title: Ideal screw entry point and projection angles for posterior lateral mass fixation of the atlas: an anatomical study.
abstract: Although various posterior insertion angles for screw insertion have been proposed for C1 lateral mass, substantial conclusions have not been reached regarding ideal angles and average length of the screw yet. We aimed to re-consider the morphometry and the ideal trajections of the C1 screw. Morphometric analysis was performed on 40 Turkish dried atlas vertebrae obtained from the Department of Anatomy at the Medical School of Ankara University. The quantitative anatomy of the screw entry zone, trajectories, and the ideal lengths of the screws were calculated to evaluate the feasibility of posterior screw fixation of the lateral mass of the atlas. The entry point into the lateral mass of the atlas is the intersection of the posterior arch and the C1 lateral mass. The optimum medial angle is 13.5 +/- 1.9 degrees and maximal angle of medialization is 29.4 +/- 3.0 degrees . The ideal cephalic angle is 15.2 +/- 2.6 degrees , and the maximum cephalic angle is 29.6 +/- 2.6 degrees . The optimum screw length was found to be 19.59 +/- 2.20 mm. With more than 30 degrees of medial trajections and cephalic trajections the screw penetrates into the spinal canal and atlantooccipital joint, respectively. Strikingly, in 52% of our specimens, the height of the inferior articular process was under 3.5 mm, and in 70% was under 4 mm, which increases the importance of the preparation of the screw entry site. For accommodation of screws of 3.5-mm in diameter, the starting point should be taken as the insertion of the posterior arch at the superior end of the inferior articular process with a cephalic trajection. This study may aid many surgeons in their attempts to place C1 lateral mass screws.
authors: Ministry of Health, Ankara Diskapi Training and Research Hospital, 2nd Neurosurgery Clinic, Sehit Osman Avci Mah., 46645 Ada, B28, KC Lale Evleri, Eryaman, Ankara, Turkey.
authors: Serkan
authors: S
authors: Simsek
authors: Kazim
authors: K
authors: Yigitkanli
authors: Hakan
authors: H
authors: Seçkin
authors: Ayhan
authors: A
authors: Comert
authors: Halil I
authors: HI
authors: Acar
authors: Deniz
authors: D
authors: Belen
authors: Ibrahim
authors: I
authors: Tekdemir
authors: Alaittin
authors: A
authors: Elhan
mesh_terms: D000886:Anthropometry; D001269:Atlanto-Occipital Joint / Q000033:anatomy & histology / Q000601:surgery; D001863:Bone Screws / Q000592:standards*; D002102:Cadaver; D001270:Cervical Atlas / Q000033:anatomy & histology* / Q000601:surgery*; D004867:Equipment Design; D005593:Fracture Fixation, Internal / Q000295:instrumentation / Q000379:methods; D006801:Humans; D016268:Internal Fixators / Q000592:standards*; D004204:Joint Dislocations / Q000601:surgery; D007593:Joint Instability / Q000601:surgery; D008422:Materials Testing; D016343:Monitoring, Intraoperative; D019838:Neck Injuries / Q000601:surgery; D011183:Postoperative Complications / Q000517:prevention & control; D019919:Prosthesis Implantation / Q000295:instrumentation / Q000379:methods; D013115:Spinal Canal / Q000033:anatomy & histology / Q000601:surgery; D016103:Spinal Fractures / Q000601:surgery; D013123:Spinal Fusion / Q000295:instrumentation* / Q000379:methods; D013124:Spinal Injuries / Q000601:surgery; D013314:Stress, Mechanical