A Rigid Spine at Risk: Chalk-Stick Fracture With Dural Sac Entrapment After Ground-Level Fall.

Ankylosing spondylitis predisposes patients to unstable spinal fractures following low-energy trauma because of progressive spinal ankylosis and loss of flexibility. These fractures may be difficult to recognize clinically and can be associated with delayed neurologic complications. We present the case of a 65-year-old male patient with ankylosing spondylitis and severe kyphosis who developed progressive left lower extremity pain, weakness, and numbness five days after a ground-level fall. Computed tomography (CT) of the lumbar spine revealed marked widening of the L1-L2 intervertebral disc space with a transverse fracture extending through the ankylosed spine and posterior elements. Magnetic resonance imaging (MRI) further characterized the injury, demonstrating disruption of the anterior and posterior longitudinal ligaments, disc-space widening, posterior element involvement, and entrapment of the anterior dural sac and ventral nerve roots within the fracture defect. Associated intradural hemorrhagic products, nerve root clumping, and paraspinal soft tissue edema were also identified. This case stresses the diagnostic difficulty of spinal trauma in ankylosing spondylitis and accentuates the complementary roles of CT and MRI in evaluating unstable chalk-stick fractures. Although CT readily identifies osseous injury, MRI can reveal clinically significant soft-tissue and neural element involvement that may not be fully appreciated on CT alone. Early recognition of these injuries is essential to reduce the risk of delayed neurologic deterioration and guide appropriate management.