All-on-4 refers to utilizing 4 implants per arch to replace a missing set of teeth. The technique was developed to achieve lower cost and immediate teeth placed the day implants are placed. It can be done when there is adequate bone to place the four implants. Usually this is the case when teeth are being extracted, or their still is a fair amount of bone left in an edentulous jaw bone (no teeth). In the upper jaw, it is not used in cases of extreme atrophy of the jaw bone.
In an effort to decrease the cost of replacing all the teeth in a dental arch, the all on 4 technique has been developed. By angling the implants up to 30 degrees in a front-back direction, it spreads them out, allowing a more even distribution of bite forces on just 4 implants. In some cases it allows the elimination of bone grafting procedures.
With all-on-4, the 4 implants per arch are placed. A temporary prosthesis is usually fashioned from a denture to fit on the implants, and 3-6 months later you get a final set of teeth. The teeth typically do not extend much further back than a second premolar, to prevent cantilever forces on the prosthesis, which could break it.
There are some compromises with the all-on-4 technique. With the All-on-4 technique, long-term success for implant maintenance at only 10 years is low — at about 94%. Studies do not exist for longer periods. In addition, the prosthesis with the fewer implants also typically requires more repairs. In addition, the angled implants require costly angled abutments, utilize small screws with lower torque values, breaking more often and requiring repairs.
An additional compromise is the prosthesis typically has less teeth, maybe 10 per arch instead of 12-14. This is one of the ways to improve longevity — by decreasing the span of teeth.
The small number of implants — 4 — can lead to overloading of the implants during their initial integration phase, and lead to loss of the prosthesis. The same can occur with the final prosthesis. In the upper arch, the maxilla, this decreases the overall longevity and long-term success of the procedure, compared to when 6 or 8 implants are placed.
For the above reasons, typically we aim for 6 implants per dental arch — to replace a full set of teeth on either the upper or lower arch. Now the catch term is “All-on-X”, since it ranges most often from 4-to-6 dental implants per arch. Dr. Loetscher typically plans for 5 or 6 implants per arch — however there are certain situations where 4 implants will be adequate.
Typical prosthetic guidelines as determined by engineering analysis usually calls for 5 or 6 implants in the lower arch, and 6 implants in the upper arch. With the additional implants being placed posteriorly in the molar area where the large bite forces exist. With these traditional reconstructions using 6 lower and 6 upper implants, long-term success after 25 years is high, 98-99% for being trouble free. The usual issue is the teeth wearing out after 20 plus years. For this reason, this becomes the better value than the all-on-4. The All-on-4 is an effective functional solution if you understand the shortcomings and likely need to eventually rebuild your arch of teeth in 8-12 years, or sooner, placing additional implants as well.