COMMON SENSE MECHANICS 101

Dr. Rabab Khursheed

CONTENTS • JCO 1979-1980‡ Common Sense Mechanics Part 1

-Introduction -Visual inspection -Simple rule

‡ Common Sense Mechanics Part 2-Forces and Moments -Cue Ball concept -Translation, Rotation-forces and moments on teeth -Lingual root torque

‡ Common Sense Mechanics Part 3-Static Equilibrium -Requirements

‡ Common Sense Mechanics Part 4-Cross Bites -Expansion/contraction overlays

‡ Common Sense Mechanics Part 5-Diving board concept -Cantilever principle -Constant load versus constant deflection

‡ Common Sense Mechanics Part 6-Clinical application of the diving board concept

‡ Common Sense Mechanics Part 7-Distalization with differential torque -Class II correction without head gear or elastics

‡ Common Sense Mechanics Part 8-Wire bracket relationships

‡ Common Sense Mechanics Part 9-Extraction mechanics

‡ Common Sense Mechanics Part 10

‡ Common Sense Mechanics Part 11-Simultaneous cuspid and bicuspid retraction

‡ Common Sense Mechanics Part 12-Applying principal to total treatment -Class I non extraction

‡ Common Sense Mechanics Part 13-Class II division 2

‡ Common Sense Mechanics Part 14-Class III malocclusion

‡ Common Sense Mechanics Part 15- Types of cross bites

"Common Sense Mechanics" Thomas F. Mulligan- series of articles in JCO (Sep. 1979 – Dec. 1980)

Based on fact that no appliance exists which allow an orthodontist to treat Maloclussion without adding the necessary ingredient of "Common Sense" to the mechanics instituted for correcting the malocclusion

• Refinement in appliances may ↓ the physical effort, but will not eliminate the need for the orthodontist to • Think• understand• Apply basic principles of mechanics

in a common sense manner

Visual Inspection Method• Often confuses the orthodontist in attempting to

determine with reliability what forces are present• The visual method seems to be so obvious, but faulty

conclusions.

• Not to be misled by determining forces present through the visual inspection method

Common Sense Mechanics, Part 1 Volume 13 : Number 09 : Pages (588-594)

1979

A. Vertical Plane of Space.

B. Horizontal Plane of Space.

•What is the force acting on the Molar ??

A SIMPLE RULE • If the bend is located off center, there will be a long segment and a short

segment. When the short segment is engaged into the bracket or tube, the long segment will point in the direction of the force produced

• Another way to think of it is this: The short segment points in the opposite direction of the force. This is certainly different than visual inspection might lead us to believe.

FORCES AND MOMENTS

• Force is nothing but push or pull

When line of action of force passes through any point other than the COR

This turning effect of force is also known as “Moment of Force”

MOMENT= FORCE X DISTANCE

• You can "sense" a force when you bend a wire, but you cannot "sense" torque. Because the latter is simply a product of force times distance, as previously discussed, the distance (length) is just as effective as the force. If the force passes through the center of resistance, no perpendicular distance is involved. Therefore, regardless of the magnitude of the force, there is no moment.

What does all of this mean?

CUE BALL CONCEPT

FORCES AND MOMENTS ACTING ON TEETH

Differential Torque

• Extrusive force present on the molar teeth acts at the molar tubes which lie, buccally to COR. This force times distance results in molar Lingual Crown Torque.

LINGUAL ROOT TORQUEWhen lingual root torque is placed into the incisor section, a long segment & a short segment is produced as with tip back bend.

When one observes an effect, he should be able to interpret the cause and vice versa. We should also begin to recognize that such force systems should not be routinely considered as undesirable side effects except for the orthodontist who is unaware of their presence and therefore is not prepared to prevent undesirable effects as well as to utilize the systems effectively when indicated. If lingual crown torque is desired, it should be permitted to act. If undesirable, it can be prevented with a lingual arch, a rectangular wire, or whatever means the operator chooses.

