Crown Size and Shape of Mandibular Premolars

Crown Size and Shape of Mandibular Premolars from the Buccal Aspect

• The crown of the mandibular first premolar is roughly a five-sided pentagon in shape from the buccal view.
• The crown has bilateral symmetrical shape, except for the shorter mesial than distal cusp ridge and a greater distal bulge of the crown.
• The buccal cusp is relatively sharp (110°) and has a central ridge extended from the cusp tip to the cervical line, called the buccal cusp ridge.
• Mesial cusp ridge notch is more common, and the mesial contact is near the junction of the occlusal and middle thirds.
• The distal contact area is slightly more occlusal in position than the mesial contact.

The Lingual Aspect

• The crown is much narrower mesiodistally on the lingual half than on the buccal half.
• The lingual cusp is quite small and short, and is often pointed at the tip.
• Much of the occlusal surface of the tooth can be seen from the lingual aspect because of the shortness of the lingual cusp.
• The mesial marginal ridge is more cervically located than the distal marginal ridge.
• The mesiolingual developmental groove separated the mesial marginal ridge from the lingual cusp.

The Proximal View

• The mandibular first premolar has a rhomboid shape from this aspect.
• The crowns of the mandibular first premolars tilt noticeably toward the lingual surface at the cervix.
• The tip of the buccal cusp is on a line with the long axis of the root and the apex of the root.
• The lingual cusp is considerably shorter than the buccal cusp by more than one-third of the total crown length.
• The tip of the lingual cusp is on a line with the lingual outline of the root.

The Occlusal View

• Much of the buccal surface is visible from this view since the tip of the buccal cusp is slightly buccal to the tooth center.
• The outline of the crown is usually not symmetrical (more bulk in the distal half).
• The mesiolingual portion of the crown outline has been pushed in (identifying feature).
• The outline converges toward the lingual, especially on the mesial.
• The distal marginal ridge forms close to a right angle with the distal cusp ridge of the buccal cusp.
• The mesial and distal cusp ridges of the buccal cusp on the mandibular first premolars are nearly aligned in a straight line mesiodistally.

Occlusal Morphology of Mandibular Premolars from the Occlusal View

• The triangular ridge of the buccal cusp is long and slopes lingually from the cusp tip to where it joins the very short triangular ridge of the lingual cusp.
• The mesial fossa is more linear buccolingually, but the distal fossa is more frequently larger and deeper.
• Each fossa has a pit.
• Only rarely is the pronounced transverse ridge of the mandibular first premolar crossed by a fissured central groove.
• More commonly, there are mesial and distal developmental grooves running in a nearly buccolingual direction, flaring buccally from the mesial and distal fossae.

Characteristics Resembling Mandibular Canine and Second Premolar

• The buccal cusp is long and sharp, and is the only occluding cusp.
• The buccolingual measurement is similar to that of the canine.
• The occlusal surface slopes sharply lingually in a cervical direction.
• The mesiobuccal cusp ridge is shorter than the distobuccal cusp ridge.
• The outline form of the occlusal aspect resembles the outline form of the incisal aspect of the canine.
• The outline of the crown and root from the buccal aspect resembles that of the second premolar.
• The contact areas, mesially and distally, are near the same level.
• The curvatures of the cervical line mesially and distally are similar.
• The tooth has more than one cusp.
• The root of the mandibular first premolar is shorter generally than that of the mandibular second premolar, but is closer to the length of the second premolar root than it is to that of the mandibular canine.