Complicated wounds with three-dimensional deficits resulting from severe trauma, chronic infections, and tumor resections often present with superficial soft tissue defects and deep dead spaces [15,16,17]. One-stage reconstruction of the complex soft-tissue defects is essential to salvage an extremity, promote wound healing, and prevent postoperative complications. Elimination of dead space is essential, and failure to eliminate the dead space often results in necrosis of vascular flaps due to higher the risk of wound infection and hematoma. The mainstream approach uses muscle tissues to achieve the obliteration of dead space [12, 18]. The latissimus dorsi myocutaneous flap is currently the most popular traditional procedure to repair a complicated wound owing to benefits such as a concealed donor site, availability of multiple blood vessels, a large available area for incision, rich blood supply, and potent anti-infection ability [12, 19, 20]. However, this procedure failed to separate the muscle and the skin, which results in the limitation of freedom of movement; actually, it could not achieve accurate obliteration of the dead space. Besides, excessive muscle tissue incision also causes severe injury to the donor site and an unsightly appearance. The combined transplantation of free muscle tissue and skin flaps is another potential strategy for repairing complicated wounds with three-dimensional deficits [16]. However, this method requires anastomosis of two sets of blood vessels, and the muscle segment and skin flap need to be harvested from two donor sites, which prolongs the operation time, damages the donor site as well as elevates the risk of surgery.

In view of the shortcomings of the above-mentioned traditional methods, the concept of chimeric perforator flap has been proposed in recent years as a possible approach to reconstruct complicated three-dimensional tissue deficits [21,22,23]. The nomenclature was unclear until Huang [24] and Hallock [25] defined this flap as a chimeric perforator flap in 2003. Following this, Hallock further systematically introduced the chimeric concept of perforator flaps based on different blocks of tissue were nourished by different branches of vessels with a common pedicle [26]. Notably, the muscle-chimeric perforator flap offers numerous advantages for repairing complicated tissue defects: ① Only a set of vessels pedicle requires anastomosis to achieve not only successful coverage of surface soft tissue defects with skin paddle but also effective elimination of dead space with muscle segments; ② the muscle segment can be accurately selected according to the volume of the dead space while minimizing donor site morbidity; ③The muscle segment and the skin paddle can be independently and easily inserted with more spatial freedom, which facilitate to avoid the folding and twisting of the pedicle and the tissue, thereby resulting in an enhanced an aesthetic appearance; ④ Muscle tissues are known to improve local blood supply, increase oxygen tension and antibiotic delivery to wounds, leading to faster wound healing. Muscle tissues easily adapt to the shape of the defect and are thus optimal for the elimination of dead space in the wound.

Although several advantages of the muscle-chimeric perforator flap have been documented in the previous literature, the conventional design of chimeric flaps has focused on the simultaneous harvest of multiple types of tissues without paying particular attention to the pedicle length of each component and has mainly been applied for the coverage of large and extensive defects. In addition, the traditional design of chimeric flaps lacks the versatility to offer adequate tissue volume and allow precise tissue positioning to optimally cover the wound. To efficiently cover the surface defect and at the same time obliterate the underlying dead space for complex three-dimensional soft tissue defects, a variant of the design of the muscle-chimeric perforator flap is warranted. Herein, we presented our insights into utilizing individual designs of chimeric TDAP flaps to reconstruct complicated three-dimensional soft tissue defects. We also introduced a novel classification system of chimeric perforator flaps for reconstructive surgeons and trainees to gain a better understanding of chimeric flaps design and allow safe, effective, and aesthetically superior reconstruction of complex three-dimensional defects.

