a short presentation on the biomechanics in Removable Partial Denture.... a very important topic to be understood completely for easy designing of cast framework and also to know the problems in already treated conditions
INTRODUCTION Biomechanics basically deals with application ofmechanical principles to biological tissues. In the oral cavity one would find a number of sources of stressgeneration, the human body is built in such a manner that it learns to adapt to any stressful situation. However when we try to create an artificialreplacement of that natural component which is lost, we are at a loss in making it fully functional and adaptable.
Biomechanics (GPT 7): 1: the application of mechanical laws to living structures, specifically the locomotor systems of the body. 2: the study of biology from the functional viewpoint. 3: An application of the principles of engineering design as implemented in living organisms.
Mechanical principles applicable in Removable Prosthodontics − Lever principle − Inclined plane − Snowshoe principle
Lever: A simple machine consisting of a rigid bar pivoted on a fixed point and used to transmit force, as in raising or moving a weight at one end by pushing down on the other.
Inclined plane Inclined plane Forces against an inclined plane may result in deflection of that which is applying the forces or may result in movement to the inclined plane, neither of these is desirable.
Snowshoe principle This principle is based on distribution of forces to as large an area as possible. Like in a snow shoe which is designed to distribute forces on the entire base area of the shoe, a partial denture should cover maximum area possible within the physiologic limits so as to distribute the forces over a larger area.
Stress consideration in a partial denture The stresses can be divided as: − Vertical 1.Displacing stresses 2. Dislodging stresses − Horizontal − Torsional
Displacing stress− Those forces which are the result of stresses along the long axis of the teeth in a crown to apex direction and the relatively vertical stresses on the ridge mucosa. These are the least harmful and are born well if within physiologic limits.
Dislodging stress− These are the forces which tend to lift the partial denture from its rest position. Reciprocal dislodging action occurs when wide edentulous spaces are interrupted by few teeth thus inviting an antero-posterior or lateral tilt of prosthesis.
Horizontal stress− They originate as a component of rhythmic chewing stroke. These forces are effective in mesio-distal and buccolingual direction. These lateral stresses are most damaging.
Torsional stress− It is a twisting rotational type of force. It’s a combination of vertical and horizontal force. Torsion is noted most frequently where a long segment acts upon the first abutment it engages. Where the ridge mucosa has higher resiliency torque is higher. Torque applies rotation about a fixed point.
Biomechanical consideration of individual component “Perpetual preservation of what is remaining is more important than meticulous replacement of what is lost” De Van
Major connector− Connects parts of prosthesis located on one side of the arch with those on the opposite side− Should be rigid:− Distribution of force− Effectiveness of other components− If flexible forces concentrated on individual teeth or ridge damaging.
RestRelation between rest and abutment should be such that forces should be directed apically down along the long axis of the tooth Stress absorbed by fibers of PDL without getting damaged.
Indirect retainerTo prevent the DEB from moving away from its seat because of cheek and tongue forces, sticky food. It uses mechanical advantage of leverage by moving the fulcrum line farther from the force.
Contributes to support and stability of the partial denture counteracts horizontal forces applied to the denture. Long span mandibular lingual bar major connector (even if tooth supported), IR provides additional support and rigidity for lingual bar. Prevents impingement of the lingual bar on the mucosa during function.
1. Retention as a means of stress control A) forces of adhesion and cohesion B) atmospheric pressure C) Frictional contact D) Neuromuscular control
Strategic clasp positioning a) Quadrilateral configuration b) Tripod configuration c) Bilateral configuration
Clasp design a) circumferential clasp b) bar clasp c) combination clasp
4 Indirect retention5 Functional basing6 occlusion a) harmonious occlusion b) size of food table c) occlusal pattern of posterior teeth
7 Denture base a) Size & configuration b) accuracy of adaptation of base to the tissues
Removable partial dentures by design are intended to be removed from and replaced into the mouth. Because of this, they are not rigidly connected to the teeth or tissues, which means that they are subjected to movement in response to functional loads. These movement can cause generation of stresses which if not controlled , can damage the remaining teeth and supporting bone.Efforts must be always directed towards minimizing the harmful forces on the remaining teeth by proper designing of the RPD.