Significance of the Curve of Spee and compensating curves

Significance of the Curve of Spee and compensating curves

Why curve of Spee and compensating curves are important?

We have read about marginal ridges, cusp tips, and fossa in the anatomy of a crown. Let’s have a glance before understanding the curve of spee and other compensating curves

Marginal ridges are the raised convex surfaces at the mesial and distal borders of the tooth, where the occlusal surfaces meet the interproximal surface.

The most elevated portion of the marginal ridge is only slightly convex; hence the contact best described as the cusp tip contacting a flat surface.

Cusp tips penetrate through food easily and spillways provided in all directions.

Each tooth occludes with two opposing teeth-

        a) Its namesake tooth in the opposing arch

   b) Its adjacent tooth

Two exceptions

This relationship helps distribute occlusal forces to several teeth and ultimately over the entire arch.

Also helps to maintain some arch integrity, even when a tooth is lost, as stabilizing occlusal contacts are still maintained on all the remaining teeth.

The alignment of the dentition in the dental arches occurs as a result of complex multi-directional forces acting on the teeth during & after the eruption.

normal musculature

When teeth erupt they are directed into a position where opposing forces are in equilibrium. The major opposing forces that influence tooth position originate from the surrounding musculature.

eruptive


Occlusal contacts-

Prevents extrusion or supraeruption of the teeth.

Maintains arch stability

On mandible closure, the unique occlusal contact patterns reemphasize and maintain tooth position.

When lost, unopposed tooth supra erupts till the occlusal contact is established.

Intra arch tooth alignment-

Relationship of the teeth with each other within the dental arch.

Plane of occlusion – imaginary surface that touches the incisal edges of the incisors and the tips of the occluding surface of the posterior teeth

Definition: The average plane established by the incisal and occlusal surfaces of the teeth               

It is not flat.

Curved to permit the maximum use of tooth contacts during the function.


Curve of Spee

Anteroposterior curvature

curve of spee
Curve of Spee

The anatomic curve established by the occlusal alignment of the teeth, as projected on to the median plane, beginning at the cusp  tip of the mandibular canine and following the buccal cusp tip of the premolars and the molar teeth, continuing through the anterior border of mandibular ramus, ending with the anterior-most portion of the mandibular condyle     

Purpose of the curve of Spee

Alignment: Offers maximum resistance to functional loading.

purpose of curve of spee

    The long axis parallel to the arc of closure around the condyle axis

  Location in relation to condyle: Relates to the condylar path of the protrusion

relates condylar path

The curve of Spee permits

Protrusive disocclusion of the posteriors by the combination of the anterior guidance and the condylar guidance.

Incisive function to the anterior’s


Curve of Wilson

MEDIOLATERAL CURVE

Contacts the buccal and lingual cusps of one side to the lingual and the buccal of the other side.

Curve of Wilson

Functions:

Offers resistance to the loading

resistance to the loading

Help and aids in the masticatory function

Help and aids in the masticatory function


The area of the tooth between the buccal and lingual cusp tips of the posterior teeth is called the OCCLUSAL TABLE.

The major forces of mastication are applied in this area. It represents approx 50%-60% of the total Bucco-Lingual dimension of the posterior tooth.

Relationship of the teeth in one arch to those in the other

The distance of the line that begins from the distal of the third molar extends mesially through all proximal contacts around the entire arch and ends at the distal surface of the opposite third molar represents ARCH LENGTH.

      Mandibular arch (126mm) < maxillary arch (128mm)

ARCH WIDTH is the distance across the arch

arch length

 To maintain the occlusal harmony, the posterior teeth must pass close to but must not contact their opposing teeth during mandibular movements.

Disocclusion: as the separation of the opposing teeth during eccentric movement of the mandible.

Posterior controlling factor (Condylar guidance)

  1. As the condyle moves out of centric relation it descends along the articular eminence
  2. If the articular eminence is steep, the condyle describes a steep vertically inclined path and if flatter, the path is less vertically inclined
  3. The angle at which the condyle moves away from a horizontal reference plane is referred to as the condylar guidance angle
  4. The CGA is steeper for the orbiting condyle in a latero-trussive movement of the mandible as the medial wall of the mandibular fossa is steeper than the articular eminence in front
  5. The two TMJ’s provide the guidance for the posterior portion of the mandible and are largely responsible for determining the character of mandibular movement posteriorly. Thus they become the posterior controlling factor
  6. It is a fixed factor

Anterior controlling factor (Anterior guidance)

