Abstract |
Hypothesis
Anterior cervical fusion with a wedged graft and a locked plate can effectively stabilize the cervical spine after
complete anterior and posterior segmental ligamentous release.
Summary of Background Data.
Distraction‐flexion stage 3 (DFS‐3) injuries of the lower cervical spine (bilateral facet dislocations) are usually
reduced under awake cranial traction. When the MRI reveals a traumatic disc prolapse, an ACDF is usually recommended.
Most authors advise combining ACDF with posterior instrumentation to address the insufficiency of the posterior elements.
However there is clinical evidence that ACDF with a locked plate alone suffices for the treatment of these injuries especially
in young patients. Still there are no biomechanical studies on the effect of a locked plate on the complete anterior and
posterior ligament deficient young cervical spine under physiologic preload.
Methods.
A three step approach was used in this study.
STEP A: First a feasibility study was conducted with lumbar spine specimens to investigate the kinematics under preload
using an in vitro three‐dimensional (3‐D) flexibility test. A total of 21 lumbar spine specimens were utilized for this flexibility
investigation as part of other research programs.
STEP B: The results of the first experiment were then applied in the cervical spine where room for technical error is small
compared to the lumbar spine. A total of six cervical spine specimens were used. Three different preloads (0N, 150N, 250N)
and three different moments (0.8, 1.5, 2.5Nm) were used to investigate lower cervical spine kinematics under preload.
STEP C: In the main experiment eight fresh frozen human lower cervical spines (C3‐C7) from young donors (age 44.5 years,
range 21‐63 ) were used. A 3‐D flexibility test was conducted using a moment of 0.8 Nm without preload. Flexion‐extension
was additionally tested using a moment of 1.5 Nm under 0N and 150N follower preload. Spines were tested first intact,
then after complete C5‐C6 discectomy with PLL resection and ACDF with a wedged bone graft and a rigid locked plate, and
finally after complete release of the supraspinous, interspinous and intertransverse ligaments, the facet capsules and
ligamentum flavum.
Results.
STEP A: From the lumbar spine kinematics experiment it was concluded that preload significantly affects the kinematics of
human spine specimens in flexion extension. The optimization of the path provides the best conditions for near normal
kinematics.
STEP B: From the cervical preload paper it was determined that preload is not affecting only saggital plane motion.
Kinematics in lateral bending and axial rotation are severely affected by the experimental set up. Preload is therefore
recommended only in flexion extension kinematics.
STEP C: When tested under 0.8 Nm moment without preload, complete posterior and anterior ligamentous release did not
significantly increase the ROM of the ACDF construct in flexion‐extension (p>0.05), lateral bending (p>0.05) and axial
rotation (p>0.05). When tested under 1.5 Nm moment without a preload, flexion‐extension tended to increase significantly
after ligamentous release as compared to ACDF with intact posterior ligaments (p=0.08). However, this difference was
negated (p>0.05) after the application of a compressive preload. The ROM of the ACDF construct with the simulated DFS‐3
injury remained significantly smaller than intact in all tested directions (p<0.05).
Conclusions.
Preload is an important characteristic of living tissue and should always be considered if meaningful clinical messages are
to be expected from biomechanical studies.
Anterior cervical fusion with a wedged graft and a rigid constrained (locked) plate can effectively stabilize the nonosteoporotic
cervical spine after complete posterior element injury when excessive range of motion is prevented (for
example, by the use of postoperative external immobilization). Even when the construct is subjected to higher moments,
adequate stability can be achieved when physiologic preload is present.
Osteoporosis and lack of sufficient preload due to poor neuromuscular control may affect long term screw stability and
additional external immobilization may be needed until fusion matures.
Keywords. Cervical spine, flexion‐distraction injury, trauma, facet dislocation, ACDF, preload, follower load
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