Purpose: The global alignment and proportion (GAP) score was recently developed to consider proportional analysis of spinopelvic alignment and has been indicated for setting surgical goals to decrease the prevalence of mechanical complications. The goal of this study was to clarify the limitations and problems with spinal corrective surgery with minimally invasive lateral lumbar interbody fusion (LLIF) without osteotomy using GAP score, and to establish a preoperative radiographical evaluation to understand the necessity for three-column osteotomy.
Methods: We included data from 57 consecutive patients treated with spinal corrective surgery with LLIF and without Schwab grade 3-6 osteotomy for ASD. To evaluate flexibility of the pelvis and lumbar spine, we examined full-length lateral radiographs with patients standing and prone. Correlations between pre- and postoperative radiographic parameters and GAP score were determined.
Results: Most patients achieved a sufficiently ideal lumbar lordosis (87.7%), but ideal sacral slope (SS) was achieved in only 50.8% of patients. Preoperative prone SS showed a significant positive correlation with postoperative SS and a significant negative correlation with GAP score. Patients whose preoperative prone SS was larger than pelvic incidence × 0.59-7.5 tended to achieve proportioned spinopelvic alignment by using LLIF.
Conclusions: The cause of poor outcome of GAP score for ASD corrective surgery with LLIF without osteotomy is a postoperative small SS. Preoperative prone SS is useful for predicting postoperative SS. When preoperative SS in prone patients is relatively small to ideal as calculated using PI, osteotomy or other correctors should be considered to achieve satisfactory spinopelvic parameters.
Level of evidence: III. These slides can be retrieved under Electronic Supplementary Material.
Keywords: Adult spinal deformity; Global alignment and proportion score; Lordosis distribution index; Mechanical complications; Minimally invasive lateral lumbar interbody fusion; SS in prone position.