Introduction to Occlusion












1
Introduction
to Occlusion
1
Functions of Teeth
The main functions of teeth are mastication,
speech, and esthetics. Waxing teeth should be
perfected in a way that allows the nal restoration
to maintain and improve the ability to perform
these functions. Teeth do not perform these
functions alone; they work in harmony with the
other components of the masticatory system,
namely the arches, the temporomandibular joints,
muscles involved both directly and indirectly in
mastication, and the vessels and nerves supplying
these tissues. All components of the masticatory
system are under neurologic control. The mastica-
tory system can be dened as the functional unit
of the body primarily responsible for mastication,
speaking, and swallowing.
1
This chapter focuses on one component of the masticatory system, the
teeth, as this is the main component that can be controlled by the dentist. All the teeth in the mouth
assist in performing these three functions. Posterior teeth are mainly important for mastication, but
they still aect esthetics and speech indirectly, as their proper alignment provides support for the
adjacent soft tissues and muscles of the face and maintains the vertical dimension of occlusion (ie,
the distance between the maxilla and the mandible when teeth are in occlusion; Fig 1-1). Thus, proper
form and position of posterior teeth inuence the overall facial appearance, prevent tooth migration,
and maintain the stability of the dentition. Anterior teeth mainly function in providing beautiful esthetics
and proper pronunciation while still assisting in mastication through incising and biting foods as well
as guiding the mandible through lateral movements. The ability of the teeth to achieve their functions
is based mainly on three factors: (1) tooth morphology, (2) tooth alignment, and (3) occlusion (ie, the
way the opposing teeth t together, which is largely dependent on the rst two factors).
Fig 1-1 Vertical dimension of occlusion (arrows).

Introduction to Occlusion
2
1
Important Definitions
2–4
Centric relation The relationship between maxillary and mandibular teeth when the mandibular
condyles are in the most physiologic orthopedically stable position, which is the most superoanterior
position in the articular fossae. This position is independent of tooth contact and does not necessarily
coincide with maximal intercuspation. In this position, the muscles of mastication function more
harmoniously and with less intensity.
Maximal intercuspation The complete intercuspation of the opposing teeth, sometimes referred to
as the best t of the teeth independent of the condylar position. This position is dictated by the teeth
themselves, determined when the patient habitually self-closes into complete tooth intercuspation.
Maximal intercuspation coincides with centric relation in only 10% of the population.
Protrusive mandibular movement The movement of the mandible forward from the intercuspal
position.
Retrusive mandibular movement The movement of the mandible posteriorly from the intercuspal
position.
Lateral mandibular movement The movement of the mandible to the right and left sides from the
intercuspal position.
Mandibular excursive movements Movement occurring when the mandible moves away from
maximal intercuspation in any direction, which includes protrusive, retrusive, and lateral mandibular
movements.
Working side The side toward which the mandible moves in lateral excursion.
Nonworking side The side away from the direction of mandibular movement in lateral excursion.
Axial loading The process of directing forces along the long axes of teeth, which are the most toler-
ated forces by teeth.
Tooth Morphology
4–8
The following description of tooth morphology is mainly focused on the tooth surfaces that are created
during occlusal waxing, which rst-year dental students are expected to learn to perfect their wax-ups and
understand the signicance of the morphologic features on proper function. A description of the morpho-
logic features needed to create anterior esthetic wax-ups is available in the second chapter of this book.

Tooth Morphology
3
Morphology of the occlusal thirds of posterior teeth (occlusal surfaces)
Occlusal surfaces are formed of several elevations and depressions. These elevations and depressions
form an uneven surface; therefore, when an elevation on an occlusal surface comes in contact with
a depression on the occlusal surface of the opposing tooth during mastication, the food is eciently
ground. Proper knowledge of the specic form of these elevations and depressions is of great impor-
tance during occlusal waxing and tooth restoration to achieve proper masticatory function. In addition
to their role in grinding food, occlusal surfaces should be restored in a way that achieves axial loading
(ie, directs the occlusal forces along the long axes of the teeth). These are the only forces that do
not harm the periodontium and supporting structures. On the other hand, if the teeth are subjected
to lateral forces, this usually leads to frictional wear, occlusal instability, and possible tooth mobility.
Elevations of the occlusal surface
Cusps
A cusp is an elevation on the crown portion of a
tooth making up a divisional part of the occlusal
surface. Cusps resemble pyramids in shape, and
the highest point of the pyramid-shaped cusp is
called the cusp tip (Fig 1-2). Cusps are present in
the posterior teeth and the canines. Premolars
have two cusps, except the mandibular second
premolar, which may have three cusps, and molars
may have three to ve cusps. Cusps are named
according to their location (eg, the maxillary rst
molar cusp that faces the mesial and lingual
surfaces is called mesiolingual cusp). The part of
the tooth between the buccal and lingual cusp tips
is approximately 50% to 60% of the buccolingual
dimension of the tooth and is where the forces of
mastication are applied; this part is called the inner
aspect of the tooth or the occlusal table, and the
part outside the cusp tips is called the outer aspect
of the tooth (Fig 1-3). The inner and outer aspects of
Fig 1-2 Pyramid-shaped cusp and cusp tip.
Fig 1-3 Inner and outer aspects of the tooth.
