Dental Implants

Key Terms

Alloplastic Related to implantation of an inert foreign body.

Ankylosis A condition of joint or tooth immobility resulting from oral pathology, surgery, or direct contact with bone.

Bioacceptance The ability to be tolerated in a biological environment in spite of adverse effects.

Bioactive Capable of promoting the formation of hydroxyapatite and bonding to bone.

Endosteal implant A device placed into the alveolar and/or basal bone of the mandible or maxilla that transects only one cortical plate.

Epithelial implant A device placed within the oral mucosa.

Implantation The process of grafting or inserting a material such as an inert foreign body (alloplast) or tissue within the body.

Micromotion The displacement of the implant root relative to the bone when the implant is loaded as a result of lack of friction or bone integration.

Osseointegration The process by which living bony tissue forms to within 100 Å of the implant surface without any intervening fibrous connective tissue.

Osteoinductive Ability to promote bone formation through a mechanism that induces the differentiation of osteoblasts.

Replantation Reinsertion of a tooth back into its jaw socket soon after intentional extraction or accidental removal.

Subperiosteal implant A dental device that is placed beneath the periosteum and overlies cortical bone.

Transosteal implant A device that penetrates both cortical plates and the thickness of alveolar bone.

The restoration and replacement of missing teeth are important aspects of modern dentistry. As teeth are lost to decay, disease, and trauma, there is a demand for improvement of esthetics and restoration of function.

Conventional methods of restoration include a removable complete denture, a removable partial denture, a fixed prosthesis, or combinations of these approaches. There are different indications for each method, with associated advantages and disadvantages. Removable dentures have long been considered problematic because of their varying mobility/stability over time and the inconvenience of removing them each day. Also, many removable dentures were bulky, others complicated chewing, and some were not adequately esthetic. Fixed prostheses were considered more esthetic and comfortable for the patient, but they involved the preparation of adjacent teeth and were associated with problems such as secondary decay or irreversible pulpitis. If the adjacent teeth did not have restorations, the decision to prepare them for a fixed prosthesis was difficult because two or more natural teeth would have to be surgically altered to provide retention for one or more artificial teeth (a bridge).

For centuries people have attempted to replace missing teeth by implanting synthetic and natural substances. Implantation has been defined as the insertion of any object or material, such as an alloplastic substance or other tissue, either partially or completely, into the body for therapeutic, diagnostic, prosthetic, or experimental purposes. Implantation is usually differentiated from other, similar procedures, such as replantation and transplantation . Replantation refers to the reinsertion of a tooth back into its jaw socket after its accidental or intentional removal, whereas transplantation is the transfer of a body part (homogeneous or heterogeneous) from one host to another.

CRITICAL QUESTION

How did the concept of implantation evolve into a widely used restorative technique in dentistry?

History of Dental Implants

Dental implants began as far back as the early Greek, Etruscan, and Egyptian civilizations, employing various designs and materials ranging from jade and other stones and metals to bone and ivory. Some of the design concepts used then have evolved into the implants of today.

Seashells were used in place of teeth in 600 CE , evidence of which was found in Honduras, and tooth restorations made of jade and turquoise were found in Mayan skulls. Albucasis de Condue (936–1013) of France used ox bone to replace missing teeth; this was one of the early documented placements of implants. Through the centuries, various tooth transplants made of either human or animal teeth were tested. These transplants became status symbols, and they quickly replaced other artificial alternatives for restoring missing teeth. Toward the 18th century, Pierre Fauchard and John Hunter further documented tooth transplantation and specified conditions for the success of this procedure. They claimed that success was greater with anterior teeth, premolar teeth, and transplants placed in young people with healthy tooth sockets. Failure was believed to be the result of the incompatibility of the type of tooth used or the lack of conformity of the tooth to the socket.

The recognition of failure rates of transplants increased interest in the implantation of artificial tooth roots. In 1809 Maggiolo fabricated gold roots that were fixed to teeth by means of a spring. These gold implants were placed into fresh extraction sites, although they were not truly submerged into bone. The crowns were placed after healing had occurred around the implant. Many attempts followed. Harris (1887) proposed the use of a platinum post coated with lead. The post was shaped like a tooth root, and the lead was roughened for retention in the socket. Bonwell (1895) implanted gold or iridium tubes to restore a single tooth or support complete dentures. Payne (1898) implanted a silver capsule as a foundation for a porcelain crown that was cemented weeks after. Scholl (1905) demonstrated a porcelain corrugated root-shaped implant that lasted for 2 years and was anchored to adjacent teeth by pins. introduced and patented a hollow “basket” implant made of a meshwork of 24-gauge iridium-platinum wires soldered with 24-karat gold. This device was used to support single implants and fixed dental prostheses (FDPs) comprising as many as eight implants.

