permanent maxillary premolars l tooth crown & root morphology MCQs for dental students

 

tooth crown and root anatomy of maxillary premolars 

Maxillary first premolar tooth crown and root morphology

Chronology

• Eruption >> 10-11 years.
• Root completed >> 12-13 years.

 

Buccal aspect

1. The geometric outline of the crown is trapezoidal in shape with the shortest uneven sides is directed cervically. The cervical line exhibits little curvature . The crest of curvature of the cervical line buccally is near the center of the root buccally.
2. Mesial Outline: The mesial outline of the crown is slightly concave from the cervical line to the mesial contact area.   The mesial contact area lies immediately occlusal to the halfway point from the cervical line to the tip of the buccal cusp.
3. Distal Outline: The distal outline of the crown below the cervical line is straighter than that of the mesial, or it may be somewhat concave. The distal contact area shows broader curvature than is found mesially. The contact areas are more nearly at the same level.
4. The buccal cusp is long with a pointed tip. The mesial slope of the buccal cusp is straight and longer than the distal slope which is shorter and more curved. This arrangement places the tip of the buccal cusp distal to a line bisecting the buccal surface of the crown.
5. Buccal surface: The buccal surface of the crown is convex, showing strong development of the middle buccal lobe. The buccal ridge continues from the cusp tip to the cervical margin on the buccal surface. Mesial and distal to the buccal ridge, at or occlusal to the middle third, developmental depressions are usually seen.
6. The Root: The buccal root appears conical in form with blunt apex, It is shorter than canine 's root by 3 to 4 mm.                                           

 

Lingual aspect

1. From the lingual aspect the outlines of the maxillary first premolar is the reverse of the outlines of the buccal aspect.
2. The crown tapers toward the lingual. The lingual cusp is narrower mesiodistally than the buccal cusp.
3. It is smooth and spheroidal with a pointed cusp tip. The spheroidal form of the lingual portion of the crown is convex at all points, sometimes with a crest called the lingual ridge.
4. The mesial and distal outlines of the crown are convex being continuous with the mesial and distal slopes of the lingual cusp and straightening out as they join the mesial and distal sides of the lingual root at the cervical line.
5. The cervical line is regular lingually, with slight curvature toward the root and the crest of curvature centered on the root. Because the lingual portion of the crownis narrower than the buccal.
6. It is possible to see part of the mesial and distal surfaces of the crown and root from the lingual aspect.
7. Because the lingual cusp is shorter than the buccal cusp, the tips of both cusps, with their mesial and distal slopes, may be seen from this aspect.
8. The lingual portion of the root, or the lingual portion of the lingual root if two roots are present, is smooth and convex at all points.
9. The apex is more blunt than the buccal root apex.

Mesial aspect

1. The geometric outline: is roughly trapezoidal. However, the longest of the uneven sides is toward the cervical portion and the shortest is toward the occlusal portion.
2. Another characteristic that is representative of all posterior maxillary teeth is that the tips of the cusps are well within confines of the root trunk i.e. the measurement from the tip of the buccal cusp to the tip of the lingual cusp is less than the buccolingual measurement of the root at its cervical portion.
3. The cervical Iine show less curvature acclusally (about 1 mm) less than the cervical curvature on the mesial aspect of anterior teeth.
4. The Buccal Outline:Itis curved with crest of curvature located approximately at the junction of cervical and middle thirds or itmay be located within the cervical third. From the crest of curvature, the buccal outline continues as a line of less convexity to the tip of the buccal cusp which is directly below the centre of the buccal root.
5. The lingual outline: It is a smoothly curved line starting at the cervical line and ending at the tip of the lingual cusp with the crest of curvature is near the centre of the middle third.
6. The lingual cusp tip is on a line with the lingual border of the lingual root.
7. The lingual cusp is shorter than the buccal cusp by about 1 mm.
8. The mesial surface: A distinguishing feature of this tooth on the mesial aspect is the mesial developmental depression which is a marked depression found just cervical to the mesial contact area, centered on the mesial surface and continues apically beyond the cervical line to join a deep developmental depression between the two roots and ends at the root bifurcation.
9. Maxillary second premolars don't have this feature.
10. Another distinguishing feature of the maxillary first premolar is a well-defined mesial marginal developmental groove.
11. This marginal groove is continuous with the central groove of the occlusal surface, crossing the marginal ridge immediately lingual to the mesial contact area and terminating a short distance cervical to the mesial marginal ridge on the mesial surface.
12. The mesial marginal ridge is at the level of the junction of the middle and occlusal thirds.
13. The Root: Most maxillary first premolarshave two roots one buccal and one lingual.
14. The buccal outline of the buccal root above the cervical line is straight with a lingual inclination.
15. Apical to the middle third, it may take a buccal or lingual inclination.
16. The lingual outline of the lingual root, above the cervical line is also straight.
17. The root trunk is long making up about half of the root length.
18. The bifurcation begins at a more occlusal point mesially than distally.
19. The mesial surface of the root is smoothly convex buccally and lingually with a deep developmental groove and depression at or below the bifurcation.
20. When one root is present, the developmental depression is very noticeable for most of the root length with the buccal and lingual  outlines ending in a blunt apex.

