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Ultrasonic scalpel causes greater depth of soft tissue necrosis compared to monopolar electrocautery at standard power level settings in a pig model
© Homayounfar et al; licensee BioMed Central Ltd. 2012
Received: 23 October 2011
Accepted: 23 February 2012
Published: 23 February 2012
Ultrasonic scalpel (UC) and monopolar electrocautery (ME) are common tools for soft tissue dissection. However, morphological data on the related tissue alteration are discordant. We developed an automatic device for standardized sample excision and compared quality and depth of morphological changes caused by UC and ME in a pig model.
100 tissue samples (5 × 3 cm) of the abdominal wall were excised in 16 pigs. Excisions were randomly performed manually or by using the self-constructed automatic device at standard power levels (60 W cutting in ME, level 5 in UC) for abdominal surgery. Quality of tissue alteration and depth of coagulation necrosis were examined histopathologically. Device (UC vs. ME) and mode (manually vs. automatic) effects were studied by two-way analysis of variance at a significance level of 5%.
At the investigated power level settings UC and ME induced qualitatively similar coagulation necroses. Mean depth of necrosis was 450.4 ± 457.8 μm for manual UC and 553.5 ± 326.9 μm for automatic UC versus 149.0 ± 74.3 μm for manual ME and 257.6 ± 119.4 μm for automatic ME. Coagulation necrosis was significantly deeper (p < 0.01) when UC was used compared to ME. The mode of excision (manual versus automatic) did not influence the depth of necrosis (p = 0.85). There was no significant interaction between dissection tool and mode of excision (p = 0.93).
Thermal injury caused by UC and ME results in qualitatively similar coagulation necrosis. The depth of necrosis is significantly greater in UC compared to ME at investigated standard power levels.
Soft tissue dissection is a major issue in all fields of surgery as it incorporates the risk of wound healing disorder, hematoma or seroma. These adverse events potentially cause additional interventions up to reoperation resulting not only in patients discomfort and prolonged hospital stay but also in persisting morbidity and higher health care costs . The search for a dissection tool safer than standard monopolar electrocautery (ME) with its well known limitations in particular burns and carbonization, has led to the development of high-frequency ultrasonic dissection tools (UC). These instruments transform electrical power into ultrasonic waves of 55.5 kHz. The subsequently released thermal energy breaks up protein molecules leading to hemostasis and tissue dissection by cavitation and coaptation with good controllability of penetration depth . Numerous studies have evaluated the safety and feasibility of UC [3–7] and it has already been introduced into clinical routine in various subspecialties of surgery especially for laparoscopic procedures. However, there is still controversy about its potential advantages. Given the anyway limited vision in laparoscopic surgery the use of UC is favourable because of less smoke and reduced risk of thermal injury to adjacent structures as known from ME due to direct burns or capacitive coupling [5, 8]. The higher costs for UC devices could be compensated by saving operating time as demonstrated for laparoscopic cholecystectomy, especially when UC is solely used .
However, besides its potential advantages data on the extent of tissue alteration and its potential adverse effects are inconsistent. The discussion on UC has been raised again since recent studies identified a higher rate of sexual disorders after laparoscopic rectal resection compared to open procedures  where to UC may contribute. Given that tissue alteration processes are different between UC and ME we hypothesized that these dissection tools also differ in quality and extent of tissue alteration. As UC has been shown to depend not only on power level setting but individual activation time and pressure [1, 11], an experimental setup with a standardized tissue dissection technique without manual handling bias is needed to investigate the impact of UC on soft tissue morphology in comparison to standard ME. Therefore, we stepped back into an experimental pig model aiming to histopathologically evaluate the quality and extent of morphologic changes caused by UC and ME for soft tissue dissection using 2 types of dissection (manual and automated).
Animal experiments were performed in 16 male 3-6 months-old pigs with a mean weight of 44.0 ± 4.7 kg. All animal care and experimental procedures were in accordance with German national legislation on animal protection and approval was given by the Ministry of Agriculture, the Environment and Rural Areas of Land Schleswig-Holstein, Germany (V 312-72241.123-34). The animals were anesthetized using the following sedation, relaxation, and narcosis regimen: ketamine 10% with a dose of 0.25 mL/kg, xylazine 2% in a dose of 0.15 mL/kg, atropine sulfate 1% in a dose of 0.06 mL/kg. After endotracheal intubation anesthesia was continued with constant isoflurane (1.5-2 vol%) inhalation and oxygen (50 vol%) with a fresh gas flow rate of 1 L/min.
