Fulcrum lines according to planes
Rotational movements occur around three distinct fulcrum lines, each corresponding to one of the planes.
The primary fulcrum line is situated in the horizontal plane and extends between the two primary (terminal) abutment teeth close to the distal extension bases on either side of the dental arch.
This fulcrum line governs rotational movements in the sagittal plane, specifically those directed toward and away from the residual ridge (Video 1–4).
The denture base rotates around the primary fulcrum line either by sinking into the mucosa under masticatory forces or by lifting away from the tissue due to sticky foods and/or gravitational forces in the maxilla. Among the three fulcrum lines, the rotational movement around this primary fulcrum line tends to be the most pronounced. This movement generates distoapical and mesioapical tipping forces on the primary abutment teeth, with the largest force vector directed apically. Since the periodontal ligaments of the abutment teeth are more resistant to axial forces, even lower-magnitude horizontal or lateral forces may result in harmful effects (Video 1–5).
The second fulcrum line is located within the sagittal plane and extends from the occlusal rest of the primary abutment tooth on one side of the arch, passing across the edentulous ridge (Video 1–6).
In Kennedy Class I cases, a fulcrum line is established within each edentulous ridge area, facilitating rotational movement of the denture base in the frontal plane. This movement, often resembling a rocking over the residual ridge, is typically more manageable and of lower magnitude. Nevertheless, the forces generated are predominantly horizontal in direction, which are poorly tolerated by the supporting oral tissues. Therefore, such movements must be critically evaluated and minimized during the prosthetic treatment planning phase.
The third fulcrum line is located vertically near the midline, lingual to the anterior teeth (Video 1–7).
This fulcrum axis lies in the vertical direction and controls rotational forces occurring in the horizontal plane. Depending on its position, the resulting forces are almost entirely horizontal. It causes the distal extension prosthesis to slide side to side, or “fish-tail,” which can be quite harmful. Therefore, particular attention must be paid to this type of movement during the treatment planning process.