A patient is missing tooth #30. A fixed partial denture spanning from #32 to #29 is being considered but is contraindicated. What is the principal reason?

Correct answer: D. The path of insertion cannot be made common to both abutments

• A. This is too absolute. Third molars CAN serve as abutments when they are well-positioned, well-supported and reasonably aligned. The objection is to this specific tilted geometry, not to third molars as a category.
• B. A single-tooth span replacing #30 is well within the range for a conventional fixed partial denture. Span length is not the limiting factor here.
• C. Crown-to-root ratio is a legitimate general criterion for abutment selection (1:2 ideal, 1:1 the minimum acceptable), but nothing in this scenario indicates that these particular abutments have poor ratios.
• D. CORRECT. A third molar is characteristically tilted and rotated, and its long axis frequently diverges markedly from that of the premolar at the other end of the span. A fixed partial denture demands a SINGLE common path of insertion for all retainers, and achieving that across widely divergent abutments would require destructive over-preparation of one or both teeth — potentially exposing pulp — or would leave the prosthesis unseatable. The problem is geometric, and it is the classic objection to bridging across a third molar.

Note: Abutment evaluation runs on Ante's law (the periodontal surface area of the abutments should equal or exceed that of the teeth being replaced), crown-to-root ratio, root configuration, and alignment/path of insertion. When a stem features a third molar or a markedly tilted molar, path of insertion is the answer being fished for.