Does segmental alignment matter? A novel understanding of segmental compensation and reciprocal change following single-level lumbar reconstruction.

Abstract

Objective: Global and regional spinal compensatory alignment changes are well documented, but segmental compensation/reciprocation remain poorly understood. The purpose of this study was to provide a detailed analysis of adjacent segment behavior after single-level reconstruction.

Methods: Inclusion criteria were adult patients who underwent L5-S1 anterior lumbar interbody fusion for nonnormative preoperative segmental lordosis (SL), demonstrated a ≥ 5° SL increase by intradiscal angle (IDA) or motion segment angle (MSA), and had normal preoperative lumbar lordosis (LL; pelvic incidence [PI]-LL < 10°). IDA; MSA; anterior disc height (ADH) and posterior disc height (PDH) at the index, adjacent, and supra-adjacent levels; and LL, PI, and L1 pelvic angle, were measured preoperatively and at 1 month and 1 year postoperatively.

Results: A total of 100 patients met the inclusion criteria. The fusion levels increased IDA and MSA at 1 month (IDA: 11.12°, p < 0.001; MSA: 9.26°, p < 0.001) and 1 year (IDA: 11.45°, p < 0.001; MSA: 9.13°, p < 0.001). There was a reciprocal decrease in L4-5 SL at 1 month for MSA and IDA (-3.58° and -2.01°, p < 0.001) and 1 year (-3.03° and -1.91°, p < 0.001). PDH increased at 1 month (1.16 mm, p < 0.001) and 1 year (0.92 mm, p = 0.002). The L3-4 level showed postoperative reciprocal decrease in MSA and IDA at 1 month (-1.30°, p < 0.001; -0.99°, p < 0.001) and IDA reciprocation was maintained at 1 year (-1.01°, p < 0.001). ADH and PDH showed commensurate increases at 1 month (0.80 mm, p = 0.015; 0.91 mm, p < 0.001) and 1 year (0.83 mm, p = 0.049; 0.79 mm, p = 0.006). LL increased at 1 month (5.541°, p < 0.001) and 1 year (7.069°, p < 0.001). Changes in IDA and MSA at the index level showed a significant positive correlation with changes in LL at 1 month (p = 0.001, p = 0.002) and 1 year (p = 0.009, p = 0.010). At the 1-year follow-up, the reciprocal decrease in IDA at the adjacent level can be estimated using the equation -0.195(∆1-year IDA at the index level) + 0.332, while the reciprocal decrease in MSA at the adjacent level can be calculated as -0.440(∆1-year IDA at the index level) + 2.023.

Conclusions: The postoperative reciprocation of the adjacent levels implies a preoperative segmental compensatory mechanism. Normative restoration of the surgical level lordosis may therefore have a preventative impact on the otherwise negative consequences of ongoing compensation mechanisms at the adjacent segmental levels in the lumbar spine. This could relate to the risk of adjacent segment disease and deserves further long-term analysis.