Excessive stress and motion at the L5–S1 level, especially in patients with posterior instrumentation suprajacent to L5, can cause degenerative changes. The use of L5–S1 anterior lumbar interbody fusion (ALIF) to support the lumbosacral junction has gotten a lot of attention. However, there were still concerns about the efficacy of stand-alone ALIF in the presence of previous posterior instrumented fusions terminating at L5. For this study, the purpose was to see how stable an L5–S1 ALIF was as the length of the posterior thoracolumbar constructions got longer. Seven human cadaveric spines (T9–sacrum) were attached to a 6-degrees-of-freedom robot and instrumented with pedicle screws from T10 to L5. In flexion-extension, axial rotation, and lateral bending, posterior fusion construct lengths (T10–L5, T12–L5, L2–5, and L4–5) were instrumented to each specimen and torque-fusion level associations were calculated for each construct. The L5–S1 motion was then measured when a stand-alone L5–S1 ALIF was subjected to increasing pure moments (2 to 12 Nm). Motion reduction was estimated by comparing L5–S1 motion in ALIF and non-ALIF modes.

They measured average motion in axial rotation, flexion-extension, and lateral bending at L5–S1 for each fusion construct with and without ALIF. In all but one fused surgical scenario, L5–S1 mobility reduction after ALIF was added to a posterior fusion compared to the non-ALIF state (p < 0.05). In some cases, longer ALIF fusions led to bigger L5–S1 motion and higher activities than native state motion. When considering an 80% reduction in native movement as a potentially positive sign of fusion, a stand-alone L5–S1 ALIF might appropriately stabilize posterior fusion structures up to L4–5. The findings for the study helped to enable biomechanical conclusions to be taken; however, the clinical implications of these findings were not fully characterized. When viewed from a therapeutic perspective, these findings could’ve helped the doctors better manage L5–S1 pathology in individuals who have had prior posterior thoracolumbar constructions.