18 Anaesthesia for orthognathic surgery

Orthognathic surgery involves the manipulation of the facial skeleton: the maxilla, mandible, and dentoalveolar segments (the portion of the jaw supporting the teeth), these can all be surgically repositioned to treat malocclusion and deformity. The term orthognathic derives from the Greek words orquos, meaning to straighten, and gnaqos, which means jaws. Hence orthognathics is surgery to straighten the jaws.

The majority of patients are referred to the orthognathic team by an orthodontist because they have malocclusion which cannot be corrected by conservative measures (braces and appliances) alone. Abnormalities of dental alignment (the bite or occlusion) relate not only to the position of the teeth, they are intimately connected with the relative positions of the upper and lower jaw. Facial harmony depends on these relationships, but malocclusion does not only influence facial aesthetics, there are serious functional implications. Chewing, mastication, speech, temporomandibular joint (TMJ) function, the ability to nose-breathe, and the ability to maintain good oral hygiene may all be compromised. The correction of these functional elements underpins the rationale behind treatment.

A multidisciplinary team approach is standard, and treatment may take years to reach completion. An orthodontist is always involved if dental occlusion is at issue; the orthodontic workup generally starts a year or two before surgery and continues postoperatively. In addition, restorative dentists, dental hygienists, psychologists or psychiatrists, throat, nose and ear, plastic and neurosurgeons, speech therapists, specialist nurses, and audiologists may all contribute to planning and treatment^{1  –4}.

Most patients presenting for orthognathic surgery have abnormalities of dental occlusion which cannot be corrected with orthodontics alone. In all but the most extreme cases, surgery is delayed until late adolescence when bone growth is complete and the permanent teeth have all erupted, so most patients are in their teens or early twenties and have no significant medical comorbidities.

A minority have bony deficiencies secondary to cleft lips, cleft palates or craniofacial syndromic deformities such as Crouzon’s or Apert’s syndrome which may be associated with other significant medical abnormalities. Some have acquired skeletal anomalies or asymmetries, usually as a result of treatment of head and neck tumours in childhood or following trauma, and this group are much more likely to have challenging airways.

Preoperative planning for orthognathics often begins years before surgery and includes a full orthodontic workup, with imaging, dental impressions, and the construction of dental models (Figure 13.4). Model surgery is carried out to establish the feasibility of different treatment options and these models are used to generate wafers with impressions of the teeth on each surface. These interpositional occlusal wafers are important intraoperatively as they allow accurate repositioning of jaws to provide the desired occlusion. Presurgical orthodontics straightens the teeth and aligns the arches so that stable occlusion can be achieved postoperatively.

Anaesthetic input into the orthognathic or craniofacial clinic helps to identify the more complex patients who may have a history of previous anaesthetic problems and significant medical comorbidities and those in whom perioperative airway management may not be straightforward. Teenagers are a distinct group who have their own set of psychosocial problems, which make them more challenging than children in many ways. In those with severe dentofacial abnormalities there is often a long history of unenjoyable hospital visits, and the usual fears and anxieties may be heightened. The opportunity to meet these patients early to provide information to help manage their expectations about the anaesthetic and to establish a rapport can be very helpful.

On the day of surgery, the preoperative visit is useful to prescribe sedative premedication if indicated and to establish guidelines for fasting. Postoperative analgesia can be discussed and the use of adjuncts such as ice packs, which help reduce pain and swelling, explained. It is helpful to warn patients if a lubricant ointment will be used to protect the eyes, as blurred vision postoperatively can cause considerable anxiety. An explanation about the numbness and loss of proprioception associated with local anaesthetic, a warning about a sore throat, and the benefits of an early oral intake help to manage patient expectations. A detailed anaesthetic and airway management plan should be devised in conjunction with the surgeon.

