In designing a retainer on a non-carious mandibular first premolar abutment with short clinical crown, which of the following restorations is most appropriate?

# In designing a retainer on a non-carious mandibular first premolar abutment with short clinical crown, which of the following restorations is most appropriate?

A. A full crown

B. A reverse ¾ crown

C. MOD onlay

D. Inlay

The correct answer is B. A reverse ¾ crown (also referred to as an inverted or mandibular partial veneer crown).

Clinical & Biomechanical Rationale

Designing a retainer for a mandibular first premolar presents specific anatomical and biomechanical challenges, particularly when the tooth is non-carious but has a short clinical crown:

1. Anatomical Morphology: The mandibular first premolar typically features a prominent, functional buccal cusp and a small, non-functional lingual cusp. The lingual surface is small and tapers significantly toward the cervical line.

2. Conservation of Tooth Structure: Because the tooth is non-carious, preserving sound enamel and dentin is a priority. A full crown (Option A) would require aggressive, circumferential reduction, unnecessarily sacrificing intact tooth structure on a healthy tooth.

3. Retention and Resistance Form: A short clinical crown inherently reduces the surface area available for retention. In a traditional partial veneer crown, the lingual surface is reduced and the buccal surface is left intact. However, on a mandibular first premolar, the small lingual surface provides highly insufficient surface area for retention.

4. The "Reverse" Approach: A reverse 3/4 crown alters the standard preparation by covering the buccal surface (along with the occlusal, mesial, and distal surfaces) while leaving the lingual surface intact.

   • By covering the larger buccal surface, it captures significantly greater surface area, which dramatically increases the retention and resistance form necessary to compensate for the short clinical crown.

   • It places the structural retention margins on the larger aspect of the tooth, preventing the restoration from dislodging facially or occlusally under functional loads.

Why the Other Options Are Less Appropriate:

• Option A (Full Crown): While it provides excellent retention for a short clinical crown, it is unnecessarily invasive for a completely non-carious tooth. Biocompatible dental practice dictates selecting the most conservative restoration that satisfies the mechanical requirements.

• Option C (MOD Onlay): An onlay provides occlusal coverage but relies heavily on the remaining buccal and lingual walls for retention. On a short clinical crown, these short vertical walls do not offer enough resistance form against the lateral dislodging forces exerted on a bridge retainer.

• Option D (Inlay): Inlays are strictly intracoronal restorations. They do not provide occlusal coverage, offer no protection against tooth fracture under regular bridge loading, and possess the lowest retention values among the choices, making them entirely contraindicated as fixed partial denture retainers.