Summary: Investigation of selective thoracic posterior spinal instrumentation and fusion (PSIF) for adolescent idiopathic scoliosis (AIS) found that loss of thoracic kyphosis (TK) was associated with unintended compensatory loss of lumbar lordosis (LL).
Introduction: A common outcome of segmental instrumentation for AIS is loss of physiological TK. This study aims to investigate the relationship between iatrogenic loss of TK after selective spinal fusion for AIS with straightening of LL and increase in pelvic tilt (PT). This study tests the hypothesis that loss of TK will result in a compensatory and reciprocal loss of LL.
Methods: Query of a prospective multicenter database identified 123 AIS patients (Lenke 1,2,3) at least 2 years following selective thoracic PSIF with lowest instrumented vertebra equal or cephalad to L1. TK (T5‐T12), LL (T12‐S1), sagittal balance (SB) and PT at preop and 2 years postop were measured. Health related quality of life measures (HRQOL) were also examined.
Results: 31% of patients undergoing a selective fusion had a net loss of TK at 2 years postoperatively (2year TK ‐ Preop TK < 0). Patients who had decreased TK had a significantly higher rate of loss of LL than patients without loss of TK (68% vs. 32%, p<0.0001). Change in LL was positively correlated to change in TK (p<0.0001) and negatively correlated to both change in SB (p=0.002) and change in weight (p=0.04). Change in PT was negatively correlated to both change in TK (p=0.03) and change in LL (p<0.0001), and positively correlated to change in weight (p=0.01). Multiple regression analysis revealed that both TK and SB had significant predictive effect on LL (p<0.001, R2=0.31), and LL had significant predictive effect on PT (p=0.0045, R2=0.12). There were no significant associations between changes in TK or LL and HRQOL.
Conclusion: Iatrogenic loss of TK occurs commonly in selective fusion for AIS. This loss of kyphosis is strongly associated with a compensatory and reciprocal loss of LL in the unfused segments, as well as an increase in PT. Seen this way, spinal fusion can have unintentional effects on balance and sagittal alignment which may have distant effects that remain to be fully elucidated.
Significance: Although a significant difference in HRQOL at 2 years postoperatively was not appreciated in this study, the experience of adults with “flatback syndrome” suggests that loss of physiologic sagittal alignment in surgical correction of AIS may increase the risk for suboptimal clinical outcomes for these patients in the future.