"Committed to providing the highest quality crowns, bridges and implants, SCDL is Australia's only truly global dental laboratory, with worldwide locations that now includes our new Sydney laboratory and Melbourne office."
A properly planned and fabricated prosthesis is a team effort involving the experienced technician and experienced clinician. Obviously the more clinical experience a clinician has in dentistry in general, and implant dentistry specifically, the better the clinician is able to provide various treatment options to the patient and better able to advise what is the best option.
The more laboratory experience a technician has in dentistry in general and implant dentistry specifically, the better the technician is able to provide various lab options to the dentist and better able to suggest what is the best clinical option. Many inexperienced dentists turn to and rely upon their technician for advice in treatment planning implant cases. Some technicians even encourage this behaviour. The problem with the technician treatment planning the case is the limitation in clinical experience of the technician. This often results in prostheses that are excellent technically but deficient clinically.
It is important the clinician has a thorough grounding in treatment planning of implant dentistry to provide adequate input when discussing the case with the technician. Our long experience with SCDL and as clinicians has taught us that proper treatment planning involves a number of principles including:
Before you start! Ensure the dentist has discussed and documented with the patient all the possible treatment plan options, such as nothing, denture, bridge or implant, before a decision is made. The patient must be giving the operator fully informed consent before any treatment is commenced.
1. Assessment of sufficient available bone width, height and contour for the placement of endosseous implants.
2. It is the prosthesis which determines the position and number of implants. Hence plan the case from prosthetic end result down. Don’t be left with beautifully integrated, unrestorable implants. Prosecuting lawyers will have a field day when a poor result is caused by poor pre-treatment diagnostic protocol.
3. All implant systems can work and all implant systems can fail. It is not the implant system or implant fixture design that will give success or failure.
4. Treatment planning is the most important phase in implant success and must be instituted prior to surgical intervention. NOT afterwards!
5. Treatment planning is based upon choosing the prosthesis that decreases the load on the supporting implants and to the prosthesis provided by the implants, teeth and soft tissue.
6. Excessive occlusal forces will cause most implants to fail. Examine not just the available bone, but also the amount of vertical bone resorption, as this leads to a poor implant length to crown height ratio. Picture the dangerous scenario of a heavily resorbed posterior maxilla, short implants due to pneumatisation of the sinus (poorer bone in the posterior maxilla to start with) and long clinical crowns to reach the occlusal plane. Not all cases are possible!
7. Natural teeth in the opposing arch transmit greater bite forces to implants than denture supported teeth. Natural teeth exert a force of 25-950 PSI, as opposed to 14-25 PSI with dentures.
8. Loading on the prosthesis can be reduced by:
9. Support can be increased by:
10. The amount of support from the implant system is directly proportional to the amount of fixture surface area attached to the bone. Ask what you would expect of the natural tooth with the same root area. If the answer is that you would have a guarded prognosis, then consider more implants to increase the surface area.
11. Do not attempt to replicate what the natural teeth failed to do. If the patient has lost a prosthesis due to inadequate support from natural teeth, do not try to replicate the design with implants. You must consider additional support by placing more implants.
12. Implants do not handle lateral loads well; the occlusion must be free of interferences in lateral excursion. Always consider the guidance you are incorporating into your design. If you have successful guidance at the outset, avoid changing this successful configuration without a carefully assessed provisional stage. Ideal anterior guiding surfaces are concave, smooth, and gradually increasing in gradient.
13. Do not exceed crown/root ratio of 1:1. The greater the crown/root ratio, the more the lateral forced are magnified.
14. Assess torque loads. The angulation of the force into the long axis of the implant must be considered. It is unrealistic to expect implants to tolerate torque loads in excess of 30 degrees from the long axis.
15. Not only must angulation be considered in a bucco-lingual direction, but the mesio-distal as well. Implants are frequently placed too far to the buccal or lingual, resulting in the occlusal contact hitting too far to the buccal or lingual. This results in lateral torque of the implant.
