GENERAL FEATURES OF PREMOLARS

  1. The premolars are intermediate teeth between molars and canines. They succeed the deciduous molars. The term “bicuspid” is often used in place of premolars, but this is inaccurate since the mandibular second premolar may show three cusps. There are eight premolars, two in each quadrant.
  2. The premolars are developed from four lobes except the mandibular second premolar, the three cusp form, which developed from five lobes, three buccal and two lingual.
  3. The buccal cusp of the first premolar is long and sharp assisting the canine in its function of tearing. The second premolars have cusps less sharp which make them efficient in gridding teeth much like molars.
  4. The marginal ridges of the crown are in a more horizontal plane and are considered part of the occlusal surface of the crown rather than the lingual surface as in case of anterior teeth. The crown and root of the premolars are shorter than those of the canines.

 

MAXILLARY FIRST PREMOLAR

lingual, which are sharply defined. The buccal cusp is about 1 mm longer than the lingual cusp and the crown is shorter than that of the canine by 1.5-2 mm. table VII lists the chronological data of the maxillary first premolar.

This tooth is the only premolar which normally has two roots (about 80%), a buccal and a lingual, with two pulp canals, although occasionally there is only one root (20%). It resembles the canine from the buccal aspect but the root is shorter, the mesial slope of the buccal cusp is longer than the distal slope (opposite to the maxillary canine) and the mesiodistal diameter is less than that of the canine. The first premolar presents some characteristics common to all posterior teeth as differentiated from the anterior teeth. (Table VIII)

Buccal aspect

  • The crown is roughly trapezoid with the smallest uneven side directed ervically. The width of the crown mesiodistally is less at the cervix than at the level of contact areas by 2mm.
  • The mesial outline of the crown is slightly concave from the cervical line to the mesial contact area and also to the cusp tip. The mesial cusp slpe is longer than the distal and the contact area is just cervical to the junction between the occlusal middle one third.
  • The distal outline is more straight below the cervical line and the contact area is broader and more occlusally placed than the mesial. The cervical line is curved with the crest of the curvature near the center of the root and toward it.
  • The buccal cusp is long with pointed tip located distal to the midline and divide the occlusal border into a long, straight or concave, mesial cusp ridge and a short convex distal cusp ridge.
  • The buccal surface of the first maxillary premolar is convex, showing strong development of the middle lobe with a continuous ridge from the cusp tip to the cervical margin. This ridge is called the buccal ridge. Two developmental grooves, mesiobuccal and distobuccal, are located on both sides of the buccal ridge and mark the union of the developmental lobes.
  • The buccal root outline of the maxillary first premolar is similar to that of the canine but is shorter by 3-4 mm.

Lingual aspect

  • The crown converges towards the  lingual cusp which is shorter and narrower mesiodistally than the buccal one. The tip of the lingual cusp is pointed and located slightly toward the mesial side. The mesial slope is shorter than the distal.
  • The mesial and distal outlines are convex and continuous with the mesial and distal slopes of the lingual cusp and become more straight as they join the mesial and distal sides of the lingual root.
  • The crown as seen from the lingual aspect is smooth spheroid and convex at all points with no definite lingual ridge and no developmental grooves. The lingual height of contour is located at the middle third. Since the lingual cusp is shorter than the buccal one, the tips of both cusps can be seen with their mesial and distal slopes from the lingual aspect.
  • The cervical line is similar to that of the buccal aspect. The lingual aspect of the root, or the lingual aspect of the lingual root if two roots are present, is smooth and convex with blunt apex.

Mesial aspect

It is trapezoidal in shape, however, the longest of the uneven sides is toward the cervical portion and the shortest toward the occlusal portion. Another characteristic of all the maxillary posterior teeth is that the measurement between the tip of the buccal cusp to that of the lingual cusp is less than the buccolingual measurement of the root at its cervical portion, i.e. the tips of the cusps are well within the confines of the root trunk.

