failure and load at 2 mm of gapping between the barbed and conventional sutures. Post-hoc power analysis revealed that the repairs achieved 91.3% power in detecting a difference in the maximum load to failure.

Results

The load at 2 mm of suture site gapping was found to be 22.8±6.3 N for the 3-0 Ethibond suture (Kessler technique) repairs. The 2-0 Quill suture (modified Kessler-Bunnell technique) repairs had a mean load at 2-mm gapping of 22.2±4.0 N. The load to failure for the tendons repaired with the 3-0 Ethibond averaged 34.7±5.4 N, compared to only 29.6±3.6 N load to failure for the repairs performed with the 2-0 Quill (Table 1; Figure 4). A two-sample Student t test demonstrated no statistically significant difference between the means of the load at 2 mm of gapping for the repair groups (P=.723). An unrealistic sample size would have been necessary to detect a difference, had a difference existed. The maximum load to failure, however, showed a significant difference between the 2 repairs (P=.001).

Figure 4: Results of tensile testing to failure of both sutures. Mean values shown. N=20 for both groups.

The modes of failure for all tendons are reported in Table 2. Eighteen of 20 Ethibond repairs failed by the suture rupturing at the knot. The other 2 repairs failed when the suture pulled out of the tendon tissue. All of the Quill repairs failed by the suture pulling out of the tendon. Since there is no knot in this repair it was not possible for a knot rupture to be the mode of failure. With the exception of knot ruptures in the Ethibond repairs, no repairs failed by the suture breaking in either testing group.

Discussion

We hypothesized that Quill sutures would have comparable maximum tensile strength and 2-mm gapping strength to that of conventional sutures. The results demonstrate similar tensile strengths for 2-mm gapping but a statistically significant difference in maximum tensile strength. Although load to failure remains an important factor when choosing sutures for flexor tendon repairs, the surgeon must be most interested in the prevention of gapping. Theoretically, barbed sutures may limit complications such as adhesions and pulley scaring. This, combined with our results, lends promise to the use of modern barbed sutures for flexor tendon repairs.

Earlier studies have attempted to investigate the use of barbed sutures in tendon repair, but prior results and conclusions are not necessarily comparable to modern models due to great advances in biomaterials.^8-10 It is of note that in the literature, all testing of barbed sutures for tendon repairs has been conducted on animal or cadaveric models.

With barbed sutures, pullout strength is determined primarily by the working length of the suture. Practical constraints such as the pertinent adjacent anatomy restrict this crucial variable. We choose 2.5 cm as a reasonable balance between the constraints of anatomy and the need for adequate working length without parameters reported in the literature. This is a point of future investigation to determine how much working length is needed to get an appreciable increase in pullout strength.

The strength of conventional tendon repairs is governed by knot pullout, suture pullout, suture rupture, configuration or pattern of suture, tensile strength of the suture, and amount of strands crossing the repair. The purpose of this study was to provide a realistic model using known and similar suture configurations between the Ethibond and barbed suture repairs. The literature has made it apparent that multiple strands crossing the repair site greatly increase the repair. For simplicity, we chose a 2-strand repair for each group, as well as an easily obtainable suture.

Unlike conventional sutures, barbed suture tensile strength is governed by the amount of barbs that engage tissue, distribution of equal tension about the barbs that are engaged, and the angles at which the suture material is placed. Because of the lack of need for a knot with barbed sutures, some of the principles that govern conventional suture repairs do not apply to barbed suture. As demonstrated by our data, barbed suture fails by pulling out of the tendon. The question of how much suture length and how many passes of barbed suture are needed to properly engage tendon tissue remains unanswered.

When discussing the amount of barbs needed to engage tendon tissue, it is important to distinguish longitudinal length from working length. The longitudinal length is the distance traversed by the suture within the tendon on its longitudinal axis. The working length is the length of the suture used within the tendon dependent on the longitudinal length and the suture pattern used. Longitudinal length, and therefore working length, is limited by the size of the surgical field and adjacent anatomy. For example, suture material should not extend beyond a pulley or sheath if damage to either structure were to occur. This practical limitation can be partially overcome by using a suture pattern that maximizes working length while minimizing longitudinal length, such as the Bunnell stitch used in our study. A determination of the optimal suture pattern and minimal longitudinal length needed to provide adequate tensile strength is a point of future study.

During initial testing of the barbed suture, it became apparent that proper tensioning of the barbs was imperative. Tension is to be applied in a uniform manner when taking multiple passes such that an equal amount of force is placed on each barb. This is a point of technical difficulty that may limit barbed suture tendon repairs. Although this has minor significance when using barbed sutures in subdermal tissue to prevent cosmetic defects, if not performed correctly in the tendon, premature pullout of the suture will result. Similar to the sequential failure seen in nonlocking vs locking plates, if a small number of barbs are resisting a greater tension at time zero, then with force those barbs will fail first, thus distributing the tensile forces on the next group of tightly held barbs. A vicious cycle begins, leading to sequential failure of sections of barbs at a faster rate.

The angle that the barbs are engaged relative to each other is determined by the pattern of the stitch. Conventional sutures using these patterns develop a noose about the tissue that locks tissue within the suture material. This method allows for a relatively simple pattern to prevent tissue pullout. A disadvantage of this method is the strangulation of tissue with subsequent damage to the tendon. With barbed sutures, suture locking of the tissue does not become as vital for adequate fixation, although at this time its importance can only be theorized.

