Using half-hitch knots to uncouple surgical knot security and loop tautness

Introduction

Despite the importance of the security of surgical knots to patient outcomes across specialties with known knot-related complications running the gamut from wound dehiscence to massive exsanguination and death (1-10), there is no standardized knot-tying curriculum for surgical residents or medical students. Trainees commonly learn how to tie knots through makeshift didactics, educational video series, or “boot camps” for technical skills with highly variable results (11-15). Compounding the problem, these modes of teaching often focus exclusively on the principles underlying the security of flat knots but overlook those of half-hitch knots (16). Half-hitch knots are tied when asymmetric tension is applied to the suture strand ends during tightening. This is problematic because most surgeons and surgical trainees tie half-hitch knots (17,18). The lack of half-hitch knot-tying curricula sets the stage for surgeons to tie knots, at times to critical structures, without understanding how these knots can be conferred with adequate loop tautness and adequate knot security, or how these two concepts are related.

The security of half-hitch knots is primarily dictated by whether greater force is applied to only one of the two suture strand ends during tying, a technique we will refer to as the single post method, or if the strand to which greater force is applied alternates between the two strand ends with each throw, a technique we will refer to as the alternating post method. We recently demonstrated in a systematic review that the incorporation of post alternation confers significantly greater security to half-hitch knots than single post techniques, regardless of whether they are tied in square or granny fashion (5,17,19-25). The trend that emerges from the literature is that when tying half-hitch knots, square knots utilizing alternating posts are more secure than granny knots utilizing alternating posts which are more secure than square knots tied on a single post which are more secure than granny knots tied on a single post (5,17,20-27).

A common reason we have encountered for surgeons being resistant to adopt techniques that lead to increased knot security is a concern that an overly secure knot will strangulate the tissue encompassed by the loop of the suture with which the knot is tied. We hypothesized that the security of a single post half-hitch knot is more dependent on the tautness of its associated loop for security than an alternating post half-hitch knot. We aim to determine whether the tautness of a loop is more closely associated with the security of single post knots than alternating post knots.

Methods

Tying setup

For the tying of each knot, a suture was passed through two adjacent brass rings of outer diameter 37 mm and inner diameter 25 mm, and then around a piece of latex hose that was cut to 3 cm in length and had a wall thickness of 3.2 mm and an outer diameter of 9.5 mm. The latex hose was taped down on a flat surface to stabilize the setup during tying.

Tested knot configurations

Four different knot configurations were tested. For every knot, each throw was tightened in a half-hitch orientation, and none were tightened in a flat orientation. Every throw was opposite from the throw prior to it, such that every sequence of throws for every knot tied represented a square half-hitch configuration rather than a granny half-hitch configuration. All knots were tied with 0 silk and six throws were utilized for each knot. The same individual tied all of the knots.

The first knot type tested was tied using exclusively single post half-hitch throws. The first throw was tightened down to the surface of the latex hose, but not beyond it (“non-taut”), and subsequent throws were similarly tightened to the level of the knot, but not beyond it, to avoid compressing the latex hose. In Dinsmore notation, this knot type would be represented as SxSxSxSxSxS. The second knot type tested was tied in the same manner for the first three throws, which were tied on a single post and advanced only to the level of the latex hose (“non-taut”), but the subsequent three throws were tied by alternating between the two strand ends as the post to generate a knot with three alternating post throws. In Dinsmore notation, this knot type would be represented as SxSxS#S#S#S. The third knot type tested was tied with the use of single post half hitch throws (SxSxSxSxSxS), but in contrast to the first knot type, every throw was tightened beyond the level of the surface of the latex hose (“taut”). Finally, the fourth knot type tested involved the tightening of the first three half-hitch throws beyond the level of the latex hose (“taut”), followed by three alternating post throws (SxSxS#S#S#S). The exact pre-tension forces could not be measured due to the nature of the experimental setup.

