ARRANGEMENT OF TEETH
DENTAL ARCH FORM
The teeth are arranged in the maxilla and the mandible in such a way to produce a curved arch which may be a U-shaped, square or tapered.
The shape of the arch from the facial aspect of the teeth is described as being a parabolic curve. The curve is divided into three segments; anterior, middle and posterior.
- The anterior segment is presented by a curved line includes the anterior teeth and ending at the labial ridge of the canine.
- The middle segment is represented by a straight line including the distal portion of the canines, the premolars and the buccal ridge of the mesiobuccal cusp of the first molar.
- The posterior segment is represented by a straight line passing along the buccal cusps of the first, second and third molars.
The line describing the segments of the curve will overlap slightly at the canines and the first molar regions. This arrangement indicates that the canines and first molars serve as anchor supports for both upper and lower arches. The are segments of upper and lower arches are not similar, they differ in three details:
- The anterior segment of the mandibular arch is smaller than that of the maxillary arch.
- The middle segment of the mandibular arch extends distally to the distobuccal cusp of the first molar, while the maxillary one ends at the mesiobuccal cusp of the first molar.
- The posterior segment of the upper arch may be inclined palatally instead of being parallel to the median plane.
These differences between upper and lower dental arches allows the following:
- Extension of the direction of the mandibular movement.
- Protection of the checks, lips and tongue from being clipped during closure of the teeth.
- Avoiding the clashing of the incisal edges and cusps of molars during function.
Fig. 1. Parapolic curve of maxillary arch (upper) and mandibular arch (lower).
Fig. 2. Bonwill triangle of the mandible.
The mandible was described as being adapted to 4 inch equilateral triangle. The angles of the triangle are placed at the centers of each of the condyles and at the mesial contact area of the mandibular incisors. In fact the measurement of 4 inches can not be emphasized arbitrarily, but the Bonwell’s theory did emphasize bilateral symmetery of the mandibular arch.
COMPANSATING CURVATURE OF THE DENTAL ARCHES
The occlusal surfaces of dental arch do not follow a flat plane. The mandibular arch conforms generally to one or more curved planes which appear concave, while the opposing maxillary arch conforms to a curvature which appear convex. When the two arches are brought together in centric occlusion, this curved planes become identical.
Fig. 3. Curve of Spee
Curve of Spee
This curve is seen within the sagittal plane when the upper and lower dental arches are observed from a point opposing to the first molars. The incisal ridges of the anterior teeth and the buccal cusps of the posterior teeth follow a curve that end at the anterior surface of the condyles.
Fig. 4. Curve of Wilson.
Curve of Wilson
In the coronal plane, the occlusal surfaces of the posterior teeth conform to a curved plane. The crowns of the mandibular teeth must incline to the lingual, while the crowns of the maxillary teeth must incline toward the buccal to conform to the curve.
The curve is deeper posterior;ly so that the inclination of the molars is greater than the premolars. Because of this inclination the buccal cusps of the lower molars and the lingual cusps of the upper molars appear to be longer. The importance of this curve is to complement the paths of the mandibular condyle during movements.
Fig. 5. Sphere of Manson.
Sphere of Monson
It is a three dimensional curvature of the occlusal plane, which is the combination of the curve of Spee and the curve of Wilson. The mandibular arch was originally described as adapting itself to a curved surface of a sphere 4 inches radius with the center of the sphere at the glabella. However, the arbitrary 4 inches measurement was refuted because the radius of the sphere varies considerably in different individuals. The mandibular teeth are the one that establish compensating curvatures, and the maxillary teeth have to adapt themselves to the mandibular teeth.
COMPANSATING CURVATURE OF THE INDIVIDUAL TEETH - CURVED TOOTH AXIS
The axis of the teeth are not at right angle to their occlusal surfaces, accordingly the force in occlusion do not act upon the tooth in straight lines. If this is not true the arches would not be stable very long because the forces brought to the tooth units would be tangent to their axes at any time the teeth of the two arches were separated. A resistant food bolus between the teeth will exert pressure unfavorable to stabilization of the teeth. In addition it is impossible to adapt teeth with straight axes to curved occlusal planes.
Fig. 6. Curvatures of individual teeth.
Accordingly the long axes of the teeth are all curved. These axial curvature tend to be parallel with each other in centric occlusion and during jaw opening regardless the extent of the opening. Importance of curved tooth axis are listed in table 1.
Angulation of individual teeth in relation to various planes:
Each tooth is positioned in the dental arch with an angle that best withstand the forces applied to it during function. The axial inclination is also essential for proper occlusal and incisal function of teeth.
