Introduction
Every abutment tooth can carry more than its own load, but not unlimited load. Bulgarian prosthodontics answers "how much more?" with Balabanov's concept of the functional-mechanical equilibrium of the periodontium (1970), used in clinical practice for more than 40 years.
The concept
The periodontium of a natural tooth is in functional and mechanical equilibrium when the ratio between the area of its occlusal surface and the area of its active root surface stays constant.
Balabanov calculated this constant as:
Fₒ is the occlusal surface area and Fₚ the periodontal (root) surface area. In plain words: a healthy root can carry two occlusal surfaces, its own and one more. The second one is the tooth's reserve force.
Applied to fixed bridges (Popov's modification)
Planning a bridge means solving two problems: choosing the abutments and designing the restoration. For an optimal result, the total occlusal area of retainers and pontics must relate to the total root area of the abutments within:
For bridges on natural teeth this means one abutment may carry one pontic.
Worked example
Tooth 35 is missing; 34 and 36 are healthy with normal mobility.
- Abutments: 34 and 36.
- Occlusal load: Fₒ34 + Fₒ35 + Fₒ36 = 3Fₒ.
- Root support: Fₚ34 + Fₚ36 = 2Fₚ.
- Ratio: 3Fₒ/2Fₚ = 1.5, which is less than 2: acceptable.
- The occlusal tables of retainers and pontic are narrowed by about 30%, because full-width surfaces would overload the abutments.