  1. The anterior teeth guide the movement of the anterior portion of the mandible
  2. As the mandible protrudes, the incisal edge of the mandibular anterior teeth occlude with the lingual surfaces of the maxillary anterior teeth
  3. The steepness of the lingual surface determines the amount of vertical movement of the mandible
  4. It  is a variable factor

VERTICAL DETERMINANTS

Effect of condylar guidance on cusp height

Effect of condylar guidance on cusp height

Steeper the articular eminence more is the descent of the condyle, resulting in greater vertical movement. Thus allowing for steeper posterior cusps

 Effect of anterior guidance on cusp height

        Increased horizontal overlap decreases the anterior guidance angle. There is a less vertical movement of the mandible leading to flatter posterior cusps

Effect of anterior guidance on cusp height

     Increased vertical overlap increases the anterior guidance angle. There is a more vertical movement of the mandible leading to steeper posterior cusps

Increased vertical overlap increases the anterior guidance angle.


Effect of plane of occlusion on cusp height

  1. The plane of occlusion is an imaginary line touching the incisal edges of the maxillary anterior teeth and the cusps of the maxillary posterior teeth
  2. Depending on the angulation of the plane of occlusion in relations to the horizontal plane, the degree of movement of the tooth varies. If less degree of movement is seen – flatter cusps; and if more degree of movement is seen – taller cusps.

 Effect of the curve of Spee on cusp height

  A curve of Spee is an anteroposterior curve extending from the tip of the mandibular canine along the buccal cusp tips of the mandibular posterior teeth

  Degree of curvature of the curve of Spee influences the height of the posterior cusps.

Flatter the curve of Spee, greater is the angle away from the maxillary posteriors – taller cusp.

The more acute curve of Spee, smaller the angle of mandibular posterior tooth movement – flatter cusps.

  An orientation of the curve of Spee also influences cusp height

  Radius perpendicular to the horizontal reference plane

  Posterior teeth located distal to the radius need shorter cusps and those mesial to the radius need taller teeth

  •               Rotated posteriorly: Shorter cusps
  •               Rotated anteriorly: Taller cusps

Effect of mandibular lateral translation movement on cusp height

This is a bodily side shift of the mandible where the orbiting condyle moves downwards, forwards and inwards.

The degree of inward movement of the orbiting condyle is determined by two factors –

  •           Morphology of medial wall of the mandibular fossa
  •           The inner horizontal portion of TM ligament, which attaches to the lateral pole of the rotating condyle

The lateral translation has three attributes: amount, direction and timing

The amount of lateral translation depends on the tightness of the inner horizontal portion of the TM ligament attached to the lateral pole of the rotating condyle.

Looser the ligament, greater is the amount of lateral translation, posterior cusps should be shorter to permit lateral translation without contact.

The direction of shift of the rotating condyle is determined by morphology and ligamentous attachment of the rotating TMJ. Movement occurs within a 60˚ cone permitting superior, inferior, anterior and posterior movements in addition to the lateral translation.

The vertical movement of the rotating condyle during a lateral translation helps determine cusp height

                                         Latero-superior movement – shorter posterior cusps

                                         Latero inferior movement – longer posterior cusps

curve of spee on cusp

The time of lateral translation is of great influence on occlusal morphology. If late, there is less influence as the maxillary and mandibular cusps are beyond functional range.

If early, a shift is seen even before the condyle begins to translate (immediate side shift).

If it occurs along with an eccentric movement, the movement is called progressive side shift.

The more immediate the side shift, the shorter are the posterior teeth cusps to avoid interference.

HORIZONTAL DETERMINANTS

Effect of distance from the rotating condyle

 Increased distance – wider angle between laterotrussive and mediotrussive pathways, flatter centric cusps

Effect of distance from the rotating condyle


Effect of distance from the midsagittal plane

Increased distance – wider angle between laterotrussive and mediotrussive pathways, flatter centric cusps

Effect of distance from the midsagittal plane

Effect of distance from rotating condyle and fossa from the midsagittal plane

        They generally counteract each other. The combination of the two positional relationships is what determines the exact pathways of the centric cusp tips.

        The curvature of the dental arch causes the two components to counteract each other and negate their effect

Effect of mandibular lateral translation movement

        Increased lateral movement, increases the angle between laterotrussive and mediotrussive pathways

The direction of rotation of the rotating condyle also plays a role

  • Lateral and anterior direction – Increased angle (flatter cusp)
  • Lateral and posterior direction – decreased angle (sharper cusp)

Effect of intercondylar distance

        Increase in the distance reduces the angle between laterotrussive and mediotrussive pathways.



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