Fig. 7. Mesiodistal inclincation of upper and lower teeth.
The teeth angulations are usually described in mesiodistal and faciolingual directions. The axial inclination of teeth relative to the median and horizontal planes varies with each group of teeth as it depends on the function the tooth should perform. (Figs 7-8)
Section through the jaws with the teeth in centric occlusion show the mesial aspect of each tooth in both arches reflect the facio-lingual inclination of the teeth as follows:
- The incisors are placed with their axes at about 60o to the horizontal plane. The axes of the maxillary incisors form an acute angle with the axes of mandibular incisors.
- The canines are placed with their axes forming less acute angles with the horizontal plane.
- The maxillary premolars are placed with their roots slightly inclined lingual as well as the mandibular first premolar, while the mandibular second premolar the roots is inclined buccally.
- The maxillary molars display great lingual inclination of their roots, while the roots of the mandibular molars exhibit great buccal inclination. This provides parallism of the maxillary and mandibular molars axes.
- Prolongation of the line bisecting the lower first molar tends to pass between the buccal and lingual roots of the maxillary first molar.
- Prolongation of the lines bisecting the mandibular second and third molars tend to bisect the lingual roots of maxillary second and third molars.
Fig.8. Facio-lingual inclination of incisors (A), Canines (B) First molars (C) and second molars (D)
The teeth also has mesiodistal axial inclinations in relation to horizontal and vertical planes. The degree of inclination is not great but it is readily seen that the long axes of the teeth is never at right angles to horizontal plane. From the labial and buccal aspect the axial inclination appear as follows:
- The maxillary anterior teeth are placed with their long axes making less than right angle with the horizontal plane with an average of 80o. This arrangement makes the crowns of the teeth point medially while their root point distally. (Fig. 20, upper)
- The mandibular incisors are nearly straight or with slight mesial root inclination, while the canines have slight distal inclination.
- The premolars and molars either mandibular or maxillary exhibit moderate to great distal root inclination.
Fig. 9. Functional form of teeth.
The functional importance of axial inclination of teeth are shown in table 2.
FUNCTIONAL FORMS OF TEETH AT THEIR INCISAL AND OCCLUSAL THIRDS
The incisal and occlusal thirds of the teeth crowns present convex or concave surfaces at all contacting occlusal areas, this includes cusps, ridges, fossa, sulci and embrasures. In other words there are no flat plans on the incisal and occlusal surfaces of any tooth unless they are created by wear or accident.
The elevation of crowns in the dental arch interlock with depressions of crowns of the opposing arch during centric occlusion. This is termed self-occluding design of tooth crown.
The lingual surface of the maxillary incisors present concave areas where convex portion of the mandibular incisors come into occlusal contact.
Cusps of the posterior teeth will contact the sides of the sulci that are formed by the convexities that point into the developmental grooves.
In centric occlusion the teeth seem to intercuspated closely, but escapment spaces are found between the occluding surfaces. These spaces are minute when the teeth are in maximum contact and become larger as the teeth come out of occlusion but preserve some degree of occlusal contact. ■
Occlusion is the term used to describe the contact of teeth in opposing dental arches when the jaws are closed “Static Occlusal relation” and during various jaw movements “Dynamic Occlusal Relationship”. During jaw movements the two mandibular condyles under go the following possible movements:
- Bilateral symmetrical movements
- Opening and closing (elevation and depression of the mandible)
- Forward movements (protrusion of the mandible)
- Backward movement (retrusion of the mandible)
- Bilateral asymmetrical movements:
- Right and left lateral movement where one condyle acts a a bivot while the other move.
Occlusion is also defined as that situation created when the mandibular teeth come in contact with maxillary teeth in functional relations. There are groups of functional relations presented by the mandibular movements. These are centric occlusion, protrusive occlusion, retrusive occlusion and lateral occlusal relation. However, an ideal occlusion should fulfill the following criteria:
- Evenly distribute the masticatory force over the teeth in both arches during function.
- Helps to bring symmetery of facial bones, muscles and good appearance.
- Promote good pronunciation of letters during speech
- Helps the stability of jaw relation.
Development of occlusion
Occlusion develops in childhood as the primary teeth erupt. At this time oral motor behavior develops and masticatory skills are acquired.
Occlusal of the erupting permanent teeth is then dependant on that of the primary teeth as they are being shed. Improper occlusion leads to undue occlusal stresses placed on teeth resulting in occlusal disharmony which eventually leads to changes in the periodontium and weakening of the masticatory apparatus
Interrelated factors are involved in the development of occlusion. These are:
- Dental arch form and alignment of teeth.
- The associated musculature.
