Etiopathogenesis/Causative Factors

Acquired segmental defects of the mandible are most commonly secondary to ablative tumor therapy or avulsive traumatic injury. Other less common causes include inflammatory or infectious conditions that result in devitalization of the mandibular bone requiring its debridement. Segmental defects secondary to tumor therapy may result from the management of aggressive benign tumors arising within the mandible such as ameloblastoma or myxoma or from malignancies (carcinomas/sarcomas) arising in the associated soft tissue envelope that invade or extend to the mandible. Management of oral squamous cell carcinoma is the most common malignancy resulting in acquired segmental defects of the mandible.

Avulsive segmental wounds most commonly arise from high-velocity injuries such as firearms, industrial accidents, and occasionally motor vehicle collisions. Fortunately, most traumatic injuries to the jaws do not result in segmental defects because of the lower kinetic energies associated with the injury. Kinetic energy (KE = mv ² ) increases dramatically as the speed of the missile or the speed of impact increases, resulting in comminution of bone, destruction of the soft tissue envelope, and devitalization of large areas of bone and soft tissue. The extent of devitalization from high-velocity injuries may not be completely apparent at the time of presentation. The astute clinician will recognize the potential for extensive tissue loss in these patients and utilize wound care and temporary stabilization of the wound until all tissue loss has had opportunity to declare itself prior to definitive reconstruction.

Inflammatory conditions that result in devitalization of mandibular bone can be quite destructive. Osteoradionecrosis and bisphosphonate osteonecrosis represent two of the most common etiologies and may present with or without an associated acute osteomyelitis of the involved bone. Osteoradionecrosis results from a profound hypovascularity of the bone secondary to radiotherapy, whereas bisphosphonate osteonecrosis results from dysfunction of the osteoclasts caused most commonly by intravenous bisphosphonate therapy. The management of these specific etiologies is beyond the scope of this chapter; however, in the author’s experience, large segmental defects resulting from either process are amenable to reconstruction with vascularized fibular bone grafts.

Pathologic Anatomy/Classification of Defects

The location of the mandibular defect has a direct implication to management. Complex classification schemes have been proposed to help guide therapy based on bony, soft tissue, and neurologic deficits. The author prefers a simplified approach based on the position of the defect relative to the mandibular anatomy. This scheme divides the mandible into functional segments consisting of the symphysis (anterior) unit, the body (lateral) unit, the ramus-condyle (posterior) unit, and the alveolus ( Box 60-1 ).

Each of these sites has variable physiologic factors that result in different functional deficits when lost as a result of tumor ablation or traumatic disruption. Each region is also subject to significantly different forces (torsion, tension, compression) acting across the defect during function. Furthermore, numerous studies have evaluated failure rates for various techniques for mandibular reconstruction and identified differences based on the location of the defect. Consequently, reconstruction of each anatomic unit demands an individualized approach based on factors related to the patient, the reconstructive procedure, and the disease process. The impact of defect location and consideration of reconstructive technique will be further discussed later in the text.

A second factor for consideration is the importance of the soft tissue envelope. Evaluation of the soft tissue envelope should consider both the quality of the tissues and the quantity of tissue remaining or removed. Kazanjian in 1952 identified four essential factors for successful bone grafting, the first two of which were (1) implantation into healthy tissue and (2) good blood supply in the recipient area. Wounds that lack healthy soft tissue because of extensive trauma to the tissues or large volume loss of soft tissue as a result of resection are poor candidates for immediate nonvascular bone grafting. Likewise, wounds that are hypovascular due to radiotherapy or extensive scarring will provide poor wound beds for successful nonvascular bone grafting. In these situations, transfer of healthy vascularized tissue is essential for successful reconstruction. Complication rates for mandibular reconstruction have shown a tendency to increase with the increasing soft tissue resection. Loss of a significant volume of tissue precludes immediate nonvascular bone grafting in most situations. Failure rates with immediate nonvascular bone grafting techniques have been reported as high as 50% in such situations. Therefore, when a segmental defect is associated with extensive loss of soft tissues, immediate reconstruction with vascularized tissue transfer or delayed reconstruction should be considered.

Pathologic Anatomy/Classification of Defects

The location of the mandibular defect has a direct implication to management. Complex classification schemes have been proposed to help guide therapy based on bony, soft tissue, and neurologic deficits. The author prefers a simplified approach based on the position of the defect relative to the mandibular anatomy. This scheme divides the mandible into functional segments consisting of the symphysis (anterior) unit, the body (lateral) unit, the ramus-condyle (posterior) unit, and the alveolus ( Box 60-1 ).

Each of these sites has variable physiologic factors that result in different functional deficits when lost as a result of tumor ablation or traumatic disruption. Each region is also subject to significantly different forces (torsion, tension, compression) acting across the defect during function. Furthermore, numerous studies have evaluated failure rates for various techniques for mandibular reconstruction and identified differences based on the location of the defect. Consequently, reconstruction of each anatomic unit demands an individualized approach based on factors related to the patient, the reconstructive procedure, and the disease process. The impact of defect location and consideration of reconstructive technique will be further discussed later in the text.