Consistent problems with these artificial implant designs and materials supported the need for a scientific approach to implant selection and placement. Some have proposed that the “modern era” started in 1925 with the invention of the tubular extension implant by H. Leger Dorez. This implant allowed for an expansion of the screw in bone, leading to actual bone anchorage and initial stability. In 1937 Venable et al. investigated the interactions of cobalt alloy and other available metals and alloys with bone for use in dentistry. They concluded that certain metals produced a galvanic reaction, which led to corrosion when these metals contacted tissue fluids. They proposed the use of Vitallium, a cast alloy, which was composed of cobalt, chromium, and molybdenum. This alloy was considered to be relatively inert, compatible with living tissue, and resistant to the adverse reactions with body fluids. Vitallium has also been used in different forms of surgical devices, such as dental subperiosteal implant and orthopedic plates, screws, nails, and joints. Early evaluations documented Vitallium implants with survival times of 10 or more years.

The background of research, development, and applications of biomaterials for dental implants began with the application of available substances, often of metallic origin. Some separate the period before 1950 as an era of applications driven by need and availability. Examples include the early use of wires, pins, and rods fabricated from gold and other noble metals and alloys. In the 1950s, the cobalt-based alloys were cast and finished for subperiosteal and some root-form designs. The austenitic iron-based stainless steels constituted from iron-chromium and nickel (sometimes with molybdenum as well) were also used in wrought and machined conditions. Within a decade, titanium and other reactive-group metals—plus carbons, ceramics, and eventually polymers—were considered.

The more “modern” era of biomaterials arose by the 1970s, when materials known and tested in many disciplines were constituted, fabricated, and finished as biomaterials. Consensus standards rapidly evolved for these biomaterials, which resulted in more consistent control of their properties. A very important aspect of this evolution—which included biomaterials, designs, and clinical application—was the clinical research led by P. I. Brånemark in Sweden. His studies used unalloyed titanium, now known as commercially pure titanium (CPTi); a root-form design; and very controlled conditions for surgery, restoration, and maintenance. Data presented in literature reviews and consensus conferences over the period 1972 to 2002 have demonstrated significant improvements in dental implant survival statistics. When outcomes were evaluated by objective criteria, the average survival at 5 years was about 50% in the 1970s. Since the 2000s, average survival at 10 years has been above 90% for these types of implants.

Classification of Implants

Implants can be classified according to anatomical location and device design, implant properties, or implant attachment mechanism.

Anatomical Location and Implant Device Design

The first and most commonly used type of design is the endosteal (also called endosseous ) implant, a device placed into an alveolar and/or basal bone of the mandible or maxilla that usually transects only one cortical plate. These implants were formed in many different shapes, such as root-form cylindrical cones or screws. They also came as thin plates called plate or blade forms and were used in all areas of the mouth. One example of an endosteal implant was called the blade implant ( Figure 12-1 ), which was developed independently in 1967 by two groups led initially by Roberts (1970) and . Endosteal blade implants consisted of thin plates placed into bone; they were most commonly used for narrow anatomical structures, such as posterior edentulous areas, after significant resorption of bone. Because of various issues with blade implants, their application in more recent implantology has decreased. Another example of an endosteal implant was the ramus frame implant ( Figure 12-2 ), a horseshoe-shaped stainless-steel device inserted into the ascending rami (bilateral) and the anterior symphysis area of the mandible. As with the blade implants, the number of applications was limited. The most popular endosteal implant has been the root-form ( Figure 12-3 ), which was designed initially to mimic the shape of tooth roots for directional load distribution and for positioning in bone. In longitudinal studies, the root-form implant has the most documentation of the endosteal implants, although several surgical stages may be needed for completion.

• Figure 12-1
A, Blade implants embedded in bone, with some bone loss. B, Failed blade implant prosthesis that was also attached to natural teeth. (Courtesy Dr. Mickey Calverley.)

• Figure 12-2
Ramus frame blade implant that traverses entire mandible and attaches to ramus.

• Figure 12-3
A, Endosteal implants are placed directly into bone; they mimic root forms for proper placement and location in bone. B and C, Restored anterior implant blending well with adjacent teeth. ( B and C, Courtesy Drs. Will Martin and Luiz Gonzaga.)

CRITICAL QUESTION

What are the indications for the use of mini-implants in dental practice?

