Distraction osteogenesis in a severe mandibular deficiency

Distraction osteogenesis (DO) is a biologic process of new bone formation between the surfaces of bone segments that are gradually separated by incremental traction [1]. A callus forms between the separated bone segments and as long as the traction proceeds, callus tissues are stretched inducing the new bone formation [2].

DO was first introduced by Codivilla at the beginning of twentieth century and during 1950s, the studies of Ilizarov made a contribution in the development of the technique by elucidating the biological and mechanical principals in the formation of new bone [3–5]. DO has been applied to craniofacial region since McCarthy et al [6] reported the first clinical application of the technique in the treatment of four children with either unilateral or bilateral mandibular hypoplasia.

In the lengthening of the hypoplastic mandible, external or intraoral devices have been used. Although intraoral distractors are more convenient socially and leave no residual skin scars, extraoral bidirectional or multidirectional devices should be preferred in severe hypoplastic cases, when three-dimensional vector control is required and gonial angle control is necessary [1].

In this case report, we intended to present the treatment of an adult patient who had severe mandibular deficiency. An extraoral multidirectional distractor was used to achieve independent horizontal distraction of the corpus and vertical distraction of the ramus. Amount of lengthening was determined with cephalograms and stereolithographic models. Stereolithographic modelling is a well known system Moreover alterations at the posterior airway space due to elongation of mandible were also presented.

No significant complications associated with the surgical procedure were encountered postoperatively. Panoramic radiographs were taken postoperatively to monitor the new bone formation. After the consolidation phase, the gaps were all filled with new bone (Figures 6, and 7).

Cephalometric analysis revealed that overjet of 15 mm and overbite of 4 mm decreased to 4 mm and 1 mm, respectively. ANB angle decreased from 13° to 6°, distance between pogonion and NV increased from 42 mm to 24 mm, corpus length (Go-Gn) increased from 49 mm to 67 mm, and ramus length (Ar-Go) increased from 41 mm to 43 mm showing the successful elongation of ramus and corpus. Gonial angle, inclination of the mandibular plane, and Y-axis decreased revealing the anterior rotation of the mandible. Elongation of the ramus length increased the posterior face height and anterior rotation of the mandible decreased the anterior face height. These alterations changed the profile of the patient and provided more symmetric and aesthetic appearance. Cephalometric superimpositions of total face on SN plane is demonstrated in Figure 4, and and pre-treatment and post-treatment cephalometric analysis are presented in Table 1.

Alterations in the posterior airway space (PAS) were also determined. The distance from Eb to posterior nasal spina increased from 72 mm to 75 mm while the distance from Eb to tongue tip decreased from 81 mm to 74 mm. All the PAS parameters increased after treatment but the most dramatic increase was observed at N-PAS distance as it increased from 7 mm to 20 mm. N-PAS that was under I-PAS before treatment, positioned between S-PAS and I-PAS following the treatment. The perpendicular distance from hyoid to mandibular plane decreased from 19 mm to 11 mm after treatment. Cephalometric landmarks used for posterior airway evaluation, soft and hard-tissue linear measurements and reference planes are presented in Figure 5. Pre-treatment and post-treatment PAS measurements and superimpositions are presented at Table 2 and Figure 8, respectively. PM and NOA planes were used for PAS superimpositions.

	Pre-treatment	Post-treatment
VAL	72 mm	75 mm
TL	81 mm	74 mm
S-PAS	17 mm	22 mm
N-PAS	7 mm	20 mm
I-PAS	16 mm	25 mm
H-MP	19 mm	11 mm

Measurements on the pre- and post-treatment solid models of the patient revealed that ramus lengths increased from 44 mm to 45 mm at right side, from 49 mm to 50 mm at left side while corpus length increased from 69 mm to 80 mm at right side and from 68 mm to 80 mm at left side. Transversal measurements in-between right and left gonion points revealed a decrease from 101 mm to 93 mm. Preoperative and postoperative solid models are shown in Figures 9, and 10.

Satisfactory results from both aesthetic and functional standpoints were acquired via distraction osteogenesis of the ramus and corpus. Correction of the skeletal deformity improved the speech and pronounciation of the patient. Respiratory problems were resolved, wheezing and snoring ended. Although adjunctive aesthetic surgeries such as rhinoplasty, scar revision and angulus augmentation were planned for optimal satisfactory results, the patient was pleasant from his new image and he refused to undergo additional operations. Missing lower teeth were reconstructed with fixed prosthesis 3 months after the treatment. Postoperative extraoral and intraoral photographs are presented in Figures 11, 12, and 13.

Mandibular hypoplasia is one of the most common malformations of the facial skeleton. It is usually associated with a deficient gonial angle, ascending ramus, and mandibular corpus. Maxilla, zygoma, temporal bone, cranial vault and cervical spine are the other anatomic landmarks that may be affected [1, 7]. Pruzansky [7] classified mandibular hypoplasia according to the severity of the deformity. In this classification, as the severity increases the ascending ramus is affected to a greater degree.

