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ih
Fig. 9.11 (continued)
alterations in blood ow, potentially predisposing toward postoperative ap fail­ure [52].
ICGA has variable value in perforator assessment. A retrospective comparative study of 28 patients who had undergone ALT ap found that ICGA was reliable in intraoperatively identifying the dominant perforator and found the use of ICGA resulted in a decrease in operator time and an increase in distal ap survival [53]. A prospective study of 12 patients undergoing ALT perforator ap found that in com­parison to handheld Doppler CDU, ICGA was superior in identifying perforator anatomy with 100% sensitivity and 100% PPV [54]. Conversely, a study of 50 patients undergoing free ap reconstruction found the PPV and sensitivity of ICGA, multidetector-row computed tomography (MDCT), and Doppler owmetry were 84% and 76%, 100% and 70%, and 80% and 100%, respectively, in the preoperative evaluation of ap perforator anatomy [55]. In this study, MDCT was less accurate in aps with a thickness less than 8mm, and ICGA was less reliable in aps with a thickness greater than 20mm, suggesting the mode of examination should be deter­mined based on the characteristics of the ap [55].
ICGA is likely better utilized to assess ap perfusion than mapping perforators. Limitations of ICGA include providing information on arterial perfusion limited to a depth less than 1cm, issues with both underestimation and overestimation of perfu­sion, and being subject to dynamic changes affected by patient temperature, cardiac output, volume status, blood pressure, volume support, and the local microvascular environment [47]. Additionally, protocols for optimal dosing and timing of contrast administration are not standardized. As such, ICGA should be considered as an adjunct tool to be used in combination with additional examination modalities.
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183
Postoperative Flap Monitoring
The greatest risk to free ap vitality is a thrombotic occlusion of the vein or artery. Over 95% of these events occur within the rst 72h postoperatively. Early detection of pedicle thrombosis is essential for successful thrombectomy and salvage. The gold standard for free ap monitoring remains frequent intermittent clinical exami­nation and is associated with low false-negative and low false-positive rates. Clinical examination includes an assessment of ap color, capillary rell, warmth, turgor, and bleeding on pinprick (Fig. 9.12). However, clinical monitoring can be labor intensive and subjective, dependent on the experience of the examiner. Additionally, clinical exam alone has a low salvage rate of approximately 63%, potentially as a result of subclinical changes in ap perfusion occurring signicantly earlier than clinically evident manifestations of congestion or ischemia [56].
External handheld Doppler systems can supplement the clinical examination and are effective when assessing changes in arterial patency but are less sensitive when assessing venous ow. These devices are useful in monitoring an external skin pad­dle but less helpful for buried aps. Additionally, reliance on external Doppler sig­nal can delay identication of venous insufciency, as the arterial signal can be present in the setting of completed venous occlusion for up to 6h.The use of the implantable venous Doppler is now widely used for postoperative ap monitoring. Additional postoperative monitoring methods include tissue spectroscopy, intrave­nous uorescein, thermography, transcutaneous laser Doppler, photoplethysmogra­phy, and transcutaneous PO2 monitoring.
ab
Fig. 9.12 (a) Clinical signs of a viable ap. (b) Clinical signs of venous congestion
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Implantable Doppler Probe Systems
In response to challenges in monitoring buried and intraoral free aps, Hartley and Cole described the 20-MHz unidirectional ultrasound Doppler probe in 1974 [57]. In 1988, Swartz etal. described a 1.0mm Doppler probe with a 20Hz ultrasonic probe applied to a silicone cuff which could be wrapped around the vessel of inter­est [58]. This allowed for continuous, real-time monitoring of vessel blood ow through both audible and visual monitor display. More recently, Doppler probes integral to the anastomotic coupler system have been developed [59], avoiding the additional step of applying the separate monitoring cuff around the anastomosis and improving operating time.