  • The cervical line is curved and regular with average curvature of 1 mm. which is similar to all posterior teeth. The buccal outline of the crown is convex below. The crest of curvature is at the junction of the cervical and middle thirds of the crown. It continue as a line of less convexity to the tip of the buccal cusp. The tip of the buccal cusp is located directly below the center of the buccal root.
  • The lingual outline is curved smoothly from the tip of the lingual cusp to the cervical line. The crest of the curvature is near the center of the middle third. The tip of the lingual cusp is on a line with the lingual border of the lingual root.
  • The lingual cusp is always shorter than the buccal cusp by about 1 mm. the mesial marginal ridge is convex and is located at about the level of the junction of the middle and occlusal thirds.
  • The mesial contact area is circular in shape and located at the junction of the middle and occlusal thirds and slightly near the buccal.

The distinguishing features of this tooth from the mesial aspect are:

  • The presence of “Mesial Developmental Depression” which extends cervically from the contact area and continue to include the cervical line then joins a deep depression between the root bifurcation, known as “Canine Fossa”. The maxillary second premolar do not have this feature.
  • The presence of a well defined developmental grooves in the enamel of the mesial marginal ridge. This marginal groove is continuous with the central groove of the occlusal surface of the crown, crossing the marginal ridge just lingual to the contact area and terminating a short distance cervical to the mesial marginal ridge. This groove is called the “Mesial Marginal Developmental Groove”.
  • The buccal outline of the buccal root is straight above the cervical line with tendency toward lingual inclination. The lingual outline is also straight and may show buccal or lingual inclination. The root trunk is long making up about half the length of the root. The bifurcation of the roots begins at a more occlusal point mesially than distally.
  • The mesial surface of the root trunk is smoothly convex buccally and lingually with deep developmental groove and depression at or below the bifurcation. In case of one rooted tooth this depression is noticed for most of the root length.

Distal aspect

  • The crown outline is similar to the mesial aspect. The crown surface is convex at all points. The distal contact area is wider buccolingually than ocluusocervically and slightly near the buccal. It is larger and more cervically located than the mesial contact area.
  • The height of contour is in the cervical third buccally and middle third lingually. The curvature of the cervical line is less than in the mesial surface. Also there is no evidence of deep developmental grove and the root trunk is flattened above the cervical line. The bifurcation is more toward the apical third with no developmental groove leading to it.

Occlusal aspect

  • The occlusal aspect has a hexagonal outline. Two equal buccal sides, mesial side shorter than the distal and mesiolingual shorter than distolingual. It is wider buccally than lingually and buccolingually than mesiodistally. The buccal margin is convex with a prominent buccal ridge at the crest of the curvature.
  • The mesiolingual and distolingual cusp ridges are continuous with the mesial and distal marginal ridges. The crest of the buccal ridge is some what distal to the lingual ridge, while the crest of the distal contact area is somewhat buccal to  that of the mesial one.
  • The occlusal surface shows two well developed cusps, the lingual one is more pointed while the buccal one is larger. Each cusp has four cusp ridges named according to their location, buccal, lingual, distal and mesial ridges. The buccal cusp ridge descends from the cusp tip cervically onto the buccal surface. The lingual cusp ridge of the buccal cusp and buccal cusp ridge of the buccal cusp descends from the cusp tip to the central area of the occlusal surface. The two triangular ridges of the buccal and lingual cusps are separated by the central developmental groove.
  • The primary grooves on the occlusal aspect are sharp and deep. A well-defined central developmental groove divides the tooth mesiodistally. It extends from the distal to the mesial marginal ridge where it joins the mesial marginal developmental groove which crosses the mesial marginal ridge and ends on the mesial surface of the crown.
  • Two developmental grooves join the central groove just inside the mesial and distal triangular fossae. These are called the mesiobuccal and distobuccal developmental grooves. The junction of the grooves are pointed and are named the mesial and distal developmental pit. Just inside the mesial and distal marginal ridges are the mesial and distal triangular fossae.

Pulp cavity

  • In the bucco-lingual section the pulp chamber is broad buccolingually with well developed pulp horns. It presents a funnel-like opening leading to the root canals. The floor of the pulp chamber is below the level of the cement-enamel junction. The lingual root canal is larger than the buccal and both taper evenly toward the apical foramen.
  • The mesiodistal section is similar to the pulp cavity of the maxillary canine. It is relatively narrow and taper evenly to the apical foramen.
  • A transverse section at the cementoenamel junction shows the the characteristic kidney shaped root trunk and the root is wider buccolingually than mesidistally. 

