This module focuses on the concept of dental implant integration into the soft and hard tissues in the oral cavity. Integration into bone is called osseointegration and this phenomenon essentially revolutionized how missing teeth can be replaced. Osseointegration has implications for all dentists as well as for patients with any missing teeth. Implants are anchored in bone tissue and penetrate the soft tissue. It is essential to understand the soft and hard tissues around dental implant restorations since only this allows for optimal restorations.

At the end of this ITI Academy Learning Module you should be able to: define the hard and soft tissue integration of dental implants, discuss how hard tissue integration or osseointegration occurs, discuss how soft tissue integration occurs and contrast this to teeth, and relate the effects of different implant/abutment connections on marginal tissues.

Prof. Per-Ingvar Branemark from Sweden is credited with making the observation that titanium metal can integrate in bone tissue. He extended that to using implants to support missing body parts, including teeth. He termed integration in bone as "osseointegration" and defined it as bone-to-implant contact at the light microscopic level. 12 years later, Prof. Andre Schroeder from Switzerland called this integration "functional ankylosis". Both of these terms describe a phenomenon where dynamic bone tissue remodels and grows against the side of the implant.

A more modern definition of osseointegration is a phenomenon whereby any biocompatible material is incorporated into bone tissue. Osseointegration is achieved and maintained by turnover of bone adjacent to the material. This definition has evolved over the years as it has become clear that other biocompatible materials such as titanium alloys and zirconium oxide can also integrate into the bone tissue.

Dental implants must also pass through the soft tissues of the oral cavity and like teeth, the peri-implant soft tissues comprise epithelium and connective tissue. These soft tissues form a biologic seal that separates the inside of the body from the outside of the body and helps prevent microbial invasion.

Definitions, Key Learning Points: Hard tissue integration is commonly known as osseointegration. Osseointegration is a histological term describing bone-to-implant contact at the light microscopic level. Other definitions describe osseointegration as a process whereby any biocompatible material is incorporated into bone tissue. Soft tissue integration consists of an epithelial attachment supported by a connective tissue contact that together are called the Biologic Width. The soft tissues form a biologic seal that helps prevent microbial penetration from the outside environment.

In order to achieve hard tissue integration of dental implants, the preparation technique must adhere to some specific principles. Overall, the technique must minimize bone damage during preparation of the recipient site. Excess heat from the drilling process must be avoided and this can be accomplished by using low speed, high torque sharp-cutting burs. In addition, the preparation is generally prepared to depth with a small diameter bur and then the drilling hole is gradually widened with increasing diameter burs. Finally, the preparation is performed using chilled sterilized saline irrigation.

To initiate implant placement, the recipient site is prepared in the bone with a series of drills. This results in a site where the native bone tissue has been cut in such a way that the implant rests directly against the cut native bone tissue. This contact is immediate osseointegration according to the definition of bone-to-implant contact at the light microscopic level. It is termed primary bone contact. Bone is dynamic, which means that it is constantly turning over and being remodeled. The transitional phase starts as new bone begins to grow onto the implant surface. Due to many factors including the amount, location and type of native bone and how osteoconductive the implant surface is, the implant may become slightly mobile during this transition. Eventually all the primary bone is replaced by remodeled and new bone on the surface and this interface is called secondary bone contact. It must be remembered that bone constantly turns over so the interface continually undergoes remodeling.

The two histological slides from an animal model show bone formation between two implant threads taken after four and eight weeks of healing. The bone on the left is the cut native bone that is remodeling and forming new bone contact.

This short video shows how the implant is placed into the recipient site and the primary bone contact that occurs with the cut native bone represented by the light pink color. Notice how with time, this primary contact begins to be lost due to remodeling and new bone formation takes place, which is represented by the red color. These events occur simultaneously and if mobility of the implant is carefully followed, the stability originally provided by the primary contact decreases during the transition (as shown by the dips) and then increases again as new bone formation and remodeling occur to generate the secondary bone contact. A more prominent dip in stability occurs in lower quality bone and with less osteoconductive implant surfaces, as the remodeling and new bone formation are not as rapid.

