Ortho unit 1 Puri, Biomechanics

Ortho unit 1 Puri, Biomechanics

What is biomechanics in orthodontics?

Bridge between physics + biology to predict safe tooth movement.

Why is biomechanics essential?

Prevents inefficient movement, root resorption, bone loss.

What are the 4 properties of a force?

Magnitude, direction, point of application, line of action.

Define force

Push/pull with magnitude + direction along a line of action.

What determines tooth movement path?

Point of application relative to CR.

Center of mass definition?

Point where mass is concentrated; force through it → pure translation.

Center of resistance (CR) definition?

Point where force causes bodily movement without rotation.

CR location in single‑rooted tooth?

1/3–1/2 root length from alveolar crest.

CR location in multi‑rooted tooth?

1–2 mm apical to furcation.

What affects CR position?

Bone height, root length, periodontal support, bone density, age.

Define moment

Rotational force = Force × distance to CR.

What increases moment?

Larger perpendicular distance from line of action to CR.

Define couple

Two equal/opposite parallel forces → pure rotation

What movement does a single force produce?

Tipping

What produces translation?

Force + counter‑moment (force through CR).

Uncontrolled tipping COR location?

Incisal to apex.

Controlled tipping COR location?

At apex

Root movement COR location?

At crown tip.

Translation COR location?

At infinity (no rotation).

Which movement is hardest to achieve?

Pure translation

Force for tipping?

35-60 g

Force for translation?

70-120 g

Force for torque?

50-100 g

Force for intrusion?

10-20 g

Force for extrusion/ rotation?

35-60 g

Elastomerics function?

Space closure, ligation.

Stainless steel properties?

Strong, stiff, weldable, cheap.

NiTi properties?

Shape memory + superelasticity → initial alignment.

TMA properties?

Springy, formable → finishing, springs.

Elastic deformation?

Returns to original shape

Plastic deformation?

Permanent change

Ultimate tensile strength?

Max load before fracture.

Modulus of elasticity (E)?

Stiffness (slope of elastic region).

Toughness?

Area under stress‑strain curve.

Fatigue?

Fracture from repeated low stress.

Round wires used for?

Initial alignment

Rectangular wires used for?

Torque + finishing

Typical alignment wires?

.012, .014, .018 NiTi.

Working wires?

.016×.022, .017×.025, .019×.025 SS/NiTi.

Finishing wires?

.019×.025 SS or TMA.