Summary
Micrognathia is the main feature of Pierre Robin sequence. When the micrognathia is severe, the neonate typically has severe upper airway obstruction. Traditionally, these patients would require a tracheotomy and gastrostomy tube. Over the past two decades distraction osteogenesis of the mandible has proven a replacement for a tracheostomy. This chapter describes the surgical technique.
40 Mandibular Distraction Osteogenesis
40.1 Micrognathia and Pierre Robin Sequence
In the simplest terms, micrognathia is defined as a small mandible. When present at birth, etiologies fall into two main categories—from an inherent growth defect or from physical restriction of mandibular growth in utero. Micrognathia is a main feature of Pierre Robin sequence, which is a chain of developmental malformations, one contributing to the next. Pierre Robin sequence begins with micrognathia, which results in backward displacement of the tongue, causing respiratory insufficiency and feeding difficulties. The tongue’s abnormal position may prevent appropriate closure of the palate, causing a cleft palate. 1 Although Pierre Robin officially named the Sequence in 1923, it had been described by many others prior. Lannnelongue and Menard first described infants with Pierre Robin features in 1891, and Fiarbarn first included cleft palate in 1911. 1 , 2 Pierre Robin sequence can be isolated or be part of a named congenital syndrome such as Goldenhar, Treacher-Collins, or Stickler syndrome. 3
Severity of airway obstruction in Pierre Robin sequence correlates with size and retrusion of the mandible. Since upper airway obstruction hinders swallowing, many of these patients also have feeding difficulties. Often, they require placement of a gastronomy tube for sufficient nutrition. Surgical treatment is considered when conservative measures fail to improve obstruction. Surgical options include glossopexy, subperiosteal floor of mouth release, tracheostomy, or mandibular distraction osteogenesis. Success rates for the first two procedures are inconsistent. Until reversed, glossopexy worsens swallowing issues because it restricts tongue mobility. While tracheostomy is the gold standard for treatment of severe airway obstruction, it carries significant long-term morbidity, mortality risk, and overall cost. 4 , 5 Mandibular distraction osteogenesis is gaining popularity as the primary treatment for airway obstruction due to micrognathia.
40.2 Mandibular Distraction Osteogenesis
Mandibular distraction osteogenesis is a surgical technique that involves mandibular osteotomies, placement of mandibular hardware for distraction, and subsequent lengthening of the mandible by gradual separation of the segments. The Law of Tension-Stress Effect governs the principle behind mandibular distraction. Constant but gradual tension and stress on segments of healing bone promote tissue growth and regeneration, while mechanical load and blood supply influence its shape. 6
After proof of concept in animal models, McCarthy introduced mandibular osteotomy followed by distraction in humans as a treatment option for micrognathia in 1992. 7 , 8 Since then, it has gained popularity as a primary surgical option for patients with micrognathia, particularly bilateral mandibular distraction osteogenesis for pediatric patients with Pierre Robin sequence.
Over the past two decades, several advances in the mandibular distraction have been made, including options for either internal or external distractors, development of multidirectional distractors, improved surgical tools allowing for more precise and controlled osteotomies, development of 3D imaging and planning technology, and better data on long-term results. 9 Recent follow-up data has confirmed long-term benefit for patients with craniofacial abnormalities with an acceptable rate of complications. These studies have also provided more information regarding preoperative patient characteristics that may confer better surgical outcomes. 9 – 12
40.3 Preoperative Evaluation and Management
Preoperative evaluation of patients with micrognathia is multifaceted. At our institution, members of a multidisciplinary cleft and craniofacial team work together when evaluating and managing these patients to provide high-quality treatment in all aspects of their care. Consideration of eligibility for surgery is based on probable success of treatment. We deem mandibular distraction osteogenesis completely successful in children with severe airway obstruction when it prevents tracheostomy placement or results in decannulation. Improvement in associated feeding difficulties is often a corollary. Studies have shown different results, mostly depending on patient age and comorbidities. Therefore, the evaluation of neonates will be different than older children.
