Comparison of the mechanical properties and anchoring performance of polyvinylidene fluoride and polypropylene barbed sutures for tendon repair

Abstract Polyvinylidene fluoride (PVDF) has been considered as an alternative suture material to replace polypropylene (PP) due to its superior biocompatibility and mechanical properties, but it has never been examined for use in barbed sutures, particularly for tendon repair. This study fabricated size 2–0 PVDF and PP bidirectional barbed sutures and compared their mechanical properties and anchoring performance in patellar tendons. The mechanical properties were evaluated via tensile testing, and the anchoring performance of the barbed sutures was assessed by a tendon suture pullout test. Sixty porcine patellar tendons were harvested, transected to mimic a full‐thickness injury, and repaired using a cross‐locked cruciate suturing technique. The ultimate tensile force was 60% higher for the PVDF barbed sutures (22.4 ± 2.1 N) than for the PP barbed sutures (14.0 ± 1.7 N). The maximum pullout force was 35% higher for PVDF barbed sutures (70.8 ± 7.8 N) than for PP barbed sutures (52.4 ± 5.8 N). The force needed to form a 2‐mm gap, indicative of repair failure, was similar between the PVDF (29.2 ± 5.0 N) and PP (25.6 ± 3.1 N) barbed sutures, but both were greater than the 2‐mm‐gap forces for non‐barbed sutures of the same size. In this study, PVDF barbed sutures provided better mechanical properties and improved tissue anchoring performance compared to the barbed PP sutures for porcine patellar tendon repair, demonstrating that PVDF monofilament sutures can be barbed and used effectively for tendon repair.

points, reducing the maximum holding capacity of the suture and widening the cross-sectional area of the tendon. 3 Barbed knotless sutures have recently gained more attention because they have certain advantages over traditional knotted sutures. 4 With multiple barbs projecting from their surface and pointing parallel to each other in the direction away from the needle, barbed sutures can be passed through the tissue when pulled from the needle end and resist removal when pulled in the opposite direction. Since the barbs grasp the surrounding tissues, knots are unnecessary, making suturing during surgery both easier and faster. Without the presence of the knots, barbs distribute the anchoring stress along the length of the suture, applying more consistent tension across the wound and reducing the repair-site cross-sectional area, which facilitates the healing process.
In 1967, McKenzie first proposed the idea of using an internal multiple barbed suture to repair flexor tendons in a canine model, but due to poorly constructed barb configurations and unsatisfactory biomaterials, interest in using barbed sutures soon waned. 5,6 With recent developments in technology and biomaterials, a resurgence of interest has occurred, and the US Food and Drug Administration (FDA) has approved several commercial barbed nylon, polydioxanone (PDS), and polypropylene (PP) sutures over the last decade. Barbed sutures have attracted more attention and have been introduced into the surgeon's armamentarium for their specific advantages over traditional knotted sutures. In particular, they are being widely used in cosmetic and plastic surgeries, and also in other types of surgery that are space-limited, such as in laparoscopic surgery and less invasive obstetric and gynecological surgeries. [7][8][9] Ex vivo studies have investigated using size 2-0 and 3-0 PP barbed sutures for flexor digitorum profundus tendon repair in pigs and cadavers, and barbed sutures made from different materials were also used to evaluate their possible use for tendon repair in several animal and human models. [10][11][12][13][14][15][16][17] Two in vivo animal studies have examined the use of barbed sutures for tendon repair, one in 2015 using 3-0 Quill SRS PDS bidirectional barbed sutures in chickens to repair the flexor digitorum profundus tendon, and one in 2019 using 2-0 PP bidirectional barbed sutures in a canine case study to repair a complete common calcanean tendon rupture. 18,19 However, barbed sutures have not yet been used for human clinical tendon repair.
Although PP barbed sutures have been used widely for clinical skin closure and in plastic and general surgery, some reports have expressed dissatisfaction with PP monofilament sutures because of their thrombogenicity and reports of mechanical failure. 20 Polyvinylidene fluoride (PVDF) has been considered as an alternative suture material to replace PP, because several ex vivo and in vivo evaluations have reported superior biomechanical performance, improved creep resistance, and greater biocompatibility and biostability of PVDF sutures. 8,9,21,22 For example, when PVDF and PP sutures were exposed to hydrolytic conditions for 9 years, the PVDF sutures lost only 7.5% of their initial tensile strength compared to 46.6% lost for PP sutures; and in a 2-year in vivo study of a canine thoracoabdominal bypass model, surface stress cracking was visible on PP sutures but not on PVDF sutures, indicating more long-term biostable potential for PVDF than PP. 8,9 The overall goal of this study was to evaluate and compare the performance of PVDF and PP knotless barbed sutures, in particular the anchoring performance, in the repair of tendon tissue. We hypothesized that PVDF barbed sutures will have superior mechanical properties and tissue anchoring for a full-thickness tendon repair.