- The neuromuscular pattern developed with mastication.
- The development and function of the temporomaandibular joint.■
Ideal tooth relationships were described in the early 1900s by Edward H. Angle. In centric occlusion there are three relationship that can exist between the first molars. He classified ideal occlusion as class I centric occlusion and defined it based on the relationship between the maxillary and mandibular permanent first molars.
A. Class I occlusal relation (normal relation)
The permanent maxillary first molar is slightly posterior to the permanent mandibular first molar. The mesiobuccal cusp of maxillary first molar is directely in line with the mesiobuccal groove of the mandibular first molar.
Fig. 10. The key of occlusion.
B. Class II occlusion relation
The buccal groove of the mandibular first molar is posterior to the mesiobuccal cusp of the maxillary first molar.
C. Class III occlusion relation
The buccal groove of the mandibular first molar is more anterior than normal to the mesiobuccal cusp of the maxillary first molar.
Key of occlusion
The key to the intercuspal relationship of the teeth in the centric occlusal position is described as the relative position of the maxillary and mandibular first permanent molars. The permanent first molars are considered the key for occlusion for the following causes:
- They are the first penanent teeth to develop and erupt in the oral cavity.
- They are guided during eruption by the presence of the second deciduous molars.
- They are the largest teeth in the dental arch.
- Their eruption is not disturbed since they have no deciduous predecessors.
- The maxillary first molars are preferred, as the erupt in the maxilla which is fixed to the skull.
I. CENTRIC OCCLUSION
Centric occlusion is defined as the relation of the upper and lower teeth when they are in maximum intercuspation. The central occlusion position is the start and the terminal position of all physiologic mandibular movements. In class I centric occlusion teeth in the opposing arches show different relationships
Each tooth in the dental arch occludes with two teeth in the opposing arch, except the mandibular central incisor and maxillary third molar (Fig. 24). These serves to:
- Equalize the forces of contact in occlusion, thereby distributing the work.
- It preserves the integrity of the dental arch in case of loosing a tooth, since the second antagonist prevents the elongation and displacement of the opposing tooth.
Vertical alignment of teeth
The longitudinal axis of each maxillary tooth is slightly distal to the corresponding mandibular tooth so that:
- The tip of the mesiobuccal cusp of the maxillary first molar is aligned directly over the mesiobuccal groove of the mandibular first molar i.e. the key factor in definition of class I occlusion.
- The distal surface of maxiilary first molar is posterior to that of the mandibular first molar.
Relationship of anterior teeth
The maxillary anterior teeth overlap the mandibular anterior teeth both horizontal and vertical. (Fig. 25)
- Horizontal overlap (Overjet), the inscial edge of maxillary anterior teeth are labial to the incisal edges of the mandibular teeth. It is due to the larger parabolic curve of the maxillary arch than the mandibular arch.
- Vertical overlap (Overbite), the incisal edges of the maxillary anterior teeth extend below the incisal edges of the mandibular teeth. It is specific to the incisors and canines. The palatal surface of the maxillary incisors overlaps the incisal third of the labial surfaces of mandibular incisors.
With age, and as a result of attrition, the amount of overjet and overbite decrease.
Relationship of posterior teeth
The maxillary posterior teeth are slightly buccal to the mandibular posterior teeth so that:
- The buccal cusp and the buccal surfaces of the maxillary teeth are buccal to those of the mandibular teeth.
- The lingual cusps of the maxillary teeth rest in the occlusal fossae of the mandibular teeth.
- The buccal cusps of the mandibular teeth rests in the occlusal fossae of the maxillary teeth.
Contact relation of permanent teeth in centric occlusion
Figures 81 to 82 demonstrate the contact relationship of permanent teeth in centric occlusion.
A. Anterior Teeth
The incisal ridge of the upper central and lateral incisors are free of contact. All the upper incisors have a labial relation to the lower ones in contact occlusion. The following describe the contact relation of the incisors:
- Upper and lower central incisors meet at the midline and their contact areas are in alignment with each other.
- The lower lateral incisors contacts the distal third of the upper central incisor and the mesial half of the upper lateral incisor at the junction of their incisal and middle thirds of the lingual surfaces.
- The distal outline of the upper central incisor is centered above the labial surface of the lower lateral incisor.
- The distal outline of the upper lateral incisor is centered above the labio-incisal part of the lower canine.
- The cusp tip of the upper canine is free of contact and is seen in the labial embrasure between the lower canine and first premolar. The mesial outline of the upper canine is above the cusp tip of the lower canine, while its distal outline is above the buccal cusp tip of the lower first premolar.
