The radiation sparing and high-quality images of magnetic resonance angiography (MRA) make it a valuable imaging technique, particularly for soft tissues such as the breast. Currently, only 1 study has reported the use of MRA for preoperative volume estimation of various free flaps including deep inferior epigastric perforators, posterior thigh, and gluteal artery perforator flaps in 102 patients.8 Predetermined landmarks were used for virtually estimated flap volumes, which closely correlated to surgically harvested flap volumes (r = 0.97). Determining flap volume can be performed manually or with an automated reporting software based on standard flap territory. Moreover, the addition of gadolinium contrast has the advantage of simultaneously mapping perforator arteries. Although MRA appears to be accurate and safe for flap volume estimation, restricted access to this costly imaging modality limits its widespread applicability.
The use of CT for objective free flap volume measurement is a relatively new concept. Most studies describing this technique were performed in the past decade, with the oldest study published in 2005.13 A multitude of preoperative volume estimation techniques have been described using enhanced CT and CT angiography (CTA), primarily in breast and craniofacial reconstruction.9–12
CTA has been proven to be an accurate method in preoperatively estimating flap volume.9–11 Preoperative markings and radiopaque markers have been described in guiding virtual volume estimation.9 Several authors have demonstrated that preoperative CT-estimated flap volumes closely correlate to intraoperative harvested flaps (Pearson correlation coefficient = 0.88–0.99).9–12 Moreover, in breast reconstruction, premastectomy or contralateral breast volume (in delayed reconstruction) can be measured to adjust abdominal flap volume.9–11 This becomes challenging when a concomitant balancing procedure, such as a reduction mammaplasty, is planned on the contralateral breast. Preoperative volumetric measurement by means of virtual surgical planning, commonly practiced in craniomaxillofacial surgery,2–6 will play an important role in these patients.
In addition to simple volume calculation, CT results have been used to create virtual 3D models of desired structures. In a cohort of patients undergoing craniofacial reconstruction for hemifacial atrophy or following tumor resection, preoperative enhanced CT was performed before surgery and a 3D computer model of the patient’s face was constructed.12 A mirror image of the unaffected side was constructed and used as a guide for reconstruction. The preoperative evaluation of the 3D computer model allowed for the determination of the ideal size, shape, and position of the desired flap. A free anterolateral thigh flap was subsequently harvested and deepithelialized according to the 3D model dimensions and transferred to a subcutaneous pocket.
Compared with MRI, CT has the advantage of generally being more easily accessible and cost-effective. Exposure to ionizing radiation remains the main disadvantage. Like MRA, CT with contrast allows for the simultaneous mapping of perforator vessel anatomy. Preoperative CT has been proven to reduce morbidity and time in perforator flap reconstruction,24 and its efficacy in flap volume estimation for breast reconstruction has been widely reported.
US is a widely available, cost-effective, nonionizing and portable imaging modality. US has good penetration of soft tissues and can be used to measure adipose tissue thickness; however, its role in measuring flap volume is limited to date. One study reported its use for preoperative transverse rectus abdominis myocutaneous (TRAM) flap volume estimation in breast reconstruction.20 TRAM flap area was designed on the patient’s abdomen, based on approximate breast volume determined by manual examination. US was used to measure tissue thickness at various zones of the proposed flap site. These depth measurements were used to estimate volume (by multiplying area and volume). To confirm the reliability of this technique, excised flap volume was measured intraoperatively by water-displacement before detaching it from its pedicle and was modeled to match the desired flap volume. Mastectomy volume measured by water-displacement was used to further adjust flap volume. The authors found a strong correlation between US-estimated and measured flap volumes (r = 0.9258).
Breast volume assessment using ultrasound is limited somewhat by the convex breast shape and heterogeneous density. More global limitations of US imaging is the narrow field of view in a typical ultrasonic probe making it difficult to assess the volume of large areas (eg, an abdomen or thigh). Moreover, US imaging is operator dependent, where the scanned image can vary significantly depending on tissue pressure or probe angulation. As such, there is potential for US as a modality for assessing flap volumes but significant technical advances are still required.