The characteristics of the implant surface can affect how well the dental implant integrates into the bone tissue. This is because the implant surface acts as a support or scaffolding for new bone tissue formation and this is called osteoconductivity. Smoother implant surfaces like machined titanium, which are also called turned surfaces, are not very osteoconductive while rougher implant surfaces, which are also called microtextured surfaces, are more osteoconductive, which means the surface encourages more bone formation. The implant surface can also be made chemically active by maintaining the native titanium oxide bonding on the surface or by adding a texture or coating to the surface.

Most dental implants used in the 1980s and 1990s were created by machining a titanium rod which generated slight grooves in the surface of the implant. This became known as a machined or turned implant surface. In the original treatment protocol, these turned implants were mainly placed in the anterior mandible to engage the bone cortex superiorly and inferiorly, a technique referred to as bicortical stabilization. However, research soon demonstrated that rougher or microtextured surfaces were more osteoconductive. Thus rougher surfaces were created. Rough surfaces can be made by either adding material to the titanium rod or subtracting material from the titanium rod's surface. Early on, additive surfaces were more popular and included adding on melted titanium resulting in a titanium plasma sprayed or TPS surface, and hydroxyapatite or HA coatings. Another way to roughen the surface was to let the surface become highly oxidized, which creates a thick layer of oxidation. Many of these surfaces were not only rough but they created porosity where bacteria could colonize easily. So today, subtractive surfaces such as sandblasted and acid etched surfaces, are more prevalent where there is roughness without porosity.

Osseointegration is influenced by surface treatment and involves new bone formation and bone remodeling. Both of these processes depend on bone cell activity including that of osteoblasts which produce osteoid on the implant surface, which then becomes mineralized. The better the osteoblast can adhere to the surface and spread out, the more bone is produced. Surfaces that encourage such bone cell activity are called osteoconductive. Different surfaces have different degrees of osteoconductivity but the final result is bone-to-implant contact. The bone formation on the implant surface creates a bond between the implant and bone, and a measure of the strength of this bond is determined by how much force is required to break this bond. That force is the removal torque value for that implant surface and bone.

Hard Tissue Integration, Key Learning Points: Osseointegration requires a careful technique that minimizes damage to the bone. Primary bone contact and stability occur immediately with cut native bone. Over time bone remodels and the transition may result in some implant mobility. Implant surfaces can be altered to increase osteoconductivity. Rougher surfaces are more osteoconductive than smoother surfaces.

Because implants and their restorations reach from their anchorage inside the body to the outside of the body, they pierce the integument, or the outer protective layer. Thus a biologic seal coronal to the bone is required. This seal is formed by the soft tissues around the implant and consists of an epithelial and connective tissue component. This seal and its components vary depending on the type and location of the implant and abutment configuration. Similar to teeth however, the connective tissue dimension is relatively stable while the epithelial length of the seal is more variable in dimension.

The epithelial contact around an implant is similar to that found around teeth. The oral epithelium is keratinized with extensions that project into the underlying connective tissue called rete pegs. The keratinized epithelium is continuous with the non-keratinized epithelium adjacent to the peri-implant sulcus. The junctional epithelium extends from the base of the sulcus to the first connective tissue contact. The cells of the junctional epithelium attach to the titanium with hemi-desmosomes similar to teeth.

The connective tissue contact is made up of mostly collagen fibers of different sizes and extends from the apical extent of the junctional epithelium to the first bone-to-implant contact. Adjacent to the implant, there is a scar-like, circular zone of avascular connective tissue which is approximately 50-micrometer-thick followed by loose connective tissue that contains vascular elements. Around rough implant surfaces at very high magnifications, small connective tissue fibers can be observed running in multiple directions. In some rough surfaces at very high magnifications, small fibers can be observed running in more horizontal directions perpendicular to the implant surface.

