Reconstruction of Mandibular Defects

Surgical reconstruction of mandibular bone defects is a routine procedure for rehabilitation of patients with deformities caused by trauma, infection or tumor resection. The mandible plays a major role in masticatory and phonetic functions, supporting the teeth and defining the contour of the lower third of the face. Therefore, mandibular discontinuity produces se‐ vere cosmetic and functional deformities, including loss of support for suprahyoid muscles and subsequent airway reduction. Reconstruction of these severe defects is mandatory for restoring the patient’s quality of life. Surgical techniques have improved considerably in the last decade, but reconstruction of large bone defects of the mandible still pose a great chal‐ lenge in maxillofacial rehabilitation. Several things can be done to optimize the surgery; the use of prototyping modeling for instance provides a better assessment of the bone defect and pre-contouring of the fixation plates, reducing operating time. The choice of the most suitable titanium plate system is critical to the success of the procedure. Mandibular defects with loss of continuity require more robust (load bearing) systems supporting mandibular function. Many studies consider the use of plates and screws temporary treatment due to the large number of complications such as fracture of plates and screws, plate exposure and infection. Thus, the use of grafts both in the first operation or in a two-stage procedure en‐ sures a more predictable result.


Introduction
Surgical reconstruction of mandibular bone defects is a routine procedure for rehabilitation of patients with deformities caused by trauma, infection or tumor resection. The mandible plays a major role in masticatory and phonetic functions, supporting the teeth and defining the contour of the lower third of the face. Therefore, mandibular discontinuity produces severe cosmetic and functional deformities, including loss of support for suprahyoid muscles and subsequent airway reduction. Reconstruction of these severe defects is mandatory for restoring the patient's quality of life. Surgical techniques have improved considerably in the last decade, but reconstruction of large bone defects of the mandible still pose a great challenge in maxillofacial rehabilitation. Several things can be done to optimize the surgery; the use of prototyping modeling for instance provides a better assessment of the bone defect and pre-contouring of the fixation plates, reducing operating time. The choice of the most suitable titanium plate system is critical to the success of the procedure. Mandibular defects with loss of continuity require more robust (load bearing) systems supporting mandibular function. Many studies consider the use of plates and screws temporary treatment due to the large number of complications such as fracture of plates and screws, plate exposure and infection. Thus, the use of grafts both in the first operation or in a two-stage procedure ensures a more predictable result.
Bone grafts are widely used in reconstructive surgery of the mandible. Incorporation of the bone graft restores continuity, shape, and strength of the jaw to near normal function. Installation of dental implants in the grafted areas is important to restore masticatory function and maintain bone graft volume. Autogenous bone is the best choice for major reconstructions due to lack of rejection, and the presence of viable osteogenic cells that increase bone formation and incorporation at the graft site. The use of a vascularized graft is a good choice because it increases the success of the treatment. However, this technique is not available in all medical centers. Autogenous free bone (non-vascularized) is still the most used graft, even in major reconstructions [1]. The high vascularity of the soft tissues in the oral cavity has allowed the use of free bone graft in the repair of oral cavity defects; but larger grafts increase the risk of bone resorption or failure of graft take. Hyperbaric oxygen therapy is currently being used to optimize bone healing. This procedure increases bone cellular activity and capillary ingrowth, inducing new bone formation and accelerating bone healing. The aim of this chapter is to present our experience with a series of patients with extensive mandibular defects where the use of autogenous free bone grafts along with hyperbaric oxygen therapy as an important adjuvant was beneficial to the outcome.This chapter also presents other alternatives for mandibular reconstruction.

