Determination of temperature elevation in tissue during the application of the harmonic scalpel

Abstract

The temperature elevation in tissue during the application of the harmonic scalpel (UltraCision®, operating frequency 55 kHz; Ethicon Endo-Surgery, Norderstedt, Germany) was determined using thermocouples of a specific design. In experiments with and without perfusion, two different scalpel blades were applied to three different kinds of pig tissue (lung parenchyma, tongue, parotid gland) in various configurations. Temperature elevations by more than 40°C were found at 1 mm distance from the blade, whereas at distances of more than 5 mm, perfusion removed the heat very efficiently. The differences in the heating potential of the two blades were small and, at a distance of 2 mm, the temperature elevation did not exceed 6°C at all. In histological investigations, the damaged area between blade and parenchyma was determined. No morphologic indications of thermal damage were found at a distance of more than 2 mm. It is concluded that, during application of the harmonic scalpel, a safety margin of 3 mm from sensitive structures should be kept. (E-mail: [email protected])

Introduction

Ultrasonic surgery systems are widely used in open and endoscopic surgery. Although, initially, hollow tubes were employed, recently developed scalpel-type devices are increasingly used in many fields when bleeding control or minimal injury is desired. The harmonic scalpel (HS) (UltraCision®, operating frequency 55 kHz; Ethicon Endo-Surgery, Norderstedt, Germany) is a surgical instrument for cutting and coagulating tissue. A titanium knife blade is driven by a high-power ultrasound (US) transducer and oscillates longitudinally with a displacement amplitude of 10 to 50 μm. At the interface between blade and tissue, high temperatures and cavitation occur and coagulation of the tissue provides closing of vascular structures in addition to the cutting capabilities.

The ultrasonic dissection was developed especially for endoscopic surgery to avoid the risks of monopolar electrocautery, like thermal injuries or generation of smoke (Amaral 1994). Recently, UltraCision® has become ever more important due to advantages such as smoke-free operation, lower temperatures in comparison to electrocautery or laser scalpel and no current flow through the patient. The application of UltraCision® has been reported for nearly all anatomical regions, for example, in general surgery Rothenberg and Chang 1997, Amaral and Chrostek 1995, Vezakis et al 1998, gynaecology (Robbins and Ferland 1995), cardiosurgery (Ohtsuka et al 1997), urology Elashry et al 1997, Helal et al 1997, Tomita et al 1998 and thoracic surgery Eichfeld et al 2000, Friedrich et al 2000, Aoki and Kaseda 1999, Hayashi et al 1999, as well as head and neck surgery (Metternich et al 2001).

The movement of blades on a microscopic scale induced by longitudinal oscillation leads to mechanical cutting of tissue, having some similarity with, for example, the ultrasonic drilling as used in mechanical engineering Cimino and Bond 1996, Bond and Cimino 1996. The cutting process is accompanied by high temperatures on the blade surface, and the tissue changes its chemical structure primarily as a result of protein denaturation. The increased viscosity and the adhesion properties of the sectioned tissue provide closing of vascular structures and bleeding control, and support the cutting process (Amaral 1994). Using a blunt blade, coagulation can be induced without the cutting option. If the blade comes into contact with a liquid medium, which happens often during sectioning, cavitation occurs (Koch et al 2001) that further supports the denaturation of the proteins and the cutting process.

Although several mechanisms contribute to the performance of the harmonic scalpel, the tremendous temperature increase, by about 100°C and more, occurring at the interface between blade and tissue is substantial, at least for the coagulation and coaptation process. The induced heat is, however, removed from the treatment site into the surrounding tissue, resulting in a temperature increase in the tissue not involved; this may induce unintentional adverse biologic effects such as damage to vulnerable structures such as bowel, nerves or big vessels. This is the most serious undesirable side effect of all techniques for cutting and coagulating tissue Nduka et al 1994, Tucker 1995, Ata et al 1993, Lange and Payne 1998. Although UltraCision® reduces such thermal injuries to a minimum Amaral 1994, Eichfeld et al 2000, Friedrich et al 2000, Metternich et al 2001, there are reports about thermal complications during an application Awwad and Isaacson 1996, Kadesky et al 1997.

To assess the risk of harmful bioeffects, the determination of the temperature distribution in the tissue is the first important step. Magnetic resonance imaging (MRI) was used to control the temperature increase during hyperthermia Hynynen et al 1996, Diederich and Hynynen 1999, Bohris et al 2001. Because MRI is expensive and bulky, methods based on diagnostic US were developed Seip et al 1996, Maass-Moreno and Damianou 1996, Sun and Ying 1999. Both techniques allow almost noninvasive temperature determination with 2-D spatial resolution, but the technical and numerical efforts are high and the long response time impedes fast time-resolved measurements. Thermocouples embedded in the tissue require surgical operation to fix them in the tissue, but they provide reliable and fast temperature determination. Many data used for the investigation of the risk of harmful bioeffects, for example by diagnostic US, were obtained via thermocouples NCRP 1992, Barnett et al 1994, WFUMB 1998.

The temperature increase during the application of a scalpel-type ultrasonic surgical device was estimated in a very early stage of development using a thermocouple (Nowotny et al 1989). Although the exploitation of a thermographic method improved the quality of the measurement, comprehensive investigations on modern devices are rare. In the present study, the temperature increase during the use of a harmonic scalpel was experimentally determined with the aid of thermocouples in three different kinds of tissue in vitro and in vivo. Histological sections were taken at several stages of the treatments and the range in which necrosis occurred was determined and compared with the temperature elevation measurements. A safety margin from sensitive structures during harmonic scalpel operation could be defined, and so this study is to promote the safe use of the harmonic scalpel that is widely applied in surgery.