Reconstruction of the Mandible

Introduction

The importance of the mandible as the gateway to the upper aerodigestive tract makes the functional rehabilitation of this structure paramount in complex head and neck reconstruction. Throughout the 19th and majority of the 20th century, mandibular reconstruction was limited, and generally patients who suffered from the loss of the mandible were oral cripples, unable to eat, swallow, talk, and breathe in a natural manner. Through the introduction of rigid plating technologies and free vascularized tissue transfer, the mandible can now be adequately restored to allow most patients these basic functions. Panje et al. first published on free tissue transfer for mandibular reconstruction using the iliac crest bone in 1981, followed by Hidalgo et al., who published the first mandibular reconstruction using the fibula free flap in 1989. Many advances have come since that time and free tissue transfer has become the gold standard in mandibular reconstruction. However, there are still many limitations with current techniques, including difficulties with dental rehabilitation, complications arising from implantable plates and screws, donor site complications, and prolonged operative times. Although tissue-engineering techniques are being developed to regrow bone, the current state of the art does not provide results comparable to those of free tissue transfer. This chapter will cover the current state of the art of mandibular reconstruction and introduce technologies aiming to overcome current shortcomings in mandibular reconstruction.

Anatomy

The mandible is the strongest and largest bone of the face with many important functions that must be considered for functional reconstruction of the lower face. The principal function of the mandible is to support and align the lower teeth to allow for mastication with precise dental occlusion. The mandible also supports the muscles of the tongue and floor of mouth, suspending the hyoid bone in the mid neck. From the hyoid, the larynx is also suspended, thus making the mandible an important framework for both the tongue and the larynx for both swallowing and speech. The mandible also supports the lower lip, providing a stable base for the lip, allowing for oral competency. Furthermore, the mandible supports the upper airway by preventing posterior collapse of the oral tongue and oropharynx. Finally, the mandible is an important cosmetic unit of the face and the mandibular height, contour, and projection should be considered during mandibular reconstruction.

The mandible consists of a strong horizontal bony arch anteriorly, with the midline symphysis, and parasymphyseal and body regions bilaterally. This provides the inferiormost horizontal or transverse buttress of the facial skeleton. Posteriorly, there are bilateral vertical segments consisting of the rami, terminating in the coronoid process anteriorly and condylar process posteriorly ( Fig. 92.1 ). These rami form the inferior-most vertical buttresses of the facial skeleton. The coronoid process serves as an insertion for the temporalis muscle tendon. The condyle articulates with the temporal bone in the condylar fossa via a synovial sliding-ginglymoid joint. The articular surfaces are lined with fibrocartilage rather than hyaline cartilage as in other joints in the body. There is an articular disk between the condyle and the condylar fossae that cushions the surfaces and allows for the complex sliding and hinge movements of the mandible. In cross section, the mandible consists of medial (lingual) and lateral (buccal) dense cortical bone, surrounding a core of cancellous bone containing bone marrow, nerves, blood vessels, and lymphatics. The typical mandible allows for an average maximal mouth opening of approximately 40 to 50 mm, taking into account sex and body habitus. The mandible also allows for about 10 mm of protrusion, 5 mm of retrusion, and lateral shifting of 10 mm to either side. Despite this three-dimensional mobility, the intact mandible closes effortlessly into precise dental occlusion.

The muscles of mastication—the temporalis, masseter, medial pterygoid, and lateral pterygoid muscles—produce mandibular movements. The anterior belly of the digastric, geniohyoid, mylohyoid, buccinator, and infrahyoid strap muscles (through their influence on the hyoid bone) serve as accessory muscles of mastication. Understanding the influence of each of these muscles on the mandible is essential to understand the effect of releasing these muscles from the mandible and disruption of the mandibular arch as outlined in Table 92.1 . Perhaps the most important relationship is that of the medial pterygoid and masseter, forming together the masseteric-pterygoid sling. The masseter and medial pterygoid both work together to close the mouth; however, the masseter pulls on the lateral surface of the mandible and the pterygoid on the medial surface of the mandible. In defects disrupting the mandibular arch, the angle of action of the medial pterygoid allows the pterygoid to overpower the masseter, causing medialization of the angle of the mandible.

