Mandible Reconstruction

Anatomy and Physiology

The mandible, the bone that comprises the lower jaw, develops embryologically from Meckel cartilage, and the fully-developed mandible has several named regions. The condyle forms the most superior-posterior aspect of the mandible on each side and articulates in the temporomandibular joint, forming the only hinge-and-glide joint in the body and giving the mandible its functional range of motion. The condyle sits on the narrower, neck-like subcondyle. The coronoid process is a triangular projection of bone anterior to the condyle and separated from it by the sigmoid notch; the tendon of the temporalis muscle inserts on the coronoid process. The condyle and coronoid lie superior to the mandible's ascending portion called the ramus, which sits just above the angle. Projecting anteriorly from the mandible angle on each side is the body, and the two bodies of the mandible are connected anteromedially by the symphysis and parasymphyseal regions. The lateral pterygoid and masseter muscles attach along the ramus' lateral aspect and work with the temporalis muscle to open and protrude the jaw.[1][2] 

The medial pterygoid muscle, by contrast, attaches along the medial aspect of the ramus and contracts to close and retrude the jaw. The inferior alveolar nerve, a mandibular nerve posterior division branch (the third branch of the trigeminal nerve, fifth cranial nerve), traverses the inferior third of the mandible through the bony mandibular canal and exits via the mental foramen to provide sensation to the chin and lower lip.[3] 

The mental foramen is found inferior to the first pre-molar on each side. The digastric and mylohyoid muscles attach to the inner, lingual cortex of the mandible. The mandible supports sixteen adult teeth that, ideally, occlude against sixteen maxillary teeth.[4] A branch of the mandibular portion of the maxillary artery, the inferior alveolar artery, is the mandible's primary blood supply and runs through the mandibular canal.[5]

Indications

Significant destruction or absence of a section of mandible requires reconstruction with free or vascularized bone. A reconstructive titanium plate can stabilize bony segments on either side of the defect.[6] Still, if the defect is not later filled with bone, there is a significant risk of soft tissue contraction, plate extrusion, and infection that may necessitate further surgery.[7] Comminution of an otherwise healthy bone surrounded by healthy tissue with a robust blood supply is amenable to repair with free, non-vascularized bone grafting, where bone chips are packed in the area of trauma to encourage bone healing and improve strength.[8] 

Non-vascularized grafts have been used to successfully reconstruct gaps in the mandible variable length in non-irradiated tissue[9]; larger defects may carry an increased risk of graft failure and necrosis due to poor blood supply, as free non-vascularized grafts rely on the surrounding tissues for nutrition. Segmental defects in the mandible two or more cm in length are amenable to vascularized bone repair with microvascular anastomosis of vessels in the neck. Vascularized grafts are indicated for larger mandibular defects, when the tissue has previously been irradiated or otherwise has a compromised blood supply, and when the patient desires rehabilitation with dental implants. Sources of vascularized bone grafts include the fibula, scapula, iliac crest, and radius.[10][11] 

The radius is typically too thin to accommodate dental implants. However, defects of the mandibular symphysis must be reconstructed with the bone; otherwise, the patient is at significant risk of developing an "Andy Gump" deformity in which there is a severe retraction of the skin and soft tissues of the chin and lower lip, plate extrusion, oral incompetence, and chronic drooling.[12] This condition is challenging to correct and very debilitating for patients.

Contraindications

Reconstruction should be delayed whenever there is an active infection in the wound bed, as an ongoing infection risks graft compromise.[13] Defects of the condyle alone do not require bony reconstruction and may best be served with a synthetic condylar head prosthetic plate.[14] 

Non-vascularized graft reconstruction should not be performed if the patient will shortly be undergoing radiotherapy as part of cancer treatment because the resulting decrease in the surrounding tissue's vascularity substantially increases graft necrosis risk. Further, any bony reconstruction should be delayed until after radiotherapy if the surgery will delay starting definitive cancer management. Patients who are candidates for vascularized bony reconstruction should undergo thorough cardiovascular evaluation before surgery. Osteocutaneous free tissue transfer can take several hours in the operating room; therefore, patients' hearts and lungs should be medically optimized before surgery. Further, free tissue transfer often requires a blood transfusion during or after surgery, so patients should be screened and blood prepared if it is needed.[15] 

