295. Comparison of the biomechanical performances among PLF, TLIF, OLIF and XLIF: a finite element analysis

BACKGROUND CONTEXT Posterolateral fusion (PLF) and lumbar interbody fusion (LIF) are the commonly used techniques for the treatment of lumbar degenerative diseases. Although the superiority of PLF or LIF remains controversial, LIF technique provides the potential advantages of anterior column supports and 360° arthrodesis. The most commonly used LIF technique is transforaminal LIF (TLIF). Lateral approaches including extreme LIF (XLIF) and the subsequent oblique LIF (OLIF), have been accepted by an increasing number of spine surgeons due to their unique advantages. Previous studies revealed that radiological outcomes significantly varied among PLF, TLIF, XLIF and OLIF, suggesting their different biomechanical properties. PURPOSE To compare the biomechanical differences among PLF, TLIF, XLIF and OLIF construct and determine which surgical construct was most beneficialto the segmental stability and cage subsidence resistance. STUDY DESIGN/SETTING a finite element (FE) analysis. OUTCOME MEASURES The ranges of motion (ROMs) and stress peaks in the cortical endplate, cancellous bone and posterior instrumentations were compared among the five constructs. METHODS An intact FE model of the L3-L5 lumbar spine was constructed. After validation, the PLF, TLIF with a banana-shaped cage, TLIF with a straight cage, XLIF and OLIF procedures were simulated at the L4/5 level. A bilateral pedicle screw fixation was added in each construct. The TLIF banana-shaped cage was placed on the anterior one-third portion of the endplate. The TLIF straight cage was placed around the midpoint of the endplate and was diagonally positioned at an angle of 45° . The XLIF cage was placed at the mid-endplate and the OLIF cage was placed more anteriorly. An axial preload of 400 N was imposed on the superior surface of L3 to simulate the corresponding physiologic compression. Another 8 Nm was applied on L3 to simulate flexion, extension, bending and axial rotation. RESULTS The PLF construct has much higher ROMs than the LIF constructs (0.44° to -1.05° vs 0.29° to -0.72°). TLIF with a straight cage induces the highest stress peak in the endplate and cancellous bone (23.2-63.7 MPa and 0.69-2.03 MPa, respectively), followedby TLIF with a banana-shaped cage (23.7-44.8 MPa and 0.6-1.36 MPa, respectively). OLIF (16.76-32.41 MPa and 0.54-1.07 MPa, respectively) and XLIF (17.01-35.34 MPa and 0.56-1.12 MPa, respectively) constructs induces much lower stress peaks in the endplate and cancellous bone. In the PLF construct, the stress peaks in the posterior instrument range from 244.6 MPa to 377.4 MPa, which are significantly higher than those in the LIF constructs (123.9-237.4 MPa). CONCLUSIONS The PLF construct had\s less stability than the LIF construct due to the higher ROMs and stress peaks in the posterior instrument. TLIF with a banana-shaped cage outperforms TLIF with a straight cage in decreasing the stress peaks in the endplate and cancellous bone, as the banana-shaped cage is ideally located more anteriorly and placed on the strong cortical rim of the endplate. Compared with TLIF constructs, the much larger cages used in the OLIF and XLIF constructs induce significantly fewer stress peaks and increase homogeneous stress distributions in the endplate cancellous bone, which was beneficial to the subsidence resistance and maintenance of disc height and segmental angle. FDA DEVICE/DRUG STATUS XLIF (Approved for this indication), OLIF (Approved for this indication)