13.1 DXI Skeletal Trauma

Why Skeletal Trauma Patterns Matter for DXI

DXI Trauma questions on Part III rarely ask you to simply spot "a fracture." They ask you to name a specific pattern from its mechanism and imaging appearance, judge whether that pattern is biomechanically stable, and decide whether plain film is sufficient or whether CT or MRI is required before clearing the patient. Examiners build these vignettes around classic mechanism-to-pattern pairings (axial load → burst, flexion-distraction → Chance, hyperextension + axial load → hangman), so the fastest path to the correct answer is matching the story in the stem to the fracture family it describes.

The Three-Column Model and Stability

The most useful stability framework is the Denis three-column concept of the spine:

• Anterior column — anterior two-thirds of the vertebral body and anterior longitudinal ligament/annulus
• Middle column — posterior one-third of the vertebral body, posterior wall, and posterior longitudinal ligament
• Posterior column — pedicles, laminae, facet joints, spinous process, and posterior ligamentous complex

As a general rule, injury confined to one column is usually stable, two-column injury is potentially unstable, and three-column injury is unstable. This model explains why a compression fracture (anterior column only) is usually managed conservatively while a burst fracture (anterior + middle, sometimes posterior) requires much closer scrutiny.

Major Fracture Patterns

Compression and Burst Fractures

A compression (wedge) fracture involves only the anterior column: the anterior vertebral body loses height while the posterior body height and posterior wall stay intact. It is the most common fracture pattern seen on DXI films, occurring from low-energy falls in osteoporotic patients or higher-energy flexion loading in younger patients, and it is usually stable unless height loss exceeds roughly 50% or multiple contiguous levels are involved, which raises concern for progressive kyphotic deformity.

A burst fracture is the more dangerous look-alike: axial loading drives the disc into the vertebral body, fracturing both the anterior AND posterior vertebral body walls, with the posterior fragment retropulsed into the spinal canal. Plain film clues include a widened interpedicular distance on the AP view and loss of posterior body height on the lateral view, but plain film routinely underestimates canal compromise — CT is mandatory to characterize the degree of retropulsion and guide management, and neurologic injury is far more common than with a simple compression fracture.

Chance Fracture

The Chance fracture (classically the "seatbelt fracture") results from a flexion-distraction mechanism — the body flexes forward over a fixed lap belt while the posterior elements distract. The fracture line runs horizontally through the spinous process, both pedicles, and the vertebral body (either entirely through bone, or through a mix of bone and ligament in the distraction variant), most often at the thoracolumbar junction. Because the flexion point is anterior to the spine, this injury carries a strong association with intra-abdominal visceral injury (bowel, mesentery, pancreas) — any Chance fracture on imaging should prompt abdominal evaluation. The pattern can look deceptively unremarkable on the AP view; the horizontal fracture through the posterior elements is best appreciated on the lateral radiograph or sagittal CT.

Hangman and Jefferson Fractures

The hangman fracture is a bilateral fracture of the C2 pars interarticularis producing traumatic spondylolisthesis of C2 on C3. The mechanism combines hyperextension with axial loading or distraction — historically judicial hanging, more commonly today diving or motor-vehicle collisions. The Effendi classification (Types I–III) grades severity by displacement and angulation; despite the alarming mechanism, many patients remain neurologically intact because the C2 canal is wide and the fracture pattern tends to decompress rather than compress the cord, though Type III injuries with facet dislocation are unstable.

The Jefferson fracture is a burst-type fracture of the atlas (C1) from axial loading transmitted through the occipital condyles, classically fracturing both the anterior and posterior arches bilaterally (a "four-part" fracture). The key radiographic tool is the Rule of Spence: on the open-mouth (AP odontoid) view, measure the combined lateral overhang of both C1 lateral masses beyond the lateral margins of C2. A combined overhang greater than 6.9 mm (commonly rounded to 7 mm) predicts disruption of the transverse ligament of the atlas and therefore atlantoaxial instability. Because subtle arch fractures are easily missed on plain film, CT is the definitive study whenever a Jefferson fracture is suspected.

Dens (Odontoid) Fractures

Odontoid fractures are staged with the Anderson-D'Alonzo classification:

• Type I — avulsion fracture of the tip of the dens; rare, generally stable
• Type II — fracture at the base of the dens where it meets the C2 body; the most common type and the one with the highest risk of nonunion, frequently requiring surgical stabilization
• Type III — fracture line extends into the body of C2; usually heals well with immobilization because of the larger cancellous bone surface

The mechanism is typically flexion or extension force combined with axial loading, and elderly fall patients are a particularly high-risk population. CT is the primary imaging modality for suspected dens fracture because the open-mouth plain film view is frequently obscured by overlying mandible and occiput, and the Mach effect at bony margins can mimic a fracture line that is not actually present.

Stress Fractures and Spondylolysis

Stress fractures fall into two categories: a fatigue fracture occurs in normal bone subjected to abnormal repetitive stress (athletes, military recruits), while an insufficiency fracture occurs in weakened bone (osteoporosis, osteomalacia) under normal stress (common in the sacrum and pubic rami of elderly patients). Plain films are frequently normal for the first two to three weeks of a stress fracture, so MRI is the most sensitive early study, showing bone marrow edema on T2/STIR sequences before any cortical change is visible; bone scan or SPECT is also sensitive but less specific than MRI.

Spondylolysis is a stress fracture of the pars interarticularis, overwhelmingly most common at L5 (then L4), typically in adolescent athletes performing repetitive extension and rotation (gymnastics, weightlifting, fast bowling, interior linemen). On the oblique lumbar radiograph, look for the "Scotty dog" sign: a lucent defect through the dog's "neck" (the pars) represents the fracture, while the ear (superior articular process), eye (pedicle), nose (transverse process), front leg (inferior articular process), and body (lamina) form the rest of the silhouette. Bilateral pars defects can progress to spondylolisthesis, graded by the Meyerding classification (Grade I = 0–25% anterior slip, II = 26–50%, III = 51–75%, IV = 76–100%, V = spondyloptosis, greater than 100%). CT best confirms an established bony defect, while SPECT or MRI is preferred for detecting an early, active stress reaction before a complete cortical break has formed.

When CT or MRI Is Needed

• CT — any high-energy trauma with suspected fracture, especially the craniocervical junction; any burst fracture (to quantify canal compromise); equivocal or subtle plain film findings; pre-surgical planning; and polytrauma protocols, where CT has largely replaced plain film as the initial cervical spine study in many trauma centers.
• MRI — any patient with a neurologic deficit; suspected ligamentous injury without a visible fracture; suspected spinal cord injury, including cord injury without radiographic abnormality; an active stress reaction before a cortical break is visible on CT; and persistent focal pain with a normal or equivocal CT, where bone marrow edema on MRI can reveal an occult fracture.