Balancing Distraction Forces in the Mandible: Newton’s Third Law of Distraction

Vertical mandibular distraction results in translation of both proximal and distal segments. The force exerted on the condylar segment not only places unwanted force on the joint but also rotates the coronoid process into the cranial base. To prevent these sequelae, we investigate the use of a "check plate" on the condylar segment in an attempt to decrease force at the Temporomandibular joint (TMJ) and prevent unwanted rotation of the coronoid. Patients with hemifacial microsomia, seen at our Children's Hospital from 2012 to 2016 having undergone unilateral vertical mandibular distraction with placement of check plate were compared with a sample of those similarly having undergone distraction without use of the plate. Preoperative and postoperative cephalometric measures and 3-dimensional computed tomography imaging were analyzed. Three subjects were identified in each group. Age and Pruzansky-Kaban classification did not differ between groups. Vertical distance from the coronoid process perpendicular to the Frankfort Horizontal did not differ between groups (P < 0.07); however, postoperative distance significantly differed with the coronoid process rotating upward into the cranial base in subjects without a check plate (P < 0.005). Preoperative angle of the coronoid process based on the Frankfort Horizontal did not differ (P < 0.06); however, postoperative angle significantly changed, confirming upward rotation into the cranial base (P < 0.01). Total regenerate did not differ (P < 0.08). Vertical mandibular distraction results in undesirable upward rotation of the proximal segment into the cranial base and superior displacement of regenerate. This can be prevented with the use of a check plate.