24 Reconstruction of the Retromolar Trigone
Summary
The goal of reconstruction of the retromolar trigone after cancer resection is to provide bone coverage, reducing contracture that risks trismus, and maintaining occlusion when applicable. Such efforts should be able to withstand adjuvant radiotherapy as well. The surgical reconstructive options are wide-ranging and can be tailored to individual patient considerations, include comorbid status, health of tissue at donor site, and prospect of future dental rehabilitation. Thoughtful reconstruction should be paired with expert rehabilitative services of speech and swallow specialists.
24.1 Introduction
Squamous cell carcinoma of the retromolar trigone (RMT) is relatively uncommon, comprising approximately 6% of all cancers of the oral cavity. 1 Cancers of this subsite have a tendency to present in an advanced stage with a propensity for bony invasion. 2 This leads to lower rates of locoregional control and overall survival. 1 , 2 It is generally accepted that treatment for squamous cell carcinoma in this location is surgical, and adjuvant therapy is indicated in cases of advanced disease. If a functional reconstruction can be achieved via a simple approach, it should be attempted first. 3 Although local pedicled flaps can provide similar tissue, shorter operative times, and potentially lower morbidity, free tissue transfer is considered the gold standard in the reconstruction of RMT. For every patient, the goals of care, nature of the defect, and patient-specific risks must be evaluated.
24.2 Diagnosis and Evaluation
Collaboration among the involved surgeons in preoperative ablative and reconstructive planning is essential. RMT lesions are often easily visualized through oral examination. Flexible nasopharyngolaryngoscopy or mirror examination should be performed to evaluate for oropharyngeal tumor extension. The mucosa of the RMT lies directly on the anterior surface of the mandibular ramus, and bony invasion may occur quite early. 1 Patients should be assessed preoperatively with computed tomography (CT) to estimate the extent of invasion and anticipate whether a bony defect will be present after resection. Allen’s testing of both hands should be performed. If a fibular osseous or osteocutaneous (OC) free tissue transfer is needed for a planned segmental mandibulectomy, lower extremity CT angiography or ultrasound duplex study is recommended to evaluate the quality of lower extremity vessels, and to identify potential poor collateral blood flow. Three-dimensional virtual surgical planning may also be utilized for bony reconstruction.
24.3 Anatomic Considerations/Relevant Anatomy
The RMT describes the small area posterior to the 3rd mandibular molar. Gingival and buccal mucosa overlie a segment of the mandibular ramus bounded by the temporal crest, anterior border of the ramus, and posterior socket of the lower third molar. 4 Superiorly, temporalis muscle fibers attach to the coronoid process and medially, the medial pterygoid muscle inserts onto the medial angle and body of the ramus. 5 This subsite of the oral cavity is bounded by the buccal mucosa laterally, the mandibular alveolar ridge anteroinferiorly, the anterior tonsillar pillar medially, the mandibular ramus deep to the mucosa, and the maxillary alveolar ridge anterosuperiorly.
Because malignant lesions of this area typically require a 1 cm gross mucosal margin during ablation—which inevitably contracts to a smaller margin—the surgical defect can become quite large and functionally significant. If a segmental mandibulectomy is performed in the setting of cortical invasion, then an OC flap should be used—for which fibula, scapula, and potentially radial forearm OC flaps are options. In patients who are not surgical candidates for an OC free flap, allowing the mandible to swing often produces acceptable functional outcomes with soft tissue-only reconstruction. Elsewhere in this book, mandibular reconstruction is described in further detail. If a marginal mandibulectomy is performed, a soft tissue-only reconstruction is indicated.
The goals of RMT reconstruction include preservation of masticatory and optimal speech and swallow functions. Although bony surfaces like the hard palate can often heal well by secondary intention, exposed bone along the RMT should be covered by a soft tissue flap for several reasons. Healing by secondary intention along the RMT leads to unacceptable fibrosis and potentially debilitating trismus. 6 In this regard, free tissue transfer is an ideal method of reconstruction, choosing thin and pliable soft tissue of appropriate volume. 4 However, various other reconstructive options are available and may be more appropriate based on the patient’s clinical state and stated goals. Various options along the reconstructive ladder, including advantages and disadvantages of each method, are described below. Comprehensive technical details of specific flaps are beyond the scope of this chapter.
