PERFORMANCE OF VENTRAL SPONDYLODESIS SCREWS IN CERVICAL-VERTEBRAE OF VARYING BONE-MINERAL DENSITY

Study Design. Biomechanical parameters affecting the strength of screw -to-bone fixations of ventral cervical spondylodeses were determined. Objectives. The rate of implant loosening with ventral cervical spondy lodeses is high. Types of failure and how they can be avoided are pres ented. Summary of Background Data. The number of sound studies on corr esponding thoraco-lumbar spinal implants is large. However, no compara tive study has been published thus far covering the strength of screw- to-bone fixation in the cervical region. Methods. Human cervical verte brae were obtained at autopsy. Their bone mineral density related to c alcium-hydroxyapatite was determined by single energy quantitative com puted tomography scan. Standard cancellous screws with a diameter of 3 .5 mm and 4.5 mm ''rescue'' screws were screwed down to failure into 4 3 single vertebral bodies. The applied torque T and the resultant axia l force F-ax at the ''washer's'' position were measured simultaneously by a specially designed electronic testing machine. Results. A strong correlation among F-ax, T, and bone mineral density aws found (0.4636 < r < 0.7545). Application of standard screws reveals that F-ax and T linearly respond to the effective thread length, whether the posterio r vertebral cortex is engaged or not (paired t test: P < 0.05, n = 38) . When ''rescue'' screws are used and the posterior vertebral cortex i s included, a significantly higher torque T must be applied to achieve the same revolution. Surprisingly, the resulting force F-ax hardly al ters. If under similar circumstances the posterior vertebral cortex is not included in the construct, F-ax 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. Conclusions. Thus, determining bone mineral density before surgery by quantitative computed tomography is recommended to ensure a proper selection of sc rew type and thereby increase the success of surgical fixation.