Multiple studies have shown improved detection of ischemia when using the implantable Doppler probe, particularly when monitoring buried aps. A retrospec­tive study comparing 259 microvascular free aps monitored with implantable Doppler and 289 aps monitored by clinical means across a wide spectrum of surgi­cal subspecialties found improved overall success rates in the implantable Doppler group (96% vs. 89%) with the greatest benet of the device seen in head and neck procedures (95% vs. 84%) and improved salvage rates in the implantable Doppler group (95% vs. 40%) [60]. A study of 1142 free aps for head and neck reconstruc­tion using the implantable Doppler reported a 12% detection rate of incipient fail­ures in the operating room leading to immediate revision prior to closure and reported an overall ap survival rate of 98% [61]. While the implantable Doppler has a high sensitivity (87% to 100%) and specicity (99%) for detecting loss of ap perfusion, it suffers from a variable false positive rate (<1% to 88%), which poten­tially leads to a high proportion of subsequent negative surgical explorations [62,
63]. Corroboration with clinical exam and the use of a second assessment tool such
as color duplex sonography to conrm the implantable Doppler ndings has been described [63].
Arterial vessel monitoring has been described with fewer false-positive results but with greater false-negative rates, and both arterial and venous monitoring are routinely done postoperatively based on surgeon preference [64]. A systematic review of 763 aps with implantable Doppler probes on 527 arteries and 388 veins found a 74% reduction in risk of false positives and 63% reduction in risk of signal loss with arterial monitoring, and no difference in sensitivity, specicity, false nega­tive rate, salvage rate, or ap failure rate between venous and arterial implantable Doppler probes; however, the clinical signicance of these ndings is not ascer­tained due to study limitations [65].
Tissue Spectroscopy (Visible Light andNear Infrared)
Tissue spectroscopy is a noninvasive, continuous method of monitoring of ap per­fusion and can further be subclassied as either visible light spectroscopy or near infrared spectroscopy. Visible light spectroscopy (VLS) uses shallow penetrating
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visible light (475–625nm) to measure tissue hemoglobin saturation at the capillary level [66]. Decreases in tissue oxygen saturation suggest arterial inow compromise related to an isolated arterial thrombosis or a venous thrombosis impeding arterial inow. Additionally, total hemoglobin concentration is measured to quantify venous drainage, providing an indirect measure of blood volume within the ap. Increased total hemoglobin concentration suggests venous drainage compromise. A prospec­tive controlled study comparing VLS and clinical examination with intermittent Doppler in the postoperative monitoring of free aps found that VLS was able to detect compromised ap perfusion prior to changes in physical exam or handheld Doppler, resulting a 100% successful ap salvage in cases of compromised ap perfusion [67]. Limitations of VLS technology include the need for an external skin paddle larger than 2cm, which precludes its use with buried aps and the inability of the probe to adhere to mucosal surfaces, limiting its use in oral reconstructions. Additionally, instability in SpO2 values during the rst 8h following arterial anas­tomosis potentially secondary to mild ischemia-reperfusion injury with free ap transfer can obfuscate potential vascular compromise of the ap [67].
Near-infrared spectroscopy (NIRS) monitors changes in tissue perfusion and oxygenation status based on the differential scattering and absorption of infrared light by tissue chromophores contained in hemoglobin, allowing for real-time mea­surement of oxygenated, deoxygenated, and total hemoglobin concentrations and detection of altered tissue hemodynamics and potential ap compromise [68]. Postoperative NIRS monitoring allows for earlier detection of subclinical vascular compromise resulting in lower ap loss rates and improved salvage rates as com­pared to clinical assessment with handheld Doppler [69]. A recent systematic review found that in comparison to free aps with vascular compromise monitored clini­cally, aps monitored with NIRS had a signicantly higher salvage rate (89% vs. 50%), a lower rate of partial loss (15% vs. 80%), and an earlier detection of vascular compromise of approximately 82min [68].
