Predictors of segmental lumbar lordosis following midline posterior (transforaminal) lumbar interbody fusion: Does interbody device type matter?

Background: Controversy exists regarding the ability of posterior (transforaminal) lumbar interbody fusion (PLIF/TLIF) to achieve lordosis. We hypothesized that an interbody device (IBD) designed for positioning in the anterior disc space produces greater lordosis than IBDs designed for straight-in positioning. The purpose of this study is to determine if using either an anterior-position or straight-in position IBD design were associated with successful achievement of postoperative lordosis. Methods: A consecutive series of patients undergoing a undergoing a single-level, posterior open midline (transforaminal) lumbar interbody fusion procedure for degenerative spine conditions during a time period when the two types of interbody devices were being used at surgeon discretion were identified from a multi-surgeon academic training center. Patient demographics and radiographic measures including surgical level lordosis (SLL), anterior disc height, middle disc height, posterior disc height, IBD height, and IBD insertion depth were measured on preop, immediate postop, and one-year postop standing radiographs using PACS. Group comparison and regression analysis were performed using SPSS. Results: Sixty-one patients were included (n = 37 anterior, n = 34 straight-in). Mean age was 59.8 +/- 8.7 years, 32 (52%) were female. There was no difference between IBD type (anterior vs. straight-in) for mean Pre-op SLL (19 +/- 7 degrees vs. 20 +/- 6 degrees, p = 0.7), Post-op SLL (21 +/- 5 degrees vs 21 +/- 6 degrees, p = 0.5), or Change in SLL (2 +/- 4 degrees vs. 1 +/- 5 degrees, p = 0.2). Regression analysis showed that Pre-op SLL was the only variable associated with Change in SLL (Beta = negative 0.48, p = 0.000). While the mean Change in SLL could be considered clinically insignificant, there was wide variability: from a loss of 9 to a gain of 13 degrees. Gain of lordosis >5 degrees only occurred when Pre-op SLL was < 21 degrees, and loss of lordosis > 5 degrees only occurred when Pre-op SLL was > 21 degrees. Conclusions: While group averages showed an insignificant change in segmental lordosis following a posterior (transforaminal) interbody fusion regardless of interbody device type, pre-operative lordosis was correlated with a clinically significant change in segmental lordosis. Preoperative hypolordotic discs were more likely to gain significant lordosis, while preoperative hyperlordotic discs were more likely to lose significant lordosis. Surgeon awareness of this tendency can help guide surgical planning and technique.