Indications for Dental Implant Use
The general requirement for dental implants is adequate bone to support the implant with the physiologic parameters of width, height, length, contour, and density. Note that the importance of these parameters varies, depending on the specific implant type (Table 23-1). Despite the “glamour” of implant dentistry, a conservative treatment protocol must be stressed. Dental implants should not be the first treatment option considered. Unsatisfactory treatment with removable dentures or fixed partial dentures is often an important indication for implant use. A number of contraindications for implant use also exist (Box 23-1).
Unattainable prosthodontic reconstruction
Patient sensitivity to implant component(s)
Debilitating or uncontrolled disease
Inadequate practitioner training
Conditions, diseases, or treatments that may compromise healing (ie, radiation therapy)
Poor patient motivation/hygiene
Perceived poor patient compliance
Unrealistic patient expectations
|Cylindric||Adequate bone to support implant—width and height of primary concern
Maxillary and mandibular arch locations
Completely or partially edentulous patients
|Blade||Adequate bone to support implant—width and length of primary concern
Maxillary and mandibular arch locations
Completely or partially edentulous patients
|Ramus frame||Adequate anterior bone to support implant—width and height of primary concern
Mandibular arch location
Completely edentulous patients
|Complete||Atrophy of bone, but adequate and stable bone to support implant|
|Unilateral||Maxillary and mandibular arch locations|
|Circumferential||Completely and partially edentulous patients|
|Staple||Adequate anterior bone to support implant—width and height of primary concern|
|Single pin||Anterior mandibular arch location|
|Multiple pin||Completely and partially edentulous patients|
Types of Implants
1. Endosseous implants are embedded in mandibular or maxillary bone and project through the oral mucosa covering the edentulous ridge.
2. Subperiosteal implants rest on the surface of the bone beneath the periosteum.
3. Transosseous implants penetrate the inferior mandibular border and also project through the oral mucosa covering the edentulous ridge.
Root-form endosseous screw-threaded implants are the most common implants in clinical practice. This subclass of implants is the only one for which good long-term (eg, 10- to 15-year) clinical tracking of large patient populations is available. Success rates for implants placed in the mandible are approximately 95% at 5 years and greater than 85% at 15 years. For maxillary implants, success rates are approximately 85% to 90% at 5 years and 80% at 15 years. The clinician’s expertise and surgical technique are more important than the specific implant and are the primary factors dictating clinical outcome.
Unlike many biomaterials, which serve to replace as much of a tissue’s natural structure and function as possible, dental implants do not restore function by mimicking the natural function of the periodontal ligament (Fig 23-2). Instead, osseointegration, or the direct structural and functional connection between ordered, living bone and the surface of a load-carrying implant, is what should occur with a well-functioning implant. This definition was originally based on retrospective radiographic and light microscopic observations and has since been modified based on scanning and transmission electron microscopic observations. However, the general working definition of osseointegration is fundamentally the same—the host bone responds in a safe, predictable, and versatile manner, to surgical placement of an implant in a sterile wound, with a healing cascade leading to interfacial osteogenesis and mechanical stability of the implant (Fig 23-3). In a well-functioning implant, interfacial osteogenesis and clinical stability are achieved (Fig 23-4a), and a stable marginal bone level is maintained. In comparison, poorly differentiated connective tissue adjacent to an implant can lead to clinical mobility and implant failure Fig 23-4b).
There are a multitude of interrelated clinical, biologic, and engineering factors that control the oral cavity’s response and dictate the success of osseointegration.
Achieving and enhancing implant-tissue attachment
An implant must be capable of carrying occlusal stresses. In addition, stresses must be transferred to the adjacent bone. Not only must stresses be transferred across the implant-tissue interface, but they must be of a “correct” orientation and magnitude so that they mimic the normal physiologic stresses and allow tissue viability to be maintained. The ability to transmit stress from the implant to the adjacent bone is largely dependent on attaining interfacial fixation. Thus, the interface must stabilize in as short a time postoperatively as possible and remain stable for as long a time as possible.
Developing an “optimal” implant that meets all of these objectives requires the integration of material, physical, chemical, mechanical, biologic, and economic factors. While all of these properties are important, they cannot all be optimized in a given design. Optimization of one property often detracts from another. Thus, in designing a dental implant and in choosing an implant for a specific clinical scenario, a ranking of requirements and objectives is necessary.
In an ideal situation, such as that achieved with commercially pure titanium (CPTi), calcified tissue can be observed within several hundred Angstroms of the implant surface. In Fig 23-5, a layer of proteoglycans 200 to 400 Å thick lies adjacent to the metal oxide, and collagen filaments can be observed about 200 Å from the surface. Less-than-optimal surgical techniques or implant surface chemistry and relative motion between the implant and tissue can lead to a thicker zone of proteoglycans, soft connective tissue, and disordered bone.
Because a stable interface must be developed before loading, it is desirable to accelerate tissue apposition to dental implant surfaces. Material developments that have been implemented in clinical practice include the use of surface-roughened implants and bioactive ceramic coatings. Other techniques include electric stimulation, bone grafting, and recombinant growth factors.
A variety of implant surface configurations can improve the cohesiveness of the implant-tissue interface, leading to increased transfer of occlusal loads to the adjacent tissue that minimizes relative motion between implant and tissue, fibrous integration, and ultimately loosening, thereby lengthening the service life of the implant. Metal implant surfaces may be smooth, textured, screw threaded, plasma sprayed, or porous coated. By far, the most common surface configuration is the screw-threaded dental implant. Osseointegration around screw-threaded implants occurs through tissue ongrowth, or direct apposition between tissue and the implant surface. Alternative methods of implant-tissue attachment, based on tissue ingrowth into roughened or three-dimensional surface layers, yield higher bone-metal shear strength than other types of fixation. Increased interfacial shear strength results in a better stress transfer from the implant to the surrounding bone, a more uniform stress distribution between the implant and bone, and lower stresses in the implant. In principle, the result of a stronger interfacial bond is decreased implant loosening.
A progression of surfaces from the lowest implanttissue shear strength to the highest is as follows: smooth, textured, screw threaded, plasma sprayed, and porous. Two factors must be stressed, though. First, different surface structures necessitate different osseointegration times. Second, surface roughening, particularly of titanium-based materials, results in reduced fatigue strength. Thus, improvements in implant-tissue attachment strength are often countered by a loss of structural strength and must be met with design compromises to avoid material failure.
Criteria for Successful Implant Placement
Three aspects of an implant-tissue system are important in determining clinical success: (1) the implant material(s) and adjacent tissue(s), (2) the interfacial zone between the implant and tissue, and (3) the effect of the implant and its breakdown products on the local and systemic tissues. Although the interfacial zone is composed of a relatively thin (< 100 µm) layer consisting of heterogeneous metallic oxide, proteins, and connective tissue, it has an effect on the maintenance of interfacial integrity. The integrity of the implant-tissue interface is also dependent on material, mechanical, chemical, surface, biologic, and local environmental factors, all of which change as functions of time in vivo. In addition, implant “success” is dependent on the patient’s overall medical and dental status, the surgical techniques used, and the extent and time course of tissue healing. The focus of this section is on the biomaterial and biomechanical factors, summarized in Fig 23-6.