Clinical History: 55 year-old man with right leg pain. T1-weighted sagittal (1a), T2-weighted sagittal (1b) and T2-weighted axial (1c) images are provided. What is your diagnosis?
Right paracentral free disc fragment from L5-S1, resulting in mass effect on the right S1 nerve root.
An important, frequently difficult to diagnose, type of disc herniation is a free disc fragment or sequestered disc herniation. A sequestered disc herniation occurs when extruded disc material is no longer contiguous with the parent nucleus pulposus. The free disc fragment typically lateralizes and may migrate superiorly or inferiorly. Rare cases of intradural and posterior epidural extension have also been described.1,2
Sequestered disc fragments are sometimes difficult to detect with MRI. In addition to their propensity to migrate, free fragments often demonstrate high signal intensity on T2-weighted images, thought to result from increased water content.3 On T2-weighted non-fat-suppressed images, the high signal intensity disc fragment may blend with epidural fat and cerebrospinal fluid within the thecal sac. In these cases, T1-weighted images become crucial in detecting the free disc material. The sequestered fragment maintains an intermediate to low signal on T1-weighted images, allowing ready differentiation of the fragment from epidural fat.
If a free fragment is not recognized on MR images, a confusing clinical picture may result. A migrated free fragment may cause nerve root compression at a level different than the parent disc, yet changes at the parent disc may be the most obvious abnormality on the MR. Clinical findings may thus contradict the patient’s most noticeable MR findings. Careful image evaluation allows the true cause of patient symptoms to be recognized in such cases.
The diagnosis of a sequestered disc herniation may significantly alter treatment options. Free fragments are a contraindication to percutaneous discectomy and chemonucleolysis techniques.4 The presence of a free fragment often necessitates a more extensive surgical approach to ensure complete removal. Failure to recognize and remove a free disc fragment will often lead to an unsatisfactory post-operative result, underscoring the importance of their detection in pre-surgical planning.
Free disc fragments may exhibit MR signal characteristics that make their detection difficult. Reliance on T2-weighted images for the detection of disc herniations may result in decreased sensitivity for free disc fragments, which often demonstrate high signal intensity on T2-weighted images. Utilization of T1-weighted sequences greatly improves the detection of disc fragments in such cases.
1 Hodges SD, Humphreys SC, Eck JC, Covington LA:Posterior Extradural Lumbar Disk Fragment. Journal of the Southern Orthopaedic Association 1999;8(3):222-228
2 Snow RD, Williams JP,Weber ED, et al:Enhancing transdural lumbar disk herniation. Clin Imaging 19:12-16, 1995
3 Glickstein MF, Burke L, Kressel HY: Magnetic resonance demonstration of hyperintense herniated discs and extruded disk fragments. Skeletal Radiol 1989;18:527-530
4 Modic MT: Degenerative disorders of the spine. In: Magnetic Resonance Imaging of the Spine (chapter 3); Modic MT, Masaryk TJ, Ross JS (eds). Year Book Medical Publishers, Inc.; Chicago 1989.