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DOI:    [Accepted]
Published online October 14, 2018.
Biomechanical and Anatomical Validity of the Short Posterior Arch Screw
Jun Mizutani1  , Nozomu Inoue2, Yoshihisa Otsuka1, Aiharu Furuya1, Alejandro A. Espinoza Orias2, Takanobu Otsuka1
1Department of Orthopaedic Surgery, Nagoya City University Graduate School of Medical Sciences, Nagoya, Japan
2Department of Orthopaedic Surgery, Rush University Medical Center, Chicago, IL, USA
Corresponding Author:  Jun Mizutani
Tel: +81-52-853-8236   Fax: +81-52-842-0266   Email:
Received: July 9, 2018   Revised: August 29, 2018   Accepted: August 31, 2018
This study was conducted to clarify the validity of the short posterior arch screw (S-PAS). S-PAS is inserted only in the pedicle analogue portion of the posterior arch. Compared to conventional C1 lateral mass screws inserted via the posterior arch (via-PAS), the S-PAS screw length is almost half that of via-PAS. S-PAS reduces the risk of VAI because it never reaches the transverse foramen. Although the biomechanical validity of various C1 lateral mass screws (C1LMS) analyzed in young specimens are published, that of uni-cortically inserted C1LMS such as uni-cortical Harms screw, S-PAS, and via-PAS for elderly patients is concerning because of osteoporosis.
Nine fresh frozen cadavers (average age at death, 72.1 years) were used for pullout testing. Bone mineral density for each specimen was evaluated using quantitative computed tomography.
Pullout strength of via-PAS (1048.5 N) was statistically stronger than that of uni-cortical Harms screw (257.9N) (p<0.05). That of S-PAS was 720.3 N and also statistically stronger than uni-cortical Harms screw (p<0.05).
Via-PAS and S-PAS offers valid surgical options even in elderly patients. Along with sufficient biomechanical strength, S-PAS screw prevents VAI.
Keywords: lateral mass screw of the atlas; vascular complications; biomechanical strength; vertebral artery and internal carotid artery; osteoporosis; atlanto-axial fixation

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