- The cusp tip of the lower canine is directly below the linguo-incisal embrasure of the upper lateral incisor and canine.
- The mesial outline of the lower canine is centered below the lingual surface of the upper lateral incisor, while its distal outline is centered below the lingual ridge of the upper canine.
Upper first premolar
- Buccal cusp is free of contact and is located in the buccal embrasure between the lower first and second premolars.
- The distal cusp is in contact with the distal marginal ridge of the lower first premolar.
- The mesial outline is directly above the buccal cusp tip of the lower first premolar.
Upper second premolar
- The buccal cusp is free of contact and is located in the buccal embrasure between the lower second and first premolars.
- The lingual cusp is in contact with the distal triangular fossa of the lower second premolar. In three cusp type lower second premolar it contacts the distal slope of the distolingual cusp.
- The mesial outline lies directly above the buccal cusp of the lower second premolar.
- The distal outline is above the mesiobuccal cusp of the lower first molar.
Lower first premolar
- The mesial outline is placed directly below the lingual ridge of the upper canine.
- The distal outline is placed directly below the triangular ridge of the buccal cusp of the upper first molar.
- The buccal cusp is below the lingual surface of the upper canine. It contacts the mesial marginal ridge of the upper first premolar .
- The lingual cusp is short and free of contact and lies below the lingual embrasure between the upper canine and first premolar.
Lower second premolar
- The buccal cusp is mainly below the occlusal embrasure of the upper first and second premolars. It contacts the mesial marginal ridge of the upper second premolar.
- The lingual cusp is free of contact and lies in the lingual embrasure of the two upper premolars.
- In the three-cusp type, the mesiolingual cusp is free of contact, and the distal slope of the distolingual cusp strikes the lingual cusp of the upper second premolars.
The maxillary molars bear a distobuccal relation to the mandibular molars, their mesiodistal dimensions are smaller, while their buccolingual dimensions are greater than lower molars.
The occlusal slopes of the buccal cusps of maxillary teeth contact the occlusal third of the buccal cusps of the lower teeth
Upper first molar
- The mesiobuccal cusp rest on the mesiobuccal sulcus of the lower first molar.
- The distobuccal cusp rests in the embrasure of the lower first and second molars.
- The mesiolingual cusp lies in the central fossa of the lower first molar.
- The distolingual cusp contacts the mesial marginal ridge of the lower second molar.
Upper second molar
- The mesiobuccal cusp rests in the buccal sulcus of lower second molar.
- The distolingual cusp lies in the embrasure of the lower second and third molar.
- The mesiolingual cusp rests in the central fossa of the lower second molar.
- The distolingual cusp strikes the distal cusp ridge of the distolingual cusp of the lower second molar.
Upper third molar
- The triangular ridge of the mesiobuccal cusp rest in the buccal sulcus of the lower third molar.
- The distobuccal cusp may be free, or contacts through its triangular ridge the lower third molar on the distal slope of its distobuccal cusp.
- The mesiolingual cusp rests in the central fossa of he lower third molar.
- If it has a distolingual cusp, it strikes the lower third molar on its distal marginal ridge.
- Its distal outline is in the same line with the distal outline of the lower third molar.
Lower first molar
- The mesiobuccal cusp contacts the distal marginal ridge of the upper second premolar and the mesial marginal ridge of the upper first molar.
- The distobuccal cusp rests in the central fossa buccal to the central pit of the upper first molar.
- The distal cusp rests in the distal triangular fossa of the upper first molar.
- The mesiolingual cusp is free of contact and is located in the lingual embrasure between the upper second premolar and the first molar.
- The distolingual cusp is free and is located immediately lingual to the lingual developmental groove of the upper first molar.
Lower second molar
- The mesiobuccal cusp contacts the marginal ridge of the upper first and second molars.
- The distobuccal cusp is centered in the central fossa of the upper second molar.
- The mesiolingual cusp is free of contact and is located in the lingual embrasure of the upper and first and second molars.
- The distolingual cusp is free of contact and is located just lingual to the lingual groove of the upper second molar.
Lower third molar
- The mesiobuccal cusp contacts the mesial triangular fossa of the upper third molar.
- The distobuccal cusp rests in the central fossa of the upper third molar slightly distal to the central pit.
- The mesiolingual cusp is free of contact and is located in the lingual embrasure of the upper second and third molars.
- The distolingual cusp is free of contact and is located just lingual to the lingual groove of the upper second molar.
Centric occlusal relation of the deciduous teeth
The normal occlusion of the deciduous teeth is established at the age of three years as follows:
- The mesial outline of the upper and lower central incisors are in line with each other at the midline.