A second factor for consideration is the importance of the soft tissue envelope. Evaluation of the soft tissue envelope should consider both the quality of the tissues and the quantity of tissue remaining or removed. Kazanjian in 1952 identified four essential factors for successful bone grafting, the first two of which were (1) implantation into healthy tissue and (2) good blood supply in the recipient area. Wounds that lack healthy soft tissue because of extensive trauma to the tissues or large volume loss of soft tissue as a result of resection are poor candidates for immediate nonvascular bone grafting. Likewise, wounds that are hypovascular due to radiotherapy or extensive scarring will provide poor wound beds for successful nonvascular bone grafting. In these situations, transfer of healthy vascularized tissue is essential for successful reconstruction. Complication rates for mandibular reconstruction have shown a tendency to increase with the increasing soft tissue resection. Loss of a significant volume of tissue precludes immediate nonvascular bone grafting in most situations. Failure rates with immediate nonvascular bone grafting techniques have been reported as high as 50% in such situations. Therefore, when a segmental defect is associated with extensive loss of soft tissues, immediate reconstruction with vascularized tissue transfer or delayed reconstruction should be considered.

Diagnostic Studies

Diagnostic studies to assess the ability of the patient to tolerate the proposed operation or the extent of malignant disease are beyond the scope of this chapter. Diagnostic studies pertinent to mandibular reconstruction include preoperative imaging of the mandible, computer-generated models or surgical guides (tactile medical imaging), and preoperative vascular studies specific to various reconstructive techniques.

Preoperative computed tomography (CT) of the mandible is critical to establishing the anticipated defect. Plain radiographs may be useful adjuncts to clinical evaluation but lack sufficient detail alone to establish the extent of disease. CT imaging allows evaluation of disease within the marrow space and can better evaluate cortical erosions that may otherwise be missed with plain radiographs. Axial and coronal scans are the minimal requirement and may be combined with sagittal images or three-dimensional reconstructions as needed to further define the extent of tumor involvement. Magnetic resonance imaging (MRI) may be useful as an additional study to identify extension along the inferior alveolar for certain tumor biologies. Because of its poor detail of osseous anatomy, MRI is not routinely utilized alone.

Tactile medical imaging is the display, in physical form, of anatomic data derived from medical imaging studies. Stereolithography and 3D printing represent the two main technologies for tactile medical models. Stereolithography utilizes laser photopolymerization of a liquid polymer to create an accurate, durable, translucent and sterilizable model of the patient’s skeletal anatomy ( Fig. 60-1, A ), whereas 3-D printing utilizes an inkjet-based printing method to create a skeletal model by injection of liquid into a plaster powder ( Fig. 60-1, B ). Three-dimensional printing models are accurate and opaque, but they are fragile and not sterilizable. Both allow hands-on study and treatment planning, but the stereolithographic models may also be used intraoperatively to assist with prebending of surgical fixation systems, model surgery, and shaping of bone grafts ( Fig. 60-2 ).

A, Stereolithographic model of mandible. B, Three-dimensional printing model of skull.

(Courtesy of Medical Modeling Inc., Golden, Colo.)

A, Stereolithographic model of mandibular reconstruction and prebent reconstruction plate. B, Plate secured to mandibular remnants with accurate reestablishment of jaw relations.

Current technology also allows for virtual treatment planning and surgery for the creation of surgical guides from computer-based images ( Fig. 60-3 ). Intraoperative use of these guides has been shown to improve accuracy of osteotomies and decrease operative time.

Surgical guides created from CT data and stereolithographic models.

Preoperative vascular studies are at times necessary to delineate the recipient (neck) or donor (flap) vascular anatomy prior to free tissue transfer. The head and neck cancer patient may have unclear vascular anatomy to serve as recipient vessels for free tissue transfer, especially in the multiply operated neck. Preoperative angiogram of the head and neck can identify the presence or lack thereof of arterial structures suitable for microsurgery and can also identify the status of the internal jugular vein. Patients with vessel-depleted necks may require vein grafting or creation of a vascular loop from distant vessels. This information can be critical in the decision process and preparation in such patients and ultimately make the difference in a successful reconstruction. Preoperative assessment of donor vessels is helpful in certain situations as well. Some patients may demonstrate surgical scars from prior surgery that may have disrupted the potential donor vessels. An example would be patients with lateral thoracotomy scars or low transverse abdominal scars that are being considered for latissimus dorsi or rectus abdominis muscle transfer respectively. Preoperative angiography can confirm the patency of the donor vessels. Probably the most common use of preoperative angiography of donor vessels is associated with fibula transfer. Two clinical situations that are critical to identify in patients considered for fibula transfer are the presence of inadequate vascularity of the leg to allow harvest of the peroneal artery and the presence of a dominant peroneal artery system. Both conditions can result in ischemia of the foot and are contraindications to the harvest of the fibular flap. Studies have demonstrated that dominant peroneal systems are present in approximately 5-10% of the population. Peripheral vascular disease resulting in compromised vascularity of the lower extremity is common in the elderly, tobacco-using population characteristic of head and neck cancer patients. Screening of potential candidates with lower extremity pulse exams can identify patients needing preoperative lower extremity angiography. Patients with normal lower extremity pulse exams, in general, do not require preoperative angiography.