STATIC EQUILIBRIUM

• When forces acting on an object which is at rest are balanced, then we say that the object is in a state of static equilibrium. The resultant of these forces equals zero

• If we, desired to convert this "dynamic" state to a state of statics, either shift the unequal weights or the fulcrum point on the board

REQUIRMENTS FOR STATIC EQUILIBRIUM • The sum of all the Vertical Forces present

must equal zero. we must deal with extrusive components of force during overbite correction. • the sum of all Horizontal Forces present must

equal zero .This is why we cannot correct a unilateral crossbite with a single horizontal force

• The sum of the Moments acting around ANY point must also equal zero

We may produce heavy torques in a given area and little or no torque elsewhere, but when added around any given point, they should equal zero.

Golden Rule The archwire when fully engaged always relults and/or tries to achieve a sate of static equllibrium.

• With two equal moments at either end of the archwire, the system is in balance.

•With two unequal moments at either end of the archwire, the system reaches a balance, but seems to be unbalanced and with the entire unit rotating counterclockwise.

• the unequal moments create (in this case) an extrusive force on the incisor and an intrusive force on the molar. The sum of these forces is zero, but the configuration

causes the entire unit to rotate clockwise.

A full strapup with a reverse curve of Spee

ARCH LEVELLING • Do you sometimes observe the posterior teeth moving lingually for

no "apparent" reason during arch leveling procedures?• During arch leveling procedures, we frequently observe responses

that may be undesirable. They often occur unexpectedly and in various forms.• As we can see, there is a reason for all responses. • Whenever we witness responses for "no apparent reason", we have

failed to recognize the cause, and as a result made our treatment somewhat more difficult. The recognition of causes permits us to utilize as well as avoid certain types of tooth movement. • When leveling an arch, it has already been shown that in a full

strapup, intrusive forces act through the molar tubes, producing buccal crown torque on the molars. Do you sometimes observe the posterior teeth moving buccally for no "apparent" reason during arch leveling?

ARCH LEVELING

CROSS BITES

• If an individual molar, or an entire buccal segment in crossbite

• when we observe a buccal segment in crossbite, it may be a unilateral crossbite, or in most of cases a bilateral crossbite with a lateral mandibular shift

OVERLAYS

• 0.36 mm over lays in 0.45 mm head gear tubes

• The force provided will be equal and opposite— not unilateral in direction as might appear to be the case when the overlay is inserted into one tube and observed

• the over corrected or worsened side will eventually return to normal by relapse , wheras the corrected side will remain corrected.

COSMETIC OVERLAY

THE DIVING BOARD CONCEPT • It is not that we use the diving board in force control, but the mental image should permit us to recall more vividly the advantages involved in utilizing the factor of "length" in our archwires. • There is a formula that says that stiffness— or load/deflection rate— is

inversely proportional to the cube of the length. Formulas of this kind often seem confusing and of little use to the orthodontist, as well as difficult to remember.

• To make all of this useful and a little easier, let us analyze the situation more closely. First of all, stiffness is the amount of deflection we get from a given load (force).

Stiffness or load/deflection rate α 1/ L3

• The formula tells us that if we are dealing with a cantilever (such as a diving board), by doubling the length stiffness is reduced to one-eighth. By doubling the length, only one-eighth the force will be required to produce the same deflection or the same force acting at double the length will produce eight times as much deflection.

A.When the length of the diving board is doubled, only one-eighth the force is required to produce the same amount of deflection. B. The same force acting at twice the length will produce eight times as much deflection

• Load on diving board produces bending moments along the board, with the maximum moment being located closest to the point of attachment

In orthodontics, we often refer to this moment as the "Critical Moment", as it is the largest moment involved and is often responsible for breakage in an archwire at that particular point

• when we place a given bend, we must determine what angle is necessary to produce the desired load (force). It also requires that we must know the length of wire between brackets and tubes. We can resort to reference tables or we can go through "trial and error" until we arrive at the bend which gives us the force we want. If, instead, we choose to place a "constant" bend (angle), we find that we create variable loads (forces)

CLINICAL APPLICATION

• As we know, small interbracket distances can produce very high magnitudes of force with the so called "light wires

• Bypassing teeth is one method of increasing interbracket distance. Individuals often use single wing brackets for this purpose, but when all teeth are banded all of the time and an archwire engaged in every bracket automatically, there is little alternative for reducing force levels

• we recognize that length load (force). If we double the length of wire, we reduce the force per unit of deflection to one-eighth.• Therefore, if we bypass bicuspids and cuspids during overbite correction,

and use a wire with tipback bends at the molars, we have in effect created a "diving board”.

