Spinopelvic / Lumbosacral Dissociation (U-shaped Sacral Fracture)

Realigning the kyphotic sacrum matters, and the lateral view showing a sacral kyphosis/step explains why. The transverse fracture lets the upper sacrum and the spine above it flex (and often translate), producing a lumbosacral kyphosis. That angulation and translation narrows the sacral canal and tents/compresses the cauda equina and sacral roots, so it directly contributes to the neurological deficit; it also unbalances sitting and the spinopelvic sagittal alignment and, if left, causes progressive deformity and chronic pain.

So reduction is therapeutic, not cosmetic: correcting the kyphosis indirectly decompresses the canal and restores sitting/sagittal balance. The principle is to realign the kyphosis - typically prone positioning (which itself helps extend the lumbosacral junction), restoring sacral lordosis and correcting the angulation/translation, frequently using the lumbopelvic instrumentation as the reduction tool (cantilever / compression-distraction across the construct), under intra-operative neuromonitoring because reducing the deformity moves the already-compromised roots. Indirect reduction may relieve canal compromise, but a true deficit or persistent canal/root compression still warrants direct decompression.

This is the part of the operation with the best evidence behind it: in the 36-patient series, residual translational displacement and residual kyphosis of the transverse fracture both predicted the clinical outcome, while Roy-Camille type, timing and laminectomy did not. Reduction quality is the variable the surgeon controls that demonstrably changes the result. (The spinopelvic sagittal parameters are developed in Spinopelvic Parameters and the construct detail in Sacropelvic Fixation.)

A sacral neurological examination and counselling about incomplete recovery both rest on the same 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, while S1 contributes to plantarflexion. A central, canal-involving (Denis zone 3) fracture therefore characteristically produces a cauda-equina picture - urinary retention/overflow, bowel and sexual dysfunction, saddle anaesthesia and reduced anal tone and voluntary squeeze - and because the fracture is central the deficit is often bilateral. That is why the examination must include bladder and bowel function (with a post-void residual), perianal sensation, anal tone and sexual function, and a PR examination.

For prognosis: many patients recover at least partially after realignment (indirect decompression) and direct decompression of compromised roots, but recovery is unpredictable, and bladder, bowel and sexual function recover the least reliably - so counsel honestly, and note that in the largest series available 42% had a poor clinical outcome. Neurological status is tracked with the Gibbons grade (1 no deficit, 2 sensory, 3 motor, 4 sphincter), and decompression is undertaken for an established deficit or canal compromise, with wider sacral-fracture practice favouring within about 24 to 72 hours - while recognising that in the 36-patient series neither the timing of surgery nor whether a laminectomy was performed reached statistical significance, and the quality of reduction did. (General cauda equina syndrome is developed in Cauda Equina Syndrome and the root anatomy in Lumbosacral Plexus Anatomy; what follows here is the sacral-root deficit specific to this fracture.)

The injury definition, the missed-diagnosis pitfall, the imaging requirements and the lumbopelvic-fixation/triangular-osteosynthesis principle come from the cited Yi and Hak review. The S2AI lumbopelvic-fixation outcomes are the cited Luo series, which is 14 patients - small enough that its near-perfect results are quoted here alongside, not instead of, the larger figure. That larger figure, the predictors of outcome and all the negative findings - 42% poor outcome in 36 patients, the significance of initial and residual transverse displacement and residual kyphosis, and the absence of any significant association with Roy-Camille type, age, sex, ISS, timing of surgery or laminectomy - come from the cited Lindahl series, a single-centre retrospective cohort collected over 18 years.

Those negative findings are reported as measured rather than as recommendations: a series of 36 is underpowered to exclude a real effect of surgical timing, which is why an established deficit is still decompressed here, with the 24-to-72-hour window taken in substance from Sacral Fractures, where it is set out for sacral fractures generally with its own sources. The Gibbons grade definitions are likewise taken in substance from that page. The Roy-Camille classification of the transverse component and the Denis zones are standard, well-established teaching. (See also Pelvic Ring Injuries.)