The influence of the variables dissection mode (automatic versus manual) and dissection tool (ME versus UC) onto the depth of necrosis was evaluated using a two-way analysis of variance for repeated measures, including the interaction effect of the two factors as well. Effects were found to be significant if p-values were less than 0.05. All analyses were performed using the free software R (version 2.8, http://www.r-project.org).
Median depth of coagulation necrosis in single tissue samples
This study shows that both, UC and ME lead to a similar histopathologic pattern of coagulation necrosis at the resection plane. However, the chosen dissection tool significantly affects the depth of this coagulation necrosis with UC generating a greater necrotic margin than ME when used for soft tissue dissection using standard power level settings.
Few previous studies exist specifically on morphological changes of soft tissue caused by UC and ME. Addressing skin and subcutaneous soft tissue dissection in a pig model, Hambely et al.  reported significantly less extensive and more localized tissue damages with UC compared to ME. Focusing on quality in contrast to extent of tissue damage, Foschi et al.  identified coagulation necrosis to be the predominant thermal injury by scanning and transmission electron microscopy which is consistent with our results.
Examining the efficacy of UC for hemostasis, Diamantis et al.  have investigated the safety and efficacy of multiple dissection tools including UC and ME for dissection and coagulation of short gastric vessels in a New Zealand rabbit model. In contrast to our results, they reported a deeper tissue damage caused by ME compared to UC. However, they applied a different, more descriptive approach referring to histological layers but did provide neither exact measurement data nor statistical comparisons. By analyzing the efficacy of UC for the hemostasis of small-, medium- and large-sized arteries in pigs, Harold et al.  observed an increase in thermal injury concomitant to increased vessel size. This direct correlation between power level settings, activation time and thermal injury has been reported in more detail by Emam et al. shortly after .
These mentioned animal studies share a relevant limitation which is the missing description of morphometric measurement. Our data clearly indicate that both UC and ME do not cause a uniform necrotic zone at the resection margin (Figure 2), most likely not only because of dissection related but local factors like tissue quality and vessel density. This suggestion is supported by the findings of Hoenig et al.  They examined the thermal injury of laparosonic coagulating shears with either sharp or blunt tip compared to bipolar electrocautery in a porcine model and observed different extent of injury depending on the type of tissue dissected.
The special contribution of this animal study is that we tried to design an experimental setup that reduces handling related bias as much as possible. In particular, we implemented a randomization-process for sample retrieval, the comparative application of the automatic device versus manual dissection, the excision of 2 samples of each kind (A and B) per animal and multiple measurement points per sample for quantifying the depth of coagulation necrosis.
Facing our result of wider necrotic margin in UC, one might hypothesize that in terms of clinical relevance this might lead to more competent ligation of both, blood vessels and lymphatics. This is supported by Morino et al.  who investigated the safety and efficacy of UC compared to ME in laparoscopic colorectal surgery within a prospective randomized clinical trial. They found a significantly lower median intraoperative blood loss for UC. Schmidbauer et al.  also reported this convincing coagulating effect with minimal blood loss for UC for the field of liver resection. In contrast, clinical studies evaluating the postoperative seroma rate following breast cancer surgery could not confirm a significant benefit of UC on seroma formation [19, 20]. On the other hand, the greater depth of necrosis could also contribute to postoperative nerval dysfunctions when UC is used close to susceptible structures in colorectal surgery. Furthermore, based on our described experimental setup we were able neither to investigate additional relevant but later occurring aspects of tissue alteration in particular inflammatory responses or induction of fibrosis nor to examine the healing sites for differences in wound healing processes or nerval dysfunctions. Therefore, future clinical trials are needed to investigate the clinical relevance of our findings and reason practical recommendations. However, our preliminary data argue for cautious use of UC when susceptible structures are close.
Our study confirmed that both, UC and ME lead to coagulation necrosis at the resection plane. Operating at standard power levels the depth of this coagulation necrosis is significantly greater when UC is used for soft tissue dissection compared to ME.
The authors thank Dr. Torsten Berg and the European Surgical Institute, Norderstedt, Germany, for cooperation and technical support.
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