The Center of Resistance as a 3-D Point in Orthodontics: Scientific Fact or a Myth?
The center of resistance is the most fundamental concept in orthodontic biomechanics. In this lecture, we will discuss the evolution of the concept of center of resistance, from its 2D centroid origins, to a problematic 3D universal extrapolation. We will introduce the concepts of axes of resistance and show how centers of resistance of a tooth may only exist as 2D projections of 3D axes of resistance intersections. We will also demonstrate that, on a 2D projection of a rotationally asymmetric tooth, the center of rotation and center of resistance do not coincide when a moment of a couple is applied.
Relate the evolution of the center of resistance concept and the limitations of the idealization of a 3D point
Discuss the relationship between tooth/PDL asymmetry and the axes of resistance
Evaluate how a 2D center of resistance can be determined in projections of 3D axes of resistance intersections for a general tooth.
The Fallacy of Archwire Shape in Producing Force Systems
Because archwire shape is not the predominant determinant of required force systems, undesirable side-effects frequently take place during the various stages of treatment and have lead to the use of transpalatal arches, lingual attachments, crossbite elastics, etc. In over 50 years of practice, Dr. Mulligan has never used any of these and has never found it necessary.
Recognize the fallacy of allowing prescription brackets to determine force systems
Analyze how archwire shape can produce exactly the opposite tooth movements that may be expected
Describe how full appliances and partial appliances most often produce entirely different force systems with the same archwire shape.