Posterior ring apophyseal fracture or separation, also called limbus fracture, occur in the immature skeleton, most commonly in the lumbar spine. They represent bony fractures of the vertebral body rim at the site of attachment of the Sharpey fibers of the intervertebral disc.
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Terminology
Not to be confused with a limbus vertebra which is a normal anatomic variant.
Epidemiology
Typically, adolescent males practising sport activities.
Clinical presentation
back pain
sciatica
muscle weakness related with root innervation
association with Scheuermann disease
Pathology
The ring apophysis is a secondary ossification center of the vertebral endplate connected to the intervertebral disc. It is firmly attached to disc fibrous annulus through Sharpey fibers and its ossification occurs at 4-6 years old.
In the first years of life, the junction between ring apophysis and the rest of vertebral body is made through a cartilage layer, that is only completely ossified around 18 years old, and this is a weak point.
The pathophysiology remains unclear, although trauma (acute avulsion and/or chronic repetitive traction) is considered most likely. Other hypotheses suggest degenerative component, disc herniation, avascular necrosis, traction apophysitis, etc 5.
Classification
They can be classified as follows:
type I: avulsions of the posterior cortical vertebral rim
type II: central cortical and cancellous bone fractures
type III: lateralized chip fractures
type IV: span the entire length and breadth of the posterior vertebral margin between the endplates
Radiographic features
CT
CT is excellent for bony detail and is therefore usually the first line imaging modality. Findings include:
osseous fragment displaced posteriorly to endplate with rectangular or arc-shaped morphology on axial plane
posterior endplate defect
posterior disc herniation
MRI
MRI is indicated where evaluation of associated soft tissue structures is required. It is the modality of choice for evaluating neural structures. Findings may include:
high T2/STIR signal of acute fracture
corner defect on posterior endplate margin
disc degeneration and loss of height
Differential diagnosis
On imaging consider:
disc calcification/ossification
calcified disc fragment
posterior osteophyte