• If the tipback activation is constant, such as a 45° angle, then as the distance doubles, so does the deflection

• Therefore, although the load per unit of deflection is reduced to one-eighth, the unit of deflection is doubled, resulting in a net force of one-fourth (2 × 1/8 = ¼). However, it is quite evident that the length of wire is increasing much more than "twice", and therefore the net intrusive force on the anterior segment is dramatically reduced. • With wire sizes of .016, the magnitudes at times become so

low, you wonder if "anything" will happen with the overbite. It is common to have forces in the range of 20-30 grams and lower. If we apply a total force on an incisor segment of 30 grams (intrusion), for example, we produce equal and opposite forces on the molars. But, one-half goes to each molar, meaning that each molar in this example would incur only 7½ grams of force— enough to allow the molars to erupt during vertical growth, but not enough to overcome the forces of occlusion.

DIFFERENTIAL TORQUE

ROW BOAT EFFECT "rowboat effect“- tendency for the maxillary teeth to move forward during anterior lingual root torque

DISTALISATION

• The opposite of the row boat effect

• This "distalization" tendency -easy to check simply by observing the unbanded cuspids and their change in axial inclination.

• The cuspid crowns tip distally as they are forced back as a result of the thrust being received at the crown level.

CLASS II CORRECTION WITHOUT HEADGEAR OR ELASTICS

Class II correction is coincidental during overbite correction using tipback bends esp. in U/arch..

The amount of headgear treatment originally planned is either reduced, sometimes even eliminated.

WIRE / BRACKET RELATIONSHIPS

• The relationship of the archwire to the brackets and tubes, prior to engagement, offers valuable and interesting information• If a straight wire is placed

over angulated brackets, a certain angular relationship develops between the wire and the plane of the bracket slot

• The same wire/bracket relationship can be created by a bend in the wire or a straight wire in relation to a malocclusion.

CENTERED BEND

Applying the requirements for static equilibrium.

• If all four forces (activational) are equal- first requirement for static equilibrium is fulfilled.

Moment : A + D = 0Moment : B + C = 0

STEP BEND

OFF-CENTER BEND

A

B C

D

EXTRACTION MECHANICS • Diffrential torque is produced by a tip back

off-center bend • Depending on the angle at which the wire

with an off-center bend crosses the bracket, and the length of the long segment

• moment produced by the longer segment can be • clockwise • counterclockwise • nonexistent

CUSPID RETRACTION

• Bends given intraorally using a tweed loop forming plier , to create differential torque .

BICUSPID RETRACTION

4 45 E

Extraction of

MOLAR PROTRACTION

TOE IN BEND OR CURVE

• Extractions- 4 4 4 5

• This type of case requires certain compromises to be established beforehand, such as the willingness to leave "some“ lower rotations, which will encourage more overbite and overjet following treatment

TYPES OF ANTERIOR CROSSBITES

SINGLE TOOTH CROSS BITE

CONCLUSION

• Common sense mechanics is a very easy to use system if we as orthodontists understand how it works. It is much more convenient to predict precise movements of teeth and prepare in advance for any unwanted tooth movement.

• This does not require us to use different appliances, or discard a different technique, but certain modifications brought about to the mechanics can be a great deal of help.

REFERENCES • JCO 1979-1980

• Common Sense Mechanics Part 1• Common Sense Mechanics Part 2• Common Sense Mechanics Part 3• Common Sense Mechanics Part 4• Common Sense Mechanics Part 5• Common Sense Mechanics Part 6

• Common Sense Mechanics Part 7• Common Sense Mechanics Part 8• Common Sense Mechanics Part 9• Common Sense Mechanics Part 10• Common Sense Mechanics Part 11• Common Sense Mechanics Part 12• Common Sense Mechanics Part 13• Common Sense Mechanics Part 14

• Common Sense Mechanics Part 15• Common Sense Mechanics Part 16