Sacral Fracture

Sacral fracture is a break in the sacrum, usually as part of a pelvic ring injury from high energy trauma in young adults, or as an insufficiency fracture in older adults with osteoporosis, after radiotherapy or with long term corticosteroid use. Up to 45% of unstable pelvic ring injuries include a sacral fracture, and many are missed on plain radiographs.

The Denis classification describes vertical fractures by their relation to the foramina.

• Zone I is lateral to the foramina (about 50%). Nerve injury occurs in about 5%.
• Zone II passes through the foramina (about 34%). L5 and S1 root injury occurs in about 28%.
• Zone III is medial to the foramina through the central canal (about 16%). Nerve injury and cauda equina syndrome occur in over 50%.

Transverse sacral fractures (Roy-Camille types) lie caudal to the sacroiliac joints and often cause cauda equina injury.

• Type 1 is a flexion deformity without displacement.
• Type 2 has posterior displacement of the upper segment with flexion.
• Type 3 has anterior translation.
• Type 4 is a comminuted fracture from axial loading.

A U-shaped or H-shaped fracture with a transverse component and bilateral vertical limbs is spinopelvic dissociation, a lumbopelvic instability.

Complications are neurological deficit (bladder, bowel and sexual dysfunction, foot drop, saddle numbness), malunion, nonunion, chronic pain, infection after open injury, pressure injury and venous thromboembolism.

Pathophysiology.

Axial loading through the spine, a fall landing on the buttocks, or lateral compression of the pelvis breaks the thin cancellous sacral bone. The sacral foramina are weak points, and the sacral nerve roots lie close to the fracture lines.

In osteoporosis the sacral alae fail under normal loads (the insufficiency fracture). The typical H-shaped pattern (Honda sign) is seen on bone scan or MRI. Bilateral alar fractures with a transverse line produce spinopelvic instability.

Low back, buttock and groin pain, worse with weight bearing, sitting and rolling. Tenderness over the sacrum and sacroiliac joints, with bruising or a haematoma over the sacrum (Morel-Lavallée lesion if the skin is degloved). Insufficiency fractures give an insidious onset of pain after minor or no trauma.

Document the lumbosacral neurology carefully: L5 and S1 motor and sensation, saddle sensation, anal tone and voluntary contraction, bulbocavernosus reflex, and bladder function. Palpate the sacrum and perform a rectal and vaginal examination for open injury. Check pulses and look for associated pelvic, spinal and abdominal injuries.

• Anteroposterior pelvis, inlet and outlet radiographs. Findings are subtle: disruption of the arcuate lines, foraminal asymmetry and a fracture of the L5 transverse process or ischial spine.
• CT of the pelvis and lumbosacral spine is the investigation of choice and defines the pattern, displacement and canal compromise.
• MRI for insufficiency fractures with a normal CT (the H-shaped signal), for nerve root injury and for cauda equina compression.
• Technetium bone scan if MRI is not available (the Honda sign).
• Haemoglobin, lactate, coagulation and creatinine in trauma. Bone profile, calcium, 25-hydroxyvitamin D, parathyroid hormone, thyroid and myeloma screen in insufficiency fractures.
• Urodynamic and anorectal assessment when there is sphincter dysfunction.

Differential diagnosis.

• Sacroiliac joint dislocation and pubic ramus fracture.
• Acetabular fracture and lumbar fracture.
• Metastasis and pathological fracture.
• Sacroiliitis, disc herniation and coccygeal fracture.

Stable undisplaced fractures (including most insufficiency fractures).

• Analgesia, early mobilisation with a walking aid and weight bearing as tolerated, with physiotherapy. Bed rest causes deconditioning and is avoided.
• Sacroplasty or percutaneous iliosacral screw augmentation for severe pain that persists beyond 6 weeks in selected patients.

Unstable or displaced fractures.

• Posterior pelvic ring fixation with iliosacral screws (7.0 mm cannulated), transsacral screws, sacroiliac plating or tension band plating.
• Spinopelvic dissociation: lumbopelvic fixation with pedicle screws in L4 and L5 connected to iliac screws, with or without decompression of the canal. Weight bearing is restricted for 8 to 12 weeks.
• Neurological deficit with canal compromise or progressing signs: decompression by laminectomy, with fixation. Cauda equina syndrome needs urgent decompression.
• Open fractures: debridement, antibiotic cover for open pelvic injury, and faecal diversion for rectal laceration.

Open pelvic injury (wounds communicating with the perineum, vagina or rectum).