A number of studies have shown improved outcomes in younger patients, particularly those younger than 3 months. 10 In neonates, distraction is often considered if a micrognathic child has persistent airway obstruction despite conservative treatment. Preliminary airway management includes the following: lateral or prone positioning, nasopharyngeal airway placement, high flow oxygen, noninvasive positive pressure ventilation, and intubation if necessary. Additional management with antireflux medication can be considered. Often additional abnormalities can contribute to the respiratory compromise. One study found ~20% of patients with micrognathia, cleft palate, and glossoptosis had an associated syndrome. 3 Therefore, it is imperative one considers additional syndromes and comorbidities.
Multiple studies have shown poorer outcomes in patients with additional syndromes and comorbidities compared to patients with isolated Pierre Robin sequence. Patients with Pierre Robin sequence as part of a syndrome such as Stickler or Goldenhar consistently had decreased rates of decannulation and increased need for placement of tracheostomy after mandibular distraction osteogenesis. Similar results were found for children with associated neurologic, cardiac, and pulmonary disease. These patients also had increased rates of complications. Thus, performing mandibular distraction osteogenesis on these patients is less straightforward and requires careful consideration of risk-to-benefit ratio. Evaluating for additional sites of airway obstruction with flexible laryngoscopy under sleep conditions is ideal. The jaw thrust maneuver can show potential benefit of mandibular distraction osteogenesis. Unfortunately, this is not possible for many candidates as they are already intubated or their obstruction is so severe that they would not tolerate the procedure.
Prior to making a final decision on treatment, the team should monitor pulse oximetry, and the frequency and degree of oxygen desaturations should be documented. Obtaining an accurate sleep study in a neonate can be challenging and treatment decisions can often be made without a formal sleep study in this age group. 13 If obstructive symptoms are mild or moderate, a sleep study is reasonable to determine additional causes of desaturation such as central sleep apnea. Monitoring capillary carbon dioxide levels can also help determine efficacy of preliminary interventions or help confirm the need to proceed with a more invasive treatment. Typically, persistent carbon dioxide levels greater than 50 milliequivalent units per liter of blood necessitate additional intervention. However, in the setting of cardiac or pulmonary disease a higher carbon dioxide threshold can be accepted.
As aforementioned, there exist other surgical options to consider. A tongue lip adhesion surgery suspends the obstructing base of tongue by suturing the tongue to the anterior lower lip. Tongue lip adhesion is a relatively simple procedure with low surgical risk, but there is controversy as to which patients benefit from it and its negative effects on speech and swallow. Furthermore, critics of the technique contend the benefit of tongue lip adhesion is also achievable with a nasopharyngeal airway. At UC Davis, nasopharyngeal airways are rarely offered in the outpatient setting.
Although feeding difficulty is not a direct reason to proceed with mandibular distraction osteogenesis, neonates will not feed effectively or gain weight appropriately if they are struggling with upper airway obstruction. Evaluation in conjunction with feeding and swallowing specialists, monitoring for oxygenation desaturations while feeding, and monitoring for coughing, sputtering, or startling with feeding can be clues that the airway is not sufficient and is limiting feeding.
In older children, the decision for proceeding with mandibular distraction osteogenesis is often based on a combination of the degree of micrognathia (usually at least 10 mm of maxillary overjet) and severity of the sleep study (severe obstruction sleep apnea with apnea–hypopnea index greater than 10, or frequent oxygen desaturations less than 90%). It is not uncommon for a child to have a tracheotomy placed earlier in life because of upper airway obstruction and to subsequently undergo mandibular distraction osteogenesis to allow for safe repair of a cleft palate or in an attempt to improve the airway sufficiently to allow for decannulation.