A mini-implant is a type of endosteal implant with some indications in dentistry. More notably, their use in orthodontics for appliance anchorage has increased. Other uses include overdenture retention in areas where there is minimal bone available or in areas that are narrow. They can also be used to anchor temporary prostheses in the case of overdentures on an arch with newly extracted teeth. These implants were initially used as temporary implants, which helped anchor the prosthesis while the larger-diameter implants were left to osseointegrate. They could then be removed once the final prosthesis was fabricated. Over time, the success of these mini-implants has led to their evolution as final restorations for compromised cases. The diameter of mini-implants ranges from 1.8 to 2.9 mm ( Figure 12-4 ). Most mini-implants can be placed without having to reflect a flap in the gingival tissue because of their small diameter. Mini-implants are also designed for immediate loading, offering the convenience of few visits and allowing the patients to have their prosthesis immediately. Although these implants are essentially made from the same material as the larger standard implants and are constructed as root-form implants, the main difference lies in their smaller diameter. This smaller size allows placement of these implants in areas where standard implants would normally require a bone-grafting procedure, which results in additional trauma and expense to patients. As the applications of mini-implants continue to grow, they are slowly being incorporated for use with FDPs, adding additional support in the pontic areas.

• Figure 12-4
Mini-implants used for orthodontic anchorage, showing 2-mm diameter of the mini-implants compared with the 4-mm diameter of standard implants.

The second implant design is the subperiosteal implant ( Figure 12-5 ), which employs an implant substructure and superstructure. The custom-cast frame was placed directly beneath the periosteum overlying and fitting along the bony cortex. This implant was first developed by Dahl (1943) and refined by , who used a direct-bone-impression technique. These devices were used to restore partially dentate or completely edentulous jaws when there was inadequate bone for endosseous implants. Use of the subperiosteal implant has been limited because of numerous considerations, including the difficulty of retrieval.

• Figure 12-5
A, Subperiosteal implant positioned beneath the periosteum. Impression making often requires a difficult surgical technique. B, Superstructure for subperiosteal implant allowing for attachment of prosthesis. C, Denture prosthesis for subperiosteal implant. (Courtesy Dr. Joseph Cain and Dr. Richard Seals.)

The third design is the transosteal implant ( Figure 12-6 ), which combines subperiosteal and endosteal components. This type of implant penetrates both cortical plates and passes through the full thickness of the alveolar bone. Use of the transosteal implant has been restricted to the anterior area of the mandible and provides support for tissue-borne overdentures. The concept of transosseous implants was first conceived in Germany in the early 1930s; early examples were made of a cobalt alloy. Small (1968) developed the mandibular staple implant made of a titanium alloy, which was modified by , who produced the transmandibular implant (TMI) made of a gold alloy. Other names for transosteal implants have included staple bone implant, mandibular staple implant, and transmandibular implant.

• Figure 12-6
A, Panoramic radiograph of a transosteal implant showing perforation of both cortical plates, hence the name staple implant. B, Transmucosal abutment for transosteal implant allowing for placement of denture restoration. (Courtesy Dr. Joseph Cain and Dr. Richard Seals.)

The fourth implant design is the epithelial implant, which is inserted into the oral mucosa. This type was associated with a simple surgical technique where the mucosa was used as an attachment site for metal inserts placed into an acrylic denture. Several disadvantages are associated with the epithelial implant, most notably a painful healing process and a requirement for continuous wear. The current use of “mucosal inserts” or epithelial implants is very limited.

Some oral implantologists also include the endodontic stabilizer as an anatomy-specific application system. These smooth or threaded pins (or screws) were placed through endodontically treated teeth with the implant extending into the bone. They were constructed from alloys or ceramics made of alumina or zirconia. Reported difficulties included sealing the transition zone from tooth to bone and the limited strength of small-diameter pins or screws.

From a historical and applications perspective, these systems were reviewed by and subsequently in each decade by researchers who participated in professional society–based consensus conferences. To summarize the various biomaterials and designs tested for dental implants, examples of devices received for examination prior to 1990 are shown in Figures 12-7 through 12-9 .

• Figure 12-7
Collection of endosteal blade implant designs for restoring single or adjacent teeth.

• Figure 12-8
Collection of endosteal implants for partial arch restorations.

• Figure 12-9
Collection of different designs and materials used for endosseous implants.

