Allen classification of cervical injury is also called mechanistic classification and was given by Allen and associates in 1982.
The translation of kinetic energy into fractures and dislocations is determined by two independent variables
- Injury vector
- Posture of the cervical spine at the time of accident
Allen et al presumed that identical segmental failures could result from injury vectors of the same magnitude when applied to cervical spines set in similar postures.
Based on the mechanism of injury, fractures and dislocations occur in groups, with specific anatomic derangements. These families of fractures and dislocations include
- Compressive Flexion
- Vertical Compression
- Distractive Flexion
- Compressive Extension
- Distractive Extension
- Lateral Flexion
The terms describe the forces upon the cervical spine at the time of injury and the magnitude of the force vector. Within each category, a series of injuries were described from mild to severe stages.
Allen classification has been commonly used over the past two decades.
This classification does not allow comparison of neurological outcome between different categories of injuries as neurologic status is not included in this classification.
It also does not explain force vectors that cause rotation and their implication in spine stability.
SCIWORA are underrepresented and may lead to mistakes in terms of management and predicting clinical outcome.
Recently SLIC system has been advised and is the preferred classification. Allen classification is hardly used now.
Six common patterns of injury have been identified and each is further classified into stages based on the degree of injury to osseous and ligamentous structures.
Compressive Flexion Stage 1
There is blunting of the anterosuperior vertebral margin to a rounded contour, with no evidence of failure of the posterior ligamentous complex.
Compressive Flexion Stage 2
In addition to the changes seen in stage 1, obliquity of the anterior vertebral body with loss of some anterior height of the centrum [Central portion of vertebra]. The anteroinferior vertebral body has a “beak” appearance, concavity of the inferior endplate may be increased, and the vertebral body may have a vertical fracture.
Compressive Flexion Stage 3
In addition to the characteristics of a stage 2 injury, fracture line passing obliquely from the anterior surface of the vertebra through the centrum [Central portion of vertebra] and extending through the inferior subchondral plate, and a fracture of the beak.
Compressive Flexion Stage 4
Deformation of the centrum and fracture of the beak with mild (<3 mm) displacement of the inferoposterior vertebral margin into the spinal canal.
Compressive Flexion Stage 5
Bony injuries as in stage 3, but with more than 3 mm of displacement of the posterior portion of the vertebral body posteriorly into the spinal canal. The vertebral arch remains intact, the articular facets are separated, and the interspinous process space is increased at the level of injury, suggesting a posterior ligamentous disruption in a tension mode.
Vertical Compression Stage 1
Fracture of the superior or inferior endplate with a “cupping” deformity. Failure of the end plate is central rather than the anterior, and posterior ligamentous failure is not evident.
Vertical Compression Stage 2
Fracture of both vertebral endplates with cupping deformities. Fracture lines through the centrum may be present, but displacement is minimal.
Vertical Compression Stage 3
Progression of the vertebral body damage described in stage 2. The centrum is fragmented, and the displacement is peripheral in multiple directions. Most commonly, the centrum fails, with significant impaction and fragmentation.
The posterior aspect of the vertebral body is fractured and may be displaced into the spinal canal.
The vertebral arch may be intact with no evidence of ligamentous failure, or it may be comminuted with a significant failure of the posterior ligamentous complex; the ligamentous disruption is between the fractured vertebra and the one below it.
Distractive Flexion Stage 1
Failure of the posterior ligamentous complex, as evidenced by facet subluxation in flexion, with an abnormal divergence of the spinous process.
Distractive Flexion Stage 2
Unilateral facet dislocation. Subluxation of the facet on the side opposite the dislocation suggests severe ligamentous injury.
In addition, a small fleck of bone may be displaced from the posterior surface of the articular process, which is displaced anteriorly.
Widening of the uncovertebral joint on the side of the dislocation and displacement of the tip of the spinous process toward the side of the dislocation may be seen.
Distractive Flexion Stage 3
Bilateral facet dislocations, with approximately 50% anterior subluxation of the vertebral body.
Blunting of the anterosuperior margin of the inferior vertebra to a rounded corner may or may not be present.
Distractive Flexion Stage 4
Full vertebral body width displacement anteriorly or a grossly unstable motion segment, giving the appearance of a “floating” vertebra.
Compressive Extension Stage 1
Unilateral vertebral arch fracture with or without anterior rotatory vertebral displacement. Posterior element failure may consist of a linear fracture through the articular process, impaction of the articular process, and ipsilateral pedicle and lamina fractures.
Compressive Extension Stage 2
Bilaminar fractures without evidence of other tissue failures.
Typically, the laminar fractures occur at multiple contiguous levels.
Compressive Extension Stage 3
Bilateral vertebral arch fractures with a fracture of the articular processes, pedicles, lamina, or some bilateral combination, without vertebral body displacement.
Compressive Extension Stage 4
Bilateral vertebral arch fractures with partial vertebral body width displacement anteriorly.
Compressive Extension Stage 5
Bilateral vertebral arch fracture with full vertebral body width displacement anteriorly. The posterior portion of the vertebral arch of the fractured vertebra does not displace, and the anterior portion of the arch remains with the centrum.
Ligament failure occurs at two levels: posteriorly between the fractured vertebra and the one above it and anteriorly between the fractured vertebra and the one below it.
The anterosuperior portion of the vertebra below is sheared off by the anteriorly displaced centrum.
Distractive Extension Stage 1
Either failure of the anterior ligamentous complex or a transverse fracture of the centrum. The injury usually is ligamentous, and there may be a fracture of the adjacent anterior vertebral margin.
The radiographic clue to this injury is abnormal widening of the disc space.
Distractive Extension Stage 2
Evidence of failure of the posterior ligamentous complex, with the displacement of the upper vertebral body posteriorly into the spinal canal, in addition to the changes seen in stage 1 injuries.
Because displacement of this type tends to reduce spontaneously when the head is placed in a neutral position, radiographic evidence of the displacement may be minimal, rarely greater than 3 mm on initial films with the patient supine.
Lateral Flexion Stage 1
symmetrical compression fracture of the centrum and ipsilateral vertebral arch fracture, without displacement of the arch on the anteroposterior view. Compression of the articular process or comminution of the corner of the vertebral arch may be present.
Lateral Flexion Stage 2
lateral asymmetrical compression of the centrum and either ipsilateral displaced vertebral arch fracture or ligamentous failure on the contralateral side with separation of the articular processes.
Ipsilateral and compressive and contralateral disruptive vertebral arch injuries may be present.