Performance of Ventral Spondylodesis Screws in Cervical Vertebrae of Varying Bone Mineral Density

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Abstract
Biomechanical parameters affecting the strength of screw-to-bone fixations of ventral cervical spondylodeses were determined. The rate of implant loosening with ventral cervical spondylodeses is high. Types of failure and how they can be avoided are presented. The number of sound studies on corresponding thoraco-lumbar spinal implants is large. However, no comparative study has been published thus far covering the strength of screw-to-bone fixation in the cervical region. Human cervical vertebrae were obtained at autopsy. Their bone mineral density related to calcium-hydroxyapatite was determined by single energy quantitative computed tomography scan. Standard cancellous screws with a diameter of 3.5 mm and 4.5 mm "rescue" screws were screwed down to failure into 43 single vertebral bodies. The applied torque T and the resultant axial force Fax at the "washer's" position were measured simultaneously by a specially designed electronic testing machine. A strong correlation among Fax, T, and bone mineral density was found (0.4636 < r < 0.7545). Application of standard screws reveals that Fax and T linearly respond to the effective thread length, whether the posterior vertebral cortex is engaged or not (paired t test: P < 0.05, n = 38). When "rescue" screws are used and the posterior vertebral cortex is included, a significantly higher torque T must be applied to achieve the same revolution. Surprisingly, the resulting force Fax hardly alters. If under similar circumstances the posterior vertebral cortex is not included in the construct, Fax is significantly lower (paired t test: P < 0.05, n = 32). A stable fixation of 3.5 mm screws cannot be achieved if bone mineral density remains below 150 mg/ml. Thus, determining bone mineral density before surgery by quantitative computed tomography is recommended to ensure a proper selection of screw type and thereby increase the success of surgical fixation.