Outer incline
Outer incline
Inner incline

Introduction to Occlusion
4
1
Fig 1-5 Supporting cusps occlude in the opposing
central fossa. Nonsupporting cusps do not contact
opposing teeth in maximal intercuspation.
B
L
B
Fig 1-4 Buccal cusps are the functional cusps of a
mandibular rst molar and thus occupy one-third of
the buccolingual dimension. Lingual cusps are non-
functional and occupy only one-sixth of the buccolin-
gual dimension.
1/3
1/6
the teeth are made up of inclines that extend from the cusp tips to either the central groove or the height
of contour on the facial or lingual surface. Depending on the role the cusps play in the dentition, they are
divided into functional and nonfunctional cusps.
Functional cusps are given this name because they are the cusps responsible for mastication and
maintaining the vertical dimension of occlusion. They also can be called working cusps and supporting
cusps. From the occlusal aspect, their cusp tips are located approximately one-third the distance into
the total buccolingual width of the tooth (Fig 1-4). Functional cusps are the lingual cusps of maxillary
teeth and the buccal cusps of mandibular teeth. Thus, premolars have only one functional cusp, which
occludes with the opposing marginal ridge(s), and molars have two functional cusps, one that occludes
with the opposing central fossa and one that occludes with the opposing marginal ridge(s) when the
teeth are in maximal intercuspation (Fig 1-5). This contact relation may result in slightly atter cusp
tips due to possible wear. If these cusps are not properly restored to contact, the opposing tooth may
supererupt. An imaginary line can be extended through the functional cusp tips in each arch. In the
mandible the line will pass through the buccal cusp tips and is named the bucco-occlusal line, and in
the maxilla the line will pass through the lingual cusp tips and is therefore named the linguo-occlusal
line. A third imaginary line can be extended through the central developmental grooves of posterior
teeth and is named central fossa line.
4
These three lines pass smoothly and continuously and are a
major aid during cusp and central groove placement in occlusal wax-ups (Figs 1-6 to 1-8).
Fig 1-6 Bucco-occlusal line (dotted line). Fig 1-7 Linguo-occlusal line (dotted line).
BO line
LO line

Tooth Morphology
5
Nonfunctional cusps or nonworking cusps are the mandibular lingual cusps and the maxillary buccal
cusps. They do not contact the opposing teeth in maximal intercuspation; therefore, they do not maintain
the vertical dimension of occlusion or play a major role in mastication, and they tend to have a sharper
cusp tip. They do however maintain the bolus of food on the occlusal table for mastication and minimize
tissue impingement as well as guide the mandible in lateral movements; thus they are also named guiding
cusps (see Fig 1-5). Nonfunctional cusps tips are located approximately one-sixth the distance into the
total buccolingual width of the tooth (see Fig 1-4).
Ridges
Ridges are linear elevations on the surfaces of teeth. They are named according to their location
or shape. The occlusal surfaces of posterior teeth exhibit several ridges: marginal, cusp, triangular,
transverse, and oblique.
Marginal ridges. Marginal ridges are rounded elevated crests forming the mesial and distal bound-
aries of the lingual surfaces of anterior teeth and the occlusal surfaces of posterior teeth. All marginal
ridges converge from the buccal toward the lingual, making the facial half of any tooth wider than the
lingual half and the lingual embrasure wider than the buccal embrasure (Fig 1-9). The embrasures
act as a way for food to escape during mastication, allowing the food to be mixed with saliva to form
the food bolus, and then the soft tissue of the lips, cheeks, and tongue carries the food back to the
occlusal surface. This dynamic reduces the forces brought to bear on the teeth during the reduction
of any material that oers resistance and thus allows more ecient mastication while protecting the
teeth from excessive forces and stimulating the surrounding soft tissues. The tongue, being one of
the strongest muscles in the body, is more ecient than the lips and cheeks in performing this func-
tion; hence a wider lingual embrasure is not only natural but also required to maximize the benet
from the powerful tongue action. Marginal ridges play an essential role in occlusion, as their maximal
convexity occludes with the opposing functional cusp tip (Fig 1-10). During waxing it is important to
create smooth, slightly convex marginal ridges, with a small crest of curvature that coincides with
the opposing functional cusp tip in static closure. The remainder of the marginal ridge is created
smooth and convex mesiodistally, which allows disocclusion (passage of cusps) in mandibular lateral
movements. All adjacent marginal ridges should be at the same height to prevent food impaction
and retention. This should be perfected during waxing to make restorations more ecient and allow
for proper masticatory function.
Fig 1-8 Using bucco-occlusal, linguo-occlusal, and central fossa lines as a guide in occlusal
waxing.

