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Preoperative perforator localization in anterolateral thigh free flap using acoustic doppler and computed tomography angiography

Abstract Objectives Our aim in this study was to investigate if we could predict perforator localization during ALTF elevation, using information from acoustic Doppler (AD) and computed tomography angiography (CTA). Methods Prospective observational data were collected from H&N cancer patients who received reconstruction with ALTF in Ajou University Hospital Cancer Center from June to December, 2021. Total of 21 cases were included in the analysis. Lower extremity angio‐CT scans were used to determine the course and depth of the perforator before surgery. During intraoperative design of the ALTF, the possible location of the perforator was identified by AD. After flap elevation, the distance between the actual and Doppler‐identified location of the perforator was measured. Results The average distance from the actual location to the Doppler‐identified location was 1.29 ± 1.26 cm. Among 21 cases, almost all perforators (20 cases) were identified in a circle with a radius equivalent to the depth of the perforator. Perforator depth measured by CTA showed a significant positive correlation with the distance from the actual to Doppler‐identified location, regardless of skin thickness or body mass index (BMI). Conclusions A circle with a radius equivalent to the CTA‐assessed depth of the perforator successfully predicted the location of the perforator in almost all cases. Depth of the perforator measured by CTA combined with Doppler‐identified location can help safely locate the perforator during ALTF harvesting. Level of Evidence: 4.


| INTRODUCTION
Since the instruction of the anterolateral thigh free-flap (ALTF) by Song et al. in 1984, 1 it has frequently been used for head and neck (H&N) reconstruction due to its numerous advantages. These include (1) a large skin paddle, (2) sufficient volume for reconstruction of H&N defects, (3) a long pedicle, (4) good matching between the branch of the lateral circumflex femoral artery (LCFA) and the caliber of H&N donor vessels such as superior thyroid artery or facial artery, (5) fair chance of primary closure and low donor site morbidity, and (6) ability to use multiple tissue components in various combinations. 2,3 However, ALTF also has significant drawbacks, with the most severe being, variability in pedicle anatomy. Identifying the location of the perforator can be a serious hurdle for appropriate flap harvesting, especially in patients with a high BMI. approaches. An ALTF acquired through subfascial dissection can provide sufficient volume and deep fascia for large and complex soft tissue defect on oral cavity or pharynx as are present after H&N cancer surgery. 4 Harvesting of a thin or super-thin ALTF via suprafascial dissection decreases the donor site morbidities such as deterioration of range of motion, ambulation, and contour aesthetics, but the possibility of partial and/or complete flap loss due to damage to the perforasome higher. 5,6 Although efforts have been made to understand the perforasome, 7 an ideal imaging technique for evaluating the perforasome in ALT territory has not yet been identified. Moreover, lipocutaneous shrinkage may occur after adjuvant radiotherapy. It therefore remains challenging to use thin or super-thin suprafascial dissection for H&N cancer reconstruction surgery.
Furthermore, incidental damage during suprafascial dissection of the subcutaneous tissue for perforator identification can lead to flap failure. Therefore, several tools including acoustic Doppler (AD), color duplex ultrasound, computed tomography (CT), and magnetic resonance (MR) imaging have been used to confirm the location of the perforator. 8 AD is preferred because of its non-invasiveness, low costs, small size, and portability. 9 AD can detect vessels with a diameter of 0.2 mm, with a sensitivity 100% for a skilled operator. 8,10 However, because of this high sensitivity, it sometimes detects vessels other than the perforator, resulting in a discrepancy between the Doppler-detected location and the actual perforator as determined by surgical dissection. In addition, it is difficult to track the actual course of the perforator by an AD. CT angiography (CTA) has also been used to assess the anatomical details of blood vessels. 11 Preoperative CTA can be used to detect atherosclerotic disease, anomaly and the approximate course of the perforator. However, it is difficult to determine the real-time location of the perforator at the surgical site using CTA. Therefore, there is no current foolproof clinical method for identifying the location of the perforator. In this study, we investigated techniques to determine perforator localization with a focus on using CT-identified perforator depth and Doppler-identified location during ALTF elevation.

| Study participants and data collection
We performed a prospective observational study using data from a total of 21 patients who underwent H&N cancer surgery with ATLF reconstruction from June through December 2021 at Ajou University Hospital. Inclusion criteria were patients who underwent

| Surgical procedure
All ALTFs were harvested by subfascial dissection. Preoperative marking of the ALT perforator was performed using an AD according to

| RESULTS
Characteristics of perforators from 21 cases were evaluated (Table 1).
Mean patient age was 60 years old and average BMI was 23.49. The actual perforator was observed near the midpoint between the anterior superior iliac spine (ASIS) and patella in 20 of 21 cases. In one case, the actual perforator was not found near the midpoint, but  showed a significant positive correlation with DA distance. In addition, a regression analysis was performed between DA distance and perforator depth, and the p value was .022 (Figure 4).

