Biomechanical effects of interbody cage height on adjacent segments in patients with lumbar degeneration: a 3D finite element study

Objective To investigate the biomechanical effects of interbody cage height on adjacent segments in patients with lumbar degeneration undergoing transforaminal lumbar interbody fusion (TLIF) surgery, so as to provide references for selection of interbody cage. Methods The finite element model of normal lower lumbar spine (L3-S1) was built and validated, then constructed three different degenerative segments in L3-L4, and the cages with different height (8, 10, 12, 14 mm) were implanted into L4-L5 disc. All the twelve models were loaded with pure moment of 7.5 N m to produce flexion, extension, lateral bending and axial rotation motions on lumbar spine, and the effects of cage height on range of motion (RoM) and intervertebral pressure in lumbar spine were investigated. Results The RoM of adjacent segments and the maximum stress of intervertebral discs increased with the increase in cage height, but this trend was not obvious in mild and moderate degeneration groups. After implantation of four different height cages (8, 10, 12, 14 mm), the RoM of L3/L4 segment reached the maximum during extension. The RoM of mild degeneration group was 2.07 degrees, 2.45 degrees, 2.48 degrees, 2.54 degrees, that of moderate degeneration group was 1.79 degrees, 1.97 degrees, 2.05 degrees, 2.05 degrees, and that of severe degeneration group was 1.43 degrees, 1.66 degrees, 1.74 degrees, 1.74 degrees. The stress of L3-L4 intervertebral disc reached the maximum during flexion. The maximum stress of L3-L4 intervertebral disc was 20.16 MPa, 20.28 MPa, 20.31 MPa and 20.33 MPa in the mild group, 20.58 MPa, 20.66 MPa, 20.71 MPa and 20.75 MPa in the moderate group, and 21.27 MPa, 21.40 MPa, 21.50 MPa and 21.60 MPa in the severe group. Conclusion For patients with mild-to-moderate lumbar degenerative disease who need to undergo TLIF surgery, it is recommended that the height of fusion cage should not exceed the original intervertebral space height by 2 mm, while for patients with severe degeneration, a fusion cage close to the original intervertebral height should be selected as far as possible, and the intervertebral space should not be overstretched.