In a recent special-topic section of the European Journal of Oral Implantology (Volume 7, supplement 2), the editors presented a report of a consensus conference on the optimal number of implants in the treatment of edentulism. On those pages, John Brunski (Senior Research Engineer at Stanford University and Professor Emeritus at Rensselaer Polytechnic Institute) approached the topic from a biomechanical perspective. He summarizes his findings here.
Figure 1. In the finite element (FE) models presented here, the distal ends of the bar are loaded bilaterally by 100 N vertically-downward forces. Please note that the ends of the mandibles are constrained in these FE simulations, as in the other images below.
Since it was first put into practice in 1998 by Dr. Paulo Malo, the All-on-4® treatment concept approach has been used successfully around the world. A large literature supports the approach (see references below).
Some of the more biomechanically-focused papers about the All-on-4® treatment concept are also considered in recent articles in Nobel Biocare News. Nobel Biocare’s summary of the physiological benefits of using the All-on-4® approach is as follows:
“By tilting the two posterior implants, longer implants can be used. This increases bone-to- implant contact and avoids vertical bone augmentation. In addition, the tilted implants can be anchored in better quality anterior bone, reduce cantilevers, and help avoid important anatomical structures.”
It’s worth taking a closer look at some of the biomechanical details alluded to in the above statement of Nobel Biocare’s rationale.
For example, some results from finite element (FE) computer models (Figure 1) indicate ways in which a typical All-on-4® treatment concept approach has important advantages over an upright 4-implant approach.
First, some degree of distal tilting of the distal implants (e.g., 30 degrees in this example) in the All-on-4® case has the effect of reducing the length of the cantilever segments of a typical prosthetic bar.
(The cantilever segment is the region of the bar between the most distal implant and the distal end ofthe bar. The bar in this FE simulation is about 4 mm in thickness and 6 mm in width, which is similar to bars in clinical use.)
Consequently, when the distal ends of the bar are loaded bilaterally by 100 N vertically-downward forces, the All-on-4® case shows a substantial reduction (relative to the “upright” 4-implant case) in the maximum tensile bending stresses on the top surface of the bar—e.g., from about 80 MPa to 45 MPa (Figure 2, column A).
Figure 2. Comparative stresses in different locations of the bars, implants and abutments of an upright 4-implant model vis-a-vis an All-on-4® treatment concept model. Key: Red = Upright 4-implant. Blue = All-on-4® treatment concept.
For peace of mind
Now, this is a significant reduction in stress; since the above FE model is linear, a tripling of the 100 N bilateral loading to 300 N—which is a moderately strong but not uncommon level of biting force—causes the maximum tensile stresses in bars of the All-on-4® and “upright” cases to increase to 240 MPa and 135 MPa, respectively.
Assuming the bars were made of commercially pure Titanium—with a fatigue endurance limit of about 300 MPa—stresses of about 240 MPa in the “upright” case would start to provoke concern about potential fatigue fracture of the bar, whereas stresses of about 135 MPa in the All-on-4® case would not cause concern.
This result illustrates an advantage of the All-on-4® arrangement insofar as lowering stresses that relate to fatigue fracture of bars. (The stress reduction also helps in avoiding delamination of acrylic over the bar.)
What’s more, an All-on-4® approach can also decrease stresses in the supporting implants. For instance, the maximum tensile bending stresses in the distal (Figure 2, column B) and anterior (Figure 2, column C) implants are lower for the All-on-4® case compared to the “upright” 4-implant case.
While this idealized FE model doesn’t account for the detailed internal structure of screw joints, implants, and abutments in the system, it nevertheless provides insight into how the All-on-4® arrangement can lead to lower stresses in the implants and related parts.
When it comes to the stresses at the sites where the implant abutments connect to the undersurface of the bar, the All-on-4® situation also shows lower stresses than in the “upright” case (Figure 2, column D).
Figure 3. How forces on all four implants change depending on the distal tilt of the posterior implant. (Red = No tilt).
Reduced stress on all implants
It is useful to look at the vertical forces that develop on the implants together with related stresses and strains in interfacial bone for the All-on- 4® vs. “upright” cases.
Concerning the vertical forces: For the same bilateral loading on a bar supported by All-on-4® vs. “upright” implants (see Figure 3), the vertical force on each implant is actually less in the All-on-4® case. (This result is predicted by FE methods as well as by Skalak’s analytical model4)
Concerning the stresses and strains in the bone: Generally speaking, smaller forces on an implant mean smaller stresses and strains in the bone around the implant. This follows from the general notion in mechanics that stress is equal to a load (force) divided by the load-supporting area. (This is why stress has the dimensions of force per unit area, and is measured in units such as the megapascal, MPa, which equals 1 Newton per square millimeter, i.e. N/mm2.)
Therefore, if one has two implants with the same bone-implant contact area, but one implant is heavily-loaded and the other is lightly-loaded, the implant with the lighter load will produce the smaller stress and strain in the surrounding bone. And since excessively large stresses and strains lead to bone microdamage, it’s an advantage to limit interfacial stresses and strains to levels below such danger thresholds.
The detailed analyses behind this issue are complicated, but the general notion of a) using slightly longer, tilted implants (as noted in Nobel Biocare’s remark about the All-on 4® treatment concept at the start of this article) plus b) reducing the loading per implant (as explained above) is an advantageous way to decrease interfacial stresses and strains.
More to explore
1 Krekmanov, L., et al., Tilting of posterior mandibular and maxillary implants for improved prosthesis support. Int J Oral Maxillofac Implants, 2000. 15(3): p. 405-14.
2 Aparicio, C., P. Perales, and B. Rangert, Tilted implants as an alternative to maxillary sinus grafting: a clinical, radiologic, and periotest study. Clin Implant Dent Relat Res, 2001. 3(1): p. 39-49.
3 Malo, P., B. Rangert, and M. Nobre, “All-on- Four” immediate-function concept with Branemark System implants for completely edentulous mandibles: a retrospective clinical study. Clin Implant Dent Relat Res, 2003. 5 Suppl 1: p.
4 Brunski, J.B. “Biomechanical aspects of the optimal number of implants to carry a cross-arch full restoration” Eur J Oral Implantol 2014;7(Suppl):S111–S132.