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.
Removable partial dentures by design are intended to be placed into and removed from the mouth. Because of this, they cannot be rigidly connected to the teeth or tissue. This makes them subject to movement in response to functional loads, such as those created by mastication. It is important for clinicians who provide removable partial denture service to understand the possible movements in response to function and to be able to logically design the component parts of the removable partial denture to help control these movements. Just how this is accomplished in a logical manner may not be clear to a clinician who is new to this exercise. One method of helping to organize design thought is to consider it as an exercise in creating a design solution.
Designing a removable partial denture can be considered similar to the classic, multifaceted design problem in conventional engineering, which is characterized by being open ended and ill structured. Open ended means that problems typically have more than one solution, and ill structured means that solutions are not the result of standard mathematical formulas used in some structured manner. The design process, which is a series of steps that lead toward a solution of the problem, includes identifying a need, defining the problem, setting design objectives, searching for background information and data, developing a design rationale, devising and evaluating alternative solutions, and providing the solution (i.e., decision making and communication of solutions) (Box 4-1).
Forces of occlusion, tissue “load-displacement” character and potential for movement, biomechanical principles applied to specific features of this unique mouth, removable partial denture component parts assigned to control movement
The rationale for design should logically develop from analysis of the unique oral condition of each mouth under consideration. However, it is possible that alternative design “solutions” could be applied, and it is the evaluation of perceived merits of these various designs that seems most confusing to clinicians.
The following biomechanical considerations provide a background related to principles of the potential movement associated with removable partial dentures, and the subsequent chapters covering the various component parts describe how these components are applied in designs to control the resultant movements of prostheses.
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.