Measurement of the stresses that occur in the bone around the posterior angled implants caused by bucco-palatal angle deviations in the All-on-four concept applied in the maxilla with finite element analysis

Dental implants are a common treatment option used in order to treat patients with tooth loss. Providing the patient with fixed restorations, especially in fully edentulous patients, is made possible with implants. In fully edentulous patients with insufficient alveolar ridge volume or a pneumatized maxillary sinus, various augmentation techniques or sinus augmentation surgery may be required before implant surgery could be performed. This results in prolongment of both the duration of the surgery and the treatment process. Recently, the All-on-four treatment protocol, a relatively new treatment alternative, was developed to prevent these unfavorable situations and it has become more frequently applied by dentists. The All-on-four method applied to the maxilla is a form of treatment in which four implants are placed: two at a distal angle in the posterior region, parallel to the morphology of the anterior wall of the maxillary sinus, and two in a vertical direction in the anterior region. In this treatment protocol, it is difficult to place the implants at the correct angle and position. And because guided surgeries are not widespread enough, implants may be positioned differently than planned before the surgery. This changes the amount of stress on the implant and peri-implant bone under chewing forces. Nowadays, in addition to the clinical studies performed on the All-on-four treatment, many studies are carried out with finite element analysis. In the present study, implants were placed in the maxilla models created by finite element analysis according to the All-on-four concept. Nine models were created by insertion of the posterior implants that are mesiodistally angled 17, 30 and 45 degrees in buccally and palatally angled positions and vertical position in the buccopalatal direction. In the models created, 150 Newtons in the vertical and oblique directions, thus a total of 300 Newtons force was applied to the cantilever part of the prosthesis placed on the implants in order to measure the stresses in the peri-implant bone and perform the bone resorption risk analysis. The findings of the study revealed that the main factor affecting the stress values in the peri-implant bone was associated with the relationship between the long axis of the implants and the direction of the force. It has been determined that as the angle of the implants in the mesiodistal direction increases, the risk of resorption increases. Moreover, in the implant models placed at the same angle mesiodistally, the stress in the peri-implant bone and the resorption risk values were measured higher in the implants of which the apices were angled palatally.