Patients desire teeth that are in good health, with good function, esthetics, and phonetics. When indicated for missing teeth, dental implants are a means to fulfill these requirements. This Learning Module will outline the implant design features and characteristics that are aimed at facilitating integration with the hard and soft tissues, allowing them to serve as anchorage elements for prosthetic reconstructions whilst also withstanding occlusal loading forces.

The body design of implants can generally be divided into three parts: the endosseous part, which lies within the bone, the transmucosal section, which lies in the soft tissues between the bone and the oral cavity, and the interface to the prosthodontic components. Within each of these three parts there are further design characteristics and variations in shape, features, and dimensions. There are also variations in implant body material and surface technology. These variations will be described and discussed in the following Learning Objectives.

After completing this ITI Academy Module, you should be able to: describe variations in the implant prosthodontic interface, describe variations in the implant transmucosal section, describe variations in the implant endosseous part, and describe variations in implant body materials and surface technology.

The primary function of dental implants is to serve as anchorage elements for prosthetic reconstructions whilst also withstanding occlusal loading forces. As such, implants need an interface with the prosthodontic components of the reconstruction. Variations in design of the interface give rise to different configurations. These variations are related to the extent to which the implant directly contributes to this interface or prosthodontic platform for a dental prosthesis and also to the design of the connection between the prosthodontic components and the implant. The relationship of the implant prosthodontic interface to the neighboring hard and soft tissues varies as well. These variations will be discussed in the following slides.

The extent to which the implant directly contributes to this interface or prosthodontic platform for a dental prosthesis can generally be divided into three types or configurations. In the first variation, shown on the left, the prosthodontic platform is an integral part of the implant. In the second variation, shown in the middle, the implant provides part of the platform and a separate component called an abutment provides the rest. An abutment is defined as a part or component that serves as support and/or retention for a dental prosthesis. In the third variation on the right the implant does not provide any of the platform. The link between the implant and the prosthesis is therefore provided entirely by the abutment. A more detailed description of abutments is covered in a separate Academy module titled 'Abutment Selection for Fixed Dental Prostheses'.

Implants that provide all or part of the prosthodontic platform are referred to as one-piece implants. Typically one-piece implants have a transmucosal section that extends into the oral cavity with a fixed neck design and built-in emergence profile.

Implants where the prosthodontic platform is provided by a separate abutment component are called two-piece. The two-piece implant is designed to stop at bone level and does therefore not have a transmucosal section. The emergence profile and prosthodontic platform are established by the abutment, which gives more prosthetic flexibility. This can be advantageous in sites with relatively small dimensions, as well as in esthetic areas because of the option to select a tooth-colored abutment.

The design of the implant connection to the prosthodontic components falls into two main configurations, which are referred to as internal connections or external connections. Most connections of either type have some form of anti-rotational index that the abutment can engage. Examples can be seen with flat sides in the tapered internal connection and with a hexagonal profile in the external connection.

The design of internal connections vary from straight-sided to tapered and can be a combination of both. Tapered designs have a greater intimacy of fit to the abutment and provide greater stability under load. External connection implants generally offer a flat shoulder for a butt fit against the abutment. This connection design offers less precision of abutment fit and stability under load, and there is a general shift towards internal connections across most implant systems.

The relationship of the implant prosthodontic interface comes in three general configurations. In one-piece implants, the prosthodontic interface is typically 2-3 mm above the bone crest. The distance to the bone crest is referred to as a vertical offset. Two-piece implants can have matching or non-matching abutment diameters. When the diameter of the abutment is the same as the diameter of the implant, a butt-joint type connection is created at the bone level. Two-piece implants with non-matching abutment diameters are known as horizontal offset implants.

The design of the implant prosthodontic interface has a bearing on stability of the marginal bone levels around the implant. The vertical offset avoids inflammation at the bone crest and allows for stable bone levels. A matching diameter at the crestal bone is associated with 1.5-2.0 mm bone loss due to a bacterial infiltrate of the butt joint and a corresponding inflammatory response. Horizontal offset moves the bacterial infiltrate away from the bone crest and reduces bone loss to 0.5 mm.

Variations in Implant Prosthodontic Interface, Key Learning Points: One-piece implants provide all or part of the prosthodontic platform, and their transmucosal design offers a vertical offset that promotes stable bone levels. Two-piece implants are designed to stop at bone level, and the prosthodontic platform is provided by a separate abutment, which offers more prosthodontic flexibility. A matching diameter of the two-piece implant to abutment interface is associated with greater crestal bone loss than a horizontal offset. An internal tapered connection to the abutment offers a more intimate fit and greater stability under load than internal straight side and external connections.