Soft tissue integration, Key Learning Points: Soft tissue integration varies with different implant/abutment configurations. In a similar way as in teeth, connective tissue length is relatively stable while junctional epithelium length is more variable. Epithelial contact is mediated by hemidesmosomes, similar to teeth. Connective tissue contact is predominantly composed of circular fibers in a scar-like avascular zone surrounded by loose fibers with vascular elements.

To connect a prosthesis to an implant, an intermediary component known as an abutment is inserted into the implant. A small gap - referred to as the microgap - is present at the interface between the abutment and the implant. The microgap varies in size depending upon the design of the implant. The size of the microgap may change during functional loading, with some implant systems showing greater stability than others. The microgap also varies in location in relation to the bone crest depending upon the design features of the implant.

These factors relating to the implant/abutment interface and resultant microgap influence the marginal hard and soft peri-implant tissues. Therefore, the type and location of the implant/abutment configuration should be considered during the treatment planning process. Bacteria infiltrate some of these interfaces and can cause inflammation and bone loss. Three general implant/abutment configurations are available. They include one-piece implants, two-piece implants with matching abutment diameters which create a butt-joint type connection and two-piece implants with non-matching abutment diameters commonly known as platform-switched implants.

One-piece implants have a transmucosal section which extends into the oral cavity. These implants are also referred to as tissue level implants. The transmucosal section is manufactured as part of the implant and has an inbuilt fixed prosthodontic platform. Typically, the top of the implant is 2 to 3 millimeters above the bone. The margin of the prosthesis contacts the top of the implant. No interface exists at the bone crest. This is important since no bacteria will infiltrate close to the bone tissue and cause any inflammation. This lack of inflammation at the bone crest allows for stable bone levels, as was shown by Buser and coworkers.

Two-piece implants, also referred to as bone level implants, are placed with their shoulders at the level of the bone. These two-piece implants can have matching or non-matching abutment diameters. The matching diameters have an interface or micro-gap at the crestal bone. This creates a butt-joint connection directly adjacent to the bone where bacteria colonize and release products that create an inflammatory response by the host. This inflammation creates marginal bone loss of 1.5 to 2 millimeters or more if the interface is placed below the alveolar crest. The bone loss with this implant/abutment configuration is so predictable that Albrektsson and coworkers in 1986 considered this bone loss a criterion of success for these implants. The junctional epithelium is located apical to this interface on the implant surface with the connective tissue and bone tissue below the epithelium.

Two piece implants with non-matching abutment diameters also have an interface at the bone crest, but this interface has a horizontal offset with smaller diameter abutments. These abutments generally have internal tapered connections that also provide enhanced stability of the implant/abutment junction. The length of the horizontal offset varies between implant systems but there does not appear to be any significant advantage to shorter or longer horizontal offsets. All of these are commonly referred to as platform-switched or shifted implants. They are generally associated with around 0.5 millimeters of marginal bone loss.

Implant/abutment configurations, Key Learning Points: Three implant/abutment configurations are available. They are one piece with the top of implant 2-3 mm above bone, two-piece with matching abutment diameters and two-piece with non-matching abutment diameters. The choice of the implant/abutment configuration and its location can alter marginal tissue levels and the health of the peri-implant tissues.

Clinically, one-piece tissue level implants and two-piece bone level implants with smaller diameter abutments (platform-switched design) generally demonstrate stable bone levels. Histologically, the two-piece platform-switched implant designs have 0.5 millimeter bone loss, and junctional epithelium can attach to the abutment. Two-piece implants with matching abutment diameters (butt-joint design) result in approximately 1.5 to 2 mm of crestal bone loss when an abutment is placed, and then this bone level can become stable.

Module Tissue Integration of Dental Implants, Summary: Osseointegration is a phenomenon whereby any biocompatible material is incorporated into bone tissue. Osseointegration can be a highly predictable outcome if the protocols are carried out with care. Rougher implant surfaces are more osteoconductive than smoother surfaces. A biologic width is found around implants consisting of a tooth-like junctional epithelium and connective tissue contact which is different from teeth. The choice and location of the implant/abutment configuration is significant in regard to crestal bone levels and tissue health.