ORTHODONTICS HAND OUT

ORTHODONTICS HANDOUT

BIOMECHANICS

CENTRE OF MASS: Each body has a point in its mass, which behaves as if the whole mass is concentrated at that single point, which we call the center of mass in a gravity free environment.

The same is called the CENTER OF GRAVITY in an environment where gravity is present.

When a force is applied on a body either above or below its center of mass, it TIPS and moves forward.

FORCE is a load applied to an object that will move it to a different positioning space and is measured in units of GRAMS or OUNCES.

If the same body is acted upon by two forces one above and one below the center of mass, which are equal in magnitude and opposite in direction, it will spin the body around the center of mass and this situation is called a COUPLE, which may be clockwise or anticlockwise.

If the force is passing through the center of mass, the whole body moves in a straight line, in a parallel fashion. Here all the points of the body get displaced equally from the initial position. This is called BODILY MOVEMENT or TRANSLATION.

The point around which a tooth rotates when a force is applied to it is called the CENTER OF ROTATION. Center of rotation can be at the Cres, apical to it, at the root apex or at infinity.

UNCONTROLLED TIPPING: Crot at Cres, crown and root move is opposite directions.

CONTROLLED TIPPING: Crot at the tooth apex, crown moves in the direction of applied force but root gets minimally displaced.

TRANSLATION: Crot at infinity, both crown and root move in the same direction equally.

TORQUE: Crot at the incisal edge, root moves in the direction of the applied force and the crown moves minimally.

Since the tooth is embedded in the alveolar bone, the force applied acts at a distance from the Cres. This is termed the MOMENT and is the product of force times the perpendicular distance from the point of application of force to the Cres, measured in gm-mm.

The ratio of moment produced to force applied is called MOMENT TO FORCE RATIO and determines the type of tooth movement.

Moment to force ratio

< 5:1	uncontrolled tipping
5 to 8:1	controlled tipping
10:1	translation
12:1	torque

If we apply two forces equal in magnitude and opposite in direction, the moment created is called the MOMENT OF A COUPLE. The moment of the couple is the product of one of the forces times the distance between the two forces. This distance is called the MOMENT ARM OF THE COUPLE.

BRACKET

PARTS:

BASE: An integral part of the bracket, which attaches the bracket to the tooth surface.

SLOT: That part of the bracket that receives the arch wire, hence bringing about tooth movement in three planes of space.

TORQUE IN THE BASE VERSUS TORQUE IN THE FACE:

Torque in the face: slot of the bracket is cut at an angle corresponding to the torque value.

Torque in the base: slot of the bracket is parallel to the Andrew’s plane but the base is angulated corresponding to the torque value.

BRACKET POSITIONING:

The features built in the bracket such as tip/ torque will be expressed ideally only if the brackets are accurately placed.

Quoting Roth “ At the heart of every excellent treatment result lies a well placed appliance, regardless of the appliance that is used.”

Andrew’s advocated a system of bracket placement where in the center of the slot of the bracket is placed on the facial axis point (FA point) which is situated midway along a vertical line on the facial surface of the tooth called the facial axis of clinical crown (FACC).

LEVELING AND ALIGNING

Comprises of the tooth movements needed to achieve passive engagement of a flat rectangular arch wire of standard arch form, into a correctly placed PEA bracket system.

KEY POINTS OF LEVELING AND ALIGNING:

Forces should be kept as light as possible
Sagittal, vertical and transverse anchorage needs should be identified for each case
Lace backs and bend backs should be used wherever necessary
Should go segmental or continuous as the case demands.

CONCEPTS OF WIRE SEQUENCING:

Variable cross-section concept
Variable modulus concept
Variable temperature concept

RETRACTION MECHANICS

SLIDING / FRICTION MECHANICS

Space closure using sliding mechanics requires a wire that produces less friction with the brackets. The arch wire and the bracket friction slow the movement of teeth along the wire. The elastic chain is the force component of the retraction assembly and the wire-bracket interaction produces the moment. The distal tipping of the tooth contributes to the retraction by causing binding of the arch wire, which in turn produces a moment that results in distal root movement. The magnitude of this moment, which causes the distal root movement, depends of the size, shape and material of the wire and the bracket width. As the tooth uprights, the moment decreases until the wire no longer binds. The crown then slides along the arch wire until distal crown tipping again causes binding. This process is repeated until the tooth is retracted or elastic force is dissipated.

LOOP / FRICTIONLESS MECHANICS

In frictionless mechanics, teeth are moved without the brackets sliding along the arch wire. Retraction is accomplished with the loops. Activation of the loop (pulling the distal end of the wire through the molar tube and cinching it back) produces the force in frictionless mechanics. Bends placed on the mesial and distal legs of the loop are called the alpha and beta bends and these produce the alpha and beta moments respectively when engaged into the brackets. The material, configuration of the loop and the preactivation of the loop influence the moment-to-force ratio. The use of TMA wires with a modulus of elasticity approximately 2/5^th of stainless steel allow the use of larger preactivation bends. The TMA wires generate low forces and have a greater range of action when compared to stainless steel.