Nasal tubes are the mainstay of maxillofacial anaesthesia and the most versatile method of managing the airway in orthognathic surgery. The most common procedures are maxillary or mandibular osteotomies, or a combination of the two (the bimaxillary osteotomy). All these procedures use dental occlusion as a reference point for accurate fixation of the bony fragments. An oral tracheal tube is therefore contraindicated and a nasal tube is the usual method of securing the airway and providing optimal surgical access (Figure 18.1). If the surgical plan involves mid or upper facial work such as a nasal manipulation, if a bicoronal flap is planned, or if the nasal passages are occluded, submental intubation is preferable (Figure 18.2)⁵. On rare occasions, when the airway is challenging and is likely to be further compromised for a significant period postoperatively, a tracheostomy may be necessary to provide secure airway control.

Preformed north-facing nasal tubes are ideal as they are stable once positioned and can be secured with strapping across the forehead which means that the bony contours and symmetry of the face are not obscured. Care must be taken to ensure that the tube lies passively and does not exert any traction on the nostril. This is particularly important for maxillary impaction as upward traction may cause distortion of the nasal septum and this may not be apparent until the patient is extubated and found to have a bent nose.

A throat pack prevents soiling of the trachea with blood and debris and the passage of blood into the stomach. The pack should be inserted under direct vision avoiding excess bulk which can anteriorly displace the mandible, undesirable if chin or jaw surgery is taking place. The use of a pack must be recorded (Chapter 12) on the anaesthetic chart and a sticker on the tracheal tube to help ensure that it is removed at the end of surgery (Figure 18.1).

Suction under direct vision prior to extubation is important, to clear clots which may have formed in the mouth or behind the palate, the so-called coroner's clot. If the patient has had a mandibular osteotomy, it is very important not to apply forward traction to the mandible with a laryngoscope as it may displace the screws or plates which have been applied to the osteotomized segments.

The eyes are vulnerable during surgery. Eye pads may not be welcomed by the surgeon as they obscure the bony contours of the face and compromise the surgical field. A barrier ointment should be applied and further protection provided with the application of transparent eye tapes, Steri-Strips or surgical eye shields.

Venous thromboembolism is rare following orthognathic surgery⁶ but it does occur, and standard prophylactic measures should be used.

Peripheral or rectal temperature probes avoid the difficulties of siting a probe in the nose. It is sensible to maintain normothermia to avoid shivering during emergence, which is both uncomfortable for the patient and increases their oxygen demands. It is easy to cause overheating during longer maxillofacial procedures as only the head and neck are exposed so it is essential to monitor the temperature.

Optimizing the surgical field involves mutually satisfactory airway management and techniques to reduce blood loss. Bleeding at the saw insertion site obscures the bony cuts and makes a precise incision more difficult during the osteotomy. There is always significant bleeding when the maxilla is down fractured during a Le Fort I and this will continue to some extent into the postoperative period. A head-up position to improve venous drainage, the use of adrenaline containing local anaesthetic or tumescent solutions to infiltrate the field prior to incision and moderate hypotension, are all helpful^{7  ,8}. The antifibrinolytic, tranexamic acid, can be used prophylactically to reduce bleeding⁹.

A smooth safe emergence is desirable. Coughing and agitation on emergence increase both arterial and venous pressure, and the risk of haematoma formation, suture disruption, and bleeding at the surgical site. Forceful coughing or mouth opening can disrupt mandibular fixation, and airway manoeuvres such as a jaw thrust or the application of a tight-fitting facemask may damage the surgical field. In general, there will be blood in the mouth at emergence even after meticulous oropharyngeal suction under direct vision, so it is safest to extubate patients awake with airway reflexes intact.

A remifentanil infusion can be used to provide an awake, cooperative patient who is tube-tolerant and in whom the cardiovascular responses to extubation are obtunded.

After some procedures, the tracheal tube can be exchanged for a laryngeal mask or supraglottic airway device before anaesthesia is lightened to provide a smooth emergence, but this technique is unsuitable following mandibular surgery when forceful mandibular manipulations are undesirable¹⁰.

If the airway was challenging at the start of anaesthesia or if there is any uncertainty about the ability to reintubate or reoxygenate following extubation or in the immediate postoperative period, the tracheal tube should be removed over an airway exchange catheter. This can be left in place in the trachea, providing a conduit for oxygenation and reintubation should it be needed (Figure 18.3)^{11  –13}.