 

MAXILLARY SECOND PREMOLAR

maxillary premolar in function and very similar to it.  The second premolar is single rooted in 85% of cases and has a less angular and  rounded crown from all aspect than that of the first premolar. The crown is shorter cervico-occlusally but the root is little longer than that of the first premolar. Table IX list the chronological data of the maxillary second premolar.

Buccal aspect

The buccal cusp is shorter and less pointed than in the first premolar. The mesial slope of the buccal cusp is shorter than the distal slope, which is the opposite for the first premolar.

Lingual aspect

Very similar to the first molar with very little variations.

Mesial aspect

Cusps are shorter than in the first premolar and are almost of the same length. The distance between the cusp tips is wider which widens the occlusal surface buccolingually. Crown is convex with no developmental depression and a shallow developmental groove is seen on the root. No developmental groove crosses the mesial marginal ridge.

Distal aspect

The distal aspect shows very little variations from that of the first premolar.

Occlusal aspect

The occlusal surface outline is more rounded or oval than angular, as the occlusal outline of the first premolar. The distance between the cusp tips is wider, the central developmental groove is shorter and more irregular with multiple supplemental grooves radiating from the central groove. The mesial and distal triangular fossae are nearer to each other.

Pulp cavity

On buccolingual section the pulp chamber is wide with well defined pulp horns. There are two pulp canals, buccal and lingual. The canals are very wide at their junction with the pulp chamber and then narrow until reaches the mid-root where it constricts rapidly to the apical foramen.

The tooth may show single broad pulp canal divided at mid-root into two canals by dentin island then the canals join again near the apical foramen. Mesiodistally the pulp chamber is similar to that of the first premolar. Cervical cross section display an oval root trunk instead of kidney shaped rott trunk of the first premolar. 


 

MANDIBULAR FIRST PREMOLAR

They are four in number, two in each side of the mandible. As a rule the mandibular first premolars are always smaller than second premolars. The opposite is true for maxillary premolars.

 

The mandibular first premolar has many of the characteristics of a small canine. It has one sharp buccal cusp which is the only part occlude with maxillary teeth. The lingual cusp is small and nonfunctioning resembling a well developed cingulum. (Table X) On the other hand the mandibular second premolar has more of the characteristics of a small molar.

Buccal aspect

  • The outline is roughly trapezoidal with the smallest of the uneven sides cervically.
  • The mesial outline of the crown is straight or slightly convex till it joins the curvature of the mesial contact area. The outline of the mesial slope of the buccal cusp usually shows some convexity. 
  • The distal outline of the crown is slightly concave above the cervical line to a point where it is continuous with the curvature of the distal  contact area. This curvature is broader than that of the mesial contact area. The distal slope of the buccal cusp shows some concavity.
  • The tip of the buccal cusp is pointed and is located a little mesial to the central of the crown buccally. The cervical line is curved toward the root.
  • The buccal surface of the crown is more convex than the maxillary premolars.
  • The middle buccal lobe of the crown is well developed, resulting in a large pointed buccal cusp with a buccal ridge continuous from the cervical margin to the cusp tip. Mesial and distal to the ridge a developmental depression may be seen separating the three lobes. The mesial cusp ridge is shorter than the distal one.
  • The contact areas are broad and are almost at the same level being just cervical to the junction between the occlusal and middle thirds.
  • The root outline resembles that of the canine but is shorter by about 3-4 mm an shows a distal curvature at the apex.

Lingual aspec

  • Lingual aspect is much narrower mesiodistally than buccal since the crown and root taper markedly toward the lingual side. The lingual surface is evenly convex in all directions.
  • The mesial and distal outlines are concave between the cervical line and the contact areas. The contact areas and marginal ridges are pronounced and extend out above the narrow cervical portion of the crown. The mesial marginal ridge is more cervically located than the distal one.
  • The cervical line is slightly convextoward the root. The occlusal surface slopes greatly toward the lingual surface down to the short lingual cusp. Accordingly most of the occlusal surface is seen from this aspect.
  • A mesiolingual developmental groove separates the mesial marginal ridge from the lingual cusp. The groove fads out at the junction of the middle and cervical thirds.
  • The root tapes evenly from the cervix to a pointed apex. A narrow smooth convex ridge is seen through the full length of the root. Often developmental depression in the root with developmental grooves mesially and distally are seen.