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1Introduction to Occlusion1Functions of TeethThe main functions of teeth are mastication, speech, and esthetics. Waxing teeth should be perfected in a way that allows the nal restoration to maintain and improve the ability to perform these functions. Teeth do not perform these functions alone; they work in harmony with the other components of the masticatory system, namely the arches, the temporomandibular joints, muscles involved both directly and indirectly in mastication, and the vessels and nerves supplying these tissues. All components of the masticatory system are under neurologic control. The mastica-tory system can be dened as the functional unit of the body primarily responsible for mastication, speaking, and swallowing.1 This chapter focuses on one component of the masticatory system, the teeth, as this is the main component that can be controlled by the dentist. All the teeth in the mouth assist in performing these three functions. Posterior teeth are mainly important for mastication, but they still aect esthetics and speech indirectly, as their proper alignment provides support for the adjacent soft tissues and muscles of the face and maintains the vertical dimension of occlusion (ie, the distance between the maxilla and the mandible when teeth are in occlusion; Fig 1-1). Thus, proper form and position of posterior teeth inuence the overall facial appearance, prevent tooth migration, and maintain the stability of the dentition. Anterior teeth mainly function in providing beautiful esthetics and proper pronunciation while still assisting in mastication through incising and biting foods as well as guiding the mandible through lateral movements. The ability of the teeth to achieve their functions is based mainly on three factors: (1) tooth morphology, (2) tooth alignment, and (3) occlusion (ie, the way the opposing teeth t together, which is largely dependent on the rst two factors).Fig 1-1 Vertical dimension of occlusion (arrows). Introduction to Occlusion21Important Definitions 2–4Centric relation The relationship between maxillary and mandibular teeth when the mandibular condyles are in the most physiologic orthopedically stable position, which is the most superoanterior position in the articular fossae. This position is independent of tooth contact and does not necessarily coincide with maximal intercuspation. In this position, the muscles of mastication function more harmoniously and with less intensity.Maximal intercuspation The complete intercuspation of the opposing teeth, sometimes referred to as the best t of the teeth independent of the condylar position. This position is dictated by the teeth themselves, determined when the patient habitually self-closes into complete tooth intercuspation. Maximal intercuspation coincides with centric relation in only 10% of the population.Protrusive mandibular movement The movement of the mandible forward from the intercuspal position.Retrusive mandibular movement The movement of the mandible posteriorly from the intercuspal position.Lateral mandibular movement The movement of the mandible to the right and left sides from the intercuspal position.Mandibular excursive movements Movement occurring when the mandible moves away from maximal intercuspation in any direction, which includes protrusive, retrusive, and lateral mandibular movements.Working side The side toward which the mandible moves in lateral excursion.Nonworking side The side away from the direction of mandibular movement in lateral excursion.Axial loading The process of directing forces along the long axes of teeth, which are the most toler-ated forces by teeth.Tooth Morphology 4–8The following description of tooth morphology is mainly focused on the tooth surfaces that are created during occlusal waxing, which rst-year dental students are expected to learn to perfect their wax-ups and understand the signicance of the morphologic features on proper function. A description of the morpho-logic features needed to create anterior esthetic wax-ups is available in the second chapter of this book. Tooth Morphology3Morphology of the occlusal thirds of posterior teeth (occlusal surfaces) Occlusal surfaces are formed of several elevations and depressions. These elevations and depressions form an uneven surface; therefore, when an elevation on an occlusal surface comes in contact with a depression on the occlusal surface of the opposing tooth during mastication, the food is eciently ground. Proper knowledge of the specic form of these elevations and depressions is of great impor-tance during occlusal waxing and tooth restoration to achieve proper masticatory function. In addition to their role in grinding food, occlusal surfaces should be restored in a way that achieves axial loading (ie, directs the occlusal forces along the long axes of the teeth). These are the only forces that do not harm the periodontium and supporting structures. On the other hand, if the teeth are subjected to lateral forces, this usually leads to frictional wear, occlusal instability, and possible tooth mobility.Elevations of the occlusal surfaceCuspsA cusp is an elevation on the crown portion of a tooth making up a divisional part of the occlusal surface. Cusps resemble pyramids in shape, and the highest point of the pyramid-shaped cusp is called the cusp tip (Fig 1-2). Cusps are present in the posterior teeth and the canines. Premolars have two cusps, except the mandibular second premolar, which may have three cusps, and molars may have three to ve cusps. Cusps are named according to their location (eg, the maxillary rst molar cusp that faces the mesial and lingual surfaces is called mesiolingual cusp). The part of the tooth between the buccal and lingual cusp tips is approximately 50% to 60% of the buccolingual dimension of the tooth and is where the forces of mastication are applied; this part is called the inner aspect of the tooth or the occlusal table, and the part outside the cusp tips is called the outer aspect of the tooth (Fig 1-3). The inner and outer aspects of Fig 1-2 Pyramid-shaped cusp and cusp tip.Fig 1-3 Inner and outer aspects of the tooth.Outer