Asymmetric Outer Bow Length and Cervical Headgear Force System: 3D Analysis Using Finite Element Method

This study sought to assess distal and lateral forces and moments of asymmetric headgears by variable outer bow lengths.

Four 3D finite element method (FEM) models of a cervical headgear attached to the maxillary first molars were designed in SolidWorks 2010 software and transferred to ANSYS Workbench ver. 11 software. Models contained the first molars, their periodontal ligament (PDL), cancellous and cortical bones, a mesiodistal slice of the maxillae and the headgear. Models were the same except for the outer bow length in headgears. The headgear was symmetric in model 1. In models 2 to 4, the headgears were asymmetric in length with differences of 5mm, 10mm and 15mm, respectively. A 2.5 N force in horizontal plane was applied and the loading manner of each side of the outer bow was calculated trigonometrically using data from a volunteer.

The 15mm difference in outer bow length caused the greatest difference in lateral (=0.21 N) and distal (= 1.008 N) forces and also generated moments (5.044 N.mm).

As the difference in outer bow length became greater, asymmetric effects increased. Greater distal force in the longer arm side was associated with greater lateral force towards the shorter arm side and more net yawing moment.

A difference range of 1mm to 15 mm of length in cervical headgear can be considered as a safe length of outer bow shortening in clinical use.