Assessment of Maximum Extension Reservoir of the Lumbar Spine Using Fulcrum Hyperextension Radiographs in Severe Sagittal Spinal Malalignment

Study Design. Retrospective study. Objectives. To report on the usefulness of fulcrum hyperextension radiograph to assess the maximum extension reservoir (MER) of lumbar spines in adult spinal deformity (ASD) surgery. Summary of Background Data. The maximum degree of lumbar lordosis (LL) by lateral lumbar interbody fusion (LLIF) will be affected by the MER, which is determined by combination of severity and flexibility of kyphotic deformities of lumbar spine. Although LLIF and anterior column realignment (ACR) are used to treat severe sagittal spinal malalignments, no clear guidelines exist regarding LLIF alone versus ACR. Materials and Methods. We included patients with severe sagittal malalignment undergoing greater than or equal to five-level fusion including the sacrum for ASD. The patients were divided into two groups according to performance of ACR: LLIF group (LLIF alone) and ACR group. Preoperative LL was compared according to patient’s positions; standing, active extension, supine, and fulcrum hyperextension. The offsets between postoperative and preoperative fulcrum hyperextension LL were calculated and compared between the groups. Results. Altogether, 161 patients were included in the study (mean age: 70.2 y; total levels fused: 7.3). Preoperative LL was significantly greatest in fulcrum hyperextension, followed by supine, active extension, and standing positions (37.2°, 26.5°, 23.8°, and 11.7°, respectively, P <0.001). The offsets between postoperative and preoperative fulcrum LL were significantly different between the LLIF and ACR groups (-0.7° vs. 17.8°, P <0.001). Subgroup analysis using patients with an LL offset >0° revealed that the mean LL offsets were 7.6° and 19.4° in the LLIF and ACR groups, respectively. Conclusions. Fulcrum hyperextension radiographs best represented the MER. Therefore, it can be used to predict the maximum LL by LLIF alone, which can be estimated as fulcrum hyperextension LL+7.6°. This threshold can guide the selection between LLIF alone and ACR in deformity correction using the lateral approach.