Background: Many factors have been related to the appearance of an incisional hernia. The level of tension in the sutured laparotomy is one of them so an experimental animal study was carried out to understand the forces applied during the closure of a laparotomy.
Method: Twenty-seven female pigs were divided into three groups depending on their weight 20 kg, 50 kg and 100 kg, respectively. Each group was subdivided into three subgroups depending on the distance measured from the fascial edge to the stitch (5 mm, 10 mm and 15 mm). All animals underwent a laparotomy and the closure forces were measured at different levels and distances with a digital dynamometer (designed by our group). Variables were divided into 3: response (closure force), experimental (weight, distance from the fascial edge to the suture points, level of the laparotomy) and confounding (abdominal circumference., fascial thickness, laparotomy length, laparotomy width) variables.
Results: Closure forces were measured along the different levels showing higher closure forces in the 5 mm subgroup than in the other 2 subgroups. The statistical analysis showed that the weight, the distance from the fascial edge and the level of the laparotomy, were all statistically significant (p=0.0001, 0.04 and <0.0001, respectively). Neither the fascial thickness nor the abdominal circumference affected the closure forces. The behaviour of the closure forces was different in the supra-umbilical and in the infra-umbilical areas.
Conclusion: Closure forces along a midline laparotomy are variable since a different behaviour in both supra-umbilical and infra-umbilical areas was described.
Many factors have been related to the appearance of an incisional hernia and despite the development of new techniques and technological advances, its incidence remains high. These factors depend on multiple variables, including patient comorbidities, surgical technique and peri-operative conditions (materials, immediate wake-up, postoperative ileus, etc.). One important parameter, that has not yet been taken seriously into account as a risk factor for developing an incisional hernia, is the tension of the suture. The literature is scarce regarding this issue, but it has been demonstrated that sutures with high tension can cause failure of the fascial tissue  by producing tears or hypoxia of the tissue. The tension of a suture is one of the mechanical vectors involved during the closure of a laparotomy. Unfortunately, there are no available tools for its measurement, which is also one of the reasons why studies are so scarce. Previous to measuring the tensile force that is needed to approximate the fascial edges, two main causes that are involved must be considered: Intra-Abdominal Pressure (IAP) and musculoaponeurotic forces. These forces, described by Förstemann et al , are involved in the development of an incisional hernia. On the other hand, the suture is responsible for supporting the tension between the fascial edges during the first 3 weeks . After this period, the healing tissue should support the tensile forces. A mechanical overload on the fascial edges over time contributes to an alteration of the organization of collagen fibers  and finally leads to failure in wound healing. The closure of a laparotomy with excessive tension of the suture is considered the most prevalent intra-operative risk factor in developing an incisional hernia .
For a better understanding of the forces involved, consider that the resulting suture forces (Closure forces) during the closure of a laparotomy are the addition of two force vectors: the tensile force needed to approximate the wound edges and the compression force in the contact area between them. These concepts are defined later on (Figure 1). In the literature, different authors [6-10] have studied the closure of a laparotomy focusing on the design of the technique (short stitches, continuous or interrupted suture, absorbable or non-absorbable sutures, needle size etc.), some have analyzed the tensile strength after closure in animals after increasing the IAP with a balloon and others by removing a tissue sample for study in the lab [11-15]. Studies about closure tension are limited. Some authors have described results in human cadavers [16-18], some in experimental settings [19-21] and others in patients with incisional hernias [22,23], but none have described the tension of the closure forces during an abdominal wall closure. Jenkins et al  described a mechanical approach to the burst abdominal wound, considering there is a 30% of sutured wound stretch after abdominal distension. They concluded that the length of the suture material (LS) should be four times greater than that of the wound (LW) to have sufficient suture to avoid high tension on the fascial edges when stretching occurs. Israelsson et al  in a prospective trial, demonstrated that the incidence of incisional hernia was lower when the closure of a laparotomy was sutured with a ratio of LS to LW ≥ 4. Sanders et al , based on the description of Nelson and Denis in 1938, studied, in rats, the tensile strength in laparotomies closed with tight and loose sutures, demonstrating that the strongest closure was with large bites (1 cm from the fascial edge) and loose ties.
Cengiz et al  described, in an animal study in rats, a higher strength to wound bursting with stitches placed 3 to 6 mm from the wound edge than with those at a distance of 10 mm, 4 days after closure of a midline laparotomy. Höer et al , using a digital dynamometer clamped to the tissue, analyzed the tensile strength of a laparotomy closure in rats, with different SL/ WL ratios. They concluded that a ratio above 4:1 avoided high suture tension and provided a significantly positive effect on the mechanical strength of the suture. They further concluded the necessity for experimental studies dealing with the development of initially applied suture tensions, thus allowing surgeons to do a suture according to tissue-specific tension. Unfortunately, as shown above, there are few studies related to the measurement of closure forces using a dynamometer in patients or in cadavers. Villalobos et al  analyzed the closure forces in patients that underwent a midline laparotomy using a force gauge (Mecmesin AFG 1000N®) and concluded that the closure forces are variable along a midline laparotomy, as was described in human cadavers. The closure force is one of the few variables that can be assessed and controlled intra-operatively by a surgeon, but neither a suitable device to measure this force nor studies about this parameter along a laparotomy exist. This article describes an animal experimental study that uses a new digital dynamometer designed by our team , to analyze the closure forces along a midline laparotomy and thus describe a new parameter that could decrease the risk of developing an incisional hernia. The study was approved by the ethics committee for animal experimentation number 61/13 of the Vall d´Hebron Research Institute (VHIR), given by the Ministry of Agriculture, Ranching, Fishing and Food of the Generalitat de Catalunya (B69900062), Barcelona, Spain.
