Transforaminal lumbar interbody fusion: the effect of various instrumentation techniques on the flexibility of the lumbar spine.

STUDY DESIGN: In vitro comparison of four reconstruction techniques following transforaminal lumbar interbody fusion in a human cadaveric model.

INTRODUCTION: Transforaminal lumbar interbody fusion (TLIF) is a relatively new technique that avoids the morbidity of an anterior approach and the nerve root manipulation of a posterior interbody fusion. This study measured the effects of a TLIF on the overall and segmental flexibility of the lumbar spine using four different spinal implant configurations.

SUMMARY OF BACKGROUND DATA: Anterior lumbar interbody fusion, posterior lumbar interbody fusion, and combined anterior-posterior spinal procedures are gaining wide acceptance for the treatment of selected patients with segmental spinal instability and spondylolisthesis with associated degenerative changes. Each fusion technique may have different effects on the overall flexibility of the lumbar spine. The unilateral TLIF procedure with adjunctive pedicular fixation is one variation of an interbody fusion technique that requires less bony and soft tissue dissection and minimizes nerve root manipulation compared with other interbody fusion methods.

METHODS: Five fresh-frozen, human lumbar spines were nondestructively subjected to flexion, extension, lateral bending, and axial rotation moments using a previously validated spine flexibility tester, and displacements were measured. Testing the intact lumbar spine was followed by testing of a unilateral L4-L5 TLIF using a single ramp carbon fiber cage without adjunctive internal fixation. The single carbon fiber (Brantigan) cage was inserted obliquely in a posterolateral to anteromedial position in the L4-L5 disc space. Following testing of the cage alone, three different adjunctive stabilization techniques were tested. Posterior stabilization involved one of the following: a contralateral translaminar facet screw, single side/ipsilateral nonsegmental pedicle screw fixation, and bilateral nonsegmental pedicle screw fixation. The overall flexibility of each lumbar spine was calculated from load-displacement curves for each axis of rotation. The flexibility of the L4-L5 segment of each spine was computed from kinematic motion data acquired via attached LED sensors to the L4 and L5 vertebral bodies. Statistical testing was performed with paired t tests.

RESULTS: The flexibility of the entire (T12-S1) destabilized spine after TLIF with interbody cage alone and with all three reconstructive techniques was comparable with the intact spine. However, the motion at the L4-L5 segment was significantly increased for the TLIF with interbody cage alone in axial rotation (299% of intact, P < 0.01), with no significant change in flexion-extension (79% of intact, P = 0.22) or lateral bending (87% of intact, P = 0.39). With the addition of a contralateral translaminar facet screw, the motion at the L4-L5 segment remained significantly more flexible in axial rotation (250% of intact, P = 0.06) although less than with the cage alone. With the unilateral pedicle screw construct, the L4-L5 segment remained more flexible in axial rotation (182% of intact, P = 0.07) although significantly less than with the facet screw construct (P < 0.05). The addition of bilateral pedicle screws most closely reapproximated the flexibility of the intact spine with no significant difference in axial rotation (91% of intact, P = 0.30), flexion-extension (93% of intact, P = 0.19), or lateral bending (99% of intact, P = 0.47). The motion at the L4-L5 segment with bilateral pedicle screws was not significantly different than for the intact specimen in axial rotation (144% of intact, P = 0.17), flexion-extension (81% of intact, P = 0.21), or lateral bending (86% of intact, P = 0.27).

CONCLUSIONS: TLIF reconstruction with a solitary cage did not increase overall spine flexibility from the intact condition but significantly increased segmental flexibility at L4-L5 in axial rotation. A unilateral translaminar facet screw had minimal stabilizing effect at L4-L5. Unilateral pedicle screws frews further increased stiffness at the L4-L5 segment. However, TLIF with bilateral pedicle screws most closely approximated the L4-L5 segmental flexibility of the intact spine.