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Obstetrics & Gynecology 1999;94:83-88
© 1999 by The American College of Obstetricians and Gynecologists
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ORIGINAL RESEARCH

Entry Force and Intra-abdominal Pressure Associated With Six Laparoscopic Trocar-Cannula Systems: A Randomized Comparison

CHRISTOPHER M. TARNAY, MD, KAREN B. GLASS, MD and MALCOLM G. MUNRO, MD

From the Department of Obstetrics and Gynecology, University of California, Los Angeles, Los Angeles, California.

Address reprint requests to: Malcolm G. Munro, MD, Olive View UCLA Medical Center, Department of Obstetrics and Gynecology, 14445 Olive View Drive, Suite 2B-163, Sylmar, CA 91324-1495, E-mail: mmunro{at}obgyn.medsch.ucla.edu


   Abstract
Top
Abstract
Materials and Methods
 Results
 Discussion
 References
 
Objective: In trocar-cannula systems, increased entry force could result in loss of operator control, a potential cause of serious visceral and vascular injuries. We developed a system to measure entry force and intraperitoneal pressure to evaluate and compare trocar-cannula systems.

Methods: Six laparoscopic trocar-cannula systems of similar diameter (12 mm) were tested (two pyramidal, two cutting-dilating, and two blunt conical) using a white swine model. All six systems were inserted into each of 12 subjects with location designated by random allotment (72 insertions). During each insertion, intraperitoneal pressure and entry force were measured using a system consisting of a gas-gas transducer, a 50-lb load cell, and a multichannel data acquisition board. Mean entry force and intraperitoneal pressure were compared using mixed-model analysis of variance.

Results: Mean entry force measurements were as follows: pyramidal 9.01 lb and 13.48 lb, cutting-dilating 9.94 lb and 16.46 lb, and blunt conical 19.15 lb and 31.91 lb. Intraperitoneal pressure changes generally reflected measured entry force.

Conclusion: The system successfully measured both entry force and resultant intraperitoneal pressure. Pyramidal trocar-cannula systems required the lowest force for entry. These differences in entry force have potential clinical implications related to the risk of visceral and vascular injury. Intraperitoneal pressure measurement could be used as a surrogate for insertional force measurement.

Laparoscopically directed procedures are now widely used by many disciplines including gynecology, general surgery, and urology. With the expanded use of this modality has come a greater appreciation of the nature and frequency of associated complications, primarily those associated with peritoneal entry. Complications of laparoscopic trocar-cannula system insertion that can confer significant morbidity include bleeding from the abdominal wall, injury to the great vessels of the pelvis, and damage to intraperitoneal viscera including the bowel and urinary tract.1

Catastrophic hemorrhage could occur if the sharp tip or edge of a laparoscopic trocar or insufflation needle injures one of the great vessels of the lower abdomen or pelvis. These injuries virtually always cause conversion to laparotomy, and if bleeding is massive or diagnosis is delayed, they comprise a major cause of mortality associated with laparoscopic technique.2,3 Although most gynecologic laparoscopists use a blind or closed approach to trocar-cannula systems insertion, the possibility of such vascular complications has led many surgeons to advocate insertion of the primary cannula after doing a minilaparotomy, an approach often referred to as open laparoscopy. However, in a review of 47 laparoscopic cases complicated by vascular injury secondary to insufflation needle or trocar-cannula systems insertion, three were associated with open laparoscopic technique and four with insertion of ancillary trocar-cannula systems.4 Injury to the bowel or urinary tract secondary to insertion of insufflation needle or trocar-cannula systems occurs despite the use of open laparoscopy.5 Consequently, it seems important to identify additional or alternative means by which the risk of visceral and vascular trauma can be minimized. The nature and extent of injuries associated with peritoneal entry could depend on a number of factors, including the design of the trocar tip, patient position, entry technique, and the amount of mechanical force necessary to pass the trocar-cannula systems through the abdominal wall.6–8

The type of trocar tip that has been used most commonly is the three-sided pyramidal design, which facilitates entry because of the three sharp edges that can slice easily through the tissue of the abdominal wall. However, there exist a number of other trocar designs, including those with blunt conical and hybrid tips. These differences in design might alter the entry characteristics and influence the resultant risk to the patient during introduction of the trocar-cannula systems. Indeed, one important comparative animal study suggests that pyramidal tips have a larger zone of injury than those with a conical design.9