Defect evaluation
In mandibular reconstruction, the restoration of bone continuity is not the only criteria for success. The ultimate goals constituting success is attaining near normal morphology and appropriate relation to the opposing jaw, adequate bone height and width, good facial contour and support for overlying soft tissue structures and restoration of jaw of function.
Bony reconstruction planning begins with evaluation of the patient's anatomy in order to define the full extent of the existing defect (both bone and soft tissues) and select the best reconstruction technique for each particular case. The size of the defect will define the magnitude of the reconstruction [2,3]. Some defects may not need to be restored to original size and shape. Loss of a significant portion of a mandibular ramus, for example, may be adequately managed by providing continuity from the condyle to the body of the mandible without restoring the coronoid process.
The quantity and quality of the soft tissues are both important when choosing the reconstructive method. The complete closure of the soft tissue without tension is essential for success. If the tissue is inadequate in quantity, the use of horizontal incisions in the periosteum must be used to guarantee tissue flexibility when needed. This ensures good (tension-free) repair, minimizes postoperative discomfort and reduces dehiscence (one of the most commonly observed complications after grafting in the oral cavity).
On the other hand, if the quantity of soft tissue is adequate but the quality is poor, the reconstruction will be compromised or limited. Tissue with extensive scarring provides a poor host bed for any grafting procedure. When considering the use of non-vascularized bone grafts, the ideal soft and hard tissue bed should have enough bulk, vascularity, and cellularity in order to permit bone graft incorporation. In several cases, tissue loss, scar contracture, and previous irradiation will hamper secondary reconstruction. In this setting, the use of hyperbaric oxygenation should always be considered, because it promotes vascularization and angiogenesis.
Preoperative radiographic evaluation of patients undergoing reconstructive bone surgery aims to evaluate the nature and extent of the lesion and provide the surgeon with anatomic mapping of important structures. Also, follow-up examinations to confirm healing and to discover complications at an early stage are paramount. The selection of the most appropriate imaging method in each case must take into account the diagnostic capability and costeffectiveness. Radiographic analysis, computed tomography with three-dimensional (3D) images and magnetic resonance can provide important information. With the development of rapid prototyping methods, such as stereolithography, fused deposition modeling and selective laser sintering, 3D reconstruction based on biomodels have become indispensible tools both for mandibular resection and bony reconstruction.
The use of 3D biomodels, may help delineate the osteotomy area, improving the accuracy of marginal resection. Pre-modeling of reconstruction plates according to the mandibular anatomy is also facilitated. At the time of the secondary reconstruction, the individual plate gives the surgeon a clear direction where the bone should be ideally placed. Another important possibility with these models is the reproduction of the anatomy of the resected area based on mirror imaging of the contralateral side of the mandible. This procedure guides the surgeon as to where to cut the bone graft in the donor area and enhance visualization of the points to be remodeled in the graft prior to fixation to reproduce the new mandible.

Reconstruction plates
Mandibular reconstruction plates and screws (2.4 System) are the most widely used devices for mandibular reconstruction; however 2.0 plates can be used in selected cases. With the conventional fixation technique, the tightening of the screws presses the plate against the bone (load sharing). This pressure generates friction, which may contribute to resorption of the grafted bone. However, with the locking systems (load bearing), additional threads within the screw head allows the plate to be anchored to the intraosseous screw instead of being compressed onto the bone. This reduces interference to the bone blood supply underlying the plate, prevents bone pressure necrosis and decreases the potential for plate failure at the screw-bone interface. These plates and screws provide an excellent rigid frame construction with high mechanical stability which is extremely useful in bone grafting (Figures 1-6).    Reconstruction plates are usually shaped before the mandibular resection and applied afterwards. By bending these plates and placing drill holes in the proximal and distal mandible segments before complete mandibular resection, the surgeon can more confidently maintain the proper occlusion and relationships of the remaining mandibular segments after removal of the involved bone. Even in edentulous cases, this planning maintains a more natural con-tour and good joint function. With the currently available low-profile locking reconstruction plates, the contoured plate can closely approximate the natural mandibular projection without sacrificing durability and strength, even when used in conjunction with bone grafts. If, however, there is involvement of the buccal cortex of the mandible, direct plate contouring to the bone is not always possible. In these cases, removal of the buccal part of the lesion to allow plate positioning before complete resection is a possible option with satisfactory results. Post-resection freehand plate contouring and fixation is another possibility, however it is difficult, presumes the need of inter-maxillary fixation (IMF) and often yields suboptimal symmetry.  It is important to understand the appropriate possibilities for bone graft fixation. In our experience, adequate internal fixation using reconstruction locking plates and, subsequently, free autogenous bone grafts seem to be most satisfactory.

Free bone grafting
During harvesting, tissue connections between the bone graft and surrounding tissues are transected. In the recipient site, the bone must be revitalized mainly via tissue ingrowth, although it is known that many cells within free bone grafts are able to survive after transplantation. The revitalization goes along with a process of initial remodeling and bone resorption, which is associated with bone volume loss. The amount of resorption depends on many factors, such as the quality of the bone (cortical, cancellous), bone graft fixation to surrounding bone, biomechanical properties (functional loading), the dimensions of the bone graft (it takes longer to revitalize large bone grafts, and therefore, usually they show greater percentage of bone loss) and tissue qualities at the recipient site (vascularization). The amount of bone formed is directly proportional to the number of viable osteogenic cells transferred. The next phase involves revascularization, remodeling, and reorganization of the previously formed bone by osteoblasts and osteoclasts.
Non-vascularized autogenous bone grafts can be harvested from the patient's calvarium, rib, ilium, tibia or fibula [4]. They can be successfully used for reconstruction of small to medium size mandibular defects with favorable prognosis. However, in large mandibular defects, bone reconstruction is still challenging.
Cancellous bone grafts, consisting of medullary bone and bone marrow, contain the highest percentage of viable cells. These grafts become rapidly vascularized due to their particulate structure and large surface area. In contrast, cortical grafts consisting of lamellar bone, provides more resistance to the graft. Cortico-cancellous bone grafts contain both cortical and underlying cancellous bone providing both viable cells and necessary strength for bridging discontinuous defects. The combination of particulate cortical bone and cancellous marrow provides the best potential for osteogenesis.
Bone harvesting should always be performed with sharp instruments under abundant irrigation, and the surgical time must be as short as possible to minimize tissue necrosis and preserve cell viability [5]. The same principles are required during the bone adaptation in the recipient site. The lack of adaptation of the bone block onto the recipient site and the presence of gaps can generate fibrous tissue interposition, which can be avoided with filling the gap with particulate autogenous bone, platelet rich plasma (PRP) or biomaterials.
The recipient site preparation should facilitate the subsequent adaptation of the graft and also expose the bone marrow, favoring revascularization, since the vessels from the periosteum were compromised when it was displaced. The cortical bone in the recipient site can be perforated or even removed with drills to enable contact of the marrow spaces of the graft [6].
Graft fixation is essential to allow its revascularization and incorporation. Movement of the bone block during the healing period results in fibrous tissue between the graft and the recipient site or graft resorption [5,6]. The fixation screws can be used in a passive or compressive manner, however, in the latter case, excessive compression must always be avoided. In cases of mandibular reconstruction decortication is extremely important before the placement of the grafts to support revascularization and facilitate the graft adaptation.

