Design of Implants
The modern endosseous dental implant was invented by the Swedish orthopedic surgeon Per‐Ingvar Brånemark in 1969. Brånemark’s concept was for a cylindrical surgical fixture to be placed into the jawbone and allowed to fuse with the bone over a few months, replacing the root of a missing tooth. After osseointegration, the root‐form implant serves as a substrate to hold a replacement tooth or other prosthesis although it should be mentioned that the original Brånemark system was designed for the edentulous jaw rather than single‐tooth replacement [1–3]. Because the dental implant is fused to the jawbone, it is very stable and mimics a natural tooth root in that it, and its associated prosthesis, can be independent of adjacent teeth.
Osseointegration, as discussed in earlier chapters, is governed by many factors such as the patient’s bone quality, the presence of infections such as peri‐implantitis, the type and magnitude of external loading modes, the implant design, the surgical procedure and even corrosion interactions between implant and the abutment or prosthetic crown. Any of these factors, alone or in combination with others, can increase the failure rate of osseointegration. This, in turn, can lead to the implant being lost owing to unsatisfactory or imperfect osseointegration between bone and implant.
Most dental implants are fabricated from titanium or one of its alloys although, as reviewed in the Chapter 2, many other materials have been evaluated as implant materials in the past. The major advantage of titanium and titanium alloys is that they are virtually chemically inert, have great strength and are biocompatible. It is the latter property that allows them to integrate with bone without being recognized as a foreign object by the body.
Today, most implant‐supported prostheses replace single missing teeth or address partially edentulous areas; in the latter situation, the final prosthesis resembles a fixed bridge (see Chapter 4). There is also a growing use of implants for full arch prostheses, either fixed or removable.
The Endosseous Implant
The basic design of an endosseous implant, abutment, and prosthetic crown is shown in Fig. 3.1 and the intended function of the dental implant is shown in Fig. 3.2.
The body of the implant, also known as the fixture, is the component that is inserted into bone where it osseointegrates. The upper (coronal) end of the fixture is the collar or crest module, and this is surmounted by the platform or abutment interface.
The Implant Body and Surface
The function of the osseointegrated endosseous implant is to transfer occlusal and masticatory forces from the prosthesis to its surrounding biological tissues. Thus, its primary functional objective is to manage biomechanical loads by dissipating and distributing the applied forces such that the functions of the implant‐supported prosthesis are optimized.
Achieving this objective depends on three factors:
- Successful osseointegration with the surrounding bone.
- The nature, magnitude and directionality of the applied forces.
- The surface area of the implant/bone interface over which the force is distributed.
The cylindrical body of the implant may be smooth, threaded, hollow, or vented. Under compressive (masticatory) loading, a smooth (non‐threaded) cylindrical fixture may essentially experience a shear‐type force at the implant‐bone interface. To alleviate such forces, the fixture surface must be modified to ensure effective and retentive osseointegration. Such surface modifications include providing micro‐retention characteristics such as surface roughening, hydroxyapatite (HA) coatings and titanium‐plasma spraying. Such micro‐retention surface modifications tend to be important for both plane cylindrical fixtures and the more common threaded implants.
Providing the fixture surface with threads came about through further studies by Brånemark and co‐workers on osseointegration . This study indicated that the live bone would remodel into the screw shape of an implant, leading to a functional contact for the implant. Consequently, the most common approach to the interfacial stress problem is to provide the implant surface with threading and to taper the implant. It should be noted, however, that although implant fixtures customarily having a threaded, conical shape, they are not “screwed” into the bone but rely upon a retentive fit into a pre‐drilled hole or osteotomy cavity.
Many thread designs or shapes are used for dental implants, Fig. 3.3:
Table 3.1 Purpose of thread designs.
|Thread Shape||Designated purpose|
|Square||Optimal surface area for transmitting compressive load to the bone/implant interface|
|Buttress||Optimal for resistance to both pullout (tensile) and/or compressive forces|
|Reverse buttress||Greater resistance to pullout (tensile) forces|
|V‐shape||Standard (universal) thread pattern|
Table 3.2 Thread profiles used by various implant manufacturers.
|V‐thread||Brånemark system (Nobel Biocare)
Screw‐vent (Zimmer Dental)
Certain (Biomet 3)
|Buttress||Inclusive tapered implant (Glidewell Laboratories)
Straumann Standard (Straumann USA)
|Reverse buttress||NobelReplace (Nobel Biocare)|
Each shape has certain advantages and/or designated purpose, Table 3.1, and each manufacturer has their preferred thread profile, several which are shown in Table 3.2. The V‐shape thread, however, is not common with oral implants.
Manufacturers often indicate the characteristics of the threads used on their fixtures, Fig. 3.4. The pitch of a thread is the number of threads per inch (TPI) applied to a single diameter cylinder; a fine‐pitch screw has more TPI than a coarse pitch thread. The angle of the thread indicates the steepness of the screw thread (i.e., the “sharpness”) whereas the depth indicated is the height of the individual thread above the shank of the screw. Sharper threads have smaller angles and greater thread angles that tend to facilitate bone expansion for greater implant stability. Greater thread depths and coarser thread fixtures are often used to ensure better osseointegration with a dense bony socket.