It is noteworthy to mention some technical issues of difficulty when handling barbed sutures on the field or placing them in tissue. To maintain the integrity of the barbs, no direct handling of the suture is to be performed with fingers or instrumentation. Additionally, the surgeon must minimize the use of wiping or dabbing the field and place the suture on the drapes until the suture is placed in the tissue to curtail the snagging of barbs. Another point of technical difficulty is that when placing barbed suture in tissue, the surgeon must not back up the suture to rethrow a stitch because it jeopardizes breaking the barbs. These technical points can easily be overcome as demonstrated by many surgeons who currently use barbed suture with subcutaneous closers.

In 1967, McKenzie¹ reported on the use of nylon and metal-barbed sutures in a dog and cadaveric model. He concluded that metal sutures were too brittle for tendon repair and that nylon, with its resistance to distortion and its high tensile strength, proved to be a suitable substitute. He stressed the need for proper insertion of barbed sutures into the tendon and recognized early that a learning curve exists when using such material. In 1968, Shaw² reported on the function of metallic barbed sutures demonstrating load to failure at 1.8 to 2 kg (ie, 17.7-19.6 N) for a repair of cadaveric profundus tendons. It was concluded that barbed suture tendon repairs offered greater tensile strength than suture wiring.

Several biomechanical studies have reported similar maximum load to failure for tendons repaired with Ethibond sutures. Smith and Evans¹¹ reported an average tensile strength of 33 N for 10 porcine tendons repaired with 4-0 Ethibond using the modified Kessler technique. The repairs were tensile tested at a rate of 2 cm/min, slower than the rate of 5 cm/min used in this study. They did not report the mode of failure of the repair.

Barrie et al¹² showed an ultimate tensile strength of 39 N for a 2-strand Kessler repair with a 4-0 Ethibond suture when tendons were strained at a rate of 4 cm/min. They also reported that all failures occurred by rupture of the suture; however, it was not specified where on the sutures the rupture occurred (whether it failed at the knot).

Abathi et al¹³ reported a mean maximum load to failure of 31.25 N for cadaveric and porcine tendons repaired with a 3-0 Ethibond suture using a modified Kessler or modified Bunnell technique. This study also reported a mean maximum load of 37.46 N for 2-0 barbed nylon repairs performed with modified Kessler or modified Bunnell techniques. Abathi et al¹³ determined the 2-0 barbed suture repair performed better than the 3-0 Ethibond suture repair. The specific suturing technique used for these repairs was not reported for the study; therefore, it is difficult to make a direct comparison to the results of our study.

Stein et al¹⁴ tested tendon repair tensile strengths using a cadaveric model with a loading rate similar to ours. They reported a maximum load to failure for a Kessler repair performed with 4-0 Ethibond sutures of 37.68 N with a dorsally placed stitch, and 33.06 N for a volarly placed stitch. These results are similar to our maximum load to failure for the Kessler repair; however, this study used a different caliber of Ethibond suture.

More recently, McLarney et al¹⁵ tested a 2-strand Kessler repair on 10 human cadaveric flexor tendons using 4-0 braided polyester sutures. They reported a load of 22 N at 2 mm of gapping at the repair site. This compares well to our mean load of 22.8 N at 2 mm of gapping for 3-0 Ethibond suture repairs. In McLarney et al’s¹⁵ study, 9 of 10 Kessler repairs failed by suture pullout, whereas in our study 2 of 20 repairs failed in this manner.

Multiple limitations in our study must be acknowledged. The first limitation was a relatively low sample size to determine if a difference existed within the 2-mm gapping group. Power analysis demonstrated the need for an unrealistic sample size on the order of >1500 specimens to show sufficient power to determine a statistically significant difference. McLarney et al¹⁵ demonstrated a 22 N force needed for 2-mm gapping using a modified Kessler stitch. Our results corroborated this and help substantiate our model in proving the barbed suture 2-mm gapping at 22.2 N. Another limiting factor to the study was reproducibility because of the operator dependency on suturing the repair. To ensure some degree of reproducibility, 2 dozen repairs were conducted prior to the actual testing.

Conclusion

The role of barbed sutures in flexor tendon repair appears to be promising but remains a point of further study. Our findings demonstrate similar tensile strength for both the barbed suture and Ethibond suture groups, although the Ethibond stitch showed a statistically significant difference in maximum load to failure. No definitive recommendations as to the use of barbed sutures for tendon repair can be made at this time without additional biomechanical and clinical studies. In the context of the literature, this study serves to demonstrate the first comparison between barbed and conventional sutures with flexor tendon repairs and proposes a new material in tendon repair to limit adhesions and scarring.

References

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Authors

Drs Trocchia and Sobol and Ms Aho are from the Department of Orthopedic Surgery, William Beaumont Hospital, Royal Oak, Michigan.

Drs Trocchia and Sobol and Ms Aho have no relevant financial relationships to disclose.

Correspondence should be addressed to: Aron M. Trocchia, MD, William Beaumont Hospital, Ste 744, 3535 W Thirteen Mile Rd, Royal Oak, MI 48073.

doi: 10.3928/01477447-20090818-12