Impact of lumbar pedicle subtraction osteotomy (PSO) level on global alignment and proportion (GAP) score in revision adult iatrogenic flatback spinal deformities.

Purpose: To explore the impact of different lumbar pedicle subtraction osteotomy (L-PSO) levels on Global Alignment and Proportion (GAP) scores.

Methods: Adults at a single center who underwent lumbar PSOs with revision instrumentation [thoracolumbar junction (T9-L1) to pelvis] and a minimum 2-year follow-up were reviewed. The patients were divided by level of PSO (L2, L3, L4, and L5) and compared with respect to demographic and surgical data, sagittal parameters, GAP scores, and mechanical complications requiring revision operations.

Results: 152 patients (average age 64.4 ± 10.6 years, average follow-up 9.0 ± 4.1 years) were included for analysis. L3 (40.8%) and L4 (45.4%) PSOs were more common than L2 (4.6%) and L5 (9.2%) PSOs. Average pre-op GAP scores (9.8 ± 2.8) were similar and improved significantly for all L-PSO levels, although post-op GAP scores (7.1 ± 2.2) remained "disproportioned" for all L-PSO levels. Post-op Lumbar Distribution Index (LDI) scores were significantly better for lower PSOs (L4 + L5) given better improvement of L4-S1 lordosis. The revisions for mechanical failures were higher in L2 and L3 PSOs. Average post-op GAP scores were not different for patients who did and did not undergo mechanical failure revisions.

Conclusions: L-PSOs, irrespective of the level, improve GAP scores. While residual disproportionate post-operative alignment was observed for all L-PSO levels, distal PSOs improved L4-S1 lordosis and LDI scores to a greater extent than proximal PSOs. Although more distal lumbar PSOs also had lower rates of revision operations for mechanical complications, other patient and surgical factors also likely played a role in the observed rates of mechanical failures.