Introduction

A primary report about lengthening a fractured or shortened leg with traction, comes from the time of the ancient Greek physician Hippocrates (Peltier 1990). However, only relatively recently, the Russian orthopedic surgeon Gavriil Abramovich Ilizarov developed a successful method to lengthen the lower extremities (fibulas and tibias) by performing an osteotomy, which is followed by gradual dissociation of the bone sides by external fixation (Ilizarov et al. 1969). The efficacy of the method of distraction osteogenesis (DO) which since then has been scientifically proved (Ilizarov et al. 1980), gained popularity and gave the idea to plastic and maxillofacial surgeons to apply the same principles in cases of mandibular deficiencies (McCarthy et al. 1992). McCarthy was the first surgeon who, following extensive experimental studies to the jaws of dogs, applied an extraoral device to lengthen a malformed mandible. Since then DO has been established as an early treatment of asymmetry in the maxillofacial region, mainly in cases of syndromes (Diner et al. 1996; Moore et al. 1994; Wangerin and Gropp 1994, 1999). DO presents important advantages compared to conventional grafting surgery; especially in mandibular asymmetries, it can safely be applied early in life, in neonates, infants and young children, achieving a significant elongation of the bone usually up to 2.5 cm or even up to 5.0 cm (Rossini et al. 2016), without jeopardizing nerve vitality; new bone develops without the need of a donor site for bone autograft, operation that has a relative morbidity.

Biological Basis

The biological procedure in distraction osteogenesis, that leads to bone production and lengthening, is similar to what follows in case of bone fracture, incidental, or surgical as in corrective osteotomies. Bone formation starts right after osteotomy and stabilization of the distraction device (distractor). A hematoma is formed at the osteotomy region and inflammatory cells and infiltrates arrive there due to the presence of cytokines IL‐1 and IL‐6. In the beginning, an osteoclastic activity is observed at the osteotomy edges and then pluripotent cells create new tissue and new vessels (angiogenesis). Several chemical agents and particularly transforming growth factors, as TGF‐β1, are found in increased quantities, which seem to play a major role in creating collagen and non‐collagenous matrix. Mineralization and remodeling follows, ending up to new bone formation (Singh et al. 2016).

Gradual distancing of the bony edges at the osteotomy site does not seem to influence the final ossification result of the produced gap, provided that the distractor offers stability to the osteotomized bony edges.

Staging of Distraction Osteogenesis

DO is a lengthy process, performed in four stages. The first stage refers to the operation, under general anesthesia. Following surgical procedures, the osteotomy of deficient bone is performed and the distractor is fixed in place. A latency phase usually lasting 1–3 days follows for soft tissue wound healing and callus formation procedure.

The second stage is the activation of the device, which is initiated by the gradual widening, 1 mm per day, of the gap created between the bone fragments. There is no need for any type of anesthesia during this phase.

The third stage is a consolidation phase which is initiated once the distracted bone has reached the preselected length and usually lasts twice as long as the activation phase. In maxillary cases, this phase can last up to 1 year.

There is also a short‐lasting fourth stage, where the distraction device is removed; it is performed under general anesthesia in children under 14 years of age.

Indications for Distraction Osteogenesis

DO has a wide range of applications in maxillofacial region, such as in cases of asymmetries associated with syndromes, trauma, rheumatoid arthritis, or tumor resection, as well as in pre‐implant surgery. DO is most often applied in children, taking additional advantage of the fast‐growing process at this period of life, offering both esthetic and functional improvement to young patients.

Mandibular DO most often associated with airway obstruction, is applied in cases of severe retrognathia, micrognathia, mandibular asymmetry, craniofacial syndromes such as Goldenhar, Treacher Collins and Nager syndrome, hemifacial microsomia and P. Robin sequence. As reported in the literature, DO was initially used in children for mandibular elongation and since then has been most often applied in this region, compared to maxillary or mid‐facial procedures (Swennen et al. 2001); long‐term follow‐up results of early mandibular DO cases have been reported and discussed (Ow and Cheung 2008). Findings showed a degree of relapse and the return of asymmetry with growth. In these cases, repetition of DO has been suggested (Ascenceo et al. 2014; Meazzini et al. 2012; Nagy et al. 2009).

Main indications for maxillary DO include moderate and major dentofacial deformities, severe maxillary deficiencies of various etiologies, such as syndromic children, with sleep apnea, or with cleft palate as well as cases with failure of previous attempts to solve those problems. Maxillary DO was first reported in 1999; external distractor was applied in the beginning and later an internal device was also introduced (Figueroa et al. 1999; Figueroa and Polley 2007). In cases of maxillary/mid‐facial distraction, the main aim is a gradual advancement of the maxilla or the frontomaxillary complex in the posterior–anterior direction. Frontofacial monobloc advancement, in cases of craniosynostoses, has also been reported (Marchac and Arnaud 2012).

