Maxillomandibular distraction osteogenesis in orthodontics

Introduction

Distraction osteogenesis (DO) is a biological process and a technique to stimulate the new bone formation between vascularised margins of bone segments created by osteotomy and gradually separated by incremental traction. The application of traction forces induces tension in the callus that bridges the bone segments. This tension stimulates bone growth. Similarly, distraction forces create tension in various soft tissues, such as muscles, fascia, tendons, ligaments, blood vessels, nerves, mucosa, skin and gingiva, causing soft tissue changes. These changes are thus appropriately termed as ‘distraction histiogenesis’. The process of DO follows the creation of osteotomy cuts at safe anatomical locations, and placement of distraction devices. After a latency period, distraction is initiated, followed by a period of no distraction, and finally, a remodelling process at the site of bone lengthening occurs. Table 81.1 and Fig. 81.1 depict the terms used in the DO course of events and the step-by-step procedure. The DO is indicated in multiple congenital anomalies and acquired defects of the maxilla and the mandible ( Table 81.2 and Fig. 81.2 ). The hypoplastic cleft maxilla and severely retarded growth of the mandible due to temporomandibular joint (TMJ) ankylosis and Pierre Robin syndrome are major frequent indications.

TABLE 81.1

Biological basis and steps in the distraction protocol

Osteotomy	Osteotomy is a surgical sectioning of the bone at a preferred anatomical location. Osteotomy for DO should be done by controlled microtrauma, aiming for maximum preservation of the periosteum and endosteum. The periosteum is the most viable structure for successful bone regeneration. Each bone surface that is being distracted must contain viable osteocytes in the bony lacunae with an intact blood supply to initiate and perpetuate the DO process.
Latency period	Latency is the period of initial healing after which distraction is inodiated. If the distraction of the soft callus is not initiated, it will follow the usual sequence of fracture healing: hard callus formation followed by calcification. Initial latency period recommended is between 5 and 7 days.
Distraction period	The distraction rate is usually of 1 mm/day. A slower rate of 0.5 mm is recommended for transverse maxillary distraction. The histological sequence during the latency period is like that seen during fracture healing. It is characterised by inflammation and the formation of soft callus. The initial clotting around the osteotomised segments is converted into granulation tissue in 3 days. The granulation tissue is marked by inflammatory tissue and fibroblasts that become more fibrous as the days progress due to the presence of collagen and the appearance of new capillaries. It is at this early stage the mesenchymal bone cells from the adjacent medullary bone and periosteum are recruited. The collection of cells and biological fluids around is also referred to as regenerate . During DO, the usual process of fracture healing is interrupted by the application of gradual traction to the soft callus. A dynamic microenvironment is created that encourages new tissue formation in a direction parallel to the vector of traction. Mechanical tension is one of the primary factors responsible for enhanced angiogenesis during natural growth and development. As distraction begins, the fibres of the soft callus and the spindle-shaped fibroblasts-like cells located between the collagen fibres become oriented longitudinally along the axis of distraction. These cells form collagen fibrils that are grouped into fibres at the distal and proximal ends of the inter-fragmentary tissues. During distraction, there is increased and prolonged angiogenesis with the proliferation of spindle-shaped fibroblast-like cells. These spindle-shaped cells are situated peripherally and throughout the vessels, producing more and more collagen parallel to the distraction vector. The collagen produced is type I, and the bone formed is intramembranous type. The cells express osteocalcin, osteopontin and alkaline phosphatase, indicating osteoblastic differentiation. The application of tension through distraction on the regenerate stimulates the transdifferentiation of chondroblasts and fibroblasts into osteoblasts. The tension on the regenerate results in chondroblasts expressing type I collagen instead of type II collagen. Vascular growth during distraction is tenfold compared to conventional repair. Distraction period lasts for 10–15 days (1 mm/day) with a total of 10–15 mm distraction.
Consolidation phase	During this phase, regenerate converts into bone by the process of primary mineralisation that begins at either end of the fibrous central zone and advances to the central part, making a bridge of immature bone across the distraction segments. The greater the distance of distraction, the longer the duration of the consolidation phase. The duration of the consolidation phase has been suggested as 3–8 weeks for children and 8–14 weeks for adults for the maxillofacial skeleton. In general, the period of consolidation should be twice the duration of the distraction phase.
Remodelling	The remodelling period is the time from the application of full functional loading to the complete remodelling of the newly formed bone. It takes a year or more before the structure of newly formed hard tissue is comparable to that of the pre-existing bone.

