28 Reconstruction of Mandibular Defects
Summary
The mandible is a horseshoe-shaped facial bone that has complex aesthetic, phonatory, masticatory, and swallowing functions. Reconstruction of the mandible is complex depending on the location of the defect, dentition of the patient, functional status, and medical status of the patient. Mandibular defects can be divided into segmental or marginal defects. Segmental defects can be divided into either anterior or lateral defects based on the mental nerve. Marginal defects are often reconstructed with soft tissue; however, segmental defects may require bony reconstruction, especially anteriorly. The most common free tissue options for bony reconstruction include fibula, scapula, iliac crest, and osteocutaneous forearm. Fibula flaps are the most commonly used flaps for mandibular reconstruction to date. When reconstructing the mandible with a bony flap, consideration should be given to plate selection, dental rehabilitation, and the need for virtual surgical planning (VSP), in select cases.
28.1 Introduction
The mandible is a horseshoe-shaped facial bone articulating with the skull that has muscular attachments corresponding to complex aesthetic, masticatory, phonatory, and swallowing functions, placing both rotational and translational forces on the bone itself. 1 , 2 Masticatory forces can be immense, with absolute maximal loading force of the mandible near 1,000 Newtons. 3 As a symmetric, rigid organ, the mandible needs proper alignment of bite in the dentulous patient to perform its function. In addition, the complex shape of the mandible lends itself to the framework of the shape of the lower one-third of the face and the transition of the head onto the neck. Because of the complex functional as well as aesthetic components, mandibular reconstruction is one of the most challenging in the head and neck surgery. 4
28.2 Diagnosis and Evaluation
Discontinuities and mandibular loss are secondary to a myriad of different medical and traumatic conditions. In oral cavity cancer, the treatment of both the primary disease and sequelae may require reconstruction. Mandibular defects can be divided into partial (i.e., a marginal mandibulectomy) or segmental (Flowchart 28.1 ). In addition, the loss of mandible may include the loss of additional subsites of the oral cavity, facial skin, and/or oropharynx, with or without dental loss. A marginal mandibulectomy generally preserves a rim of bone, usually about 1 cm in vertical height. 5 , 6 These defects generally do not require reconstruction other than soft tissue coverage from any additional defect created. At certain centers, titanium reconstruction plates are placed to help prevent further pathologic fracture, although there is limited literature on this practice.
Segmental mandibular defects create a discontinuity of the bone with significant functional and aesthetic consequences and will be discussed herein in the rest of the chapter. Such consequences vary with the location and the size of the defect, which can be further classified as medial or lateral to the mental foramen. Defects that are medial to the mental foramen are classified as “anterior” defects, while “lateral” defects are those that are lateral to the mental foramen. Both types of segmental defects result in two independent segments that act accordingly to the remaining musculature. 7 , 8 Anterior defects result in the medial pterygoid pulling the remaining segments upward, posteriorly, and medially. The lip loses sensation and the tongue loses anterior support and is displaced posteriorly. These defects result in an appearance classically known as the Andy Gump deformity and generally require bony reconstruction for function and appearance. Lateral defects displace the mandible toward the site of the defect secondary to the unopposed actions of the ipsilateral muscles of mastication. Reconstruction of these defects is complex and involves considering soft tissue components of the resection, patient comorbidities and prognosis, and dental rehabilitation. 9 The current techniques for primary reconstruction of discontinuous mandibular defects will be reviewed in the following.
28.3 Anatomic Considerations/Relevant Anatomy
The mandible itself is a continuous single bone that has two syndarthrotic joints present at each attachment in the glenoid fossa establishing a connection with the temporal bone. It consists of seven paired segments, namely, condyle, subcondyle, coronoid, ramus, angle, body, and parasymphysis, and one unpaired segment, namely, the symphysis (▶ Fig. 27.2 and ▶ Fig. 28.1). The body to the symphysis of the mandible contains two cortices with a narrow marrow space. The superior aspect of the body contains the alveolar process, which contains the dental sockets for teeth. The mandible extends toward the angle then rises to become the ramus and ends anteriorly with the coronoid and posteriorly with the condyle, the point of articulation. The mandibular notch separates the condyle and the coronoid.
The mandible derives its blood supply from the external carotid system which includes the facial artery, the lingual artery, and the inferior alveolar artery, which is a branch of the maxillary artery that enters through the mandibular foramina. The foramen also serves as the conduit for the inferior alveolar nerve, a branch of the mandibular nerve, which traverses the alveolar canal providing sensation to the lower dentition and to the chin and lower cheek through the mental nerve, which exits through the mental foramen. The mental foramen is approximately at the midway point of the mandible height and at the level of the second premolar. In an edentulous mandible, the mental foramen may be present at the upper border of the mandibular body.
