IMPLANT DESIGN CONSIDERATIONS
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.