COMPARATIVE IN-VITRO STUDY OF CYLINDRICAL AND CONICAL PEDICLE SCREWS

Abstract

Introduction: With non-fusion stabilization surgery technique, the demands on the pedicle screw system have increased. The screw implants require a high press fit for optimum bone integration and a high pullout strength to guarantee primary stability. We compared the cylindrical screw design from a pedicle screw system with the conical design in regard to the insertional torque and pullout strength.

Methods: Three human cadaver specimens L1 – L5 (15 vertebrae, 30 pedicles) were fitted with pedicle screws on both sides. The pedicles were randomized to one of two screw types: 1) cylindrical pedicle screw, 2) conical. A computer tomographic bone density measurement was performed beforehand. The insertional torque was measured while inserting the pedicle screws. The correct position of the screw was verified using computer tomography. In order to test the pullout strength, the preparations were divided up into individual vertebrae and fixed.

Results: The mean peak insertional torque for the conical screws was significantly higher than that for the cylindrical screws. The pullout strength showed no significant difference between the two types of pedicle screws. With both the conical and the cylindrical pedicle screws, the pullout strength and insertional torque significantly decreased with decreasing bone density in the preparations.

Conclusions: Statements in the literature about the effect on insertional torque and pullout strength of using a conical or cylindrical pedicle screw design vary. The two screw designs in our study only differed in regard to their internal diameter. The significantly higher insertional torque values found for the conical screws could describe the improved screw press-fit behavior. The pullout strength was not significantly affected by the conical design. No correlation was found between the insertional torque and pullout strength. In order to optimize the adjustment of insertional torque and pullout strength, we recommend the conical pedicle screw design for non-fusion surgical techniques for the surgical treatment of degenerative diseases of the lumbar spine.