Image, AltText currently not available — Figure 81.1

TABLE 81.2

Indications and contraindications of DO in the facial region

S. no.	Indications		Contraindications There are no absolute contraindications for distraction. Relative contraindications are:
S. no.	Congenital disabilities and acquired conditions	Congenital disabilities and acquired conditions
1.	Pierre Robin sequence. DO is required to prevent asphyxia and to correct the mandibular deficiency.	Post-traumatic growth disturbance of the mandible. Mandibular hypoplasia due to TMJ ankylosis ( Fig. 81.2 )	Poor nutrition and lack of soft tissue
2.	Treacher Collins syndrome and Goldenhar syndrome.	Malunion of fracture	Inadequate bone stock, as in neonates
3.	Craniofacial microsomia: unilateral/bilateral.	Atrophy of edentulous segments	Geriatric patients due to a decreased number of mesenchymal stem cells
4.	Congenital micrognathia (non-syndromic).	Oncologic mandibular osseous defects	Irradiated bone
5.	Skeletal facial asymmetry.	Rapid canine distraction	Osteoporotic bone
6.	Midface hypoplasia.	Distraction for ankylosed teeth	Systemic disease which affects bone metabolism
7.	Severely constricted maxilla in adults.	Maxillary deficiency in operated cases of cleft lip and palate	Systematic conditions that contraindicate general anaesthesia (GA)
8.	Severely constricted mandible in children and adults.	Obstructive sleep apnoea (OSA) syndrome due to micrognathia or maxillary hypoplasia
9.	Mandibular distraction is used for the lengthening of the mandible where due to loss of bone substance or unavailability of bone contraindicates orthognathic surgery. In situations where a large amount of bone lengthening is required, distraction is the preferred therapeutic modality over orthognathic surgery.

Although orthognathic surgery (OGS) is now a well-established discipline, it has several limitations in managing severe deformities in those situations where the volume and amount of bone-associated soft tissues are insufficient. In such cases, OGS produced less than optimum outcomes with greater chances of relapse. Inadequate bone contact, insufficient fixation, stability and partial or total regression due to excessive muscle stretching were often observed. DO is a viable option for large movements and clinical conditions where OGS is not possible. The relative advantages and disadvantages of the DO are summarised in Table 81.3 .

TABLE 81.3

Advantages and disadvantages of distraction osteogenesis over orthognathic surgery

S. no.	Advantages	Disadvantages
1.	The need for orthognathic surgery is minimised, and so are the complications associated with orthognathic surgery.	Poor 3D control on the segments with current distractors. The 3D distractors are being continuously modified for desired results.
2.	A shorter hospital stay. Reduced postoperative pain and swelling compared to orthognathic surgery.	Vector control is important for the success of DO. Intraoral DO devices offer poor control in vector management as compared to extraoral devices, whereas splints in orthognathic surgery offer optimised bone positions during the surgery.
3.	A significant maxillomandibular advancement is possible compared to orthognathic surgery.	DO cannot provide movements for the setback or compression of bone.
4.	Increased stability. A gradual bone lengthening allows soft tissue adaptation in form and function, and therefore, muscle pull causing relapse in orthognathic surgery can be avoided in distraction osteogenesis.	Multiple segmentation of bone for correction is not possible as it will make distraction segments unstable.
5.	The new bone formed via DO is more native and permits orthodontic tooth movement.	Manipulation of the corticotomy site once or several times a day could give rise to pain.
6.	Allows complete bone sculpting that changes the shape and form of bones to maximise the 3D structural, functional and aesthetic needs of the patient.	Difficult access for the orthodontist during distraction and consolidation stages as the distractor could obscure the buccal segments.
7.	In paediatric patients, severe cases of mandibular deficiency, maxillary hypoplasia and OSA can be treated early.	Damage to TMJ due to incorrect vector orientation.
8.	Reduced need for inter-maxillary fixation.	Equipment-dependent operation.
9.	No bone graft is required, unlike conventional orthognathic surgery, thus eliminating donor site morbidity.	Technique-sensitive surgery.
10.	The need for space creation through extraction or inter proximal reduction (IPR) in transverse discrepancy is not required in DO.	Need for a second surgery to remove distraction devices.
11.	A lower likelihood of nerve injury.	Poor oral hygiene and plaque control in intraoral distraction.
12.	A less likelihood of idiopathic condylar resorption.

