Although the precise prediction of the results before distraction is important, performing three-dimensional (3D) simulations for all distraction osteogenesis patients is not practical. Formulating general guidelines based on the factors affecting the 3D results of distraction treatment is recommended. This study was performed on a 3D mandible based on a finite element method. Three surgical cuts (oblique, vertical and horizontal) were made in the right side of the mandible. The amount and direction of movement of proximal and distal segments were evaluated after simulation of 15 mm of distraction. In the distal segment, the maximum displacement in the pogonion occurred in the vertical cut. In the proximal segment, the maximum displacement occurred in the coronoid process in horizontal and oblique cuts in a superior direction. The condylar process rotated in the clockwise direction when the vertical cut was used and the coronoid process moved inferiorly. To make the gonial angle more prominent the vertical cut should be used. A horizontal cut is used to lengthen the ramus. Vertical and oblique cuts can be used in patients with long anterior facial height, but all other conditions being equal horizontal cuts are better used in short faced patients.
Mandibular lengthening by distraction osteogenesis (DO) has become an accepted and popular method for treating patients with craniofacial developmental defects. The use of DO dates back to 1905 when C odivilla used this method for the first time.
I lizarov popularized DO for correcting limb deformities in congenital defect and trauma patients. Later, S ynder in 1973 and M ichieli & M iotti in 1977 used DO for mandibular lengthening by gradual distraction in experimental animals. In 1992, M c C arthy reported the use of mandibular lengthening by DO in a patient with hemifacial microsomia and this useful method has been applied to different bones of the craniofacial structures .
In DO, one or more surgical cuts are made in the specified bone and this bone and its associated soft tissues are stretched gradually by applying tensile force by a distractor. There is usually no need for bone grafting because new bone is added in the gap between bone segments . In DO, all the soft tissues (including skin, muscles, nerves, and vessels) are stretched and it is concluded that they are expanded in addition to the bone and this expansion reduces the tendency for relapse . In comparison with the traditional treatment of autogenous bone grafting or osteotomies, DO appears to have fewer complications and less tendency to relapse, so it has gained popularity over the years .
To achieve a good result in DO, precise treatment planning is paramount. It is now possible to create three-dimensional (3D) images of craniofacial structures using imaging techniques. By performing virtual surgery on these 3D images, clinicians are able to plan and implement multiple procedures on patients and see the results of these treatment modalities beforehand . It has become evident that precise control of the direction of distraction vector is mandatory for gaining predictable results in this procedure.
Prediction in mandibular distraction is more important than in other craniofacial distractions because the mandible generally goes through rotational as well as linear movements during the distraction process .
Intraoral and extra-oral distraction devices have been used for maxillofacial DO. The advantages of intraoral devices include lack of visibility and facial scars. Although some multidirectional distraction devices have been invented, many of the distracters used routinely are unidirectional and they are largely dependent on the direction of the distraction force or ‘distractor orientation’.
Precise prediction of the results before distraction is of paramount importance and different attempts have been made to simulate the distraction procedure preoperatively but performing these simulations for all patients is not practical. Formulating general guidelines, based on the factors affecting the 3D results of distraction treatment, is recommended.
A few attempts have been made to formulate guidelines including that by M ikhail et al. , which investigated the effects of distractor orientation on the treatment results in a two-dimensional (2D) computer model. Mandibular changes during DO are 3D and many patients needing DO are affected by asymmetric conditions such as hemifacial microsomia. Analyzing the changes made during DO in asymmetric mandibles using a 3D computer model would help more accurate treatment planning in these patients.
Method and materials
This study was performed on a 3D model of a desiccated mandible, separated from a young human skull. For transferring the dimensions of this mandible to the computer and making a virtual model the following steps were followed based on the finite element method: measurement of the dimensions; 3D modelling using Nisa II software; defining the boundaries of the model; and analyzing the forces applied on the model.
Measurement is carried out with a contact technique using a coordinate measuring machine. The mandible is fixed on the platform of this machine and a suitable sensor is selected and installed on the arm of the machine. Each time the sensor touches the surface of the mandible, a signal is sent to the control unit and the Cartesian coordinates of the points are saved as a file in this unit. These data are processed by the microprocessor unit and viewed in the display window of the Nisa II software as a 3D virtual mandibular model.
To make this model more accurate, two boundaries on the outermost parts of both left and right condyles were specified resembling glenoid fossae ( Fig. 1 ). Using this specification, the movement of proximal segments of the mandible during distraction was made possible in all directions except in the vertical direction (resembling the restriction imposed by the glenoid fossae on this movement). All other movements including rotational movements were possible.
In the next step, three surgical cuts were made on the right side of the mandible, to include all the surgical cuts necessary for correction of different combinations of bone defects of the body and ramus ( Fig. 2 ). The first was made obliquely from the distal line angle of the right third molar to the gonial angle. The other two cuts were made with a 45° angle to the first one. So, the second surgical cut was from the distal line angle of the right third molar to the lower border of the mandible in a vertical direction and the third was from the former to the posterior border of the ramus in a horizontal direction.
The distractor pins were placed perpendicular to the surgical cuts, with a 5 mm distance on either side. They were opened first for 15 mm and then for 30 mm. Both the proximal and distal segments moved the same amount (that is for the 15 mm opening, 7.5 mm movement for each segment).
To make the measurement of the amount and direction of movement of distal segments easier, multiple points were highlighted on the lower border of the mandible, from surgical cut to a point corresponding to this point on the lower border of the other side ( Fig. 3 ). By evaluating the movements of these points the movement of the distal segment was elucidated more easily and accurately.
Assuming the most anterior point of the chin (that is the pogonion) as the zero coordinate, the amount and direction of movement of proximal and distal segments and pogonion were evaluated in all three planes of space. The movements in the transverse dimension were traced on the X -axis so that the negative values represented movements to the distraction side and the positive values indicated the movement to the other side. The movements in the sagittal dimension were depicted on the Y -axis, with positive values indicating posterior movements. The movements in the vertical axis were shown on the Z -axis in a way that positive values indicated upward movement. The movements are shown first in three planes separately in their respective diagrams and then the resultant displacement is shown three dimensionally for distal and proximal segments.
Comparison of oblique, horizontal and vertical cuts in the transverse dimension is shown in Fig. 4 . In the distal segment, there is more movement of the pogonion with the oblique cut compared with the vertical and horizontal cuts. With the oblique cut the pogonion moves approximately half the amount of its movement with the vertical or horizontal cuts ( Table 1 ).
|Surgical cut||X -axis displacement (mm)||Y -axis displacement (mm)||Z -axis displacement (mm)||Result displacement (mm)|
When the vertical cut was used, maximum displacement occurred in the chin region from inferior border to the alveolus. With the oblique and horizontal cuts, the maximum displacement is limited to just inferior to the border of the ramus and not the alveolar crest. In all the conditions the chin moved in a positive direction.
In the distal segment, the amount of displacement was increased as the distance to the osteotomy site was increased up to the pogonion. After this point, this variable decreased progressively to the condylar process of the contralateral side, such that the condylar process of the distal segment goes through the minimum displacement.
In the proximal segment, with oblique and horizontal cuts, maximum displacement was seen in the coronoid process and in the negative direction. This process moved less in the horizontal cut. In contrast, maximum displacement was seen in the inferior border adjacent to the osteotomy site with the vertical cut.
Comparison of the oblique, horizontal and vertical cuts in the anterior–posterior dimension is shown in Fig. 5 . In the distal segment, the maximum displacement of the pogonion occurred when the vertical cut was used and this was in the anterior direction (with negative values). The amount of pogonion displacement in different cuts can be seen in Table 1 .