A perforator flap is a type of skin flap or subcutaneous flap that is supplied by one (or more) perforator blood vessels that branch from a deeper blood vessel. The isolated perforator is moved and freely dissected together with the overlying tissue, enabling the flap to be moved. Simple translation or transposition of the flap is sufficient in some cases, but when the flap needs to be rotated by more than 90°, it is usually deployed in the manner of a propeller with the perforator used as the axis of rotation, which is called a propeller flap [3].
The propeller flap is a special form of the perforator flap. The advantages of the propeller flap are as follows: (1) because the donor site is located near the defect, the flap is composed of tissue that is similar to the tissue of the recipient site; (2) the donor site arteries and muscles are used to completely or partially close the donor site defect and reduce the morbidity rate of the donor site; (3) lower technical requirements and faster transfer than free tissue flap reconstruction [18–22]. The propeller flap is gaining popularity as a reliable technique for repairing soft tissue defects in the distal leg and ankle; the postoperative appearance is satisfactory, and the procedure is simple, easy to master, and requires a short operation time [11, 24]. The main source arteries for the propeller flap in the distal leg are the posterior tibial artery, anterior tibial artery, and peroneal artery [11].
Although the propeller flap is an established surgical technique, serious complications may still occur and cause failure if not adequately addressed. As the propeller flap is a type of perforator flap, the occurrence of venous return disorder cannot be completely avoided. Venous return disorder is the most common complication of propeller flaps, and is one of the main causes of flap necrosis. Necrosis mainly occurs at the distal end of the flap, but may lead to necrosis of the entire flap in severe cases. Flap necrosis reportedly occurs in 10.77–24.00% of cases [25–28]. In the present study, the rate of partial flap necrosis was 20.7%. Flap necrosis may be more likely to occur when the flap perforator is located in the injured area, but this hypothesis lacks objective evidence. There are many reasons for skin flap necrosis. It is currently believed that propeller flap necrosis is influenced by the flap size, pedicle length, and angle of rotation [3, 29].
The first factor causing propeller flap necrosis is the flap size. The size of the propeller flap, especially the size of the large paddle, has a large effect on venous return. After the flap is rotated, if the length of the large paddle is insufficient, the flap is stretched so that it barely covers the wound and causes excessive tension of the vascular pedicle. In addition, flap rotation and other factors increase the risk of venous return disorder. Adequate preoperative preparation and flap design effectively reduce this problem. In the present study, preoperative Doppler examination was performed to determine the locations of the perforating vessels in the distal leg. The rotation point of the perforator propeller flap depends on the position of the perforator fulcrum. In theory, the closer the perforator fulcrum is to the wound, the greater the length of the flap that can be cut. The size of the skin flap should be designed in accordance with the size of the wound, and the position and diameter of the perforating vessels. The large paddle is located near the axis of rotation, and its length should be 0.5 to 1.0 cm longer than the distance from the point of rotation to the most distal end of the wound. The small paddle is located between the rotation point and the wound surface. The width of the flap should be 0.5–1.0 cm wider than the wound surface, and is determined by the thickness of the subcutaneous fat. However, the closer the perforator is to the wound, the greater the impact on the wound. Wound inflammation reportedly damages the perforator blood vessels and causes necrosis [30]. Our study found that the necrosis rate was significantly higher when the perforator was farther away from the flap. Furthermore, ROC curve analysis showed that the flap necrosis rate increased when the distance from the perforator to the wound was less than 3.5 cm (sensitivity 69.7%; specificity 82.4%), Table 4). We consider that there were three reasons for the increased risk of flap necrosis in cases where the perforator was closer to the flap. 1. The degree of inflammation. The inflammatory response of the flap is related to the injury mechanism/wound contamination severity. In the present study, the injuries were caused by high-energy trauma, while contaminated skin defects were excluded. 2. The shape of the skin wound. The present study assessed the distance from the perforator to the center of the flap, while the distance from the perforator to the wound center did not significantly affect the flap necrosis rate (Fig. 2B). 3. The time from injury to wound coverage, and the inflammatory reaction period. As inflammation peaks at 7–12 days after injury, it is optimal to cover the wound before or after this period. The exposed wound needs to be treated with standard dressing changes.
Previous studies have also shown that the flap width affects the survival of the flap, as the anastomoses between the perforators of the main blood vessels in the calf are almost all choke anastomoses [31]. Therefore, the wider the flap, the farther the edge of the flap will be from the axis of rotation, and the greater the decrease in the diameter of the vascular network and the pressure of the blood flow; furthermore, due to the special anatomical structure of the lower leg, when the flap position is lower, the wider edge of the flap approaches or even surpasses the midline of the front and rear of the calf, which directly leads to partial necrosis of the flap.
The second factor affecting propeller flap necrosis is the length of the vascular pedicle. After the flap is rotated, the pedicle must be kept under appropriate tension. A vascular pedicle that is too short causes excessive local tension due to traction, which causes venous return disorder. Intraoperatively, the pedicle vessel should be dissected in the direction of the source vessel as much as possible to create a length of at least 3 cm and a width of at least 1 mm [32]; this significantly reduces the risk of blood vessel deformation after rotation. The caliber of the blood vessel must also be considered. Preoperative Doppler examination must be performed to locate the perforator position and select a perforator with a suitable caliber as the direct nutrient vessel for the flap [33].
The third factor affecting propeller flap necrosis is the flap rotation angle. The propeller flap needs to rotate at a large angle of up to 180°. The perforating vessels, especially the perforating veins, are easily compressed by the surrounding deep fascia fiber bundles due to their thin wall and low pressure.
Therefore, the vascular pedicle usually needs to be naked, and different rotation directions (clockwise or counterclockwise) should be assessed intraoperatively; the rotation direction that causes the smallest twist of the pedicle should be selected [35]. The complication rate of propeller flap reconstruction is higher in the extremities than in the trunk. This is because the trunk has relatively abundant perforators and large perforator areas connected by blood vessels, which may aid in the safe harvest of flaps, thereby reducing the incidence of complications [36–41].
Finally, previous studies suggest that other risk factors for flap necrosis include patient age over 50 years, smoking history, and diabetes mellitus [6, 29], but these factors did not significantly affect the development of flap necrosis in the present study.
This study has some limitations. First, the sample size was relatively small, so a significant relationship between potential risk factors and complications may be hidden. Second, this was not a prospective study. Third, we did not perform this kind of surgery on a series of patients with bone defects in other parts, which means that there may be selective bias in choosing this surgical technique. Fourth, there was no control group who underwent other surgical techniques, such as free flap reconstruction. However, although we did not compare the results of perforator and free flap reconstructions, a recent meta-analysis study reported that the overall failure and complication rates are similar for free flaps (19.0%) and perforator flaps (21.4%) [42]. Despite these study limitations, to the best of our knowledge, this is the first quantitative and accurate study of the relationship between the position of the propeller flap and the wound surface. This research will help surgeons identify potential risk factors and choose the appropriate surgical method to repair soft tissue defects of the lower limbs.