How to create burn porcine models: a systematic review

Summary

Management of burn patients is challenging, and requires a lot of knowledge and experience gained through research. However, experimental study on humans is not morally and ethically accepted. Porcine is the most frequently used experimental model because pig skin is anatomically and physiologically similar to human skin. We included systematic reviews, meta analyses, and experimental studies of burns using porcine models. We excluded studies conducted more than 10 years ago and which included only thermal injuries. Burn injury made to the porcine model had to be second or third degree. Searches of Ovid MEDLINE, Pubmed, Burns Journal and Cochrane Library revealed 21 relevant studies. Three methods used to create full thickness burn wounds were found: aluminium, brass, and immersion in hot water. The aluminium bar was heated to 200°C and placed on the pig’s skin for 20 seconds. For deep dermal burns, there are different methods: modified glass bottle and aluminium bar. The bottle is filled with sterile water and heated to the desired temperature (92°C), then placed on the pig’s skin for 15 seconds. To measure burn wound depth, almost all the studies used histopathological evaluation. There was no standardised method to create burn wounds in porcine models. Nevertheless, for deep dermal burn wounds, we can use the modified glass bottle method and for full thickness burn wounds, we can use aluminium or brass. There are no previous studies discussing how to make burn porcine models, nor any studies in this review that focused on creating the burn wound alone. Further studies are needed to achieve better results in creating burn wounds in porcine models.

Introduction

Burn management is challenging for every surgeon because it has high mortality, volume and risk. Research and experimental study is needed to create a new management, a new method of burn care. Animals are often used in experimental studies rather than humans, as using humans is more often considered unethical. Therefore, this study was conducted with animals. Animals often used in experimental studies are swine/pigs, guinea pigs, rats/mice, dogs, rabbits and sheep. Porcine models are more frequently used as experimental models because pig skin is anatomically and physiologically more similar to human skin.¹ The cornified layer and epidermis, dermis and subcutaneous region of the pig is similar to human skin. The hair and tubular apocrine of the pig is also similar to that found in humans. Other similarities between porcine and human skin include epidermal enzyme patterns, epidermal tissue turnover time, the character of keratinous proteins, and the composition of the lipid film of the skin surface.² Sullivan et al.¹ reported that the porcine model is an excellent tool for the evaluation of therapeutic agents destined to be used in human wounds.

For these reasons, the porcine model is a suitable burn model. It is useful for studying burn symptoms and treatment, such as dressing, tangential excision, skin grafting, scar formation, wound healing, diagnostic tools, fluid resuscitation, etc. There is still no gold standard for the procedure to make burn models. Therefore our aim is to conduct a systematic review of how second and third degree burns in porcine models can be used in further studies.

Methods

Types of study

We included systematic reviews, meta analyses and experimental studies of burns that used porcine models for their investigations. Two reviewers identified titles and abstracts of the studies to be included. The full text paper was acquired when there was any uncertainty about its inclusion. We excluded studies that were more than 10 years old and included only thermal injuries.

Types of participant

Porcine model, pig model and swine models that underwent a burning procedure.

Types of intervention

The grading of burn injury had to be second or third degree burns. The grading had to be confirmed using a procedure.

Types of outcome measures

Characteristics of the porcine models (age, weight, kind), area of burn, tool, sources of heat, temperature, temperature measurement device, duration, dimension, validation method and dressing.

Search methods for identification of studies

Online searches of multiple databases from Ovid MEDLINE, Pubmed, the Burns Journal and the Cochrane Library were conducted on 6th July 2015, at 10:00 am: searches from Ovid MEDLINE(R) database at http://ovidsp.ovid.com (2006 until present), from PubMed database at http://www.ncbi.nlm.nih.gov/pubmed (2006 until present), from the Burns Journal database at http://www.burnsjournal.com (2006 until present), and lastly from the Cochrane Library at http://onlinelibrary.wiley.com (2006 until present). No language restrictions were applied. The online search ended on Tuesday, 6th July 2015 at 4:00 pm. The search strategies used search terms and keywords such as “Third degree burn”, “Full thickness burn”, “Deep dermal burn”, “Porcine models”, “Swine models” and “Pig models”. Boolean operator was incorporated into the search terms to distinguish more specific studies, in general, using the following strategy: ((Third degree burn OR Full thickness burn OR Deep dermal burn) AND (Porcine models OR Swine models OR Pig Models)). Searches were performed on all fields, and no search limitation was applied initially.

Quality assessments tools

To assess the methodological quality of the studies, we used a checklist for the assessment of the methodological quality both of randomized and non-randomized studies of health care interventions.³ This tool consists of 27 aspects of trial validity and the scoring of them. It is shown in Table I.

Table I.

Results

Description of studies

A search of Ovid MEDLINE(R) database (http://ovidsp.ovid.com) from the year 2006 until 2017 revealed 12 studies; the search of the PubMed database resulted in 28 studies from 2006 until 2017; 108 studies between 2006 and 2017 were retrieved from the Burns Journal database, and lastly the COCHRANE LIBRARY search found 2 studies for the period 2006 until 2017. The online search ended on Monday, 6th July 2015 at 4:00 pm. A total of 150 studies were retrieved from 4 databases. We identified and excluded studies based on our inclusion and exclusion criteria, thus yielding 23 remaining studies. Finally, after we had read the abstracts and excluded two based on full text availability, the remaining 21 studies were selected for the final analysis. See Fig. 1 for a detailed search history and Table II for results of the search.

