In patients who have lost their posterior teeth many years ago, the maxillary sinus cavities in the upper jaw sag downwards, while in the lower jaw, the main nerve canal (inferior alveolar nerve) approaches the very top border of the bone. In this situation, placing vertical implants in the posterior regions becomes impossible. In All-on-4 or All-on-6 concepts, to avoid these anatomically dangerous structures in the posterior region, implants are placed not vertically, but at angles of approximately 30 to 45 degrees in the posterior region. The biomechanical rationale for this angled placement is: to ensure that the implants attach to the more anterior and harder (cortical) bone tissue of the jawbone, to distribute the chewing loads in the posterior region evenly across the entire jaw by widening the distance between the implants (emergence profile), and to allow the patient to have a fixed prosthesis fitted on the same day without undergoing posterior bone grafting (sinus lift) surgery.