The transmucosal section lies in the soft tissues between the bone and the oral cavity. In a one-piece implant the transmucosal section is built into the implant and is typically 2 to 3 millimeters in height. In a two-piece design the transmucosal section is part of a separate abutment that attaches into the implant abutment interface. When the abutment is seated in the implant, the transmucosal section is indicated by the arrow. The design of the transmucosal section is aimed at soft tissue integration and establishing a biologic width similar to teeth. A comparison of this similarity in biologic width can be seen in the schematic image of a tooth versus a one-piece implant with a vertical offset.

The transmucosal section can vary in a number of different aspects. First it may vary in dimensions, with different lengths and diameters. The clinician selects the length of the transmucosal section depending upon the clinical situation and, in particular, the thickness of the mucosa. The appropriate diameter also depends upon the specific clinical situation, especially the dimensions of the edentulous space. Both one-piece and two-piece implants offer variations in length and diameter but the two-piece designs are likely to offer greater flexibility of choice.

The transmucosal section can also vary in design of the surface. Most implant designs have a smooth or machined transmucosal part. Some manufacturers have added textured surfaces to the transmucosal part, as there is some evidence that this may enhance soft tissue attachment. Similarly, some manufacturers have added microthreads with the aim of improving soft tissue attachment.

Variations in Implant Transmucosal Section, Key Learning Points: In a one-piece implant the transmucosal section is built into the implant, while in a two-piece design the transmucosal section is part of a separate abutment. The length and diameter of the transmucosal section is selected to suit the specific clinical situation. The transmucosal section may be textured to enhance soft tissue attachment.

The endosseous part or implant body that is inserted into the bone may be designed in one of several overall shapes or configurations. Implants may be cylindrical or conical in shape, or a combination of cylindrical and conical. When viewed in longitudinal section, the external walls of cylindrical implants are parallel to each other, whereas the walls of conical implants taper towards the apical end of the implant. Implants designed with a combination of cylindrical and conical shapes have features of both parallel and tapered walls.

The configuration of the implant is selected according to the clinical needs at the site of implant placement. These clinical needs include the amount, quality, and morphology of available bone. In general, implants with conical features tend to achieve higher stability when initially placed into the same bone type and density compared to implants with a cylindrical shape. Higher stability is a requirement if the treatment plan involves immediately loading the implant with a temporary prosthesis within 1 week of surgical placement. A variety of shapes can be observed in these examples of commercially available implants. The overall shape of the implant determines the steps taken in preparation of the implant bed or osteotomy as well as the drilling instruments to be used.

Implant threads are an important body design feature. The threads are helical ridges that begin near the apex of the implant and continue up the body towards the top, usually in a clockwise direction. In between the threads is a groove. Implant threads have several functions. First, threads allow the implant to be inserted into the implant bed preparation, or osteotomy. When a rotational force is applied to the implant, the threads bind to the surrounding bone, drawing the implant into the osteotomy. Once inserted, the thread design contributes to the initial or primary stability of the implant. Normally, initial bone-to-implant contact is established at the tip of the threads. Implant threads are also believed to have a positive function in directing loading forces into the bone.

One feature of threads that can vary between implant designs is their pitch. 'Thread pitch' is the term used to refer to the distance between the threads. In this composite image of two implants, the thread pitch is wider on the right side implant than on the left side implant. Manufacturers vary the thread pitch to suit different bone types. As a general rule, implants with a wider thread pitch require fewer revolutions to seat the implant. This can be an advantage in bone of low density by reducing the risk of crushing the bone at the junction between the implant and the osteotomy, therefore preserving primary implant stability. It should be noted that some implants are designed without threads, as shown in this example. These implants are placed by pushing or tapping the implant into the osteotomy. These designs are less common today, with most implants featuring threads.

There are many variations to the design of the threads themselves. Their configuration may consist of a single thread or double threads spiraling around the implant. The grooves between the threads may be relatively flat or deep. Threads may be self-tapping or non-self-tapping. Implants with self-tapping threads may be inserted without first having thread grooves tapped into the osteotomy. For non-self-tapping designs, tapping is normally required before the implant is inserted. In situations with low-density bone, placing a threaded implant without prior tapping may help to increase primary stability. In summary, the amount, quality, and morphology of the available bone influences the clinician's selection of an implant of a certain thread design.

Similar to the walls of the implant body, implants are designed with different apical configurations. These configurations include flat, rounded, or pointed shapes at the apical termination of the implant. Furthermore, the implant may be designed with or without threads at the very end of the implant, even if the implant has threads along its body. Yet another aspect is the ability of the implant apex to cut bone, compress bone, or be passive with respect to contact with the bony bed when being inserted. The amount, quality, and morphology of the available bone influences the selection of the implant apex design. In turn, the shape of the implant apex affects the steps taken in preparation of the osteotomy as well as the drilling instruments to be used.