Knot testing

After each knot was finished, the setup was first inspected to ensure that there was no space between the two brass rings and the latex hose. Then the apposition of the suture with the hose was observed to confirm that the non-tautly tied first and second knot types did not compress the lumina of the hoses and that the third and fourth knot types did compress the lumina of the hoses as evidenced by crimping of the hoses where the sutures were in apposition with them (Figure 1). Each hose was then carefully divided sharply and removed from the loops of the knots. Afterwards, the pairs of brass rings tied together were mounted on a manual tensiometer. Knots were distracted at a rate of 12.5 millimeters per minute. Distraction was performed to the beat of an electronic metronome with a plate mounted below the hand crank divided into octants. The tensiometer display was recorded by two video cameras during knot testing. The maximum forces generated by knots by 1 and 2 mm of displacement, noted with audio and visual cues during the video recordings, and to ultimate failure, were documented upon review of the video recordings. Ultimate failure was defined as the knot either slipping to the point of unraveling, or the suture breaking.

Figure 1 Latex hoses tied to brass rings for each of the four different knot configurations. From left to right: tautly tied alternating post knot, tautly tied single post knot, non-taut alternating post knot, non-taut single post knot. The suture loop of the non-taut knots does not compress the edge of the latex hose, whereas for the tautly tied knots, crimping can be appreciated at the interface of the suture with the wall of the latex hose.

Statistical analysis

Two-sided t-test was performed to compare mean knot securities between each of the four knot types tested at 1 mm, 2 mm, and ultimate knot failure. Statistical significance was declared for P less than 0.05. All statistical analysis was performed in R (version 3.5.1, R Foundation for Statistical Computing, Vienna, Austria) using RStudio (version 1.1.463) and package “tidyverse” (version 1.3.0).

Results

The latex hoses associated with all of the tautly tied knots were crimped where the sutures encircled them, and none of the latex hoses for any of the non-tautly tied knots were crimped at the site where the suture encircled them.

The mean of the maximum forces achieved by each knot type at 1 and 2 mm of distraction and ultimate failure are shown in Table 1. By 1 mm of distraction, significant differences were noted between the non-taut alternating post knot and the non-taut single post knot (16.2 vs. 2.8 N, P<0.001) and between the tautly tied single post knot and the non-taut single post knot (14.2 vs. 2.8 N, P<0.001). The tautly tied alternating post knot was more secure than the tautly tied single post knot (24.0 vs. 14.2 N, P=0.04) and the non-taut single post knot (24.0 vs. 2.8 N, P=0.002) but was not significantly different from the non-taut alternating post knot.

By 2 mm of distraction (Figure 2), significant differences were seen between the tautly tied single post knot and the non-taut single post knot (17.6 vs. 3.0 N, P=0.003). Differences were also observed between the non-taut alternating post knot and the non-taut single post knot (39.0 vs. 3.0 N, P<0.001) and between the non-taut alternating post knot and the tautly tied single post knot (39.0 vs. 17.6 N, P=0.001). Differences in security were also identified between the tautly tied alternating post knot and the non-taut single post knot (50.0 vs. 3.0 N, P<0.001), the tautly tied single post knot (50.0 vs. 17.6 N, P<0.001) and the non-taut alternating post knot (50.0 vs. 39.0 N, P=0.03).

Figure 2 Knot security by 2 mm of slippage for each of the knot types tested. The mean security of the taut alternating posts knots (50.0 N) was significantly higher than for any of the other knot types. The security of the non-taut alternating posts knots (39.0 N) was significantly higher than that of the taut single post knots (17.6 N) and the non-taut single post knots (3.0 N), and the taut single post knots were more secure than the non-taut single post knots.