| Materials
Suture sizes 2-0 and 3-0 are commonly used for human flexor tendon repairs, and these sizes of barbed sutures were used in previous cadaver and animal in vivo repair studies. 11,18,19,[23][24][25] Manufacturers report that the mechanical properties of barbed sutures are similar to those of non-barbed sutures of one size smaller. 26 Therefore, size 2-0 (non-barbed and barbed) and 3-0 (barbed) sutures were examined in this study. PVDF and PP surgical sutures were obtained from G. Krahmer GmbH (Buchholz, Germany).
Sixty porcine knees (30 pairs) from 3-to 4-year-old female pigs with an average weight of 561 lbs (423-716 lbs) were obtained from City Packing Company through Neese Country Sausage, Inc.
(Burlington, NC). The knees were dissected to expose the patellar tendons, which were transected in the middle to mimic a full-thickness injury ( Figure 1) and then stored at À7 C until surgical repair and testing. Each tendon was randomly assigned to one of six different groups

| Tendon suture pullout test
To examine the anchoring performance of the barbed suture within the surrounding tissue, the maximum force during a tendon suture pullout test was calculated as an indicator for how much force the repaired tissue could support during early active motion, such as post-surgical physical rehabilitation. The tendons were mounted between flat, stainless steel clamps and, after applying a preload of 2 N, were tested to failure in uniaxial tension using a crosshead speed of 20 mm/min (Instron 5584 mounted with a 2000-N load cell). The maximum force was recorded.
Gap formation was also monitored as a measure of the surgical efficacy of the repair and the anchoring strength of the suture material. Clinically, if the gap at the repair site widens more than 2 mm, then tendon function is lost, and the repair is considered a failure. 17,28 To monitor the gap size formed at the repair site during the pullout test, a dial caliper set to a 2-mm gap was placed adjacent to the tendon. Gap formation was recorded with a high-definition video camera (Canon VIXIA HF R62, Canon Inc., Tokyo, Japan) that was manually synchronized with the load cell output. After testing, the video images were analyzed frame-by-frame using ImageJ software (National Institutes of Health, Bethesda, MD) to determine the force required to generate a 2-mm gap, which was defined as failure of the tendon repair.

| Suture tensile test
The ultimate tensile force decreased significantly after cutting the barbs ( Figure 6,  Figure 7), and 3-0 non-barbed sutures (23.1% greater, p < .001). The ultimate tensile force of PVDF 2-0 barbed sutures was similar to that of PVDF 3-0 non-barbed sutures (p = 0.90), consistent with the information provided by the manufacturers stating that the tensile properties of a barbed suture are equivalent to the properties of a non-barbed suture of the same material that is one size smaller. 26 However, different from PVDF, PP 2-0 barbed sutures required significantly less peak force to break than did PP 3-0 non-barbed sutures (p < .001).

| Tendon suture pullout test
The maximum force during the tendon suture pullout test represented the force required either to break the suture or pull it out of the tendon tissue. Consistent with the suture ultimate tensile force, the maximum pullout test force was greatest for the 2-0 non-barbed sutures (  The force required to form a 2-mm gap, which was defined as failure for the tendon repair, was 32.6% larger for the PP 2-0 barbed sutures than for the PP non-barbed sutures of the same size (p < .001, Table 2). The 2-mm-gap force of PVDF 2-0 barbed sutures was similar to that of PVDF 2-0 non-barbed sutures (p = .12). In addition, the 2-mm-gap force was equivalent between the PVDF and PP materials for the 2-0 barbed sutures (p = .11, Figure 7) but was marginally greater for PVDF than for PP in 2-0 non-barbed sutures (p = .046) and marginally lower for PVDF than for PP in 3-0 non-barbed sutures (p = .028). Also, significantly more force was required to form the 2-mm gap in 2-0 barbed sutures than in 3-0 non-barbed sutures, both for PVDF (p < .001) and PP (p < .001) materials.

