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BIOMECHANICAL CONSIDERATIONS IN IMPLANT DENTISTRY Dr. Niyati SinghMDS IIIrd yearUNDER THE GUIDANCE OFDR. AJAY SINGH
CONTENTSIntroductionDefinition Types of biomechanicsImportance in field of dental implants Methods to analyze and visualize stresses in bone Biomechanics of natural teethElements of mechanical propertiesForce delivery and failure mechanism
Clinical moment arms and crestal bone lossBone as reactive recipient materialThe bone repair mechanismEndosseous implantBiomechanical problems of implant-supported prosthesesEffects on treatment planningSummary and conclusion
Biomechanics is one of the most important considerations affecting the design of framework for an implant borne prosthesis. In general, the forces that participate in both the masticatory process and parafunction must be considered in the design of the prosthesis. These considerations act as determining factors of the devices success or failure.INTRODUCTION
WHAT ARE BIOMECHANICS?Biomechanics is the scientific study of the load-force relationships of a biomaterial in the oral cavity. (Ralph Mc Kinney).GPT-8It is the relationship between the biologic behavior of oral structures and the physical influence of a dental restoration.
TYPES OF BIOMECHANICS TWO TYPES:-1) Reactive Biomechanics: is the interaction of isolated biomechanical factors which when combined, produce an accumulative effect. 2) Therapeutic Biomechanics is the clinical process of altering each biomechanical factor to reduce the cumulative response causing implant overload.
IMPORTANCE IN THE FIELD OF DENTAL IMPLANTS:First, to know the loading (bite forces) exerted on the prosthesis. Secondly, to know the distribution of the applied forces to the implants and teeth supporting the prosthesis. Thirdly, the force on each implant must be delivered safely to the bony tissues which in turn depend on the shape and size of the implant. In all these, the aim of biomechanical analysis is to foresee failure of any part of the system, including the prosthesis, the supporting implants and the biological tissues.
BIOMECHANICS OF NATURAL TEETH The tooth is suspended within the alveolar bone by dentoalveolar bundles of collagen fibers. Because of its elasticity there is measurable horizontal, vertical, and rotational tooth mobility. Basically, tooth movement can be subdivided into two phases:-Desmodontal phase:- referred to as the first stage of movement, taking place when a load of up to 100 N acts on the tooth. During this phase, the tooth moves slightly within the socket. Some periodontal fiber bundles are stretched, while others are relaxed. However, the alveolar process does not undergo marked deformation.
Periodontal phase:- commences at loads exceeding 500 N. After the desmodontal phase has passed and the periodontal fiber bundles have been stretched to their full length, these strong forces cause deformation of the entire alveolar process, which subsequently offers more resistance to further tooth deflection. The degree of tooth deflection may vary from one individual to another and ranges between 10 and 50 m.
Calculations of the stress distribution around natural teeth have shown that physiologic stresses result in a relatively even stress distribution. The exception is in the cervical region of the tooth, where horizontal loading with transfer of the compressive forces into the periodontium results in moderately high tensile stresses, and the lamina dura is affected by axial compressive stresses because of bending of the alveolar bone.
Mass, is the degree of gravitational attraction the body of matter experiences. Force was described by Newton. Newton's second law, states that the acceleration of a body is inversely proportional to its mass and directly proportional to the force that caused the acceleration. A f \ m Therefore, F = maWeight is the gravitational force acting on an object at a Specified location. Weight and Force therefore can be expressed by the same units, newtons (N) or pound force (Ibf)
ELEMENTS OF MECHANICAL PROPERTIES
FORCESForces may be described by magnitude, duration, direction, type, and magnification factors.Forces acting on dental implants are referred to as vector quantities; that is, they possess both magnitude and direction. A force applied to a dental implant is rarely directed absolutely longitudinally along a single axis. In fact, forces are three dimensional with components directed along one or more of the three clinical coordinate axes i.e. :-
Components of Forces (Vector Resolution)A single occlusal contact most commonly result in a three-dimensional occlusal force. The process by which three-dimensional forces are broken down into their component parts is referred to as vector resolution Types of Forces Forces may be described as :-Compressive forces . Tensile forces Shear forces
Compressive forces tend to maintain the integrity of a bone-to-implant interface, whereas tensile and shear forces tend to disrupt such an interface.
Shear forces are most destructive to implants and bone when compared with other load modalities.
Compressive forces, in general, are best accommodated by the complete implant-prosthesis system.
The implant body design transmits the occlusal load to the bone. Threaded or finned dental implants impart a combination of all three force types at the interface under the action of a single occlusal load.Cylindrical implants are at highest risk for harmful shear loads under an occlusal load directed along the long axis of the implant body. As a result, cylinder implants require a coating to manage the shear stress at the interface through a more uniform bone attachment along the implant length. Compressive forces should typically be dominant in implant prosthetic occlusion.
STRESSThe manner in which a force is distributed over a surface is referred to as mechanical stress. Thus stress is defined by the familiar relation: Stress = F/A The internal stresses that develop in an implant system and surrounding biologic tissues have a significant influence on the long-term longevity of the implants in vivo. As a general rule of thumb, a goal of treatment planning should be to both minimize and evenly distribute mechanical stress in the implant system and the contiguous bone.
The magnitude of stress is dependent on two variables:- force magnitude and cross-sectional area over which the force is dissipated.
Force magnitude rarely be completely controlled by a dental practitioner. The magnitude of the force may be decreased by reducing the significant "magnifiers of force :- cantilever length, offset loads, and crown height. Night guards to decrease nocturnal parafunction, occlusal materials that decrease impact force, and overdentures rather than fixed prosthesis so they may be removed at night are further examples of force reduction strategies.
Functional cross-sectional area is defined as that surface that participates significantly in load bearing and stress dissipation. It may be optimized by :- Increasing the number of implants for a given edentulous site, and Selecting an implant geometry that has been carefully designed to maximize functional cross-sectional area.
DEFORMATION AND STRAINStrain is defined as the change in length divided by the original length. The deformation and strain characteristics of the materials used in implant dentistry may influence interfacial tissues, and clinical longevity. Elongation (deformation) of biomaterials used for dental implants range from 0% for aluminum oxide (Al2O3) to up to 55% for annealed 316-L stainless steel.
Related to deformation is the concept of straina parameter believed to be a key mediator of bone activity.
All materials (both biologic and nonbiologic) are characterized by a maximum elongation possible before permanent deformation or fracture results.
STRESS-STRAIN CHARACTERISTICS:A relationship is needed between the applied force (and stress) and the subsequent deformation (and strain).
If any elastic body is experimentally subjected to an applied load, a load-vs.-deformation curve may be generated. If the load values are divided by the surface area over which they act and the change in the length by the original length, a classic engineering stress-strain curve is produced.
Such a curve provides for the prediction of how much strain will be experienced in a given material under an applied load. The slope of the linear (elastic) portion of this curve is referred to as the modulus of elasticity (E), and its value is indicative of the stiffness of the material under study.
The closer the modulus of elasticity of the implant resembles that of the biologic tissues, the less the likelihood of relative motion at the tissue-to-implant interface.Once a particular implant system (i.e., a specific biomaterial) is selected, the only way to control the strain is to control the applied stress or change the density of bone around the implant.
When two bodies collide in a very small interval of time (fractions of a second), relatively large forces develop. Such a collision is described as impact. In dental implant systems subjected to occlusal impact loads, deformation may occur in the prosthetic restoration, in the implant itself, or in the interfacial tissue. The higher the impact load, the greater the risk of implant and bridge failure and bone fracture. Rigidly fixed implants genera