| DISCUSSION
Two approaches are currently used to find the perforator for ALTF: subfascial and suprafascial approaches. Suprafascial dissection can provide a thin, pliable flap and is associated with fewer donor site morbidities such as compartment syndrome, muscle herniation/ bulging or abnormal sensations in the lower extremity, even though a skin graft might be required. 13,14 However, a major surgical risk associated with the suprafascial approach is related to the perforasome, a unique vascular territory supplied by a single perforator. 15 Schaverien et al. classified perforasomes into three types. 7 In type 3 perforasomes, the perforator is divided into several branches at the suprafascial level, and can be easily damaged by suprafascial dissection, resulting in marginal necrosis of the flap. 16 In reconstruction after H&N cancer surgery, marginal necrosis can lead to a fistula, which adversely affects clinical outcomes. According to Chen et al., fistulas were more frequent in patients who had undergone suprafascial dissection than those who had undergone subfascial dissection (10% of cases versus 6.5%, respectively), suggesting that suprafascial dissection in H&N reconstruction may be less safe than subfascial dissection. 13 A subfascial approach can preserve the perforasome, and is often simpler than suprafascial dissection decreasing operation time.
In addition, the deep fascia obtained with subfascial dissection may be useful in the reconstruction of H&N defects. A waterproof suture line can be made with deep fascia, and in cases of an unstable hyoid due to suprahyoid muscle resection, the deep fascia can also be used for hyoid suspension. 3 One of the main hurdles in ALTF is anatomical variation of the perforator, which cannot be predicted preoperatively. In a systematic review conducted by Smith et al., the overall mean number of perforators was 2.3, but no perforator was found in 2% of patients. 17  flow, the sensitivity is reported to be higher than that of AD, but scanning skill is required. 20 CTA had a sensitivity of 90.4% in a meta-analysis. 21 However, it is not conducted in real-time and has disadvantages such as contrast medium allergy, potential carcinogenic effects due to radiation exposure, and its time consuming nature. 11 Catheter angiography using indocyanine, indocyanine green angiography, also showed perforator localization of nearly 100%, and it has the advantage of perforator mapping and perfusion in real time, but it is expensive and invasive. 11,22 Each test has advantages and disadvantages, so one or two tests are usually performed considering situations such as patients, preference of surgeon, and environment of operating room.
Among the methods used to evaluate the location of the perforator before surgery, AD is simple to use and provides relatively easyto-interpret results, but can detect non-usable blood vessels and only flow can only be assessed, making it difficult to trace the actual perforator. 23 Moreover, due to Doppler's principle, the Doppler-detected location may differ from the actual perforator location. AD uses the principle that the frequency changes when ultrasonic waves collide with a moving object and are reflected. This changed frequency is called the "shifted frequency" and is affected by the speed of the moving object and the angle between the emitted ultrasonic wave and the moving object. As the angle approaches 90 , the shifted T A B L E 2 Correlations between the distance from the Dopplerdetected location to the actual location of the perforator, perforator/ skin depth, and BMI F I G U R E 4 Correlation between the Doppler-detected location and actual location of the perforator and perforator depth frequency decreases, and as the angle approaches 0 , the shifted frequency increases. Therefore, in the case of a blood vessel parallel to the transducer, the sound produced by AD increases. Blood vessels parallel to the transducer include the perforator and communicating branches that connect to the subdermal plexus as well as the perforator ( Figure 5). In addition, blood vessels in the muscle can also be detected because the 8 MHz transducer usually used in ALTF has a penetration depth of 3.5 cm. 7,24 However, if the perforator is located deep within the tissue, the course of blood vessel can be meandering. 11 To reduce the sensitivity of AD to deep blood vessels, Mun We demonstrated that in 95% of cases we evaluated, the actual perforator was within the perforator depth distance from Dopplerdetected location. These findings indicate that AD and CTA can be used as complementary tools to identify perforator location.
In the current investigation, among 21 cases, 20 perforators increased. In patients with thick subcutaneous tissue, the skin perforator would appear more oblique than in thin patients, resulting in a discrepancy between the Doppler-detected location and the actual perforator location. 26 However, the accuracy of the AD is less than 100% even in Asians with a lower BMI than Western populations. 27 Thigh circumference is utilized to measure total or regional muscle mass in anthropometry. 30 It is specific to the thigh, and contains information on both muscle and subcutaneous fat. Therefore, it will be worth to study the effectiveness of thigh circumference in predicting perforator depth.
Even if the existence of a perforator has been sufficiently evaluated preoperative, there may be no perforator near the midpoint of thigh. Surgeons should proceed with the operation keeping in mind that there may not be a perforator, even if there is preoperative evidence that a perforator is present.
This study had several limitations. First, this study is a retrospective study with a small sample size at a single institution. Second, factors such as operation time were not included in this study. This study is a preliminary study, with a small number of cases, but it was conducted on a scale enough to show statistical significance. 31 In this study, it was concluded that the depth and location of the perforator were significant, so in future studies, more cases can be analyzed including variables such as operation time. Third, all patients were Asian. Since the thickness of the anterolateral thigh is known to be different between Asians and Westerners, if the relationship between the depth and position of the perforator is studied in Westerners, the perforator can be found more safely.

| CONCLUSION
We demonstrated that CTA and AD can be used as complementary tools to identify perforator location. The actual location of the F I G U R E 5 Schematic illustration of points that can be detected by acoustic Doppler perforator in 95% of cases we examined fell within perforator depth distance from Doppler-identified location. The perforator depth on CTA combined with the AD can help surgeons safely locate the perforator during ALTF harvesting. The error between the predicted location and the actual location of the perforator was confirmed through CTA in this study, but in further studies, it would be beneficial to both the surgeon and the patient if the error could be confirmed with a simpler and more intuitive test such as a physical examination.