The use of surgical textile implants (so-called “mesh”) for laparoscopic or open repair of ventral, incisional and inguinal hernia is an accepted standard. To reduce the recurrence rate, more than 2 million polymer-based meshes are inserted each year worldwide . Beside a recurrent hernia, shrinkage and deformation of the implant are common complications. They may cause mesh-related problems such as chronic pain, migration, penetration into abdominal organs, or fistula formation. [2, 3]. To assess mesh-related problems more precisely, to plan a surgical reoperation or a subsequent abdominal operation more accurately, and to get more information for further mesh development, a clear and complete visualization of the mesh would be very helpful.
Nevertheless, the radiologic visibility depends amongst others on the density, structure, and thickness of the material used to produce the mesh. One might say more mass implies greater visibility . However, in order to optimize tissue integration and biocompatibility modern mesh structures tend to be thin with reduced material (low-weight large-pore-sized) , which increases the difficulty for high-resolution imaging techniques such as computed tomography (CT) and magnetic resonance imaging (MRI). In some cases no signal from the polymer can be acquired using conventional MRI techniques, because of too short T2 constants, due to the small diameter of the filaments. Rakic and LeBlanc explored in a review that the visibility of meshes varies from not visible at all, to hardly discernible, to readily seen, to always visible. Beside the specific properties of the mesh itself, visibility often is determined by the host inflammatory reactions induced by the implanted prostheses . Therefore, tissue integration without contrasting seroma, abscess or hematoma is a challenging condition for mesh imaging.
Furthermore, Zinther, Wara and Friis-Andersen studied the extension of adhesions to intraperitoneal onlay mesh 5–7 years after laparoscopic ventral hernia repair using transabdominal ultrasound and cine MRI. They could give detailed information about the restricted slide of the bowel loops along the anterior abdominal wall. But especially in case of adhesion-free areas the delineation of the mesh itself could be better . According to this Fischer et al. explored functional cine MRI for direct visualization and assessment of implanted mesh in patients after incisional hernia repair. They found that only one of the used mesh types, a ePTFE mesh, was clearly visible in MR images, allowing an accurate assessment of the mesh itself and its fixing. By contrast the PP mesh, could not be identified on MR images several months after the operation. In the laparoscopic surgery group they found a large number of patients in whom the visible net fixation seemed not to be intact and the edges of the mesh were not found in the proper position in MRI .
Especially under complex anatomic conditions as in vaginal reconstructive surgery, in reinforcement of pelvic floor structures or in inguinal hernia repair, mesh imaging is limited. Svabik et al. had problems to monitor a textile implant after anterior vaginal reconstruction for postoperative follow-up. Even with vaginal ultrasound – the method of choice in this context- they were unable to accurately measure transverse mesh diameters and consequently have not attempted to calculate area due to the uneven shape and width of the mesh. . Furthermore, visualization under complex anatomic conditions becomes more complicated, using preshaped mesh implants with three-dimensional (3D) structure. To get full information about 3D meshes and its surroundings, a computer-aided 3D reconstruction of MRI-data is mandatory.
Being aware of common complications after mesh implantation on the one hand, and inadequate imaging of mesh implants and its problems on the other hand, the development of a MR-visible mesh is absolutely essential. A MR-visible textile implant should be able to clarify the following aspects:
Do the implant show a loss of functional properties (e.g. elasticity) after tissue integration?
What is the extent of shrinkage or migration?
Differentiation of reasons for patients complaints after mesh implantation: Are these complaints mesh-related? Is a reoperation necessary?
Result testing: Is the implant in a correct postoperative position? Is there a sufficient overlap? Does the implant cover the fascial defect?
In 2007 a research project had started in Aachen, including the University hospital and the FEG-Textiltechnik, with the aim to study whether polymer implants can become visible in MRI by supplement of iron particles. In the following we outlined the major steps in the development of “visible” textiles, which can be used in patients.
Materials and methods
The polymer used in this project was polyvinylidene fluoride (PVDF), a material known to have an excellent biocompatibility. To achieve MR-visability, ferrooxide particles (nanoparticles with a core size of 9.4 nm) were incorporated into the base material during the spinning process (Figure 1). The applied extruding process guaranteed a homogeneous distribution of the ferrooxides over the threads. These ferrooxides caused signal voids during MR-scanning (Figure 2; FEG-Textiltechnik Forschungs- und Entwicklungsgesellschaft mbH, Aachen, Germany ). Testing different diameters of threads and different concentrations of ferrooxides, we could investigate the different contrast modulation. Concentrations of 9 mg ferrooxides per g polymer showed best results concerning contrast modulation, outshining the real dimension of the mesh by only up to 2 mm. Furthermore, concentration of ferrooxides could be influenced by the amount of MR-visible fibres of the used mesh structure. With 10% of MR-visible fibres the current meshes enabled a sufficient contrast modulation and a computer added three-dimensional (3D) reconstruction and analysis of MRI data. To optimize the discrimination of the MR-visible mesh against all kinds of abdominal structures, including signal free tissues or air, we used a positive contrast imaging. With this special MR sequence, the MR-visible mesh provided an optimal hyperintense signal [9–11].
