Anterior cervical decompression and fusion for Samartzis Type I Klippel-Feil syndrome complicated by cervical radiculopathy with radiographic spinal cord compression: a case report of sandwich-type interposed mobile segment.

Background: Klippel-Feil syndrome (KFS) is a rare congenital cervical malformation caused by embryonic segmentation defects leading to vertebral fusion, which alters cervical biomechanics and predisposes to premature spinal degeneration. The "sandwich-type interposed mobile segment" refers to a solitary mobile cervical segment trapped between congenital bony fusion and iatrogenic surgical fusion. It bears concentrated mechanical stress and carries a high risk of early adjacent segment degeneration (ASD). Current epidemiological data for postoperative ASD are mostly derived from general spinal cohorts, whereas targeted clinical evidence for KFS patients remains limited. This study systematically retrieved KFS cervical surgical cases from Web of Science and PubMed to explore clinical features and long-term degenerative risks of this unique biomechanical lesion. Case presentation: A 44-year-old female with Samartzis Type I KFS presented with intractable cervical radiculopathy and radiographic spinal cord compression. After 6 months of failed conservative management, she underwent C6/7 anterior cervical discectomy and fusion (ACDF). Radicular symptoms resolved immediately, with full neurological recovery at the 12-month follow-up. Symptomatic C5/6 ASD occurred 40 months postoperatively. Revision ACDF was performed after repeated conservative treatment failure, and complete neurological remission was achieved 6 months after revision. No extra-spinal congenital anomalies were identified. The C5/6 level represented the only functional mobile segment sandwiched between congenital C4/5 fusion and iatrogenic C6/7 fusion, forming a high-risk stress-concentrating biomechanical construct. Conclusion: ACDF yields favorable shortterm outcomes for Samartzis Type I KFS patients with cervical radiculopathy and spinal cord compression. Nevertheless, KFS patients with sandwich-type interposed mobile segments may face an increased risk of early symptomatic ASD. Combined with nine retrieved KFS surgical cases, our findings suggest the necessity of individualized surgical planning and lifelong imaging surveillance owing to persistent biomechanical abnormalities. This case supplements scarce clinical and biomechanical evidence for KFS related early postoperative degeneration and provides references for preoperative risk stratification in similar clinical scenarios.