| DISCUSSION
Although barbed sutures have been available for surgeons to use for several decades, the US FDA has only approved the use of biodegradable barbed sutures for plastic and cosmetic surgeries (e.g., rhytidectomy and breast reconstruction) and for soft tissue endoscopic surgeries (e.g., laparoscopic and urologic procedures) but not yet for tendon repair. Because the commonly used suture material PP is associated with the problems of thrombogenicity, creep, and long-term mechanical fatigue, PVDF is considered an attractive alternative suture material due to less thrombogenicity and superior longterm mechanical fatigue performance. In this study, we successfully fabricated PVDF barbed sutures using a manual barb cutting machine.
These novel PVDF barbed sutures showed superior mechanical properties (8.4 N greater ultimate tensile strength for size 2-0 sutures) and anchoring performance (18.4 N greater tendon suture pullout strength) compared with PP barbed sutures when used for tendon repair. A sufficiently strong tendon repair is needed so that the tendon can tolerate the forces generated during early active motion in rehabilitation programs. Previous studies have reported that tenomalacia at the suture-tendon junction site caused 50% loss of initial strength of the immobilized sutures within the first week, whereas the breaking force for the tendon repair depends on the mechanical properties of the sutures, as well as the local anchoring performance of the suture-tissue interface. [29][30][31] Our study demonstrated that PVDF non-barbed sutures had a higher ultimate tensile force than PP non-barbed sutures of the same size. The greater tensile properties of PVDF non-barbed sutures may be related to the higher crystallinity of PVDF (59%) compared to PP (43%), since with higher crystallinity, the intermolecular bonding is more significant, which would lead to increased strength. 32  Multiple suture strands crossing the repair site are needed to allow early postoperative movements without damaging the tenorrhaphy, and currently 4-strand repairs are the minimum recommendation for tendon repairs. 24 A previous study of flexor tendon repair in human cadavers suggested that the number of core suture strands across the repair site played a more important role in the strength of tendon repair than did suture size; they found that an 8-strand repair using 4-0 sutures was 43% stronger than a 4-strand repair using 3-0 sutures, even though the 3-0 suture was 49% stronger than the 4-0 suture made from the same material. 17 In other studies using a 4-strand repair, locking cross-stitches had significant benefits compared to looped techniques with regard to greater gap formation force. 7,24,25 One limitation of this study was that, while the barb cutting machine provided the desired geometry and uniformity of barbs, the procedure was performed manually, since PVDF barbed sutures are not yet commercially available. More studies are needed to evaluate the efficacy of commercial barbed sutures, which are created on a continuous production machine, in tendon repair. In addition, the repairs were performed by a researcher under the advisement of an experienced orthopedic surgeon, rather than directly by the surgeon, which likely introduced more variable results. Nevertheless, this study showed that although the mean 2-mm-gap formation force did not differ significantly between the PVDF and PP 2-0 barbed sutures, the combined greater degree of bending in PVDF barbs and superior tendon pullout force indicate that the PVDF barbed sutures show promise for use in tendon repair.

| CONCLUSIONS
This study showed that barbed sutures can be fabricated with PVDF monofilament sutures and have the potential to be used for tendon repair. The PVDF knotless barbed sutures showed superior mechanical properties, with higher ultimate tensile strength and stiffness and higher maximum tendon pullout force, compared with PP barbed sutures. They also showed excellent anchoring performance, with higher 2-mm-gap formation force than non-barbed sutures of the same size. Future work will focus on improving the efficiency and reliability of the barb cutting process, as well as performing cyclic tests that simulate long-term in vivo loading to assess the clinical advantage of using PVDF barbed sutures over traditional knotted PP and PVDF sutures.

ACKNOWLEDGMENTS
We thank Dr. Russell R. Gorga for providing the laboratory mechanical testing equipment and Dr. Hui Cong for providing technical support in the Biomedical Textiles Research Laboratory.

DATA AVAILABILITY STATEMENT
The data that support the findings of this study are available from the corresponding author upon reasonable request.