Movement of the mandible is dependent on two opposing muscle groups (▶ Fig. 28.2a, b). The elevator group is composed of the masseter, medial pterygoid, and temporalis muscles. The masseter muscle inserts on the outer aspect of the ramus of the mandible, the medial pterygoid muscle inserts on the medial surface of the angle, and the temporalis muscle inserts on the coronoid. The depressor musculature consists of the geniohyoid, digastric, and mylohyoid muscles. The geniohyoid, digastric, and mylohyoid muscles all insert onto the medial aspect of the symphyseal region onto the mylohyoid line, which extends along the internal aspect of the body. Lastly, the lateral pterygoid muscle inserts on the anterior condyle. Overall, the elevators are much stronger than the depressors and the masseter muscle is the strongest of the elevators. In the case of mandibular discontinuity, the forces of medial pterygoid muscles will dominate due to vector of force compared to other elevators.
28.4 Surgical Considerations and Approaches
As stated above, goals of reconstruction may differ depending on the extent of mandible and soft tissue envelope and concurrently resected oral cavity subsites. Defects discussed in this chapter will be limited to segmental defects and bony reconstruction. Ideally, mandibular reconstruction would allow for: (1) dental rehabilitation, and if possible, restoration of the natural occlusal relationship; (2) restoration of the natural shape of the mandible and aesthetic appearance; and (3) restoration of oral competency including sensation and motor function. As defects become more complex or as other factors, such as poor prognosis or patient health, become more prominent factors in decision-making, complete functional restoration may not be a realistic goal and thus more basic needs such as wound closure may become a higher priority. In addition, when considering the type of reconstruction, several factors must be considered including the quality of the tissue vascularization, previous surgery and/or radiation history, dental health, cancer prognosis, and general patient health.
28.5 Reconstruction Methods
Current reconstruction methods consist of five options: (1) primary closure with no plate (“letting it swing”), (2) reconstruction plate alone with no soft tissue or osseous flap, (3)?reconstruction with a free bone graft and plate, (4) reconstruction with a soft tissue flap with or without plate reconstruction, or (5) reconstruction plate with an osseous free flap. Each of these strategies will be reviewed, including the advantages and disadvantages of each.
28.5.1 Bone Grafts
Mandibular reconstruction has its roots in bone grafting with the first bone graft being described as a canine xenograft in 1668 and the first autograft used in 1821. 1 Grafts became increasingly important during World War I due to the high prevalence of gunshot wounds. 10 These grafts were fixed with wire and long periods of intermaxillary fixation. The technique of free bone grafting improved in the 1970s with the use of rigid fixation using cortico-cancellous grafts from the iliac crest. Because revascularization takes place from the surrounding soft tissue, graft stability is of paramount importance. Currently, most free bone grafts are taken from the anterior or posterior iliac crest. Bone grafts as large as 5 cm can be used for mandibular reconstruction with acceptable results. Graft take is less likely with large soft tissue defects and previous radiation therapy precludes their use. 10 Free bone grafts should not be used with intraoral defects as the risk of infection, and thus graft failure, is high. For these reasons, the use of free bone grafts in cases of malignancy or postradiation osteonecrosis is limited.
28.5.2 Pedicled Flaps
Pedicled flaps were introduced in bony and soft tissue reconstruction in the 1970s. Conley reported on the use of the sternocleidomastoid flap with clavicle in 1972. 11 Demergasso and Piazza reported the first use of the trapezius osteomyocutaneous flap for mandibular reconstruction in 1979. 12 Cuono and Ariyan introduced the pectoralis major flap with the 5th or 6th rib in 1980. 13 These pedicled bone grafts have the advantage of not needing a skilled microsurgeon for reconstruction; however, their blood supply can be tenuous, and the bone quality may be suboptimal. Survival of the bone graft in these instances has been reported to be as low as 50%. 14 In addition, these techniques can lead to poor cosmetic results.
