Though it is known well that hemifacial microsomia is a congenital malformation in most cases, there are only a few presumptions that it can be caused by natural mutation rather than heredity and/or by drugs like thalidomide, primidone, or retinoic acid [5]. Some other authors suggested that hemifacial microsomia can be caused by the stapedial artery hematoma [1] or abnormal neuroectodermal cell migration during embryogenesis [6]. The former theory explains that the hemorrhage from the stapedial artery produces hematoma to induce the pressure around the first and second pharyngeal arch. Thus, the size and shape of hematoma can be related to the phenotypic variability. The latter theory proposes that retinoic acid changes the pattern of migration and/or the distribution of neural crest cells, which finally incur the deformity of the facial tissues from pharyngeal arches. However, the exact pathogenesis for hemifacial microsomia is still unclear even with these hypotheses.
There is also a controversy about the growth pattern of hemifacial microsomia. Some authors reported that patients have mild facial abnormality at birth, but their asymmetry becomes more distinct as the non-affected side grows faster than affected side does [11]. However, others suggested that the degree of facial deformity is not accelerated during growth [16]. Thus, it is not clear again about the growth pattern of hemifacial microsomia, as in its pathogenesis [6]. We hope our analysis with functional unit in large sample size can be of help to support them.
Variety of clinical and supplementary data, including the facial photos, plaster dental models, and radiographic images, can be used for diagnosis of hemifacial microsomia. Especially, the two-dimensional cephalometric radiography has been the main diagnostic tool. However, it has inevitable limitations such as the image expansion and distortion and the blurring of superimposed anatomical structures. So there are difficulties in precise diagnosis and treatment planning of three-dimensional craniofacial structures [17–19].
Since the first introduction of CT in 1979, three-dimensional CT (3D CT) became a major imaging tool for craniofacial evaluation and treatment planning. Even though 3D CT needs high-dose radiation and expensive cost, it can allow us to observe the craniofacial structure at the various perspectives and to analyze 3D length and angle more precisely than two-dimensional cephalometric radiography does [18, 19]. Furthermore, there are no image distortions, and the deep structures can be directly observed by controlling images. Nowadays, 3D imaging software for CT can be easily accessed with the personal computer environment [17], and the development of high-quality CT machine such as the multi-detector CT and the cone-beam CT make it possible to acquire thin sliced CT image (being less than 0.5 mm) and to reduce radiation dose with the special low-dose protocols. Therefore, the 3D CT is expected to be more popular for craniofacial imaging in the future [15].
The 3D imaging technology for craniofacial deformity is developing rapidly up to the level, which can measure the length and angle for anatomic structure at the complex craniofacial region, make a 3D simulational operation, and predict the outcome after the simulational surgery [17–19]. But the 3D technology for diagnosis and treatment planning has not been applied enough to the field of hemifacial microsomia and congenital dysmorphosis. We will need more works for the 3D understanding of biological structures, the confirmation of etiopathogenic mechanism and region, and the simulational planning to reconstruct the craniofacial structure to be normal.
The mandible is the main anatomic structure on the lower part of craniofacial region, and it can have a strong influence on the development of malocclusion and craniofacial deformity. At 10th week of human embryonic development, the membranous bone begins to be formed near the mental foramen after Meckel’s cartilage development and calcification [20]. It is progressed along the inferior alveolar nerve to mandibular foramen. Then, a primitive mandible is completely formed with the additional development of secondary cartilage at the condyle, coronoid process, and symphysis region [14]. After the fetal isometric growth and its birth, the mandibular growth is attained by the longitudinal growth, similar to that of the long bones, at the condylar region and also by the superficial apposition and resorption of bones [14].
Based on this process of the mandibular development and growth, Moss and Simon assorted functional units using functional matrix theory [13]. Precious and Delaire additionally proposed that the mandibular growth is the sum of independent growth of each mandibular functional unit [21]. Based on these theories, the mandible can be divided into the unit of the condyle, coronoid, body, angle, symphysis, and dentoalveolus. And the functional matrix, best exampled by the masseter muscle, can affect the growth of these units while being affected reciprocally.
