The role of sacral slope in lumbosacral fusion: a biomechanical study.
OBJECTIVE
Abnormal sacral slope (SS) has shown to increase progression of spondylolisthesis, yet there exists a paucity in biomechanical studies investigating its role in the correction of adult spinal deformity, its influence on lumbosacral shear, and its impact on the instrumentation selection process. This in vitro study investigates the effect of SS on 3 anterior lumbar interbody fusion constructs in a biomechanics laboratory.


METHODS
Nine healthy, fresh-frozen, intact human lumbosacral vertebral segments were tested by applying a 550-N axial load to specimens with an initial SS of 20° on an MTS Bionix test system. Testing was repeated as SS was increased to 50°, in 10° increments, through an angulated testing fixture. Specimens were instrumented using a standalone integrated spacer with self-contained screws (SA), an interbody spacer with posterior pedicle screws (PPS), and an interbody spacer with anterior tension band plate (ATB) in a randomized order. Stiffness was calculated from the linear portion of the load-deformation curve. Ultimate strength was also recorded on the final construct of all specimens (n = 3 per construct) with SS of 40°.


RESULTS
Axial stiffness (N/mm) of the L5-S1 motion segment was measured at various angles of SS: for SA 292.9 ± 142.8 (20°), 277.2 ± 113.7 (30°), 237.0 ± 108.7 (40°), 170.3 ± 74.1 (50°); for PPS 371.2 ± 237.5 (20°), 319.8 ± 167.2 (30°), 280.4 ± 151.7 (40°), 233.0 ± 117.6 (50°); and for ATB 323.9 ± 210.4 (20°), 307.8 ± 125.4 (30°), 249.4 ± 126.7 (40°), 217.7 ± 99.4 (50°). Axial compression across the disc space decreased with increasing SS, indicating that SS beyond 40° threshold shifted L5-S1 motion into pure shear, instead of compression-shear, defining a threshold. Trends in ultimate load and displacement differed from linear stiffness with SA > PPS > ATB.


CONCLUSIONS
At larger SSs, bilateral pedicle screw constructs with spacers were the most stable; however, none of the constructs were significantly stiffer than intact segments. For load to failure, the integrated spacer performed the best; this may be due to angulations of integrated plate screws. Increasing SS significantly reduced stiffness, which indicates that surgeons need to consider using more aggressive fixation techniques.
Departments of 1 Neurosurgery and.
Doniel
D
Drazin
Globus Medical, Inc., Valley Forge Business Center, Audubon, Pennsylvania; and.
Mir
M
Hussain
Globus Medical, Inc., Valley Forge Business Center, Audubon, Pennsylvania; and.
Jonathan
J
Harris
Globus Medical, Inc., Valley Forge Business Center, Audubon, Pennsylvania; and.
John
J
Hao
Globus Medical, Inc., Valley Forge Business Center, Audubon, Pennsylvania; and.
Matt
M
Phillips
Orthopedics, Cedars-Sinai Medical Center, Los Angeles, California;
Terrence T
TT
Kim
Departments of 1 Neurosurgery and.|Department of Neurosurgery, University of California, Davis, Sacramento, California.
J Patrick
JP
Johnson
Globus Medical, Inc., Valley Forge Business Center, Audubon, Pennsylvania; and.
Brandon
B
Bucklen
D000328:Adult; D002102:Cadaver; D006801:Humans; D008159:Lumbar Vertebrae; D065289:Pedicle Screws; D016059:Range of Motion, Articular / Q000502:physiology; D012447:Sacrum; D033081:Shear Strength / Q000502:physiology; D013123:Spinal Fusion; D016474:Weight-Bearing / Q000502:physiology
ATB = anterior tension band plate with interbody spacer; PPS = posterior pedicle screws placed with interbody spacer; SA = stand-alone integrated spacer with self-contained screws; SS = sacral slope; anterior interbody fusion; biomechanical study; biomechanics; integrated spacer; sacral slope; stand-alone