In fibular free tissue transfer, patients must undergo imaging evaluation of the blood supply to their feet. Commonly, the peroneal, anterior tibial, and posterior tibial arteries all perfuse the foot. However, in some patients with peripheral vascular disease, prior leg surgery, or other conditions, one or more of these arteries may be compromised. In that case, if a fibula is harvested for osteocutaneous free tissue transfer with its peroneal artery, the patient is at risk for foot necrosis. If a patient does not have "three-vessel runoff" to the foot or appropriate collateral circulation, he should not undergo fibula harvest.[16] Scapula and iliac crest osteocutaneous flaps are alternatives in these patients.[17][18][11]

Equipment

A standard mandible reconstruction set, as provided by many different vendors, is useful in mandibular reconstruction. Regardless of whether vascularized or non-vascularized grafts are used, a titanium reconstruction plate is needed to stabilize the bony segments. In some cases, a patient-specific plate can be planned before surgery to minimize surgical time and maximize the accuracy of bony positioning intra-operatively. Patient-specific plates can be expensive; however, a standard 2.0 mm thickness bar is sufficient in all cases; it simply requires intra-operative bending to fit a patient's jaw anatomy. Often, 10 mm to 18 mm bicortical screws are used to fixate the reconstruction bar, depending on the mandible thickness. Often monocortical, 8 mm titanium screws are used to fixate an osseous free flap, as bicortical screws increase the graft's risk of devascularization with subsequent necrosis. In the case of non-vascularized bone grafts, a titanium tray can be useful to hold the bone chips in position while they are healing. Some vendors also provide an alloplastic bone matrix that can be used alone or to enhance autologous transfer.[19] 

A bone mill is also helpful in grinding a single, solid piece of harvested bone into suitable chips. For patients with an infected wound bed, the surgeon may want to avoid placing a reconstruction bar to stabilize the mandibular segments, as the bar itself can become a repository for bacteria and a source for ongoing infection. In these cases, the mandible can be stabilized with an external fixator, a series of steel bars connecting posts seated in the different bone segments, and projects through the skin.[20]

An external fixator, or "ex fix," provides excellent mandibular stabilization with minimal foreign body burden within the wound bed and can allow for infection resolution before definitive internal fixation and bone grafting. Whenever placing plates and screws, a drill is often needed to make pilot holes for the screws. Also, mechanical saws and rongeurs are used to shape and size the native mandible ends and transferred autologous bone.

Personnel

Mandible reconstruction is performed in the operating room under general anesthesia. A minimum of four personnel is required to carry out the procedure:

• A qualified surgeon
• Anesthetist or anesthesiologist
• Circulating nurse
• Scrub tech
• Often, a retractor holder or first-assist is helpful to the surgeon and frees the scrub tech to pass instruments more efficiently

Preparation

Pre-operatively, imaging of the mandible helps demonstrate the location and extent of disease and surgical planning. A pre-operative computed tomography (CT) scan without contrast and with fine, 1 mm cuts can be used for intra-operative navigation or, more commonly, to make patient-specific plates and cutting guides to be used intra-operatively.[21][22] 

When planning for osteocutaneous free fibula transfer, pre-operative arterial imaging is recommended, as discussed above. The vessels of the leg can be evaluated with a Doppler sonogram or CT angiogram. If a CT with fine cuts is performed, cutting guides and patient-specific planning can help shape the bony flap to fit the anticipated defect.[23] As mentioned above, a pre-operative electrocardiogram (EKG) or echocardiogram may be necessary to evaluate and optimize the patient from a cardiovascular standpoint before a long procedure.

