Biomechanical evaluation of pedicle screws versus pedicle and laminar hooks in the thoracic spine.
BACKGROUND
Pedicle screws have been shown to be superior to hooks in the lumbar spine, but few studies have addressed their use in the thoracic spine.


OBJECTIVE
The objective of this study was to biomechanically evaluate the pullout strength of pedicle screws in the thoracic spine and compare them to laminar hooks. STUDY DESING/SETTING: Twelve vertebrae (T1-T12) were harvested from each of five embalmed human cadavers (n=60). The age of the donors averaged 83+8.5 years. After bone mineral density had been measured in the vertebrae (mean=0.47 g/cm(3)), spines were disarticulated. Some pedicles were damaged during disarticulation or preparation for testing, so that 100 out of a possible 120 pullout tests were performed.


METHODS
Each vertebra was secured using a custom-made jig, and a posteriorly directed force was applied to either the screw or the claw. Constructs were ramped to failure at 3 mm/min using a Mini Bionix II materials testing machine (MTS, Eden Prairie, MN).


RESULTS
Pedicle claws had an average pullout strength of 577 N, whereas the pullout strength of pedicle screws averaged 309 N. Hooks installed using the claw method in the thoracic spine had an overwhelming advantage in pullout strength versus pedicle screws. Even in extremely osteoporotic bone, the claw withstood 88% greater pullout load.


CONCLUSIONS
The results of this study indicate that hooks should be considered when supplemental instrumentation is required in thoracic vertebrae, especially in osteoporotic bone.
Cordista
Andrew
A
Department of Orthopaedics and Rehabilitation, University of Florida, 1600 SW Archer Road, Gainesville, 32610, USA.
Conrad
Bryan
B
Horodyski
MaryBeth
M
Walters
Sheri
S
Rechtine
Glenn
G
D001696:Biomechanical Phenomena; D001863:Bone Screws; D002102:Cadaver; D004867:Equipment Design; D006801:Humans; D008422:Materials Testing; D013314:Stress, Mechanical; D013524:Surgical Flaps; D013904:Thoracic Vertebrae / Q000601:surgery*