Volume 200, Issue 5 , Pages e18-e20, May 2009
An evaluation of knot integrity when tied robotically and conventionally
Article Outline
Objective
The purpose of this study was to evaluate the knot integrity of 3 commonly used sutures in sacrocolpopexy that were tied conventionally (by hand) and robotically.
Study Design
Knots were tied with polyglactin 910, polypropylene, and polyester, with 5-6 knots tied, depending on the suture used. We compared the knots that were subjected to tensile force until the suture broke or untied.
Results
The mean force that was required for the suture to untie was 47.7 ± 18.8 (SD) Newtons and was seen only among the robotically tied polyglactin 910 knots. Robotically tied polyglactin 910 knots were significantly weaker than all other robotic and conventional knots that were tested (P < .05). The tying modality and material interaction was significant (P < .001), which suggests that the effect of suture material varied, depending on the tying modality.
Conclusion
Knot failure rates for conventional or robotically tied suture varied based on the suture material that was used.
Key words: procedure, square knot, suture, tensile strength
One of the fundamental steps of sacrocolpopexy is suturing a graft to the native tissue. During a typical abdominal sacrocolpopexy, approximately 20 knots are tied, which requires significant time and effort. If each knot takes 10 seconds to tie, then approximately 5% of the procedure is spent suturing and tying.
Abdominal sacrocolpopexy is the gold standard for treatment of vaginal vault prolapse. Unfortunately, laparoscopy has technical challenges that do not lend it to this procedure. Tying the large number of surgical knots, with as many as 6 throws, is difficult because of limited dexterity of laparoscopic instruments.1 Tasks like adhesiolysis, suturing, and knot tying are enhanced with the robotic suturing system.2 Our primary aim was to evaluate the effects of tying modality on knot tensile strength.
Materials and Methods
Three suture materials were studied: polyester coated with polybutyrate (Ethibond; Ethicon, Inc, Somerville, NJ), polyglactin 910–dyed suture (Vicryl; Ethicon, Inc), and polypropylene (Prolene; Ethicon, Inc). All strand-to-strand nonidentical square knots were tied on a jig made of 2 hex-head screws 50 mm on center (Figure).3 For each suture type, knots were tied by each of the following methods: (1) da Vinci robotic surgery or (2) pulled tight by hand, with maximum tension (conventional). Because it is common practice to tie some permanent suture materials with additional throws, 6 throws were tied in polypropylene and polyester. Current studies show that polyglactin 910 needs 5 knots for security.4 For all materials 0-gauge United States Pharmacopeia size suture was used. Because surgeon fatigue could produce variation in knot strength, knots were tied over multiple days, and a single author tied each knot (R.F.C.H.).
FIGURE.
Device for tying knots that released suture loop without disrupting throws.
Muffly. Knot integrity when tied robotically and conventionally. Am J Obstet Gynecol 2009.
The tied suture was transferred to a tensiometer, where each loop was subjected to tensile forces; failure was defined as either breakage of the suture or tail slippage of > 3 mm. To determine the effects of knot type and material on knot strength, an analysis of variance was conducted, with tests of main effects for both factors and the interaction between the factors. The number of knots for each of the 6 knot and material type combinations was determined with a power analysis that was based on effect size estimates from previous literature
Results
A total of 41 knots were tied and tested for tensile strength. The robotically tied knots were strongest with polypropylene sutures and weakest with polyglactin 910. Conventionally tied knots were strongest with polyglactin 910 and weakest with polypropylene. The analysis yielded a main effect for suture material (P = .004) and no main effect for tying modality (P = .2). Levene's test for equality of error variances revealed that the standard deviation for the polyester sutures was significantly lower than the other materials (P < .001). After correcting for this variance inequality with the Kruskal-Wallis test, the main effect of material was not statistically significant (P = .248).
However, the knot by material interaction effect was statistically significant (P < .001), which suggests that the effect of suture material varied depending on tying modality (Table). This interaction effect remained significant after correction for unequal variances (P < .001).
