BIOMECHANICAL ANALYSIS OF BONE-MINERAL DENSITY, INSERTION TECHNIQUE, SCREW TORQUE, AND HOLDING STRENGTH OF ANTERIOR CERVICAL PLATE SCREWS

The bone mineral density (BMD) of 99 cadaveric cervical vertebral bodi es (C3-7) was determined using dual x-ray absorptiometry. The vertebra l bodies were randomly assigned to receive either a unicortical (51 bo dies) or bicortical (48 bodies) Caspar cervical plating screw. The ini tial insertion torque was measured using a digital electronic torque w rench, and the force required to withdraw the screw from the vertebral body was determined. The mean BMD for the total group of 99 was 0.787 +/- 0.154 g/cm(2), the mean insertion torque was 0.367 +/- 0.243 newt on-meters, and the mean pullout force was 210.4 +/- 158.1 newtons. A s ignificant correlation was noted between BMD and torque (p < 0.0001, r = 0.42), BMD and pullout force (p < 0.0001, r = 0.54), and torque and pullout force (p < 0.0001, r = 0.88). Although the BMD of the unicort ical and bicortical groups was equivalent (p = 0.92), the insertion to rque and pullout force differed significantly (p = 0.02 and p = 0.008, respectively) for the unicortical and bicortical groups. A holding in dex for each screw and insertion technique was defined as the product of the BMD and insertion torque. The calculated holding index and resu ltant pullout force were significantly correlated for both techniques of screw insertion (r = 0.92), and a significant difference in holding index was observed with unicortical versus bicortical screw placement (p = 0.04). The determination of BMD and measurement of insertion tor que to create a unique holding index provides an assessment of bone-sc rew interaction and holding strength of the screw, both of which impac t on the resultant stability of cervical instrumentation. As the numbe r of cervical plating systems increases, the determination of a holdin g index for various screws and insertion techniques may assist in the comparison of cervical instrumentation.