Permanent maxillary first premolar  l tooth crown & root morphology revision for dental students

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Morphology of tooth crown and root of Maxillary First Premolar

Buccal Aspect 

• Pentagon or trapezoidal buccal outline of the crown is broader at contact areas and narrow at the cervix.
• Maxillary first premolar possesses two cusps – one buccal cusp and the other palatal cusp. Buccal cusp is usually longer by 1 mm to palatal cusp. Similar to other posterior teeth, this tooth is wider bucco-palatally.
• Maxillary first premolar also exhibits three buccal lobes on the convex buccal surface and one palatal lobe on the palatal surface respectively. Buccal surface develops into a central buccal ridge running cervico-occlusally from buccal cusp to cervical line bounded by two developmental depressions mesial and distal to it (mesiobuccal and distobuccal lobes).
• Pointed cusp tip is positioned slightly distal to the vertical axis bisector. (Mesial cuspal ridge of the buccal cusp is longer than the distal cuspal ridge).
• Proximal contours from the cervical region to the contact areas appear straight or convex in overall shape. However, with an exception mesial outline of the crown is slightly concave between the mesial contact area and the cervical line. This may be attributed to developmental depression on the root that extends over the cement-enamel junction onto the crown surface better described from the mesial aspect. Distal outline of the crown is straighter than mesial outline.
• Mesial proximal contact is present at the junction of middle and occlusal third while distal contact area is present slightly more cervically.
• Crest of curvature of the cervical line and apices of both the buccal and palatal roots on premolar crowns will be centered or slightly distal to the mesiodistal long axis bisector.
• Convex root is usually bifurcated into a buccal and a palatal root. Buccal root is comparatively wider than palatal root. Apical third of both roots is distally titled.

Palatal Aspect

• Palatal outline is an inverse image of the buccal outline. Crown appears narrower mesio-distally on the palatal aspect than buccal aspect exhibiting a palatal convergence. Hence, a portion of the mesial and distal surfaces are also evident.
• Similar to the buccal ridge on the buccal aspect, a smooth elevated portion is evident on the palatal surface running cervico-incisally. (palatal ridge)
• Palatal cusp tip is less pointed and about 1.0 mm lesser than the buccal cusp. It is placed mesial to the mesiodistal long axis bisector exhibiting a shorter mesial cuspal ridge. Due to this, both buccal cusp tip and palatal cusp tip can be viewed from the palatal aspect of the tooth.
• Overall, mesial and distal outlines of the palatal aspect of the crown appear slightly more convex.
• Cervical line on the palatal aspect appears less curved, regular and located central to the root surface.
• Palatal root appears smooth, convex and often with blunt apex than buccal aspect of the tooth.

Mesial Aspect

• Trapezoidal outline of the crown with the longer parallel side at the cervix and the shorter parallel side at the occlusal aspect of the tooth.
• Buccal outline of the crown is slightly convex and outwardly placed from the buccal cusp tip to the crest of the cervical ridge. Palatal outline is convex from the palatal cusp tip to the cervical line of the tooth. Both the buccal and palatal outlines with respective cusp tips are within the confines of the root.
• Crest of curvature on the buccal surface of crown appears at the junction of cervical and middle third of the crown. Crest of curvature of the palatal outline appears at the center occlusogingivally or sometimes at the middle third of the crown.
• Buccopalatal width of the occlusal surface is about half that of the total buccopalatal width of the tooth crown. This makes the cusp tips stay well within the confines of the root trunk. Buccal cusp tip lies exactly below the apex of the buccal root while palatal cusp tip is present below the palatal border of the palatal root.
• Occlusal outline between cusps reflects the outline of the triangular ridges which meet at the central groove hidden by the outline of the mesial marginal ridge. Mesial marginal ridge is placed slightly more occlusally than the distal marginal ridge.
• Mesial cuspal ridges of the buccal and palatal cusps converge cervically from their respective cusp tips to meet the buccal and palatal segments of the mesial marginal ridge respectively. Mesial marginal developmental groove crosses the ridge from the occlusal surface and extends onto the mesial surface of the crown.
• Mesial developmental depression extends on the root surface, crosses the cervical line and extends to the level of the junction of the cervical and middle thirds of the crown. After crossing the cervical line onto the crown, the depression continues to become wider bucco-palatally as it progresses occlusally. The entire mesial developmental depression is triangular in outline with the base of the triangle at the junction of the middle and cervical thirds of the crown and its apex at the root bifurcation. This shallow depression
• is bordered buccally and palatally by mesiobuccal and mesiopalatal line angles.
• Mesial proximal contact area is located just cervical to the buccal segment of the mesial marginal ridge, and buccal to the mesial marginal developmental groove. The area of this crown cervical to the mesial contact area will be slightly more concave due to the developmental depression.
• Mesial developmental groove crosses the mesial marginal ridge and stays closely in association with the mesial developmental depression on the mesial surface of the root. It remains continuous with the central groove from occlusal aspect, across mesial marginal ridge into palatal aspect of the mesial contact area on the mesial surface of the tooth.
• Cervical line curves to a lesser extent occlusally on the mesial aspect of the tooth.
• Mesial aspect of the tooth shows two roots – one buccal and other palatal root. Buccal root exhibits a buccal inclination in the cervical third and buccal or palatal inclination in the middle to apical third of the tooth. Palatal outline shows either a buccal or palatal inclination. Root appears to be bifurcated almost at the central portion of the root.