inclineOuter inclineInner incline Introduction to Occlusion41Fig 1-5 Supporting cusps occlude in the opposing central fossa. Nonsupporting cusps do not contact opposing teeth in maximal intercuspation.BLBFig 1-4 Buccal cusps are the functional cusps of a mandibular rst molar and thus occupy one-third of the buccolingual dimension. Lingual cusps are non-functional and occupy only one-sixth of the buccolin-gual dimension.1/31/6the teeth are made up of inclines that extend from the cusp tips to either the central groove or the height of contour on the facial or lingual surface. Depending on the role the cusps play in the dentition, they are divided into functional and nonfunctional cusps.Functional cusps are given this name because they are the cusps responsible for mastication and maintaining the vertical dimension of occlusion. They also can be called working cusps and supporting cusps. From the occlusal aspect, their cusp tips are located approximately one-third the distance into the total buccolingual width of the tooth (Fig 1-4). Functional cusps are the lingual cusps of maxillary teeth and the buccal cusps of mandibular teeth. Thus, premolars have only one functional cusp, which occludes with the opposing marginal ridge(s), and molars have two functional cusps, one that occludes with the opposing central fossa and one that occludes with the opposing marginal ridge(s) when the teeth are in maximal intercuspation (Fig 1-5). This contact relation may result in slightly atter cusp tips due to possible wear. If these cusps are not properly restored to contact, the opposing tooth may supererupt. An imaginary line can be extended through the functional cusp tips in each arch. In the mandible the line will pass through the buccal cusp tips and is named the bucco-occlusal line, and in the maxilla the line will pass through the lingual cusp tips and is therefore named the linguo-occlusal line. A third imaginary line can be extended through the central developmental grooves of posterior teeth and is named central fossa line.4 These three lines pass smoothly and continuously and are a major aid during cusp and central groove placement in occlusal wax-ups (Figs 1-6 to 1-8).Fig 1-6 Bucco-occlusal line (dotted line). Fig 1-7 Linguo-occlusal line (dotted line).BO lineLO line Tooth Morphology5Nonfunctional cusps or nonworking cusps are the mandibular lingual cusps and the maxillary buccal cusps. They do not contact the opposing teeth in maximal intercuspation; therefore, they do not maintain the vertical dimension of occlusion or play a major role in mastication, and they tend to have a sharper cusp tip. They do however maintain the bolus of food on the occlusal table for mastication and minimize tissue impingement as well as guide the mandible in lateral movements; thus they are also named guiding cusps (see Fig 1-5). Nonfunctional cusps tips are located approximately one-sixth the distance into the total buccolingual width of the tooth (see Fig 1-4).RidgesRidges are linear elevations on the surfaces of teeth. They are named according to their location or shape. The occlusal surfaces of posterior teeth exhibit several ridges: marginal, cusp, triangular, transverse, and oblique.Marginal ridges. Marginal ridges are rounded elevated crests forming the mesial and distal bound-aries of the lingual surfaces of anterior teeth and the occlusal surfaces of posterior teeth. All marginal ridges converge from the buccal toward the lingual, making the facial half of any tooth wider than the lingual half and the lingual embrasure wider than the buccal embrasure (Fig 1-9). The embrasures act as a way for food to escape during mastication, allowing the food to be mixed with saliva to form the food bolus, and then the soft tissue of the lips, cheeks, and tongue carries the food back to the occlusal surface. This dynamic reduces the forces brought to bear on the teeth during the reduction of any material that oers resistance and thus allows more ecient mastication while protecting the teeth from excessive forces and stimulating the surrounding soft tissues. The tongue, being one of the strongest muscles in the body, is more ecient than the lips and cheeks in performing this func-tion; hence a wider lingual embrasure is not only natural but also required to maximize the benet from the powerful tongue action. Marginal ridges play an essential role in occlusion, as their maximal convexity occludes with the opposing functional cusp tip (Fig 1-10). During waxing it is important to create smooth, slightly convex marginal ridges, with a small crest of curvature that coincides with the opposing functional cusp tip in static closure. The remainder of the marginal ridge is created smooth and convex mesiodistally, which allows disocclusion (passage of cusps) in mandibular lateral movements. All adjacent marginal ridges should be at the same height to prevent food impaction and retention. This should be perfected during waxing to make restorations more ecient and allow for proper masticatory function.Fig 1-8 Using bucco-occlusal, linguo-occlusal, and central fossa lines as a guide in occlusal waxing. Introduction to Occlusion61Fig 1-9b Fig 1-9a Fig 1-10 Contacts between opposing functional cusps and marginal ridges.Cusp ridges, triangular ridges, and transverse ridges. The pyramid-shaped cusp: A pyramid has four walls, but contrary to the at-walled pyramid, each cusp wall has a linear convexity in the center that runs in an occlusocervical direction; these are called the cusp ridges. Each cusp ridge is named according to the location/surface it faces: buccal, lingual, mesial, and distal. The cusp ridges that descend from the cusp tip toward the central groove, which could be the lingual cusp ridge of a buccal cusp or the buccal cusp ridge of a lingual cusp, are also named triangular ridges because their shape resembles a triangle, with the apex at the cusp tip and the base at the central groove (Fig 1-11). The triangular ridge is convex in both faciolingual and mesiodistal directions, and when the triangular ridges are waxed to their proper contour and convexity, minimal eort is needed to emphasize the grooves and depressions on the occlusal