CHAPTER THREE: THE PRACTISE OF ORAL IMPLANTOLOGY USING MOSTDIS
BASIC INFORMATION ON MOSTDIS
OSSEOINTEGRATION IN THE USE OF MINI-IMPLANTS
Mini-implants,
narrow diameters, small diameters, reduced diameters, MOSTDIs are all made from
titanium and they all osseointegrate, just like conventional sized dental
implants. Mini-implants, due to their smaller diameter and minimally invasive
placement technique results in a much smaller healing challenge to the body and
therefore probably integrate even faster. In addition, they are usually placed
up to a torque of 50 ncm as compared to 30 ncm in the case of conventional.
This can be done because minis are solid one piece implants and are actually
stronger in resisting fracture than conventionals that have a hole within the
fixture which weakens it considerably and does not allow the driver to apply a
high torque to it. Thus minis can and are torqued to a higher level than
conventionals, giving it a stronger primary stability that allows immediate
loading and faster finishing than conventionals.
RETRIEVABILITY
IN MINI- IMPLANTS
The
ongoing debate in oral dentistry is whether or not a prosthesis should be
screwed on or cemented. In his landmark lecture in 1982 in TORONTO, CANADA,
Professor Branemark showed successful cases of endosseus screw titanium dental
implants, all of which were of the two piece design with the prosthesis screwed
on. The screwed on technique was designed to enable retrievability so that if
any complication like a fracture or infection occurs, the prosthesis can be
removed by unscrewing the connecting screw. The implant could then be examined
and remedial action may be taken with direct visual information.
Over
the years, it has been observed again and again that in general, oral implants
have an average success rate of up to 95%. Therefore, as argued by some practitioners,
since there is only a 5% to 10% failure rate, the prosthesis ought to be screwed
on so that they can be retrieved when necessary in order to treat the oral
implants directly. This advantage, they feel, offsets the disadvantage of loose
screws, broken screws, the need for high precision, the resulting microgap, and
the high costs of manufacturing and maintenance of such small precision
components.
The
other school of thought feels that the combined disadvantages do not justify
the advantage of ease of retrievability that is gained with a screw retained
prosthesis. After all, if worse comes to worse, the prosthesis can be cut off with
a high speed air rotor or a diamond disc with a guard. In addition, dental practitioners have been
cementing crowns and bridges for years over natural teeth, even though after 10
years, almost 30% of crowns and bridges that are built on natural teeth fail
for one reason or another. The logic therefore is that if we insist on
cementing on our crowns and bridges even though there is a 30% failure rate
after 10 years, then based on the same logic, there is even greater
justification in cementing on our crowns and bridges over dental implants
instead of screwing them on.
One
solution to this problem is to use a temporary cement like temp-bond and even
dycal to cement the implant prosthesis on. Thus if necessary, the prosthesis
can be removed with minimal guided force. Temporary cements with its weak
bonding strength works over implants because implants are osseointegrated, i.e.
ankylosed to the bone and does not move. Natural teeth are attached to the bone
via the periodontal ligament which allows the natural teeth to move in tandem
with the applied forces, much like the suspension of a car. Therefore, if a
temporary cement is used, the cement bond is broken easily especially when
there are eccentric forces acting on the prosthesis.
Another
school of thought is that when in doubt, use a screw, and when you are very
sure of your implant fixtures, cement them in. This philosophy can often cause
a crisis of choice when the time to fix the prosthesis comes.
The
ideal system seems to be one that would allow easy retrievability without the
accompanying disadvantages of a screw retained system. Currently, because of
the numerous inherent disadvantages of screwing on the prosthesis, the trend is
more and more towards cementation with temporary cement. And after a period of
time, if the temporary cement fails, then a permanent cement to fix on the
prosthesis is used, the logic being that adequate time has been allowed to ensure
the health and stability of the implant.
In
the case of mini-implants, retrievability have always been a problem since most
crowns and bridges were cemented on permanently. Temporary cements may be a
solution but because of the smaller size of the implant head as compared to
conventional implants, the prosthesis tends to become unstuck more often. A prosthodontic
solution for mini-implants may be to use prefabricated abutments that fit over
the small implant head. The resulting abutment is then larger and less likely
to fracture in the lab and easier to build a prosthesis on. In addition, the depth
of porcelain used will not be so thick as to compromise its ability to resist
fracture. The prosthesis can then be cemented permanently on to the abutment
and then cemented with temporary cement over the small implant’s head. In this
case, the prosthesis can be removed when necessary with a crown remover, thus
solving the problem of retrievability. A corresponding technique is to cement
the abutments permanently on the MOSTDIs and the final prosthesis can be
cemented on the abutment with temporary cement.
The
success of oral implantology has been built on the solid advances of crown and
bridge prosthodontics that began in earnest since the early 20th
century. Initially, crown and bridgework were frowned upon because they were
poorly constructed and paid little heed to hygiene, maintenance and material
biocompatibility to oral tissues.
