Biomechanics in orthodontics obeys several fundamental laws, amalgamating physics and biology. This article on Centre of Rotation is the last in a series of five articles covering the fundamentals of biomechanics in orthodontics. It will enable you to better understand the physics and biological responses governing tooth movement.
Understanding the principles of biomechanics in orthodontics is essential to controlling the outcome of tooth treatment, and confidently managing your patients’ cases. While it is possible for a dentist beginning in the field of orthodontics to lean on a technician’s skill to develop a clear aligner treatment plan for basic cases, as a professional is important to understanding how to control orthodontic outcomes and to lead the development of treatment planning, particularly when attempting to correct complex malocclusions such a cross bite. For more on the complexity of treating cross bites, read this article.
Each article in this series covers the following key principles for biomechanics:
The orthodontic force
Centre of resistance
Moments of force and couples
Equivalent forces
Centre of rotation
Centre of rotation
This series on biomechanics in orthodontics has shown you the elementary concepts required to understand teeth movement. This last article will describe a key concept for analysing different tooth movement types, the centre of rotation (CRot).
Knowing the CRot enables clinicians to better understand a tooth’s movement.
The CRot is the point around which the tooth rotates when it is moved. The CRot point may be located at any point, inside or outside the tooth.
The CRot is denoted as a Red dot.
To achieve controlled tooth movements in clinical practice you need an understanding of how to determine what moment of force ratio (M/F) to apply at the point of application in order to achieve the correct M/F regarding the Centre of Resistance (CR).
To build this understanding let’s compare four different types of movements with a standard incisor using a single sagittal force at different levels, noting their equivalent force systems at the CR, and how movements differ according to the position of the CRot with respect to the CR, either in relation to a specific M/F at the point of application, or a M/F at the CR.
Note:
Examples only represent a limited view of movement types.
Patients’ movements may be a combination of translational and rotational.
The distances represented are for a tooth with standard root anatomy and marginal bone level.
Click here for more detailed view of movements: Movement Difficulty Classification Guidelines.
Summary
Orthodontic tooth movement is the result of a complex interaction of forces generated during the simultaneous movement of several dental units and biological events (apposition and resorption), combined with a complex chemical composition of the oral environment. This brief series had provided you an introduction to key the principals required develop your understanding of these complexities.
To better explore the biomechanical behaviour of clear aligners Proligner enables you to explore movements in vitro in the Introduction to Clear Aligners Course. Students can apply these key principals in a hands-on practical with models that realisticaly simulate orthodontic tooth movements’ reaction to forces.
Dental professionals interested in gaining a greater understanding of orthodontics and clear aligners are encouraged to look out for a Proligner Introduction to Clear Aligners Course near you or join the course online via live streaming: Education.
Look out for future articles that will explore biomechanics further through the application of clear aligners.
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