surface. All posterior teeth will have at least two triangular ridges facing each other and separated by the central groove; the union of these ridges is called a transverse ridge. This ridge is especially obvious in the mandibular rst premolars (Fig 1-12). The mesial and distal cusp ridges together with the marginal ridges form the boundaries of the occlusal surfaces of the teeth and therefore are built in the early steps of occlusal waxing to guide creation of the remaining wax-up (Fig 1-13). Tooth Morphology7Oblique ridge. The oblique ridge is the ridge that exists only on maxillary molars and runs obliquely from the distobuccal cusp tip to the mesiolingual cusp tip. The oblique ridge is most prominent on permanent maxillary rst molars and is often crossed by the oblique ridge groove (Fig 1-14). Oblique ridges are not in contact in the intercuspal position. The central fossa of the maxillary molars lies mesial to the oblique ridge.Fig 1-12 Mandibular rst premolar with transverse ridge.Fig 1-13 (a) Boundaries of the occlusal surface (cusp ridges in black and marginal ridges in blue). (b) Waxing the boundaries of the occlusal surface.aabbFig 1-14 (a) Oblique ridge in a maxillary rst molar with central fossa mesial to it. (b) Oblique ridge in a wax-up with central fos-sa mesial to it.Fig 1-11 (a) Cusp ridges of the buccal cusp of a maxillary pre-molar. (b) Cusp ridges of the buccal cusp of a maxillary premolar wax-up.a b Introduction to Occlusion81Depressions of the occlusal surfaceCentral fossaA central fossa is a concave circular shallow depression found in the center of the occlusal surface of molars that occludes with the opposing functional cusp. The food bolus can be eciently crushed between the functional cusp and central fossa because the concavity of the central fossa is wide enough to allow movement of the food. The food is ground between the walls and the oor of the fossa and the cusp tip, similar to the action of a mortar and pestle (Fig 1-15; see also Fig 1-5).a bFig 1-15 (a) Circular central fossa and triangular fossae. (b) Occlusal wax-up showing fossae.Triangular fossae/pitsThese are two small fossae on the occlusal surface of posterior teeth located mesial and distal to the marginal ridges and triangular in shape. The base of the triangle is at the mesial/distal marginal ridge, and the apex faces the center of the occlusal surface; the triangular fossae end at small pinpoint depressions named the mesial/distal pits. The key to creating these fossae in an occlusal wax-up is to create the proper convexity of the marginal ridge. If the marginal ridge is waxed to proper form and given the necessary mesiodistal rounding, minimal carving adjacent to the marginal ridge with an acorn burnisher will result in the perfect triangular fossae, and the same principle applies when carving amalgam restorations. It is important to precisely duplicate the small width and depth of these fossae; exaggerating their size may result in food impaction (see Fig 1-15).GroovesThere are two types of grooves: (1) developmental and (2) supplemental. Developmental grooves separate the lobes or the primary parts of the crown and are sharply dened. They are named according to their location. The central groove runs at the center of the occlusal surface of posterior teeth, separating the buccal and lingual cusps. The buccal developmental groove runs on the buccal surface, separating the two buccal cusps of molars, and the lingual developmental groove runs on the lingual surface, separating the two lingual cusps (Fig 1-16a). Supplemental grooves are small, irregular, and less distinct. They do not mark the junction of primary parts of the tooth and are not necessarily a repetitive feature. Teeth with the most supplemental grooves in the permanent dentition are the third molars and the maxillary second premo-lar (Fig 1-16b). Grooves play a role in masticatory function by allowing passage of the cusps in excursive Tooth Alignment9movements; therefore, carving grooves in occlusal wax-ups in their proper position with the proper width and depth ensures ecient mastication. Grooves are not emphasized by deep and exaggerated carving of the wax-up. If the occlusal elevations, specically the triangular ridges, are given the proper convexity, slope, and form, the grooves appear naturally and can be enhanced with minimal carving (Fig 1-17).a bFig 1-17 (a and b) Slope of the triangular ridges produces the groove pattern (blue lines).a bFig 1-16 (a) Blue lines pointing to the buccal developmental groove, and black lines pointing to the central groove. (b) Third molar with mul-tiple supplemental grooves.Tooth Alignment 4The way the teeth are aligned can be divided into two components: (1) intra-arch alignment, which is the relationship of the teeth to each other within the dental arch, and (2) interarch alignment, which is the relationship of the opposing teeth (ie, occlusion). There are four important factors that stabilize tooth alignment and maintain arch integrity:1. Tooth size and arch width relation (discrepancies could lead to crowding or spacing).2. Proximal contact between the adjacent teeth, which helps to maintain the mesiodistal position of the teeth. Introduction to Occlusion1013. Equal opposing forces of the lips and cheeks on the buccal side and the tongue on the lingual side, which maintain the buccolingual position of the teeth, also called neutral tooth position (Fig 1-18).4. Occlusal contact between opposing teeth, which maintains the occlusocervical position of teeth. Because the proximal contacts maintain the mesiodistal position of the teeth and the occlusal contacts maintain the occlusocervical position of teeth, extraction of any tooth will disrupt these relations and lead to drifting of the adjacent teeth and supereruption of opposing teeth (Fig 1-19).Fig 1-20 Root axis angulations (lateral view, maxilla).Fig 1-21 Root axis angulations (lateral view, mandible).Single roots & Buccal rootsPalatal rootsFig 1-18 (a and b) Neutral tooth position.a bFig 1-19 Drifting of adjacent teeth (arrows) following extraction and supereruption of the opposing tooth.Tooth angulationsIn this section, tooth angulations are