There are two common types of mandibular second premolars: a two-cusp type with one lingual cusp (frequency 43%), and a three-cusp type with two lingual cusps (frequency 54,2%). The buccal cusp on first premolars is longer, sharper and more pointed than the buccal cusp on second premolars.
The cusp slopes meet at a nearly right angle – 110º, while the cusp slopes of the second premolar meet at a more obtuse angle of about 130º. A common phenomenon is the occurrence of shallow notches on both cusp ridges on unworn premolars, called Thomas notches (named after a dentist P.K. Thomas). The lingual cusp of the first premolar is very small and is often pointed at the tip. It is nonfunctional.
On mandibular first premolars there is frequently a mesiolingual groove separating the mesial marginal ridge from the mesial slope of the small lingual cusp. This groove is called the mark of Adloff. Due to this groove the mesial marginal ridge of the first premolar slopes cervically at nearly a 45 degree angle from the buccal cusp and is nearly parallel to the triangular ridge of the buccal cusp.
An exception to all other permanent teeth is the mandibular first premolar, the only tooth where the mesial marginal ridge is more cervically located than the distal marginal ridge. As on all mandibular posterior teeth, the crown of the mandibular first and second premolar tilts noticeably toward the lingual surface at the cervix.
From the occlusal aspect the crown of the mandibular first premolar has the shape of a diamond, converging lingually from the contact areas. The outline of the crown is not symmetrical. It often looks as though the mesial side of the crown has been pushed inward on the mesiolingual corner. The distal crown outline is considerably more convex.
The crown of the two-cusp second premolars is round or oval shaped and tapers to the lingual. On the three-cusp second premolars, the occlusal surface is more square because the crown is broad on the lingual side. When the lingual cusps are large, the occlusal surface is broader on the lingual side than on the buccal side.
The triangular ridge of the buccal cusp is long and slopes lingually from the cusp tip to the line where it joins the short triangular ridge of the lingual cusp. The grooves on the first premolars are fewer in number but deeper than those on the second premolars. The mesial and distal fossae are circular, not triangular, each fossa has a pit. The distal fossa is usually larger or deeper.
On the two-cusp type mandibular second premolars, the lingual cusp is smaller than the buccal cusp, with a large triangular ridge on the buccal cusp and a smaller one on the lingual cusp. These two ridges form a transverse ridge.
The curved central developmental groove extends mesiodistally across the occlusal surface and ends in the circular mesial and distal fossae, where it joins the supplemental grooves.
The three-cusp second premolar has three triangular ridges converging toward the central fossa. There is no transverse ridge. The large central fossa is located quite distal to the centre of the occlusal surface.
Three-cusp second premolars do not have a central groove. There is a long mesial groove extending from the central fossa to a small mesial triangular fossa. The short distal groove extends from the central fossa to the very small distal triangular fossa.