Biological aspects of oral implants
The macroscopic configuration of implants has varied widely; the most common types are:
1. Endosseous implants
A.
Bladelike
B.
Pins
C.
Cylindrical (hollow and full)
D.
Disklike
E.
Screw shaped
F.
Tapered and screw shaped
2. Subperiosteal framelike implants
3. Transmandibular implants
Currently, most endosseous implants have a cylindrical or tapered, screw-shaped, threaded design.
1. Endosseous Implants
A. Blade implants: Blade implants were inserted into the jawbone after mucoperiosteal flap elevation. They were tapped in place in a narrow trench made with a rotary bur. One or several posts pierced through the mucoperiosteum after suturing of the flaps. After a few weeks of healing, a fixed prosthesis was fabricated by a classic method and cemented on top of it.
Because the high-speed drilling leads to ample bone necrosis at the histologic level, fibrous scar tissue formation occurs. This allows downgrowth of the epithelium, which leads to marsupialization of the blade implants. If a bacterial infection occurs, it can lead to an intractable periimplantitis with ample bone loss. More importantly, removal of such implants after complications implies sacrificing surrounding jawbone. Because of its retentive geometry, the blade implant cannot simply be extracted or removed by a trephine, as with a cylindrical or screw-shaped implant.
B. Pins. Although seldom used at present, in the classic technique, three diverging pins were inserted either transgingivally or after reflection of mucoperiosteal flaps in holes drilled by spiral drills. At the point of convergence, the pins were interconnected with cement to ensure the proper stability because of their divergence. On top of this arrangement, a single tooth could be installed.
In edentulous jaws, several of these pin triads could be used to interconnect with a fixed prosthesis.
As with blade implants, the bone necrosis during drilling leads to fibrous encapsulation, marsupialization, and loss of the implants because of infections. A positive aspect, however, is that when such implants must be removed, removing the connection at the place of convergence is sufficient to allow easy extraction of each individual pin. Thus, bone loss from removal is minimal.

C. Cylindrical Implants. When discussing cylindrical implants, it is important to distinguish between hollow and full cylindrical implants. The idea was that implant stability would benefit from the large bone-to-implant surface provided by means of the hollow geometry. It was also thought that the holes (vents) would favor the ingrowth of bone to offer additional fixation. Although it was not clarified whether the cause was geometry or the associated surface characteristics (titanium plasma sprayed surface, titanium alloy), survival statistic were disappointing with the hollow cylinders.
Full cylindrical implants were used available under the name IMZ, referring to the "internal mobile shock absorber." Although early results. were encouraging, especially in the symphyseal area, the long-term survival rates became unacceptable, lweading to the limited use of this implant type currently.
Even when an intimate bone apposition is achieved, extraction forces on such cylindrical implants lead to strong shear forces at the bone-to-implants interface. Only the microscopic surface irregularities offers some mechanical retention by interdigitation of bone growing onto the implant surface. With a screw based geometry forces acting parallel to the long axis of the implant are dispersed in many direction forces acting parallel to the long axis of the dispersed in many directions.
D. Disk Implants. Disk implants are rarely used at present. Once introduced into the bone volume, therefore, the implant has strong retebtion against extraction forces. Implants have been used with one, two, and even three disks.
Unfortunately, the cutting of the bone by means of high-speed drills leads to a fibrous scat tissue surrounding the implant, as revealed frequently by periimplant radiolucencies.
E. Screw-Shaped (Tapered) Implants. Currently, the most common implant is the screw-shaped. Threaded implant. Even systems that started with cylnrical geometries (hollow or not) progressively adopted screw shaped implants, For example, a decrease in interthread distance at the coronal end of the implant has proposed to enhance the marginal bone level adaptation.
Tapered implant forms have been used primarly because they require less space in the apical region, a relevant issue in some partially edentulous patient (i.e.
between roots that approximate one another) and areas such as the anterior maxilla where apical concatives are common. No clinical data tend to suuort a superior success rate with tapered versus straight oral implants. Presumably, the problems with a tapered implant result from a poor fit in the osteotomy site at the time of implant placement (e.g., implant too loose or too tight as a rsylt of the vertical placement position being different than the vertical preparation of bone.)
Subperiosteal Implants
Subperiosteal implants are customized according to a plaster model derived from an impression of the exposed jawbone, prior to the surgery planned for

