Imaging plays a critical role in the diagnosis, staging, and management of segmental mandibular defects. Imaging allows mandibular defects to be classified which aids in microvascular free flap reconstruction. This review serves to complement the surgeon’s clinical experience with image-based examples of mandibular pathology, defect classification systems, reconstruction options, treatment complications, and Virtual Surgical Planning.
Key points
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Imaging plays a key role in diagnosis, preoperative planning, and disease surveillance.
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Proper surgical decision-making requires accurate imaging protocols and interpretation.
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Virtual surgical planning is used routinely for mandibular resection and reconstruction including autologous harvest site planning.
Introduction
Imaging plays a critical role in the preoperative assessment of patients undergoing mandibular reconstruction. Segmental mandibular defects can arise from neoplasm, trauma, and less commonly infection and congenital deformities. Deformities of the face can result in impaired function, increased psychosocial stress, isolation, and depression. The main goal of mandibular reconstruction is to reestablish mandibular function and to return patients to their pre-disease state. The surgeon’s tasks include preserving the normal contours and structure of the lower third of the face while restoring the normal functions of swallowing, mastication, and speech production. Proper dental occlusion and temporomandibular joint (TMJ) function should also be preserved to allow for dental rehabilitation. , Oral cavity anatomy consists of different types of tissue including skin, bone, mucosa, connective tissue, and muscle. Free tissue transfer allows resected tissue to be replaced with the same tissue types harvested from a distant site of the patient’s own body. This “like-with-like” resection and reconstruction strategy has been shown to achieve the goals and objectives of oral cavity reconstruction while preserving structural integrity, function, and aesthetics. Microvascular free tissue transfer is typically reserved for large mandibular defects usually spanning greater than 6 cm, whereas nonvascularized grafts and reconstruction plates can be used for smaller defects. ,
Several classification systems for detailing mandibular defects have been proposed, although none have gained widespread acceptance. The main benefit of adopting a standardized classification system is the ability to develop an algorithmic treatment strategy that encompasses both reconstruction and harvest site surgical planning.
This article serves as an imaging-based review of the different etiologies of mandibular defects, including major proposed defect classification systems. We also review and illustrate reconstruction options including the advantages and morbidity of different flap sources, including options for mandibular condyle reconstruction. Imaging examples related to pathology, management decisions, and complications are provided. Lastly, we describe recent advances in presurgical planning, specifically the utility of imaging in computer-aided virtual surgical planning (VSP).
Mandibular reconstruction indications
Segmental mandibular defects can be caused by many etiologies, the most common of which is surgical resection related to oral cavity squamous cell carcinoma (SSCA). Tumor can be localized to the mucosal layer of the oral cavity or invade deeper tissue layers including bone, muscle, connective tissues, and overlying skin. Because mucosal lesions are amenable to visual inspection, smaller or lower-grade lesions can be staged primarily by clinical examination and are frequently not evident on imaging. However, the overall extent of a tumor may be underestimated on physical examination due to the depth of invasion of adjacent osseous and soft-tissue structures. Different classification systems have been proposed to take into account the different tissue types that must be resected and reconstructed. SSCA can arise from any mucosal or submucosal surface within the oral cavity. The anatomic subsites of the oral cavity include the lips, floor of mouth, the oral tongue, the buccal mucosa, the upper and lower gingival surfaces, the hard palate, and the retromolar trigone. Approximately 75% of oral cavity SSCA arises from the lower lip, oral tongue, or floor of mouth, and the mandible can be involved secondarily from any subsite by direct extension. The presence of osseous involvement indicates a T4 lesion, and as such, imaging can be used to assess for both cortical and marrow space involvement. Computed tomography (CT), MRI, and/or PET/CT are commonly used for complete assessment and staging. On CT, the most sensitive findings of osseous involvement include cortical erosion adjacent to the musical primary lesion, aggressive periosteal reaction, abnormal marrow space attenuation, and pathologic fractures ( Fig. 1 ). CT has a sensitivity of 95% when section thickness of 1 mm and multiplanar reformats are obtained in bone windows. However, these osseous changes are often difficult to discern due to the presence of beam hardening related to dental amalgam and hardware.