There is also the question of whether to use an internal or external distraction device (image/table). External devices have pins that extrude from the mandible to the skin. The main advantages of the newer internal devices are the lack of a large cumbersome external device and scar formation from the pins. 14 However, internal devices require removal of the device in the operating room. Not only is the second incision larger as the device has increased in size with distractions, the fascial planes are distorted after healing from the initial surgery (image). Theoretically, this would increase risk of injury to the marginal mandibular nerve. Additionally, the internal device only has a unidirectional vector of movement and thus placement of screws has very little room for error. Because of this, 3D computerized planning is typically used with the internal device. A copy of the patient’s computerized topography scan with 3-dimensional re-formatting is sent to a company that makes a model of the mandible. The surgeon collaborates with an engineer at the company to determine vector of distractions, osteotomy, and pin sites. The model is created along with distraction guides, all of which are then needed to perform the surgery. This usually takes 3 weeks. Therefore, at our institution, we only offer internal distraction to patients who do not need immediate results. For example, if we are trying to avoid tracheostomy in an infant, then external devices are placed. For children who require distraction for safe palate repair or for mild/moderate airway obstruction, there is time to plan for internal distraction. ▶ Table 40.1 shows internal vs. external device.
40.4 Surgical Technique
The following is the surgical technique employed by the senior author (CS):
Room preparation
Equipment
Distractor, pins, screws.
Cutting guides if using internal device with official 3-dimensional planning.
Saw:
i. Ultrasonic bone saw.
ii. Reciprocating saw.
Straight Osteotomies: 3 to 6 mm.
Malleable retractors.
Soft-tissue instruments:
i. 15 blade:
ii. Needle tip Bovie.
iii. Dissectors and pickups.
iv. Wide double prong retraction needles.
Sterile pencil.
Nerve hook.
Periosteal elevator (Molt #9, freer, Cottle).
Drill.
Discuss airway with anesthesia
Nasotracheal vs. orotracheal intubation:
i. Nasotracheal intubation allows for better monitoring of occlusion and mandibular symmetry during the distraction phase.
Turn bed 180 degrees.
Perioperative antibiotics within 30 minutes of the incision.
No ongoing neuromuscular paralysis during the case.
Determine postoperative airway plan. At our institution, if used for airway obstruction, we leave the patient intubated until appropriate amount of distraction performed (see postoperative section).
Positioning, prepping, and draping
Place a shoulder roll.
Iodine based prep.
Drape to include the following in the field:
Upper and lower lips with a clear adhesive over the mouth.
Ear lobule.
Mastoid process and cervical skin back to the anterior edge of trapezius.
Inferiorly to the clavicles.
The anesthesia circuit can be passed through a sterile circuit bag over the surgical field allowing for easy visualization of the circuit and turning the head from side to side.
Incision planning
Submandibular incision
2 cm below the inferior boarder of the mandible to stay below the marginal mandibular branch of the facial nerve for neonates, 2 finger breadths for older children and adults.
2 to 3 cm long in a skin crease if present.
Extend about 5 mm posterior to the mandibular angle.
Inject local anesthetic and vasoconstrictive agent (0.25% bupivacaine with 1:200,000 epinephrine using appropriate dosing).
It is helpful to mark out both incisions at the same time for symmetry.
Surgical approach to the mandible
Sharply make a skin incision and carry the incision through the subcutaneous fat down to the level of the platysma.
Either sharply or using electrocautery divide the platysma the entire length of the skin incision.
Identify submandibular gland. The marginal mandibular nerve will run in the fascia overlying the submandibular gland. Often the facial vein and artery need to be divided to reflect the fascia off the submandibular gland. Staying deep to the vessels will protect the nerve. It is imperative to not inadvertently transition to a more superficial plane in order to keep the nerve safe.
Maintaining a plane under the submandibular fascia, the pterygomasseteric sling is identified.
Bovie or bipolar cautery is used to divide the pterygomasseteric sling on the inferior boarder. Dividing along the inferior boarder as opposed to the lateral mandibular boarder adds additional protection to the marginal mandibular nerve. A 2 cm opening is usually sufficient, since the opening can often be safely stretched with retractors while preforming the subperiosteal dissection.
