The goal of this study was to determine whether a difference in cycles to failure or mode of failure would be observed among specimens of 3 high-strength suture materials, and whether different suture configurations would affect knot security. Ten representative specimens of Ethibond (Ethicon, Inc, Somerville, New Jersey), FiberWire (Arthrex, Inc, Naples, Florida), MaxBraid (Biomet, Inc, Warsaw, Indiana), and Orthocord (DePuy Orthopaedics, Warsaw, Indiana) were tied in 6 different knot configurations commonly used in orthopedic procedures. Each specimen was cyclically loaded between 9 and 180 N at a rate of 1 Hz until the specimen failed or reached a maximum of 3500 cycles. Each suture material was subjected to tensile loading until failure at a rate of 1.25 mm/s. The 3 most secure knots all included the 3 reverse half-hitch on alternating posts (3-RHAP) configuration. All specimens tied with these 3 knot types failed by suture rupture. All knots using the overhand with 3 of the same half-hitches on the same post (O-3SHSP) configuration failed by knot slippage regardless of suture material. When the 3 strongest knots were combined, FiberWire resisted a significantly greater number of fatigue cycles than Orthocord or MaxBraid. In the single load to failure tests, Orthocord, FiberWire, and MaxBraid all had significantly higher ultimate strength than Ethibond.
Knots using the 3-RHAP configuration provide security superior to that of those without this configuration. All 3 high-strength sutures tested outperformed Ethibond in single load to failure testing, with FiberWire resisting the greatest number of cycles. Postoperative strength and reliability of a soft tissue repair is inherently dependent on the properties of the suture materials used.
Suture material may be chosen by a surgeon for a variety of reasons, including permanence vs absorbability, strength, knot security, ease of handling, cost, and even color or markings that can aid in suture management. Recently, multiple high-strength permanent suture materials have become available and currently play a large role in both open and arthroscopic orthopedic interventions. While handling characteristics, color, and markings will remain in the realm of personal preference, knowledge of suture strength and resistance to failure under clinical conditions are vital to the surgeon and the patients.
Knot type and suture type are both critical for soft tissue repairs. Arthroscopic knots were historically considered inferior to hand-tied knots because half-hitches result from the uneven tension applied using a post and a pusher through a cannula rather than square knots, which can be placed during open procedures.1 However, in 2006, Elkousy et al2 reported results demonstrating that arthroscopic knots were equivalent or better than open knots with regard to cyclic loading and load to failure tests. Furthermore, certain material characteristics that may improve ultimate failure strength and knot security may also lead to a higher tendency to cutout through the repaired tissue. Complex suture techniques, such as the Mason-Allen technique, decrease soft tissue cutout but are not yet commonly used with arthroscopic techniques.3
Recently published literature reports several studies of various suture types tested with multiple arthroscopic knot configurations.2,4-9 No studies have yet evaluated MaxBraid (Biomet, Inc, Warsaw, Indiana), FiberWire (Arthrex, Inc, Naples, Florida), and Orthocord (DePuy Orthopaedics, Warsaw, Indiana) head-to-head and compared results with those of Ethibond (Ethicon, Inc, Somerville, New Jersey) tied with the knot configurations used in this study. At the outset of this study, we were unaware of any previous publications that directly compared high-strength materials in terms of knot security under a high-tension (9 to 180 N) cyclic loading condition and load to failure.
The goal of this study was to evaluate the material properties of 3 high-strength suture materials and compare them with Ethibond in 6 knot configurations. The properties evaluated included mechanism of failure (suture rupture vs knot failure vs excessive suture elongation) and load at failure. The hypothesis was that no significant difference in cycles to failure or mode of failure would be observed between representative specimens of 3 high-strength suture materialsFiberWire, MaxBraid, and Orthocordand that all 3 materials would require a higher number of cycles than Ethibond to fail.
Materials and Methods
The 3 high-strength suture materials tested were MaxBraid, FiberWire, and Orthocord. MaxBraid is composed entirely of ultra-high-molecular-weight polyethylene, known as Dyneema Purity.8 FiberWire is a composite material that contains 2 components: a multi-strand polyethylene core and a braided polyester jacket.9 Orthocord is also a composite suture; its makeup includes Dyneema Purity and dyed polydiaxanone. The material is coated with caprolactone and glycolide to increase the ease of handling.