implant insertion. Several posts, typically four or more for an edentulous jaw, are passed through the gingival tissues. Subperiosteal implants are designed to retain an over denture, although fixed prostheses have also been cemented onto the posts. As a result of epithelial migration, the framework of subperiosteal implants usually beomes surrounded by fibrous connective tissue (scar), including the space between the implant and the bone surface. The marsupialization, often leads to infectious complications, which often necessitates removal of the implant. Furthermore, while being loaded by jaw function, jawbone resorption occurs rapidly, resulting in a lack of adaptation of the frame to the bone surface. As a result of this type of outcome, subperiosteal implants are now rarely used.
Transmandibular Implants
Transmandibular implants were developed to retain dentures in the edentulous lower jaw. The implant was applied through a submandibular skin incision and required general anesthesia. Two models were available. The 1 st , called the
"Staple-Bone" implant, developed by small. consisted of a splint adapted to the lower border of the mandible, to which it is fixed by stabilizing pins.
Two transmandibular screws were driven transgingivally into the mouth. The reported implant survival rate exceeded 90% after 15 years. The other model, introduced has two metal splints, one below the lower border of the mandible and one intraorally to connect the 4 posts piercing through the soft tissues. The transmandibular implants are rarely used at present.
IMPLANT SURFACE CHARACTERISTICS
A key element in the reaction of hard and soft tissues to an implant involves the implant's surface characteristics, that is, the chemical and physical properties.
Some materials are not suitable for implantation because they have toxic cellular side effects. Some materials are biocompatible because they do not provoke an immune reaction and are "passive" toward the tissue-healing process (I,e. they do not cause adverse reactions, and they do not cause reactions that promote healing).
The proper surgical steps to be taken to obtain an intimate bone-to-implant contact, known as osseointegration. The quest was for biocompatible if not bioactive surfaces, achieved through additive or subtractive processes.
Titanium, preferably commercially pure titanium, became the standard endosseous implants both in orthopedics and in perioodontology. Actually, titanium is a very reactive material that would not become integrated in tissues.
However, its instantaneous surface oxidation creates a passivation layer of titanium oxides, which have ceramic-like properties, making it very compatible with tissues.

Additive Processes
The chemical nature of the implant surface can be modified dramatically by coating its surface. Calcium phosphates, especially hydroxyapatite, have been a popular coating material because of its resemblance to bone tissue. The content of this hydroxyapatite seems to be that the cell response, exemplified by osteoblasts, varies according to the calcium-to-phosphate ratio and impurities.11
Other by increased or modified titanium oxide (TiO2) layers to enhance or accelerate the bone formation. This is achieved by anodizing or chemical processing. The oxide content of the TiO2 layer is essential for nucleation processes to form calcium phosphate precipitates, which lead to mineralized bone formation. Another involves integrating fluoride in the TiO2 layer. These ions can be displaced by oxygen originating from phosphates, thus achieving a covalent binding between bone and implant surface. Fluoride release is also known to inhibit the adhesion of proteoglycans and glycoproteins on the hydroxyapatite surface, two macromolecules known to inhibit mineralization.
Besides this chemical change, surface additives primarily modify the microstructure of the implant surface, which influences adhesion of molecules and cells. The latter implant characteristics, with "micro-roughness" as a predominant aspect, are often the result of subtractive processes during manufacturing.
HARD TISSUE INTERFACE
Stages of Bone Healing and Osseointegration: Cell Kinetics and Tissue
Remodeling
Any bone wounding leads to an inflammatory reaction with bone resorption and subsequently the activation of growth factors and attraction by chemotaxis of osteo-progenitor cells to the site of the lesion. The differentiation toward osteoblasts will lead to a reparative bone formation, which, in the case of a bone fracture, will lead to fusion of both ends. In the case of an implant insertion into a prepared hole, this will lead to bone apposition onto the implant surface if the latter is nontoxic. A "coupling" occurs between bone resorption and bone formation; the latter can occur only if an intercellular matrix is produced that favors apatite crystal integration in the collagen network. For example, osteocalcin, an extracellular matrix protein, modulates apatite crystal growth.
The osseointegration process observed after implant insertion can be compared to bone fracture healing. Logically, a certain immobility of the implant surface toward the bone should be maintained. A mild inflammatory response, as triggered by movements or appropriate electrical stimuli, may enhance the bonehealing response, but above a certain threshold, this is detrimental. It has been reported that when micromovements at the interface exceed 150u,m, differentiation to osteoblasts will not occur; rather, a fibrous scar tissue lead down between the bone and implant surface. Therefore, avoiding such forces as

