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A modified version of the Ilizarov method of bone lengthening, known as distraction osteogenesis (DO), is now being used worldwide[1]. Of interest, here are its applications in the maxillofacial region, especially in the dental alveolus[2]. Many patients present for implantal treatment with complex ridge deformities. A common prerequisite treatment for these patients is the regeneration of sufficient vertical and horizontal alveolar support for implant placement. As a new and successful technique for ridge augmentation, alveolar ridge distraction based solely on a centrally fixed distractor has come into popular use since 1997[3]. It allows for minimally invasive surgery by using a single-stage surgical technique for distraction and implant placement, and can provide a much more predictable outcome in regenerating ridge height in many cases[4]. This technique is based on a “minidistractor” that is first used for distraction and then can be transformed into a dental implant[3].
The stability of distractor relies on favorable stress distribution at the bone-distractor interface[5]. Clinically, the distractor was inserted through osteotomy and the alveolar ridge was divided into two parts, the basal section and the transportable section. After 5-7 days’ latency period, activation of the distractor was initiated under the condition of lack of osseointegration between the bone and the distractor[1-3, 5]. The subsidence of distractor and the bending of bone would be unavoidable. Relating to the reasons above, some complications were reported, such as the fracture or deformation of the basal and transportable section, the loosing of distractor, the failure to achieve prospective height of alveolar ridge, etc[6]. The alveolar bones were classified to 4 classes, based on the ratio of cancellous and cortical bone and rarefaction of cancellous bone[7]. In an attempt to clarify the relation between bone quality and stress in the bone and distractor, a three-dimensional finite elements analysis was performed to investigate stress distribution and deformation in bone and distractor in the presence of a bone defect of various qualities and dimensions.

Unlike other finite element analysis in dental implant, the models used in this study were different and complex. The bones were split into two parts, and osseointegration did not occur between the distractor and bones. When the distraction was performing, the distractor did not bear the occlusal force. That is why the interface conditions and loads were set[15-17].
The aim of this study was to find the pure effect upon the bone stresses and deformations of variations of the bone quality. So it was assumed that all the parameters of the models were identical except the composition of the bone and the elastic prosperities of cancellous bone. This makes it possible to make a comparison between different bone qualities[9, 11].
In each model, the highest EQV stress of the bone was observed around the distractor neck in transportable section. It confirmed the conclusion that previous research in dental implant achieved[8-9]. Therefore, unlike the previous literatures, the highest EQV stress was observed in the cancellous bone in Class Ⅲ and IV. This might caused by the insufficient thickness of cortical bone[3]. From the results of EQV stress, it could be seen that stress augmented as the bone quality decreased. Because the bone in molar area was always Class Ⅲ and Ⅳ, this disadvantage would certainly affect the achievement ratio of implantal distraction[4]. During the distraction, the tensile stress caused by basilaris substantia and tension caused by oral mucosa were gradually increasing. All the disadvantages would raise the incidence rate of transportable segment fracture[5].
Based on the results of distribution of deformation, subsidence of the bone and distractor increased when the bone quality decreased. As a strong internal fixation, the distractor made stress trending to the central area, which leading to the severely deformation of margin and less deformation of center[6]. The tendency was inevitable. So to the patient with unfavorable bone quality, the initial movement of distractor would be increasing and the completion of distraction would be delayed because of the subsidence[5, 7]. 

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