Divergent Configuration Improves Insertion Torque and Pullout Strength of Anterior Cervical Screws

Abstract

Anterior cervical discectomy and fusion (ACDF) is commonly performed with cancellous screws placed a 30o convergence. However, there is no consensus on the relationship between regional variation in vertebral body bone mineral density (BMD) and screw orientation for optimal stability and fusion. Twelve cervical (C6 and C7) and thoracic (T6 and T7) vertebrae were harvested from two fresh human cadavers (40- year-old male and 32-year-old female). BMD was measured using dual energy x-ray absorptiometry (DEXA), computerized tomography (CT), and Faxitron X-ray. Each vertebra was casted in epoxy resin and two self-tapping (4.2 mm diameter) cancellous screws of the same length were implanted for side-by-side comparison. One screw was placed at 30o convergence and another at 10o divergence. The screw length was measured to be 85% of the vertebral antero-posterior diameter. The insertion torque was measured using a torque dynamometer. The specimens were then mounted on a mechanical testing machine (Instron) for pullout testing. The central region of the cervical vertebrae was 14% less dense than the lateral region. Cervical convergent screws mean insertion torque, pullout strength, and stiffness were 58 N.cm, 1094 N, and 654 N/mm, respectively. Cervical divergent screws showed an increase in torque (32%), pullout strength (2%), and stiffness (10%). However, the center was denser and mechanically stronger than the periphery at the thoracic level. These preliminary data showed regional variations in biomechanical properties within the vertebral bodies of young adults. The lateral region of the cervical vertebrae was denser and stronger than the central one.