Long-bone distraction osteogenesis as developed by Ilizarov, Codvilla, and others used a circumferential distraction device that provided controlled linear bone elongation. The distraction device could be applied directly parallel to the bone’s long axis, which was the desired vector of distraction. The varying curvilinear form of the facial bones and their relationship to the soft tissues and specialized structures of the face create limits on device placement and vector realization. Difficulty has been experienced in designing devices for craniofacial distraction that offer the same control and stability that circumferential distraction devices provide in lengthening long bones. Although designs for craniofacial devices have been improved substantially, the limitations for corticotomy/osteotomy placement and device placement imposed by the complexity of human facial anatomy probably never will be overcome completely. For this reason orthodontic/orthopedic control of the distal segment is important to the quality of outcomes achieved with distraction osteogenesis.
The principal concern addressed in this article is the position of the distal segment during and following the activation and consolidation phases of distraction. Once consolidation is complete, the position of the distal segment, good or bad, is fixed. The goal, therefore, is to maximize the capabilities of the distraction device to deliver the desired vector(s) by selecting the correct device, applying it at a proper orientation, and taking advantage of whatever adjustability the device offers. In addition, having control of the dentition allows manipulation of the distal segment position to optimize occlusion, facial esthetics and function.
Although not inclusive, situations requiring special consideration in control of the distal segment include (1) delivering bilaterally symmetric advancement of the mandible, maxilla, and/or midface, (2) correcting asymmetry with unilateral vertical and anteroposterior distraction of the mandible, (3) recognition of premature consolidation as the source of distal segment misdirection and its management, (4) providing a level of precision in distal segment position that is comparable to traditional jaw osteotomies when used as the definitive procedure on skeletally mature patients, and (5) compensating for inadequacies in osteotomy and/or device placement.
Distal-segment positional issues observed with bilateral advancement of the mandible, maxilla, or midface include incomplete correction, unintended asymmetric advancement, anterior open-bite formation, or other malocclusion. These issues may be result from incomplete presurgical preparation, improper treatment planning, dental interferences during activation, inadequate corticotomy/osteotomy implementation, errors in device position, hardware limitations, and/or inadequate control of the occlusion.
If the distraction device is not long enough, the desired position of the distal segment cannot be achieved. The ratio of skeletal lengthening to device activation is not 1:1 but instead varies based on the distance from the device to the bone surface (along with other factors): the closer the device is to the bone, the closer the ratio is to 1:1, and the ratio decreases as the device is distanced from the bone surface ( Fig. 1 ). Distance from the device to the bone must be taken into consideration when selecting the device length. For an “average” device placement, a conservative rule of thumb is to select a device that has twice the lengthening capacity of the desired result. For example, if the desired linear anteroposterior correction at the incisors is 15 mm, a device with a length of 30 mm or more should be selected. It also is important to understand that additional length is needed in a device that offers three-dimensional adjustment if the angular component will be activated.
When the angular component of a multidimensional device is activated in the mandible, the lateral surface of the mandible maintains or increases the distance between the distal and proximal segments. Conversely, the medial surface of the ramus sees a convergence of the proximal and distal segments as the angulation dialed into the distraction device is increased ( Fig. 2 ). To prevent convergence and interference of the proximal and distal segments at the medial surface, a distraction gap must be established before the angular component of the device is activated. It is widely accepted that a 10-mm distraction gap is necessary before angular activation to avoid this interference. When angular activation is added, a net loss of linear length is observed. Hence additional linear activation is required to achieve the desired anteroposterior result, and therefore the device must be longer.
Multidirectional mandibular distraction devices offer the ability to correct emerging asymmetry with transverse adjustments. This capability is not used frequently, as illustrated in the following example. If the distal segment is moving asymmetrically to the right, one intuitively would consider activating the device to the left to correct this asymmetry. Unfortunately, the result of left-transverse adjustment is not movement of the distal segment to the left. Instead, the condyle segment, offering less resistance to force than the distal segment, moves left with the left-transverse adjustment. This movement of the condyle segment results in a counterintuitive movement of the distal segment to the right in accord with left-transverse adjustment. More predictably, interarch elastics may be used to manipulate the distal segment orthopedically during active distraction to improve the sagittal relationship of the distal segment and to control misdirection. This manipulation is accomplished with heavy elastic traction on distraction-stabilization appliances or similar rigid tooth-borne appliances to “guide” the mandible to the desired position ( Fig. 3 ).
In addition to elastic traction, distal-segment control may be achieved in many ways. Adequate predistraction arch preparation (removing potential dental interferences), including osteotomy position and device orientation, careful surgical execution, and having orthodontic/orthopedic appliances in place before surgery will help mitigate unfavorable distal segment movement when it occurs. As mentioned, manipulating the distal segment by applying orthopedic force via dental appliances often is the most effective and precise means of correcting misdirection. Force may be applied to the advancing skeleton in any direction that change is indicated. A common example is the use of class III elastics placed unilaterally during a class II distraction to control a developing asymmetry ( Fig. 4 ). In this scenario, additional linear lengthening may be needed to compensate for the period of class III elastic wear ( Fig. 5 ). In addition to elastics, other orthopedic modalities (ie, high-pull headgear, protraction headgear) may be used 24 hours per day with elastics changed frequently throughout the day to maintain peak force. Close monitoring of orthopedic forces by the surgeon and/or orthodontist is necessary to assure that distal segment correction is adequate. It is common to alter the configuration of elastics at every appointment, twice per week, to achieve the desired result ( Fig. 6 ). Angular activation of the multidirectional distraction device may be used carefully in concert with orthopedic forces to achieve optimal distal segment control ( Fig. 7 ).