Biomechanics of posterior instrumentation in L1-L3 lateral interbody fusion: Pedicle screw rod construct vs. transfacet pedicle screws.

BACKGROUND: The use of pedicle screws is the gold standard for supplemental posterior fixation in lateral interbody fusion. Information about the performance of transfacet pedicle screws compared to standard pedicle screws and rods in the upper lumbar spine with or without a lateral interbody fusion device in place is limited. METHODS: Fifteen fresh frozen human cadaveric lumbar spine segments (T12-L4) were studied using standard pure moment flexibility tests. Specimens were divided into two groups to receive either bilateral transfacet pedicle screws (n=8) or bilateral pedicle screws (n=14). Stability of each motion segment (L1-L2 and L2-L3) was evaluated intact, with posterior instrumentation with an intact disc, with posterior instrumentation and a lateral interbody fusion device in place, and following cyclic loading with the interbody device and posterior instrumentation still in place. Both raw values of motion (range of motion, lax zone and stiff zone) and normalized mobility (ratios to intact) were analyzed for each case. FINDINGS: In terms of immediate stability, transfacet pedicle screws performed equivalent to similarly sized pedicle screws, both with intact disc and with lateral interbody fusion device in all directions of loading. Stability following cyclic loading decreased significantly during lateral bending and axial rotation. INTERPRETATION: Posterior fixation with transfacet pedicle screws provides equivalent immediate stability to similarly sized pedicle screws. However, in the presence of a lateral interbody fusion device, pedicle screws seem to resist loosening more and may be a better option for fusion in the upper lumbar spine.

Stability of transforaminal lumbar interbody fusion in the setting of retained facets and posterior fixation using transfacet or standard pedicle screws.

BACKGROUND CONTEXT: The transforaminal lumbar interbody fusion (TLIF) technique supplements posterior instrumented lumbar spine fusion and has been tested with different types of screw fixation for stabilization. Transforaminal lumbar interbody fusion is usually placed through a unilateral foraminal approach after unilateral facetectomy, although extraforaminal entry allows the facet to be spared. PURPOSE: To characterize the biomechanics of L4-L5 lumbar motion segments instrumented with bilateral transfacet pedicle screw (TFPS) fixation versus bilateral pedicle screw-rod (PSR) fixation in the setting of intact facets and native disc or after discectomy and extraforaminal placement of a TLIF technology graft. STUDY DESIGN: Human cadaveric lumbar spine segments were biomechanically tested in vitro to provide a nonpaired comparison of four configurations of posterior and interbody instrumentation. METHODS: Fourteen human cadaveric spine specimens (T12-S1) underwent standard pure moment flexibility tests with intact L4-L5 disc and facets. Seven were studied with intact discs, after TFPS fixation, and then with TLIF and TFPS fixation. The others were studied with intact discs, after PSR fixation, and then combined with extraforaminally placed TLIF. Loads were applied about anatomic axes to induce flexion-extension, lateral bending, and axial rotation. Three-dimensional specimen motion in response to applied loads during flexibility tests was determined. A nonpaired comparison of the four configurations of posterior and interbody instrumentation was made. RESULTS: Transfacet pedicle screw and PSR, with or without TLIF, significantly reduced range of motion during all directions of loading. Transfacet pedicle screw provided greater stability than PSR in all directions of motion except lateral bending. In flexion, TFPS was more stable than PSR (p=.042). A TLIF device provided less stability than the intact disc in constructs with TFPS and PSR. CONCLUSIONS: These results suggest that fixation at L4-L5 with TFPS is a promising alternative to PSR, with or without TLIF. A TLIF device was less stable than the native disc with both methods of instrumentation presumably because of a fulcrum effect from a relatively small footplate. Additional interbody support may be considered for improved biomechanics with TLIF.