Background: Posterior C1 ring-osteosynthesis (C1-RO) as treatment for displaced Jefferson fractures is a promising C1-C2 function-preserving operation but not yet a common method. Since the first description of anterior C1-RO by Ruf et al., 11 studies were published and included 50 cases of posterior C1-RO, 40 cases of anterior C1-RO and 8 cases combined anterior and posterior osteosynthesis. Two biomechanical studies have shown that even in cases of ruptured transverse ligament, reduction followed by C1-RO can lead to C1/C2 stability without need for C1/C2 fixation. Methods: In this work we present two cases of displaced Jefferson burst fractures who were treated with closed reduction followed by posterior C1-ring Osteosynthesis. We present a description of the intra-operative manual assessment of atlantoaxial stability. Results: The surgical plan involved fluoroscopically controlled repositioning and fixation of the fracture over a Mayfield clamp in the typical prone position and inclination, followed by 180-degree scan with 3D reconstruction to assess the reduction of the atlas. Then fluoroscopically controlled insertion of 3.5 mm lateral mass polyaxial shaft-screws bicortically. Compression was applied over a 3.5-mm transverse rod. C1/C2 stability was manually assessed intraoperatively to assure sufficient stability of the C1/C2 complex after reduction and osteosynthesis. This was achieved by simultaneously applying manual pressure on the transverse rod applied by a clamp and pulling on the C2 spinous process with an attached Kocher clamp under dynamic fluoroscopic control. Postoperative flexion-extension images of the cervical spine were performed to prove atlantoaxial stability. Conclusion: In a case of isolated unstable Jefferson fracture, a surgeon familiar with a C1/C2 fixation according to Harms/Goel might start with repositioning and C1-ring fixation and only add C2 screws if the C1/C2 segment fails to provide stability intraoperatively.