Force transfer in implant dentistry: Basic concepts and principles

1993(1)10. Force transfer in implant dentistry: Basic concepts and principles
Bidez M.- Misch C.E.
September 1992 - Journal of Oral Implantology 1992: - Vol. 18 No. 3 pp 264-274
© Specialist Dental Summaries Ltd. 1997. All rights reserved
The primary function of virtually all dental implants is to transfer and distribute the forces generated by functional activities to the bone of the maxilla or mandible in which they are placed.
This article describes these forces, their components, moments, force transfer mechanisms, deformation and strain, stress-strain relations, impact loads and impulse that implants and the superstructures are subjected to.
Compressive forces are the most easily understood generally acting perpendicular to the surface or plane of interest, and tending to maintain the integrity of the bone/implant interface, whereas tensile forces tend to disrupt or distract such an interface.
Compressive and tensile forces will occur at the same time on opposite sides of threads or implants when occlusally loaded.
Sheer forces typically act parallel to the surface or plane of interest and are destructive with respect to maintenance of the contiguous bone to implant interface. Where high sheer components of this force exist the interface is clearly at risk from mechanical failure.
Implants are most vulnerable to shear forces soon after exposure when subjected to tightening of screws or attachments.
The positions of occlusal contacts on the prostheses are directly related to the types of force components distributed throughout the implant system. Craig (1980) reported a range of biting forces in adults (vertical component) with several teeth of 200-2440 N. Molar bite forces can exceed four times the magnitude of bite forces excerpted in the incisor region.
Moments: The moment of a force about a point tends to produce rotation or bending about that point. The imposed moment load is also referred to as a torque or torsional load, and these may be destructive to implant systems.
Torques or bending moments imposed on implants due to long cantilever or bar sections may result in interface breakdown. Torsional loading on dental implants should be minimised since the magnitude of deleterious moment load is currently unknown.
Occlusal height of the prostheses determines the moment arm for forces directed along the facio-lingual axis e.g.. tongue thrusts and passive loading by cheek and oral musculature. Osteoplasty before implant placement or excessive resorption bone loss will increase the occlusal height directly affect the loading of the system.
Cantilever prosthesis attached to splinted implants result in a complex load reaction. Under ideal conditions it is suggested empirically that the distal cantilever should not extend beyond two and a half times the anterior posterior distance between implants. The implant length, bone density, arch form and the amount and frequency of the distal load will greatly influence this. Minimal cantilevering and thereby minimising torquing forces is always recommended.
Cantilever example: Two implants placed 10 mm apart with a simple 20 mm cantilever extension and a 100 N force applied to the end of the cantilever, will subject the anterior implant to a force of 200 N and the distal abutment to a force of 300 N. By moving the distal implant 5 mm closer to the anterior implant cantilever load arm increases from 20 to 25 mm. The force on the anterior implant increases by 2.5 x to 500 N and on the distal implant 2.0 x to 600N.
Structurally the maxilla is less suited to cantilever prostheses than the mandible such that distal implants are usually advisable. Sinus augmentation to allow ideal implant placement may often be necessary.
Impact Loads and Impulse are forces transferred to the bone and superstructure which are dependant upon the prostheses materials and the occlusal impact velocity. Progressive loading of implants is a means of controlling the response of maturing bone to functional impacts.
Discussion and Conclusion Basic concepts and principles of engineering mechanics may be applied to the solution of clinical problems in implant dentistry. Understanding the character of and manner in which forces are transmitted by dental implants to the surrounding bone will enable the clinician to design implant supported prostheses that observe basic biomechanical principles.
The original paper is excerpted from Contemporary Implant Dentistry published by CV. Mosby Inc. with modifications.
Correspondence and reprint requests to: Dr. Bidez, The University of Alabama, School of Engineering, UAB Station, Birmingham, AL 35294-4461, USA.