Implant Properties

Implant biomaterials can also be classified according to their composition and their physical, mechanical, chemical, electrical, and biological properties. These classifications often include ranked comparisons of properties, such as elastic moduli, tensile strength, yield strength, and ductility, to determine optimal clinical applications ( Table 12-1 ). These properties are used to aid in the design and fabrication of the prosthesis. For example, the elastic modulus of the implant is inversely related to the strain present at the implant–tissue interface during loading of the implant; that is, the greater the elastic modulus of an implant, the greater the stress in the implant and the lower the stress distributed to bone.

Table 12-1
Mechanical Properties and Density of Metallic and Ceramic Implant Materials
Material Grade or Condition Yield Strength (MPa) Elongation (%) Modulus of Elasticity (GPa) Tensile Strength (MPa) Density (g/cm 3 )
CP titanium 1 170 24 102 240 4.5
2 275 20 102 345 4.5
3 380 18 102 450 4.5
4 483 15 104 550 4.5
Ti-6AI-4 V 860 10 113 930 4.4
Ti-6AI-4 V ELI 795 10 113 860 4.4
Ti-15Zr 953 103.7
Co-Cr-Mo Cast 450 8 240 700 8.0
Stainless steel Annealed 190 40 200 490 8.0
Cold-worked 690 12 200 860 8.0
Aluminum oxide Polycrystalline 400 * (500/flexure) 0.1 380 220 3.96
Zirconium oxide Y 2 O 3 (stabilized) 1200 (flexure) 0.1 200 350 6.0
Cortical bone N/A 1 18 140 0.7
Dentin N/A 0 18.3 52 2.2
Enamel N/A 0 84 10 3.0

* ASTM Standard: minimum values.

An implant material with an elastic modulus comparable to that of bone should be selected to produce a more uniform stress distribution across the interface. Ductility is another important property because this relates to the potential for some plastic deformation of abutments or fixtures without fracture in areas where there is high tensile stress. Metals possess both high strength and ductility, whereas the ceramics and carbons are brittle materials.

CRITICAL QUESTIONS

What is the preferred implant-to-tissue interaction? How has this influenced the popularity of implant applications?

Implant Attachment Mechanism

Periodontal structures, which attach teeth to bone, consist of highly differentiated fibrous tissue. These fibers are replete with numerous cells and nerve endings that allow for functional force transfer, sensory function, bone formation, and tooth movements. Although this is the ideal form of integration, there are no known implant biomaterials or designs at present that can stimulate the growth of these fibers and can fully mimic the function of the periodontal ligament and a natural tooth.

In the past, implant attachment to fibrous connective tissue was accepted as a measure of successful implant function (the pseudoligament concept). This type of interaction was reported to be very susceptible to acute or chronic inflammatory responses, which were accompanied by pain and eventual loss of the implant. This type of attachment has also been called a pseudoperiodontium. Despite numerous reports of implant success, clinical studies indicate that this type of interaction is susceptible to progressive loosening and infection, with subsequent loss of the implant construct.

In contrast to the pseudoperiodontium, the implant-to-bone interaction (called osseointegration) is characterized by direct contact between bone and the surface of a functional implant after 1 year. This central theme of the Brånemark group was called “direct anchorage to bone” and has become a major attribute of dental implants. This mode is described as the direct adaptation of bone to implants without any other intermediate nonbony tissue and has been described by some as similar to tooth ankylosis, where no periodontal ligament or fibrous tissue exists. The strength of this contact has been shown to be stable, which is advantageous compared with the soft tissue interface described previously.

This type of osseous interface has been described extensively and includes a process wherein bone-producing cells migrate along the implant surface through the connective tissue scaffolding that forms adjacent to the implant surface. Integration at the implant interface is highly dependent on the implant surface’s chemistry and design. Bone apposition has been reported at higher rates when microscopic surface ridges are present. Osseous integration has also been achieved through the use of bioactive materials that stimulate the formation of bone along the surface of the implant. Another way of achieving osseous integration involves de novo bone formation, wherein a mineralized interfacial matrix and/or active growth factors are deposited along the implant’s surface. The implant’s surface topography and chemistry influence the bone development around the implant.

CRITICAL QUESTION

What is the implication of having direct bone contact and the absence of a periodontal ligament on the prosthetic superstructure?

