# Functional Evaluation of a Novel Multi-Axial Alveolar Distractor--Preliminary In Vivo Animal Study
## 4 Discussion  
The main alveolar bone distractor principle is to generate new bone by gradually pulling the periosteum from the bone. Several kinds of alveolar bone distractors have been circulating in clinical applications. However, the current alveolar bone distractors only achieve axial direction distraction along the transport screw or biaxial adjustment with buccal-lingual adjustable angulation intra-operatively [13]. These existing distractors do not permit free-angle adjustment in different directions and cannot be applied in conjunction with clinically complex situations and irregular alveolar bone defect shapes. In our previous study a new multi-axial alveolar bone distractor design was presented based on the high degree of freedom mechanism for the ball joint to attain multi-axial distraction directions [18]. However, only static mechanical function tests were performed in the previous in vitro safety and effectiveness experiments. It remains unclear whether this new distractor can be applied to living bone for distraction and whether the ball and socket joint can withstand the bone growth force during distraction without causing the ball joint surface to slip, causing the distraction direction to change. Further studies are required to determine whether the bone growth direction determined with a novel multi-axial distractor can be maintained during distraction. We therefore investigated the feasibility and stability of the novel multi-axial distraction function by conducting in vivo pilot experiments using the rabbit mandible angulus. The primary objective of this in vivo experiment was to observe whether the bone growth direction determined before surgery with a multi-axial distractor can be maintained during distraction. The choice of suitable animals and their corresponding bone regions for fixing a novel multi-axial distractor was challenging because the distractor ball and socket joint opposing force should be higher than the bone distraction and growth forces during the experimental period. The bone distraction force might exceed the capability of our new multi-axial distractor if it is used on the mandibles of large animals such as pigs and sheep. Therefore, the mandible angulus of the New Zealand rabbit was selected because of its circular shape and thin cortex. These features make it better suited to planning multi-axial distraction, in a predetermined direction, compared with a mandible body with only straight geometric features. To the best of our knowledge, a ball and socket joint mechanism with the capability of such a high-degree-of-freedom cone trajectory motion was first applied to the alveolar bone distraction by our research team. Whether our novel multi-axial distractor can be further applied to the living body remains the most substantial challenge despite performing rigorous mechanical functional tests. The distractor direction adjusted to 25\({}^{\circ}\) at the buccal side was based on two relative pieces of literature [22; 23]. The angle of 15\({}^{\circ}\) set at the mesial side was smaller than that at the buccal side because of the mandible and muscle interference angulus space limitation. However, bone growth direction and deformation prediction at the angulus region following distraction was challenging, because its thin circular shape is completely different compared with other study animals. Therefore, two rabbits that were not subjected to distraction were retained as a control group to facilitate subsequent morphological comparisons. The computed tomography (CT) image resolution of the distraction bone was not sufficiently accurate to represent the final distraction bone proximal to the bowl shape with uniform thickness because of the irregular CT scan intervals. Moreover, the large test sample size impeded the use of a micro CT scan. The bone growth measurement, in terms of length or volume, cannot be performed using medical image reconstruction.