July 1, 2000
by Blake Nicolucci, BSc., DDS
There are many Implant dentists who believe that any given case might be ‘over treated,’ while the next case is considered ‘under-treated.’ The number of implants used in each individual case can vary with:
a) What does the patient wants or what can they afford?
b) What the dentist is trying to accomplish?
c) How long is the particular prosthesis required to function? (i.e. what age is the patient?).
To be the ‘Architect’ and/or ‘Draftsman’ for any given case is an essential element when focusing on a treatment plan for ‘long term success.’ Pre-planning a case can make life a lot easier for both dentist and patient.
It should be quite obvious that an implant born prosthesis in a 20-year-old person will be required to function at higher stress levels (and for a much longer period of time) than an implant born prosthesis in a 70-year-old individual.
To some dentists, over-treating every case is a standard of practice. (Figures 1 & 2) This may seem to be the safest way to go, but are they really doing their patients a service –or are they taking unfair advantage of the situation (from a financial point of view) (Figures 3-5). There are a group of Implant Dentists out there that feel “the more–the better!” This is not all together incorrect. Although placing “extra” implants may allow you to sleep better at night, at what stage is it considered ‘Over-kill?’ Should every implant patient be treated for “Titanium Deficiency”? (Figures 6 & 7)
The reverse, (reducing the number of implants solely to reduce the fee for the patient), may attain more surgical cases for the dentist but when the case is completed, will it be as bio-mechanically stable, and/or biologically sound? (Figures 8 & 9) Dentists usually apply this approach when the patient tries comparison-shopping and the dentist wants to keep the case by undercutting the competition. But will the under-treated case cost you more to retreat in the long run when costs escalate due to repairs and remakes (Figures 10 & 11)? Unfortunately, the dentist who under-treats the case will not know these answers until it is too late. There is an old saying in Implant Dentistry–“It is better to see the back of a patient’s head once and lose the case (because of fees) than it is to see the patients face thirty or forty times (for repairs and corrective procedures).”
So the question still remains–“How many implants is ‘Enough’?” Well, there are a few factors that must be taken into consideration when determining how many implants are required to properly engineer a specific case. Some are listed below and briefly reviewed.
As stated by Misch, there are basically four bone Densities–D1, D2, D3, and D4. The D1 density bone is considered to be the densest, and the D4 density bone is considered to be the least dense. If we were to use an analogy of bone density to wood density, they would compare out as D1 having the density of ‘Oak,’ D2 having the density of say ‘Maple’ or ‘Pine’, D3 having the density of ‘Balsa Wood’, and D4 would have a density similar to ‘Styro-foam’. This ‘wood’ analogy should give the reader a very easy understanding of why the number of implants for a specific prosthesis will vary from patient to patient. It should also be noted that different areas of the mouth have not only different quantities of bone, but different qualities or Densities of bone as well. (i.e. the anterior mandible generally has D1 or D2 bone whereas the posterior maxilla generally has D4 type bone.) An Implant Dentist might also expect that the pre-maxilla and the posterior mandible would have a D3 density of bone. These are generalities, but it should indicate to the reader that in the extreme case, an implant in the anterior mandible has much more support to offer a prosthesis than an implant in the posterior maxilla. Since the maxilla has bone densities of generally lesser densities than those in the mandible, one can conclude that a maxillary appliance or prosthesis will require more implants for support than would be necessary in the mandible. But bone density is only one part of the equation.
Since the laws of levers dictate that the bio-mechanics of a hinged system delivers different forces at different positions on the lever, it can be deducted–and rightly so–that implants placed more posterior in the arch are subjected to increased bio-mechanical forces than implants in the anterior segments of the arch. A more specific example would be cracking nuts with a ‘nut-cracker.’ The nut is easier to break open (and with less force) if it is placed close to the hinge of the instrument. The same principle applies to implants. Those placed in the posterior segments of the jaws undergo greater biomechanical forces than do those in the front of the mouth. This can be considered one of the ‘Saving Graces’ for implants placed in the Pre-maxilla. Because of the lower forces of mastication in the anterior segments, the necessity to place implants at a slightly protrusive inclination does not have as great a deleterious effect (as it would have if it were in the posterior segments).
There are two basic categories of implant prosthesis. They would be a fixed prosthesis, or a removable prosthesis. The fixed category can again be broken down into three sub-categories, and the removable into two sub-categories. [We will not go out on a tangent about the specifics on prosthetics at this juncture.] It should stand to reason that a fixed prosthesis that is implant supported and implant retained will require substantially more support than a removable prosthesis that is implant retained, but tissue supported (since the bio-mechanical forces of the system are born by the tissues – not the implants.). Another analogy is this. You wouldn’t build a 10,000-sq-ft house on a foundation that was meant to support a 2,000-sq-ft house. At this point, the Implant surgeon and the restoring dentist must be a team and have constant communication between each other.
Age and Sex
There are obvious differences in the chewing capacities of men vs. women, and young vs. old individuals. Just as ‘arch position’ for an individual can designate how many implants should be used (and what size), it is no surprise that a 300-pound male wrestler would have developed stronger biting force than a woman of average or slight stature. It should also make sense that an 80-year-old man would not be able to bite with as much force as a 25-year-old man. As the muscles of the trunk of the body deteriorate with age, so do the muscles of mastication.
Different forces caused by different diet types can be demonstrated in the following values. An Eskimo can develop compressive forces exceeding 300 psi. [during function on his posterior teeth]. The forces of mastication of the average adult North American male turn out to be as little as one half of this value!
This might be considered a good reason to reduce the number of implants placed. In reality, if the case is under-engineered from the beginning, it will probably increase costs to the patient in the long term for repairs, or a change in treatment plan (and in some instances increase costs to the restoring dentist). If money is a concern to the patient, the treatment plan should be changed. If a ‘best’ or ‘ideal’ treatment plan is placed in front of the patient, and the patient indicates that the cost is too prohibitive, then the patient should be asked “What are you willing to give up to reduce the cost?” (Which could possibly indirectly reduce the number of implants). A fixed prosthesis can become a removable prosthesis. An implant retained and implant-born prosthesis can become an implant retained and tissue supported prosthesis. Even a tissue born prosthesis can have the number of implants reduced, but this can sometimes jeopardize the long-term prognosis of the system The educated and well-informed Implant dentist can readily make these decisions as long as the directions of movement of the prosthesis on its attachments have been taken into consideration.
“How many implants are ‘enough’?” As can be seen from the above passage, every case must be ‘blue-printed’ on its own merits. Again, there is no single answer for any given general situation. The number of implants required to achieve an excellent result must be tailored to each case. One must remember that if you reduce the number of implants, you must change the prosthetic design accordingly.
Blake Nicolucci is Oral Health’s Board member, Implantology.
Oral Health welcomes this original article.