The Duncan loop and SMC base knots were chosen as arthroscopic knots with intermediate internal friction.10 Multiple previous analyses have shown that the most secure arthroscopic knot configurations include 3 reversed half-hitch throws on alternating posts.1,10-12 Therefore, this study included 3 alternating half-hitches on reversed posts on the Duncan loop and SMC base knots (Figure 1). Some surgeons clinically use a half-hitch on the same post as the initial knot in an effort to cinch the knot down more securely. Because of this, a group of sutures with the Duncan loop and SMC base knots with a single half-hitch on the initial post followed by 2 reversing half-hitches on alternating posts was included (Figure 2).
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|Figure 1: The Duncan loop with 3-RHAP knot configuration. Figure 2: The Duncan loop with DL-HHSP-2RHAP knot configuration. |
Each suture was hand tied in similar fashion over a metal post measuring 21 cm in circumference. All knots were tied by 1 senior resident physician who had completed a clinical rotation in arthroscopic surgery (J.F.). He was not blinded as to the type of suture material being tied. It was not believed that this knowledge would significantly influence the results of the test performed. The physician did not wear gloves. He did not use a knot pusher or cannula to minimize the possibility of suture abrasion,12 and because studies have shown no difference in the mechanical properties of knots that are hand tied and knots that are tied arthroscopically.13,14 The large suture loop post was chosen because it was readily available in the laboratory and could be easily tested on a servohydraulic materials test frame (Bionix 858; MTS Corp, Eden Prairie, Minnesota). Although a length of 21 cm was significantly larger than a typical suture loop used clinically, it was not felt that this would alter load to failure or mode of failure results.
We identified 6 knot configurations for testing. These knot configurations were applied to the 6 suture materials, respectively, creating 24 knot/material combinations. Ten specimens were created for each of these 24 knot/material combinations, for a total of 240 specimens. The suture materials were #2 Ethibond, FiberWire, MaxBraid, and Orthocord. The knot configurations included: (1) overhand with 3 reverse half-hitches on alternating posts (O-3RHAP); (2) overhand with 3 of the same half-hitches on the same post (O-3SHSP); (3) Duncan loop with a single half-hitch on the initial post followed by 2 reverse half-hitches on alternating posts (DL-HHSP-2RHAP); (4) Duncan loop with 3 reverse half-hitches on alternating posts (DL-3RHAP); (5) SMC knot with a single half-hitch on the initial post followed by 2 reverse half-hitches on alternating posts (SMC-HHSP-2RHAP); and (6) SMC knot with 3 reverse half-hitches on alternating posts (SMC-3RHAP). Thus a total of 240 sutures were tested.
Individual suture and knot combinations were tested and analyzed for mechanism of failure. This analysis was performed with regard to each knot type, each suture type, and the combinations of each knot and each suture type.
Suture tails were trimmed to approximately 4 mm in length. Each loop was passed around 2 rings attached to the testing machine and pretensioned to 9 N (Figure 3). The loops were then cyclically loaded between 9 and 180 N at a rate of 1 Hz until failure or a maximum of 3500 cycles. Failure was defined as suture breakage, elongation of 10%, or knot slippage. The endpoint of 3500 cycles was chosen because Burkhart et al15 reported that beyond this point, failure by cyclic loading is unlikely to occur. The maximum force of 180 N corresponds with the load produced by the rotator cuff at approximately two-thirds maximal contraction.16 Displacement, number of cycles prior to failure, and mode of failure were recorded.
|Figure 3: The suture loop being tensioned on the material testing device between 9 and 180 N. |
Kaplan-Meier survival estimates were calculated and plotted both by knot type (with Ethibond sutures excluded) and by suture material within the 3 knot types with the best fatigue performance. Log-rank tests between knots or materials were performed to evaluate differences in survivorship curves.
Additional strands of each type of suture material were cut to equal lengths and subjected to loading to failure at a rate of 1.25 mm/s.17 The ultimate strengths were recorded and a load-deformation curve was generated.
