Posterior ligamentous complex injury

Last revised by Arlene Campos on 4 Jun 2024

Posterior ligamentous complex injury refers to tears/ruptures of the spinal posterior ligamentous complex, which consists of the ligamentum flavum, interspinous ligaments, supraspinous ligament, and facet joint capsules. Posterior ligamentous complex disruption is a central part of the popular classification systems for spinal injuries including:

Patients present following blunt trauma to the spine, such as a fall or motor vehicle accident. On physical examination, they may have a palpable interspinous defect or focal posterior tenderness 1.

Posterior ligamentous complex injury is inferred according to bony landmarks and injuries on radiography or CT and is directly assessed on MRI.

Plain radiography for primary evaluation of spinal trauma is outdated because CT is more sensitive. In settings where CT is not readily available, the following radiographic signs have been described in association with posterior ligamentous complex injury 1,2:

  • vertebral translation or rotation, including perched or dislocated facets

  • interspinous spacing (splaying) greater than that of levels above or below

  • focal kyphosis without vertebral body injury

  • severe (>50%) compression of the anterior vertebral body without fracture of the posterior wall

  • facet joint diastasis

  • avulsion fracture of the superior or inferior aspect of a spinous process

Among these signs, the first two (vertebral displacement and widened interspinous space) are considered the most accurate for inferring the presence of posterior ligamentous complex injury, while the other signs are less reliable.

Plain radiographs remain more useful than CT/MRI in the evaluation of alignment in positions other than supine (e.g., upright, flexion and extension), which can dynamically reveal a pathologic listhesis or distraction.

CT is the first-line imaging study for spinal trauma. CT can detect all the aforementioned signs on plain radiography with greater sensitivity.

In addition, cervical spine CT can detect obliteration of the paraspinal fat pad (posterior perivertebral space underlying the nuchal ligament) by hemorrhage/edema, which is another indirect sign of posterior ligamentous complex injury 3.

MRI directly evaluates the posterior ligamentous complex. The primary findings of injury are the following 1,2:

  • T1: disruption of the hypointense band ("black stripe") of the ligaments on sagittal images

  • STIR or T2 FS: edema (high signal intensity) in the region of the posterior ligamentous complex elements on sagittal images

  • T2: facet joint fluid on axial images; disruption of the hypointense band ("black stripe") of the ligaments on sagittal images

The five components of the posterior ligamentous complex can be separately evaluated at each motion segment:

  • for the cord-like components (supraspinous ligament and ligamentum flavum), the status is usually binary as ruptured/torn or intact based on the continuity of the black stripe 4

  • the interspinous ligaments are membranous and often show interspersed areas of high signal on STIR/T2 FS without total discontinuity, which may be described as indeterminate, incomplete disruption, partial tear, or edema 2,4‐6

  • the facet joint capsules are also difficult to evaluate definitively (unless there is wide diastasis, subluxation, or dislocation) and the main abnormal but nonspecific finding is the presence of joint fluid

To assess the status of the posterior ligamentous complex as a whole, it has been proposed that the supraspinous ligament is the single most important indicator 4,7. In a sequential model of posterior ligamentous complex injuries 7, facet diastasis/fluid and interspinous ligament edema occur first and may be present with injuries that are considered stable. Rupture of the supraspinous ligament and then the ligamentum flavum occur in potentially unstable injuries. Vertebral displacement is associated with complete ruptures of the posterior ligamentous complex and facet fracture or dislocation.

Injury of the posterior ligamentous complex is often biomechanically unstable, resulting in vertebral displacement/translation and subluxation or dislocation of the facet joint:

In the long term, posterior ligamentous complex injuries have a tendency not to heal and, due to imbalanced compressive forces on the anterior vertebral body, can promote progressive kyphosis and vertebral body collapse.

Thus, posterior ligamentous complex injuries often are treated with surgical stabilization.

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