CHAPTER THREE: THE PRACTISE OF ORAL IMPLANTOLOGY USING MOSTDIS

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 20^th 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 21^st 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.