To achieve not only predictable osseointegration but also a functional and esthetic restoration, each implant system needs to have nine essential design features. Each characteristic and its purpose are explained in detail in this article.

1.Implant Material

Fig. 1

Gold, carbon, titanium and zirconia have been utilized as implant material. Titanium (Fig.1, right) has proven to be the choice of material due to excellent biocompatibility and strength properties. Titanium is produced in different grades of purity and  in some cases alloyed with other metals to enhance material properties. Manufacturers use different grades of titanium for their implants. Grade 5 (Ti-6Al-4V) titanium is 78% stronger than its next weaker Grade 4 cousin. 

2.Outside Thread Design

Fig. 2

Proper implant design should allow for a high primary stability in the receptor site to facilitate osseointegration. The tighter the implant surface is in contact with the bone at the time of placement the more predictable its success will be. Early versions of implants (Calcitek®, Spline) were produced without outside thread pattern (Fig.2, left) by some implant manufacturers. This design quickly fell out of favor due to a reduced BIC (Bone to Implant Contact) and primary stability concerns. Modern implants are designed with a outside thread pattern in various configurations.

Fig. 3

The thread geometry itself may be designed as a common V-thread (Fig.3, "A", Zimmer®, Tapered Screw-Vent), trapezoid (Fig.3, "B", NobelActive™), square (FIG.3, "C", BioHorizons®) or buttress (Fig.3, "D", Straumann®) profile. Each manufacturer can produce scientific studies supporting their preferred thread geometry in regards to stress distribution capabilities and cutting efficiency.

Fig. 4

Threads may be machined as a single (Fig.4, "A", BioHorizons®, external hex), double (Fig.4, "B", Implant Direct™, Legacy™) or triple lead thread (Fig.4, "C", Zimmer®, Screw-Vent). This feature has no bearing on osseointegration, BIC or stress distribution. It simply increases the speed at which the implant is driven in to the osteotomy at the time of insertion.

Thread pitch is the distance from the crest of one thread to the next. The smaller the thread pitch (Fig.1, right) the greater the number of threads and the greater the surface area of the implant. More threads equal better stress distribution and greater primary stability.

Fig. 5

Thread depth can be even along the length of the implant or vary. A diminishing or non existing thread depth (Fig.5, left) towards the apical of the implant will reduce its self tapping capability and potentially diminish primary stability compared to a deep thread implant (Fig.5, right) design due to the reduced BIC.

3.Body Geometry

Fig. 6

The implant geometry (Fig.6) may be machined with a parallel, parallel with a tapered apex or tapered body. A parallel body geometry provides the greatest surface area and stress distribution while a tapered body may be more suitable in situations with limited apical space due to anatomical constraints, but sacrifices surfaces area. Parallel body implants with a tapered apex try to bridge the advantages and disadvantages of purely parallel and tapered implant geometries.

4.Surface Treatment

Fig. 7

To further enhance osseointegration, the outer surface of the implant that is in direct contact with the surrounding bone should be roughened to increase the surface area. Resorbable media blasting followed by acid etching (Fig.7, center) are currently the most common modes of surface roughening. Some implants are titanium (TPS) or hydroxylapatite (HA) plasma sprayed (Fig.7, right). Both intended to aid osseointegration. HA and TPS coated implants are mostly used in poor quality bone such as type D4 bone.

5.Implant Length

Fig. 8

Implants are offered in several lengths (Fig.8). 5mm implants are currently the shortest implants on the market while Biomet 3i™ offers the longest at 20mm, excluding zygoma implants. The longer the implant the greater the surface area and stress distribution. Length beyond 15mm has little beneficial effect.

6.Implant Diameter

Fig. 9

Choice of Implant diameter (Fig.9) is a important factor in stress distribution and greater surface area. The BioHorizon® Laser-Lok®3.0 implant is currently the smallest three component (implant, abutment, screw) implant system on the market while the 8mm Rescue® implant from MegaGen is the widest. Different platform diameters will allow for a more suitable emergence profile congruent with the tooth to be replaced. A Diameter greater than 6mm may be detrimental due to a stress shielding effect.

If all the above criteria are met within an implant system, the chances for successful and predictable osseointegration are significantly increased. But osseointegration alone does not portend a predictable, long lasting and functional prosthesis. For a successful restoration after osseointegration, it is also important to look at factors that have a favorable influence on the fabrication, delivery, function, esthetics and maintenance of the final restoration.

7.Abutment Indexing

Fig. 10

The implant should have a built in indexing design such as an external or internal hex at the connecting surface to the abutment. Indexing aides in proper implant position registration and transfer through impression taking to restoration fabrication and delivery of the prosthesis. An internal indexing design is favored because the screw head of the abutment screw can be designed to seat closer to the head of the implant (Fig.10, right). Effectively lowering the stack height of the components and allowing greater freedom in abutment design and favorable stress distribution from prosthesis/abutment to the implant. A internal indexing feature can be designed with a grater length (up to 1.5mm) as compared to a external indexing feature (up to 1mm). The greater length of the indexing feature gives not only better tactile feed back when seating the abutment/restoration but also provides greater torsional resistance.

8.Abutment Screw

Fig. 11

A removable and replaceable abutment screw (Fig.11) that holds the abutment securely connected to the head of the implant is a design necessity because of potential future screw failure. A screw down connection design will also allow for a simplified disassembly in case of prosthesis failure or prosthesis design changes in the future. Screws are implant brand specific and may vary in design and torque requirements from implant diameter to implant diameter within a given brand and abutment material. The majority of fixation screws are torqued to 30Ncm and up. Screw heads are designed for various tool connections from square (Robertson) to multiple hex dimensions and more recent Torx™ (hexalobular) configurations. Some manufacturers plate their screws with gold (Biomet 3i™), titanium nitride (Keystone) or carbon (Nobel Biocare™) to reduce friction during the torque procedures, effectively increasing pre-loading of the screw.

9.Abutment Material

Fig. 12

Abutments (Fig.12) should be produced a wide variety of styles and materials: straight, angled, different collar height, lengths, emergence profile diameters and UCLA type to allow the greatest freedom and choice to create the most ideal and cost effective abutment for the most ideal restoration even in a less than perfect implant placement situation. Abutment material choice has become an important factor because of consumer demand for high esthetics and "metal free" restorations. A implant system that offers abutments manufactured out of titanium and zirconia will meet the demands of even the most discerning patient. Zirconia abutments may be produced with a friction fit or pre-bonded titanium implant interface and as a through and through zirconia abutment. Smaller (3.3mm to 3.5mm) implant/abutment interface (platform) diameter zirconia abutments may only be suitable for small, single anterior restorations due to strength concerns.

To summarize the most important elements, titanium grade, screw thread design, roughened surface, geometry, choice of length, several diameters, indexing, separate and replaceable 30N-cm abutment screw, the ability to manufacture custom abutments out of titanium and zirconia can lead to a more predictable and highly esthetic final restoration. Additional features may improve implant and component handling and are discussed in the expanded design article.