Elevate in a subperiosteal plane to expose the outer mandibular angle, inner mandibular angle, sigmoid notch, subcondylar mandible, posterior ½ to 1/3 of the mandibular body, and medial surface of the mandible. The periosteum needs to be circumferentially elevated off the mandible along the planned osteotomy site. For external distractors, a minimum of 1.5 cm of exposed bone should be visible on either side of the osteotomy. Wider exposure is required if cutting templates or an internal distractor will be used. For internal distractors, enough elevation should be done to allow for the entire footplate to sit directly on mandibular bone.
Close attention to the medial surface of the mandible often will reveal the location where the inferior alveolar nerve enters the mandible just lateral to the lingula.
Design the osteotomy and mark it with a pencil. Confirm there is adequate subperiosteal dissection. Osteotomies can be angulated, reversed L-shaped, or vertically oriented. Preoperative imaging can be used to create cutting guides or to help determine the ideal locations for the osteotomies. The senior author prefers an angulated, cortical osteotomy from the inner angle to just anterior to the outer angle in neonates and children. When the mandible is larger, a through-and-through reversed osteotomy gives best results. Distraction works best when the osteotomy creates a large surface area for healing. The ramus is relatively thin in neonates and young children.
An angulated osteotomy (▶ Fig. 40.1) extends from the inner mandibular angle. The osteotomy can overlap with the posterior tooth bud, by design the majority of the bud should remain in the anterior segment. With distraction, the buds should move with the anterior segment. It allows for multidirectional distraction. The final part of the osteotomy is completed by fracturing the bone to prevent injury to the inferior alveolar nerve (see Step 7). A criticism of this osteotomy is that after distraction is complete, the mandibular angle is not well defined.
A reversed L-shaped osteotomy (▶ Fig. 40.2) is through and through, and it starts with its horizontal segment above the lingual nerve and extends posteriorly. Then it turns inferiorly for its vertical segment after the lingua nerve and tooth buds are passed. The degree of angulation at the turn can be as much as 90 degrees, but in younger patients a less acute turn is used (i.e., 30 degrees) to help maintain better posterior bone stock. L-shaped osteotomies are more technically demanding and difficult in neonates.
The senior author does not perform vertical osteotomies.
Distractor placement: The distractors need to be positioned before the osteotomies are completed in order to optimally control their orientation.
External distractors
Pin placement
i. Pin site choice is important (▶ Fig. 40.3). The pins should be at least 0.5 mm from the osteotomy site. Ideally, they are placed in thick bone just posterior to the inferior alveolar nerve and at the inferior borders of the mandible. With distraction, the pins will migrate with the direction of the vector of distraction. The posterior pins will migrate posteriorly and the anterior pins anteriorly. However, since the inferior border of the mandible is solid thick bone, it is unlikely that they will erode through the bone during migration. The posterior pins should be placed just posterior to the course of V3 in the subcondylar buttress about 8 mm from the posterior border of the mandible. If preoperative imaging is available, the course of the inferior alveolar nerve can be approximated to help determine pin site placement that will not violate the nerve intraoperatively or via pin migration during distraction.
ii. Place posterior pins first. The ideal orientation for the pins is perpendicular to the midsagittal plane. This orientation can be difficult to accurately identify and therefore we find it more reliable to place the pins perpendicular to the plane of the ascending ramus. Placing the pins perpendicular to the body will create an angulated vector and cause binding and tension on the pins (▶ Fig. 40.4).
iii. A small hemostat is bluntly passed from internally and a small skin incision is made. The path is stretched by opening the hemostat. The hemostat tines are placed into the hole of the trocar which guides the trocar through the soft tissue.
iv. Using the trocar to maintain correct orientation, a drill is used to make a shallow cortical opening. Without the cortical opening, the self-drilling pins often create small microfractures.
v. A self-drilling pin is then placed bi-cortically through the trocar, taking care to maintain perpendicular orientation in both the superior–inferior and anterior–posterior planes. It is helpful to have a second observer confirm correct orientation. A malleable can be placed medial to the mandible during pin placement for protection and for tactile feedback. The posterior pins are threaded for 10 mm, while the anterior pins need to be threaded about 15 mm.