By ultimate failure (Figure 3), significant differences were noted between the tautly tied single post knot and the non-taut single post knot (17.6 vs. 3.2 N, P=0.003). Differences were also identified between the non-taut alternating post knot and the non-taut single post knot (61.6 vs. 3.2 N, P<0.001) and between the non-taut alternating post knot and the tautly tied single post knot (61.6 N vs. 17.6 N, P<0.001). Differences were also identified between the tautly tied alternating post knot and the tautly tied single post knot (62.2 vs. 17.6 N, P<0.001) and the tautly tied alternating post knot and the non-taut single post knot (62.2 vs. 3.2 N, P<0.001), but not between the tautly tied alternating post knot and the non-taut alternating post knot.

Figure 3 Knot security by ultimate knot failure. The mean security of the taut alternating posts knots (62.2 N) and the non-taut alternating posts knots (61.6 N) were significantly greater than those of the taut single post knots (17.6 N) and the non-taut single post knots (3.2 N). The taut single post knots were more secure than the non-taut single post knots.

Discussion

Herein we demonstrate that the knot security of single post knots are more dependent on the tautness of their associated loops than are knots that incorporate post alternation, and the knots tied with post alternation were more secure than the single post knots, regardless of the tautness of the loop. This dispels the notion that a more secure knot will have a tauter associated loop. A tauter loop could cause undesired consequences such as tissue strangulation. However, a single post half-hitch knot can be made more secure with the use of alternating posts than it can be by maintaining a single post but applying more pre-tension to create a tauter loop. The security of a non-taut single post knot by 2 mm of slippage could be increased thirteen-fold by incorporating post alternation compared to a six-fold increase by tying the knot more tautly. By ultimate failure, the non-taut alternating post knots were nineteen-fold more secure than the non-taut single post knots, and the tautly tied single post knots remained six-fold more secure than the non-taut single post knots.

The ability to uncouple knot security from loop tautness by appropriately applying sequences of single and alternating post throws into a knot is valuable because there are many scenarios in which a secure but non-taut knot is desired. For example, in an enterotomy repair, it is desirable to have a knot that is non-taut to avoid strangulation of the tissue which could induce a leak or prevent the enterotomy from healing. However, it is also desirable for the knot to be secure, so that if the patient develops an obstruction and the bowel becomes distended, the increased force on the knot does not cause it to slip and allow for a leak. While flat square knots accomplish these objectives, in some cases they are impractical to tie due to limitations of space or exposure. In such cases, a knot can be initiated with two or more single post throws advanced to the bowel wall far enough that the wall is well apposed but not so far that it becomes constricted by the suture loop, and then throws with alternating posts can be added to that base knot to give it security so that the knot to prevent knot slippage. Such non-taut but secure knots are often desirable—in an animal study of the healing of aponeurotic tissues, knots that were tied in a non-taut fashion were associated with improved tissue strength relative to those tied tautly when tested at 3, 4, 7, 10 and 14 days after closure (28). In other cases, such as the ligation of a blood vessel, a tautly tied knot is more desirable to occlude the lumen of the structure being tied, in which case beginning the knot with a series single post throws until desired tautness/occlusion is achieved would be optimal. By understanding these principles, the knot can be tied in a manner fitting for any scenario encountered.

Commonly in the knot-tying literature, clinical failure of a knot is defined as slippage by more than 2 mm (2,17,19,20,29,30) or, frequently in the orthopaedic literature, slippage by more than 3 mm (21-25,31). We selected the former threshold, but we also assessed the security achieved by 1 mm, as well as the maximum security achieved by the time of ultimate failure of the knot by either complete slippage or breakage of the suture. We checked securities at these additional levels of distraction because we hypothesized that the differences between the taut and non-taut knots would increase as distraction progressed, which was confirmed by our findings. By 1 mm of distraction, the tautly tied single post knots were comparable in security to the non-taut alternating post knots, but by 2 mm and ultimate failure the non-taut alternating post knots were significantly more secure. This is because throws that are tightened on an alternate post principally derive security from their configuration rather than how tautly they are tied. When such knots are placed under tension, the alternation of post strands can be thought of as blocking the throws closer to the loop from slipping backwards—such terminology is actually incorporated into two of the many half-hitch knot nomenclatures that have been developed, including that of German surgeon Meiss who dubbed such knots “blockierenden Standardknotens” (standard blocking knots) and that of Italian surgeon Romeo who refers to alternating post knots as “blocking half-hitches” designated by the symbol Sb (32,33).

