All-On-Four Dental Implants
The All-On-Four Dental Implants Technique was developed by a dentist in Portugal in cooperation with Nobel Biocare. It is an attempt to allow dentists to place an almost full arch of teeth (no molars are included) in patients with poor bone levels by using highly angled implants in only the anterior portion of the jaw, where there is more bone density. The idea is to provide a shortcut around bone grafting.
Dr. Chal has taken courses on the All-On-Four Technique. In fact, he is personally acquainted with Dr. Palo Malo, the dentist who developed the technique. They have had breakfast, lunch, and dinner together. This opportunity allowed Dr. Chal to ask Dr. Palo Malo, “who is the appropriate patient for his technique”. He answered, it is a person who did not want any form of simple or complex bone grafting. Often it is an elderly patient just looking for some greater stability than a conventional removable denture.
Dr. Chal believes patients should be cautions and well informed about the risks associated with this technique. His primary concern is the gigantic risk that if one of the dental implants fail, the entire restoration fails and has to be redone. A common saying at dental implantology conferences is “all on four but none on three”.
The diagram above (taken from a Nobel Biocare training manual) clearly shows that there are no molars in an All-On-Four restoration. Remember, back teeth (molars) do 80% of your chewing.
Here is a panographic x-ray of the four angled implants that have been placed.
A second concern when this technique is used is the jaw atrophy that occurs in the posterior region of the jaw. (For more information on this jaw atrophy, please see our page on facial collapse). This atrophy creates a risk of jaw fracture. Since the All-On-Four technique only places implants in the anterior of the jaw, the bone in the posterior is not stimulated and thus continues to be resorbed by the body. The presence of implants, on the other hand, has a piezoelectric effect that prevents bone resorption.
Another risk is the possible loosening of the screws of the prosthesis, leading to infection and failure.
Please see more information in this website on dental implant failure.
The all-on-four dental implants technique is promoted as a way to simplify providing dental implants for patients with no remaining natural teeth. Often these patients are missing a lot of bone. The theory is that by using only four dental implants, with two of them at extreme angles, the patient can eliminate the need for expensive and time-consuming bone grafting surgery.
You can see that the length of the threads of the all-on-four screw is half that of the conventional dental implant screw, and that the threads aren’t as deep. Over time, under the heavy stresses of chewing, these factors increase the risk of the screws coming loose. Dr. Carl E. Misch, author of the authoritative textbook, Dental Implant Prosthetics, lists twelve possible causes of screw loosening. Eleven of those twelve possible causes are related directly to the ability of the components of the dental implant system to resist the forces of the function of the replacement teeth (Misch, Dental Implant Prosthetics, p. 454). And a loose screw will lead to the trapping of foreign matter, subsequent infection, and the failure of the restoration.
Also notice that the diameter of the shank of the all-on-four screw is about 70% that of the conventional dental implant screw. This means that the cross-sectional area and thus the shear strength of the screw is slightly less than half that of the conventional screw. Over years of function under the heavy stresses of biting that are present in the mouth, these screws will be subject to metal fatigue and weakening.
The package has strong sales appeal. The patient can have the entire treatment done in one appointment. And it’s profitable for the dentist, because it commands a handsome fee without taking a lot of the dentist’s time.
Dr. Arthur Chal, a prominent implant dentist in Phoenix, has done extensive study of the all-on-four dental implants technique, and has had personal conversations with Dr. Palo Malo, the Portuguese dentist who developed it. He sees some weaknesses that would be helpful for prospective patients to know about. In some cases, the all-on-four technique may be appropriate for the patient. However there needs to be considerable judgment and discretion in its use. The patient also needs to be aware of the potential risks before agreeing to the treatment.
One of the weaknesses is in the structural strength of the restorative system. Dr. Chal seriously questions whether this treatment is appropriate for patients with large facial muscles that can generate heavy biting forces. Below is an enlarged photograph of a screw used to fasten a conventional implant to an implant overdenture, next to a screw used to fasten an all-on-four dental implant to its overdenture. The dime is used to help you judge the actual size.