All on Four Dental Implants
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.
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.
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.
For more information on this subject, please see our other page about all-on-four dental implants.