Biomechanics of a lumbar interspinous anchor with anterior lumbar interbody fusion

Laboratory investigation

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Object

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.

Abbreviations used in this paper: ALIF = anterior lumbar interbody fusion; ISA = interspinous anchor; ROM = range of motion; SPP = spinous process plate.

Article Information

Address correspondence to: Dean G. Karahalios, M.D., Division of Neurosurgery, NorthShore University Evanston Hospital, Burch Building, 2nd floor, 2650 Ridge Avenue, Evanston, Illinois 60201. email: dkarahalios@northshore.org.

© AANS, except where prohibited by US copyright law.

Headings

Figures

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    Diagram depicting the sequence of constructs tested.

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    A and B: Photographs of the top and side of the Aspen ISA. C: The experimental loading apparatus for applying pure moments. Strings and pulleys in conjunction with a standard servohydraulic test frame were used to induce flexion (shown), extension, axial rotation, and lateral bending. D: The experimental flexion-compression apparatus. A stepper motor linked to the rostral-most potting fixture of the specimen with a heavy-duty belt allows precise control in the sagittal plane of the top fixture's angle relative to the base. Weights hung from the motor apply a constant compressive preload. Because of the orientation of the pulleys, the direction of the preload remains aligned with the axis of the specimen throughout the ROM. In both apparatuses (C and D), optical markers attached to the anterior and right sides of the specimen track segmental motion at each level.

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    Bar graphs depicting mean ROM for each construct in flexion/extension, lateral bending, and axial rotation (upper) and a combined analysis equally weighting and averaging all loading modes (lower). PS = pedicle screw.

  • View in gallery

    Bar graphs depicting the angle of kyphosis (flexion) or lordosis (extension) at the level instrumented with the ISA and at the adjacent levels above and below without load (left) and after application of a 400-N load (right).

  • View in gallery

    Bar graphs depicting foraminal height change following instrumentation in the upright position (A), in flexion (B), and in extension (C).

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