It is important to prevent postoperative nausea and vomiting in maxillofacial surgery. Vomiting is always distressing but is particularly so after surgery to the oral cavity. It is unpleasant for the patient and, since the forced involuntary muscular contraction of retching and vomiting raise the venous pressure, increasing the risk of bleeding and suture disruption, it should be avoided if possible.

Antiemetics with dexamethasone, a liberal preoperative fluid regimen, and encouragement to start oral fluids early after surgery are all helpful. There is a lower incidence of postoperative nausea and vomiting following total intravenous anaesthesia with propofol than after volatile anaesthesia¹⁴.

In general, most orthognathic procedures are not very painful postoperatively. A combination of intraoperative local anaesthetic, simple analgesics, and morphine followed by regular paracetamol and a non-steroidal anti-inflammatory postoperatively is often sufficient. Oral medications should be prescribed in liquid or soluble form to make them easy to swallow. A patient-controlled analgesic (PCA) regimen is rarely necessary. Instructions to nurse the patient in a head-up or sitting position, the use of regular dexamethasone, and the application of ice packs reduce postoperative swelling and enhance patient comfort. Early resumption of oral fluid intake is also beneficial as it eases the sore throat and dry mouth which follows anaesthesia.

If supplemental oxygen is needed postoperatively humidification prevents drying and crusting of blood and secretions in the nose and the mouth.

The commonest orthognathic procedures are Le Fort I maxillary osteotomy, bilateral sagittal split osteotomy of the mandible, and a combination of the above—the bimaxillary osteotomy, and genioplasty or osteotomy to the chin (Figure 18.4).

A Le Fort I osteotomy involves complete mobilization of the maxilla which is then repositioned using the occlusal surface of the upper and lower teeth as a reference point, and stabilized with titanium screws and plates. An intraoral mucosal incision is made from the first molar tooth on one side to the same point on the other and a mucoperiosteal flap is raised. An oscillating saw is used to make the osteotomy cuts which extend from the lateral wall of the nose at the pyriform fossa into the pterygomaxillary fissure behind the posterior aspect of the maxilla. At the nasal floor, the septum is separated from the maxilla, the lateral nasal walls are osteotomized, and the inferior turbinates are trimmed if necessary. This allows down fracture of the maxilla which, although freely mobile, remains attached and viable by its soft tissue pedicle. The jaws are temporarily wired together over an interpositional acrylic wafer which has impressions of the teeth in perfect occlusion on each surface, and the maxilla is fixed in place using miniplates and screws.

The maxilla can be mobilized in any direction, forwards (advancement), upwards (impaction), downwards, backwards (setback), and it can be rotated. In some circumstances, a vertical maxillary split may be carried out to widen the upper dental arch or segmental osteotomy, dividing the maxilla into separate fragments which can be repositioned independently.

Postoperative rigid intermaxillary fixation (IMF) (jaw wiring) is rarely needed but light elastic traction attached to arch bars or orthodontic brackets is sometimes used to encourage correct final positioning¹⁵. Indications of Le Fort I: maxillary osteotomy are included in Table 18.1 and anaesthetic considerations in Table 18.2.

The bilateral sagittal split mandibular ramus osteotomy (BSSO) is the most versatile and popular mandibular osteotomy. It allows forward or backward movement of the lower teeth and some rotation of the jaw is also possible.

The intraoral incision is made halfway down the vertical ramus of the mandible and extends as far as the last molar tooth. The neurovascular bundle is identified as it enters the mandibular canal and a horizontal osteotomy is made on the lingular surface with an oscillating saw. The vertical osteotomy cut is made on the buccal surface of the body of the mandible. The vertical and horizontal cuts are then joined with a sagittal osteotomy (parallel with the internal oblique line), the split is then completed using osteotomes, and the procedure repeated on the other side. The toothbearing portion of the mandible is now mobile and can be repositioned using mini plates and screws or screws alone. An interpositional wafer is often used to permit accurate repositioning and may be left in place at the end of surgery. IMF may be necessary if the splits are considered unstable but is rarely used routinely. Anaesthetic considerations are included in Table 18.3.

