Comparison of liver parenchymal ablation and tissue necrosis in a cadaveric bovine model using the Harmonic Scalpel™, the LigaSure™, the Cavitron Ultrasonic Surgical Aspirator® and the Aquamantys® devices

Abstract

Objectives

The amount of tissue that is ablated or necrosed at the line of parenchymal transection is of clinical significance in the interpretation of resection margin status following hepatic resection. The aim of this study was to define the extent of parenchymal ablation and necrosis in liver tissue using the Harmonic Scalpel™, the LigaSure™, the Cavitron Ultrasonic Surgical Aspirator® (CUSA®) and the Aquamantys® dissector ex vivo.

Methods

Mounted blocks of non-perfused bovine liver were transected using the Harmonic Scalpel™, LigaSure™, CUSA® and Aquamantys® devices. Outcome measures included parenchymal ablation (ablation band widths and weights) and tissue necrosis band widths along the line of transection. Each experiment was replicated five times.

Results

All devices were associated with parenchymal ablation (Harmonic Scalpel™, 4.73 ± 1.62 mm; LigaSure™, 4.55 ± 2.02 mm; CUSA®, 7.16 ± 2.87 mm; Aquamantys®, 4.75 ± 1.43 mm) and tissue necrosis (Harmonic Scalpel™, 1.07 ± 0.46 mm; LigaSure™, 1.36 ± 0.36 mm; CUSA®, 0.81 ± 0.21 mm; Aquamantys®, 0.81 ± 0.36 mm).

Conclusions

The Harmonic Scalpel™, LigaSure™, CUSA® and Aquamantys® devices were associated with bands of tissue loss along the hepatic parenchymal transection line in this benchtop cadaveric model. This should be taken into account in the interpretation of resection margin status following liver resection.

Introduction

Liver resection provides the only potential cure for patients with colorectal liver metastases. Defining an adequate margin of resection (the distance from the tumour edge to the cut surface of the liver) is contentious.¹ Historically, an inability to achieve a 10-mm margin of clearance was considered a contraindication to resection.² Several studies now indicate that a resection margin of < 10 mm should not preclude a patient from undergoing liver resection, provided the margin itself is clear of tumour (i.e. there is no tumour within 1 mm of the cut surface of the liver).^3–9 However, interpreting these data is difficult because the evidence from prospective trials is limited. In addition, the various studies have used different parenchymal transection techniques and many of the large-cohort studies span the pre- and post-chemotherapy eras.

The method of liver transection has a bearing on how margin positivity is interpreted.^3,5,9,10 A range of non-ablative (finger fracture, Kelly clamp/crush and vascular stapling devices) and ablative techniques using the Harmonic Scalpel™ (Ethicon Endo-Surgery, Inc., Johnson & Johnson Medical Ltd, Wokingham, UK), the LigaSure™ (Covidien Ltd, Fareham, UK), the Cavitron Ultrasonic Surgical Aspirator® (Integra CUSA EXcel®; Integra Neurosciences Ltd, Andover, UK) and the Aquamantys® (formerly TissueLink™) (Medtronic Advanced Energy, Portsmouth, NH, USA) devices are available for hepatic parenchymal transection.¹¹ When an ablative transection technique is used, a band of tissue is lost at the line of transection. If this is adjacent to the edge of the tumour, it may lead to the false reporting of a positive margin. The extent of tissue loss that occurs during liver transection with each of the Harmonic Scalpel™, LigaSure™, CUSA® and Aquamantys® devices has not previously been reported. The aim of this study was to define the extent of parenchymal ablation and tissue necrosis achieved using each of these devices in a non-perfused benchtop cadaveric model.

Materials and methods

The instruments were assembled according to the manufacturers' instructions. A 15-mm Harmonic Scalpel ACE™ was connected to the Harmonic Generator 300™ (Ethicon Endo-Surgery, Inc.). The 5-mm LigaSure™ V Sealer/Divider was connected to the Force EZ™ generator (Valleylab Inc., Boulder, CO, USA). The EXcel™ 8 Ultrasonic Surgical Aspirator System (Integra Neurosciences Ltd) was connected to the 23-kHz CUSA® handpiece, and the Aquamantys® Pump Generator was connected to the Aquamantys® 9.5 XL Bipolar Sealer (Medtronic Advanced Energy). All devices were used at the manufacturers' recommended settings for liver transection. The Harmonic Scalpel™ was set at a minimum of 3 and a maximum of 5. The LigaSure™ was used at a maximum of 350 W. The CUSA® was used at 2* setting at 100% amplitude (30 ms on/10 ms off), 60% suction (≍ 400 mmHg) and 5 ml/min irrigation. The Aquamantys® was used at 170 W at a medium flow rate of 22 ml/min.

