Orthodontists have been taught that the mandible is displaced downward and forward, that the condyle grows up and back, and that the greater the backward growth of the condyle, the greater the anterior displacement of the chin. This notion, which serves as the foundation for functional appliance therapy, is wrong. The greatest chin projection in normally growing subjects occurs in those whose condyles grow forward the most. This course will provide the background necessary for orthodontists to understand these relationships. It will be shown that the key determinant of AP and vertical chin position is the true mandibular rotation that takes place during growth. Traditionally, most orthodontists have underestimated how important rotation really is because they have not been adequately taught to distinguish between true mandibular rotation – which changes greatly - and the rotation of the mandibular plane – which changes little. Changes in tooth position – over which orthodontists have great control – plays an important role in determining the true rotational changes that occur. Understanding the relationship between rotation and changes in tooth position is particularly important for treating retrognathic, hyperdivergent Class II cases. Clinically, orthodontists must be aware of and able to use the various centers of rotation to prevent deleterious changes from occurring during treatment, and to orthopedically correct the problems that previously developed. They must understand that true rotation is not only the primary determinant of chin position, but also a major determinant of condylar growth direction, mandibular modeling, and dentoalveolar compensations. They must learn how to take advantage of the mandible’s ability to adapt to rotational changes.
After this lecture you will be able to:
Explain why true rotation of the mandible is the primary determinant of chin position;
Discuss that true rotation depends on changes in the vertical positions of the teeth;
Apply this new appreciation of growth to treat hyperdivergent, retrognathic Class II patients orthopedically.