Table II.

Fig. 1.

Risk of bias among the studies included

The methodological quality assessment is a method to assess both randomized and non-randomized studies. This checklist consists of 5 subscales:

• reporting, to allow a reader to make an unbiased statement of the findings of the study;

• external validity, whether the study could be generalized to the population from which the study subjects were derived;

• bias, assessed biases in the measurement of the intervention and the outcome;

• confounding, addressed bias in the selection of study subjects;

• power, to assess whether the negative findings from a study could be due to chance.

A summary of methodological quality assessment is presented in Table III.

Table III.

Table III shows the results of assessment through 27 questions representing 5 subscales that are presented in Table I. Out of the 21 studies assessed, 16 have a good score and 5 have a fair score, with a median of 21 and averaging 20.5. This indicates that most of the studies are in the considerable range, and averaging a good score of assessment, making them methodologically a suitable source for this review.

Fig. 2 describes the frequency of methodological quality assessment scoring for each study. Twelve of the studies used metals (including aluminium and brass) as a tool of intervention, 11 of which have a good score and only 1 has a fair score. The other tool used for intervention was a glass bottle filled with hot water, in 9 studies in total, 6 of which have a good score and 3 have a fair score.

Fig. 2.

Effects of intervention

The method that resulted in partial thickness burn wounds in the porcine model is presented in Table IV, while the method that resulted in full thickness burn wounds in the porcine model is presented in Table V. The temperature and duration of exposure used to create partial thickness and full thickness wounds is listed in Fig. 3 and Fig. 4, respectively.

Table IV.

Table V.

Fig. 3.

Fig. 4.

The data show that the same number of studies (7) used metal or glass bottle as a tool. The most common heat source was heated water (6), followed by microwave (4) and hot plate (1).

The average exposure temperature to create partial thickness burn wounds was 94.21oC, with a median of 92oC, and the average duration was 19.79s, with a median of 15.5s. The above data show that the most used tool was metal (4), followed by glass bottle in just one study less (3). There are an equal number of studies that used heated water and gas burner (2), with none using microwave or hot plate. The average exposure temperature to create full thickness burn wounds was 139.14oC, with a median of 100oC, and the average duration was 48.14s, with a median of 30s. Therefore, if we compare both data sets, we see that it takes a higher temperature and a longer duration of exposure to create a full thickness burn wound in the porcine model, ignoring the heat source and tools used.

The porcine models weighed 15-65 kg. Female and male pigs were used. Not every study described the age of the pigs. The age range was 6-42 weeks. Out of 22 studies, 18 made the burn injury on the back and flank of the pig’s body. Prior to any surgical intervention, the hair on the back and flank was clipped with an electric clipper and the skin was rinsed sequentially with an antiseptic solution and clean water.

Three methods were used to create full thickness burn wounds: aluminium, brass and immersion in hot water. Regarding the aluminium method, an aluminium bar was heated to 200°C and placed on the pig’s skin for 20 seconds. As for the brass comb/bar method, the brass was heated to 100°C and placed on the pig’s skin for 20-30 seconds.

The tools used to make the burn wound were mostly heated in boiling water to the desired temperature. Regarding immersion in hot water, the water was heated to 70-97°C and the pigs were immersed in the water for 17-180 seconds.

Various methods were used to create deep dermal burns, the most frequent being a modified glass bottle. The bottom of the bottle-heated device was removed and replaced with a plastic wrap. Then the bottle was filled with sterile water and heated to the desired temperature (92°C). The bottle was placed on the pig’s skin for 15 seconds. The other method used an aluminium bar heated to 80°C and then placed on the pig’s skin for 20 seconds with gravity pressure.

To measure the depth of the burn wound, almost all of the studies used histopathological evaluation. This evaluation was achieved from biopsies done periodically. Singer et al.⁵ did the biopsies at 7, 9, 11 and 13 days after injury. Tennyson et al.²³ used Histologic Assessment Tools, which consists in 8 tissue measurements.

Discussion

A burn wound model should have proper extent and depth of wound. However, the burn wound model still has to be safe, simple and reproducible. The important elements needed to produce a burn wound model are the tools, temperature, duration of exposure and pressure. Animal models should have similar characteristics to human skin and have a similar outcome of type and extent of burn when exposure is applied.^2,25

From our perspective, the safest, simplest and most reproducible tool for creating deep dermal burns is the modified glass bottle. It is safe because we can monitor burn appearance during creation from the bottom of the bottle.²⁶ However, there are no studies that compare the modified glass bottle with the metal bar to make full thickness burn wounds. To our knowledge, they are caused by the better conduction of metal compared to glass. To date, the metal bar (aluminium and brass) is still used to make full thickness burn wounds in the porcine model. The other technique is to immerse the porcine models in hot water, but this is ethically harder to accept. The weaknesses of the studies are the small sample sizes and the fact the sample cannot be randomized.

Conclusion

A review of the studies above showed that there was no standardized method to create burn wounds in the porcine model. Nevertheless, for deep dermal burn wounds we can use the modified glass bottle method and for full thickness burn wounds we can use metal tools (aluminium or brass). There are no previous studies that discuss how to make porcine burn models. There are also no studies in this review that focus on creating the burn wound alone. Further studies are needed to achieve better results in how to create burn wounds in porcine models.