16. The patient must have an intact dentition in the opposing arch. Missing and decayed teeth, periodontally involved teeth, over erupted teeth, maloccluded teeth, etc. must all be correct before implants are placed.
17. The prognosis of a totally implant borne prosthesis in the maxilla is much less favourable and possibly contraindicated, if occluding against natural teeth. The bone in the maxilla is weaker, generally less dense and lesser in quantity than that found in the mandible. The addition of more implants (as many as 7 or 8) must be considered to support a totally implant borne prosthesis occluding against natural teeth.
18. Especially for the beginner, presurgical diagnostic waxups on mounted study models and surgical guide stents are recommended.
19. Once osseointegration is attained, the bone formed around the implant is a weak, immature woven type. During the next 12 months the immature bone converts to stronger more mature lamellar bone. Excessive occlusal loading during the transition period can result in the loss of osseointegration and replacement by a connective tissue interface. Mature bone can withstand greater load.
20. Flexure of the mandible: when the mandible opens, the lateral pterygoid muscles pull the mandible medially resulting in mandibular flexure. The flexure is thought to occur in the area of the bicuspids, not in the midline. Therefore, any fixed prosthesis that crosses from distal to the bicuspids to a point anterior to the bicuspids can result in a splitting of the flexure zone which can lead to the patient experiencing pain and/or metal fatigue and a broken prosthesis. Lateral stress breaking features should be incorporated into the design of the prosthesis in the flexure area. If implants are kept anterior to the mental foramina, flexure does not seem to pose a problem.
21. Allow adequate clearance under the metal superstructure for patient hygiene. If the prosthesis is totally implant supported, then the case must be designed to allow the patient to clean around and under the superstructure. Generally, 3mm is adequate. However, with a high lip line in the maxilla, this poses a problem with aesthetics. Some patients complain that they whistle and spit saliva though the spaces under a maxillary prosthesis.
22. Implants and their abutment posts should have sufficient distance between them to allow for proper cleaning of the prosthesis and to maintain the viability of the bone. Allow a minimum of 3mm between implants and 2mm between an implant and a tooth.
23. Parallelism, while not critical, should always be attempted. Most implant systems are designed to allow for non parallelism, but the greater the divergence of the implants, the greater the destructive torquing forces exerted on the implants and this is subsequently transmitted to the bone.
24. Although cantilevering is possible with osseointegrated implants, avoid it. Each case must be assessed individually to assure that over-leverage of the implants does not occur. Such factors as: the number of implants, the surface area of the implants, the opposing dentition being natural teeth vs. dentures, the depth of arch that the implants form and the length of cantilever all enter into the judgement as to how far one can cantilever. If bone is available, place an implant abutment behind the mental foramen and/or sinus for support to avoid cantilevering.
25. Porcelain teeth have very little stress absorbing capacity and should be used with caution in crown and bridge cases, especially when opposed by natural teeth. Composite crowns such as Adoro and BelleGlass NG on metal sub stuctures are ideal semi-permanent crowns that absorb stress effectively. However, studies have also shown that porcelain teeth when used in conjunction with removable dentures have sufficient stress absorption ability to not overload the supporting implants.
26. Proper balanced occlusion is one of the more important aspects to be determined in prosthetic design.
27. Where possible, decrease the occlusal table by:
28. We always use metal analogues in the lab working models. Metal analogues offer greater accuracy than one poured in dental stone.
29. The accuracy of the fit of the metal framework that seats onto implants is more critical with implants than with natural teeth. With teeth, there is a certain degree of adaptability by the periodontal ligament to accommodate minor discrepancies with metal framework construction. The ligament is not present with implants eliminating the “adapting factor”, necessitating the need for greater accuracy in the casting joining the implants.
30. The metal framework must fit passively on implants. Otherwise several untoward results can occur:
31. The mating surfaces of machined components is generally more accurate than a cast surface. When possible use a machined part rather than a cast one.