A Novel Metric for Assessing Long-Term Outcomes in Adults with Chiari Malformation Type I: Occipitocervical Dura Angulation (ODA)-Applications and Value.

Foramen magnum decompression with duraplasty (FMDD), a common surgery for adult Chiari malformation type I(CM-I) with a 60-70% improvement rate, lacks assessment metrics. This study applied the concept of occipitocervical dura angulation (ODA) and aimed to investigate ODA's role in evaluating long-term FMDD outcomes. The ODA normal range was measured in 230 healthy individuals. We included 160 CM-I adults who underwent FMDD over 10 years. Long-term outcomes (> 1 year) were evaluated using the Chicago Chiari Outcome Scale(CCOS). The cohort was grouped to analyze ODA's role in assessing and predicting FMDD outcomes. The normal range of ODA was 104°-145°, with an average value of 124.2 ± 10.4°. The median patient age was 46 years (range: 18-64 years). The median follow-up period was 64.5 months (range: 18-123 months). The cohort was divided into two groups based on preoperative ODA: group A (n = 90) with ODA < 125° and group B (n = 70) with ODA ≥ 125°. Group B had a significantly better prognosis than that of group A (P = 0.012). Logistic regression analysis revealed that an increase in ODA was associated with clinical outcomes in both groups. The receiver operating characteristic curve showed that ODA increases of > 10° and > 3° could be considered as credible threshold values for groups A and B, respectively. ODA is a reliable predictor of prognosis. FMDD should result in an increased ODA, which is one of the criteria for surgical validity, especially for those with a preoperative ODA of < 125°.