It has been assumed by many investigators that practicing surgeons would be resistant to change their established knot-tying techniques. The innovator who first rigorously studied surgical knots, Taylor, noted in 1939 the hubris of surgeons and their assumptions that only other surgeons tie faulty knots (34). The pioneer of the study of half-hitch knot security, Trimbos, expressed doubt that surgeons would change their practice in response to scientific evidence (35). This was likely reflective of surgical culture in these eras and may have dampened enthusiasm by these early investigators for attempting to popularize their findings. Today, however, evidence-based practice in surgery is highly valued.

The major strength of this study is its design which allows for a straightforward uncoupling of measures of a knot’s security from the tautness of its associated loop. To our knowledge, this is the first study to demonstrate that the influence of loop tautness on security is mediated through knot configuration for half-hitch knots. This is important because in our anecdotal experience, a common reason offered for resistance to changing knot-tying technique is concern regarding resultant loop tautness causing tissue strangulation. Our results explain the reason behind this concern—surgeons tying single post half-hitch knots are correct in their concern that these knots are made significantly more secure by tying them tautly. However, knot security can be more appropriately increased, and independently of the loop tautness, using post-alternation. Another strength is the testing of multiple different one-handed alternating-post tying techniques. Finally, while neither a strength nor a weakness, the decision to tie and test five knots per configuration was made after review of other studies on half-hitch knot securities, which ranged from 5 to 16 knots tested per unique combination of configuration and suture material and size (5,20-23,29-31,36,37).

There are several limitations to our study. First, while care was taken to remove the latex hose from the suture loop with minimal effect on the knot, given that the loop was tied directly around the latex hose, it is very likely that every knot was exposed to a small amount of tension prior to testing. This tensioning should not have influenced results, but the manipulation is nonetheless a limitation. Another limitation was the use of a manual tensiometer, however we attempted to minimize this limitation by turning the hand crank to the beat of a metronome with a marker piece mounted on the tensiometer to attune to the beats. One final limitation was the tying of knots in a dry environment. Environmental moisture has been shown to influence knot security (38), so tying in a moist environment likely would have influenced the securities, but there is no evidence to suggest that the effect of moisture is mediated through knot configuration, so we opted not to incorporate this into our study.

Conclusions

Surgeons who tie single post half-hitch knots should consider adopting methods that mix single and alternating post throws. Single post throws should be performed during the initiation of a knot in order to achieve a desired degree of loop tautness. Beyond that point, the knot can be made significantly more secure without further increasing the loop tautness by changing the knot configuration to one involving post alternation. In this way a surgeon can independently modulate the knot security and the loop tautness as needed for every encountered scenario.

Acknowledgments

This article has been presented at the 2024 Association of VA Surgeons Annual Meeting.

Footnote

Peer Review File: Available at https://atm.amegroups.com/article/view/10.21037/atm-25-2/prf

Funding: None.

Conflicts of Interest: All authors have completed the ICMJE uniform disclosure form (available at https://atm.amegroups.com/article/view/10.21037/atm-25-2/coif). The authors have no conflicts of interest to declare.

Ethical Statement: The authors are accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved. This research meets the ethical guidelines and no approval or informed consent was necessary, and the study adhered to US legal requirements.

Open Access Statement: This is an Open Access article distributed in accordance with the Creative Commons Attribution-NonCommercial-NoDerivs 4.0 International License (CC BY-NC-ND 4.0), which permits the non-commercial replication and distribution of the article with the strict proviso that no changes or edits are made and the original work is properly cited (including links to both the formal publication through the relevant DOI and the license). See: https://creativecommons.org/licenses/by-nc-nd/4.0/.