Results and development
After having proven the MR visibility of this new textile implant in different phantoms, we started to explore this new tool in vivo.
Visible mesh in abdominal wall of rats
First, we used a rat model to demonstrate and quantify time dependent mesh shrinkage in vivo by MRI. The MR-visible mesh with a simple experimental textile construction was implanted as abdominal wall replacement in 30 rats. On days 1, 7, 14, or 21, MRI was performed to measure the length, width, and area of the device. The MRI presented a mean shrinkage of the mesh area in vivo of 13% on day 7, 23% on day 14, and 23% on day 21. Being aware of approximately 8% additional shrinkage during ex vivo measurements, these results were in line with the postmortem findings. We could prove a precise visualization of the MR-visible textile implants during MRI and an accurate in vivo assessment of shrinkage (Figure 3) .
Visible mesh in abdominal wall of rabbits
Subsequently, we tested these implants as intraperitoneal onlay mesh (IPOM) in a rabbit model. In this study positive-contrast imaging (PCI) was implemented to separate susceptibility-induced voids of the ferrooxide particles from proton-deficient voids like inflated intestine. We implanted IPOM’s laparoscopically in 10 rabbits without creating any additional peritoneal lesion. At days 1, 30, and 90 after surgery, conventional gradient echo, turbo spin echo (TSE) sequences, and PCI were performed. Images were scored with respect to mesh visibility, delineation of the surrounding tissue, differentiation from other structures, and overall diagnostic use. The mesh shape, possible deformation and possible mesh migration were evaluated on the different pulse sequences and compared with the results at surgery and autopsy. The MR-visible meshes appeared as hypointense signal voids on gradient echo sequences, as a hyperintense line on PCI, and as a very thin dark line on TSE images. In all animals, a precise depiction of the mesh location and its spatial configuration and integrity was possible by MRI and confirmed by surgical and autopsy results. The standard gradient echo imaging was best suitable to assess implant location, integrity, and configuration. PCI was helpful to achieve a complete delineation of mesh borders .
In a second set of experiments we explored whether application of a mesh in presence of pneumoperitoneum may cause deformation or wave formation when gas is released. We therefore objectified the extent of mesh deformation and shrinkage by MRI. Laparoscopic intraperitoneal onlay mesh (IPOM) implantation, again without any additional peritoneal trauma, was performed in 10 female rabbits using our ferrooxide loaded PVDF meshes. MRI measurements were performed postoperatively at days 1 and 90. After three-dimensional (3D) reconstruction of all MRI images the total surface and the effective surface of the implanted mesh were explored and calculated computer-assisted. We could identify the mesh in MRI in all cases. Answering the aspects of the introduction section, the subsequent 3D reconstruction always allowed a calculation of the mesh area. In vivo investigation of mesh surface via MRI could exclude a significant initial reduction of the effective mesh surface after release of pneumoperitoneum in this IPOM rabbit model. A further subsequent shrinkage of these MR-visible, large pore PVDF implants could be excluded, as well (Figure 4) .
Visible mesh in abdominal wall of pigs
After successful visualisation of plane MR-visible surgical meshes, we started to evaluate MRI visualization of preshaped implants with complex 3D structure and direct contact to the intestine. Therefore, terminal sigmoid colostomy has been done with laparoscopic implantation of a MRI-visible prophylactic intraperitoneal stoma mesh with a flat part and a central funnel in a porcine model. MRI investigations were done after 1 week, 6 months and in case of clinical impairment. These findings were compared to endoscopy and makroscopical investigation of the preparation. The first animal has to be killed because of an ileus 4 weeks after operation. The second animal has to be killed after 7 weeks because of recurrent obstipation. In all cases MRI investigation could identify the 3D implant and could clearly separate between mesh and intestine. MRI revealed a big bowl ileus due to a funnel dislocation in the first animal. In the second animal, MR diagnostic explored a functional stenosis because of a too small diameter of the central funnel in combination with sticky feces and distension of the terminal sigmoid before discharging into the funnel. Endoscopy and makroscopical investigation of the preparation supported MRI findings. Although complicate clinical course was a diagnostic challenge exploring 3D implants, visualization of this new MRI-visible mesh could be proved and turned out as an effective diagnostic possibility .