discussed according to the angulation of the root axis line, which is an imaginary line that on the facial and lingual surfaces of teeth divides the root at the cervix into mesial and distal halves and on the mesial and distal surfaces divides the root at the cervix into facial and lingual halves. The teeth are not all the same occlusocervical length and do not have the same occlusal/incisal morphology; therefore, if they did not exhibit any angulation it would be impossible for them to t together during closure and achieve proper mastication. From a lateral view, maxillary teeth are inclined mesially, except the second and third molars, which are slightly inclined distally in relation to the alveolar bone, while the mandibular teeth are all inclined mesially (Figs 1-20 and 1-21). Tooth Alignment11The mesial inclination of the anterior teeth in the lateral view represents the faciolingual angulation of these teeth; therefore, anterior tooth crowns are positioned labial to the roots (labial inclination), and the maxillary anterior crowns exhibit slightly more labial inclination than the mandibular ante-rior crowns. From a frontal view, all the maxillary posterior crowns are inclined buccally, while all the mandibular posterior crowns are inclined lingually, which allows the mandibular buccal cusps to t in the central fossae of the maxillary posterior teeth and the maxillary lingual cusps to t in the central fossae of the mandibular posterior teeth. All anterior teeth exhibit slight mesial inclination of the root axis line in the frontal view (the root apices are toward the mesial). The mesial inclination is of varying degrees in dierent mouths but tends to increase from the central incisors to the canines (Figs 1-22 and 1-23).Fig 1-22 Root axis angulations (frontal view, maxilla). DB, distobuccal; MB, mesiobuccal; PM, premolar.Fig 1-23 Root axis angulations (frontal view, mandible).Single rootsMesial rootsDistal rootsAnterior roots and PM buccal rootsPalatal rootsMB rootsDB rootsPlane of occlusionThe dental arches are U-shaped, so the incisal edges and buccal cusp tips follow a curved line around the outer edge of the arch. The line can be extended to reach the lingual cusp tips and continues across the arch to the opposite side. This line is referred to as the plane of occlusion. When the teeth are viewed from a lateral view from a point opposite the rst molars, an imaginary curve can be drawn through the buccal cusp tips of maxillary posterior teeth. The tooth angulations in the lateral view are mesially inclined except for the second and third molars, which are distally inclined, making this curve convex. If the same line is drawn through the mandibular buccal cusp tips, the line would be concave because all the mandibular teeth are inclined mesially. This anteroposterior curve is referred to as the curve of Spee (Fig 1-24a). When the teeth are viewed from the frontal view, an imaginary curve can be seen connecting buccal and lingual cusp tips of the right and left sides. This curve is convex for the maxillary arch due to buccal inclination of the teeth and concave for the mandibular arch due to their lingual inclination. This curve is referred to as the curve of Wilson (Fig 1-24b). When the arches come in contact during occlusion, the maxillary convex curve ts perfectly in the mandibular concave curve, allowing for maximal utilization of tooth contacts during function. Introduction to Occlusion121Occlusion 4,5Occlusion has been dened as “the contacts between teeth.” These contacts can be considered stat-ically or dynamically, as teeth slide over each other during mandibular movement. In other words, it is the relationship between maxillary and mandibular teeth as they approach each other at rest or during function. This relationship is largely dependent on tooth alignment, arch length, and occlusal morphology. In normal occlusion, the mandibular arch is slightly shorter than the maxillary arch, approximately 126 cm versus 128 cm, respectively. This dierence is due to the narrower mesiodistal width of the mandibular anterior teeth. In addition, the maxillary posterior teeth are inclined buccally, while the mandibular posterior teeth are inclined lingually, and the maxillary anterior teeth are more labially inclined than the mandibular anterior teeth. This results in the maxillary teeth overlapping the mandibular teeth facially, which allows the opposing cusps and fossae to come together perfectly. It is important to understand that there are deviations from these normal patterns. However, this section will discuss the normal and most commonly found occlusal relationship that would be developed when creating occlusal and diagnostic wax-ups as a dental student.Anterior tooth relationshipsDue to the labial inclination of both maxillary and mandibular anterior teeth (the maxillary teeth are more labially inclined) and the larger length of the maxillary arch, the maxillary anterior teeth overlap the mandibular anterior teeth in both horizontal and vertical directions. There may be light contact or no contact at all between maxillary and mandibular anterior teeth in maximal intercuspation, which serves these teeth well, as the presence of heavy occlusal forces could lead to displacement of the maxillary teeth further labially. The anterior teeth do not need to be in contact in maximal intercuspation to maintain the vertical dimension of occlusion; this function is carried by the poste-rior teeth. However, the anterior teeth do come in contact during mandibular protrusive and lateral movements and they play an essential role in guiding the mandible during these movements. The ability to perform this function is dependent on the degree of anterior teeth overlap mentioned abFig 1-24 (a) Curve of Spee. (b) Curve of Wilson. Occlusion13previously. The vertical overlap is called overbite; it is the distance between the incisal edges of the opposing anterior teeth, which is usually half the length of the mandibular crowns and can range from 1 to 5 mm. If this distance is increased it is described as deep bite, and if there is a lack of vertical overlap it is described as open bite. The horizontal overlap is