The 3 strongest knot configurations combined performed significantly better than the other 3 knots combined (P<.001) (Figure 4). The superior knots all included the 3- RHAP configuration. All knots in the O-3SHSP groups failed by knot slippage regardless of suture type, indicating incompetence of knot strength rather than poor suture performance. The SMC and Duncan loop knots both failed most commonly by knot slippage, but this mode was only significant for the SMC knot. Neither of the SMC and Duncan loop knots that failed were backed up by the 3-RHAP configuration. The O-3SHSP knot failed by this same mode at a significantly higher rate.
|Figure 4: The 3 most reliable knots (all with the 3-RHAP configuration) are compared with the 3 other knot configurations. The 3-RHAP demonstrated greater survival. |
It was rare for any knot/suture combination to be stretched to failure. Although the difference in suture breakage between the SMC and Duncan loop knots was not significant, there was a significant difference shown in the number of SMC knots reaching the maximum number of cycles compared to the Duncan loop. In this case, the Duncan loop was superior with regard to reaching the maximum number of cycles before failure.
When the 3 superior knots were combined, FiberWire resisted a greater number of cycles than Orthocord (P<.001) and MaxBraid (P=.014). FiberWire, MaxBraid, and Orthocord all outperformed Ethibond (all P<.001) with regard to the number of stress cycles before suture failure.
Failure mechanism by suture type is shown in the Table. The most common mode of failure by far was knot slippage for all suture types except for Ethibond, which experienced suture breakage most commonly. MaxBraid had a significantly higher rate of knot slippage (90.9%) than FiberWire (78.6%) and Orthocord (63.3%), respectively. Orthocord had a significantly higher rate of suture breakage than FiberWire or MaxBraid at 36.7%.
In the single load-to-failure tests, Orthocord (218 N), FiberWire (200 N), and MaxBraid (184 N) all had significantly higher load to failure than Ethibond (136 N) (P=.01). The difference between Orthocord and FiberWire compared to MaxBraid approached but did not reach significance. As previously stated, the most common mode of failure for all 3 high-strength sutures tested was knot slippage. For Ethibond, suture breakage was seen most commonly by an overwhelming margin.
Although knot slippage was shown to be the most common failure mechanism for FiberWire, MaxBraid and Orthocord, FiberWire showed a clear superiority over the other suture materials with regard to reaching maximum number of cycles prior to failure. FiberWire reached maximum cycles in 3 of 4 knot types, which is more than any other suture type. The O-3SHSP knot did not produce any completions with FiberWire.
Orthocord, however, showed clear inferiority to the other sutures in the same regard. None of the Orthocord specimens tested reached the maximum number of cycles. MaxBraid was the only suture type other than FiberWire that reached completion, although this occurred in only 2 knot types, compared with 3 in FiberWire and fewer specimens overall compared to FiberWire. With regard to Ethibond, the suture broke in every trial with every type of knot except the O-3SHSP configuration, which slipped in all specimens tested.
This study evaluated 6 arthroscopic knot configurations applied to secure 3 high-strength suture materials and tested these specimens in single load-to-failure and cyclic loading. To our knowledge, no other study has tested this combination of suture materials and knot configurations in this manner. Individually, FiberWire performed the best of all suture type materials, and it did so with the Duncan loop and the overhand alternating configurations. This combination created the highest number of sutures reaching the maximum number of cycles. MaxBraid produced individual sutures reaching completion with the Duncan loop and overhand alternating knot but at a much lower rate. Orthocord and Ethibond produced no specimens reaching the maximum number of cycles, with Ethibond sutures breaking or slipping in all tests.
In 2006, Barber et al16 reviewed numerous suture materials, including Orthocord, Ultrabraid (Smith & Nephew, Memphis, Tennessee), Force Fiber (Teleflex Medical OEM, Kenosha, Wisconsin), Hi-Fi (Conmed Linvatec, Largo, Florida), MagnumWire (ArthroCare, Austin, Texas) and MaxBraid. The study evaluated single load-to-failure mode at 12.5 mm/s. This rate was 10 times that which was used in the current study. Their results showed that Orthocord and FiberWire4 failed at lower tensile loads than the other materials tested. These other materials included MaxBraid. The study further found that Orthocord, FiberWire, and MaxBraid failed at 92 N, 188 N, and 256 N, respectively.