Various donor sites, including the thoracodorsal artery (TDA), subscapular vessel [27, 28], lateral circumflex femoral vessel (LCFV) [29], and deep inferior epigastric vessel (DIEV) [30] systems, have been described in the literature for harvesting chimeric perforator flaps. Among them, the gold standard for the reconstruction of complicated soft tissue defects in adults remains the vastus lateralis muscle-chimeric anterolateral thigh perforator (ALTP) free flap [29]. However, the ALTP flap is well known for variations in its vascular pedicle, and failure to understand its variability can lead to vascular flap embarrassment and tissue loss [31, 32]. Recently emerge researches have demonstrated that a suitable cutaneous perforator vessels for free ALTP flap transfer in the thigh was absent in 5–6% of patients [33, 34]. Muscle-chimeric TDAP flap possesses the inherent advantages of latissimus dorsi myocutaneous flaps, such as a concealed donor site, large available area for incision, strong anti-infection ability, and rich blood supply, and thus is a useful alternative for reconstructing complex three-dimensional soft tissue defects. However, the majority of studies have reported that the muscle component was predominantly used to increase the dimensional of flap for covering huge soft tissue defect, and providing a well-vascularized bed for skin grafts when the wound could not be covered with the skin perforator flap alone. Studies regarding the use of this flap for reconstructing complex three-dimensional extremity defects are limited. Recently, Lee KT et al. [12] reported the use of free latissimus dorsi muscle-chimeric TDAP flaps for the reconstruction of complex soft tissue defects and emphasized on obtaining a Y-shaped pedicle configuration with adequate lengths for the skin paddle and the muscle segment; as expected, meticulous intramuscular dissection was needed for both tissue segments. Therefore, a time-consuming intramuscular dissection was unavoidable for the acquisition of the pedicle, which led to higher donor site morbidity. In this study, three types of flap designs were developed for the customized reconstruction of complex three-dimensional tissue defects. Our report focused on the novel design of chimeric TDAP flap and its various designs for the individualized reconstruction of three-dimensional defects to minimize the donor site morbidity and shorten the operation time. To the best of our knowledge, these are the largest series to date reporting microvascular reconstruction of complicated three-dimensional soft-tissue defects in the extremities using customized designed chimeric TDAP flaps.

Based on our previous clinical work experience, a working algorithm can be introduced for the reconstruction of complex three-dimensional defects using the TDAP chimeric flap (Fig. 5). The type of surgical procedure is determined by several factors, including the wound size, location of dead space, and the degree of morbidity at the donor site. When the dead space located on the center of wound, only a short vascular pedicle which linked the muscle component and the skin paddle was required to achieve freedom mobilization or rotational of the skin paddles. It was facilitated to precisely insert the muscle into dead space and place the skin paddles to cover the superficial soft tissue defect. Thus, the primary objective of reconstruction is to reduce intramuscular dissection of the perforator to reduce donor site morbidity and shorten the operation time. In this context, type A design is the optimal choice. A short operation time, as well as limited site morbidity, are the expected benefits. In contrast, when the dead space is located on the edge of the wound, a longer vascular pedicle is required for the muscle component and the skin paddles to effectively reconstruct the three-dimensional soft tissue defect. Moreover, to reduce pedicle tension and twisting, the length of the vascular pedicle between components should also be completely taken into consideration. In this situation, a type B design might be more appropriate which presented with a longer vascular pedicle between components and more freedom to avoid pedicle kinking and twisting. It is worthwhile to point out that even though the type B design was still unable to avoid high morbidity of the donor site and time-consuming intramuscular dissection. Eventually, ischemic necrosis of the distal flap portion remains a commonly encountered complication in clinical practice for the reconstruction of larger complicated wound, and the balance between donor-site morbidity and optimal reconstruction of recipient sites needs to be carefully assessed. Type C may potentially be more suitable in this situation. This type design presented with multiple perforator vessels to supply blood to the large skin paddle which have been proven to enhance the flap's viability in clinical practice. However, considering that the muscle segment is located on a side branch whereby no tedious intramuscular dissection is required, whilst other perforator vessels still need to be meticulously dissected to harvest sufficiently long vascular pedicles to be freely three-dimensional insets.

Diagram illustrated a working algorithm for the reconstruction of complex three-dimensional defects by using the TDAP chimeric flap

If the three types of flap design were suitable for the wound, the primary aim of the approach is to reduce the donor site's morbidity and scar-related disorders. We prioritized choosing a transverse-designed skin paddle flap on this donor site for the moderate-sized soft tissue defects. Comparing transverse-oriented flaps to non-transverse-oriented flaps, several studies have found that transverse-oriented flaps on the backside significantly reduce the risk of developing scar-related issues at the donor site [35]. Our previous study also demonstrated that the transverse design of the scapular artery perforator flap presented a more aesthetic appearance. Moreover, the transverse design causes less lateral movement of breast mounds and nipples during donor-site closure. Considering the donor site's cosmetic appearance improvement, the recent practice showed that using a transverse designed flap was more effective than using a non-transverse one. Also, for female patients, the scar can be readily concealed with a brassiere.

The limitations of this study include the small number of cases as well as its retrospective character. And other limitations include the absence of a direct comparison group and a standardized assessment tool for the objective assessment of the result following reconstructive surgery. Thus, further studies, such as prospective case–control or randomized studies, are needed in future study.