Background:

Adjacent level degeneration (ALD) is a precursor to construct failure in adult spinal deformity surgery, but whether construct design affects ALD risk remains unclear. Here we present a biomechanical profile of common deformity correction constructs and assess adjacent level biomechanics.

Methods:

Standard nondestructive flexibility tests (7.5 Nm) were performed on 21 cadaveric specimens: 14 pedicle subtraction osteotomies (PSOs) and 7 anterior column realignment (ACR) constructs. Ranges of motion (ROM) at the adjacent free level in flexion, extension, axial rotation, and lateral bending were measured and analyzed.

Results:

ACR constructs had a lower ROM change on flexion at the proximal adjacent free level than constructs with PSO (1.02 vs 1.32, normalized to intact specimen, P<0.01). Lateral lumbar interbody fusion (LLIF) adjacent to PSO and 4 rods limits ROM at the free level more effectively than transforaminal interbody fusion (TLIF) and 2 rods in correction constructs with PSO. Use of 2 screws to anchor the ACR interbody further decreased ROM at the proximal adjacent free level on flexion, but adding 4 rods in this setting added no further limitation to adjacent segment motion.

Conclusions:

ACR constructs have less ROM change at the adjacent level compared to PSO constructs. Among constructs with ACR, anchoring the ACR interbody with 2 screws reduces motion at the proximal adjacent free level. When PSOs are used, LLIF adjacent to the PSO level has greater reduction in adjacent-segment motion than TLIF, suggesting that deformity construct configuration influences proximal adjacent-segment biomechanics.