Biomechanics of a lumbar interspinous anchor with anterior lumbar interbody fusion.
OBJECTIVE
An interspinous anchor (ISA) provides fixation to the lumbar spine to facilitate fusion. The biomechanical stability provided by the Aspen ISA was studied in applications utilizing an anterior lumbar interbody fusion (ALIF) construct.


METHODS
Seven human cadaveric L3-S1 specimens were tested in the following states: 1) intact; 2) after placing an ISA at L4-5; 3) after ALIF with an ISA; 4) after ALIF with an ISA and anterior screw/plate fixation system; 5) after removing the ISA (ALIF with plate only); 6) after removing the plate (ALIF only); and 7) after applying bilateral pedicle screws and rods. Pure moments (7.5 Nm maximum) were applied in flexion and extension, lateral bending, and axial rotation while recording angular motion optoelectronically. Changes in angulation as well as foraminal height were also measured.


RESULTS
All instrumentation variances except ALIF alone reduced angular range of motion (ROM) significantly from normal in all directions of loading. The ISA was most effective in limiting flexion and extension (25% of normal) and less effective in reducing lateral bending (71% of normal) and axial rotation (71% of normal). Overall, ALIF with an ISA provided stability that was statistically equivalent to ALIF with bilateral pedicle screws and rods. An ISA-augmented ALIF allowed less ROM than plate-augmented ALIF during flexion, extension, and lateral bending. Use of the ISA resulted in flexion at the index level, with a resultant increase in foraminal height. Compensatory extension at the adjacent levels prevented any significant change in overall sagittal balance.


CONCLUSIONS
When used with ALIF at L4-5, the ISA provides immediate rigid immobilization of the lumbar spine, allowing equivalent ROM to that of a pedicle screw/rod system, and smaller ROM than an anterior plate. When used with ALIF, the ISA may offer an alternative to anterior plate fixation or bilateral pedicle screw/rod constructs.
Karahalios
Dean G
DG
NorthShore University Health System, Evanston, Illinois 60201, USA. dkarahalios@northshore.org
Kaibara
Taro
T
Porter
Randall W
RW
Kakarla
Udaya K
UK
Reyes
Phillip M
PM
Baaj
Ali A
AA
Yaqoobi
Ali S
AS
Crawford
Neil R
NR
D000328:Adult; D000368:Aged; D001696:Biomechanical Phenomena; D001858:Bone Nails; D001860:Bone Plates; D001863:Bone Screws; D002102:Cadaver; D004867:Equipment Design; D005260:Female; D006801:Humans; D066298:In Vitro Techniques; D008159:Lumbar Vertebrae / Q000601:surgery*; D008297:Male; D008875:Middle Aged; D009984:Orthopedic Fixation Devices; D016059:Range of Motion, Articular; D013123:Spinal Fusion / Q000379:methods*; D016474:Weight-Bearing