occlusal load during the early healing period seems to be a safe approach. If the neighboring bone has been overheated or crushed during drilling, however, the necrotic area will prevent ingrowth of stem cells, and a scar formation or sequestered formation will result. The critical temperature for bone cells is as low as 47° C at an exposure time of 1 minute. This corresponds to the denaturing temperature of alkaline phosphatase, the main bone cell enzyme. Implant placement thus implies profuse cooling with intermittent moderate-speed drilling with sharp drills. Another complicating factor, well recognized from open wound fractures, is that microbial contamination jeopardizes the normal bone repair.
Thus, when oral implant are placed strict aseptic techniques should be maintsined.
The limited damage to bone tissue must be cleared up by osteoclasts before the bone healing. These multinuclear cells, originating from the blood can resorb bone at a pace of 50 to 100 um per day. There is a coupling between bone apposition and bone resorption. The preosteoblasts, derived from the invading primary mesenchymal cells, depend on a favorable oxidation-reduction (redox) potential of the environment. Thus a proper vascular supply and oxygen tension are needed. This explains why, in the case of compromised oxygen supply, such as after local radiotherapy, hyperbaric oxygen therapy can be a key issue. If the favorable conditions are not met because of one of the interfering factors previously cited, the primary stem cells may differentiate into fibroblasts, and the implant (or the bone fracture surface tissue) will be facing scar tissue. Although this may be acceptable for endosseous implants, such as femoral implants. Oral implants will eventually be connected to the outer environment with the potential for infection.
First, woven bone is quickly formed in the gap between the implant and the bone. Second, after several months this is progressively replaced by lamellar bone under the load stimulation. Third, a steady is reached after about l.5 years.

Ofeten for oral implant, occlusal load allowed as soon as 2 to 3 months, while mostly woven bone is present.
Woven bone grows fast, up to 100 um per day, and in all directions. It is characterized by a random orientation of its collagen fibrils, high cellularity, and limited degree of mineralization. Limited mineralization means that the bone's biomechanical capacity is poor, and thus occlusal load should be controlled.
Woven bone can grow by apposition, originating from the bone lesion or by conduction, using the implant surface as a scaffold. The implant surface characteristics, such as material properties, surface free energy, and roughness profile, are determining factors that influence bone apposition. Altered implant surface topographies, such as those created by acid etching, blasting, combinations of acid etching and blasting, or increasing the titanium oxide layer

appear to result in greater bone apposition to the implant surface as compared to a "turned" or machined surface.
After 1 to 2 months, under the effect of load, the woven bone surrounding the implant will slowly transform into lamellar bone. Parallel layers of collagen fibrils characterize the latter, each with their own orientation, which explains the typical polarized light aspect. Contrary to the fast growing woven bone, lamellar bone apposition occurs at a slow pace of a few microns per day.
An ongoing debate concerns the nature of the established bone-to-implant interface. At the light microscopic level, an intimate bone-to-implant contact has been reported extensively60. Depending on the bone type at the implantation site and on the type of implant surface, large variations exist in the percentage of the implant surface that actually contacts mineralized trabeculae. The remaining interface consists of vessels and bone marrow. Such analysis should also consider time. Thus, some implant surfaces may have an exponential bone apposition to stabilize after some time, whereas others may demonstrate a linear increase over months. Some eventually show a full coating of the implant surface by bone.