TABLE. Mean ultimate load (in Newtons) until knot failure for each tying modality and material combination
| Tying modality | Material | N | Mean (Newtons) | SD |
|---|---|---|---|---|
| da Vinci robotic | Polyglactin 910a | 5 | 57.4 | 27.1 |
| Polyester | 5 | 86.8 | 3.4 | |
| Polypropylene | 4 | 119.9 | 24.9 | |
| Total | 14 | 88.0 | 32.3 | |
| Conventional | Polyglactin 910 | 10 | 112.2 | 4.0 |
| Polyester | 8 | 85.1 | 5.5 | |
| Polypropylene | 9 | 82.9 | 4.1 | |
| Total | 27 | 93.4 | 14.6 |
aFour of 5 da Vinci robotic polyglactin 910 knots slipped apart; all other knots broke apart. |
An examination of failure for each suture type showed that polyglactin 910 knots that were tied with the da Vinci robot came untied at a rate of 80%. In all cases, once a da Vinci robot–tied polyglactin 910 knot began to untie, it completely unraveled at a mean force of 47.7 Newtons. Mathematically, 1 Newton is equal to the amount of force required to give a mass of 1 kg an acceleration of 1 m/second2. A 100-g object like a medium-sized apple weighs 1 Newton on the earth's surface. Physiologically, sustained intraabdominal pressure over 25 mm Hg leads to renal failure from inferior vena cava compression. Based on this information, we used Laplace's law to convert knot tension (Newtons) to intraabdominal pressure (millimeters of mercury) and showed that the knots would fail at a force of > 25 mm Hg. All other suture and modality combinations broke at the knot instead of slipping apart like the da Vinci robot–tied polyglactin 910 suture. There was no statistically significant difference in polyglactin 910 suture diameter.
Comment
The factors that contribute to knot strength are well established.4 With the use of the recently suggested guidelines for polyglactin 910, it was determined that 6 throws would be optimum for this study. The decision to use 5 knots for polyglactin 910 was based on the study by Behm et al,5 which showed an additional throw was needed in the monofilament knot for security. Additionally, the study by Kadirkamanathan et al6 discussed the wider distribution of strengths between laparoscopic knots. There is a paucity of literature about the tensile strength of strand-to-strand nonidentical square knots that are tied robotically. Our results indicate that knots that are tied with either modality failed at supraphysiologic forces, because all the knots untied at pressure of > 25 mm Hg.7 In some pelvic tissues, we suggest that the suture will tear through the sutured tissue before the knot breaks.8 However, we feel that it is safe to approximate tissue using conventional or robotic surgical means.
Although we initially hoped to study knot-tying modalities, there was an additive effect of modality and suture. The braided polyester suture had very similar values for both modalities (ie, 86.8 vs 85 N), so it does not appear that knot-tying modality had any effect on knot strength. The polyglactin 910 sutures that were tied with the da Vinci robot were only approximately one-half as strong as sutures that were tied conventionally (ie, 57 N vs 112 N). We were able to tie strong knots with polyglactin 910 conventionally, but 80% of the robotically tied knots untied at a low force. We believe that the polyglactin 910 slipped at a higher rate than the stiffer monofilament suture because it is difficult for the surgeon to gauge how tightly the throws are tied. Monofilament suture stiffness often requires that the surgeons use a surgical clamp on the end of a strand to provide extra leverage when tightening throws. The impressive grip strength from the da Vinci robot arms allowed the throws to be tightened more efficiently. Also, this stiffness showed each throw tightening, which provided visual feedback that replaced tactile feedback. Once the knots are tied, the polypropylene holds the knots better than other monofilament synthetic materials. The surgeon is sitting at a robotic console where he or she determines the force that is applied based on 3-dimensional visual feedback, which may be challenging. Incorporating tactile feedback into future da Vinci consoles may increase precision and knot strength.
In conclusion, both conventionally and robotically tied knots are effective in the approximation of tissue, and future studies are needed to determine whether ligation of high-pressure vascular pedicles with the use of a robotic system is wise.
References
- . Assessment of the durability of robot-assisted laparoscopic sacrocolpopexy for treatment of vaginal vault prolapse. J Robotic Surg. 2007;1:163–168
- . Total laparoscopic hysterectomy utilizing a robotic surgical system. JSLS. 2005;9:13–15
- Mechanical performance of knots using braided and monofilament absorbable sutures. Am J Obstet Gynecol. 2002;187:1438–1442
- . Security of various knots commonly used in surgical practice. Obstet Gynecol. 1984;64:274–280
- Flat square knots: are 3 throws enough?. Am J Obstet Gynecol. 2007;197:172.e1–172.e3
- . A comparison of the strength of knots tied by hand and at laparoscopy. J Am Coll Surg. 1996;182:46–54
- . Factors relating to the volume of surgical knots. Int J Gynaecol Obstet. 1989;30:355–359
- . Tissue strength of structures involved in musculoaponeurotic layer sutures in laparotomy incisions. Acta Chir Scand. 1976;142:349–355
Accepted for the 29th American Urogynecologic Society Annual Scientific Meeting, Sept. 4-6, 2008, Chicago, IL.
Authorship and contribution to the article is limited to the 5 authors indicated. This study was funded through an institutional Grant from the St. Luke's Hospital Foundation.
PII: S0002-9378(08)01027-2
doi:10.1016/j.ajog.2008.08.058
© 2009 Mosby, Inc. All rights reserved.
Volume 200, Issue 5 , Pages e18-e20, May 2009
