Hi alaa,, nice to see you here

regarding your concern ,, i really like the guidelines provided by Carl Misch in this regard

in short it depends on many factors including crown height space (vertical cantilever), amount of BL bone surrounding implants and whether bone grafts were used or not , type of occlusion and any habits , number and distribution of implants , design of full mouth reconstruction (splinted or segmented)

Having said that, the criteria provided in Misch's book are almost ideal and deviation of his recommendation is inevitable,, but at least it gives a general outline in which you can try to make your best judgment

i would recommend you strongly to put as little cantilever as possible ,, especially if the patient is less concerned about esthetics

I hope my answer was usefull

best of luck !

Thank you Huda 

I would not cantilever more than one small bicuspid or anterior distal to a single or multiple implant. And I would minimize the occlusal forces on the cantilever as well. Rotational forces will weaken the implant quite rapidly.

There are a lot of variables that need to be considered:

what is the distance from the fulcrum (a line extended from the distal of the terminal implant on the right side extending to the left side) and the anterior most implant (identified at the center of the retaining screw)?  The anterior implant will be under tensile stress and the posterior implants will be under compression with loading of the cantilever.

What is the anticipated load that will be exerted on the cantilever?  what is the mean load, what is the maximum load?

What is the duration and number of tooth to tooth (restored tooth) contacts during the day/night?  

Is there only functional contact or is there parafunctional contact?

What alloy is used for the frame that is being supported by the implants?  

Will the prosthesis be metal ceramic, all ceramic or resin and metal?

What are the dimensions of the frame?

What is the opposing occlusion?

What is the proximity of the most posterior tooth to the TMJ?

What is the muscular level of the patient?

Classic descriptions of 1.4 to 2.5 times the fulcrum to anterior abutment are obviously flawed because they are so simplified.  1.4 times might work when factors above are not as favorable while 2.5 might be fine if factors are quite favorable.

You have to also ask what it is that you are trying to prevent.  Indeed if you have a short and narrow posterior implant that is in a patient with little bone surrounding that implant and in a patient who exhibits bruxism and habitually chews at a "heavy" level then the implant itself might be at risk.  Now I have to say that I am not a believer in occlusal overload causing bone loss but that statement is based upon the notion that the occlusion is distributed among prosthetic and natural teeth, that there is not an obscene overload (the Isidor study showing implant failure secondary to overload was a primate study with 6 mm of high occlusion....this was excessive---{Isidor, 1996 }), that the prosthesis was made correctly and that it fit well.  If you are considering extreme hyper occlusion there is not much that I can say relative to maximum levels of force so there is not much to say about maximum cantilever level.

So if you are trying to prevent implant failure secondary to gross overload the first thing to consider is ways to reduce the gross overload.  If the patient is a habitual overloaded then any cantilever might be a problem.

If you are trying to prevent frame fracture then the material of the frame, dimensions of the frame, type of veneering material, etc all come into play.

Simple formulas are therefor oversimplified.  Must consider all variables thereby making the answer anything but simple.

So, what type of prosthesis? What dimensions of frame? What veneer?  Are you worried about implant failure, implant fracture, screw loosening, screw fracture, frame fracture, veneer fracture or some other concern?

I agree with Dr Eckert BUT to simplify the matter for the General Practitioner who is making a chairside clinical decision, the suggested parameter is a good rule to follow. 

I think it is important to remember that cantilevers have been used and continue to be used but we need specifics regarding the application that is being discussed.  

The simple formula of quantifying the distance from the fulcrum line to the anterior abutment on a bilaterally supported full arch prosthesis and multiplying this by a factor seems to be comforting.  Charlie English suggested the factor of 1.4 (this is the smallest factor so this is why it gets quoted) but if the frame is made of a weak material (Grade 1, 2, 3 or 4 CP Ti) this might be too much cantilever.  With 6-4 alloy it might work fine if the connectors are large enough.  

When Charlie talked about 1.4 x the world was simpler.  Frames were bulky and the veneer was resin.  Today we need to fill in a lot more blanks before offering an opinion and then we qualify so much that it sounds like there is no answer.  

I think Dr Freedman is correct, a suggested parameter is a good thing, we just need topic a parameter that is truly safe.  I like the English factor and look at it as a default but I still do Ti 6-4 alloy frames  and the anterior abutment to fulcrum line is usually long.  So for me the risk is frame fracture and know that a long cantilever (even if the formula allows) will take its toll on the integrity of the frame if overloaded because there will be a high stress at the connector to the terminal implant because of the lever arm (moment of force).

Thanks Dr.Eckert, very informative answers .

Having considered all factors to avoid damaging forces ensure that the cantilever arm is shorter than anchorage arm.

Never encouraged. Always try and avoid these over implants. Decreased success rates have been recorded !

http://www.ncbi.nlm.nih.gov/pubmed/23057026

It is very difficult to agree with the statements "Never encouraged.  Always try and avoid these over implants. Decreased success rates have been recorded !"  The research in biomechanics, cohort studies and in clinical case series do not support this rigid recommendation.

Assuming that we are discussing implant retained prostheses with bilateral support and a curvilinear pattern the data are pretty consistent and direct. Actually the science is pretty consistent.  Cantilevers do indeed work and there are few complications (caveat is that the implants are bilateral, the frame fits well, the dimensions of the frame are such that rigidity is maintained, that the cantilever is appropriate for the material of the metal frame, etc.) are observed.  Indeed there are exceptions but those exceptions usually fall into the realm of too much cantilever length, too small of frame dimensions, unfavorable frame material (CP Ti rather than 6-4 alloy), unacceptable fit of frame to implants, inappropriate insertion torque, inappropriate retentive screw material, debris eliminated from access chambers, etc.  When there is a problem, there is almost always a reason for the problem when cantilevers are used.  So if you play by the rules the results are quite predictable.