Distraction osteogenesis is one of the ideal treatment strategies for hemifacial microsomia, as described previously. In order to apply this treatment strategy to the treatment of hemifacial microsomia, it is necessary to understand the long-term effectiveness of this distraction modality. On a report written by Meazzini et al. [22], the distraction was performed at the ramus of the mandible for eight patients with type I and II hemifacial microsomia at an average age of 5.6 years old. Five years postoperatively, the ratio between affected and non-affected rami returned to 77 % of the correction obtained by the distraction. Huishinga-Fisher et al. [23] also reported that the distraction osteogenesis was performed in eight children and about 50 % of cases seemed to have relapse, which occurred 1 year after distraction osteogenesis. And these relapses seemed to progress up to 3 years after distraction osteogenesis.
Thus, the decision about the optimal timing of distraction osteogenesis should consider the mandibular growth pattern and the effect of distraction osteogenesis to the mandibular growth pattern. If facial asymmetry and deformity in hemifacial microsomia become worse during the growth period, the early application of distraction osteogenesis will be necessary to prevent secondary deformity [11]. However, if the degree of deformity is not worsened during the growth period, the surgical correction should be delayed until the growth is finished [6]. There are few studies about the growth pattern of hemifacial microsomia to acquire the conclusion about this. But Grayson reported in his long-term follow-up study that the vertical bone growth in hemifacial microsomia is not definite after distraction osteogenesis [24]. Moreover, Marquez reported that the growth ratio of affected side is reduced after distraction osteogenesis [25]. Considering all these reports, the early stage treatment seems to have less advantage in terms of long-term treatment effect.
It is also controversial at which the distraction osteogenesis can be applied. Mommaerts and Nagy suggested that the results of distraction osteogenesis at the mandibular body are more stable, and the body part may be more important than the ramus does [26]. But Kusnoto et al.’s report does not allow clear conclusion about the vertical stability of the distraction osteogenesis-induced new bone between the mandibular body and ramus [27].
The functional treatment goal for hemifacial microsomia is to restore the normal function and structure and to induce the normal growth by recovering the affected mandibular functional unit. So we tried in this study the analyses of hemifacial microsomia for 3D mandibular shape to find the affected functional unit, at which we can apply the distraction osteogenesis. Though the sample size is too much limited, we could obtain the result saying that the size differences between affected and non-affected side were observed at the condyle, angle, and body in descending order. So we can assume that the ramus, especially the condylar and angular unit, is the most etiopathogenic or affected area in hemifacial microsomia. Particularly, the lack of length at the affected the condyle unit reached about 70 % as compared with that of the non-affected side. Based on these results, the affected ramus may be treated by distraction osteogenesis to lengthen the short condyle unit and surrounding muscles. Additional treatment to the angular unit also needs to be considered because the insufficient size of mandibular angle may not be resolved by distraction osteogenesis alone. The distraction at the angular region or the free bone graft may be a possible candidate solution.
Meanwhile, the size difference of mandibular body unit at the affected and non-affected side was not as evident as had been expected in this study. The mandibular body unit is intimately related to the development of the inferior alveolar neurovascular bundle during the developmental period. Neiva et al. measured the length of inferior alveolar canal with 3D CT for hemifacial microsomia [28]. And they found no significant difference of the bony canal length between the affected and non-affected side of Pruzansky’s type I group subjects, while those of Pruzansky’s type II to be significantly different. According to their finding, we could assume that the insignificant difference of body length might be related to the small-sized sample of this study. Three subjects out of four belonged to Pruzansky’s type I deformity in this study. So we cannot expect the result of this study to be applied to the general mandibular shape of hemifacial microsomia, and more studies for this point will be necessary in the future. Furthermore, the individualized analysis of each hemifacial microsomia has to be performed to make customized treatment planning for individual patients. Nevertheless, the mandibular functional unit analysis to find the major etiopathogenic area can be a useful diagnosis tool for hemifacial microsomia.
