Does the direction of pedicle screw rotation affect the biomechanics of direct transverse plane vertebral derotation?
METHODS
In vitro biomechanical investigation using human cadaveric vertebrae.


OBJECTIVE
Evaluate the biomechanical differences in transverse plane vertebral body derotation maneuvers of thoracic pedicle screws in both medial and lateral directions.


BACKGROUND
Thoracic pedicle screws are thought to have better vertebral rotation control and better segmental scoliosis correction compared to hooks and wires. Little data exists regarding the biomechanical stability of pedicle screws when derotated in either medial or lateral directions.


METHODS
Vertebral bodies (T4-L5) from 12 cadavers were instrumented with appropriate length pedicle screws while measuring insertion torque. Each body was anchored for independent loading in medial or lateral directions. Each screw was rotated around a rod using a constant length lever arm (30.5 cm) rigidly attached to the screw head simulating the posterior vertebral derotation maneuver. Yield torques (Nm) were analyzed using a one-way analysis of variance (P < 0.05).


RESULTS
Yield torques for both directions were significantly related to screw insertion torque (both P < 0.01). There were no statistical differences in yield torque between medial (12.0 +/- 4.9 Nm) or lateral (11.5 +/- 5.1 Nm) directions. There were no significant differences after normalization for insertion torque or screw length. Tests rotating the screw tip laterally demonstrated structural failure in the following percentages (anterolateral failure = 67%, posterior element failure = 33%, additional screw bending = 42%). Rotation medially demonstrated structural failures in the following percentages (canal penetration = 51%, posterior element failure = 49%, additional screw bending = 44%).


CONCLUSIONS
From these data, a surgeon performing a direct vertebral derotation using a 30 cm (12 in) lever would need to apply roughly 40 N (9 lbs) to causeanatomic failure. Adolescent patients would likely tolerate a greater force without bone failure given a greater bone density, yet, extreme caution is still recommended to prevent screw rotation either medially into the spinal canal or laterally into the chest.
Parent
Stefan
S
Orthopaedics Unit, Ste Justine Hospital, Montreal, Quebec, Canada.
Odell
Tim
T
Oka
Richard
R
Mahar
Andrew
A
Newton
Peter
P
D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D001696:Biomechanical Phenomena / Q000502:physiology*; D001863:Bone Screws / Q000009:adverse effects; D006801:Humans; D008159:Lumbar Vertebrae / Q000502:physiology* / Q000601:surgery; D008875:Middle Aged; D012399:Rotation / Q000009:adverse effects; D013904:Thoracic Vertebrae / Q000502:physiology* / Q000601:surgery; D019415:Torque