The results of Barber et al16 are in contrast to those of our study, given that in our protocol Orthocord outperformed FiberWire, and FiberWire outperformed MaxBraid in single load-to-failure testing (218 N, 200 N, and 184 N, respectively). The difference in ultimate strength results found in these 2 studies may be due to a difference in strain rates. Multiple previous studies have found higher load to failure with increased strain rates.18-20
In 2006, Mahar et al21 compared the performance characteristics of the Duncan loop, the Weston knot, and the San Diego knot tied with #2 Ethibond and #2 Force Fiber sutures. Specimens were pretensioned to 10 N and mechanically loaded from 10 to 45 N for 1000 cycles. Intact knots with no evidence of slippage were subjected to load-to-failure testing to determine ultimate strength of the knot/material combination. One-third of the Ethibond knots slipped by >3 mm during cyclic loading. For Ethibond sutures, no significant differences were observed between knot configurations. However, with Force Fiber sutures, the San Diego knot was statistically similar in ultimate failure strength to the Weston knot but was significantly stronger than the Duncan knot. In contrast, our results showed Ethibond suture breaking in nearly every trial. The tension placed on the suture as well as the number of cycles attempted were significantly higher than those of Mahar et al21 (9 to 180 N and 3500 cycles, respectively).
Our study found the ultimate strength of Orthocord, FiberWire, and MaxBraid were greater than Ethibond, but there was no significant difference between the 3 high- strength sutures. Under cyclic loading conditions, however, FiberWire had a significantly longer fatigue life than the other suture materials tested.
With regard to knot security, our findings were consistent with those of several other studies. Knots with 3-RHAP configurations significantly improve knot security.1,10-12 The O-3SHSP was not a secure, reliable knot regardless of suture material used. Based on these findings, these knots should be used with caution.
Jo et al22 in 2008 studied 4 commonly used arthroscopic knots: Duncan loop, SMC, Weston, and SP. The first half-hitch was either placed on the same post or on the loop of the sliding knot. Loop security and knot security were measured for Ethibond or #1 PDS II (Ethicon) sutures. It was found that knot configurations, number of RHAPs, or mode of placement of the first half-hitch did not have a clinically significant effect on loop security for either suture type. The study further demonstrated that switching the post just after the sliding knot could save 1 half-hitch without compromising knot security.
Results of these investigations further showed a significant difference in fatigue life using a base knot with HHSP-2RHAP compared to the base knots with 3-RHAP. The technique of creating knots impacts the strength of the knot. Using the initial half-hitch on the same post as the base knot to cinch the knot down followed by 2 RHAPs should not be considered equal to using 3-RHAP knot configurations.
This study had several limitations. The suture loops that were tied around the hydraulic machine prior to stress were much larger than a suture loop used during an operative procedure. Only 3 knot configurations were used in the final evaluation of the 4 sutures tested, and the loads used in our testing module (9 to 180 N) may not perfectly replicate those that sutures are subjected to in vivo. Finally, the fact that the physician tying the knots was not blinded to the suture materials that he was tying may have introduced bias into the final results.
Based on the results of this study, knots using the 3-RHAP configuration provide security superior to that of those without this configuration. All 3 high-strength sutures tested (FiberWire, Orthocord, and MaxBraid) outperformed Ethibond in single load-to-failure testing, with FiberWire resisting the greatest number of cycles.
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Drs Miller, Feinblatt, Litsky, and Flanigan and Mr Craw are from the Department of Orthopedics, The Ohio State University, Columbus, Ohio.
Drs Miller, Feinblatt, Litsky, and Flanigan and Mr Craw have no relevant financial relationships to disclose.
Correspondence should be addressed to: David C. Flanigan, MD, Department of Orthopedics, The Ohio State University, 2050 Kenny Rd, Columbus, OH 43221 (firstname.lastname@example.org).