The multidirectional distractor is adjusted, and then positioned on the pins in the intended orientation and position. Make sure the distractor driver is positioned along the distraction arm in a way that allows for sufficient subsequent distraction (i.e., the distractor driver can inadvertently be positioned at the completion end of the arm leaving no additional room for distraction).
Place the distractor on the pins in the intended final position and mark the pins with a surgical site marker.
Remove the distractor and complete the osteotomies (see below for details on completing osteotomies).
Internal distractors
Determine the desired distraction vector—usually, parallel to or in a plane slightly inferior to the plane of the inferior border of the mandible is chosen.
During 3D computer planning:
i. Determine the intended orientation of the distraction arm. Anteriorly positioned distractor arms will leave a more visible anterior scar but are further away from the facial nerve. Posteriorly positioned distraction arms are preferred unless the vector and osteotomy would irritate the auricle.
ii. Determine screw location and length. Ideally, four screws locations on each plate are available in case one location is not useable intraoperatively.
iii. The distractor footplates are customized by the surgeon. The inferior set of screw holes is typically removed. Using the 3D model, the footplates are pre-bent to exactly fit the mandible.
Confirm there is adequate mandibular periosteal dissection for the distractor footplate
The guide is placed over the mandible and two screw locations are marked by a monocortical drill hole.
Preform the osteotomies. Preoperatively osteotomy guides can be created to help avoid the inferior alveolar nerve, tooth buds, and allow for precise internal distractor positioning.
Make the cortical osteotomy with the ultrasonic bone or reciprocating saw:
At the superior and inferior border of the mandible, it is through both cortices.
Over the tooth buds and the inferior alveolar nerve, it is monocortical on both the lateral and medial surface. Because of exposure, the entire medial cortical osteotomy is often not feasible (▶ Fig. 40.5 and ▶ Fig. 40.6).
A straight osteotome is wedged into the osteotomy site and gently twisted to break the cancellous bone. Care should be taken to not fracture cortical bone while twisting the osteotome. If it appears that adjacent cortical bone is beginning to fracture, one should confirm the cortical osteotomies are complete. Often there is additional cortical bone along the superior and/or inferior border of the mandible that needs to be cut. This final portion of the cortical osteotomy can be finished with the piezo-electric saw or sharply with the osteotome.
The mandible should be freely mobile at the osteotomy site. It is important to verify a complete fracture, as a greenstick fracture will not distract.
A right angle nerve hook can be used to confirm integrity of the inferior alveolar nerve.
Occasionally, there are tooth buds visible in the osteotomy line. If upon widening the osteotomy they stay in the ramus segment, they can be pushed anteriorly into the body to keep them in a more anatomic location after distraction.
Place the distractor
External distractor
The distractor is placed at the marked lines on the pins.
Activate the distractor and confirm there is uniform distraction along the osteotomy.
Close the osteotomy gap back down to 1 mm.
Internal distractor
Place the distractor across the osteotomy by loosely placing the previously marked screws. Typically, a 3 to 7 mm hex head screws are used depending on the proximity of tooth buds or the inferior alveolar nerve.
Ideally the foot plates are secured with three screws per plate.
Activate the distractor and confirm there is uniform distraction along the osteotomy. Uneven distraction suggests a greenstick fracture.
Close the osteotomy gap back down to 1 mm.
Place a multidirectional elbow and distraction arm. The distraction arm can exit through the incision, but we prefer a separate stab site. Ideally the arm is positioned to allow for the elbow to be completely under soft tissues, so after distraction is completed and the distraction arm is removed, no additional hardware is exposed.
Adjust the distractor pin to ensure nonreversing ratcheting.
Close the incisions in multiple layers
The pterygomasseteric sling does not necessarily need to be closed.
Close the platysmal layer.
Close the skin with deep dermal sutures and a subcuticular suture followed by skin glue.
A drain is not usually needed for external distractors. If needed for internal distractors, a quarter inch Penrose drain is preferred.
Dressing
Antibiotic ointment and gauze is placed around the pin sites or distractor arm followed by xeroform gauze for external distractors (▶ Fig. 40.7).
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