The recent availability of blunt trocars that are capable of penetrating the abdominal wall potentially offers an opportunity to reduce the risks associated with both blind peritoneal entry and the positioning of ancillary ports. There are, however, legitimate concerns that the entry force required to insert such a blunt-tipped instrument might be excessive. Some have postulated that high entry force results in loss of operator control and thereby contributes to an increased risk of vascular and visceral injury, particularly if the trocar-cannula systems suddenly pops into the peritoneal cavity.5

To address those speculations, we designed a study to objectively evaluate and compare the entry performance of various trocar-cannula systems with trocars of similar internal diameter but different tip design. To do the study, we designed a multicomponent computerized system to allow the acquisition, storage, and analysis of longitudinally acquired streams of data relating to entry force and intraperitoneal pressure. Using this system we sought to compare the entry performance of several different trocar-cannula systems, hypothesizing that blunt conical trocars would be associated with higher entry force and intraperitoneal pressure compared with both pyramidal and hybrid designs.


   Materials and Methods
Top
Abstract
 Materials and Methods
Results
 Discussion
 References
 
To measure entry force, a system was designed to acquire, store, and process a continuous stream of force and intraperitoneal pressure data (Figure 1Go). The system consisted of a 50-lb compression load cell (Omega Engineering, Stamford, CT) that was fixed to a universal adapter to accommodate the six types of trocar handles. To best simulate operative conditions, the palmar interface consisted of a modified trocar handle that was bonded to the load cell. Intraperitoneal pressure was measured by using a PX170 pressure transducer (Omega Engineering, Stamford, CT) affixed to an insufflation needle inserted into the peritoneal cavity. During trocar-cannula system insertion, the electronic signals were received by a multichannel strain gauge signal conditioning board (SC-2043-SG; National Instruments, Austin, TX). The data were analyzed using a data acquisition card (DAQCard-1200; National Instruments, Austin, TX) then processed using Measure (National Instruments, Austin, TX) software and stored on an Excel spreadsheet (Microsoft, Roselle, IL).



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  		Figure 1. Force and pressure measurement and data acquisition system. The controller box contains the gas-gas transducer and signal conditioning board.

 
Six different, disposable, laparoscopic trocar-cannula systems were selected for comparative analysis, all with an internal diameter of 12 mm according to the manufacturer (Figure 2Go). Included were two pyramidal tip designs, Ethicon Endopath TriStar (Ethicon Endosurgery Inc., Cincinnati, OH) and the Autosuture Surgical Surgiport (United States Surgical Corp., Stamford, CT). Both of these devices have a pyramidal metal cutting blade with a sprung outer sleeve or obturator that retracts during insertion and then redeploys upon entry into the peritoneal cavity, thereby covering the otherwise exposed sharp trocar tip. The second group of trocar-cannula systems included two blunt conical designs, the ConMed TroGard (ConMed Corp., Utica, NY) and a radially expanding design, the Innerdyne Step (Innerdyne Inc., Sunnyvale, CA). Neither of these systems has movable metal parts, and both are built around a blunt plastic trocar. The Innerdyne system requires that a radially expanding sheath be inserted using a standard 15 cm insufflation needle that is subsequently removed. Then the tapered blunt conical trocar with cannula is inserted, expanding the sheath and the tract in the abdominal wall. The third group of trocar-cannula systems could be considered a hybrid of the first two. These trocars have smaller leading linear blades to create incisions that are then dilated by a blunt component of the trocar. Included were the Ethicon Endopath Dilating Tip (Ethicon Endosurgery Inc., Cincinnati, OH), which has a retractable blade housed within a blunt dilating trocar tip, and the Dexide System (Dexide Inc., Fort Worth, TX), characterized by a curvilinear cutting blade that partially surrounds a retractable blunt-tipped dilating trocar.



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  		Figure 2. Composite photograph of the six trocar-cannula systems tips demonstrating the groupings and design differences. A) ConMed TroGard. B) Innerdyne Step. C) Ethicon Endopath Dilating Tip. D) Dexide System. E) Autosuture Surgical Surgiport. F) Ethicon Endopath TriStar.

