Sacral Insufficiency Fracture
Overview & Epidemiology
The sacral insufficiency fracture is an increasingly common fragility fracture, part of the fragility fracture of the pelvis (FFP) spectrum, and a frequently missed cause of disabling pain in older people. It is high-yield because it is easy to miss, easy to mistake for cancer, and a sentinel marker of untreated bone fragility.
Who gets it. Elderly patients, predominantly postmenopausal osteoporotic women, and also patients after pelvic radiotherapy, on chronic steroids, or with rheumatoid, renal or metabolic bone disease. Incidence is rising with an ageing population, and the fracture is commonly under-diagnosed because the radiographs look normal.
Why it matters. Morbidity is high. Reported series give loss of independence in 64-89% and mortality of 13-27%, comparable to other major fragility fractures. It is also a care-gap condition, because most patients are not started on secondary fracture prevention, and that is the examiner's governance angle.
Pathophysiology: insufficiency vs fatigue & risk factors
Two kinds of stress fracture. A stress fracture is a fracture from repetitive sub-maximal load. In an insufficiency fracture, abnormal (weakened) bone fails under normal physiological load; in a fatigue fracture, normal bone fails under abnormal, repetitive load, as in a runner, dancer or military recruit. Do not swap them.
The sacral insufficiency fracture is the classic insufficiency type: an osteoporotic sacrum in an elderly patient, often after minimal or no trauma.
What weakens the bone. Osteoporosis is the commonest cause, but not the only one.
- Postmenopausal or senile osteoporosis
- Pelvic radiotherapy (radiation osteitis)
- Chronic glucocorticoids
- Rheumatoid arthritis
- Renal osteodystrophy and osteomalacia
- Hyperparathyroidism
- Paget disease
The fracture pattern. Weak bone fails under normal load at the ala first. The complete pattern is an H: bilateral vertical fractures through the sacral alae, lateral to the foramina, joined by a horizontal fracture across the sacral body. On a bone scan the same shape gives the Honda sign, and the complete H is the minority (see Investigations).
Classification — fragility fractures of the pelvis
The FFP classification. Rommens and Hofmann grade fragility fractures of the pelvis by anterior and posterior ring involvement and by stability, and the grade guides treatment: conservative for stable patterns, fixation for unstable or progressive ones. Progression from a stable to an unstable pattern is common, influenced by bone density, pelvic morphology and sarcopenia, so follow-up and treating the bone matter.
Anterior pelvic ring only (e.g. pubic rami) with an intact posterior ring. Stable, and managed conservatively.
The escalation above, conservative at FFP II and fixation at FFP III, rests on a distinction that observers apply inconsistently. In 100 pelvic CTs read twice by four observers, overall interobserver reliability was only moderate (kappa 0.42-0.59, 61% agreement). The subgroup that matters most was far worse: for FFP IIc, IIIc and IVb, the combined anterior-and-posterior injuries involving a complete non-displaced or displaced sacral fracture, interobserver kappa fell to 0.10-0.52 (Krappinger, PMID 30482410). Those are the patterns sitting on the conservative-versus-operative boundary.
So do not let a grade label make the decision on its own. In a borderline case, weigh displacement on reformatted CT, whether the patient can mobilise with analgesia, and whether serial imaging shows progression. The classification's own author notes that most FFPs are minimally displaced and need no surgery, so the default is non-operative care with bone-health treatment, and the burden of proof lies with escalating, not with waiting.
Denis zones and the sacral roots. FFP grades the pelvic ring and its stability; the Denis classification grades the sacral fracture itself by its medial–lateral position, and so predicts the neurological risk. A sacral fracture answer that skips the nerve-root examination loses marks.
- Zone 1 (alar). Lateral to the neural foramina, through the ala, the usual site of the insufficiency fracture's vertical limbs. Lowest neurological risk; if any, the L5 root, which runs over the ala.
- Zone 2 (transforaminal). Through the foramina. Intermediate risk, with unilateral L5/S1/S2 radiculopathy; a vertical-shear zone-2 pattern can be unstable.
- Zone 3 (central). Medial to the foramina, into the central sacral canal. Highest risk, with bilateral deficits and bowel, bladder and sexual dysfunction (cauda equina). The transverse component of the H or U pattern, spinopelvic dissociation, is a zone-3 injury.
Neurological injury rises with the zone, roughly 6%, 28% and 57% across zones 1-3 in the original Denis series.
Examine the sacral roots in every case. Test L5 (dorsiflexion, EHL), S1 (plantarflexion, ankle jerk) and the S2-S4 sphincteric roots (perianal sensation, anal tone, urinary function). A low-energy insufficiency fracture with a transverse or H (zone-3) pattern can still cause cauda equina, and is a surgical red flag.
Clinical Presentation: the 'great mimicker'
The story. Low back, buttock or groin pain, sometimes radiating to the hip or thigh, with difficulty weight-bearing and mobilising; the pain is worse with standing and walking. Onset is often insidious with no recalled injury, or only trivial trauma, in an elderly and often osteoporotic patient, or one who has had pelvic radiotherapy.