Material and Methods
There are some mechanical concepts related to the forces at the fascia, before and after a laparotomy, that must be defined for a better understanding of this study:
-Tensile force (Ftens): the fascia at the midline has to withstand a tensile or traction force in a transversal direction. This force can be evaluated as the addition of the musculoaponeurotic traction force (Fmusc) and the circumferential force due to intra-abdominal pressure (FIAP).
Ftens = Fmusc + FIAP
When a midline laparotomy is performed, this force is responsible for the separation between the edges of the incision.
-Closure Force, (Fc) is the force requested to maintain the abdominal wound edges just in contact with each other at the time of surgery. In this study, a digital dynamometer was constructed in order to measure this variable. This force was measured when the dynamometer’s tines grabbing the tissue, placed it at the same distance from the incision as prior to the laparotomy
Formula: Fc = Ftens
-Compression force (Fcomp): after the borders of the cut tissues are brought together by the suture, or by other means, the tissue of both fascial edges starts compressing with each other, creating a new force called compression force. This force rises exponentially as the suture is stretched and as the distance between the suture points becomes shorter.
-Suture force (Fsut): the force exerted by the suture to close the fascial edges. Although there is no commercial device to measure it, the surgeon applies a certain tension depending on the suture caliber, based on his experience, in order to ensure the contact between the wound edges. In general, the suture force can be defined as the sum of the closure force plus a certain compression force.
Fsut = Fc + Fcomp
This suture force is transmitted by the caliber of the suture to the healing tissue, distributing the tension along the incision, depending on the suture technique applied.
The variables in the study were divided into 3 categories as follows:
-Response variable: Closure force (Fc)
-Experimental variables: Pig weight, distance from the fascial edge to the suture entry points, level of the laparotomy (distance upward or downward from the umbilicus)
-Confounding variables: Abdominal circumference, fascial thickness, laparotomy length, laparotomy width.
Accurate and reliable measurements of the closure force were key factors for this study. So, our group developed and validated a digital dynamometer designed for this purpose . It is a compass-like instrument with two articulated legs that can bring closer or move further away two sets of tines that grab the tissue allowing both sides of the incision to be pulled together in a way similar to that of the suture during closure of the laparotomy. The dynamometer measures both the forces applied when approximating the tissues between the opposite grabbing points as well as the distance between them. Our team named this device DynaSurg (Figure 2) Patent ES-2630977_B1.
Twenty-seven female pigs were divided into three groups, A, B and C, depending on their weight (20 kg, 50 kg and 100 kg, respectively). Each group was in turn subdivided into three subgroups depending on the distance measured from the fascial edge to the suture entry point (5 mm, 10 mm and 15 mm). All these suture points were marked every 20 mm and were named SU2, SU4 ….SU16 and IU2, IU4…. IU16, if they were localized in the Supra-Umbilical (SU) or in the Infra-Umbilical (IU) area, respectively.
The study was initially carried out at the Vall D´Hebrón Research Institute (VHIR) in Barcelona and was later concluded at the Applied Biomèdic Experimental Research Center (CREBA) in Lleida. Once the pigs were under general anesthesia, transversal lines, named levels, were drawn at the suture points, every 20 mm, starting from the umbilicus (U) and moving upwards (supra-umbilical, or SU) and downwards (infra-umbilical, or IU). Next, a measurement of the abdominal circumference was taken at every level using a measuring tape. The animals then underwent a xiphopubic laparotomy with dissection of the skin,
the subcutaneous fat and the cutaneous trunk muscle, in order to expose the anterior rectus sheath, 5 cm away from the midline on each side. The resulting subcutaneous flap was attached laterally with 3 silk stitches on each side. Transversal lines were marked on the fascia, in the same way as the ones previously described on the skin (levels). The linea alba was subsequently incised, sparing the peritoneum to avoid evisceration that could interfere with the measurements (Figure 3). Afterwards, small orifices were made at every level at varying distances from the fascial edge, depending on the subgroup (5, 10 or 15 mm), followed by a measurement of the fascial thickness at these points. Finally, the closure forces at every level were measured individually (Figure 4). These forces were measured as soon as the fascial edges contacted each other, in order to avoid or minimize the compression force.