Influence of Lumbar Fusion Alignment and Sitting Postures on the Biomechanics of Adjacent Segments

Progressive adjacent segment degeneration (ASD) leading to new pain and disability is a complication of lumbar fusions with an annual incidence of 2%-3% and 10-year prevalence of 20%-25%. The long-term goal of this research is to improve outcomes of lumbar fusion surgery by including the patient’s preoperative spinal and pelvic alignment, spinal flexibility, and occupation in the surgical planning process. Individuals suffering from chronic low back pain (LBP) may require spinal fusion when other modalities fail to bring symptomatic relief.1 Unfortunately, a subset of fused patients develop progressive adjacent segment degeneration (ASD), that sometimes requires chronic use of pain medications or further surgery.2-6 The proposed study acquired data to delineate how patient-specific factors such as active flexion-extension range of motion, lumbar fusion levels, fusion alignment, and postural parameters (pelvic incidence, sacral slope, lumbar lordosis) influence stresses in adjacent segments during standing and sitting postures. A patient’s ability to make up for the lost mobility in fused segments may depend on the motion available from the remaining segments with symptomatic patients likely being those with the least available motion. The central hypothesis is that adjacent segment stresses will depend on patient-specific spinal flexibility, patients’ spinopelvic alignment in post-fusion postures, lumbar fusion levels, and fusion alignment. Part one of the experiment measured standing and sitting sagittal spinal alignments in 9 patients with fusions in the L3-S1 region. We did experiments on 4 cadaveric specimens and specimen-specific kinematic modeling to assess adjacent segment stresses. The results of the human subject’s study and the cadaver study support the claims that post-fusion sitting may be a risk factor for adjacent segment level breakdown (increased degeneration). However, more evidence and a larger sample size is needed in both aims to support these outcomes further.