24.4 Surgical Considerations and Approaches
24.4.1 Skin Grafts
Split or full thickness skin grafts may be considered for small defects of the RMT if a vascular bed is still present over mandibular cortex. Without a vascular bed, skin grafts serve only as a short-term biological dressing. 4 With low morbidity, an appropriately sized graft can be bolstered to the surgical bed, and may promote neovascularization and reepithelialization from the wound edges. 7 Advantages include an abundance of donor tissue, ease of harvest and inset, and the resultant short operative time. In certain patients who have submucosal fibrosis—for example, due to betel nut chewing—poor vascularity will reduce skin graft viability. 8 Disadvantages of skin grafts include donor site pain, lack of tissue bulk, and wound contracture across the entire surgical bed that will result in trismus. To avoid donor site complications, acellular dermal matrix grafts can be used in place of skin for similar results; however, a vascularized wound bed is often lacking after RMT cancer excisions. 9 As discussed next, skin grafting is rarely a first-line option for quick reconstruction, given the availability of various pedicled local and regional flaps.
24.4.2 Local and Regional Pedicled Flaps
Local and regional flaps are an excellent reconstructive option in patients who have multiple medical comorbidities that preclude longer periods of general anesthesia required for free tissue transfer. With the majority of oral cancer patients presenting in the sixth decade of life and upward, medical comorbidities, functional status, and vessel quality become major considerations. Pedicled flaps are reliable options requiring minimal specialized equipment, training, and postoperative vascular monitoring. Many local and regional pedicled flaps have been described in RMT reconstruction including the buccinator musculomucosal flap (BMF), facial artery musculomucosal flap (FAMM), palatal island flap, tongue propeller flap, submental island flap, superficial temporal artery island flap, melo- and nasolabial flaps, platysma island flap, supraclavicular artery island flap (SCAIF), and pectoralis musculocutaneous flap (PMMC). Several of these reconstructive options are described in further detail below.
Buccinator Musculomucosal Flap (BMF)
The posteriorly-based BMF provides well-vascularized tissue for reconstruction of small RMT defects. 10 This flap provides similar, sensate tissue that is easy to harvest in close proximity to the RMT defect, facilitating tension-free insetting for small defects. The flap is pedicled upon the buccal artery and vein, which enter the buccinator muscle at the posterior-inferior muscle edge on its lateral side, medial to the buccopharyngeal fascia. These are vascular tributaries of the internal maxillary system. 5 , 10 , 11 Preservation of the buccopharyngeal fascia protects the more lateral facial nerve branches, and prevents herniation of the buccal fat pad into the surgical wound. The superior limit of the flap lies 3 mm inferior to Stenson’s duct and the anterior limit lies 1 cm posterior to the oral commissure. This flap can be harvested to a maximal height and length of approximately 4 × 7 cm. The donor site defect may be closed through primary closure or left to heal by secondary intention.
Facial Artery Musculomucosal Flap (FAMM)
The FAMM is a reliable and versatile flap first described by Pribaz et al in 1992, extrapolating from techniques used to harvest nasolabial and buccal flaps. 12 The flap is pedicled upon the facial artery running axially. A superiorly-based FAMM receives retrograde facial arterial flow, and begins within the ipsilateral gingivolabial sulcus adjacent to the nasal alar margin. An inferiorly-based FAMM begins adjacent to the RMT and receives anterograde facial arterial flow. Careful design requires a broad mucosal base during elevation, as the venous outflow of the FAMM is via submucosal tributaries. The maximal dimensions of this flap are approximately 3 × 8 cm, which can be rotated over a gentle axis to cover the RMT and adjacent defects. As with the BMF, the Stenson’s duct is protected, in this case posterior to the flap margin. Flaps greater than 3 cm in width may have a higher risk of symptomatic cheek contracture, and thus should be avoided. 13 The rotation of the FAMM into the RMT does not require crossing a dentate area, an issue that poses challenge in other oral cavity sites. The donor site can be closed primarily with acceptable contracture with the above dimension limits.