called overjet; it is the distance between the incisal edge of the maxillary incisors and the labial surface of the mandibular incisors (Fig 1-25). Incisal thirds of anterior teeth thus enable them to have a direct and indirect eect on masticatory function. The direct eect is providing a means for the anterior teeth in incising and biting food, and the indirect eect is due to their role in guiding the mandible in protrusive and lateral movements, which protects the posterior teeth from horizontal damaging forces and therefore facilitates their masticatory function.Posterior tooth relationshipsMesiodistal relationshipThe mesiodistal relationship of posterior teeth can be described by Angle’s classication. This clas-sication depends on the relationship between the opposing rst molars. The rst molars are the rst permanent teeth to erupt, and their alignment and occlusion plays a major role in the stability of the dentition (Fig 1-26).Fig 1-25 Normal overlap of anterior teeth.Horizontal overlapOverjet OverbiteVertical overlap• Angle Class I: The most common relationship found in natural dentition whereby the mesio-buccal cusp of the maxillary rst molar is aligned directly over the mesiobuccal groove of the mandibular rst molar.• Angle Class II: The mesiobuccal cusp of the maxillary rst molar is aligned mesial to the mesiobuccal groove of the mandibular rst molar.• Angle Class III: The mesiobuccal cusp of the maxillary rst molar is aligned distal to the mesiobuccal groove of the mandibular rst molar.Fig 1-26 Angle classication.Class IMesiobuccal cusp tipMesiobuccal grooveClass II Class III Introduction to Occlusion141Buccolingual relationshipThe maxillary arch normally overlaps the mandibular arch buccally due to the longer length of the maxillary arch and the fact that the maxillary posterior teeth are inclined buccally and mandibular posterior teeth are inclined lingually. This relationship results in the maxillary lingual cusps contacting the central fossae of the mandibular molars and the mandibular buccal cusps contacting the central fossae of the maxillary molars (see Fig 1-5), which allows ecient mastication while protecting the adjacent soft tissues and maintaining normal facial esthetics. When this overlap is reversed and the mandibular teeth are buccal to the maxillary teeth, it is described as crossbite (Fig 1-27).Fig 1-27 Left side showing normal arch relation. Right side showing anterior and posterior crossbite. (Courtesy of Dr Sara Abdallah Mohamed, Cairo, Egypt.)Occlusal relationship (location and types of contacts)Knowledge of contact locations in maximal intercuspation and mandibular excursions is essential during occlusal waxing, as the existence of additional contacts in improper locations provide inter-ferences during mastication and should be eliminated from the wax-up.Occlusal contacts in intercuspal positionIt was discussed earlier that the functional cusps contact either central fossae or marginal ridges. Two occlusal schemes exist with respect to marginal ridge contact:1. The cusp tip contacts one marginal ridge on the opposing tooth, resulting in one contact on the cusp tip. This relationship is the most common and also the most simple and predictable when it comes to creating resto-rations and occlusal wax-ups.2. The cusp tip contacts two marginal ridges on adjacent teeth, resulting in two points of contact on the cusp tip (Fig 1-28). Fig 1-28 Occlusion15Table 1-1 is a summary of occlusal contacts. Points of contact on the maxillary posterior teeth exist on (1) mesial marginal ridges, (2) central fossae (which contact the opposing mandibular buccal cusps), and (3) maxillary lingual cusp tips. Points of contact on the mandibular posterior teeth exist on (1) distal marginal ridges, (2) central fossae (which contact the opposing maxillary lingual cusps), and (3) mandibular buccal cusp tips. The central fossae of the mandibular molars are in contact with the mesiolingual cusps of the maxillary molars, and the central fossae of the maxillary molars are in contact with the distobuccal cusps of the mandibular molars. If the cusp tips are in contact with two adjacent marginal ridges instead of one, all marginal ridges in the posterior teeth will exhibit contact areas.Table 1-1 Occlusal contact locations on posterior teeth in case of a cusp tip contacting one marginal ridgeMandibular buccal cusps Opposing contact on maxillary toothFirst premolar Mesial marginal ridge of rst premolarSecond premolar Mesial marginal ridge of second premolarMesiobuccal of rst molar Mesial marginal ridge of rst molarDistobuccal of rst molar Central fossa of rst molar Distal of rst molar No contactMesiobuccal of second molar Mesial marginal ridge of second molarDistobuccal of second molar Central fossa of second molarMaxillary lingual cusps Opposing contact on mandibular toothFirst premolar Distal marginal ridge of rst premolarSecond premolar Distal marginal ridge of second premolarMesiolingual of rst molar Central fossa of rst molar Distolingual of rst molar Distal marginal ridge of rst molarMesiolingual of second molar Central fossa of second molarDistolingual of second molar Distal marginal ridge of second molarOcclusal contacts in protrusive mandibular movementIn protrusion, the main contact between teeth occurs anteriorly between the incisal edges of the mandibular incisors and the incisal edges and lingual fossae of maxillary incisors. These areas are considered the guiding inclines of anterior teeth (Fig 1-29). The steepness of these surfaces deter-mines the amount of vertical movement of the mandible in protrusion. This is called anterior guidance and aects cusp placement and height in the following way: The greater the vertical overlap (steeper Introduction to Occlusion161angle) and the smaller the horizontal overlap, the longer the posterior cusps may be without causing interference in protrusion. The smaller the vertical overlap and the greater the horizontal overlap, the shorter the posterior cusps are to avoid interferences (Fig 1-30). Restorations and wax-ups of anterior teeth should be completed with this consideration and knowledge of the eect on posterior occlusion. Posterior protrusive contacts