MRI is used for staging with relatively high sensitivity (96%) but relatively low specificity (54%). The most common finding of tumor invasion into the mandible is replacement of normal hyperintense fat signal on T1-weighted images by lower signal intensity and tumor and enhancement within the tumor and bone marrow. T2-weighted and short tau inversion recovery (STIR) images may show intermediate to hyperintense signal depending on the degree of tumor cellularity ( Fig. 2 ). False positive findings on MRI can occur due to similar signal changes related to recent tooth extraction, or post-treatment-related changes such as radiation-induced fibrosis and osteoradionecrosis (ORN) ( Fig. 3 ).
When invasion of the mandible is present, the preferred treatment is a partial mandibulectomy. The degree of resection depends on the extent of osseous involvement by tumor. In marginal mandibulectomy, the tumor is excised with the adjacent cortex without sacrificing the whole segment. Marginal mandibulectomy can be performed if a lesion abuts the mandible but is freely mobile on examination, or if minimal cortical invasion is present on physical examination and imaging. However, if there is gross cortical and marrow space invasion, or involvement of the inferior alveolar nerve or mental foramen, then a segmental mandibulectomy is required followed by segmental reconstruction. The specific type of mandibular reconstruction performed is based on the surgical margins after tumor resection.
The timing of reconstruction following mandibular resection due to tumor has been historically debated among reconstructive surgeons. In the past, a delayed or staged approach was used with reconstruction after a period of observation to evaluate for the development of recurrent disease. Delayed reconstruction was assumed to be critical for wound bed maturation to allow for graft placement. However, due to the evolution of microsurgical techniques it has become widely accepted that immediate reconstruction can be performed without risk of recurrent disease. Healing of the reconstruction flap is essential because adjuvant radiation therapy must be performed within 6 weeks of surgery to improve patient survival. , Additionally, most patients prefer immediate reconstruction as it results in higher quality of life as compared with staged reconstruction.
The types of defects caused by trauma and ORN are similar to those resulting from tumor ablative surgery; however, each poses unique challenges. Avulsive segmental fractures can occur from gunshot wounds, industrial accidents, and occasionally motor vehicle collisions ( Fig. 4 ). Blunt trauma usually does not have enough kinetic injury to cause the kind of segmental defects that necessitate reconstruction. The kinetic energy associated with missile projectiles, such as bullets, dramatically increases the impact on bone and soft tissues and results in devitalization due to vascular compromise. For complex mandibular defects due to trauma, microvascular free flap reconstruction has become the mainstay of treatment.
ORN is a severe complication following radiation therapy for head and neck malignancies. ORN is defined as a condition in which the necrotic bone becomes exposed through a wound in the overlying mucosa or skin. Owing to its tenuous blood supply, ORN is more likely to occur in the mandible than the other bones of the head and neck. The presence of dental decay, minor trauma, and tooth extraction can increase the chances of developing ORN. ORN-related mandibular necrosis results from radiation-induced vascular compromise, and therefore carries a less favorable prognosis for reconstruction than from other etiologies. On CT, ORN is characterized by lytic bony destruction, cortical disruption, and a loss of marrow trabeculations ( Fig. 5 ). On MRI, ORN is characterized by bone destruction without a soft-tissue mass. The bone may be low or intermediate in signal intensity on T1-weighted-images and hyperintense on T2 or STIR. Adjacent soft tissues may show hyperintense T2/STIR signal and enhancement ( Figs. 6 and 7 ). ORN with or without superimposed osteomyelitis can be difficult to distinguish from tumor recurrence.
Recurrent tumor often occurs at the surgical resection margins. The margin between the resection site, also known as the recipient bed, and the flap is the most common site of local disease recurrence ( Fig. 8 ). The presence of a solid or cystic soft-tissue mass is strongly associated with tumor recurrence ( Fig. 9 ). Other complications along the margins of a flap include dehiscence and fistula formation. Soft-tissue infections can also occur with nonspecific imaging features including soft-tissue swelling and stranding. Distinguishing recurrent tumor for infection or post-treatment changes from tumor recurrence may be difficult on imaging alone, and therefore surgical debridement or biopsy may be necessary ( Fig. 10 ). Early stages of ORN are treated conservatively with antibiotics, daily chlorhexidine rinses, and occasionally hyperbaric oxygen therapy. However, more advanced stages of ORN often require surgical intervention including mandibular resection with microvascular reconstruction.
