Trocars and Cannulas

Trocars and Cannulas

Nicole J. Buote

The Trocar, Cannula, and Sheath

In its simplest configuration, a trocar is a pen-shaped instrument with a sharp triangular point at one end, typically used inside a hollow cylinder, known as a cannula or sleeve, which provides an access port into a cavity during surgery. Rigid telescopes must be placed through a cannula in order to gain access to the body cavity. The literature shows a notable inconsistency of terminology; often trocar is used to describe the assembly of a cannula with its associated obturator. A cannula–trocar assembly is made up of three components: a cannula, seal and obturator.


A cannula is a tube-shaped metal or plastic shaft placed in the patient to allow access into the abdominal cavity during a laparoscopic procedure. Cannulas are sometimes sutured in place to the body wall or thoracic wall to ensure they do not migrate; they can also be screwed in place or held in place by inflatable balls and plastic flanges. Sheaths are protective shafts that are usually locked in place on the telescope, such as used with cystoscopes and arthroscopes.1


A seal is located at the top of the cannula, which allows instruments to pass through the cannula while preventing carbon dioxide (CO2) from escaping from the abdominal cavity. A gas-tight valve is located at the top of the cannula to allow instruments to be inserted and removed during a procedure without permitting the insufflated carbon dioxide escape. Various types of valves are available (spring loaded, magnetic trap door, trumpet, silicone, and so on), offering different characteristics in terms of leakage, mode of operation, and location on the cannula. More recently, a valveless cannula has been designed that makes use of a pressurized curtain of gas at the top of the instrument, eliminating the need for a valve altogether. This approach has the dual benefit of significantly reducing carbon dioxide leakage and smudging of the laparoscope lens, which is problem commonly associated with traditional valve types.2


An obturator is the tool (either sharp or blunt) that allows the cannula to penetrate the abdomen for initial placement. Although it once was used to refer solely to the piercing tool (obturator), as mentioned earlier, the term trocar is now often used to refer to the whole assembly. The pointed pyramidal tip from which the trocar gained its name is now one of several different types available, with outer diameters ranging from around 2 to 15 mm.3 Other designs include flat double-edged blades and pointed conical tips. Bladed trocars reduce the amount of force needed for the instrument to pass through the abdominal wall. For increased safety, some designs now include a spring-loaded plastic shield that automatically covers the blade as it enters the abdominal cavity. Conical tips can be either metal or plastic and require a small initial incision to be made using a scalpel. They pass through the tissues of the abdominal wall by stretching rather than cutting them. This leads to improved sleeve retention because it is surrounded by intact tissue layers that help hold it in place.3

Today, a very wide range of precision-engineered laparoscopic trocars exists, which has revolutionized patient care as we know it. Surgical trocars in human medicine are most commonly a single-patient use instrument and have graduated from the “three–point” design that gave them their name to either a flat bladed “dilating tip” product, or something that is entirely blade free. In veterinary medicine, the most commonly used trocar assemblies are reusable, reautoclavable configurations made of stainless steel or plastic or silicone materials.

Trocar–Cannula Placement

Laparoscopic entry is covered in detail in Chapter 8. However, a brief description as it relates to instrumentation follows here.

Initial trocar placement can be done after insufflation of the peritoneum or preceding it depending on the method chosen. Creating a pneumoperitoneum, or insufflation of the abdomen, allows for separation between the body wall and internal organs and allows for increased internal working space for manipulation of organs by surgical instruments. Cannula placement can be done by one of three methods: Veress needle, direct trocar insertion, or the open Hasson method. Insufflation can be performed using a Veress needle before placement of the primary trocar or via the trocar itself through a gas intake port, typically located on the side of the outer cannula. After the laparoscope has been introduced, secondary trocars can be placed under direct laparoscopic observation to minimize the risk of injury.

Veress Needle Technique

The Veress needle technique is the oldest and most traditional technique. One large retrospective study revealed that 81% of 155,987 gynecologic laparoscopic procedures used the Veress needle technique, but only 48% of 17,216 general surgical laparoscopic procedures used this method of insufflation.4 The Veress needle is a specially designed instrument with an outer diameter of approximately 2 mm (Figure 4.16). The outer cannula consists of a beveled needle point for cutting through tissue and an inner spring-loaded dull-tipped stylet (Figure 4.17). After the sharp outer needle passes through the abdominal wall, the spring-loaded stylet springs forward to protect the inner organs. The needle can then be attached to insufflation tubing and CO2 used to inflate the cavity. Blind placement of a Veress needle remains an important risk factor for complications. There are multiple reported safety tests to confirm that the Veress needle is properly placed before insufflation, including the “double click sound,” “aspiration test,” “hiss sound test,” “waggle test,” and “hanging drop test.” Unfortunately, most of these tests have been shown to be unreliable.5 In fact, waggling the Veress needle from side to side was considered contraindicated in clinical guidelines for MD surgeons because of the risk of organ laceration.6 Two recent studies have looked at the diagnostic accuracy of tissue impedance measurement interpretation for correct Veress needle placement in cats and dogs.7,8 In dogs, the impedance measurement had a 89.7% sensitivity, 100% specificity, and 90% accuracy.7 In cats, tissue impedance measurement resulted in 94.7% sensitivity, 20% specificity, and 79.2% accuracy.8 The differences between cats and dogs were thought to be due to the overdeveloped retroperitoneal fat pad in cats as well as their small size.


Figure 4.16 Veress needle. (© 2014 Photo courtesy of KARL STORZ GmbH & Co. KG.)


Figure 4.17

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Sep 27, 2017 | Posted by in GENERAL | Comments Off on Trocars and Cannulas
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