Insertion torque profile during pedicle screw insertion of the thoracic spine with and without violation of the pedicle wall: comparison between cylindrical and conical designs.
METHODS
Biomechanical study in human cadaveric thoracic spines.


OBJECTIVE
To compare the insertion torque profile of cylindrical and conical screws with normal insertion tract, and with violation of the pedicle.


BACKGROUND
Previous studies revealed higher insertion torque for conical screws. No study has investigated the torque profile of both screws when there is violation of the pedicle.


METHODS
The pedicle screws were inserted intrapedicularly (Type 1), violating the lateral cortex of the pedicle (Type 2), and violating the medial cortex of the pedicle (Type 3). Type 2 screws were further subdivided into those having contact with the lateral vertebral body (2a) and those without (2b). Type 3 screws were subdivided into those having contact with the medial pedicle wall (3a) and without (3b). RESULTS.: During initial insertion, the torque increased as they penetrated into the pedicles. With violation of the lateral cortex of the pedicle, insertion torque for the cylindrical screw decreased as the screw advanced anteriorly out of the pedicle. The insertion torque for the conical screw continued to increase at a constant rate after violation of the lateral cortex. With medial breach of the pedicle wall, torque profile of the conical screws remained the same as those inserted in the normal trajectory. There was a decline in the insertion torque for the cylindrical screws when medial violation of the pedicle wall occurred, the decrease being greater for Type 3b as compared with Type 3a screws.


CONCLUSIONS
Surgeons may be unable to perceive the occurrence of medial or lateral breach of the cortex when inserting a conical screw. Surgeons inserting cylindrical screws with Type 3a violation may not be able to perceive a significant decrease in torque.
Hee
Hwan T
HT
Department of Orthopaedic Surgery, National University of Singapore, Singapore. doshht@nus.edu.sg
Khan
Mohammad Shazad
MS
Goh
James C
JC
Wong
Hee K
HK
D001863:Bone Screws / Q000592:standards; D004867:Equipment Design / Q000295:instrumentation / Q000379:methods; D006801:Humans; D016268:Internal Fixators / Q000592:standards; D008422:Materials Testing / Q000295:instrumentation* / Q000379:methods; D011859:Radiography; D013904:Thoracic Vertebrae / Q000000981:diagnostic imaging / Q000502:physiology* / Q000601:surgery; D019415:Torque