Adjacent segment motion after a simulated lumbar fusion in different sagittal alignments: a biomechanical analysis.
METHODS
An in vitro biomechanical study of adjacent segment motion (at L3-L4 and L5-S1) after a simulated lumbar interbody fusion of L4-L5 in different sagittal alignments was carried out.


OBJECTIVE
To test the hypothesis that an L4-L5 fixation in different sagittal alignments causes increased angular motion at the adjacent levels (L3-L4 and L5-S1) in comparison with the intact spine.


BACKGROUND
Clinical experience has suggested that lumbar fusion in a nonanatomic sagittal alignment can increase degeneration of the adjacent levels. It has been hypothesized that this is the result of increased motion at these levels; however, to the authors' knowledge no mechanical studies have demonstrated this.


METHODS
Eight fresh human cadaveric lumbar spines (L3-S1) were biomechanically tested. Total angular motion at L3-L4 and L5-S1 under flexion-extension load conditions (7-Nm flexion and 7-Nm extension) was measured. Each specimen was tested intact, and then again after each of three different sagittal fixation angles (at L4-L5): (1) in situ (21 degrees lordosis), (2) hyperlordotic (31 degrees lordosis), and (3) hypolordotic (7 degrees lordosis). The simulated anterior/posterior fusion was performed at L4-L5 with pedicle screws posteriorly, vertebral body screws anteriorly, and an interbody dowel.


RESULTS
The averaged values for flexion-extension motion at L3-L4 were as follows: intact specimen 2.0 degrees, in situ fixation 4.0 degrees, hyperlordotic fixation 1.7 degrees, hypolordotic fixation 6.5 degrees. The averaged values for flexion-extension motions at L5-S1 were as follows: intact specimen 2.3 degrees, in situ fixation 2.6 degrees, hyperlordotic fixation 3.6 degrees, hypolordotic fixation 2.9 degrees.


CONCLUSIONS
Hypolordotic alignment at L4-L5 caused the greatest amount of flexion-extension motion at L3-L4, and the differences were statistically significant in comparison with intact specimen, in situ fixation, and hyperlordotic fixation. Hyperlordotic alignment at L4-L5 caused the greatest amount of flexion-extension motion at L5-S1, and the difference was statistically significant in comparison with intact specimen but not in situ fixation or hypolordotic fixation.
Akamaru
Tomoyuki
T
Emory Spine Center, Department of Orthopaedics, Emory University School of Medicine, Decatur, Georgia, USA. aka@p2332.nsk.ne.jp
Kawahara
Norio
N
Tim Yoon
S
S
Minamide
Akihito
A
Su Kim
Keun
K
Tomita
Katsuro
K
Hutton
William C
WC
D000368:Aged; D000369:Aged, 80 and over; D001696:Biomechanical Phenomena; D002102:Cadaver; D005260:Female; D006801:Humans; D066298:In Vitro Techniques; D008159:Lumbar Vertebrae / Q000502:physiology* / Q000601:surgery; D008297:Male; D008875:Middle Aged; D009038:Motion; D013123:Spinal Fusion / Q000379:methods*