After laparotomy, treating large incisional hernias (width >= 10cm) proves challenging due to the progressive retraction of lateral abdominal muscles and the separation of rectus muscles. This width is a significant risk factor for repair failure and recurrence. High rates of severe postoperative morbidity, up to 50%, are reported, linked to dissection extent, increased muscular tension, and abdominal pressure. Reconstructing normal anatomy by bringing muscles together may be impossible, leading to the use of complex procedures like component separation techniques (CST), involving large aponeurotomy for muscle relaxation. Intramuscular injection of botulinum toxin A (BTA) induces reversible flaccid paralysis, with potential benefits in hernia closure, known as "chemical CST." Retrospective studies suggest reduced muscle retraction and facilitated closure without specific morbidity. Prehabilitation with BTA aims to reduce surgical morbidity compared to repair and CST. The prospective evaluation of BTA's clinical benefits, including reduced postoperative morbidity, pain, successful abdominal closure, and decreased IH recurrence risk, is lacking. A prospective randomized double-blind placebo-controlled trial is proposed to demonstrate BTA's efficacy. The hypothesis is that BTA injection before IH repair is more effective than a placebo in reducing postoperative morbimortality. Secondary expectations include a significant reduction in complete closure of the abdominal wall without CST.

Almost 20% of patients will develop an incisional hernia (IH) after laparotomy. Each year in France, around 30,000 patients undergo IH repair with mesh [PMSI, 2017]. The treatment of large IH (width>=10cm) is difficult due to the progressive retraction of the lateral abdominal muscles associated with the separation of the rectus muscles. The IH width is a major risk factor of failure of the repair and recurrence. Furthermore, high rates of severe postoperative morbidity, up to 50%, have been reported and related to the extent of dissection and increase of muscular tension and abdominal pressure. Thus, bringing the muscles together to reconstruct the normal anatomy may be impossible and lead the surgeon to use complex and morbid technical procedures, such as component separation techniques (CST), consisting in large aponeurotomy for relaxation of the lateral muscles. The intramuscular injection of botulinum toxin A (BTA) makes it possible to obtain a reversible flaccid paralysis of the striated muscle fibers and its advantage has been demonstrated for the treatment of neurological spasticity. Its use to obtain a relaxation of the lateral muscles of the abdomen, so-called "chemical CST", reduce their retraction and facilitate hernia closure, as studied in retrospective studies, without specific morbidity. In particular, prehabilitation with BTA injection, is supposed to reduce surgical morbidity in comparison with surgical repair and CST. The expected clinical benefit, in terms of reduction of postoperative morbidity and pain, successful closure of the abdomen, and reduction of the risk of recurrence of the IH, has never been evaluated prospectively. Thus, a prospective randomized double-blind placebo-controlled trial would be the best method to demonstrate the benefit of BTA injection. The investigators hypothesize that BTA injection in the lateral muscles before IH repair is more effective than placebo injection in reducing postoperative morbimortality. Secondarily, the investigators expect that BTA injection is associated with a significant reduction of complete closure of the abdominal wall without CST.

Inclusion Criteria:

1. Patients between 18 and 79 years;
2. BMI < 35 kg/m²;
3. Midline anterior primary or recurrent IH (subxiphoidal to suprapubic), of width >= 10 cm, on the abdominopelvic CT without injection of contrast agent, performed in the 6 months before inclusion (EHS W3);
4. IH without loss of domain, defined by the ratio: (volume of the peritoneal sac) / (total peritoneal volume) < 25%, on the abdominal CT without injection of contrast agent, performed in the 6 months before inclusion;
5. Written informed consent;
6. Scheduled surgery for an open IH repair;
7. For female of childbearing potential: using highly effective contraception.

Exclusion Criteria:

1. Other types of IH (lateral, groin, para-stomal, portsite);
2. VHWG grades 3 or 4 for the risk of surgical site infection;
3. Ongoing skin infection or inflammation at the IH site or at the BTA injection site;
4. Planned IH repair with slowly absorbable mesh;
5. IH with loss of domain (volumetric ratio > 25%);
6. Emergency IH surgery;
7. ASA score > 3;
8. Pregnancy or breastfeeding;
9. Ongoing treatment with aminoglycosides;
10. Severe hemostasis disorder or non-weaning treatment with curative dose anticoagulant;
11. Active tobacco use (or cessation inferior to 3 months);
12. Use of another investigational product within 6 months or 5 half-lives (whichever is longer), or currently participating in a prospective study with an investigational product, whether it concerns an experimental drug or a medical device;
13. Patient not covered by social insurance;
14. Patient under legal guardianship;
15. Hypersensitivity to the active substance (Clostridium Botulinum neurotoxin type A) or to any of the excipients (Human albumin, sucrose);
16. Generalized disorders of muscle activity (e.g. myasthenia gravis, Lambert-Eaton syndrome, peripheral motor neuropathic diseases (e.g. amyotrophic lateral sclerosis or motor neuropathy), facial nerve disorders, underlying neurological disorders) and history of dysphagia and aspiration);
17. Patient with ongoing treatment with medicinal products that interfere with the transfer of an impulse from a nerve to a muscle, e.g. tubocurarine-type muscle relaxants that weaken the muscles;
18. Patient with severe and uncontrolled cardiovascular diseases;
19. Patient has received BTA within 12 weeks;
20. Patients with a history of seizures.

The injection physician and surgeon will be blinded to the randomization group due to the transparency of BTA and the placebo.

The surgeon will not know the patient's treatment during surgery as BTA does not affect muscle appearance.

A blinded CT-scan, four to six weeks post-injection, will be performed; results will be kept from the surgeon to avoid unblinding, and patients will remain blinded to their randomization group.