Chapter 1. Biomechanics of Removable Partial Dentures
The supporting tissues respond to the movements that occur during the functioning of a removable partial denture (RPD) based on the strength, frequency, direction, and duration of the functional forces. When the stresses exceed the strength of the supporting tissues, irreversible damage may occur.
RPD is supported by two different tissues. These are the tooth support (alveolar bone through the periodontal ligaments of the abutment teeth) and the tissue support (the remaining crestal bone along the soft tissue covering it).
Tooth-supported, Cl III prostheses, like fixed partial dentures, receive all of their support from natural teeth (Video 1-1).
In tooth-supported RPDs, forces are transmitted along the long axis of the abutment teeth through the periodontal tissues to the alveolar bone. However, since they are not rigidly attached to the abutment teeth as in fixed prostheses, their limited movement during function exposes the abutment teeth to non-axial forces.
Tooth-tissue supported, Cl I, II, IV prostheses are supported by teeth in the edentulous area close to the abutment tooth and by edentulous crest tissue in the distant area (Video 1-2).
Movement of the denture base is influenced by factors such as the degree of resiliency of the underlying mucosa, the amount of bone resorption, the fit and extension of the denture base, and the characteristics of the masticatory force.
Movement of the prosthesis results in damaging tilting movements of the abutment tooth adjacent to the distal-extension denture base and bone resorption in the edentulous ridge. The quality and supporting properties of the mucosa in resorbed crests also deteriorate.
When planning prosthetic components for RPD, the capacity of the abutment tooth to withstand functional forces should be well known. Since the periodontal ligament fibers surrounding the teeth are arranged to absorb axial forces along their axis, they are not resistant to non-axial tilting and rotational forces. When planning the prosthesis, the teeth should be kept away from non-axial forces.
As De Van states, “The preservation of the remaining tissues is more valuable than the meticulous replacement of what has been lost.”