References

1. Tera H, Aberg C. Relaparotomy. A ten-year series. Acta Chir Scand 1975;141:637-44. [PubMed]
2. Tera H, Aberg C. Tensile strengths of twelve types of knot employed in surgery, using different suture materials. Acta Chir Scand 1976;142:1-7. [PubMed]
3. Melinek J, Lento P, Moalli J. Postmortem analysis of anastomotic suture line disruption following carotid endarterectomy. J Forensic Sci 2004;49:1077-81. [Crossref] [PubMed]
4. Waitayawinyu T, Martineau PA, Luria S, et al. Comparative biomechanic study of flexor tendon repair using FiberWire. J Hand Surg Am 2008;33:701-8. [Crossref] [PubMed]
5. Balgobin S, Hamid CA, Brown SA, et al. Mechanical performance of surgical knots in a vaginal surgery model. J Surg Educ 2013;70:340-4. [Crossref] [PubMed]
6. Johanns P, Baek C, Grandgeorge P, et al. The strength of surgical knots involves a critical interplay between friction and elastoplasticity. Sci Adv 2023;9:eadg8861. [Crossref] [PubMed]
7. Kumar DA, Agarwal A, Agarwal A, et al. Long-term assessment of tilt of glued intraocular lenses: an optical coherence tomography analysis 5 years after surgery. Ophthalmology 2015;122:48-55. [Crossref] [PubMed]
8. Li S, Li M, Wong KK, et al. Laparoscopically assisted simple suturing obliteration (LASSO) of the internal ring using an epidural needle: a handy single-port laparoscopic herniorrhaphy in children. J Pediatr Surg 2014;49:1818-20. [Crossref] [PubMed]
9. Ma X, Cao DY, Dai YX. Experience in the Management of Vaginal Cuff Dehiscence and Evisceration: A Retrospective 37-Year Single-Center Study. Front Surg 2022;9:880875. [Crossref] [PubMed]
10. Mäkelä JT, Kiviniemi H, Juvonen T, et al. Factors influencing wound dehiscence after midline laparotomy. Am J Surg 1995;170:387-90. [Crossref] [PubMed]
11. Stewart RA, Hauge LS, Stewart RD, et al. A CRASH course in procedural skills improves medical students' self-assessment of proficiency, confidence, and anxiety. Am J Surg 2007;193:771-3. [Crossref] [PubMed]
12. McMillan R, Redlich PN, Treat R, et al. Incoming residents' knot-tying and suturing skills: Are medical school boot camps sufficient? Am J Surg 2020;220:616-9. [Crossref] [PubMed]
13. Bochenska K, Milad MP, DeLancey JO, et al. Instructional Video and Medical Student Surgical Knot-Tying Proficiency: Randomized Controlled Trial. JMIR Med Educ 2018;4:e9. [Crossref] [PubMed]
14. Raythatha J, Hameed A, Lee T, et al. Technical skills teaching to MD students: a blinded, randomized controlled trial investigating video assistance in the education of the single-handed knot tie. Discov Educ 2024;3:29. [Crossref]
15. van Empel PJ, Verdam MG, Huirne JA, et al. Open knot-tying skills: resident skills assessed. J Obstet Gynaecol Res 2013;39:1030-6. [Crossref] [PubMed]
16. Davis CR, Toll EC, Bates AS, et al. Surgical and procedural skills training at medical school - a national review. Int J Surg 2014;12:877-82. [Crossref] [PubMed]
17. Trimbos JB. Security of various knots commonly used in surgical practice. Obstet Gynecol 1984;64:274-80. [PubMed]
18. Ching SS, Mok CW, Koh YX, et al. Assessment of surgical trainees' quality of knot-tying. J Surg Educ 2013;70:48-54. [Crossref] [PubMed]
19. Trimbos JB, Klopper PJ. Knot security of synthetic absorbable suture material; a comparison of polyglycolic acid and polyglactin-910. Eur J Obstet Gynecol Reprod Biol 1985;19:183-90. [Crossref] [PubMed]