Distraction osteogenesis versus histiogenesis

DO osseous regeneration is associated with a slow separation and remodelling of osteotomised bone segments and simultaneous stretching and adaptation of soft tissues, including blood vessels, nerves, ligaments, fascia, muscles, skin, mucosa, cartilage and periosteum. The tension generated during distraction causes soft tissue to adapt to the new position of the underlying bone; therefore, it is more aptly called distraction histiogenesis. Distraction histiogenesis shares many characteristics with embryonic growth, foetal development and the formation of limbs in neonates. However, the specific cellular and molecular mechanisms underlying distraction histiogenesis still need to be fully understood. The reduced tendency for relapse in DO compared to conventional OGS is thought to be linked to this phenomenon of distraction histiogenesis.

Bouletreau and coworkers have shown that several factors, including cytokines and extracellular matrix proteins, are involved in the process of mineralisation of regenerate in the distraction gap. Messenger RNA and protein expressions of the regulatory factors fluctuate at different stages of distraction. Newly formed bone during distraction can benefit from drugs and physical stimulation to enhance bone quality and accelerate the healing process. Various techniques have been tried, including the use of bone marrow–derived mesenchymal stromal cells, growth factors like recombinant human bone morphogenetic protein (BMP) and osteoinductive growth factors to promote bone formation. Physical stimulation techniques like photonic stimuli, such as low-level laser therapy using Ga-Al-As laser, electromagnetic stimulation, electrical stimulation, such as extracorporeal shock wave therapy, direct current electrical waves, and mechanical stimulation methods such as low-intensity pulsed ultrasound (LIPUS) have also been found to enhance bone cell activities and bone formation. Electrical and ultrasound stimulation of the regenerate have been found to have a stimulatory effect on osteogenesis and improve bone quality.

Historical perspective and philosophy of maxillomandibular distraction osteogenesis

DO has revolutionised the management of severe maxillomandibular deformities. Integration and amalgamation of various philosophies like extraoral orthodontic appliances, orthodontic expansion with jackscrews, craniofacial osteotomies, skeletal fixation methods, limb lengthening procedures and appropriate applications for use on face and jaws have resulted in the evolution of contemporary craniofacial DO devices and protocols.

However, applying tensile and compressive forces to the skeleton has been introduced previously. An attempt at skeletal traction was described as long ago as 460–377 BC during the Hippocrates era when traction on long bones was performed using rubber straps. In the 18th and 19th centuries, mechanical forces were used to separate the bones of the maxilla at the mid-palatal suture. In 1905, Codivilla introduced a surgical procedure for lengthening the lower limbs, but this procedure had many complications.

The most path-breaking work in the field of DO is credited to Dr Gavril IIizarov (1951) of Russia. He developed DO for limb lengthening based on bone biology and surrounding tissues’ ability to regenerate under tension. He performed corticotomy, which is a surgical bone division with maximum preservation of the periosteum and the endosteum. His distraction protocol utilised a 5–7 days latency period, and the bone segments were separated at 1 mm/day. Following distraction, a consolidation period is commenced, during which the tissue in the distraction gap gets mineralised and remodelled.

Distraction in craniofacial region

Successful use of DO in the field of orthopaedics and the associated development of armamentarium was noticed by maxillofacial surgeons, and experimentation on the craniofacial region began. A miniaturised orthopaedic device was successfully used by J.G. McCarthy et al. in four children (average age of 78 months) with craniofacial anomalies. The amount of mandibular lengthening ranged from 18 to 24 mm. Later, McCarthy’s group developed a multidirectional mandibular distractor, which enabled the correction of defects in all three planes of space. Their team has continued to report work related to distraction and is considered highly esteemed in this field. Inspite of advances, patients were apprehensive about the placement of extraoral distracters as these are socially inconvenient and often result in permanent facial scars. This led to the evolution of intraoral distraction devices.

Development of intraoral distractor

K. Wangerin, in the late 1990s, developed the first intraoral titanium mandibular distraction device. The device consisted of miniplates for bone fixation connected by a distraction cylinder. Intraoral devices significantly improved mandibular distraction techniques. The main advantages are the absence of extraoral scars and the inconspicuous nature of the device. Dessner and group demonstrated an intraoral, tooth-borne distractor for lengthening the mandible. They are available in five different designs and are used as per the required clinical situations. Commercially known as ROD distractor. The designs include tooth-borne, bone-borne and hybrid models. The ROD distractor can be utilised both for maxillary and mandibular advancement. They are also available as pre-programmed devices and have been reported to be successful.

The distractor designs continued to evolve with changes in size, shape, modes of attachments and so on. Some manufacturers developed universal distraction systems that could be applied to different parts of the craniofacial skeleton, while others developed specific devices based on anatomic locations. Most of the intraoral distractor systems are unidirectional. Lately, a three-dimensional (3D) intraoral distractor has been developed. Other developments in the hardware include curvilinear, motorised and hydraulic distraction devices. The latter two are characterised by remote activation, monitoring and precise directional control. Reports on successful intraoral distraction devices for mandibular lengthening with reasonably successful outcomes are now available.

