Effect of the Total Facet Arthroplasty System after complete laminectomy-facetectomy on the biomechanics of implanted and adjacent segments.
BACKGROUND
Lumbar fusion is traditionally used to restore stability after wide surgical decompression for spinal stenosis. The Total Facet Arthroplasty System (TFAS) is a motion-restoring implant suggested as an alternative to rigid fixation after complete facetectomy.


OBJECTIVE
To investigate the effect of TFAS on the kinematics of the implanted and adjacent lumbar segments.


METHODS
Biomechanical in vitro study.


METHODS
Nine human lumbar spines (L1 to sacrum) were tested in flexion-extension (+8 to -6Nm), lateral bending (+/-6Nm), and axial rotation (+/-5Nm). Flexion-extension was tested under 400 N follower preload. Specimens were tested intact, after complete L3 laminectomy with L3-L4 facetectomy, after L3-L4 pedicle screw fixation, and after L3-L4 TFAS implantation. Range of motion (ROM) was assessed in all tested directions. Neutral zone and stiffness in flexion and extension were calculated to assess quality of motion.


RESULTS
Complete laminectomy-facetectomy increased L3-L4 ROM compared with intact in flexion-extension (8.7+/-2.0 degrees to 12.2+/-3.2 degrees, p<.05) lateral bending (9.0+/-2.5 degrees to 12.6+/-3.2 degrees, p=.09), and axial rotation (3.8+/-2.7 degrees to 7.8+/-4.5 degrees p<.05). Pedicle screw fixation decreased ROM compared with intact, resulting in 1.7+/-0.5 degrees flexion-extension (p<.05), 3.3+/-1.4 degrees lateral bending (p<.05), and 1.8+/-0.6 degrees axial rotation (p=.09). TFAS restored intact ROM (p>.05) resulting in 7.9+/-2.1 degrees flexion-extension, 10.1+/-3.0 degrees lateral bending, and 4.7+/-1.6 degrees axial rotation. Fusion significantly increased the normalized ROM at all remaining lumbar segments, whereas TFAS implantation resulted in near-normal distribution of normalized ROM at the implanted and remaining lumbar segments. Flexion and extension stiffness in the high-flexibility zone decreased after facetectomy (p<.05) and increased after simulated fusion (p<.05). TFAS restored quality of motion parameters (load-displacement curves) to intact (p>.05). The quality of motion parameters for the whole lumbar spine mimicked L3-L4 segmental results.


CONCLUSIONS
TFAS restored range and quality of motion at the operated segment to intact values and restored near-normal motion at the adjacent segments.
Phillips
Frank M
FM
Rush University Medical Center, Chicago, IL, USA.
Tzermiadianos
Michael N
MN
Voronov
Leonard I
LI
Havey
Robert M
RM
Carandang
Gerard
G
Renner
Susan M
SM
Rosler
David M
DM
Ochoa
Jorge A
JA
Patwardhan
Avinash G
AG
D019643:Arthroplasty, Replacement / Q000295:instrumentation* / Q000379:methods; D001696:Biomechanical Phenomena / Q000502:physiology*; D005260:Female; D006801:Humans; D007796:Laminectomy / Q000295:instrumentation* / Q000379:methods; D008159:Lumbar Vertebrae; D008297:Male; D008875:Middle Aged; D016059:Range of Motion, Articular / Q000502:physiology*; D013131:Spine / Q000601:surgery*