Along the superior portion of the anterior mandible lies the alveolar process. This bony ridge supports the mandibular dentition. Below the dentition runs the inferior alveolar nerve in the mandibular canal. This canal begins on the medial aspect of the mandibular ramus, just posterior to the bony lingula at the mandibular foramen. The nerve then curves anteriorly below the mandibular dentition, traveling along the trajectory of the mylohyoid line, until it reaches the mental foramen. The mental foramen is typically located near the interspace of the first and second premolar and is usually midway between the upper and lower border of the mandible. Here the nerve enters the mental soft tissue as the mental nerve to provide sensation to the parasymphyseal dentition, gingiva, and lower lip. In the edentulous mandible, the alveolar process that supports the teeth recedes, decreasing the overall vertical height of the mandible and placing the mental foramen close to the gingival surface ( Fig. 92.2 ). Understanding the course of this nerve allows one to preserve its function when possible during mandibular resection and reconstruction.

Goals of Mandibular Reconstruction

The mandible provides major structural support for the lower face and upper aerodigestive tract, as described in the preceding section. When faced with reconstruction of a mandibular defect, the surgeon must consider the functional consequences of each unique defect. The overall goals of mandibular reconstruction are to (1) maintain oral competency, (2) maintain occlusal relationships of remaining teeth, (3) maintain the temporomandibular joint alignment and architecture, (4) maintain the contour of lower third of the face, and (5) allow for dental rehabilitation. Depending on the size and location of the defect, the nature of the pathology, the condition of dentition, the goals of the patient, and the overall health of the patient, the reconstructive surgeon should be able to determine a method for successful reconstruction.

There are two basic types of mandibulectomies: marginal and segmental ( Fig. 92.3 ). Marginal mandibulectomies leave the mandibular arch intact. To prevent fracture of the mandible, it is generally recommended that a height of at least 1 cm of bone should be left intact at the lower border of the mandible and the osteotomy should be created in a curvilinear fashion. The appropriateness of marginal mandibulectomy has been questioned in oncologic resections. However, marginal mandibulectomy in the setting of periosteal invasion only or limited cortical invasion without marrow space invasion appears to be oncologically equivalent to segmental resection. Reconstruction of marginal defects is generally achieved by soft tissue reconstruction alone either with a soft tissue flap or skin graft. Some authors have recommended placement of a reconstruction plate spanning the segmental bone resection to reinforce the region of the defect. The main disadvantage of this maneuver is the phenomenon of stress shielding, whereby the bone that no longer is experiencing normal loading forces can weaken, as loading forces are a stimulus needed for maintaining bone density. This can increase the load on the plate and cause plate and bone fracture over time. If there is a concern about the strength of the remaining bone, plating is not recommended and a segmental resection should be considered.

Segmental defects involve the entire height of the mandibular bone. The reconstructive options for these defects become more limited, as the resection proceeds more anteriorly. Defects of the ramus can be successfully rehabilitated with soft tissue only, plate and soft tissue, or bone containing free tissue transfer. Defects incorporating the body of the mandible generally require hard tissue reconstruction in the form of a plate or bone, while anterior defects need bone containing free tissue transfer to maintain a durable reconstruction. Segmental defects involving bone only are generally the consequence of benign mandibular cysts and tumors, trauma, osteomyelitis, and osteoradionecrosis of the mandible. Occasionally, gingival malignancies with limited mucosal disease can invade deeply into the mandibular bone, requiring segmental mandibulectomy while allowing primary closure of the oral cavity mucosa.

Composite defects, including mandibular bone along with gingival, buccal, labial, lingual, floor of mouth, and/or external facial tissue, are common in oral cavity malignancies. Successful reconstruction in these cases requires bone and soft tissue replacement. Through-and-through oral cavity defects are particularly challenging, requiring both internal and external soft tissue to cover the bony reconstruction. Multiple flaps or chimeric flaps (multiple flaps connected via one vascular pedicle, as afforded by the subscapular arterial system) may be required to achieve success in these cases. Goals of associated soft tissue reconstruction include maintaining the position, shape, bulk, and mobility of the tongue and restoration of the buccal soft tissues.