Classification of distraction osteogenesis

A comprehensive coded classification of DO was devised to provide information about the DO procedure, including the jaws, site of osteotomy, side, type of distractor and vectors involved through a distraction osteogenesis coding (DOG Code).

An example of a coding: Mn (R, EO): II-B would mean the distraction would be done in the mandible on the right side with an extraoral distractor with biplanar vectors. The site of osteotomy will be the angle of the mandible.

Distraction devices are categorised as external or internal based on their location and usage, including the vectors they provide.

Based on location in craniofacial region

External distraction devices
•
Internal distraction devices

Maxillary rigid external distractor

Rigid external distractor (RED) is used for mid-face advancement ( Fig. 81.3.i ). It consists of a halo frame fixed to the cranium with titanium screws. A mid-sagittal rod with distraction cylinders is suspended from the halo frame. The distraction is usually set at the level of the centre of mass of the osteotomised maxilla. The osteotomised maxilla is connected to the distraction cylinders with an intraoral splint and stainless steel ligature wires. Titanium mini bone plates at the level of the centre of resistance (C _Res) of the maxilla can be used in place of the intraoral splint. The distraction cylinder is activated with the help of an Allen key provided by the manufacturer. Activation is performed in the counter clockwise direction.

RED was first experimented on animal models before its use in clinical practice. Rachmiel and coworkers in the late 1990s successfully performed mid-face gradual advancement. During the same years, Polley, Figueroa and colleagues used RED, an external fixed cranial halo, to distract the mid-face. They continued to use and improve the techniques and report the findings on patients with mid-face deficiency syndrome and cleft patients.

Mandibular external distractor : They are placed using bone pins. They are much easier to set and simpler to replace during distraction if necessary or to remove at the completion of bone lengthening ( Fig. 81.3.ii ). The disadvantages of extraoral distractors include skin scarring and poor acceptance by the patient. The external distractors for the mandible have virtually been replaced with intraoral distractors. However, on the maxilla, external distraction devices like the RED appliance have been successful.

Internal distractors

They are placed either sub-mucosally (buried) or extramucosal (intraoral). They may be tooth-borne, bone-borne or hybrid. Internal devices have a unique advantage of intraoral placement; they do not produce facial scarring and hence do not have the negative psychosocial impact of the external devices ( Figs 81.4 and 81.5 ).

Mandibular intraoral distractor : The most commonly used distraction device is the intraoral titanium mandibular distraction device. The design varies for a horizontal distraction and vertical distraction ( Fig. 81.4 ). These are implantable devices intended for single use only. The device consists of miniplates which enable bone fixation with miniscrew implants. The most important part of the distraction device is the distraction cylinder. The distractor cylinder is activated with the help of an Allen key provided by the manufacturer. The activation is performed by counter clockwise turns. A complete rotation corresponds to a distraction of 0.5 mm.
Modular internal distractors (MID) : They are used for mid-face advancement but are not popular as they need better vector control. This design is a universally adaptable distractor comprising adaptable titanium mesh plates and a flexible activation cable.
Customised tooth-borne distractors : The orthodontist can develop and fabricate a tooth-borne distractor for maxillomandibular distraction in the transverse plane. Unidirectional micro screws can be incorporated into a custom-orthodontic appliance for the distraction of ankylosed teeth or rapid canine distraction.
Alveolar distractor : These distractors are primarily used to increase the height of the alveolar ridge ( Fig. 81.5 ). It is a contemporary pre-prosthetic surgical procedure for correcting grossly resorbed edentulous alveolar ridge. Dental implants can be optimally placed after alveolar distraction.

Distraction devices based on vector

Uniplanar and multiplanar devices : Uniplanar devices accomplish the distraction in a single direction, while multiplanar devices permit distraction in more than one plane using separate distraction arms. The type of deformity dictates the type of distractor. The uniplanar device is appropriate if the skeletal deficiency is mainly in a single plane. However, multiplanar devices or separate distractors are placed sequentially for complex deformities requiring distraction in more than one plane. The distractors are available commercially, along with tools required for their fixation and activation. An extraoral device is more versatile. It can accomplish a greater magnitude of distraction in multiple planes and permit changes in the vector of distraction as it is being carried out. However, such a device is cumbersome and leaves pin-track scars on the skin. The intraoral appliance, in contrast, is inconspicuous and does not interfere with day-to-day activities, making the treatment more acceptable to the patient.