Three-dimensional Modeling and Material Templates
Three-dimensional laser scanners are an accurate tool to determine a structure’s dimensions and volume. The technology has been successfully used in breast and craniofacial reconstruction.13–15 , 17 , 18 The low cost and portability of the scanner is unique compared with other imaging techniques such as MRI and CT. Although radiographs and CTs provide pertinent tissue information, their ability to analyze surface anatomy is limited compared with 3D photogrammetry. Before this technology, physical templates of craniofacial defects have been described using wax or alginate molds.16 , 19 These molds can be used to guide intraoperative flap volume and design. The development of virtual planning with 3-dimensional imaging has led to more practical methods of measuring defect volume.13–15 , 17 , 18
Due to its portability and small size, laser scanners can be used intraoperatively. One study described the use of intraoperative 3D scans for autologous breast reconstruction with TRAM flaps.13 After mastectomy, scans of the reconstructed and contralateral breasts were performed and compared. Corrections were made by excising excess flap tissue until acceptable volume differences and a symmetrical appearance wass established. Such a technique can be highly useful for less experienced surgeons.
Three-dimensional laser imaging has also been used to create a mold for determining flap volume.14 , 15 In delayed breast reconstruction, the use of a cast of the unaffected breast has been described as a template for msTRAM and deep inferior epigastric perforator flap reconstruction. The excised flap was placed into the cast, which provided surgeons with the target flap volume and orientation. Excess flap volume was excised before flap anastomosis. Intraoperative use of the cast showed reduction in surgery time and improved symmetry.
Laser scanning has also been used to monitor tissue expansion. One study used 3D digital color scanning of a facial skin graft contracture induced defect.17 Expanders were placed in preparation for cervicofacial and scalp flaps. The expanders were progressively inflated until the expanded area reached a similar value to that of the facial defect, confirming the availability of adequate flap area to over the defect of the excised facial scar. The contralateral side of the face can also be used as a guide to determine adequate expansion volume.18
The weight of 1 g of abdominal adipose tissue has an approximate volume of 1 cm3.9–11 , 25–28 Under this assumption, scales can be used intraoperatively to convert the weight of abdominal flaps to corresponding volumes. The size, portability, low cost, and ease of use of scales are incomparable with any other volume measuring technique. The disadvantage of this technique is that over- or underestimation of flap volume can occur. Subcutaneous adipose tissue density varies from 0.925 ml/g to 1.32 ml/g9-11 , 25-28. Moreover, unlike previously described techniques, flap shaping and molding is not possible.
Scales have been used to estimate flap volume and improve postoperative symmetry in breast reconstruction. Excised mastectomy specimens can be weighed and raised flaps trimmed to match the mastectomy specimen’s weight.21 , 22 This technique has proven to be quick and can help improve postoperative breast symmetry.21 , 22 Scales have also been used in conjunction with body mass index measurements to construct an equation aimed at estimating the required latissimus dorsi flap weight based on a patients’ body mass index or body weight.23
Limitations and Future Direction
Volumetric measurement is only 1 aspect of computer-assisted 3-dimensional representation. For example, with more complex 3-dimensional procedures such as breast reconstruction, the skin envelope, pocket, projection, and breast footprint are also critical variables and must be accounted for. There is no guarantee of a symmetric outcome by simply estimating the volume in breast reconstruction, for example with a scale. Three-dimensional imaging is a promising modality for breast reconstruction due to its relatively low cost, excellent topographical surface measurements, and its proven utility in reduction mammaplasty and alloplastic breast reconstruction.29 , 30 The current review focuses on adipose tissue measurement; however, practical imaging modalities must also discriminate between various tissues, such as muscle and skin. Future advancements in the field must focus on overcoming the above-mentioned limitations. Future studies must also evaluate the impact of objective quantification in reconstructive surgery on symmetry, patient satisfaction, procedure length, and cost-effectiveness.
This systematic review provides a summary of various published techniques for objective pre- or intraoperative quantification of flap volume in reconstructive surgery. Potential benefits include improved symmetry, increased patient satisfaction, decreased procedure length, and revision rates when compared with subjective measurements. Potential risks may include exposure to ionizing radiation (eg, CT scan) and increased cost or time. The preliminary results from this review are promising, and we believe that 3-dimensional representation and objective quantification is the future of reconstructive flap surgery. More studies are needed to study the clinical relevancy and impact of the various imaging modalities reviewed and to develop automated volumetric measurement technology with improved accuracy, efficacy, and reproducibility.
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Copyright © 2018 The Authors. Published by Wolters Kluwer Health, Inc. on behalf of the American Society of Plastic Surgeons. All rights reserved.
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