The treatment of abdominal wall defects, including various hernias, has made important advances in the last half century. However, the treatment of complex abdominal wall defects is still a major challenge that surgeons must face, and their understanding is still being refined. The aim of surgical treatment is not only to restore the anatomical integrity of the abdominal wall, but also to restore the function of the abdominal wall, and to achieve the desired therapeutic effect of abdominal wall defect repair through abdominal wall reconstruction. 1. Definition of complex abdominal wall defect Complex abdominal wall defect mainly contains four conditions: (1) size, volume and location of abdominal wall defect: width of defect ≥ 10 cm, ratio of hernia to abdominal volume ≥ 15%, abdominal wall defect with abdominal wall insufficiency, defect occurring in special areas such as suprapubic and subcostal margins. (2) Local conditions of the abdominal wall soft tissues: with contamination or infection of the defective abdominal wall tissues, after extended abdominal wall tumor resection, after severe trauma, after multiple surgeries, after skin grafting, with ulcers or difficult to heal wounds or with enterostomy and intestinal fistula. (3) Patient’s general condition and history of previous diseases: with obesity, diabetes mellitus, chronic obstructive pulmonary disease, steroid hormone use, poor nutritional status, history of incisional dehiscence, after repair with patching or tissue structure separation techniques. (4) Concomitant conditions of the patient: the need for simultaneous bowel resection, the need for patch removal or multiple hernias. The accurate staging of complex abdominal wall defects is the basis for selecting the appropriate surgical plan and the prerequisite for postoperative outcome assessment. However, there is a lack of unified staging criteria for complex abdominal wall defects. The author classifies abdominal wall defects as: (1) Type I: involving only skin and some subcutaneous tissue loss. (2) Type II: the abdominal wall myofascial layer is mainly missing, but the abdominal wall skin integrity still exists, such as a giant abdominal wall incisional hernia is a typical type II abdominal wall defect. (3) Type III: total abdominal wall loss, including total abdominal wall loss after extended tumor resection or after severe trauma are all type III abdominal wall defects. The vast majority of complex abdominal wall defects are type II or type III abdominal wall defects. According to the location of the defect, the author divided the abdominal wall defect into 3 zones: abdominal wall defect in the midline (M zone), abdominal wall defect in the upper outer quadrant (U zone) and abdominal wall defect in the lower outer quadrant (L zone). The abdominal wall defect typing is described by the degree of abdominal wall defect combined with the site of occurrence. This abdominal wall defect typing is simple and practical, and can provide an important reference for the choice of surgical treatment of abdominal wall defects. 3. Functional repair techniques for complex abdominal wall defects The ideal abdominal wall repair is one in which the abdominal wall is re-covered by myofascial tissue with vascular and neurological innervation. The repaired abdominal wall not only provides mechanical support of sufficient strength to protect the abdominal cavity contents, but also has a good appearance to achieve a true sense of abdominal wall reconstruction. (1) Patch-reinforced abdominal wall defect repair techniques The use of various patches for abdominal wall defect repair has become the basis of modern abdominal wall surgical defect repair since 1958, when American physicians first used the synthetic material polypropylene for hernia repair. The use of patches has reduced the recurrence rate of abdominal wall defects by at least 50%. The application of patches includes both patch-reinforced repair to close the abdominal wall defect and bridging repair with direct fixation of the patch to the edge of the defect. The reinforced repair involves the closure of the abdominal wall defect and the placement of the patch in the following three locations: anterior myofascial, retrofascial and intra-abdominal. Since the patch can be better fixed to the abdominal wall under intra-abdominal pressure, the two positions of pre-fascia and post-fascia and intra-abdominal cavity are more commonly used in clinical practice. Bridging repair is mainly performed by intermuscular patch placement. The former has significant advantages over bridging repair in reducing the recurrence rate and complications of abdominal wall defects. The tension on the abdominal wall after closure of the abdominal wall defect is shared between the patch and the myofascial tissue in front of it, and the intra-abdominal pressure is uniformly distributed throughout the abdominal wall. In contrast, in bridging repair, the intra-abdominal pressure is not uniformly distributed between the patch and the abdominal wall, and this uneven distribution of tension predisposes to recurrence of the hernia, especially at the patch-fascia junction. In addition, the subcutaneous adipose tissue over the patch in bridging repair is compared with the myofascial tissue over the patch in strengthening, with insufficient vascular ingrowth in the former and also with pulling of the