Hyperbaric oxygen therapy
The hyperbaric oxygen (HBO) is a therapeutic modality performed within devices called pressurized containers, in which the patient breathes pure oxygen at a high pressure. The HBO promotes an increase in the amount of dissolved oxygen in the blood due to increased pressure inside the chamber, aiding tissue oxygenation [7] (Figure 7). For years, conventional medicine thought of HBO only as a treatment for decompression sickness and air embolism. However, the use of HBO is becoming increasingly common in general practice. HBO has already been used in the treatment of carbon monoxide poisoning, cerebral arterial gas syndrome, decompression sickness, osteoradionecrosis and clostridial gas gangrene. It is also beneficial to improve the healing of a variety of compromised or hypoxic wounds including diabetic ulcers, radiation-induced tissue damage, gangrene, and necrotizing anaerobic bacterial infections [8].
Complications of HBO can be due to either O 2 toxicity or barotrauma. O 2 toxicity is due to formation of superoxide, OH-and H 2 O 2 . Signs and symptoms of O 2 toxicity mainly involve respiratory system and central nervous system with symptoms like anxiety, nausea, vomiting, seizures, vertigo and decreased level of consciousness. Patients also show respiratory discomfort ranging from dry cough and substernal pain to pulmonary edema and fibrosis [7].
HBO is contraindicated in a patient with pneumothorax due to increased risk of gas embolism. It is also contraindicated in epileptics, hyperthermia and acidosis due to increased risk of seizures. Chronic obstructive pulmonary disease, malignant tumors, pregnancy, claustrophobia, hereditary spherocytosis and optic neuritis are other relative contraindications for the use of HBO therapy [9].
Following maxillofacial trauma there is a vascular disruption which leads to the formation of a hypoxic zone. While hypoxia is necessary to stimulate angiogenesis and revascularization, extended hypoxia will blunt the healing process. HBO may be used to aid in the healing of these compromised wounds by increasing oxygen diffusion from the capillaries to tissues [10]. The available oxygen also has bacteriostatic and bactericidal activites, enhances the phagocytic capacity of white blood cells and promotes differentiation of fibroblasts by interfering with the synthesis of collagen. Important biological events such as angiogenesis and osteogenesis are also stimulated by HBO [11], improving tissue repair and increasing the overall success of reconstruction procedures.
The stimulation of osteogenesis by HBO has been reported in animal experiments and clinical cases. In 1996, Sawai et al. conducted a study to evaluate the effect of hyperbaric oxygen therapy on autogenous free bone grafts transplanted from iliac crest to the mandibles of rabbits and the results indicate that HBO accelerates the union of autogenous free bone grafts [12]. Other studies also demonstrated that HBO elevates alkaline phosphatase activity, a marker of bone formation, in rats following mandibular osteotomy [13], increased osteoblastic activity and angiogenesis in irradiated mandibles undergoing distraction [14] and increased vascular endothelial growth factor expression during bone healing [15].

A hyperbaric oxygen protocol in mandibular reconstructions
The following treatment steps are included in these sessions: 10 minutes of ventilation to fill the chamber with 100% oxygen, 10 to 15 minutes of diving (0.06 to 0.12 kgf/cm 2 in 1 minute), the patients are exposed to 2.4 ATA (Atmosphere Absolute) pressure for 90 minutes, 10 minutes of re-surfacing and 10 minutes of air ventilation. HBO is given every day and the treatment starts 10 days before bony reconstruction and continues for another 40 days after the surgical procedure.