VLS confers several advantages over NIRS.The absorption of visible light by hemoglobin is 100 times that of infrared light, allowing for more sensitive evalua­tion of variation in tissue oxygenation. Additionally, the smaller VLS probe, as nar­row as 6 mm, allows for easier adaptation to tissue surfaces for monitoring as compared to the larger NIRS probe. Conversely, NIRS can monitor to greater depth as compared to VLS, up to 30mm, which may be useful for evaluating some buried aps [70].
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Conclusion
The eld of microsurgery has signicantly evolved over the past century. Advances in preoperative imaging have allowed for more sensitive assessment of donor site vessel anatomy and perforator planning. Venous coupler systems decrease operative time while maintaining low rates of venous thrombosis. Implantable Doppler probe systems allow for improved ap monitoring, resulting in earlier detection of
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microvascular thrombosis and ap compromise. Multiple technologies are available to the microvascular reconstructive surgeon to optimize the preoperative assess­ment, intraoperative microvascular anastomosis, and postoperative monitoring of free aps; however, these should be considered as adjunctive tools to supplement rather than replace clinical judgment and excellent surgical technique. These advances have allowed for improved ap predictability and a shift toward custom personalized reconstructions with the goal of providing improved functional and esthetic outcomes in microvascular reconstruction.
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Chapter 10
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Use ofThree-Dimensional Technology forVirtual Surgical Planning inOral andMaxillofacial Surgery
SalahAlDinAl Azri , YohaannAliGhosh , andJonathanShum
Introduction
The use of virtual surgical planning has had a signicant impact on oral and maxil­lofacial surgery. Virtual planning has been utilized since 1980 and has now been implemented into all aspects of osseous surgery and reconstructions of the face. The success of these virtually planned cases depends on each step of the workow pro­cess: patient workup, quality of the image modality, data acquisition, virtual plan­ning, and surgical execution. Each component of the process should be thorough and meticulous in order to minimize the possibility of error during the surgical execution. The overwhelming utility of virtual surgical planning and the ability to create guides and implants to translate virtual reconstructions into reality have led to increased efciency, reduce costs, and ultimately improve surgical outcomes.
S. A. D. Al Azri (*) Maxillofacial Oncology and Microvascular Reconstructive Surgery, Houston, TX, USA
Department of Oral and Maxillofacial Surgery, The University of Texas Health Science Center at Houston, Houston, TX, USA e-mail: salah.al.din.alazri@uth.tmc.edu
Y. A. Ghosh School of Medicine and Dentistry, Grifth University, Gold Coast, Australia
Integrated Prosthetics and Reconstruction, Department of Head and Neck Surgery, Chris O’Brien Lifehouse, Sydney, NSW, Australia e-mail: yohaann.ghosh@lh.org.au
J. Shum Maxillofacial Oncology and Microvascular Reconstructive Surgery, Houston, TX, USA
Department of Oral and Maxillofacial Surgery, The University of Texas Health Science Center at Houston, Houston, TX, USA
The University of Texas Health Science Center at Houston, Houston, TX, USA e-mail: jonathan.shum@uth.tmc.edu
© The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 J. C. Melville et al. (eds.), Advancements and Innovations in OMFS, ENT, and Facial Plastic Surgery, https://doi.org/10.1007/978-3-031-32099-6_10
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S. A. D. Al Azri et al.
These processes and applications are outlined in this chapter to provide an overview of the advancements that allow for accurate maxillofacial reconstructions.
Imaging Modality Hard Tissue Considerations
The initial step of virtual surgical planning necessitates obtaining the appropriate images that are processed, used for planning sessions and construction of the appropriate surgical guides and hardware. Different imaging modalities have dif­ferent spatial and contrast resolution. Spatial resolution is the ability for an image modality to differentiate between two separate objects in a radiographic image, whereas contrast resolution is the ability to differentiate image intensities between two areas (i.e., fat stranding vs. normal adipose tissue) [1] (Fig.10.1).
Fig. 10.1 (a) Inferior alveolar nerve location within the mandible rendered from computed tomography (CT) scan. (b) Depth determined in millimeters from mandible buccal cortex. (Courtesy of
KLS Martin group)
a
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