abdominal wall muscles on the deficient side. It has been shown that the rate of abdominal wall bulge or hernia recurrence after bridging repair with biopatch alone can be even as high as 80-90%. The choice of patch material is also another important issue that must be considered when performing abdominal wall defect repair. The materials currently used clinically for abdominal wall defect repair are divided into two main categories: synthetic nonabsorbable materials and biomaterials. The ideal patch material should have sufficient mechanical strength, good biocompatibility, and support the long entry function of its own tissue, but no material can fully meet this requirement at present. Synthetic nonresorbable materials can provide strong support for abdominal wall defects, but their use is limited in cases of abdominal wall defects with severe contamination or infection; biomaterials that support neovascularization and host cell ingrowth are a more suitable choice, but their long-term efficacy needs to be verified by results of long-term follow-up studies. Based on the risk assessment of the occurrence of unexpected events at the surgical site, the current investigators believe that (1) for patients without comorbidities and wound infections, etc., the choice of patches can be determined by the physician on a patient-by-patient basis. (2) For patients with comorbidities such as diabetes mellitus and malnutrition, the use of biomaterial patches is potentially advantageous because of the close association between comorbidities and the occurrence of surgical site accidents. (3) For patients with contaminated wounds, suspected contamination, and previous incisional infections, the use of permanent synthetic non-absorbable material patches is not recommended due to the increased risk of infection that may result from contaminated wounds, and the choice of biomaterial repair is potentially advantageous. (4) For patients with significant existing infection, biomaterials are recommended for abdominal wall reinforcement repair. (2) Tissue separation technique The core of abdominal wall strengthening repair is to close the abdominal wall defect. The histologic separation technique offers the possibility of closure of large abdominal wall defects in the M region. Theoretically, the bilateral histologic separation technique can achieve up to 20 cm of abdominal wall defect coverage at the umbilical level, which not only reconstructs the abdominal white line and significantly expands the abdominal cavity volume, but also preserves the vascular nerve bundles that innervate the abdominal wall muscles. Therefore, this technique allows for a more physiologically correct reconstruction of the abdominal wall function. However, its recurrence rate can be as high as 30% after abdominal wall defect repair when applied alone. Therefore, the technique of tissue structure separation based on patch reinforcement has been extensively tried and applied clinically, and the recurrence rate of postoperative abdominal wall defects has been reduced to less than 10%. In addition, the traditional tissue structure separation technique requires extensive subcutaneous tissue separation, which inevitably causes damage to the peritoneal wall penetrating vessels. In contrast, tissue structure separation techniques that preserve the peritoneal vessels and endoscopic tissue structure separation techniques are being increasingly used in the repair of complex abdominal wall defects with good results because they can protect the peritoneal vessels and reduce the incisional complications. (3) Tissue flap technique Type III abdominal wall defects are difficult to repair by conventional treatment methods because of the presence of defects in the entire abdominal wall, especially the absence of skin. Autologous tissue flap repair of abdominal wall defects has become an important option for the treatment of type III abdominal wall defects. Small to medium-sized abdominal wall defects can be repaired with a tipped tissue flap, and the choice of tipped tissue flap is determined mainly by the site of the defect. However, large abdominal wall defects often require free tissue flap repair, and vascular anastomosis technique is an important technical guarantee for successful free tissue flap grafting. The tissue flap technique is actually often used in combination with structural tissue separation techniques and patching techniques to achieve better repair and reconstruction results. Its autologous nature also allows it to be used for tissue repair in contaminated or infected conditions. However, the tissue flap technique also has the disadvantage of being a complex surgical technique that can cause new damage in the donor area. Therefore, the principle of simplicity, practicality, and minimizing the sacrifice of normal tissues should be followed when performing tissue flap techniques for the treatment of complex abdominal wall defects. 4. Conclusion The typing of abdominal wall defects is the basis for the development of surgical protocols for complex abdominal wall defects, and the realization of abdominal wall anatomy and functional reconstruction is the purpose of treatment. The author believes that with the rapid progress of surgical and material science techniques, the treatment of complex abdominal wall defects will definitely achieve better results on the existing basis.