Finally, the endosseous part may vary in its overall dimensions, with different lengths and diameters. For standard indications, the length typically ranges from 6 to 14 millimeters, and the diameter ranges from 3 to 6 millimeters. The clinical decision on the appropriate implant length and diameter is determined by prosthodontic and surgical assessment of the planned implant site. This is covered in more detail in a separate Academy module titled 'Surgical Assessment of the Implant Site'.

Variations in Implant Endosseous Part, Key Learning Points: The amount, quality, and morphology of available bone as well as the need for higher initial stability influence the selection of implant body shape. The thread design contributes to the initial stability of the implant. The configuration of the implant apex has an impact on the bony bed during insertion. Prosthodontic and surgical assessment of the planned implant site determines the appropriate length and diameter of the implant.

Implants are made of various materials. Common to all materials are two requirements; they must be biocompatible, and they must promote osseointegration. Commercially pure titanium is the material of choice due to its clinical results and long history of successful outcomes in everyday clinical practice.

A significant amount of research has been conducted on implant materials. In addition to commercially pure titanium additional metal materials with promising clinical results include alloys of titanium such as titanium-aluminum-vanadium and titanium-zirconium. Alloying of titanium with certain other metals can increase the tensile strength of the material. Non-metallic zirconia is also available. The primary advantages of zirconia implants over titanium and titanium alloy implants are their esthetic appearance and metal-free composition. The choice of implant material depends upon the clinical situation. It may also be influenced by specific patient desires.

The implant surface is another design parameter to consider. Surface properties generated as part of the manufacturing process have an effect on osseointegration of the implant.

Implant surfaces may exhibit different degrees of surface roughness that may be categorized as smooth, minimally rough, moderately rough, or rough.

Surface roughness greatly influences the process of osseointegration. A multitude of studies have shown that smooth surfaces are not well integrated by the host bone. Surfaces with a very high roughness are not optimally integrated either. The host bone best integrates implants exhibiting a moderately rough surface. In these studies, integration was measured histologically by the rate of bone formation or by the amount of bone in contact with the implant surface.

In clinical terms the higher rate of bone apposition onto moderately rough surfaces allows loading such implants earlier, thus decreasing the time span between implant placement and delivery of the prosthodontic reconstruction. Similarly, the higher amount of bone-to-implant contact at implants with moderately rough surfaces enables such implants to be of shorter length while transferring the same amount of loading forces to the surrounding bone as implants with other types of surface roughness.

In the initial step in fabricating an implant body, a piece of titanium undergoes a grinding process to produce the desired macrostructure. The result is an implant exhibiting the surface roughness resulting from this manufacturing process, sometimes referred to as a machined or turned surface. Manufacturers can then alter the native surface roughness of an implant body by applying additional manufacturing processes to achieve the desired microstructure. These additional steps typically involve subtracting material from or adding material to the existing implant body. Mechanical or chemical processes are utilized to achieve this roughness. These additional processes produce implant surfaces with distinct features.

Chemical processes are not only suitable to alter the surface roughness of the implants, they can also influence other parameters. Importantly, chemical processes can help to clean the implant surface of contamination. In addition, these processes may add specific chemical compounds with the aim of improving bone formation.

One successfully applied chemically induced alteration involves sandblasting the implant surface followed by acid etching. This improves the wettability of the implant, enhancing contact with blood and its components; the subsequent cellular attachment and proliferation leads to faster osseointegration. This chart shows the results of an experimental study comparing the original SLA surface with an SLActive surface that has been produced under continuous nitrogen protection, which reduces the degree of carbon contamination from the atmosphere. At 2 and 4 weeks, the SLActive surface shows higher bone-to-implant contact than the original SLA surface.

Along these same lines of thought are processes that add biologic compounds aimed at improving osseointegration, such as hydroxyapatite, to the implant surface. Much hope has been placed on these developments for improving patient care. To date, however, they have not led to any tangible results from a clinical perspective.

Variations in Implant Material and Surface Technology, Key Learning Points: Implants may exhibit different degrees of surface roughness as well as distinct surface features. These features are produced by a range of mechanical, chemical, and biologic manufacturing processes that may be additive or subtractive. Surface roughness influences the rate and amount of osseointegration; greater bone-to-implant contact has been observed for implants with moderately rough surfaces. Chemical and biologic alterations of the implant surface have the potential to increase the process of bone formation at the implant surface.

Implant Designs and Characteristics, Module Summary: Implant design features affect clinical outcomes; therefore, clinical aims govern the choice of implants with specific design features. Implant design features encompass both macro- and microstructural characteristics. Implant design features are further associated with chemical and biologic processes. The amount, quality, and morphology of available bone influences selection of implant design features. Primary implant stability is influenced by mechanical and biologic elements.Secondary implant stability is also influenced by mechanical and biologic elements.