Palatal Island Flap
The palatal island flap was first described by Gullane and Arena in 1977 for local oral cavity reconstruction after oncologic resection. 14 The flap is raised as an axial mucosal flap based on the ipsilateral greater palatine vessels. It can be rotated 180 degrees. 3 Further mobilization can be achieved by removing the hamulus of the medial pterygoid plate. Nearly the entire hard palate mucosa can be transferred for reconstruction, with healing of the donor site by secondary intention. An alternative local flap should be considered for patients with a prior radiation volume including the hard palate, or prior hard palate surgery. This flap can be utilized for RMT patients even with full dentition. Because this flap requires the borrowing of maxillary mucosa to reconstruct a mandibular defect, alternative flaps may have a lower risk of promoting trismus.
Deep Lingual Artery Axial Propeller Flap
Although there is concern that borrowing tissue from the intrinsic tongue poses an unacceptable risk to oral function, 3 the deep lingual artery axial propeller flap is feasible for RMT reconstruction. 15 This flap is based on the ipsilateral deep lingual vessels and includes a long musculomucosal paddle. Retrograde dissection of the pedicle is performed to its origin between the genioglossus and hyoglossus muscles. The flap can span the entire length of the tongue and is approximately 6 mm thick with a maximal height of 4 cm. After harvest the donor site can be closed primarily or allowed to close secondarily. Depending on the RMT defect, intervening mucosa between the recipient and donor site may require excision, or incorporation into the flap advancement. This flap provides excellent soft tissue and reach to the RMT; however, the risk of tongue contracture, dehiscence from tongue movement, and speech impairment may again be greater than alternatives.
Superficial Temporal Artery Island Flap
The superficial temporal artery island flap can be raised as a fasciocutaneous or temporoparietal fascial flap based on either the anterior or posterior terminal branches of the superficial temporal artery. 16 The branches of the artery are mapped using a handheld Doppler probe, with the flap centered over the desired pedicle. The pedicle is dissected proximally into the parotid gland, a safe dissection that passes posterior to uppermost facial nerve divisions. After blunt dissection between the masseteric and zygomatic muscles below the zygomatic arch, this pliable flap can be tunneled medially into the oral cavity. If fasciocutaneous in design, the bridging skin segment will require deepithelialization. Flaps as large as 8 × 16 cm can be harvested, and the incisional scar can be camouflaged well within hair-bearing skin. This flap has a reliable vascular supply, but its use may have been limited due to probable contracture and resulting trismus.
Supraclavicular Artery Island Flap (SCAIF)
The SCAIF is a robust, pedicled fasciocutaneous regional flap that has regained popularity in recent years. 7 This axial flap is supplied by the supraclavicular artery, a terminal confluence of transverse cervical arterial tributaries. The pedicle runs reliably within 1-2 cm of the posterior clavicular border, and can be followed with Doppler probe. The accompanying venous outflow runs in the same plane as the external jugular vein. The skin paddle can be designed as large as 12 × 35 cm, and is easily deepithelialized for tunneling to the RMT and surrounding defect. The SCAIF is elevated in a subfascial plane over the deltoid muscle from distal to proximal. The proximal pedicle can be precisely skeletonized for mobilization, or broadly protected. Complications include wound dehiscence with wider skin paddles, although defects as wide as 8 cm can be closed primarily. Surgical drains should be placed given the large potential space formed after harvest. Distal flap loss has been reported in 8-15% of cases. 17 , 18 The use of intraoperative Novadaq SPY fluorescence imaging can aid in predicting distal flap viability. While studies have reported distal flap viability up to 5 cm beyond the detectable pedicle Doppler signal, intraoperative perfusion imaging techniques such as SPY are likely more precise estimators. With ease of harvest, a long and wide skin paddle, and low donor site morbidity, this flap is an excellent alternative to free tissue transfer for RMT reconstruction. Patients with multiple comorbidities and poor vascular status may have a more expeditious reconstruction via this approach. 19
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