• Give intravenous co-amoxiclav 1.2 g every 8 hours with metronidazole 500 mg intravenously every 8 hours. Alternatively give cefazolin 2 g every 8 hours with metronidazole.
• Start within 1 hour of injury. Continue for 24 hours after definitive wound control and for no more than 72 hours.
• Co-amoxiclav maximum is 3.6 g daily. Give every 12 hours at creatinine clearance 10 to 30 mL/min. Check liver tests, and reduce the metronidazole dose in severe hepatic impairment.
• For gross contamination add gentamicin 5 mg/kg intravenously once daily for up to 72 hours. Check creatinine daily and a level before the second dose. Avoid, or extend the interval, at eGFR under 60 mL/min/1.73 m2.
• With severe penicillin allergy use clindamycin 600 mg intravenously every 8 hours (maximum 2.7 g daily) with gentamicin.
• Give a tetanus booster 0.5 mL intramuscularly if the last dose was over 10 years ago.
• Add tetanus immunoglobulin 250 units intramuscularly when immunisation is incomplete or unknown. Give 500 units if over 24 hours since injury or heavily contaminated.

Operative antibiotic prophylaxis.

• Cefazolin 2 g intravenously (3 g when weight is 120 kg or over) within 60 minutes before incision, as a single dose. Repeat after 4 hours or blood loss over 1500 mL.
• With severe penicillin allergy, vancomycin 15 mg/kg intravenously starting 90 to 120 minutes before incision, as a single dose. Check levels and creatinine in prolonged cases.

Thromboprophylaxis.

• Enoxaparin 40 mg subcutaneously once daily. Start within 24 to 48 hours once bleeding is controlled and haemoglobin is stable. Continue for at least 28 days or until the patient is mobile.
• Use 20 mg once daily at creatinine clearance under 30 mL/min. Adjust by anti-Xa level in obesity and renal impairment.
• Check the platelet count on days 5 to 14 for heparin induced thrombocytopenia.
• Use mechanical prophylaxis until enoxaparin starts.

Analgesia.

• Paracetamol 1 g orally or intravenously every 6 hours (maximum 4 g daily; 3 g daily when weight is under 50 kg, with chronic alcohol use or hepatic impairment).
• Oral morphine 2.5 to 5 mg every 4 hours as needed for severe pain in older adults, and 5 to 10 mg in younger adults, for up to 5 days. Avoid exceeding 60 mg in 24 hours without senior review. Start 2.5 mg with age over 75 or eGFR under 30 mL/min/1.73 m2. Monitor sedation and respiration, and prescribe a laxative.
• Ibuprofen 400 mg orally every 8 hours with food for up to 5 days (maximum 1200 mg daily) once bleeding is excluded. Avoid with eGFR under 30 mL/min/1.73 m2, peptic ulcer, heart failure or anticoagulant use.
• Neuropathic pain from root injury: gabapentin 300 mg orally at night, increased by 300 mg every 1 to 3 days to 300 to 600 mg three times daily (maximum 1800 mg daily). Reduce at creatinine clearance under 60 mL/min. Monitor sedation and dizziness.

Secondary prevention after a fragility fracture.

• Alendronate 70 mg orally once weekly (maximum 70 mg weekly), for 5 years then reassess. Take fasting with a full glass of water and stay upright for 30 minutes.
• Check calcium, 25-hydroxyvitamin D and creatinine before starting and yearly. Avoid with eGFR under 35 mL/min/1.73 m2, uncorrected hypocalcaemia or oesophageal stricture.
• Zoledronic acid 5 mg intravenously over at least 15 minutes once yearly (for 3 years, then reassess) when oral therapy is not tolerated and eGFR is 35 mL/min/1.73 m2 or over. Check creatinine and calcium before each dose.
• Colecalciferol 800 to 2000 IU orally daily with dietary calcium to a total of 1000 to 1200 mg daily (maximum total 2000 mg daily). Correct vitamin D deficiency first.

Monitoring.

• Neurology and bladder function every 4 hours in the first 24 hours, then daily in hospital.
• Post-void residual volume when bladder function is uncertain.
• Radiographs after fixation, at 2 weeks, 6 weeks, 3 months and 12 months.
• Bone mineral density at 3 months after an insufficiency fracture.
• Nerve recovery with electromyography at 3 to 6 weeks when a deficit persists.

Referral and escalation. Refer every sacral fracture beyond a stable undisplaced insufficiency fracture to orthopaedics or spinal surgery the same day. Emergency escalation for cauda equina syndrome, progressive neurological deficit, shock, open wounds, or a Morel-Lavallée lesion. Refer insufficiency fractures to bone health and falls services.

Special groups.

• Older adults: insufficiency fractures cause prolonged immobility. Mobilise early, treat pain, assess bone health and investigate the fall.
• Pregnancy: left lateral tilt of 15 degrees, early obstetric input and foetal monitoring when over 24 weeks.
• Children: sacral fractures are uncommon and accompany pelvic ring injury; associated visceral injury is frequent.
• Kidney impairment: avoid ibuprofen and bisphosphonates at eGFR under 35 mL/min/1.73 m2, reduce gabapentin and extend gentamicin intervals.