Mesial aspect

  • Crown outline is rhomboidal which is a characteristic of all mandibular posterior teeth from mesial and distal aspect.
  • The surface of the crown presents an overhang above the root in a lingual direction. The tip of the lingual cusp is on a line with the lingual border of the root (both the buccal and lingual roots of the maxillary premolars are within the confines of the root trunk).
  • The tip of the buccal cusp is nearly centered or a little buccal to the center of the root.
  • The buccal outline of the crown is convex with the crest of curvature near the cervical one third. This feature is a characteristic for buccal surface of all posterior teeth. The lingual outline is less convex and the crest of curvature approaches the middle third.
  • The mesial marginal ridge has a sharp inclination lingually in a cervical direction (45o), the lingual border of which merges with the developmental depression mesiolingually. This depression harbors the mesiolingual developmental groove.
  • The contact area is ovoid in shape and is located to the junction of the occlusal and middle thirds and slightly buccally.
  • Some of the mesial portion of the occlusal surface is seen from this aspect, this include mesial portion of the buccal cusp triangular ridge and mesial marginal ridge.
  • The cervical line curves occlusally and the crest of curvature is about 1mm in extgent and is centered buccolingually.
  • The mesia surface is convex at the mesial contact area then becomes sharply concave till the cervical line.
  • The root outline tapers to a pointed apex in line with the tip of the buccal cusp. Deep developmental groove is shown in the center of the root which may end in bifurcation at the apex.

Distal aspect

In differes from the mesia aspect in the following:

  • Distal marginal ridge is more occlusally located and nearly at line angle to the axis of the crown and root.
  • The distal contact area is broader than the mesial and is centered buccolingually in the middle one third.
  • Curvature of cervical line is nearly the same as in the mesial aspect.
  • The root surface is more convex distally.
  • A shallow developmental depression is centered on the root but with no developmental groove

Occlusal aspect

This aspect shows considerable variations on the outline. It may be diamond-shaped, similar to the incisal aspect of canine, or circular similar to some second premolars. The characteristics common to all mandibular first premolars from the occlusal aspect regardless the type are listed in table XIII.

The pulp cavity

  • It resemble that of the canine but smaller in dimensions.
  • In buccolingual section the pulp chamber is wide with clear pulp horn under the buccal cusp. The pulp horn directed to the lingual cusp is missing. The root canal is broad tapering evenly and is more constricted at halfway point of root length.
  • Mesiodistally the pulp chamber and root canal are narrow and the canal taper evenly until the constricted apical foramen. This is typical for all premolars.
  • Cervical cross section shows wider pulp canal bucculingually ehan mesiodistally. The outline form may be oval, rectangular or triangular. Occalsionally two cnals may be seen.

 

MANDIBULAR SECOND PREMOLAR

The crown of the mandibular second premolar is wider buccolingually by 0.5 mm than the mandibular first premolar. Its lingual cusps are more developed, and both marginal ridges are higher. This produces a more efficient occlusion with the maxillary antagonist. Therefore, a mandibular second premolar functions more like molar than a canine.

There are two common forms which this tooth assumes;

  • The three cusp type, which is the most common and appears more angular from the occlusal aspect.
  • The two-cusp type appears more rounded from the occlusal aspect.

The single root of the second premolar is longer (by 0.5 mm) than that of the first premolar with developmental groove buccally in many cases.

Buccal aspect

From the buccal aspect the crown resembles a first premolar in its general shape. It presents shorter buccal cusp than the first premolar with the mesiobuccal and distobuccal cusp riges more rounded and less pointed cusp tip which is little mesial to the tooth center. The mesial and distal contact areas are nearly at the same level but are broader and placed higher occlusally. The buccal ridge isles prominent that than that of the mandibular first premolar.

The root is longer and broader mesiodistally than the first premolar and ends in more blunt apex. The single root may show a developmental groove.

