Lordosis Distribution Index in Short-Segment Lumbar Spine Fusion – Can Ideal Lordosis Reduce Revision Surgery and Iatrogenic Deformity?

OBJECTIVE: The demand for spinal fusion is increasing, with concurrent reports of iatrogenic adult spinal deformity (flatback deformity) possibly due to inappropriate lordosis distribution. This distribution is assessed using the lordosis distribution index (LDI) which describes the upper and lower arc lordosis ratio. Maldistributed LDI has been associated to adjacent segment disease following interbody fusion, although correlation to later-stage deformity is yet to be assessed. We therefore aimed to investigate if hypolordotic lordosis maldistribution was associated to radiographic deformity-surrogates or revision surgery following instrumented lumbar fusion. METHODS: All patients undergoing fusion surgery (≤ 4 vertebra) for degenerative lumbar diseases were retrospectively included at a single center. Patients were categorized according to their postoperative LDI as: “normal” (LDI 50–80), “hypolordotic” (LDI<50), or “hyperlordotic” (LDI>80). RESULTS: We included 149 patients who were followed for 21±14 months. Most attained a normally distributed lordosis (62%). The hypolordotic group had increased postoperative pelvic tilt (PT) (p<0.001), pelvic incidence minus lumbar lordosis (PI–LL) mismatch (p<0.001) and decreased global lordosis (p=0.007) compared to the normal group. Survival analyses revealed a significant difference in revision surgery (p=0.03), and subsequent multivariable logistic regression showed increased odds of 1-year revision in the hypolordotic group (p=0.04). There was also a negative, linear correlation between preoperative pelvic incidence (PI) and postoperative LDI (p<0.001). CONCLUSION: In patients undergoing instrumented lumbar fusion surgery, hypolordotic lordosis maldistribution (LDI<50) was associated to increased risk of revision surgery, increased postoperative PT and PI–LL mismatch. Lordosis distribution should be considered prior to spinal fusion, especially in high PI patients.