Planning distraction osteogenesis for the craniofacial region

The orthodontist has a vital role to play in the diagnosis and treatment planning after conducting a comprehensive clinical examination of the face and oral structures. With 3D digital surgical planning, the procedure can be planned with precision, and the outcome can be visualised. A 3D virtual planning software can be immensely helpful in considering vectors of the distraction and its effect on occlusion and treatment outcome. The surgeon and orthodontist must plan for the exact location of surgical cuts and placement of the distractor device and predict treatment outcomes.

Orthodontic considerations during treatment planning

The following points are considered necessary while planning a case for DO:

1.
Records: Complete records (pre-, mid- and post-treatment) are essential. The cone beam computed tomography (CBCT) has virtually replaced the need for all other X-rays. The decision to undergo CBCT should be made in consultation with the surgeon to avoid duplication of the orthopantomogram (OPG), cephalogram and additional X-rays.
2.
Treatment planning sessions: Joint sessions between the orthodontist and the team of surgeons are advisable.
3.
When planning for mandibular lengthening, it is essential to consider the nature and type of advancement involved.
- a.
  Unilateral versus bilateral
- b.
  The vector of distraction: vertical versus horizontal versus oblique
- c.
  The site of distraction: corpus versus ramus
4.
Maxillary advancement in hypoplasia: sagittal, sagittal combined with transverse.
5.
Maxillary advancement in cleft lip and palate patients: velopharyngeal considerations/speech considerations are of paramount importance.

Orthodontic and distraction protocol

•
Pre-distraction orthodontics
•
Osteotomy
•
Distraction protocol
•
Orthodontics during the distraction and consolidation phase
•
Post-distraction orthodontics
•
Retention

Pre-distraction orthodontics

This involves preparing the upper and lower arches by levelling and aligning, decompensation and correction of the curve of Spee. The teeth should be moved to such positions about the basal bone so that existing dental compensations do not compromise a perfect maxillomandibular relationship.

The standard distraction orthodontic treatment protocol includes appropriate transverse arch width coordination of both maxillary and mandibular arches followed by placement of passive rectangular arch wires with hooks for engaging inter-arch elastics during and after distraction.

Before DO, the orthodontist should jointly plan the objectives with the surgeons to arrive at a consensus on the type and extent of orthodontic tooth movement and desired vector(s) of DO. The osteotomy design, distractor device orientation, masticatory muscles influence, occlusal interferences and orthodontic forces during the distraction and consolidation phases govern and affect the vectors of distraction. In the case of DO in children and young adolescents, future growth prediction and requisite overcorrection are recommended. However, overcorrection should not have an adverse psychosocial impact due to poor aesthetics.

In the case of mandibular corpus/ramus distraction, the occlusal plane, mandibular plane and ramus plane (posterior border of ramus) are of significant relevance in the planning. These planes are used as guiding planes to orient the distractor device. If the device is placed parallel to the ramus plane, it will result in an oblique distraction vector. If greater sagittal advancement is desired, the distractor should be put parallel to the occlusal plane. If the distractor device is placed parallel to the mandibular plane, a clockwise rotation of the mandible will result, leading to an anterior open bite and an increase in the lower anterior face height. Vector planning will have a profound effect on the treatment outcome.

The vectors for mandibular distraction and their impact can vary depending upon its vertical and horizontal relationship with border of mandible, occlusal plane and ramus.

When the distraction vector is planned parallel to the maxillary occlusal plane, the mandible is moved forward with little to no change in anterior face height, as illustrated in Fig. 81.6 A. Conversely, when the vector is set to bisect the maxillary occlusal plane and the lower border of the mandible, the mandible is distracted both forward and downwards. This results in an increase in anterior face height and a reduction in deep bite, as shown in Fig. 81.6 B. Additionally, Fig. 81.6 C explains that when the distraction vector is aligned parallel to the lower border of the mandible, the mandible is again moved forward but with a greater downward shift. This leads to an increase in anterior face height and the creation of an open bite.

Careful planning is essential for vector planning in ramus distraction. If the vector is perpendicular to the occlusal plane, it will result in significant vertical movement and create a posterior open bite ( Fig. 81.7 A). Conversely, if the vector is oblique to the occlusal plane and oriented more or less parallel to the posterior border of the ramus, it will lead to distraction of the ramus downwards and forward, resulting in the formation of an anterior open bite ( Fig. 81.7 B).

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Maxillomandibular distraction osteogenesis in orthodontics

Introduction

Distraction osteogenesis versus histiogenesis