Similar to ORN, the mandible is the most commonly affected bone for medicine-related osteonecrosis of the jaw (MRONJ). MRONJ is related to the systemic uptake of pharmaceuticals, most commonly bisphosphonate medications. However, it has also been associated with immunosuppressive and antiangiogenic drugs such as Avastin. , , MRONJ is primarily diagnosed on CT. Imaging findings are nonspecific, and this entity can be radiographically indistinguishable from ORN. A wide spectrum of findings has been reported in MRONJ including bone sclerosis, erosion of cortical bone, periosteal reaction, disruption of trabecular bone, and pathologic fractures. The presence of a bony sequestrum and adjacent soft-tissue swelling are found in approximately 50% of patients ( Fig. 11 ). In contrast to ORN of the mandible, treatment of MRONJ is primarily palliative with fastidious dental hygiene with mouthwashes and antimicrobials. Surgery is generally avoided because it can aggravate the degree of necrosis.
Mandibular defect classification
When the mandible is invaded by tumor, segmental resection is the first and most important step in the treatment process. Mandibular and other oral cavity defects resulting from tumor resection determine the type of reconstruction that can be offered. The goal of a classification is to aid in the development of a reconstructive treatment strategy. An ideal classification system would be logical and simple, with defects falling into categories that are reflective of increasing complexity and difficulty of achieving the objectives of surgical reconstruction. Although there is no established standard, classification systems are helpful as they can highlight the size and location of the defects, as well as the associated functional and esthetic deficits. The primary difficulties in mandibular reconstruction arise from the anatomic complexity of the mandibular arch, the relation to the opposing maxillary dentition, as well as its articulation with the temporal bone. The mandible can be divided into three major sections: the horizontal body, the vertical ramus, and the anterior (central) segment. The horizontal or lateral body extends from the mandibular angle to the canine teeth. Lateral resections that spare the ramus or central mandible portend good functional and esthetic outcomes , . The symphyseal or anterior portion of the mandible is located between the canine teeth, including the incisors, and serves as the attachment of the paired geniohyoid and digastric muscles, which are important for mouth opening. Anterior symphyseal defects present greater challenges in successful reconstruction related to the functional objectives of swallowing, mastication, speech, and breathing. Tumor involving the anterior segment of the mandible may necessitate resection of either the anterior floor of mouth or lower lip, which can cause problems with speech, swallowing, incompetent oral cavity closure, and esthetics. Vertical segments extend from the mandibular angle to the ramus including the condyle and coronoid process. Condylar resection poses unique problems, as stability of the TMJ is necessary for proper dental occlusion, oral intake and speech.
Pavlov published the first mandibular defect classification in 1974. In this classification system, mandibular defects were placed into three different classes depending on the number of remaining mandibular arch fragments (1, 2, or 3) after resection. This classification recognized the functional problems posed by resection of the condyle and the symphysis, and technical difficulties in achieving the goals of reconstruction of these anatomic regions. Since that time, many classification systems have been proposed, of which, the most widely cited is the HCL classification by both Jewer and Boyd and later modified by Boyd. , According to the Jewer classification, central defects between both canines are designated as “C,” and lateral segments that exclude the mandibular condyle are designated as “L.” When the condyle is resected together with the lateral mandible, the defect is designated as “H,” or hemi-mandibular ( Fig. 12 ). Mandibular defects can be combined into eight combinations––C, L, H, LC, HC, LCL, HCL, and HH––which can then be used for surgical planning. A lateral defect can be reconstructed with a single osteotomy, or segment of bone, whereas a central or larger defect would require multiple osteotomies and potentially mandibular condyle reconstruction. Boyd modified this system to include soft-tissue defects, with “t” representing a large tongue defect, “m” a mucosal defect, and “s” an external skin defect. Further classification includes the mucosal and soft-tissue components of the defect with “o”, no muscle/skin, “m” muscle only, and “s” skin only.
