Alignment and Occlusion

Summary of Alignment & Occlusion:

Alignment & Occlusion refers to how your teeth come together when you bite down

If we were to put down an imaginary surface/ or a piece of paper on your bottom teeth (mandibular teeth), it would not be a flat surface. It would be curved due to the different inclines of the teeth. Orthodontists, dentists, and researchers have looked at different alignments of the mouth and they created theoretical shapes to fit the mouth. The following shapes are described below.

Bonwill’s Triangle: theoretically, there is a triangle in the mouth. There’s four inches in between each condyle (end points of the lower jaw that articulate in the TMJ) and this forms the base of the triangle. The sides of the triangle are formed by a line that stretches from one condyle to the middle of the lower central incisors and each side is also 4 inches long.

Monson’s Sphere: theoretically, there is a sphere in the mouth and the radius of this sphere is 4 inches. The center of Monson’s sphere is located in the gabella area of the brain. 3 radii that extend from the the center of the sphere touch each respective condyle and the occlusal (top) of the posterior teeth. Monson’s sphere can be broken apart into the Curve of Spee (shows the alignment of cusp tips from posterior to anterior and is viewed in the sagittal view) and the Curve of Wilson (curve that touches one opposing tooth to another and is viewed in the frontal or posterior plane).
^The above curves seem rather intuitive, however, the Curve of Spee can be tricky depending on the view you are looking at. Normally, looking at a lateral (sagittal) view the maxillary (upper arch) is convex and the mandibular (lower arch) is concave. *I remember it by thinking maXillary=conveX and mandibular is like a ‘man going into a cave.’ (weird..i know). The trick with the Curve of Spee is that if you are viewing the curve from a superior view than the maxillary arch will also appear concave because you have to remember that you are looking down at a ball.

*when you think about it, Bonwill’s triangle and Monson’s sphere are in harmony with one another. The upper jaw is in harmony with the lower jaw . All teeth are tipped outward (or mesially) except the maxillary 3rd molars. The upper teeth fit outside the lower teeth because they are inclined buccally (toward your cheek) whereas the the lower teeth are inclined lingually (toward your tongue). **This form follows the function of the teeth**

Focusing on Alignment:
There are 3 alignment lines of the mouth; B-O (buccoocclusal), L-O (linguoocclusal) and C-F (central fossa line).
B-O line (on mandibular arch): occludes with C-F line on max arch
L-O line (on max arch): occludes with C-F line on man arch
C-F line  (on both arches): has proximal contacts that are slightly buccal to C-F line

There’s also an inter-arch relationship (inter=b/t both arches) for the anterior teeth. Usually, the incisal edges of the lower teeth hit the backside of the upper teeth in a position called the lingual fossa (fancy term for a little groove half way up tooth).  *Max anterior teeth overlap the mandibular anterior teeth almost 1/2 a man. crown length. The function of these anterior teeth is anterior guidance, or guidance of the mandible. Some deviations can occur within the inter-arch relationship. Two of these deviations are an “over-jet” and an “over-bite.”
Overjet: is a horizontal overlap (remember that a jet plane fly’s horizontally)
Overbite is a vertical over lap

Focusing on Occlusion:

Angles Classification (Orthodontic Principles)- based on molar relationships
Class 1: ideal alignment- (mesial part of upper first molar sits in groove of lower molar) each mandibular tooth occludes with it’s counterpart and the adjacent tooth mesial to it; technical definition- MB cusp of maxillary first molar sits in buccal groove of of mandibular first molar. Also, MB cusp of mandibular first molar occludes in embrasure space b/t maxillary second premolar and first molar. Finally, ML cusp of maxillary first molar occludes with central fossa of mandibular first molar. Class I Contacts: contacts on premolars occur b/t cusp tips and opposing marginal ridges
Class II: Forward movement – over-bite; the back part of the maxillary first molar sits in groove of mandibular first molar. Technical def: DB cusp of maxillary first molar occludes with developmental groove of mandibular first molar.  Lower 1st molar is 1/2 behind it’s counterpart. In ‘Mild class II occlusion’  the first molars are ‘end-to-end’.
Class III:Backwards movement. Technical def: DB cusp of mandibular 1st molar occludes in embrasure space of max 2nd premolar and max 1st molar. MB cusp of maxillary 1st molar occludes in embrasure space b/t man first and second molars. ML cusp of maxillary 1st molar occludes in mesial pit of mandibular second molar.

Some definitions to know-
Maximum Intercuspation (denoted as MI or ICP)- relationship b/t maxillary and mandibular teeth which produces the maximal contact b/t their respective occlusal surfaces.


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