In many cases optimal correction of orthognathic deformities can only be achieved with surgery of both the mandible and maxilla—this is termed a bimaxillary osteotomy or bimax.

The maxillary osteotomy is performed first; an intermediate interpositional wafer (which has been generated using patient-specific dental models as part of the orthodontic workup) is placed between the upper and lower teeth. These are then wired together and the maxilla fixed in its new position, effectively using the original position of the lower teeth as a reference point. The IMF is released and the bilateral mandibular sagittal splits carried out. A second, ‘final’ wafer is used to reposition the mandible, so that this stage of the surgery uses the new position of the maxillary teeth as the reference point. Although a bimax takes longer than a single jaw osteotomy, the anaesthetic considerations are the same.

The chin is critical to facial harmony. It influences the facial height, facial centre lines, and cosmetic appearance of the nose. Genioplasty can be used to augment or reduce chin size, to straighten an asymmetric face or to lengthen a short face. It can be carried out as a single procedure or combined with other osteotomies or soft tissue facial procedures.

After infiltration with adrenaline containing local anaesthetic solution, an intraoral incision is made behind the lower lip; the osteotomy is performed with an oscillating saw and the bony fragment repositioned using titanium screws. Anaesthetic considerations are included in Table 18.4.

Distraction osteogenesis, a technique to generate bone and soft tissue, is gaining in popularity for the surgical correction of hypoplasias of the craniofacial skeleton. In the 1950s Gavril Ilizarov, a Russian orthopaedic surgeon, showed that osteogenesis could be induced if bone is expanded (distracted) along its long axis at the rate of 1 mm per day¹⁹. A corticotomy is used to fracture the bone into two segments which are gradually moved apart during the distraction phase. New bone forms along the vector of pull, avoiding the need for a bone graft, and offers the additional benefit of expansion of the overlying soft tissues, which are frequently deficient in these patients²⁰.

Distraction osteogenesis is used to augment a hypoplastic mandibular or maxillary alveolar ridge prior to dental implant insertion, distraction of the mandible to produce lengthening where there is hypoplasia or deformity, and midface distraction for syndromic conditions associated with midface hypoplasia such as Crouzons and Aperts. The first two procedures involve a simple corticotomy to divide the bone and the fixation of the distractor plates to each of the two segments with screws. After a brief latent period, the distractor is adjusted via its external component to extend the gap gradually. Midface distraction is a much more complex, technically demanding, procedure, and is generally the preserve of specialist craniofacial centres (see Le Fort II and III osteotomies).

The alveolus is the part of the jaw that supports the teeth. Dentoalveolar surgery is concerned with management of the diseases of the teeth and the hard and soft tissues which support them.

This is a subspecialty of dentoalveolar surgery which involves the restoration of oral and facial form and function which has been lost through the loss of teeth and related bony structures. This may be as the result of extractions at an early age or following trauma, surgery or radiotherapy. Without reasonable dentition, normal speech, mastication, and swallowing are not possible, and there is soft tissue collapse so that both form and function suffer. If the teeth are removed or lost, bony resorption occurs, and the alveolar ridge becomes hypoplastic and will not provide adequate support for a denture.

The most widely used implant is the osseo-integrated implant, based on the discovery that titanium can be successfully fused into bone when osteoblasts grow on and into the rough surface of the implanted titanium. Osseo-integrated implants can be used either to provide anchor points for dentures or obturators or to allow construction of artificial teeth—they have the beneficial effect of stimulating bone growth. If there is insufficient bone to anchor a dental implant, bone grafting or distraction surgery are needed to augment the jaw prior to insertion.

Depending on their site and number, implants can be placed under local anaesthesia, with or without sedation, or under general anaesthesia. A submucoperiosteal flap is raised, the bone is drilled, and the preliminary fixture inserted into the bone. A titanium healing cap (Figure 18.5) is screwed into the implant to prevent ingrowth of the oral mucosa and a period of months is allowed for integration. The second stage is much less invasive and can usually be carried out under local anaesthesia. The healing cap is removed and a prosthetic tooth moulded onto a titanium post is screwed into place.

