## Why Flexion-Extension Views is Right The Atlanto-Dental Interval (ADI) — the distance between the anterior arch of C1 and the dens of C2 — is normally < 3 mm in adults and < 5 mm in children. This 6-year-old's ADI of 4.5 mm is at the upper limit of normal but raises concern for atlantoaxial instability, particularly in Down syndrome where transverse ligament laxity is a known association. Flexion-extension lateral cervical spine views are the standard radiographic method to dynamically assess for atlantoaxial instability and are mandatory for pre-sport clearance in Down syndrome patients. These views reveal whether the ADI widens excessively (> 5 mm in children or > 3 mm in adults) during flexion, indicating pathologic instability that would contraindicate contact sports. [Maheshwari Orthopedics 10e; Harrison 21e Ch 461] ## Why Each Distractor is Wrong - **Reassure parents that this measurement is normal for age and allow unrestricted sports participation**: While 4.5 mm is technically within the pediatric upper limit (< 5 mm), a borderline ADI in a Down syndrome patient requires dynamic assessment. Static measurement alone is insufficient; flexion-extension views are the gold standard for detecting instability that may not be apparent on neutral films. - **Proceed directly to MRI cervical spine to evaluate for transverse ligament integrity**: Although MRI can assess soft tissue structures including the transverse ligament, it is not the first-line dynamic assessment tool for ADI. Flexion-extension X-rays are simpler, faster, more cost-effective, and specifically designed to detect functional atlantoaxial instability. MRI would be reserved for cases where instability is confirmed or for evaluating cord compression. - **Recommend immediate surgical fusion without further imaging**: Surgical intervention is premature without documented instability on flexion-extension views. Many Down syndrome patients have borderline ADI measurements but remain stable dynamically and do not require fusion. Surgery is indicated only if flexion-extension views demonstrate pathologic widening or if there is evidence of cord compression or myelopathy. **High-Yield:** In Down syndrome, a borderline or elevated ADI mandates flexion-extension lateral cervical spine views for pre-sport clearance; static measurement alone is insufficient. [Maheshwari Orthopedics 10e; Harrison 21e Ch 461]
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