In the presented case, the patient had a severe mandibular hypoplasia due to short ramus and mandibular body. Since mandibular lengthening procedure required vertical and horizontal components of distraction, an extraoral multiplanar distractor was selected. Intraoral devices usually apply unidirectional force due to the limited space in the oral cavity. Problems such as malocclusion and deficiencies in lower facial contours have been encountered in use of unidirectional devices for mandibular distraction [8]. Extraoral bidirectional or multidirectional distractors should be preferred in the correction of severe deformities including ramus, corpus and the angle of mandible [9, 10].

Molina and Ortiz-Monasterio [9] were the first to use bidirectional osteodistraction in the mandible to lengthened both ramus and corpus of the mandible simultaneously. Anatomically, the mandible consists of two halves that are fused at an acute angle in the midline forming V-shaped bone structure. Therefore, in order to correct severe mandibular deformities in three dimensions, independent lengthening of mandibular corpus and ramus must be combined with gradual angular adjustments. Because of this, several multidirectional extraoral distractors have been developed in the last decade [11, 12]. Multidirectional distractors are advantageous in distraction of the mandible in all three planes of space [13]. Linear or angular correction in the sagittal and vertical planes and angular correction in the transverse plane can be obtained with a multidirectional distractor and movement of bone segments or shape of the regenerate bone may be changed during distraction process [12]. In our patient, we used a multidirectional distractor and performed two ipsilateral osteotomies to shorten the total distraction period by creating two callus sites. This also allowed the development of a mandibular angle [1, 2, 12]. In the cephalometric analysis and model measurements we couldn't observe a significant increase in ramus length. Although the vertical component of the distractor was activated for 7 days, it did not reflect to ramus length exactly. Similarly, horizontal component was activated 45 mm but corpus length increased only 18 mm according to the cephalometric analysis. In our opinion the reason of the relapse, which was more than expected, was probably depending on the angular alterations performed during distraction period on days 10, 20, 30 and 40. These angular alterations were made in order to decrease the gonial angle and create anterior rotation of the mandible.

In the treatment of the patient, distraction osteogenesis was preferred as orthognathic surgery has relapse risk in severe mandibular deficiencies requiring lengthening of the mandible more than 8–10 mm. Even when the surgical technique is modified, 15 mm is approximately the outer limit of predictable surgical mandibular advancement. In our patient, lengthening of the ramus was also needed but after conventional orthognathic surgery procedures, pterygoid muscle usually does not adapt to the elongation of ramus. However, during distraction osteogenesis, active histiogenesis occurs in different tissues including gingiva, blood vessels, ligaments, cartilage, muscles and nerves [15, 16]. These adaptive changes in the soft tissues decrease the relapse risk and allow the treatment of severe facial deformities.

DO has also some risks such as infection, loosening of the distractor, paraesthesia, and excessive skin damage caused by the pins of the extraoral device. Strategic mistakes such as inappropriate distractor configuration or inadequate calculation of distraction parameters and technical mistakes like misalignment of the distractor leading to displacement of the bone segment, insufficient rate of the lengthening, premature consolidation may cause undesired results. These complications are usually related with the experience of the surgeon [17–19]. In the presented case, only facial scar was observed that was the inevitable result of the extraoral distractor.

Severe mandibular hypoplasia can lead to a reduction of oropharyngeal capacity and glossoptosis because of the posterior location of the insertion of the suprahyoid muscles into the mandible. As a result, upper airway obstruction, feeding difficulties, gastroesophageal reflux may occur. Several authors have reported that these conditions could be resolved by the help of mandibular distraction [20–22]. Similarly, following the advancement of the mandible, respiratory problems, snoring, and difficulties during feeding and talking improved in our patient, due to the increase in the PAS and correction of the excessive overjet.

Elongation of the corpus length increased the volume of the oral cavity therefore tongue was relieved. Before treatment, tongue tip was positioned over the incisal edges of the lower incisors because of insufficient space. However, following the treatment, it retruded behind these teeth to a normal position. This retrusion decreased the value of TL (distance between tongue tip and Eb). The tongue accompanied the advancement of the mandible and postural alterations occurred. Since the tongue moved in the anterior direction, all the PAS parameters increased. The most significant increase was in the N-PAS distance which was threefold higher than the initial value. Translation of the tongue affected not only the width of the N-PAS but also changed the position of this area. N-PAS was under I-PAS before treatment but it positioned between S-PAS and I-PAS following the treatment and this was the place where it should be. During the distraction of the corpus, mandible revealed anterior rotation and probably this rotation affected the hyoid bone through the muscles and caused an elevation at the hyoid so the perpendicular distance from hyoid to mandibular plane decreased after treatment.

During the treatment planning, both cephalometric analysis and solid model measurements were utilized. Solid model revealed the exact shape and size of the skeletal structures and provided the advantage of measuring the length of ramus and corpus directly and separately for right and left sides. Model surgery may be performed on the solid model so as to determine the osteotomy sides, inclination of the osteotomy and distraction vector. Distractors may be placed on the model after the model is cut at the osteotomy side with a saw and it may be activated to observe the results that would occur at the end of treatment and to determine the distraction period. In our opinion solid models could be used successfully in the treatment planning of distraction osteogenesis or orthognathic surgery cases.