Implant-supported restorations differ from tooth-supported restorations in that the former lack a periodontal ligament, which reduces shear stress and strain, provides shock absorption, and reduces the development of dangerously high occlusal forces. The benefits of the periodontal ligament reduce the potential for inadvertent damage to the tooth or restoration. Ceramic materials are brittle by nature, and because of inherent processing flaws, they cannot withstand any stress above their yield point, which is equivalent to their tensile strength. These stresses can be generated during occlusal loading. Implants, on the other hand, are directly anchored to the bone and cannot readily flex laterally under extremely high occlusal loads to relieve some of the excessive loading. analyzed the fracture strength of all-ceramic prostheses on abutment teeth and implants in vitro . They concluded that implant-supported ceramic prostheses fractured at higher loads than those supported by natural teeth because of the lack of a periodontal ligament. This indicates that the supporting structures, in this case the implants, are subjected to higher stress levels. However, clinical studies have shown that the complication rate for implant-supported FDPs is 30% over 5 years, whereas that for tooth-supported FDPs is 27.6% over the same time period, indicating no real difference in survival of the restoration between the two types of support.

Conversely, studies have analyzed whether the type of prosthetic material can affect the longevity of an implant. Brånemark and Skalak advocated the use of acrylic for the prosthetic superstructure to act as a shock absorber and essentially dissipate the load on the implants and the bone surrounding them. Acrylic, noble metal alloys, and ceramic are being used in implant superstructures. In the absence of a periodontal ligament, minimizing the load along the bone–implant interface is logical. However, there is no evidence to associate the use of any prosthetic material with the longevity and survival of the implants.

CRITICAL QUESTION

What is the purpose of pretreating implant surfaces?

Implant Components

To understand the material characteristics and function of an implant, one must first be knowledgeable about the numerous component parts. Although each implant system varies, the parts are basically consistent. The body of the implant (called a fixture for the Brånemark system) ( Figure 12-10, A ) is the implant component that engages with bone. Depending on the implant system, the body section can have different surfaces—threaded, grooved, perforated, plasma-sprayed, sand-blasted, or coated. These characteristics are often classified as subtraction (threading, grooves, perforations, sand-blasting, or acid-etching) or addition (plasma spray and coating) types. Each surface type is meant to enhance bone integration and primary stability; that is, increased surface area enhances bone integration, and better cortex engagement plays an important role in immediate and long-term bone anchorage. The coated or plasma-sprayed biomaterials, discussed later in this chapter, are used to enhance attachment to bone. The second component ( Figure 12-10, B ) is the transmucosal abutment, which provides the connection between the implant body and the intraoral prosthesis to be fabricated ( Figure 12-10, C ), which will provide intraoral function. The abutment is usually connected to the implant body by means of a screw; however, the abutment can also be cemented or connected by a Morse taper-type design. Abutments can become engaged to the implant body either by an internal or external geometry (initially a hexagon) within the implant body, which also serves as an antirotation device and is particularly important for single-unit restorations. The last part of an implant is the prosthesis. The prosthesis can be attached to the abutments through the use of screws, cement, precision attachments, magnets, or other designs, such as those used for removable implant overdentures.

• Figure 12-10
Diagram of implant components. A, The implant fixture (endosteal root-form). B, Transmucosal abutment serving as the attachment between fixture and the actual prosthesis. C, The actual prosthesis, which can either be cemented, screwed, or swaged.

Implant Surgical Placement

Placement and restoration of implants intended to integrate with bone are usually performed in stages. The first stage, or stage 1, occurs when the implant is surgically placed into the bone ( Figure 12-11 ). The implant is left within the bone (passive) for a period of months, depending on the bone quality, and allowed to heal and become integrated. A secondary surgery, or stage 2, is sometimes required whereby the implant is uncovered and exposed through the oral environment ( Figure 12-12 ). This exposure is performed 4 to 6 months after implant placement and after proper bone healing has occurred.

• Figure 12-11
Implant placement, Stage 1 surgery. A, Sequential twist drills used for osteotomy. B, Depth pins used to confirm osteotomy and implant parallelism. C, Implant placement following the correct angulation. (Courtesy Dr. Luiz Gonzaga.)

• Figure 12-12
Implant placement, Stage 2 surgery. A, Tissue evaluation pre uncovering. B, Healing abutment on implant after uncovering. Minimal crestal incision, tissue punch or laser can be used as long as keratinized mucosa is present. C, Impression coping on implant to facilitate fabrication of provisional restoration at the same appointment. (Courtesy Dr. Luiz Gonzaga.)

The restorative phase then follows the placement of abutments and a crown, a partial denture, or a removable denture, with or without a bar construct. Some implant systems use only one surgical intervention, where the implant is immediately placed in contact with the oral environment and is sometimes able to function in a limited way within days. Some of these systems have even been advocated for immediate functional loading, with reports of relative success.

Aug 11, 2021 | Posted by in Dental Materials | Comments Off on Dental Implants
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