Treatment/Reconstructive Goals

The goal of mandibular reconstruction is the restoration of form and function of the mandibular apparatus. Specifically, this requires reestablishing an acceptable appearance and mandibular continuity, maintaining a pain-free mandibular range of motion and relatively normal soft tissue relationships within the oral cavity to facilitate speech and deglutination, and providing a foundation on which to reconstruct the dentition.

Treatment options for reconstruction of segmental defects of the mandible include collapse of the defect, mandibular reconstruction plates (MRP) with or without soft tissue flaps (no osseous reconstruction), nonvascularized bone grafts, and vascularized bone flaps. Choice of technique is influenced by several factors including (1) surgeon training/preference, (2) location of defect, (3) length of bony defect, (4) extent of oral mucosal or facial skin loss, (5) quality/vascularity of the wound bed, and (6) patient medical comorbidities.

Anterior Defects

Defects in this region represent the most difficult to reconstruct for many reasons. The symphysis has multiple forces acting across the region depending on the mandibular function. Both compressive and tensile forces are present as well as torsional forces placing significant stress on any construct. The curved shape of the symphysis tends to be more difficult to re-create compared to the relative straight segments of the posterior and lateral regions. Its shape may influence the appearance of the lip and vertical height of the lower face. The symphysis also serves as the site for attachment of the suprahyoid and tongue musculature. When nonosseous reconstructions are performed in this region, the force of these muscle attachments is essentially conferred to the soft tissue envelope. This results in increased pressure of the soft tissues against the reconstruction plates and the opportunity for pressure necrosis of the overlying tissue with subsequent hardware exposure. As previously mentioned, anterior defects reconstructed with reconstruction plates and soft tissue flaps have been shown to be at high risk of failure. Although vascularized bone flaps may have some limitations in contouring, the high failure rates associated with MRPs in the anterior region (without immediate osseous reconstruction) make them the procedure of choice for the anterior region.

Lateral Defects

Lateral defects characteristically are less complicated to reconstruct. They are relatively straight, do not have as significant muscle attachments, demonstrate lower complication rates compared to anterior defects, and in general have less impact on facial form. Lateral defects smaller than 6 cm and associated with benign disease are candidates for nonvascular bone grafting in patients not desiring immediate reconstruction. Lateral defects 6 cm and larger, associated with larger soft tissue loss, and having a history or need for radiation therapy should be reconstructed with vascularized bone flaps. Two important considerations present themselves when planning reconstruction with dental rehabilitation of lateral defects: the lateral and vertical position of the osseous construct. The normal mandibular body contour tends to be wider than the position of the alveolar bone. Therefore, when planning the contour of reconstruction plates and the position of the bone graft or flap there is often a balance between placing the bone in a location to recreate the ideal facial contour versus a location that is ideal or at least usable for dental implants. Vertical position of bone flaps must also be considered. Fibula bone flaps have a much lower profile compared to the dentate mandible. Setting the reconstruction plate and the bone flap at the inferior border will recreate the most ideal jawline but may create a significant prosthetic challenge when one considers the location of the bone in relation to the occlusal platform. In the edentulous mandible, the fibula is an excellent size match and vertical height does not usually pose a problem.

Posterior Defects

Posterior defects, similar to lateral defects, involve a relatively straight segment of the mandible and are therefore less complicated to reconstruct unless involving the condylar process. They can impact facial form, including posterior vertical height of the mandible, and may create a more significant impact on mandibular range of motion because of the relationship with the temporomandibular joint and muscles of mastication. Posterior defects do represent a region that is amenable to allowing collapse of the defect without reconstruction when dictated by the patient’s clinical situation. Depending on local and patient factors such as volume of tissue loss, disease process, status of the temporomandibular joint, and radiation therapy defects of the posterior unit may be reconstructed with costochondral/corticancellous grafts, alloplastic prostheses, or vascular bone flaps. Alloplastic joint prostheses probably do not have a role for true segmental defects involving the posterior unit and should be reserved for isolated loss of the condylar process in the absence of radiation therapy and significant tissue loss. Marx and colleagues reported the successful use of reconstruction plates with condylar prostheses for defects of the posterior unit including the condylar process. Complications including plate fracture and erosion into the glenoid fossa have been reported ( Fig. 60-5