Visible mesh in abdominal wall of humans
After clinical approval of these implants in different animal studies, we pursued to evaluate the MRI conspicuity of such ferrooxide-loaded mesh implants in patients treated for inguinal hernias and explored the postsurgical mesh configuration by MRI. All implants used in humans are CE- approved after extensive biocompatibility testing according to DIN EN ISO 10993 with special attention to the MRI-suitability according to ASTM-standards (American Society for Testing and Materials). After approval by the Ethics Committee, we perfomed a prospective cohort study with 13 patients treated for inguinal hernia, receiving MR-visible mesh implants (Figure 5). The implants were applied via laparoscopic transabdominal preperitoneal technique (TAPP; n=8, 3 patients with bilateral hernia treatment) or via open surgical procedure (Lichtenstein surgery; n=5). MRI were performed 1 day after the surgery using different sequences. Three radiologists independently evaluated mesh conspicuity and diagnostic value of the sequences. Furthermore, mesh deformation and coverage of the hernia were visually assessed and rated using a semi-quantitative scoring system. MRI visualized all 16 implants successfully. Overall, in both implantation techniques, the meshes exhibited mild to moderate deformations. In this context we could address more answers to the aspects at the beginning. According to our findings we propose a combination of different gradient echo sequences (GRE) and T2-weighted turbo spinecho sequences (T2wTSE) for mesh configuration. T2wTSE for anatomy assessment, and GRE3 for evaluation of hernia coverage and mesh localization (Figure 6) .
In the past MR visualization of modern surgical textile implants was not or was only inadequately possible. Using ferrooxide particles in the base material PVDF, we could improve contrast modulation and delineation of the mesh material from the surrounding anatomical structures, thereby getting visible in MRI.
After proofing good results in the development stage, we could establish the effectiveness of such implants in different animal models. Even in IPOM situation, with inflated intestine in direct contact to the MR-visible mesh, we could give precise depiction of the mesh. From clinical point of view, the most substantial progress has been the application of this new device during human inguinal hernia repair. We can demonstrate, that ferrooxides-loaded implants are visualized in patients using MRI; and that it is possible to assess mesh deformation in detail. Appraisal of mesh shrinkage, dislocation or defect-covering and 3D reconstruction now is possible.
Nevertheless, results of these animal models have to valued reluctant. Tissue integration and shrinkage of a small implant (2×3 cm) in a young rat might be different from a multimorbide old patient with a large implant (20×30 cm), which furthermore has a distinct textile construction. As it is difficult to find suitable animal models to explore complex mesh geometry or challenging anatomical surrounding like pelvic floor reconstruction, future studies should focus on meshes that have been implanted in humans.
MR-visible mesh will help patients and surgeons to examine potentially mesh-related complaints, to evaluate the need for revision surgery, and to plan a reoperation. Furthermore, this tool can be helpful to address important issues in mesh-based hernia treatment, such as deformation, shrinkage, migration, or adhesions. Questions according to mesh fixation, elasticity, or pore-size of textile implants might be answered with in vivo MRI results instead of post-mortem measurements. Particularly changes or deformation of textile implants in long time observation period will help to improve ingrowth and biocompatibility of meshes. In addition, this tool will generate information for development and improvement of complex 3D-preshaped textile implants, like Dynammesh® Endolap-3D. This new implant for endoscopic inguinal hernia repair (TAPP) base upon MRI in vivo information of our prospective cohort study (code no. 194/11) using MR-visible meshes for inguinal hernia repair in human . With the MR-visible mesh even motion-picture visualization seem to become feasible, which might give additional information about mesh related problems like adhesion with corresponding transit disorder or pain because of fixation material.
Moreover, the combination of ferrooxide particles and MRI offers varies perspectives for other application area like vascular implants or catheter-technique. Maybe visualisation of implanted MR-visible implants becomes a standard postoperative quality control like x-ray documentation in trauma surgery.
In this study we described the development and first results of a ferrooxide-loaded MR-visible mesh. In animal model and in clinical use such implants turned out to be a valuable tool for diagnostic and development.
This project was supported by the German Federal Ministry of Education and Research (Ref. 01 EZ 0849) and the German Federal Ministry of Economics and Technology (Support Code KF2545603AJ1).
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