may occur between mesial inclines of mandibular teeth and distal inclines of maxillary teeth.Fig 1-30 The anterior guidance angle is altered by variations in the horizontal and vertical overlap. (a) Horizontal overlap (HO) varies while vertical overlap (VO) remains constant. (b) Vertical overlap varies while horizontal over-lap remains constant. (Redrawn from Okeson.4)a bVOVOHOHOFig 1-29Guiding inclinesGuiding inclinesOcclusal contacts in mandibular lateral movementsIn the intercuspal position, the maxillary arch overlaps the mandibular arch horizontally (ie, the maxillary buccal cusps are buccal to the mandibular buccal cusps). Therefore, when the mandible moves laterally, there are two potential areas of contact on the working side: (1) between the inner inclines of the maxillary buccal cusps and the outer inclines of the mandibular buccal cusps and (2) between the outer inclines of the maxillary lingual cusps and the inner inclines of the mandibular lingual cusps. These contacts are termed laterotrusive or working-side contacts. On the nonwork-ing side, the potential sites of contact are between the inner inclines of the maxillary lingual cusps and the inner inclines of the mandibular buccal cusps. These contacts are termed mediotrusive or nonworking-side contacts (Fig 1-31). As the mandible moves further laterally, contact may occur Occlusion17between the labial surface and cusp of the mandibular canine and the lingual surface and cusp of the maxillary canine. When the maxillary and mandibular canines disocclude the posterior teeth during mandibular lateral movement, thus protecting them from horizontal damaging forces, this is called canine guidance (Fig 1-32). Another occlusal relationship can exist in lateral movements, known as group function, where further lateral movement results in contact between all posterior teeth on the working side rather than just the canines (Fig 1-33). When the forces are distributed on all posterior teeth, the damage is minimized or nonexistent. However, if the contact exists on one tooth only, no teeth other than the canines are designed to tolerate these forces. It is of great importance to achieve either canine guidance or group function occlusion during occlusal waxing and esthetic wax-ups to achieve proper masticatory function. The relationship between the anterior and posterior teeth where light or no contact exists between anterior teeth in maximal intercuspation and light or no contact exists between posterior teeth in mandibular excursions is called mutually protected occlusion.Occlusal contacts in retrusive mandibular movement The retrusive movement is quite limited as it is restricted by ligaments. As the mandible moves distally in retrusion, contacts will occur between the distal inclines of mandibular teeth and the mesial inclines of maxillary teeth (opposite contacts of protrusive movement) (Fig 1-34).Fig 1-31 (a) Laterotrusive contacts. (b) Mediotrusive contactsa bB BL LFig 1-32 Canine guidance. Direction of mandibular movement (arrow).Fig 1-33 Group function. Direction of mandibular movement (arrow).Lateral canine guidanceGroup function Introduction to Occlusion181Fig 1-35 Small centric contacts in an occlusal wax-up.Types of centric contacts8Properly waxed occlusal surfaces should allow for small centric contacts (approximately 1 mm) that disocclude completely during excursive movements of the mandible, thus minimizing lateral stresses and frictional wear (Fig 1-35). If the contacts are large as in the case of an occlusal surface formed only by inclined planes and lacking the proper convexities of the occlusal elevations and proper width, depth, and position of grooves and other occlusal depressions, teeth will not disocclude completely during the excursive movements. As mentioned previously, centric cusps occlude with either the central fossa or the marginal ridge of the opposing tooth in maximal intercuspation, and when the contact is between the cusp tip and the marginal ridge, the result is always one small area of contact between the cusp tip and the maximum convexity of the marginal ridge. However, when the cusp tip occludes with the central fossa, two options exist in the natural dentition and may be reproduced during occlusal waxing: (1) tripod contact and (2) cusp tip–to–fossa contact.Fig 1-34 (a) Retrusive and (b) protrusive contacts.aD MbDM Occlusion191. Tripod contact: The cusp tip never touches the opposing tooth. Instead, contact is made on the sides of the cusps, which are convexly shaped. Therefore, the resulting contact on the central fossa is a tripod on the walls rather than the depth of the fossa (Fig 1-36).2. Cusp tip–to–fossa contact: The cusp tip directly touches the deepest point in the concavity of the central fossa, resulting in one area of contact on the cusp tip and one area of contact in the depth of the fossa (Fig 1-37).Fig 1-37 Cusp tip–to–fossa contact.Fig 1-36 Tripod contact.It is important to understand that both types of centric contacts can achieve stable functional occlusion and direct the forces along the long axes of the teeth. It was believed earlier in the literature that tripod contacts were superior and necessary to achieve axial loading, but modern literature does not agree with this concept.4,8 In addition, achieving a tripod contact for every cusp in the dentition may be clinically challenging during restorative work and is almost never performed in clinical practice. For these reasons, the type of contacts created during occlusal waxing and restorations are cusp tip–to–fossa contact. That being said, in some cases the cusps are quite convex and the central fossae are deep; therefore, contact can only be created by tripodization. These cases are not clinically challenging; on the contrary, a tripod can be easily achieved. It is also worth mentioning that if an attempt was made to create a tripod contact during waxing and only two out of the three contacts were created, the results may be harmful to the dentition as the forces may not be directed along the long axes of teeth, resulting in damaging lateral stresses on the teeth and possible wear.

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