 
White swine animals, each 60–65 kg, were selected as test subjects, both because of their size and their dual layer lower abdominal wall fascia that, in pilot studies, was most representative of human thickness with regard to the layers of muscular aponeuroses. The protocol was reviewed and approved by the University of California, Los Angeles Chancellor’s Animal Research Committee, and the animals were cared for in compliance with the University of California, Los Angeles Office for Protection of Research Subjects. Animals were anesthetized with ketamine and butorphanol and maintained with inhaled halothane, titrated to effect. The abdomens were shaved, an insufflation needle was placed supraumbilically in the midline, and CO2 gas was insufflated to maintain pressure between 10 and 12 mmHg. A 10-mm trocar-cannula system was inserted to allow a laparoscope to be introduced for observation of experimental trocar insertion and to confirm free intraperitoneal placement of the insufflation needle. In pilot studies we determined that more than six trocar-cannula insertions (in addition to that made for the laparoscope) compromised our ability to maintain adequate intraperitoneal pressure. Consequently, six trocar-cannula systems were inserted by direct vision through one of six transverse 1.8-cm paramedian incisions separated by 8 to 10 cm that were made in a uniform manner below the level of the umbilicus. Each trocar-cannula system was introduced by the same experienced operator (MGM), at a 45-degree angle to the horizontal, with attention paid to rotational orientation, if indicated by the manufacturer. Insertion was done with the operator’s dominant hand holding the load-cell handle, taking care to maintain the load cell collinear to the long axis of the trocar-cannula system and ensuring that the nondominant hand used to stabilize the instrumentation did not add force. Entry force and intraperitoneal pressure were measured continuously, sampling at 100 times per second for the 10-second interval encompassing each trocar-cannula system insertion. At the end of all testing, the trocar-cannula systems were removed and wound and incisional measurements were done as part of a consecutive study using the same study subjects. The animals were then euthanized according to institution protocol.

Swine were assigned for all possible combinations of trocar and location. The assignment comprised a Latin square across trocar-cannula and location in a random order.10

Means of each outcome variable (entry force and intraperitoneal pressure change) were compared using analysis of variance. The analysis of variance is a mixed model analysis of variance with two fixed factors (trocar and location) and one random factor (swine). Pairwise comparisons among the six trocar-cannula system groups were done using the Fisher-Tukey (least significant difference) criteria.


   Results
Top
Abstract
 Materials and Methods
 Results
Discussion
 References
 
The mean peak entry force (lb) for each trocar-cannula system was as follows: Ethicon Endopath TriStar 9.01, Ethicon Endopath Dilating tip 9.94, Autosuture Surgiport 13.48, Dexide System 16.46, ConMed TroGard 19.15, and Innerdyne 31.91. Statistical significance, with an alpha error of 0.05, was reached between trocar-cannula systems where indicated in Figure 3Go by the error bars that show the standard error of the mean. Special mention of the Innerdyne Step product is warranted because the needle insertion pressure was low (mean 5.78 lb), but the trocar-cannula entry force was very high (mean 31.91 lb). This high force caused damage to the load cells in the force gauge, necessitating replacement, and therefore we abandoned acquisition of Innerdyne force data after six insertions. Nevertheless, despite the premature cessation of force data acquisition, statistical significance was attained.



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  		Figure 3. Mean peak entry force (lb) during the trocar-cannula system insertion. A) ConMed TroGard. B) Innerdyne Step. C) Ethicon Endopath Dilating Tip. D) Dexide System. E) Autosuture Surgical Surgiport. F) Ethicon Endopath TriStar.

 
Change in intraperitoneal pressure is expressed as the mean of the differences between the baseline and the maximum (or peak) intraperitoneal pressures during insertion of each system (Figure 4Go). These mean intra-peritoneal pressure changes mirrored those of the entry force data and were as follows (in mmHg): Ethicon Endopath TriStar 5.50, Ethicon Endopath Dilating Tip 6.40, Autosuture Surgiport 7.32, Innerdyne Step 10.53, Dexide System 11.11, and ConMed TroGard 12.16. Overall trocar-cannula design had a significant effect on pressure (P = .0029). The effect between trocar and location also was significant (P = .0323). As such, location on the abdomen could not be excluded as an influence in pressure measurements. Any differences between trocar-cannula systems could be attributable to location as well as inherent design.



