CHAPTER 4 Biomechanics of Removable Partial Dentures

As was stated in Chapter 1, the goal is to provide useful, functional removable partial denture prostheses by striving to understand how to maximize every opportunity for providing and maintaining a stable prosthesis. Because removable partial dentures are not rigidly attached to teeth, the control of potential movement under functional load is critical to providing the best chance for stability and patient accommodation. The consequence of prosthesis movement under load is an application of stress to the teeth and tissue that are contacting the prosthesis. It is important that the stress not exceed the level of physiologic tolerance, which is a range of mechanical stimulus that a system can resist without disruption or traumatic consequences. In the terminology of engineering mechanics, the prosthesis induces stress in the tissue equal to the force applied across the area of contact with the teeth and/or tissue. This same stress acts to produce strain in the supporting tissue, which results in load displacement in the teeth and tissue. The understanding of how these mechanical phenomena act within a biological environment that is unique to each patient can be discussed in terms of biomechanics. In the design of removable partial dentures, with a focus on the goal of providing and maintaining stable prostheses, consideration of basic biomechanical principles associated with the unique features of each mouth is essential. Oral hygiene and appropriate prosthesis maintenance procedures are required for continued benefit of optimum biomechanical principles.

Biomechanical Considerations

As Maxfield states, “Common observation clearly indicates that the ability of living things to tolerate force is largely dependent upon the magnitude or intensity of the force.” The supporting structures for removable partial dentures (abutment teeth and residual ridges) are living things that are subjected to forces. Whether the supporting structures are capable of resisting the applied forces depends on (1) what typical forces require resistance, (2) what duration and intensity these forces have, (3) what capacity the teeth and/or mucosae have to resist these forces, (4) how material use and application influence this teeth-tissue resistance, and (5) whether resistance changes over time.

Consideration of the forces inherent in the oral cavity is critical. This includes the direction, duration, frequency, and magnitude of the force. In the final analysis, it is bone that provides the support for a removable prosthesis (i.e., the alveolar bone by way of the periodontal ligament and the residual ridge bone through its soft tissue covering). If potentially destructive forces can be minimized, then the physiologic tolerances of the supporting structures are not exceeded and pathologic change does not occur. The forces that occur with removable prosthesis function can be widely distributed and directed, and their effect minimized by appropriate design of the removable partial denture. An appropriate design includes the selection and location of components in conjunction with a harmonious occlusion.

Unquestionably the design of removable partial dentures necessitates mechanical and biological considerations. Most dentists are capable of applying simple mechanical principles to the design of a removable partial denture. For example, the lid of a paint can is more easily removed with a screwdriver than with a half dollar. The longer the handle, the less effort (force) it takes. This is a simple application of the mechanics of leverage. By the same token, a lever system represented by a distal extension removable partial denture could magnify the applied force of occlusion to the terminal abutments, which would be undesirable.

Tylman states, “Great caution and reserve are essential whenever an attempt is made to interpret biological phenomena entirely by mathematical computation.” However, an understanding of simple machines applied to the design of removable partial dentures helps to accomplish the objective of preservation of oral structures. Without such understanding, a removable partial denture can be inadvertently designed as a destructive machine.

Machines may be classified into two general categories: simple and complex. Complex machines are combinations of many simple machines. The six simple machines are lever, wedge, screw, wheel and axle, pulley, and inclined plane (Figure 4-1). Of the simple machines, the lever, the wedge, and the inclined plane should be avoided in the design of removable partial dentures.

Figure 4-1 The six simple machines include lever, wedge, inclined plane, screw, pulley, and wheel and axle. The fulcrum, wedge, and inclined plane are matters of concern in removable partial denture designs because of the potential for harm if they are not appropriately controlled. F, Fulcrum.

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