Distal Aspect

1. Trapezoidal outline of the tooth with the longer parallel side at the cervix and the shorter parallel side at the occlusal aspect of the tooth.
2. Surface of the crown cervical to the contact area on both sides will be smoothly convex.
3. Crest of the distal marginal ridge will be positioned more cervically. No evidence of supplementary groove crossing the distal marginal ridge.
4. Distal contact area will still be placed more occlusally (middle third of the crown occlusogingivally) than the mesial contact area.
5. Cervical line shows very little or no curvature toward the occlusal aspect of tooth.
6. Root trunk in the cervical third is usually convex. Depression on the distal root surface appears less prominent.
7. Developmental groove is less conspicuous.

Occlusal Aspect

1. Hexagonal occlusal outline appears wider bucco-palatally than mesiodistally. Crown appears broader on the buccal aspect than palatal aspect of the tooth. Hexagonal shape refers to following six sides of the tooth – mesiobuccal, mesial, mesiopalatal, distopalatal, distobuccal and distal surfaces on the teeth.
2. Mesial and distal cusp slopes of buccal and palatal cusp meet at the cusp tips at the center and marginal ridges laterally. Mesial marginal ridge meet the mesial cuspal ridge of the buccal cusp at a right angle while the distal marginal ridge meets the distal cuspal ridge of the buccal cusp at an acute angle.
3. Mesial cuspal ridge of the palatal cusp will be shorter than its distal cuspal ridge.(mesiopalatal cusp outline and mesiopalatal angle is shorter than the distopalatal cusp outline and distopalatal angle)
4. Triangular ridges from the buccal and palatal cusps meet near the center of the occlusal table but slightly palatal to the buccopalatal bisector of the crown. Buccal triangular ridge adjacent to central
5. groove joins the buccal cusp tip. Palatal triangular ridge present adjacent to central groove joins the palatal cusp tip.
6. Buccal cusp tip is positioned distal to the mesiodistal bisector and is about half the distance from the buccal outline to the buccopalatal bisector of the crown. The buccal outline show evidence of the three lobes that make up the buccal half of the tooth exhibiting indentations in the outline corresponding to the
7. developmental depressions between the lobes. Mesiobuccal and distobuccal line angles are angular in appearance. Palatal cusp tip appears mesial to the mesiodistal bisector of the crown.
8. Crest of curvature of distal contact area appears buccal to mesial contact area. Buccal crest of curvature appears distal to the palatal ridge.
9. Mesial marginal ridge is intervened by mesial marginal developmental groove and distal marginal ridge is longer and slightly curved.
10. Mesial segment of the outline measures less bucco-palatally than the distal outline. Mesial marginal ridge developmental groove is evident. Distal segment of the outline is slightly convex, and blends smoothly into the arc of the palatal segment.
11. Central developmental groove divides the occlusal table into buccal and palatal halves. It ends in the mesial and distal pits of the mesial and distal triangular fossae adjacent to mesial and distal marginal ridge. In the mesial triangular fossae, the central groove meets the mesiobuccal and mesiopalatal grooves and corresponding mesial pit. In the distal triangular fossae, the central groove meets the distobuccal and distopalatal grooves and the corresponding distal pit.
12. Depression on the mesial portion of distal marginal ridge is distal triangular fossa and depression on the distal aspect of mesial marginal ridge is mesial triangular fossa.
13. Supplemental grooves are relatively less seen.
14. Overall effect of the shortness of the mesial surface versus the distal surface measurement and the position of the buccal and palatal cusp tips cause the crown appear “twisted” or “rotated” around its long axis.