Contemporary Dental Implants

The discipline of biomedical engineering, which applies engineering principles to living systems, has unfolded a new era in diagnosis, treatment planning, and rehabilitation in dental patient care. This is especially true in dental implant care as it continues to advance in complexity and design.

A principal aspect of engineering, 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 such as teeth and bone. Advancements in prosthetic, implant and instrument design have been realized by use of mechanical design optimization theory.

Occlusal Force on Dental Implants

It is fundamental that dental implants are subjected to occlusal loads when placed in the function. Such loads may vary dramatically in magnitude, frequency, and duration, depending on a particular patient’s parafunctional habits. Passive mechanical loads also may be applied to dental implants during the healing stage because of mandibular flexure, contact with the first-stage cover screw, and second-stage permucosal extension. Perioral forces of the tongue and circumoral musculature may generate low but frequent horizontal loads on implant abutments, as well. These loads may be of greater magnitude with parafunctional oral habits of tongue thrust. Application of non-passive prostheses to implant bodies may result in mechanical loads applied to the abutment, even in the absence of occlusal loads.

Biomechanics of Dental Implant Treatment

So many variables exist in implant treatment that it becomes almost impossible to compare one treatment philosophy with another. However, basic units of mechanics may be used to provide the tools for the consistent description and understanding of such physiologic (and non-physiologic) loads on the dental implants. Two different approaches can often render a similar short-term result; however, a biomechanical approach still can determine treatment renders more risk over the long-term life of the dental implants.

Biomechanics is related very firmly to engineering and often employs traditional engineering sciences to understand biological systems. Leonardo da Vinci (1452–1519), the father of modern biomechanics, was principally concerned in the anatomy of the human body and interested in movement. His dynamic anatomical drawings of the musculoskeletal system are today still regarded as special. Dentistry continues to depend upon physical replicas, such as plaster casts and models, to duplicate the human dentition in diagnosing patients and in the design of dental prostheses. The development of technologies plays a key important role in devising diagnostic methods, clinical and surgical methods, as well as understand the biomechanics of the entire stomatognathic system of patients. 

How Does My Implant Surface Affect the Results?

The dental implant design and surface condition both influence the dynamics of osteointegration, a crucially important process in the success of the implant. The following four distinct phases occur in the development of the bone-implant interface:

• Surgical integration
• Healing dynamics
• Early loading period
• Mature loading period

The overall implant design and the surface condition affect these four processes, often as independent features. The surgical process of implant dentistry requires initial fixation and lack of relative movement during the initial phases of the development of the implant-bone interface. The implant design is of primary importance to accomplish this initial step; however, the surface condition of the implant may also be a contributing factor. For example, rougher implant surfaces will aid the implant to have more friction and fixation during insertion of the implant. When the overall design of the implant is a cylinder and/or the bone quality is poor, the surface roughness of the implant will improve the surgical fixation of the implant. The initial healing period of an implant (which is not immediately loaded) is the phase of the osteointegration process that is primarily affected by the surface condition of the implant.

“As a general rule, roughened surfaces increase the bone-implant contact (BIC) percent during the initial bone-healing process.”

In one study, clinicians compared three thread shapes with endosteal implants of similar width, length, thread number, thread depth, and surface condition. The V-shaped and reversed buttress thread had similar BIC and reverse torque values. The square threaded implants had higher BIC and higher reverse torque values. Therefore, thread shape may also influence the implant-bone interface during the healing period of an implant, but it is a secondary factor.

Implant Surface Condition

The implant surface condition alone may also decrease the biological width, which causes a marginal bone loss when the implant extends through the mucosal tissues. One team of dentists inserted a combination of smooth- and rough-surface implants with smooth collars of 1.5 mm below the bone and other implants with the rough surface placed at the bone crest. Within a month the implants with smooth collars lost 1.5 mm of bone, even though the implants received no occlusal load. The implants with a roughened surface to the crest of bone and no occlusal load maintained the bone for the six months of the study. Therefore, roughened surfaces improve the initial BIC during initial healing and decrease the marginal bone loss when the implant extends through the soft tissue, before occlusal load.

Early Loading The Implant

The early loading period of an implant has considerations from an implant body design and the surface condition of the implant, and both criteria are similar in importance. Stress equals force divided by the area over which it is applied and is directly related to the strain observed in the bone. Therefore, BIC will directly relate to the amount of strain at the bone-implant interface. When the strain conditions are within the physiologic zone of bone, the bone-implant interface may maintain a lamellar bone organization, which is organized and mineralized and is best to resist occlusal loads to the interface.

Dental Implant Surface Roughness

The roughened surface of an implant that improves unloaded healing may also help bear the initial occlusal load to an implant interface. For example, hydroxyapatite (HA)-coated cylinder implants reported a high BIC after healing. This implant design also reported high survival rates during the three initial years of loading. The implant design also may affect the early loading period of an implant. For example, smooth-surface cylinder implants do not respond favorably to occlusal load. On the other hand, smooth-surface threaded implants have early loading success, especially in good bone types. The mature loading period of an implant begins to occur after three to five years and continues throughout the implant the lifespan. In general, the surface condition of the implant is least important during this time frame. For example, roughened surfaces on cylinders often lose bone after five years of an occlusal load. The roughened surface on a cylinder can withstand the initial loads, but fatigue factors of the bone interface begin to break down over continued cycles. The higher the hone turnover rates are because of high strain conditions, the more likely the bone microstrain condition is found to be in the pathologic overload range.

Dental Implant Design

On the other hand, the implant design is the major feature of an implant body during the mature loading period and is primarily responsible for the bone turnover rates adjacent to the implant. Therefore, the implant design becomes most important in the mature loading period. Before the advent of osteointegration, the use of implant-supported prostheses often resulted in the formation of a fibrous structure around the implants, resulting in mobility and subsequent loss of the implant. The observation that bone can grow over titanium structures started a new pathway in the study of the intraosseous anchorage for dental prostheses. As a result, long-term implant success rates are reported to be as high as 90 percent to 100 percent.

Why Do Mass, Force and Weight Matter in Dental Implant Design?

Mass, a property of matter, is the degree of gravitational attraction the body of matter experiences. As an example, consider two cubes composed of hydroxyapatite (HA) and commercially pure titanium, respectively. If the two cubes are restrained by identical springs, then each spring will deflect by a certain amount relative to the attraction of gravity for the two cubes. The two spring deflections in this example can be made equal by removing part of the material from the titanium cube. Even though the cubes are of completely different composition and size, they can be made equivalent with respect to their response to the pull of gravity. This innate property of each cube that is related to the amount of matter in physical objects is referred to as mass.

Weight is simply a term for the gravitational force acting on an object at a specified location. Weight and force can be expressed by the same units, newtons or pound force. If a titanium cube is considered as though placed on the moon, then its weight (force caused by gravity) is different from its weight on the Earth. The mass in the cube has not changed, but the acceleration caused by gravity has changed.

Forces of three kinds are involved in the biomechanics of implants. They 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 shear. Additionally, cement and retention screws, implant components, and bone-implant interfaces all accommodate greater compressive forces than tensile or shear.