| ABSTRACT: |
Fresh frozen human cadaveric spinal specimens
(T8-S1) were subjected to pure flexion extension bending moment and pure
axial torque loadings while intervertebral rotations were recorded at the
L3-L4, L2-L3, and Ll-L2 discs. A standardized unstable defect was created
at the L3-L4 disc, and loading tests were repeated after application of
bilateral Steffee plates in 2 configurations: a short plate with 2 pedicle
screws (spanning the defect) and a longer plate with 3 pedicle screws (spanning
the defect and 1 disc above). Each plating configuration was tested in the
unlocked state (nuts compressing the plate down onto the spine) and locked
state (nuts above and below the plate tightened against each other to clamp
the plate to the screws). Locking the plates to the screws had no effect
on any intervertebral rotation at any disc level. Use of a longer plate
that also spanned the disc above the defect offered no advantage in controlling
flexion extension rotations at the defect site. However, mean torsional
rotation at the defect site with the 3-screw plate was approximately 50%
of the mean for a 2-screw plate. Extension and torsional rotations at the
L2-L3 disc (1 level above the defect site) were unaffected by application
of a 2-screw plate; flexion rotation at this level increased slightly after
plating. All motions at the L2-L3 disc were reduced (as would be expected)
when the 3-screw plate spanned this uninjured disc. Plating the defect had
no effect on disc rotations at the L1-L2 disc (2 levels above the fracture
site). |