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  		Figure 4. Mean peak intraperitoneal pressure changes (mmHg) during trocar-cannula system insertion. A) ConMed TroGard. B) Inner-dyne Step. C) Ethicon Endopath Dilating Tip. D) Dexide System. E) Autosuture Surgical Surgiport. F) Ethicon Endopath TriStar.

 

   Discussion
Top
Abstract
 Materials and Methods
 Results
 Discussion
References
 
The system we designed to acquire continuous streams of data on insertion force and intraperitoneal pressure functioned well for all but one of the trocar-cannula systems tested. The Innerdyne Step system, which required excessively high insertional forces, caused damage to one of the load cells in our force measurement system. By acquiring and displaying the continuum of data points it was possible to determine both the peak force and change in intraperitoneal pressure measurements used for comparative analysis in this study. The force and pressure profiles also provide a potentially useful source of information regarding the performance of a given instrument during the entire insertion process. Such data could be useful to engineers to show which design aspects impair or facilitate the insertion of a given instrument.

Of the six disposable systems evaluated, those with pyramidal trocars were associated with the lowest mean peak entry force. Currently, pyramidal trocar-cannula systems comprise the majority used in laparoscopic procedures, at least in North America. The low mean entry force was not unexpected, because the cutting tip is designed to incise both muscle and fascia during penetration of the abdominal wall. The Ethicon cutting and dilating trocar-cannula system had entry force requirements that were comparable to those of the pyramidal systems. The other cutting-dilating design, Dexide, required a mean peak entry pressure of 16.46 lb, which was significantly higher than that associated with the other three trocar-cannula systems that have cutting blades. We believe that a substantial component of the force required to position this device successfully within the peritoneal cavity was that needed to overcome the step created at the abutment of the trocar and the thick wall of the cannula. Indeed such a step could be identified in many of the force profiles.

Blunt-tipped trocars were designed to avoid some of the perils involved with blind or even visually directed insertion of a sharp instrument into the peritoneal cavity. Two such systems were evaluated in this study. The Innerdyne Step had entry force requirements that were substantially higher than those needed for insertion of the ConMed TroGard, which had the next highest required mean entry force. The shape of the leading tip of the Innerdyne and ConMed trocars is similar, which leads us to conclude that most of the extra force requirements for the Innerdyne system are related to the need to dilate the radially expanding sheath with the blunt obturator. This observation has also been reported by others.11 The maneuvers required to insert the 12-mm diameter Innerdyne Step trocar-cannula system directly after insertion of the insufflation needle and radial sheath are, at best, awkward, even for an experienced operator, and in the present study caused considerable difficulty with control. Additionally, the need to place an insufflation needle does not circumvent the associated risk for visceral and vascular injury, many of which have been reported to be caused by the Veress needle.4,5

The risk of injury to intraperitoneal structures might be associated with the surgeon’s losing control of the sharp trocar as it penetrates the abdominal wall. For sharp instruments, such control might be diminished as the force required to penetrate the abdominal wall increases. When the abdominal wall is finally breached, the trocar can suddenly "pop" into the peritoneal cavity in an uncontrolled fashion and with considerable force. Differences in control could relate to instrument design or, in the case of nondisposable trocar-cannula systems, suboptimal maintenance resulting in dull trocar tips. In a survey of Canadian gynecologists, done when non-disposable trocar-cannula systems were ubiquitous and most were pyramidal-tipped, up to one third admitted to having difficulty in applying sufficient force during insertion of either the primary or ancillary trocar-cannula systems. Injuries occurred twice as often among those who had such difficulty.5 Although it is impossible to determine the cause of the difficulty, it is reasonable to conclude that contributing factors could comprise technique, instrument design, poorly maintained and dull trocar tips, or a combination of those.

Many factors other than entry force contribute to the impact of trocar design on clinical outcomes such as intraperitoneal trauma. Differences in the design of the trocar that facilitate penetration of the abdominal wall might also contribute to a greater relative risk of injury to intraperitoneal structures. In a study examining trocar-associated vessel injury in an animal model, Hurd et al9 demonstrated that sharp, conical-tipped trocars created less lateral damage than did pyramidal-tipped devices of similar diameter. This difference in injury zone implies that pyramidal tips have a greater potential than conical tips for blood vessel injury should the retroperitoneal space or mesentery be entered. The Hurd study9 evaluated sharp conical tips, which leads one to infer that blunt conical trocars might have even less potential for causing vascular and visceral injury. As a result of these considerations, incremental increases in entry force required to insert trocar-cannula systems with blunt conical trocars, even if statistically significant, might not translate into an increase in entry-related complications. It could be hypothesized that such complications will be reduced with the use of the blunt trocars, at least when compared to sharp trocars, and particularly when they are of a pyramidal design. Nevertheless, the present study does not provide data to support or refute such a hypothesis.