- The upper central incisors have a labial position to the lower incisors which strike the upper teeth lingually above the level of the incisal ridge.
- The upper central incisor occludes with the lower central incisor and the mesial third of the lower lateral incisor.
- The upper lateral incisor occludes with the distal two-thirds of the lower lateral incisor and the portion of the lower canine which is mesial to the tip of its cusp.
- The upper canine occludes with the portion of the lower canine distal to the cusp tip and the portion mesial to the tip of the mesiobuccal cusp of the lower first molar (about one third of the molar)
- The upper first molar occludes with the distal two-thirds of the lower first molar and the mesial portion of the lower second molar represented by the mesial marginal ridge and the mesial triangular fossa.
- The upper second molar occludes with the remainder of the lower second molar. The distal surface of the upper molar projects slightly over the distal portion of the lower second molar (distal step).
Fig. 11. Angle's classification of occlusion
II. PROTRUSIVE OCCLUSION RELATION
When the mandible moves from centric occlusion, the only teeth that should touch are the anterior. The mandibular four incisors should glide across the maxillary four incisors. The canines may touch slightly. No posterior teeth should come in contact during the protrusive movement.
Occlusal cycle of the anterior teeth
This cycles occurs during the protrusive movement in the following steps:
- Starting from centric occlusion position, the mandible moves downward to free the cusps.
- The mandible moves forward for biting by the anterior teeth, while the balancing side at the posterior teeth.
- Then the mandible moves backward and upward to centric occlusion.
This alternating protrusion to working and back to centric is called “Occlusal cycle of anterior teeth”.
Fig. 12. Class I centric occlusal relationship of upper and lower teeth.
Fig. 13. Horizontal (black arrow) and vertical (white arrow) ovelap (left) and Mesial view of upper and lower first molars with maximum intercuspation during centric occlusion. (right)
Fig. 14. Contact relations of permanent teeth in centric occlusion.
Fig. 15. Incisors relations (left) and canine relations (right) in centric occlusion
Fig. 16. Centric occlusion relationship of premolars.
Fig. 17. Centric occlusion relation of molars, facial relation (upper), lingual relation (middle) and mesial relation (lower).
Fig. 18. Centric occlusion relationship of deciduous teeth.
Fig. 19. Occlusal cycle of anterior teeth (left). Step 1 mandible moves down and forward, step 2 mandible moves back and upward and occlusal cycle of posterior teeth (right)
III. RETRUSIVE OCCLUSAL RELATION
The mandibular teeth show posterior relation to centric occlusion with the maxillary arch. Actually retrusion is very limited and not performed during mastication. It is a reference mandibular movement used by dentists.
IV. LATERAL OCCLUSAL RELATION
In lateral occlusal relation, the mandible moves toward the right or left side until the canines on that side are in cusp to cusp relationship.
The side to which the mandible moves is referred to as the working side while the other side is referred to as the non-working side. On artificial teeth this non-working side is referred to as the balancing side.
Occlusal cycle of posterior teeth
This cycle occurs during the lateral movement of the mandible which starts from the centric occlusion and ends also in centric occlusion as follows:
- From centric occlusion the mandible moves downward to free the cusps.
- The mandible moves to lateral side, e.g. right side. This is the working side while the left side is tne non-working side.
- In the working side the buccal cusps of the maxillary and mandibular teeth are in contact
- In the nonworking side, the lingual cusps of the maxillary teeth come in contact with the buccal cusps of the mandibular teeth.
- Then the mandible moves back to centric occlusion.
Forms of malocclusion
There are three common forms of malocclusion which are crowding, anterior open bite and cross bite.
Crowding of teeth: Crowding is the term used to describe the condition where the teeth are markedly out of the line of the dental arch. Usually these condition is due to disproportion between the size of the arch and the size of the teeth.
Anterior open bie: This condition occurs when there is no incisors contact and no incisor overbite. It may be caused by thumb sucking habit, abnormal swallowing pattern or skeletal abnormalities.
Fig. 20. Anterior open bite.
Cross bite: This is a transverse abnormality of the dental arches where there is an asymmetrical bite. The condition may be unilateral or bilateral. The condition is usually related to discrepancies in the width of the dental bases and may involve displacement of the mandible to one side to obtain maximum intercuspation. In this condition the mandibular teeth have facial relation to maxillary teeth. There are two main forms of cross bite:
- Anterior cross bite which is usually associated with class III occlusal relation, or it may be due to improper faciolingual inclination of the whole segment of an individual tooth.
- Posterior cross bite which is usually related to discrepancies in width of the dental base and may involve displacement of the mandible to one side. This condition can be unilateral or bilateral.■