Technique or Treatment

The mandible can be exposed through an intra-oral incision made in the gingivobuccal sulcus or through a cutaneous incision in the neck. With the trans-oral approach, care should be taken to preserve the mental nerve exiting at the mental foramen, unless it has already been resected. Trans-cutaneous approaches can include a pre-auricular approach that reflects the parotid gland anteriorly and allows access to the ramus and condyle.[24] In this approach, care must be taken to preserve the main trunk of the facial nerve. If damaged, disfiguring facial palsy can result. A retromandibular incision can also be used. In this approach, the incision is positioned posterior to the angle of the mandible.[25] 

The parotid gland's inferior aspect often needs to be retracted superiorly to allow access to the angle and ramus of the mandible. Still, the risk of main trunk facial nerve injury in this approach is less. Access to the body and symphysis of the mandible can be accomplished through a transverse incision made in a skin crease 2-4 cm inferior to the jaw's inferior edge. In this approach, care must be taken to preserve the facial nerve's marginal mandibular branch not to cause lower lip weakness. Classically, the marginal mandibular branch may be preserved by reflecting the facial vein superiorly; the vein will, in turn, retract the nerve and protect it. The mandible must be stabilized with a reconstruction bar; ideally, four screws should be placed on either side of the defect for maximal stabilization. Three screws are adequate when four are not achievable. Two screws are sufficient but not as robust as three. One screw will lead to rotation of the bone around the screw and insufficient bony stabilization. The reconstruction bar should be shaped and plated along the mandible's inferior edge so as not to disrupt any tooth roots.

Before plating, the jaw should be reduced to its native position; the wear facets of any teeth present should be appropriately opposing to make sure the jaw is set in its pre-traumatic or pre-operative configuration. Temporary maxillary-mandibular fixation can be achieved with screws and 24 gauge wires, arch bars, and other methods. If the area being reconstructed has comminuted fractures, then the bone fragments should be removed, and the wound irrigated before reconstruction. If a section of the mandible is missing, then the remaining bony edges should be freshened to improve healing; a reciprocating saw can be used to remove an additional 1-2 mm of bone until fresh, bleeding edges are seen.

Non-vascularized bone grafts may be harvested from many sites throughout the body and commonly include the iliac crest and anterior tibia.[26] In the iliac crest harvest, an incision is made in the skin lateral to the anterior superior iliac spine (ASIS). The soft tissue is rolled medially, and dissection is continued with monopolar electrocautery (settings of 40-40) to reveal the ASIS's surface. A #9 elevator or similar instrument is then used to lift the periosteum from the medial aspect of the superior ilium; the surrounding muscles are then retracted medially. A reciprocating saw is used under copious saline irrigation to cut a rectangle of cortical bone of whatever size is desired. The deep aspect of the bone can be released with an osteotome and mallet. After removal, additional bone can be scraped from the wound bed with a curette, if necessary. Harvest from the anterior tibia and other bone sites is similar; a skin incision facilitates access to the bone, and a monocortical section is removed, leaving the bulk of the bone intact and not compromising the donor bone's load-bearing ability. The harvested bone can be used as a single strip if desired, but for mandible reconstruction, the bone is often passed through a bone mill to create medium-to-fine sized chunks of fresh bone. These fresh bone chips are then packed into a tray to fill the mandibular defect. In some cases, harvested bone can be mixed with allograft bone to augment the available volume.

For osteocutaneous free tissue transfer, an incision is made in the neck, and suitable vessels for microvascular anastomosis are isolated first. Commonly, the facial artery and vein provide a suitable caliber match. The jaw and the defect are exposed through the neck, if possible, to prevent the possibility of saliva leaking onto the graft site. However, if necessary, an additional transoral incision can be made for enhanced exposure of the area to be reconstructed. The harvested osteocutaneous graft is fitted into the defect and secured to the plate with monocortical screws. In this case, two screws per segment of the graft are often sufficient, placing too many screws risks devascularization and subsequent graft failure. To restore the mandible's natural arc, wedge-shaped closing osteotomies of the bony flap can be made to permit restoration of the arched mandibular contour. However, individual segments should not be less than 1.5 to 2 cm in length, or there is an increased risk of devascularization and graft failure. Osteocutaneous grafts are harvested with their periosteum intact, as the periosteum is an important source of perfusion for the bone. Care must be taken not to disrupt the periosteum, even if making osteotomies, to maximize flap survival chances.[27] 

