A cusp of the mandibular left first premolar (tooth 20) has fractured. Which cusp must be restored to full form and function?

Correct answer: D. The buccal cusp

• A. Mandibular buccal cusps are the principal supporting cusps of the mandibular arch.
• B. They are not equivalent; the functional/non-functional distinction is fundamental to occlusal design.
• C. The lingual cusp of a mandibular first premolar is small, often vestigial and NON-functional — it does not contact in centric occlusion. It is restored for contour and to protect the tooth, but it is not the cusp that must be re-established functionally.
• D. CORRECT. The BUCCAL cusp of a mandibular premolar is the FUNCTIONAL (centric, supporting) cusp. Mandibular buccal cusps and maxillary lingual cusps occlude into the opposing fossae and marginal ridges, and they carry the load of centric occlusion and maintain the vertical dimension. A fractured functional cusp must be restored to correct contour, height and contact.

Note: The mnemonic is MaLBu — MAxillary LINGUAL and mandiBUlar BUCCAL cusps are the supporting cusps. This determines cuspal coverage decisions too: a functional cusp with an undermined base needs onlay or cuspal coverage, whereas a non-functional cusp may often be conserved.