To measure liver tissue ablation and necrosis, blocks of homogeneous bovine liver measuring 5 × 5 × 1 cm, harvested from freshly killed animals, were mounted on graded cork blocks (Fig. 1). Pre-transection mounted and unmounted liver weights were recorded. All tissue reached the ambient controlled temperature of the laboratory (20 °C) before transection commenced. Liver tissue was transected through the centre of each block. Following transection, the distance between cut edges (ablation band width) was measured using electronic Vernier 46610 callipers (Draper Tools Ltd, Southampton, UK) at five points at 1-cm intervals along the length of transection. Transection was repeated five times by the same operator (JSH). Post-transection mounted and unmounted liver weights were recorded. The tissue was fixed in formalin, processed and stained with haematoxylin and eosin (H&E). The band of necrosis adjacent to the resection margin (necrosis band width) was measured by a blinded gastrointestinal histopathologist (AMZ). Representative sections were imaged using the Hamamatsu Digital Nanozoomer® (Hamamatsu Photonics UK Ltd, Welwyn Garden City, UK).

Figure 1.

Schematic demonstrating the benchtop cadaveric model

Data were analysed using spss for Windows Version 14 (SPSS, Inc., Chicago, IL, USA). Data are expressed as the mean ± standard deviation. The statistical significance of differences among groups was assessed using Student's t-test (unpaired). Differences were considered statistically significant at P < 0.05.

Results

Parenchymal ablation

Parenchymal ablation measurements were validated in a series of pilot studies using the Harmonic Scalpel™. Both the CUSA® and Aquamantys® devices required a second instrument to divide the small vessels along the line of transection. Measurements were made following the division of this band. Figure 2a summarizes the ablation band widths achieved using the four devices. Figure 2b shows the percentage weight loss following the transection of the blocks using the Harmonic Scalpel™ and the LigaSure™ device. This was significantly greater for the LigaSure™ tool. It was not possible to evaluate weight changes accurately following transection with the CUSA® and Aquamantys® tools because saline irrigation was required for these transections.

Figure 2.

(a) Widths of bands of tissue ablated with the various devices. Comparisons between devices show: A vs. B, P = 0.868; A vs. C, P = 0.139; A vs. D, P = 0.986; B vs. C, P = 0.117; B vs. D, P = 0.832, and C vs. D, P = 0.128. (b) Percentage loss in tissue weight after ablation. The difference between outcomes with devices A and B is significant at P = 0.006. (c) Widths of tissue necrosis adjacent to the ablation zone. Comparisons between devices show: A vs. B, P = 0.047; A vs. C, P = 0.036; A vs. D, P = 0.133; B vs. C, P = 0.003; B vs. D, P = 0.006, and C vs. D, P = 0.897

Tissue necrosis

Figure 3 shows tissue necrosis adjacent to the resection margin for each of the four devices in H&E-stained sections of bovine liver. Figure 2c summarizes the necrosis band widths obtained using each of the four tools. Significantly greater necrosis band widths were obtained using the LigaSure™ device.

Figure 3.

Haematoxylin and eosin-stained sections (original magnification ×20) of necrotic bands following transection with the (a) Harmonic Scalpel™, (b) LigaSure™, (c) CUSA® and (d) Aquamantys® devices

Discussion

Resection margin status is an important prognostic factor following liver resection for colorectal metastases and has implications for the planning of future treatment.^3,5,6,8,12 However, the accurate interpretation of margin status is difficult as the use of ablative transection techniques may encroach on the clear margin and lead to the over-reporting of false positive margins.^3,5,10 This study demonstrated that the Harmonic Scalpel™, LigaSure™, CUSA® and Aquamantys® devices were all associated with bands of tissue ablation along and tissue necrosis on either side of the line of transection.