Ideally, the osseous graft ends will have good bony contact with the native mandible after inset to enhance healing. If there is poor bone-to-bone contact between the graft and mandible, then the gap can be packed with autologous or allograft bone chips to improve healing. The peroneal artery and venae comitantes are sewn to vessels in the neck to restore perfusion, and all incisions are then closed in layers. While an oral surgeon has historically placed dental implants three to six months after vascularized bone transfer to allow sufficient healing time, some patients will tolerate a longer initial operative time – so long as malignancy is not involved – that permits placement of dental implants at the time of reconstruction. This procedure is beginning to gain traction at some academic centers and is known as "jaw in a day" surgery.[28]

After mandibular reconstruction, all patients are maintained on a no-chew diet for at least six weeks to allow for bony healing. If patients attempt to chew or put pressure on the reconstruction plate before sufficient healing has occurred, then there is a risk of non-union, infection, and failure of the reconstruction. If there are intra-oral incisions, patients are often kept nil per os (NPO) and fed via a nasogastric or gastric tube for a few days to allow mucosal healing. If there are no intra-oral incisions, patients can resume a soft, no-chew diet immediately after surgery. Patients are observed in the intensive care unit and a hospital ward for several days after osteocutaneous free tissue transfer with regular evaluation of the reconstruction so that acute vascular insufficiency can be recognized and corrected before flap failure, should it occur.[15] Patients who undergo non-vascularized reconstruction can discharge from the hospital as soon as they have tolerable pain control and adequate caloric intake.

Complications

The most feared complication of mandible reconstruction is graft failure and the need for a second reconstructive surgery. Infection, early movement of the jaw (often from early chewing), and poor nutrition either from the flap vessels or surrounding tissues, all increase the risk of reconstructive failure. Patients with a jaw radiotherapy history, hypothyroidism, protein deficiency, uncontrolled diabetes, active tobacco use, and other conditions are at increased risk of poor wound healing; efforts should be made to control these conditions much as possible before surgery.[15] 

In osseous free tissue transfer, success rates are generally above 95%; however, the most common complication is a vascular compromise with subsequent partial or complete flap failure. Venous compromise is more common than arterial compromise. Both are most likely to occur in the first 72 hours after microvascular anastomosis, as this is the time during which re-endothelialization of the vessels is occurring. Vascular compromise can occur at any time after flap transfer up until angiogenesis renders the tissue independent of its pedicle, which usually takes at least 2 to 3 weeks.

Several complications may develop after the reconstruction is fully healed; improper sizing, positioning, or shaping of the graft or reconstructive plate can cause a malocclusion, facial asymmetry, and chronic temporomandibular joint pain. Additionally, radiotherapy, major infection, or additional surgery - such as placement of dental implants - that occurs after healing can still compromise the blood supply to the area and cause partial or complete flap failure.

Clinical Significance

Accurate and robust reconstruction of the mandible is necessary to restore many essential functions and improve life quality. A thorough pre-operative evaluation, meticulous intra-operative manipulation, and close post-operative care can restore a patient's ability to chew and swallow, restore a natural facial appearance, relieve pain, and, following, improve patient's confidence and independence in their activities of daily life. Poor reconstruction of the mandible, by contrast, can be devastating and lead to lifelong disability.

Enhancing Healthcare Team Outcomes

A multidisciplinary approach is essential to reconstructive success. The process of reconstructing a mandible can take weeks to months before the final result is achieved, and patients will need follow-up, counseling, and reassurance throughout the course of recovery. If patients require dental rehabilitation, an oral surgeon should be involved in initial operative planning and throughout the reconstructive process. For patients undergoing free tissue transfer, an operative team (anesthesiologist, circulating nurse, surgical tech) comfortable with free flap physiology and microsurgical needs are critical for surgical success. Subsequently, a hospital team (intensivist, hospitalist, nursing staff) comfortable with free flap care is essential for flap survival. Effective and ongoing communication among medical personnel with a focus on patient-centered care will greatly improve patient outcomes.[29]