With one exception, the mean intraperitoneal pressure profiles reflected those of entry force. This variable has not been studied previously. We included intraperitoneal pressure measurements in this study because several systems currently available or planned for future release require insertion techniques that do not rely on the simple application of linearly aligned force to a trocar. The exception mentioned above is an example where insertional force was not indicated by intraperitoneal pressure. The Innerdyne Step intraperitoneal pressures were not higher in concert with insertional force because of the pressure-counterpressure method of inserting the obturator through the radially dilating sheath.

Although the differences between entry force and change in intraperitoneal pressure are not large, this example shows the potential value of having more than one method of measuring the impact of trocar-cannula system entry on the patient. Intraperitoneal pressure measurements are also valuable in awake patients, for comparison of patient experience with a quantified measurement of pressure during trocar-cannula insertion. In the present study, the analysis of intraperitoneal pressure data is inconclusive because of the wide variation in trocar-cannula systems resulting from location. As opposed to the other variables, location could not be excluded as an influence on trocar-cannula systems differences.

These disposable laparoscopic trocar-cannula systems vary significantly in the entry force required. There could be minor differences with nondisposable instrumentation, but we suspect that such differences would be minimal. Entry force is potentially an important factor for many surgeons as it can contribute to control when force is low. Whether better control correlates with decreased incidence of visceral and vascular injury is unknown but certainly would be a desirable feature.


   Footnotes
 
Financial support for this research has been provided by an unrestricted educational grant from ConMed Corporation, Utica, New York.

PII S0029-7844(99)00288-4

Received August 31, 1998. Received in revised form December 21, 1998. Accepted January 7, 1999.


   References
Top
Abstract
 Materials and Methods
 Results
 Discussion
 References
 
1. Smith S. Complications of laparoscopic and hysteroscopic surgery. In: Azziz R, Murphy AM, eds. Practical manual of operative laparoscopy and hysteroscopy. New York: Springer-Verlag, 1996:199–217.

2. Baadsgaard SE, Bille S, Egeblad K. Major vascular injury during gynecologic laparoscopy. Report of a case and review of published cases. Acta Obstet Gynecol Scand 1989;68:283–5.[Medline]

3. Nordesgaard AG, Bodily K, Osborne RW, Buttorff JD. Major vascular injuries during laparoscopic procedures. Am J Surg 1995;169:543–5.[Medline]

4. Soderstrom R. Injuries to major blood vessel during endoscopy. J Am Assoc Gynecol Laparosc 1997;3:995–8.

5. Yuzpe AA. Pneumoperitoneum needle and trocar injuries in laparoscopy: A survey on possible contributing factors and prevention. J Reprod Med 1990;35:485–90.[Medline]

6. Corson SL, Batzer FR, Gocial B, Maislin G. Measurement of the force necessary for laparoscopic trocar entry. J Reprod Med 1994;34:282–4.

7. Tews G. Significant reduction of operational risk in laparoscopy through the use of a new blunt trocar. Surg Gynecol Obstet 1991;173:67–8.[Medline]

8. Waxman K, Birkett DH, Sackier JM, Este-McDonald J, Duquette J. Clinical and laboratory evaluation of an electrosurgical laparoscopic trocar. Surg Endosc 1994;8:1076–9.[Medline]

9. Hurd WW, Wang L, Schemmel MT. A comparison of the relative risk of vessel injury with conical versus pyramidal laparoscopic trocars in a rabbit model. Am J Obstet Gynecol 1995;173:1731–3.[Medline]

10. Dunn OJ, Clark VA. Applied statistics: Analysis of variance and regression. New York: Wiley & Sons. 1974:115.

11. Bhoyrul S, Mori T, Way W. Radially expanding dilatation. Surg Endosc 1996;10:775–8.[Medline]




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