Lingual aspect

  • The lingual lobes are developed to a greater degree making the cusp or cusps longer. In the three cusp type, there are a mesiolingual and a distolingual cusps. The former is the wider and longer of the two cusps. The cusps are divided by a lingual groove.
  • In the two cusp type, the single lingual cusp is higher than on a mandibular first premolar. There is no groove, but a developmental depression is seen distolingually where the lingual cusp ridge joins the distal marginal ridge.
  • The lingual surface of the crown of all mandibular second premolars is smooth and spheroidal, having a bulbous form above the constricted cervical portion.
  • The root is much wider lingually than that of the first premolar. This results in less convergence towards lingual. It is smooth and convex.

Mesial aspect

The second premolar differs from the first from the mesial aspect in the following ways;

  • The crown and root are thicker buccalingually than the first premolar.
  • The buccal cusp is shorter and its tip located more to the buccal side.
  • The lingual lobe or lobes development is greater. The tip of the lingual cusp is on line with the lingual border of the root.
  • The marginal ridge is at right angle to the long axis of the tooth.
  • Less of the occlusal surface may be seen.
  • There is no mesio-lingual developmental groove on the crown portion.
  • The root is longer and in most cases slightly convex on the mesial surface with a more blunt apex.

Distal aspect

  • More of the occlusal surface can be seen because the distal marginal ridge is at a lower level than the mesial marginal ridge.
  • As a general rule, the  crown of all posterior teeth (maxillary and mandibular) are tipped distally to the long acis of the root. So, much of the occlusal surface may be seen from the distal aspect.
  • In the three-cusp type both lingual cusps are seen since the distolingual cusp is shorter than the mesiolingual cusp.
  • The root is conical in shape and may show deeper developmental depression.

Occlusal aspect

In both the two and three-cusp forms, the buccal cusp is similar, however, the outline of each type shows some variations from the occlusal aspect.

The occlusal characteristics of the three cusp type:

  • It appears square lingual to the buccal cusp ridge. It has three distinct cusps, the buccal cusp is the largest then the mesiolingual cusp. The distolongual cusp is the smallest.
  • Each cusp has a well defined triangular ridges separated by deep developmental grooves. These grooves converge in a central pit and from a ‘’Y’’ shape on the occlusal surface. Three pits may be present, a central, a mesial and distal.
  • The central pit is located in the center buccolingually and slightly distal to the midway point between the mesial and distal marginal ridges.
  • From the central pit, a mesial and distal developmental grooves travel in a mesiobucal and distobuccal directions respectively ending in the mesialand distal triangular fossae.
  • The lingual developmental groove extends lingually between the two lingual cusps and ends on the lilnguall surface of the crown just below the lingual cusps. The mesiolongualcusp is wider mesiodistally than the distolingual cusp.
  • Supplemental grooves and depressions are often seen radiating from the developmental groove.
  • The mesial and distal marginal ridges are confluent with the cusp ridges.

The occlusal characteristics of the two-cusp type as compared with the three-cusp type are:

  • The outline of the crown is rounded.
  • The lingual surface of the crown is more convex and tapers toward the lingual side (lingual convergence)
  • The mesiolingual and distolingual line angles are rounded.
  • There is only one well-developed lingual cusp located directly opposite to th buccal cusp in a lingual direction.
  • There is no lingual developmental groove.

A central developmental groove is seen on the occlusal surface traveling in amesiodistal direction. This groove may be straight and often crescent-shaped. The groove pattern can be either a ‘’U’’ or ‘’H’’ groove pattern, depending on whether the central developmental groove is straight mesiodistally or curves buccally at its ends. The central groove of the two-cusp from terminates in a mesial and distal fossae. There is usually no central pit, a mesial or distal pit is much more likely. Usually the two cusp type shows a transverse ridge.

The pulp cavity

  • In buccolingual section it resembles that of the mandibular first premolar, but is larger in size, and the roof of the pulp chamber is pointed to accommodate more than one pulp horn. The lingual pulp horn is smaller than the buccal pulp horn.
  • In mesiodistal section it resembles that of the mandibular first premolar except for the added size and length.
  • The pulp chamber has three pulp horns in the three cusp type. The largest is the buccal pulp horn then the mesio lingual then the distolingual.
  • In cervical cross section the outline of the pulp cavity follows that of the root and most commonly is rectangular.■