Compensatory Mechanism of Maintaining the Sagittal Balance in Degenerative Lumbar Scoliosis Patients with Different Pelvic Incidence

OBJECTIVE: To investigate the compensatory mechanism of maintaining the sagittal balance in degenerative lumbar scoliosis patients with different pelvic incidence (PI). METHODS: This was a retrospective imaging observation study. Patients in our department with degenerative lumbar scoliosis between 2017 and 2019 were reviewed. A total of 36 patients were eligible and included in the present study. The average age of those patients was 64.22 years, including 8 men and 28 women. The coronal and sagittal parameters were measured on full‐length spine X‐ray film, including globe kyphosis (GK), lumber lordosis (LL), thoracolumbar kyphosis (TLK), thoracic kyphosis (TK), sagittal vertical axis (SVA), sagittal shift angle, Cobb angle, coronal shift angle, and vertebra. The anterior pelvic plane angle (APPA) and pelvic parameters were also measured, including the pelvic tilt (PT), the PI, and the sacral slope (SS). PI‐LL, LL‐SS, and GK‐SS were calculated. Traditional pelvic tilt was also calculated using the following formula: cPT = PI × 0.37–7. These patients were divided into two groups according to their PI values. The patients’ PI value in Group 1 was smaller than 50°. The patients’ PI value in Group 2 was equal to or larger than 50°. RESULTS: These patients’ SS, PT, PI, LL, TLK, TK, and GK were 28.70° ± 11.36°, 23.28° ± 6.55°, 52.00° ± 11.03°, 31.66° ± 14.12°, 12.12° ± 14.9°, 17.81° ± 13.53°, and −13.17° ± 16.27°. The sagittal shift angle, the APPA, the Cobb angle, the coronal shift angle, vertebra, PI‐LL, cPT, APPA‐4, LL‐SS, and GK‐SS were 4.38° ± 5.75°, −12.55° ± 8.83°, 30.03° ± 12.59°, 2.40° ± 2.13°, 4.08 ± 0.93, 19.86° ± 10.97°, 12.35° ± 4.55°, −8.30° ± 9.07°, 3.30° ± 8.82°, and 15.53° ± 9.83°, respectively. There was no significant difference between PT and cPT + APPA‐4 or between cPT and PT‐APPA+4. There was significant difference between PT and cPT + APPA or between cPT and PT‐APPA. This demonstrated that the APPA‐4 is reliable as degree of the pelvic sagittal retroversion. There were significant differences in SS, PI, LL, TLK, GK, APPA, PT‐APPA, PT‐APPA+4, cPT, and APPA‐4 between Group 1 and Group 2. There were no significant differences in PT, TK, sagittal shift angle, SVA, Cobb angle, coronal shift angle, vertebra number, PI‐LL, cPT + APPA, cPT + APPA‐4, LL‐SS, and GK‐SS between Group 1 and Group 2. The Pearson tests showed that PI‐LL had significant correlations with TK, LL, sagittal shift angle, SVA, and LL‐SS. There was no significant correlation between PI‐LL and Cobb angle, GK, TLK, APPA, vertebra, Coronal Shift Angle, or GK‐SS. CONCLUSION: The APPA‐4 is reliable as degree of the pelvic sagittal retroversion. In degenerative lumbar scoliosis, patients with smaller PI tended to rely more on the pelvic retroversion to maintain the sagittal balance than patients with larger PI, or patients with smaller PI were likely to start up the pelvic retroversion compensatory mechanism earlier than the patients with larger PI.