# 25: Clinical Biomechanics in Implant Dentistry

Chapter 25 Clinical Biomechanics in Implant Dentistry

The discipline of biomedical engineering, which applies engineering principles to living systems, has unfolded a new era in diagnosis, treatment planning, and rehabilitation in patient care. One aspect of this field, biomechanics, concerns the response of biological tissues to applied loads. Biomechanics uses the tools and methods of applied engineering mechanics to search for structure-function relationships in living materials.1 Advancements in prosthetic, implant, and instrumentation design have been realized because of mechanical design optimization theory and practice.2 This chapter provides fundamental concepts and principles of dental biomechanics as they relate to long-term success of dental implants and restorative procedures.

# MASS, FORCE, AND WEIGHT

In 1687, Sir Isaac Newton described a force in what is now referred to as Newton’s laws of motion.3 In his second law, Newton stated that the acceleration of a body is inversely proportional to its mass and directly proportional to the force that caused the acceleration. The familiar relation expresses this law:

where F is force (newtons [N]), m is mass (kilograms), and a is acceleration (meters per second squared [m/sec2]). In the dental implant literature, force commonly is expressed as kilograms of force. The gravitational constant (a = 9.8 m/sec2) is approximately the same at every location on Earth; therefore mass (kilograms) is the determining factor in establishing the magnitude of a static load.

# FORCES

A force applied to a dental implant rarely is directed absolutely longitudinally along a single axis. In fact, three dominant clinical loading axes exist in implant dentistry: (1) mesiodistal, (2) faciolingual, and (3) occlusoapical (Figure 25-1). A single occlusal contact most commonly results in a three-dimensional occlusal force. Importantly, this three-dimensional force may be described in terms of its component parts (fractions) of the total force that are directed along the other axes. For example, if an occlusal scheme on an implant restoration is used that results in a large magnitude of force component directed along the faciolingual axis (lateral loading), then the implant is at extreme risk for fatigue failure (described later in this chapter). The process by which three-dimensional forces are broken down into their component parts is referred to as vector resolution and may be used routinely in clinical practice for enhanced implant longevity.

## Three Types of Forces

Forces may be described as compressive, tensile, or shear. Compressive forces attempt to push masses toward each other. Tensile forces pull objects apart. Shear forces on implants cause sliding. Compressive forces tend to maintain the integrity of a bone-implant interface, whereas tensile and shear forces tend to distract or disrupt such an interface. Shear forces are most destructive to implants and bone compared with other load modalities. In general, compressive forces are accommodated best by the complete implant-prosthesis system. Cortical bone is strongest in compression and weakest in shear10 (Table 25-2). Additionally, cements and retention screws, implant components, and bone-implant interfaces all accommodate greater compressive forces than tensile or shear. For example, whereas the compressive strength of an average zinc-phosphate dental cement is 83 to 103 MPa (12,000 to 15,000 psi), the resistance to tension and shear is significantly less (500 psi) (Figure 25-2).

TYPE OF FORCE APPLIED STRENGTH (MPa)* LOAD DIRECTION/COMMENTS
Compressive 193.0 (13.9) Longitudinal
173.0 (13.8) 30 degrees off axis
133.0 (15.0) 60 degrees off axis
133.0 (10.0) Transverse
Tensile 133.0 (11.7) Longitudinal
100.0 (8.6) 30 degrees off axis
60.5 (4.8) 60 degrees off axis
51.0 (4.4) Transverse
Shear 68.0 (3.7) Torsion

From Reilly DT, Burstein AH: The elastic and ultimate properties of compact bone tissue, J Biomech 8:393, 1975.

Offset loading on single-tooth or multiple-abutment restorations results in moment (bending) loads (described later under Force Delivery and Failure Mechanisms). As a result, an increase in tensile and shear force components is often found. Compressive forces typically should be dominant in implant prosthetic occlusion.

Multiple abutment restorations, particularly with distal cantilevers, produce a remarkably complex load profile in the prosthesis and in the bone-implant interface. These clinical realities underscore the need for optimizing dental implant design to provide the maximum functional surface area to dissipate such forces.

Jan 7, 2015 | Posted by in Implantology | Comments Off on 25: Clinical Biomechanics in Implant Dentistry
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