BIOMECHANICS OF REMOVABLE PARTIAL DENTURES

BIOMECHANICS OF REMOVABLE PARTIAL DENTURES
Machines may be classified into two general categories:
1- Simple.
2- Complex. (Combinations of many simple machines).

The six simple machines are lever, wedge, screw, wheel and axle, pulley, and inclined plane.

The fulcrum, wedge, and inclined plane are concerns in removable partial denture designs because of the potential for harm if not appropriately controlled. The lever and the inclined plane should be avoided in designing removable partial dentures.
In its simplest form, a lever is a rigid bar supported somewhere along its length. It may rest on the support or may be supported from above. The support point of the lever is called the fulcrum, and the lever can move around the fulcrum.

POSSIBLE MOVEMENTS OF PARTIAL DENTURE
The direct retainers are functioning to minimize vertical displacement, rotational movement will occur about some axis as the distal extension base(s) move either toward, away, or horizontally across the underlying tissue. Unfortunately, these possible movements do not occur singularly or independently but tend to be dynamic, and all occur at the same time. The greatest movement possible is found in the tooth-tissue supported prosthesis, because of the reliance on the distal extension supporting tissue to share the functional loads with the teeth.
Cantilever can be described as a rigid beam supported only at one end. When force is directed against unsupported end of beam (as in this rest placed on a cantilevered pontic), cantilever can act as a first-class lever.
Distal extension removable partial denture. Cast circumferential direct retainer engages mesiobuccal undercut and is supported by distocclusal rest. If rigidly attached to the abutment tooth, this could be considered a cantilever design, and it may impart detrimental first-class lever force to abutment if tissue support under extension base allows excessive vertical movement toward the residual ridge.
The potential for first-class lever action can also exist in Class II, mod.1 design for removable partial denture frameworks. If cast circumferential direct retainer with a mesiobuccal undercut on right first premolar were used, force placed on denture base could impart upward and posteriorly moving force on premolar, resulting in loss of contact between premolar and canine. Tissue support from extension base area is most important to minimize lever action of clasp. Retainer design could help accommodate more of an anteriorly directed force during rotation of the denture base in an attempt to maintain tooth contact. Other alternatives to first premolar design of direct retainer would be tapered wrought-wire retentive arm that uses mesiobuccal undercut or just has buccal stabilizing arm above height of contour.
More periodontal fibers are activated to resist forces directed vertically on tooth than are activated to resist horizontally (off-vertical) directed force. Horizontal axis of rotation is located somewhere in root of tooth.