Reconstructive Options

The benefits of reconstruction should be weighed against the unique factors in each individual patient and the patient's current functional status and goals. For example, a patient with osteoradionecrosis of the jaw who is already completely gastrostomy tube dependent due to effects from prior therapy to the pharynx may have little benefit to having a bony reconstruction of the mandible. The goal of therapy in this patient may best be focused on prevention of continued infection, improved pain, maintenance of oral competency, and repair of orocutaneous fistula. Small to moderate lateral defects of the mandible (<5 cm) can be reconstructed with either soft tissue alone or a plate and soft tissue, with good functional and cosmetic results in some cases, especially in edentulous patients.

When bony reconstruction or plating is not employed, the remaining mandible shifts toward the deficient side, and this alters the alignment of the temporomandibular joint, altering the facial contour and disturbing occlusion ( Fig. 92.4 ). However, many patients will continue to function, eat a soft diet, and have an acceptable outcome with such a reconstruction. Reconstruction of the soft tissue only, with either a pedicled or free flap, can provide acceptable flexibility to the tongue and buccal tissues while closing lateral and posterior oral cavity defects.

Another viable option for mandibular reconstruction in patients with significant surgical risk, or lack of adequate donor sites, is to rely on a reconstructive plate without any associated attempt at bony reconstruction ( Fig. 92.5 ). Again, this is best suited for short, lateral, and posterior defects where the stress on the plate is limited. Patients should be restricted to a soft diet to prevent loosening or fracture of the plate. Dentulous patients are at increased risk of plate fracture. These plates are also at risk of exposure, so the plate must be adequately covered with soft tissue to prevent plate exposure, either with local soft tissues, pedicled flaps, or free flaps. There are conflicting reports on the effects of postoperative radiation on hardware exposure, loosening, and fracture.

Benign mandibular lesions and traumatic defects allow for consideration of free nonvascularized bone grafting, allografting, and the use of recombinant human bone morphogenetic protein (rhBMP). Crushed or aspirated allograft or autograft bone chips in titanium mesh cribs can provide a substrate for new bone growth in these patients ( Fig. 92.6 ). If a patient is likely to receive radiation therapy following reconstruction, free autograft or allograft bone will not provide a stable bony reconstruction in the time frame prior to receiving this treatment. Furthermore, the use of growth factors in patients with a history of malignancy is dubious, as current understanding of the effects of these proteins on tumorigenesis and metastasis is incomplete.

A special mention should be made of reconstruction of the temporomandibular joint. The joint is very complex and bears a considerable amount of compressive and shearing forces with mastication. There are various methods used for reconstruction of the joint, all with varying degrees of success. The simplest method is to place a rounded end of the fibula free flap against the condylar fossa of the temporal bone and use anchoring sutures to hold this vertical segment of bone in position. As the patient heals, this generally will scar into place and provide adequate mouth opening. Other methods include removing the condyle from the resection if possible, and plating this as a free graft to the blunt end of a fibular flap. These free condylar transfers have been shown to be successful in restoring tandem temporomandibular joint function. If the condyle cannot safely be preserved from an oncologic standpoint, a rib cartilage graft can be utilized and secured to the fibular flap as a load-bearing surface. There are also various mechanical prosthetic joints, both condylar head components and condylar fossae components, which can be utilized based on surgeon experience and preference. With the advances of custom plate manufacturing, condylar components can also be manufactured onto a reconstruction plate that is custom to the patient. Complications of loosening, fracture, and wear-through (to the point of violation of the skull base) of these prostheses have been reported, so this remains a controversial topic in mandibular reconstruction.

Fibular Free Flap Planning

Preoperative evaluation for a fibular free flap involves ensuring the patency of the vasculature supplying the lower leg and foot. The posterior tibial, anterior tibial, and peroneal arteries provide three-vessel runoff to the foot. Prior to surgery, an MRA of the lower extremities is recommended to ensure that the runoff to the foot is not dependent on the peroneal artery alone. CT angiography, angiography, and color Doppler studies can also be utilized, based on surgeon preference. Newly developed high-resolution blood pool magnetic resonance angiography (MRA) has been shown to be able to accurately image small perforating vessels to better access the soft tissue viability and aid in flap design.

Once the vascular supply to the lower extremities has been evaluated, the surgeon must consider which leg best suits the defect and desired reconstructive outcome. The orientation of the vascular pedicle, degree of freedom of the skin paddle, and ability to place dental implants or cover the plate with an additional layer of soft tissue can all be modified based on which leg is utilized and how the flap is designed and oriented.