Soft tissue flaps without bone are other options for reconstruction and much more commonly used. Myocutaneous pedicled flaps such as the pectoralis myocutaneous flap or the latissimus dorsi myocutaneous flap can be used with or without a plate for reconstruction of a lateral segmental defect in an edentulous patient. However, in anterior defects or in dentulous patients, soft tissue flaps with a plate are not advised given the high rate of extrusion and plate fracture, which can be as high as 38% at 10 months. In such circumstances, a vascularized osteocutaneous reconstruction is preferred. If this is not feasible, a vascularized soft tissue flap without a plate will likely lead to less complications in the segmental mandibulectomy defect in the dentulous patient as plate fracture rates are high (26.1%). 15 In an edentulous patient with poor prognosis, soft tissue reconstruction without a reconstruction plate for lateral defects is an option with rates of plate fracture at 5.9% and rates of extrusion at 8% if the soft tissue defect is overcorrected. 15
28.5.3 Free Tissue Transfer
In the early 1990s, free tissue transfer became the dominant technique for reconstruction of segmental mandibular defects greater than 4-5 cm. Several options for reconstruction exist including a vascularized osteocutaneous fibula, osteocutaneous forearm, chimeric scapulae-based flaps, osteocutaneous anterolateral thigh flaps, and iliac crest flaps. These techniques are complex and involve the use of specialized surgeons and centers familiar with microvascular and flap monitoring techniques. However, these flaps have proven to be superior to soft tissue alone with respect to function and complication rate in particular clinical situations. 16 , 17
The osteocutaneous fibula flap is the most common free tissue transfer used for mandibular reconstruction. This graft has significant bone quality, bone length up to 25 cm, excellent and reliable vascular pedicle, and minimal donor site morbidity. However, the vertical height of the bone is somewhat limited, and generally requires a technique to increase the height such as the “double-barrel” technique or placing the fibula 1 cm above the inferior mandibular border (▶ Fig. 28.3 a-d). 18 – 21 If this limitation cannot be satisfactorily ameliorated, then consideration of other osseous free tissue options is necessary. In addition, preoperative angiographic imaging of the lower extremities is often recommended because of anatomic variation. One such variation which exists in 5% of the population is peronea arteria magna, a condition where both the anterior and posterior tibial arteries are hypoplastic and a large dominant peroneal artery supplies the entire leg and foot. Another consideration for preoperative imaging is to determine if there is any vessel narrowing secondary to atherosclerotic disease in the peroneal vessels. 22 , 23
Donor site morbidity in fibula free tissue can be minimized with careful consideration to a few axoms. Ankle stability is maintained on leaving at least 6 cm of bone on the distal aspect of the fibula. Plantar flexion is maintained with care to preserve the peroneus musculature and the peroneal nerve, proximally. The harvesting technique can be modified to include a skin paddle based on perforators from the peroneal artery system. Perforators are more robust and can be more reliable when including a cuff of soleus and/or flexor hallucis longus muscle, and multiple paddles can be designed. 24 , 25 When harvesting a skin paddle, care should be taken to prevent compartment syndrome by closing the wound without any tension. Skin grafting can help prevent this, and Kim et al described skin grafting from the flap incision itself when used for mucosal defects to prevent secondary donor site morbidity. 26
The iliac crest flap is based on the deep circumflex iliac artery and was initially popularized by Taylor and Urken. Advantages of this flap include an optimal vertical height to mimic native mandible and excellent bone quality for osseointegrated implantation. 27 , 28 However, the flap is limited by the thickness of the skin paddle, small pedicle length, and significant donor site morbidity including intractable pain, destabilization of the pelvic girdle, and abdominal herniation. While the advantages are significant, the morbidities related to this flap could be profound, limiting its popularity compared to fibula free tissue transfer.
The lateral border scapula flap is based on the circumflex scapular artery, which is part of the vasculature of the subscapular system. This flap can be harvested with several different subsystems which allows creation of a chimeric flap that can include cutaneous, muscle, and myo-osseous options making it ideal for complex defects with both intra- and extra-oral components. 29 The primary disadvantages include inadequate bone quality to support dental implants, relatively short pedicle, and difficulty in simultaneous harvest and ablation. Pedicle length can be increased using bone from the scapular tip, and modifications to harvesting technique can allow for simultaneous harvest. 30 – 32 Even so, limitations in bone quality and quantity make this a secondary option for many reconstructive surgeons.
The osteocutaneous radial forearm flap is another option that utilizes a well-known flap with a consistent soft tissue paddle, excellent pedicle length, and very reliable anatomy. Disadvantages of this flap include a limited quantity of thin bone of approximately 10-12 cm in total length, which portends to poor bone stock, and precludes the ability to place dental implants. 26 , 33 , 34 In addition, fracture risk of the radial bone increases with the amount of harvested bone and prophylactic plating can sometimes require specialized expertise. While this flap can be useful in edentulous patients or non-weight bearing defects such as the angle or ramus, its use for mandibular segmental reconstruction is not widespread due to the limited quantity of bone. Other options such as the anterolateral thigh osteocutaneous flap are described, but these techniques are less common and donor site morbidity such as pathologic fracture remains a serious concern. 35
Plate Selection
Plate selection for mandibulectomy defects varies based on the type of reconstruction used. Currently, plates are constructed using titanium, a lightweight material that is rigid and stable. Complications from the use of a plate include short-term complications such as infection and exposure and long-term complications such as dehiscence, loss of fixation, fracture, and exposure. 36 – 38 Rates of exposure in patients with bony microvascular reconstruction are between 7 and 29% depending on the plate type used. 39 – 41 These complication rates will increase in patients who have a history of smoking or radiotherapy. 42
Selection of hardware requires thorough planning and knowledge of the defect size, location, and reconstructive plan. In soft tissue reconstruction, large loadbearing plates, 2.0 mm or more, are used in reconstruction. In bony osseous free tissue reconstruction, loadbearing plates or smaller/mini-plates can be used to reconstruct gaps between osteotomies or flap and native mandible. Some advantages of smaller plates are that they are less obvious to palpation and less prone to extrusion. 39 – 43 Smaller plates allow the bone graft to carry the weight of the mandibular stresses, minimizing stress shielding and allowing for less bone reabsorption over time. If the patient undergoes dental implantation, smaller plates, especially miniplates, would make planning more simplistic and can be performed even in a single stage. 43 One study demonstrated in a cadaveric model that nonlocking miniplate systems had noninferior strength compared to the strength of a single, larger plate. 44
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