Torsional rigidity of scoliosis constructs.
METHODS
A biomechanical study of the rigidity of various scoliosis constructs instrumented with and without caudal pedicle screw anchors and with none, one, or two cross-link devices.


OBJECTIVE
To determine whether the increased torsional rigidity provided by distal pedicle screw fixation might make cross-linking unnecessary.


BACKGROUND
Pedicle screws and cross-linking devices have been shown to increase the structural rigidity of spinal constructs. Their relative contributions to scoliosis construct rigidity has not been determined.


METHODS
"Short" (T2-T11) and "long" (T2-L3) scoliosis constructs were mounted on an industrially fabricated spine model and tested in a hydraulic testing machine. Four different short and four different long constructs were tested: hooks only, hooks with concave side thoracic sublaminar wires, hooks with distal pedicle screw anchors, and hooks, distal pedicle screw anchors, and concave thoracic sublaminar wires. There were four iterations for each construct tested: no cross-links, one superior cross-link at T4-T5, one inferior cross-link at T9-T10, and two cross-links. Torsional rigidity was tested by applying a rotational torque at T2. Vertebral body motion was recorded with a three-dimensional video analysis system.


RESULTS
Constructs with distal pedicle screws were statistically more rigid in torsion than those with hooks as distal anchors. The additional torsional rigidity from one or more cross-links was negligible compared with that provided by pedicle screws.


CONCLUSIONS
With pedicle screws as distal anchors in scoliosis constructs, cross-linking with one or two devices adds very little additional rotational stiffness and may be unnecessary in many cases.
Wood
K B
KB
Department of Orthopaedic Surgery, University of Minnesota, Minneapolis, Minnesota 55455, USA.
Wentorf
F A
FA
Ogilvie
J W
JW
Kim
K T
KT
D001863:Bone Screws; D004548:Elasticity; D004867:Equipment Design; D016268:Internal Fixators; D008159:Lumbar Vertebrae / Q000503:physiopathology; D008954:Models, Biological; D012600:Scoliosis / Q000503:physiopathology* / Q000601:surgery; D013123:Spinal Fusion / Q000295:instrumentation*; D013904:Thoracic Vertebrae / Q000503:physiopathology; D014102:Torsion Abnormality / Q000503:physiopathology; D014741:Video Recording; D016474:Weight-Bearing / Q000502:physiology