Spinopelvic / Lumbosacral Dissociation (U-shaped Sacral Fracture)

The injury. Bilateral vertical sacral fractures, one through each ala or foraminal region, are joined by a transverse fracture of the sacral body. The upper-central sacrum, and the entire spine above it, is then mechanically dissociated from the rest of the sacrum and the pelvis. The way the transverse and vertical lines meet gives the characteristic shapes: U, H, Y, lambda or T.

Mechanism. High-energy axial loading, classically a fall from a height or a high-speed collision. The fall gives it the name "jumper's fracture", and it often comes with calcaneal, tibial-plateau or spinal burst fractures, so the injury is typically part of severe polytrauma.

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The transverse component: Roy-Camille. The Roy-Camille type communicates the direction in which the transverse fracture displaces the upper sacral fragment, and remains the standard way to describe the pattern. Whether it predicts anything is a separate question, taken up under outcome below.

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The vertical components: Denis zones. The Denis zones grade the vertical components of a sacral fracture by where they run relative to the foramina:

• Zone 1 - lateral to the foramina (the ala)
• Zone 2 - transforaminal
• Zone 3 - medial to the foramina, the central canal

Spinopelvic dissociation involves the central sacrum (zone 3) and the canal, the zone with the highest neurological risk. That is why the neurological-injury rate is high: the cauda equina and the sacral roots controlling bladder, bowel and sexual function are in the way.

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Suspect it. The patient is usually a polytrauma victim, after a fall from height or a motor vehicle collision, and is managed as polytrauma under ATLS. The injury is easily missed and the diagnosis often delayed, because the associated injuries are severe and the transverse component is hard to see on the AP pelvis. Actively suspect it after a fall from height or with unexplained sacral neurology.

Examination. Cauda equina and sacral-root injury is common, so every suspected case has a full sacral neurological examination and a search for associated injuries:

• Bladder and bowel function, with a post-void residual
• Perianal sensation, anal tone and a PR examination
• Sexual function
• The lower-limb roots
• The soft tissues, for degloving or a Morel-Lavallée lesion
• Associated spinal and limb injuries

Imaging.

• Pelvic radiographs - AP, inlet and outlet views
• Lateral sacral radiograph - the key to seeing the transverse component, as a sacral kyphosis or step
• CT with reconstructions - essential to define the full pattern; the diagnosis is often made here
• MRI - the lumbosacral roots and cauda equina

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Why surgery. The lumbopelvic junction is grossly unstable, so treatment is usually surgical. Minimally displaced, neurologically intact fractures may occasionally be managed non-operatively, but the unstable, displaced or neurologically involved patterns need fixation.

The three aims.

1. Realignment and reduction of the displaced, kyphotic sacrum (see the sacral kyphosis, below)
2. Stable fixation of the lumbopelvic junction
3. Sacral decompression, for example sacral laminectomy or foraminal decompression, when there is a neurological deficit or canal or root compromise

Lumbopelvic fixation. Lumbopelvic (spinopelvic) fixation connects the lower lumbar spine, through L4/L5 pedicle screws, to the pelvis, through iliac or S2-alar-iliac (S2AI) screws. It is often combined with iliosacral screws as triangular osteosynthesis. The result is robust, load-sharing fixation that resists the high shear at the lumbosacral junction and allows early mobilisation of the polytrauma patient.

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What to promise. A small series of 14 patients treated with S2AI lumbopelvic fixation reported good reduction and function, union in all, neurological recovery and low complication rates. Set that optimism against the larger series of 36 analysed under outcome below, and counsel accordingly.

What the deformity does. The transverse fracture lets the upper sacrum and the spine above it flex, and often translate, producing a lumbosacral kyphosis: the kyphosis or step seen on the lateral view. That angulation and translation narrow the sacral canal and tent or compress the cauda equina and sacral roots, so the deformity directly contributes to the neurological deficit. It also unbalances sitting and spinopelvic sagittal alignment and, if left, causes progressive deformity and chronic pain.

Reduction is therapeutic, not cosmetic. Correcting the kyphosis indirectly decompresses the canal and restores sitting and sagittal balance. It is also the part of the operation with the best evidence behind it, set out under outcome below.

How. The principle is to realign the kyphosis:

• Typically prone positioning, which itself helps extend the lumbosacral junction
• Restore sacral lordosis and correct the angulation and translation
• Frequently use the lumbopelvic instrumentation as the reduction tool, by cantilever or compression-distraction across the construct
• Intra-operative neuromonitoring, because reducing the deformity moves roots that are already compromised

When indirect is not enough. Indirect reduction may relieve canal compromise, but a true deficit or persistent canal or root compression still warrants direct decompression. The spinopelvic sagittal parameters are developed in Spinopelvic Parameters and the construct detail in Sacropelvic Fixation.

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The anatomy. The sacral canal carries the cauda equina and the sacral nerve roots. S2-S4 provide the parasympathetic control of bladder and bowel, the anal and urethral sphincters, perianal (saddle) sensation and sexual function; S1 contributes to plantarflexion. The same anatomy underlies both the examination and the counselling about incomplete recovery.

The picture. A central, canal-involving (zone 3) fracture characteristically produces a cauda-equina picture, and because the fracture is central the deficit is often bilateral:

• Urinary retention or overflow
• Bowel and sexual dysfunction
• Saddle anaesthesia
• Reduced anal tone and voluntary squeeze

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Grading the deficit. The Gibbons grade measures the deficit itself rather than the fracture's location, and the change in grade is the standard way outcome is reported after decompression and fixation:

• Grade 1 - no deficit
• Grade 2 - sensory deficit only
• Grade 3 - lower-limb motor deficit
• Grade 4 - bowel or bladder (sphincter) dysfunction

Record it alongside a documented perianal (S2-S4) examination and the bulbocavernosus reflex, so the baseline is unambiguous.

Recovery. Many patients recover at least partially after realignment (indirect decompression) and direct decompression of compromised roots, but recovery is unpredictable. Recovery is least reliable for bladder, bowel and sexual function, and in Gibbons terms for the higher grades, sphincter involvement above all. Counsel honestly. General cauda equina syndrome is developed in Cauda Equina Syndrome and the root anatomy in Lumbosacral Plexus Anatomy; this section covers the sacral-root deficit specific to this fracture.

The uncomfortable headline. In 36 consecutive H-type spinopelvic dissociations treated with lumbopelvic fixation and neural decompression over an 18-year period, with a minimum 18-month follow-up, excellent or good radiological results were achieved in the vertical components in every single patient. Yet 15 of the 36 (42%) still had a poor clinical outcome.

What that means. Getting the vertical fractures to look right on the postoperative film is not the same as getting the patient right, and a substantial minority do badly despite a technically satisfactory construct. It is the honest figure to carry into a consent discussion.

Two consequences follow, and both are examinable because they run against the intuitive answer.

Roy-Camille classifies the injury; it does not predict it. Comparing type 2 against type 3 did not predict neurological impairment. The authors instead propose the simpler and more useful split of the transverse fracture into partially displaced versus completely displaced, which did predict the rate of neurological recovery.

Accurate reduction is the variable the surgeon controls that actually matters. Initial displacement is fixed by the injury, but residual displacement and residual kyphosis are chosen in theatre, and both predicted the clinical score. This is the evidence behind reducing the sacral kyphosis rather than merely spanning it.