Biomechanical evaluation of the pins of a mandibular external distractor

Our goal was to establish whether the pins of an external distractor were capable of overcoming tissue resistance to distraction. The study was carried out in 2 parts. The first part of the study determined the bending rigidity of the distractor's pins. To accomplish this, the distractor was installed on the mandible of a stereolithographic model using 4 regular 2.0-mm steel pins. An osteotomy was not performed. The distractor was activated using a torque gauge, and the bending rigidity of the pins was recorded. The second part of the study determined the tissue resistance to mandibular distraction using fresh cadavers. Six cadavers were divided into 2 groups to determine tissue resistance to angular and linear distraction, respectively. The devices used to measure tissue resistance were a modified external multiplanar distractor and a torque gauge. In the anteroposterior direction, the tissue resistance to linear distraction clearly exceeded the bending rigidity of the pins for the first 7.5 mm of activation. After this, the opposite was true. In the vertical direction, the tissue resistance clearly exceeded the bending rigidity of the pins for the first 8.0 mm of activation. After this, the opposite was true. For the first 15.0°of angular distraction in the sagittal plane, the tissue resistance was almost identical to the bending rigidity of the pins. After this, the tissue resistance significantly exceeded the bending rigidity of the pins. For the first 7.0°of angular distraction in the transverse plane, the tissue resistance was marginally greater than the bending rigidity of the pins. After this, the bending rigidity of the pins increased significantly. The 2.0-mm steel pins used in most external multiplanar distractors are not capable of overcoming the tissue resistance to linear or angular distraction.