A 48-year-old woman presents with acute onset neck pain and right upper limb radiculopathy after a motor vehicle accident. Physical examination reveals weakness of shoulder abduction and external rotation, sensory loss over the lateral shoulder and arm, and diminished biceps reflex on the right. Plain radiographs show a unilateral facet joint dislocation at C5–C6 on the right side. Which vertebral structure is primarily responsible for preventing excessive forward displacement of C5 on C6 in this injury?

Explanation

## Role of Facet Joints in Vertebral Stability ### Anatomical Function of Facet Joints **Key Point:** The facet joints (zygapophyseal joints) are the primary restraint to forward (anterior) translation of one vertebra on another. They act as mechanical "locks" that prevent excessive sliding and maintain segmental stability. ### Mechanism of Stability The facet joints: 1. Articulate between the inferior articular processes of the superior vertebra and the superior articular processes of the inferior vertebra 2. Are oriented in the cervical spine at approximately 45° to the horizontal plane 3. Provide both a **bony block** and **ligamentous support** (joint capsule) against anterior translation 4. When disrupted (as in unilateral facet dislocation), allow the superior vertebra to slide forward on the inferior vertebra ### Comparison of Stabilizing Structures | Structure | Primary Role | Effect When Damaged | |-----------|--------------|---------------------| | **Facet joints** | **Prevent anterior translation** | **Allows forward sliding (dislocation)** | | Anterior longitudinal ligament | Prevents hyperextension, provides some anterior support | Allows excessive extension | | Intervertebral disc | Weight-bearing, shock absorption | Allows vertical collapse, not primarily anterior translation | | Ligamentum flavum | Prevents gaping of laminae during flexion | Allows excessive flexion | **Clinical Pearl:** Unilateral facet dislocation (as in this case) is a **flexion-rotation injury**. The disrupted facet joint allows C5 to slide anteriorly on C6, compressing the ipsilateral nerve root (C6 root in this case, causing shoulder abduction weakness and biceps reflex loss). **High-Yield:** In cervical spine injuries, facet joint integrity is the primary defense against anterior translation. Loss of facet joint alignment = loss of anterior stability. ### Mnemonic: FACET FUNCTION **F**acet joints = **F**orward translation prevention **A**nterior ligament = **A**nti-extension **D**isc = **D**amping/load-bearing **L**igamentum = **L**ateral/laminar support