 

Distal aspect

• The distal aspect of the crown and root differs from the mesial aspect as follows:
• The crown surface is convex at all points.
• The curvature of the cervical line is less on the distal than on the mesial surface, even showing a straight line from buccal to lingual.
• No developmental groove is evident on the distal marginal ridge.
• The root trunk is flattened on the distal surface with no developmental signs.
• The bifurcation of the root is near the apical third, with no developmental groove leading to it as found mesially.

Occlusal aspect

• The occlusal aspect of the maxillary first premolar resembles roughly a six-sided or hexagonal figure. The two buccal sides are nearly equal, the mesial side is shorter than the distal side and the mesiolingual side is shorter than the distolingual side.
• The crown is wider on the buccal than on the lingual.
• The buccolingual dimension of the crown is much greater than the mesiodistal dimension.
• When looking at the occlusal aspect of the tooth so that the line of vision is in line with the long axis, more of the buccal surface of the crown is seen than the lingual surface.
• The buccal outline is generally convex with prominence of the buccal ridge.
• The lingual margin is evenly convex or semicircular.
• The buccal triangular ridge of the buccal cusp is prominent arising near the centre of the central groove to the tip of buccal cusp.
• While the lingual triangular ridge of the lingual cusp is less prominent arising near the centre of the central groove to the tip of lingual 1 cusp.
• The mesial and distal triangular fossae are triangular depressions located just distal and mesial to the marginal ridges respectively.
• The occlusal surface has no supplemental grooves.
• A well-defined central developmental groove divides the surface evenly buccolingually.
• It extends from a point just mesial to the distal marginal ridge to the mesial marginal ridge, where it joins the mesial marginal developmental groove.
• This latter groove crosses the mesial marginal ridge and ends on the mesial surface of the crown.
• Two collateral developmental grooves join the central groove just inside the mesial and distal marginal ridges.
• These grooves are called the mesiobuccal and the distobuccal developmental grooves.
• The junctions of the grooves are deeply pointed and are named the mesial and distal developmental pits.

 

pulp cavity

• The maxillary first premolar usually have two root canals, a buccal and a lingual canals.
• Mesiodistal section: The outline of the pulp chamber and root canal is similar to the maxillary canine. The pulp cavity tapers slightly from the occlusal aspect to the apical foramen.
• Buccolingual section: The pulp chamber reveals two prominent pulp horns. The buccal horn extends further occlusally under the buccal cusp. The pulp chamber floor is below the cervical level of the tooth. The shape of the pulp chamber (excluding the pulp horns) is square or rectangular.
• Cervical cross section: The cross section at the cervical level is kidney-shaped. The pulp outline follows the mesial concavity on the tooth  surface.

 

 

 Maxillary second premolar tooth crown and root morphology

Chronology

• Eruption >> 10-12 years.
• Root completed >> 12-14 years.

 

Buccal aspect

• The buccal cusp of the second premolar is not as long as that of the first premolar and is also less pointed . The mesial slope of the buccal cusp ridge is usually shorter than the distal slope . The opposite is true of the first premolar.
• The crown and root of the second premolar are thicker at their cervical portions.
• The buccal ridge may not be so prominent when compared with the first premolar.

Lingual aspect

• The lingual cusp is longer, making the crown longer on the lingual side, so less of the occlusal surface is visible from this aspect.

Mesial aspect

1. From the mesial aspect, the cusps of the second premolar are shorter than those of the first premolar, with the buccal and lingual cusps more nearly the same length.
2. The occlusal surface is wider buccolingually because of the greater distance between the cusp tips.
3. No developmental depression is evident on the mesial surface of the crown.
4. No developmental groove crossing the mesial marginal ridge is evident.
5. The level of the mesial contact area and the mesial marginal ridge are located more cervically than those of the first premolar.
6. A single tapered root is present with a shallow developmental groove on its center.

 

Distal aspect

• The crown feature of the first & second premolars are the same except that the buccal & lingual cups of the second premolar are more even in length.
• The distal contact area is larger in size when compared to the first premolar.
• The distal depression on the root surface is deeper than the mesial depression.
• This is the opposite of that of the maxillary first premolar.

Occlusal aspect

• The outline of the crown from the occlusal aspect is more rounded or oval, rather than angular.
• The central developmental groove is shorter and more irregular with multiple supplementary grooves radiating from the central groove. Because of this, the occlusal surface is irregular with a very wrinkled appearance.

 

Pulp cavity

• Mesiodistal section: It is similar to the maxillary first premolar.
• Buccolingual section: Most maxillary second premolars have one root and one canal, although two canals may also be found.
• The pulp horns are well developed.
• The pulp chamber and root canal are very broad buccolingually.
• In the apical third, the pulp narrows suddenly and then tapers gently toward the apex.
• Sometimes the pulp canal bifurcates at the apical third of the root.
• Cervical cross section: The cervical cross section is usually oval.