Introduction
Incisional hernia is a common complication after abdominal surgery, especially after open surgery with a median laparotomy. Incidences of incisional hernia and burst abdomen after midline laparotomy range from 11 to 20% and 1 to 3%, respectively, and involve frequent reoperation [
1,
2]. These complications occur even more often in high-risk populations, like patients with comorbidities such as obesity, smoking or diabetes [
1‐
3] and are associated with discomfort or pain, which result in a lower quality of life [
4]. In the USA, over 300,000 hernia operations are performed annually, with estimated associated costs of $3.2 billion [
5]. Mesh-based and suture-based repair of incisional hernia exhibits recurrence rate from 0.8 to 24% and from 12 to 67%, respectively [
6‐
8]. Because most studies provide only short-term follow-up, these recurrence rates may be underestimated.
To prevent incisional hernia, laparotomy closure techniques have frequently been investigated in both experimental and clinical studies. Some of these showed that incisional hernia is an early complication after closure [
9]. Several decades of research led to recommend continuous suture technique with small suture bites of 5 mm from the wound edge and an inter-stitch distance of 5 mm with slowly absorbable suture material as the most efficient closure technique compared to the commonly used large bites [
2,
10‐
15]. The small bites suture technique still exhibits 13% incidence incisional hernia at 1 year [
15]. Incisional hernias remain a clinical challenge. Both biological and biomechanical mechanisms that result in the occurrence of an incisional hernia remain globally unknown.
Therefore, further research should be conducted to develop and systematically investigate closure techniques and materials. Clinical studies will give the highest level of evidence, but are expensive and in most cases not suitable to investigate new concepts. Preclinical experiments with cadaveric or animal specimens face several problems: the availability of human cadaveric tissue is limited and animal experiments tend to be more and more debated from an ethical standpoint. Moreover, the anatomy and physiology of animals are considerably different from the human ones. For example, the linea alba of a rat is relatively narrow and relatively much shorter compared to the human linea alba [
16]. The pig abdominal wall (AW) is more comparable to the human AW, but still exhibits numerous anatomical differences.
Previous research has focused on linear tensile strength testing of sutured porcine AW [
12]. Although this research provided important conclusions for further clinical investigation [
15], linear testing does not take into account the intra-abdominal pressure acting as well on the AW.
Moreover, linear testing features a flat and not a curved AW and therefore does not mimic the real physiology.
There is a strong need for a standardized way to compare different closure techniques and materials under physiological conditions. This device could be used to investigate pathophysiology and treatment of AW incisional hernia. A standardized artificial AW simulator could also be used as a training device for mechanical evaluation.
The recent study published by Deerenberg et al. [
15] clearly shows the impact of mechanical conditions of midline laparotomy closure on clinical outcomes.
The aim of this study was to develop a physical simulator to investigate the mechanical behavior of the AW under physiological conditions using 3D image stereo correlation and to demonstrate the possibility to describe the biomechanics of the AW after laparotomy closure. These experiments will provide a proof of concept of the ‘AbdoMAN’ device.
Experimental setup
Test setup repeatability
To investigate test reliability and repeatability, pressure and 3D image stereo correlation data were evaluated for a series of synthetic AW samples.
To simulate the physiological conditions, a test setup was chosen with standard IAP of 10 mmHg and to simulate coughing, actuator inner pressure, necessary to generate the lateral muscle force, was increased up to 3000 mmHg during three cycles at 1 Hz frequency.
Midline closure repeatability
One of the purposes of this part of the experiment was to investigate the repeatability and the possibilities of visualizing the biomechanical effects of bite size and inter-suture distance using 3D image stereo correlation. A 15 cm median laparotomy was carried out on five synthetic AWs. The incision was closed using PDSII 1 sutures (Ethicon, Somerville, NJ, USA) and using a continuous 5 × 5 modality (5 mm distance between suture and incision, 5 mm distance between two sutures). The suture was knotted five times on both ends. After suturing, the sample was placed on the ‘AbdoMAN’ and cough tests were performed as described above. Strain patterns and incision distension at the moment of muscle contraction were measured using the 3D image stereo correlation system to test the reproducibility of sutured samples.
Video material is available as supplemental material online.
Discussion
The ‘AbdoMAN’ is the first human AW simulator that enables dynamic testing under physiological conditions. It combines both intra-abdominal pressure (IAP) and abdominal muscle activity.
The stiffness of the synthetic materials (765–885 kPa) is equivalent to an active human AW (600–1000 kPa) [
23]. The found anisotropic rate of 1.22–1.28 is also in the same order of magnitude as that reported of human linea alba (1.47) [
24]. For coughing, the force applied by the pressure actuators, 660 N, and the resulting stress applied on the sample, 0.46 MPa, are within the range of the skeletal muscles stress (0.089–0.801 MPa) [
18,
19,
25‐
27]. Mean peak IAP was 74.9 mmHg (range 65.3–88.3 mmHg; Fig.
3b) which is entirely in the physiological range of 37–81 mmHg during coughing [
18,
19].
The use of 3D image stereo correlation in combination with a physiological biomechanical simulation model to analyze strain patterns and displacement in AW research was described before [
23,
28,
29]. However, the combination with a dynamic simulation device has not been demonstrated yet, and provides insights into the biomechanics of the sutured AW.
The midline closure part demonstrates the possibility to visualize strain patterns around the incision and the suture points. Using a combination of the three criteria described previously, it might be possible to investigate different closure modalities and to find an optimal laparotomy closure modality from a biomechanical standpoint. The criteria used in this part show consistent test results when repeating test cycles with different samples. Therefore, they can be used to compare different suture modalities (i.e., bite sizes).
The next step in this research field will be the systematic testing of different midline closure modalities using both the ‘AbdoMAN’ and the 3D image stereo correlation system. In the future, human cadaveric AW or porcine AW could also be used with the ‘AbdoMAN’ device. For this purpose, additional experiments will be needed to check if the criteria used to compare modalities on synthetic AW will still be relevant using biological tissue.
When this next step has been completed, the ‘AbdoMAN’ can be used in experiments in which (cough) cycles are being repeated numerous times. This will reflect the physiological situation in which incisional hernias develop over time after a longer period of repeated, intermitting stress.
When more will be known about strain and displacement data interpretation, the ‘AbdoMAN’ may be used for future research on finding new, ideal suture modalities. Moreover, different suture materials (such as elastic or barbed sutures) or mesh augmentation could be investigated using the ‘AbdoMAN’. Even more challenging and interesting would be the creation and closure (with or without mesh) of AW defects to investigate different treatment modalities.
Finally, the ‘AbdoMAN’ could provide an easily accessible tool for training of laparotomy closure and hernia repair. For example, the effect of a suboptimal closure technique performed by a trainee could be directly evaluated.
To our opinion, the complete test setup can be reproduced at other sites, enabling standardized, simultaneous experiments or teaching settings throughout one (or more) countries.
The ‘AbdoMAN’ has limitations. It is not possible to simulate tissue healing, as it is a mechanical simulator.
One other limitation is the fact that in this setup, although the stiffness of the synthetic materials was set up to mimic active tissue, the AW does not reproduce the material properties changes driven by the contraction. This might result in different phenomena.
Also, the synthetic AW consists of two components to provide both the strength and flexibility needed to simulate the human AW features. This may react differently than the human linea alba, consisting only of connective tissue. The dimensions of the sample, comparable to a human AW, but five times thicker than a fascia [
30], the friction between the sample and the artificial abdominal cavity could as well be limitations.
Some variance was found in IAP and strain data, which might be explained by slight stiffness differences observed between synthetic abdominal walls.