Liver parenchyma is a highly vascular tissue containing hepatocytes and non-parenchymal cells suspended in a collagen-based extracellular matrix through which runs a network of vessels and biliary structures. Successful liver transection requires the achievement of haemostasis and the sealing of ductal structures.¹¹ The Harmonic Scalpel™ uses ultrasound energy to cause coagulation and protein (collagen) denaturation to dissect and seal tissue. The tissue-cutting effect is achieved by the longitudinal vibration effect. In experimental models, the Harmonic Scalpel™ has been shown to seal vessels up to 3 mm in diameter.¹¹ The LigaSure™ system uses a combination of radiofrequency energy and compression to dissect tissue. The cutting effect is achieved with a built-in blade. In experimental models, the LigaSure™ device has proved able to seal vessels up to 7 mm in diameter.¹¹ The CUSA® dissector uses ultrasonic energy transmitted via saline released from its tip to fragment parenchymal tissue that is then aspirated to expose blood vessels and ductal structures that can be electrocauterized, ligated or clipped.¹¹ The Aquamantys® dissector uses radiofrequency energy conducted through saline released from its tip. This coagulates tissue, sealing small vascular and biliary structures, and isolating larger structures that can be electrocauterized, ligated or clipped.¹¹

The ablative band widths for both the Harmonic Scalpel™ and the LigaSure™ device correspond to the maximum diameters of the instrument tips used in this study (Harmonic Scalpel™, 5.5 mm; LigaSure™, 5.0 mm). Other tip sizes are available. Bands of necrotic tissue measuring 1.0–1.5 mm were found on either side of the line of transection. There was a significant difference in tissue weight loss following liver transection with the LigaSure™ tool compared with that performed using the Harmonic Scalpel™. This corresponds with the larger surface area of the tip of the LigaSure™ device.

The ablation/aspiration band width of the CUSA® is operator-dependent. In this cadaveric model, an arc of 7 mm was sufficient to expose small vessels and biliary structures. In an in vivo context, in which there is concern about margin clearance, the operator may widen this arc of aspiration to clear additional tissue at the adjacent margin. Because the tissue is aspirated, it is usually not possible to evaluate it formally.¹⁰

The Aquamantys® device produced a wide band of coagulation adjacent to the line of transection, but this was not reflected in either the ablative or necrotic band width. Although the device led to disruption of the extracellular matrix within this coagulative band to a distance of 2 mm from the resection margin, there was no clear evidence of tissue necrosis beyond 1 mm. The impact of this coagulation effect on tumour cell viability is not known.

The selection of liver transection technique is dependent on the surgeon's experience and preference. Non-ablative techniques remain the first choice at many centres and provide excellent perioperative and longterm outcomes.^13,14 The CUSA® is also used widely and provides flexibility when margin clearance is in question.^10,11,15,16 The advantages of newer coagulative devices (the Harmonic Scalpel™,^17–20 the LigaSure™^21–24 or the Aquamantys®^25–27) over existing transection techniques for parenchymal disease have yet to be demonstrated in large clinical trials. In laparoscopic liver resection, the Harmonic Scalpel™ and the LigaSure™ tool enable precise dissection through small vascular and biliary structures in which the use of CUSA® is less practical. However, these devices may be used in combination with other techniques to secure larger vessels.^11,28

There are several limitations to this study. The work was undertaken in a non-perfused benchtop cadaveric model. It would be valuable to compare the different ablative techniques in vivo in order to assess ablation and necrosis more accurately in the presence of the heat sink effect that occurs in perfused liver tissue. It would also be of value to assess the performance of these devices in a model for metastatic disease and in cadaveric human tissue to determine how the necrotic effect varies in tumour cells and to establish how performance changes in the presence of parenchymal disease (sinusoidal injury, steatohepatitis, fibrosis and cirrhosis).

In a quest to improve rates of resectability, a more aggressive approach to resection has been advocated. This includes downsizing with chemotherapy²⁹ and the adoption of a variety of parenchyma-preserving strategies.³⁰ The interpretation of margin status in this setting is, therefore, crucial for the planning of further treatment and for predicting prognosis. The findings of the present study demonstrate that transection with the Harmonic Scalpel™, LigaSure™, CUSA® and Aquamantys® devices is associated with bands of parenchymal ablation and necrosis at the resection margin. This must be considered when interpreting resection margin status following liver resection for colorectal liver metastases. Furthermore, when clearance of the resection margin is in question, an ablative rather than a non-ablative transection technique may be preferred in order to reduce the incidence of recurrence at the resection margin.

Conflicts of interest

None declared.