Background Atlantoaxial instability is an important disorder that causes serious symptoms such as difficulties in walking, limited neck mobility, sensory deficits, etc. Atlantal lateral mass screw fixation is a surgical technique that has gained important recognition and popularity. Because accurate drilling area for screw placement is of utmost importance for a successful surgery, we aimed to investigate morphometry of especially the posterior part of C1. Methods One hundred and fifty-eight human adult C1 dried vertebrae were obtained. Measurements were performed directly on dry atlas vertebrae, and all parameters were measured by using a digital caliper accurate to 0.01 mm for linear measurements. Results The mean distance between the tip of the posterior arch and the medial inner edge of the groove was found to be 10.59 ± 2.26 and 10.49 ± 2.20 mm on the right and left, respectively. The mean distance between the tip of the posterior arch and the anterolateral outer edge of the groove was 21.27 ± 2.28 mm (right: 20.96 ± 2.22 mm; left: 21.32 ± 2.27 mm). The mean height of the screw entry zone on the right and left sides, respectively, were 3.86 ± 0.81 and 3.84 ± 0.77 mm. The mean width of the screw entry zone on both sides was 13.15 ± 1.17 and 13.25 ± 1.3 mm. Conclusion Our result provided the literature with a detailed database for the morphometry of C1, especially in relation to the vertebral artery groove. We believe that the data in the present study can help surgeons to adopt a more accurate approach in terms of accurate lateral mass screw placement in atlantoaxial instability.

Objectives: The aim of the study was to determine the soft tissue thickness overlying the dorsal tubercle and the relationship with adjacent anatomical structures in the distal radius for using this area as an alternative intraosseous route. Methods: Contrast-enhanced MR images of 56 adult patients (28 females, 28 males) without any wrist pathology were evaluated. The shape of dorsal tubercle and its relations with neighboring tendons and vessels with a diameter larger than 2 mm was identified on the axial T1-weighted sections. The soft tissue thickness above the most protruding point of the dorsal tubercle, the distance of the dorsal tubercle to closest tendon on the radial and ulnar sides, as well as its distance to the bone edges on the ulnar and radial sides, and the cortical bone thickness of the radius was evaluated. Results: The dorsal tubercle had sharp edges in 40 cases (71.4%), blunt in 12 cases (21.4%), and hump in 4 (%7.1) cases. Branches of dorsal venous plexus were found on its surface in 11 cases, extensor pollicis longus tendon only was found superficial to the dorsal tubercle in 7 cases while both extensor pollicis longus and dorsal venous branches were found in 2 cases. Conclusion: Dorsal tubercle of the distal radius can be considered as an important alternative route for IO infusions since it can be easily accessed without having a risk of injury to important structures, and can provide effective flow.