Introduction: Focal ossification of the ligamentum flavum (OLF) is a rare cause of thoracic myelopathy. The lower thoracic spine is most frequently affected and the patients present initially with posterior column disturbances followed by progressively increasing spastic paraparesis. The pathogenesis of OLF has not been conclusively established.
Materials and methods: Five patients with thoracic myelopathy due to OLF underwent decompressive laminectomy and excision of the ligamentum flavum. Magnetic resonance (MR) images consistently revealed a linear or beak-like excrescence, uniformly hypodense on T1- and T2-weighted images, situated posterior to the thecal sac. A comparison of the preoperative neurological status and at followup was done using the Japanese Orthopaedic Association and Nurick scores modified for thoracic myelopathy.
Results: Decompressive laminectomy with excision of the OLF resulted in significant improvement in motor weakness and gait in 4 (1 excellent, 2 good, and 1 fair) patients who had slow-onset, but progressive compression, OLF. The patient who had acute traumatic spinal injury did not recover despite decompression and rehabilitation. All improved in their gait and spasticity, but 4 patients had persistent sensory deficit.
Conclusion: OLF can significantly contribute to a spatial reduction of the thoracic spinal canal, resulting in slowly progressive paraparesis or acute paraplegia after trauma to the back. The T2-weighted sagittal image of MR imaging is the modality of choice for screening the longitudinal extent of OLF, with increased diagnostic accuracy when combined with computed tomographic myelogram. Neurological improvement usually occurs following decompression laminectomy with or without excision of the OLF. However, the persistence of residual numbness or weakness at follow-up may be due to irreversible changes within the cord as a result of severe thecal compression and the delay between the onset of initial symptoms/signs and surgical decompression. Prognosis remains poor for acute myelopathy with pre-existing OLF, despite surgery.