Early DO contributes significantly to the relief of upper airway obstruction, due to the posterior position of the mandible or the maxilla, especially in cases of syndromic neonates or infants with severe retrognathia or micrognathia (Shand 2015). In these cases, the tongue not having enough space, takes a posterior position resulting in partial obstruction and early tracheostomy can become necessary. Further treatment with DO will elongate the mandible and will create space for the tongue thus providing a real functional rehabilitation. Mandibular DO is considered as the most indicated therapy to eliminate upper airway obstruction in babies to achieve their decannulation, if present (i.e. removal of their tracheostomy), and relieve them from problems associated with feeding, sleep apnea, and snoring (Iatrou et al. 2010). According to a relatively recent systematic review, obstructive sleep apnea in cases of retrognathia, treated with DO, was successful in 90–100% of the cases (Tsui et al. 2016). Especially in cleft patients, where maxillary advancement with DO has been achieved, a relatively high possibility of skeletal relapse has been reported, indicating the need for overcorrection; the latter is required mostly in cases of incorrect primary procedures and scar formation (Liu and Zhou 2018). Fronto‐maxillary distraction can be selected in synostosis patients; in these cases, brain damage is prevented, the upper airway space is enlarged, exophthalmia is corrected, and the face improves esthetically.

DO can be selected to correct mandibular defects caused by severe trauma or tumor resection in children. In addition, cases of temporomandibular joint ankylosis or those of stable juvenile rheumatoid arthritis, leading to asymmetrical development of the mandible, can be improved with DO.

The Distractors and Their Function

Nowadays the distractors are titanium made. They are fabricated in different types, sizes, and shapes to fulfill the previously described needs. Distractors are mainly categorized to those for mandibular lengthening, and those specialized for the maxillary and/or midface and upper‐face advancement. The devices are screw or pin retained. Their mechanism, depending on the type of distractor, is either internal with plates and monocortical screws or external with pins; screws or pins, most often three on each osteotomy site, are fixed on each side of the osteotomized bone, allowing the distraction osteogenesis.

The internal devices consist of an expanding mechanism, two malleable plates, one at each side of the mechanism, and a rod within a cylinder which remains approachable postoperatively. The plates, regular, L or T in shape, are scheduled for at least three monocortical screws each, for infants up to 1 year old and for 4–6 screws for older children. The screw length should not exceed 3–5 mm, to protect teeth buds or apices and the inferior mandibular neurovascular bundle. The screws, in difficult to be approached sites as the mandibular angle, can be inserted with special, long, and angulated handpiece. By manually rotating the rod with the screwdriver provided, the device is activated and space is created between the plates and consequently between the edges of the osteotomized bone.

• Mandible: Both types of distractors are used for mandibular DO, with internal devices being the first choice; they are inserted and fixed on the buccal mandibular surface intraorally or via a submandibular approach. Depending on the approach, the bar is located intraorally or extraorally. For external distraction, pins are inserted percutaneously and secured in the mandible on one end and the other end is connected to the extraoral distractor. In any case, distractors for mandibular DO are placed either unilaterally in cases of facial hemi‐microsomia or bilaterally in cases of mandibular micrognathia.
• Maxilla and midface: Maxillary DO is performed most often with external distractors and more recently with internal ones. For external distraction, the concept being to advance the maxilla, one part of the distractor is fixed to the maxilla (to the teeth or to the bone), and the other part to the cranium. During operation, an external metallic wreath is stabilized horizontally with 2–6 screws, on each lateral side of the cranium, 3–4 cm over the ear helix, where the bone is thick enough. The tooth‐borne component, in cases of permanent dentition, has been preoperatively adapted by the orthodontist; in cases of mixed dentition, a bone‐borne component is intraoperatively adapted to the maxilla. After performing the osteotomy (in LeFort I, II, or even III level), the cranial part of the distractor relates to the maxillary part with external components and shafts which are scheduled for gradual distraction. Upon activation, the osteotomized maxilla moves forward up to the pre‐selected point. The same procedure can be applied in cases of frontofacial monobloc advancement. Internal devices for anterior maxillary advancement are based on the same concept and similar devices, as those for mandibular distraction. Recent developments in maxillary distractors include an antirelapse ratchet, hooks on the plates for additional use of guiding elastics, and a swivel joint, to facilitate the cranial adaptation of the distractor and allow compensation of exercised undesirable forces.

More recent publications point out the advantages of internal distractors; they are better tolerated by the patients in comparison to the external ones, which may become intimidating and cumbersome to both patients and clinicians (Combs and Harshbarger 2014).

Resorbable distractors have been introduced for mandibular DO (Burstein 2008); nevertheless, by the experience acquired from resorbable osteosynthesis plates, they probably are not stable and strong enough for DO, compared to metallic ones.

Preoperative Patient Evaluation

A well‐organized pediatric maxillofacial surgery unit in a pediatric hospital, with the special armamentarium required for DO, as well as access to the hospital Intensive Care Unit, are the prerequisites to proceed to an osteogenesis distraction operation. For better results, a team collaboration approach between experts is advocated including pediatrician, pediatric maxillofacial surgeon, orthodontist, and pediatric dentist. The operation is performed under general anesthesia.

Patients’ selection needs to be careful, as there are several general, anesthesiologic, and surgical limitations. As in every scheduled operation, a thorough preparation is required, including medical and dental history followed by clinical and laboratory findings.

Diagnostic imaging with panoramic X‐rays (in children over 4 years of age), cone beam computer tomograms (CBCTs), and CTs with three‐dimensional (3D