Bone grafts are used to augment bony deficiencies and may be carried out in isolation, in preparation for definitive surgical procedures, or as a component of other procedures. The common donor sites for bone grafts are the iliac crest, rib, maxilla, mandible, cranium, and tibial plateau. Corticocancellous bone can be used in blocks; particulate cancellous bone can be sandwiched into osteotomies or defects. A third form includes purely cortical grafts, primarily used to form a wall or strut in a defect that is simultaneously packed with particulate cancellous bone. Cortical grafts revascularize very slowly and have minimal to no cell survival. Cadaveric bone is occasionally used.

The morbidity associated with bone grafts is related to the donor site. Iliac crest grafts are painful and an epidural catheter can be left in situ to allow boluses or an infusion of local anaesthetic to be given for the first 24 hours.

Midfacial advancement²¹ is indicated for patients with syndromic craniosynostosis and in severe developmental hypoplasia of the midface which can include cleft patients. In these patients, rigid internal fixation is an option but the degree of advancement required is often so great that restriction of the adjacent soft tissues may preclude stable advancement in one stage; distraction techniques are favoured as they allow gradual bone and soft tissue lengthening. These procedures are the preserve of specialist craniofacial centres and involve a multidisciplinary workup and treatment pathway^{22  ,23}.

The Le Fort II osteotomy is a procedure used in patients with midface hypoplasia in whom the maxilla and nose require movement together. Access is via incisions in the mouth, transconjunctival incisions behind the lower eyelids or lower eyelid (blepharoplasty). Access to the nose is via a bicoronal incision to raise the scalp.

The Le Fort III osteotomy is used to correct generalized midface hypoplasia which includes the malar complexes and the orbits. Osteotomies of the maxilla, nose, and zygomas allow the entire midface to be moved en bloc or in segments. The surgical approach is similar to the Le Fort II procedure²⁴. If a distraction technique is used, a halo device is fixed on to the skull with pins, and distraction wires link titanium plates attached to the facial bones with the frame. These devices are designed to disarticulate with relative ease but do restrict access to the airway (Figure 18.6)²⁵.

Anaesthetic considerations are included in Table 18.5.

The temporomandibular joint (TMJ) is a compound joint composed of the temporal bone, the condyle of the mandible, and the articular disc, contained within a tightly attached fibrous capsule and lined with a synovial membrane. The disc divides the joint into two compartments; the lower compartment permits hinge motion or rotation and the upper compartment permits sliding. It is a diarthrodial joint, a discontinuous articulation of two bones with the extent of movement dictated by associated muscles and ligaments. It is the most active joint in the body, moving up to 2000 times a day during talking, chewing, swallowing, yawning, and snoring²⁹.

There are two groups of patients with TMJ disorders: those with normal anatomy and abnormal function, and those with abnormal anatomy. The first group present with pain and clicking, the aetiology of which is incompletely understood but which may be triggered by abnormalities of occlusion which produce disharmonious joint movement and trigger muscle spasm. This is common, affecting over 10% of the population and representing the major cause of orofacial pain. These patients generally have a degree of trismus but this usually relaxes after induction of anaesthesia and they rarely present airway management issues. The second group of patients have anatomical derangements ranging from disc displacement, which causes symptoms but does not limit mouth opening, to ankylosis of the joint which prevents mouth opening³⁰.

An arthroscope is introduced into the joint and lavage carried out. Anaesthetic considerations are included in Table 18.6.

TMJ symptoms caused by an abnormal articular disc may be treated with an open procedure in which the joint space is accessed via a preauricular incision (Figure 18.7). Anaesthetic considerations are included in Table 18.7.

Excision of the coronoid process is sometimes performed for TMJ ankylosis. Anaesthetic considerations are included in Table 18.8.

TMJ replacement is reserved for severely deranged joints. The surgery is delicate and prolonged and generally only carried out in specialist centres³¹. Anaesthetic considerations are included in Table 18.9.

Orthognathic surgery necessitates a multidisciplinary team approach within which the anaesthetist has a pivotal role. Early communication with other members of the team and appropriate patient preparation is an important aspect of the anaesthetic care.