0
Articles   |    
Biomechanical Study of Screws in the Lateral Masses: Variables Affecting Pull-out Resistance*
JOHN G. HELLER, M.D.†; BRADLEY T. ESTES, M.S.†, DECATUR, GEORGIA; MOUNIR ZAOUALI, PH.D.‡, RANG-DU-FLIERS; AMADOU DIOP, PH.D.§, PARIS, FRANCE
View Disclosures and Other Information
Investigation performed at the Department of Biomechanical Engineering, École Nationale Supérieure des Arts et Métiers, Paris
J Bone Joint Surg Am, 1996 Sep 01;78(9):1315-21
5 Recommendations (Recommend) | 3 Comments | Saved by 3 Users Save Case

Abstract

The purpose of this study was to investigate the effects of the design of the screw, the depth of insertion, the vertebral level, and the quality of the host bone on the pull-out resistance of screws used in the lateral masses. The study included twelve fresh cervical spines from human cadavera. Radiographs were made of each specimen to ensure the absence of defects, and then the cancellous-bone density of the vertebral bodies was measured at each level with quantitative computed tomography scanning. Six commercially available screws of various diameters and thread configurations (2.7, 3.2, 3.5, and 4.5-millimeter cortical-bone screws; a 3.5-millimeter cancellous-bone screw; and a 3.5-millimeter self-tapping screw) that are currently used for fixation of the cervical lateral masses were tested for axial load to failure. A twelve-by-twelve Latin square design was used to randomize the screws with regard to level (second through seventh cervical vertebrae), side (right and left), and depth of insertion (unicortical or bicortical purchase). Each screw was then subjected to uniaxial load to failure. The data were analyzed to determine if the diameter of the screw, the thread configuration, the number of cortices engaged, the cervical level, or the bone density was associated with the load to failure.Three major subgroups (greatest, intermediate, and lowest pull-out resistance) were identified. The subgroup with the greatest pull-out resistance included only screws with bicortical purchase; the 3.2, 3.5, and 4.5-millimeter cortical-bone screws and the 3.5-millimeter cancellous-bone screw were in this subgroup. Regardless of the thread configuration, no screw with unicortical purchase was in the group with the greatest pull-out resistance. Two of the three values in the subgroup with the lowest pull-out resistance were for the 3.5-millimeter self-tapping screw (with unicortical or bicortical purchase). The cancellous-bone density of the vertebral body was not associated with pull-out resistance and it did not vary significantly according to the cervical level, with the numbers available. However, the pull-out resistance of the screws varied significantly (p = 0.004) by level: it was the greatest at the fourth cervical level, decreasing cephalad and caudad to that level.CLINICAL RELEVANCE: Posterior plate fixation of the cervical spine with screws inserted into the lateral masses may be appropriate in certain circumstances. Previous studies on cadavera and clinical experience have attested to the potential pitfalls of fixation with screws in the lateral masses. Our data suggest that the surgeon should consider not only the type and size of the screws but also whether unicortical or bicortical purchase should be achieved. Bicortical purchase engenders a greater risk of injury to local anatomical structures, but this may be an acceptable compromise at the cephalad and caudad regions of the cervical spine, where the purchase of screws is relatively weaker.

Figures in this Article
    Sign In to Your Personal ProfileSign In To Access Full Content
    Not a Subscriber?
    Get online access for 30 days for $35
    New to JBJS?
    Sign up for a full subscription to both the print and online editions
    Register for a FREE limited account to get full access to all CME activities, to comment on public articles, or to sign up for alerts.
    Register for a FREE limited account to get full access to all CME activities
    Have a subscription to the print edition?
    Current subscribers to The Journal of Bone & Joint Surgery in either the print or quarterly DVD formats receive free online access to JBJS.org.
    Forgot your password?
    Enter your username and email address. We'll send you a reminder to the email address on record.

     
    Forgot your username or need assistance? Please contact customer service at subs@jbjs.org. If your access is provided
    by your institution, please contact you librarian or administrator for username and password information. Institutional
    administrators, to reset your institution's master username or password, please contact subs@jbjs.org

    References

    Accreditation Statement
    These activities have been planned and implemented in accordance with the Essential Areas and policies of the Accreditation Council for Continuing Medical Education (ACCME) through the joint sponsorship of the American Academy of Orthopaedic Surgeons and The Journal of Bone and Joint Surgery, Inc. The American Academy of Orthopaedic Surgeons is accredited by the ACCME to provide continuing medical education for physicians.
    CME Activities Associated with This Article
    Submit a Comment
    Please read the other comments before you post yours. Contributors must reveal any conflict of interest.
    Comments are moderated and will appear on the site at the discretion of JBJS editorial staff.

    * = Required Field
    (if multiple authors, separate names by comma)
    Example: John Doe





    Related Content
    The Journal of Bone & Joint Surgery
    JBJS Case Connector
    Topic Collections
    Related Audio and Videos
    Clinical Trials
    Readers of This Also Read...
    JBJS Jobs
    03/05/2014
    Oklahoma - The University of Oklahoma
    05/03/2012
    California - UCLA/OH Department of Orthopaedic Surgery
    11/15/2013
    Louisiana - Ochsner Health System
    01/22/2014
    Pennsylvania - Penn State Milton S. Hershey Medical Center