However,
since the middle of the 21st century with rapid advances in
infection control and materials science, crown and bridgework has had
unprecedented success in function, aesthetics and maintenance. But the bugbear
of having to cut and injure vital teeth proved to be an ongoing problem. Crown
and bridgework after 10 years on the average has up to 30% failures, very often
due to sequelae and as a result of cutting abutment teeth all the way to
dentine. No matter how much cooling water spray we apply, once we cut to the
dentine, the pulp undergoes an inflammatory response showing that there is
acute damage done to it. Most of the time, the tooth repairs and defends itself
successfully. However, there is a substantial percentage of such teeth that
undergoes chronic inflammatory changes that later becomes acute if there is
bacterial invasion. With the advent of oral implantology, the necessity of
cutting vital teeth has become increasingly obsolete. Now crowns and bridges
can be built totally on non-vital artificial screw titanium implants. But it is
to be recognized that without the advanced development of crowns and bridges on
vital teeth, oral implantology will not enjoy the outstanding success it has
today.
PROSTHODONTICS FOR CONVENTIONAL 2 PIECE DENTAL IMPLANTS
Since
Professor Braunemark’s landmark presentation on his 15 year studies on large
two piece titanium screw dental implants in 1982, most implant manufacturers
have basically copied his precedents.
The precedents were:-
(1)
The use of titanium
(2)
Root sized screws
(3)
Atraumatic placement
(4)
Two piece screw dental implants
(5)
Waiting 3-6 months for osseointegration of the fixture before connecting the second
piece called the abutment
(6)
Building the crowns and bridges on the abutment and screwing them on for the
sake of retrievability
Abutments
are usually screwed on to the implant proper, usually called the fixture. The
abutment itself may have a threaded part that can be used to screw it on or it
may have a separate screw to connect it to the fixture. Some implant abutments
are designed to fit into the fixture and just tapped on and held there
basically by frictional forces. Some abutments may also be cemented onto the
fixture.
Crowns
and bridges may be screwed on as advocated by Professor Branemark and his
followers. Increasingly, many practitioners cement in the prostheses. The
rationale for cement over screwing them on are many, the top of which is since
we have all along cemented on crowns and bridges event though they have only a
70% success rate after 10 years, what is wrong with cementing on crowns and
bridges on oral implants whose success rate is 90% after 10 years.
However,
Professor Branemark has an important point in requiring retrievability for his
implants. That was why he advocated that the prosthesis should be screwed on
rather than cemented on. The additional problem in screw retained prosthesis is
the need for passive fit and therefore a very high precision fitting, failing
which the improperly fitting prosthesis will give rise to screws loosening or
fracturing or implants losing osseointegration. Cementation requires less
precision because the gaps can be filled up with cement.
Some
practitioners solve the problem of retrievability by using temporary cement.
The prosthesis can easily be removed whenever necessary by using a crown
remover. Temporary cement works on implants indefinitely because implants are
osseointegrated/ ankylosed to the bone and move very little in relationship to
other implants since they are all osseointegrated to the same bone. Temporary
cement does not work on teeth because teeth can move much more independently of
each other and this movement will break the temporary cement and cause the
bridge to come loose after a short time.
PROSTHODONTICS
FOR ONE PIECE SMALL DENTAL IMPLANTS AND THE QUESTION OF RETRIEVABILITY
There
are several problems encountered in building a prosthesis over small dental
implants. The issue of whether small dental implants are strong enough to bear
crowns and bridges will be addressed in another chapter. This chapter will
address mainly the prosthodontic restoration of small dental implants.
The
several problems are:-
(1)
The abutment portion of the SDI is small. The diameter is usually between 2.0mm
to 2.5mm. The small abutment when cast in a model can break easily and thus be
problematic to the dental technician.
(2)
Building a crown or bridge over a small 2.5mm diameter abutment will result in
large amounts of metal or porcelain used. If permanent cement is used and one
day the crown or bridge has to be removed for one reason or other, it would be
a very tedious process to cut through the thick metal and porcelain.
(3)
The small abutment results in very thick porcelain supported by a small metal
core. Such porcelain can fracture more easily.
(4)
The large surface area interface between the mucosa and the crown or bridge may
allow collection of food debris that can give rise to foul smells and
infection,
(5)
Because of the small diameter of the implant, a good emergence profile cannot
be achieved. As such, aesthetics from the anterior teeth may be a problem.
Some
of the solutions used so far for the above problems are:-
(1)
Prefabricated copings are made. These are precision made to fit over the head
of the SDI. They may be connected to the implant and prepared like for a crown
or a bridge. Impressions are taken, models are cast and the crown and bridge
built in the lab. The copings can also be placed and the impression taken with
the copings coming off with the impression. Implant analogues are inserted into
the copings stuck in the impression. The casting will have the analogues in
place. The copings are replaced and the crowns and bridges built over them.
(2)
Composite or glass ionomer material may be bonded on to the implant head and
then prepared like for a crown or bridge. The problem in this case is a smooth
surface of the composite or glass ionomer in its interface with the mucosa
cannot be obtained. The use of a rubber dam or some kind of film will leave a
tiny space that may serve as a food trap.
(3)In
place of the emergence profile for the anteriors, the anterior crowns are built
directly on to the mucosa surface, the surface around the SDIs or the models
are relieved generously from 0.5mm to 1.0mm to give the crowns and bridges a
slight compression on the mucosa surface resulting in a reasonable emergence
profile. This technique has been used very successfully on anterior pontics of
bridges. There is hardly any food debris or plaque under the pontics if
properly constructed.