20. Trimbos JB, Van Rijssel EJ, Klopper PJ. Performance of sliding knots in monofilament and multifilament suture material. Obstet Gynecol 1986;68:425-30. [Crossref] [PubMed]
21. Burkhart SS, Wirth MA, Simonich M, et al. Knot security in simple sliding knots and its relationship to rotator cuff repair: how secure must the knot be? Arthroscopy 2000;16:202-7. [Crossref] [PubMed]
22. Amortegui JD, Restrepo H. Knot security in laparoscopic surgery. A comparative study with conventional knots. Surg Endosc 2002;16:1598-602. [Crossref] [PubMed]
23. Ilahi OA, Younas SA, Alexander J, et al. Cyclic testing of arthroscopic knot security. Arthroscopy 2004;20:62-8. [Crossref] [PubMed]
24. Ivy JJ, Unger JB, Mukherjee D. Knot integrity with nonidentical and parallel sliding knots. Am J Obstet Gynecol 2004;190:83-6. [Crossref] [PubMed]
25. Ilahi OA, Younas SA, Ho DM, et al. Security of knots tied with ethibond, fiberwire, orthocord, or ultrabraid. Am J Sports Med 2008;36:2407-14. [Crossref] [PubMed]
26. Loutzenheiser TD, Harryman DT 2nd, Ziegler DW, et al. Optimizing arthroscopic knots using braided or monofilament suture. Arthroscopy 1998;14:57-65. [Crossref] [PubMed]
27. Bushong EE, Janis JE. Knot Security 101: A Comprehensive Practical Review to Optimal Knot Configuration, Pulling Direction, Throw Count, and Tail Length. Plast Reconstr Surg Glob Open 2024;12:e6047. [Crossref] [PubMed]
28. Aberg C. Change in strength of aponeurotic tissue enclosed in the suture during the initial healing period. An experimental investigation in rabbits. Acta Chir Scand 1976;142:429-32. [PubMed]
29. Babetty Z, Sümer A, Altintaş S, et al. Changes in knot-holding capacity of sliding knots in vivo and tissue reaction. Arch Surg 1998;133:727-34. [Crossref] [PubMed]
30. Babetty Z, Sümer A, Altintaş S. Knot properties of alternating sliding knots with different patterns in comparison to alternating and simple sliding knots. J Am Coll Surg 1998;186:485-9. [Crossref] [PubMed]
31. Schubert DC, Unger JB, Mukherjee D, et al. Mechanical performance of knots using braided and monofilament absorbable sutures. Am J Obstet Gynecol 2002;187:1438-40; discussion 1441-2. [Crossref] [PubMed]
32. Romeo A, Fernandes LF, Cervantes GV, et al. Which Knots Are Recommended in Laparoscopic Surgery and How to Avoid Insecure Knots. J Minim Invasive Gynecol 2020;27:1395-404. [Crossref] [PubMed]
33. Romeo A, Rocha CL, Fernandes LF, et al. What is the Best Surgeon's Knot? Evaluation of the Security of the Different Laparoscopic Knot Combinations. J Minim Invasive Gynecol 2018;25:902-11. [Crossref] [PubMed]
34. Taylor FW. Surgical knots and sutures. Surgery 1939;5:498-510.
35. Trimbos JB, Brohim R, van Rijssel EJ. Factors relating to the volume of surgical knots. Int J Gynaecol Obstet 1989;30:355-9. [Crossref] [PubMed]
36. Sharp HT, Dorsey JH, Chovan JD, et al. The effect of knot geometry on the strength of laparoscopic slip knots. Obstet Gynecol 1996;88:408-11. [Crossref] [PubMed]
37. van Rijssel EJ, Trimbos JB, Booster MH. Mechanical performance of square knots and sliding knots in surgery: comparative study. Am J Obstet Gynecol 1990;162:93-7. [Crossref] [PubMed]
38. Taylor FW. Surgical knots. Ann Surg 1938;107:458-68. [Crossref] [PubMed]