Why it is missed. The great mimicker passes for mechanical low back pain, hip pathology or metastatic disease. Examination is often non-specific, with sacral tenderness and pain on stressing the pelvis, and plain films are commonly normal, so the diagnosis needs cross-sectional imaging.
Investigations
Radiographs. Plain films are insensitive and frequently normal, because overlying bowel gas, osteopenia and the sacral curvature obscure the fracture. A normal radiograph is not a clearance: if an elderly osteoporotic patient cannot walk, the next test is MRI or CT, not another X-ray.
CT. CT is the gold standard for fracture morphology, showing a sclerotic fracture line or a cortical break, and it helps exclude a lytic, destructive lesion. It grades the FFP pattern, but it is not the most sensitive test, and MRI still finds the occult fractures.
MRI. MRI is the most sensitive test, at around 99%. It shows bone-marrow oedema in the sacral ala and the fracture line even when other imaging is normal, and it best distinguishes fracture from tumour.
Bone scan. A bone scan classically shows the Honda sign, an H-shaped pattern of increased sacral uptake. The full H is the minority: it was present in only 17 of 194 patients in one scintigraphy series, which the authors report as 4%. Unilateral, vertical-only or horizontal-only variants are common, so a missing H does not exclude the diagnosis.
Fracture or malignancy? Metastasis and myeloma present identically in this age group, so CT or MRI must distinguish a fracture from a destructive lytic lesion before you accept an insufficiency fracture. "Exclude malignancy" is the refrain, but the examiner wants the discriminating features.
- Benign insufficiency fracture. A linear fracture line, often vertical in the ala and parallel to the SI joint; marrow signal change that is band-like and tracks along the fracture line; a preserved cortex; no soft-tissue mass; sclerosis along the line on CT (healing). A bilateral-symmetric H pattern is essentially diagnostic of benign disease.
- Malignant lesion. A focal, rounded or geographic destructive lytic lesion with cortical destruction or breakthrough; mass-like marrow replacement (a nodular low-T1 focus, not a thin band along a line); an associated soft-tissue or epidural mass; often multiple lesions elsewhere.
- Genuinely uncertain. An atypical or asymmetric pattern, a focal mass, marrow change that does not track a fracture line, or a known primary calls for CT-guided biopsy, with myeloma and metastasis bloods: serum and urine electrophoresis and free light chains, calcium, PSA and the relevant tumour work-up.
Management
Conservative care is the default. The majority of sacral insufficiency fractures, the stable FFP I-II patterns, are managed with analgesia, early mobilisation, physiotherapy and fall prevention. Prolonged bed rest is harmful in the elderly, so mobilise early with analgesia.
Two things are non-negotiable in every patient: exclude malignancy, and investigate and treat the underlying osteoporosis with calcium, vitamin D and bone-protective therapy. Secondary fracture prevention is widely neglected, with about 71% of patients not receiving it, yet the fracture is a sentinel event for further fragility fractures. Arrange bone-health assessment, a falls assessment and orthogeriatric input, not just symptomatic relief.
When to escalate. Intervention is for refractory pain, or for a fracture that is unstable or progressing.
- Indication / note
- Refractory pain: significant, often rapid pain relief and reduced analgesic use. Limited stabilising capacity, so it is a pain procedure rather than a fixation for instability
- Indication / note
- Unstable or progressive posterior fractures; often cement-augmented in osteoporotic bone to improve purchase
- Indication / note
- Highly unstable patterns (FFP IV, spinopelvic dissociation, bilateral or displaced fractures), to restore spinopelvic stability
Complications
- Complication
- Prolonged pain, immobility, deconditioning
- Note
- The great mimicker is often missed on radiographs, which delays mobilisation and care
- Complication
- Loss of independence and mortality
- Note
- Figures under Overview; mandates orthogeriatric care
- Complication
- Further fragility fractures
- Note
- A sentinel event; secondary fracture prevention is the answer (Management)
- Complication
- Progression to an unstable pattern
- Note
- Stable FFP can progress, hence follow-up
- Complication
- VTE, pneumonia, pressure sores, decline
- Note
- Why early mobilisation, not bed rest, is the rule in the elderly
- Complication
- Cement leakage (sacroplasty); screw loosening/malposition
- Note
- Cement augmentation mitigates screw loosening in osteoporotic bone; sacral nerve roots at risk
Viva practice
Viva practice
Practise clinical reasoning and management decisions out loud
“An 78-year-old woman has 3 weeks of low back and buttock pain and difficulty walking, with no injury. Radiographs of the pelvis are normal. What is your differential and how do you investigate?”
“MRI confirms a sacral insufficiency fracture. How would you manage her, and when would you consider intervention or surgery?”
Exam cheat sheet
Definition
- Insufficiency stress fracture: weak (osteoporotic) bone, normal load
- vs fatigue fracture: normal bone, abnormal load
- Risk: osteoporosis, pelvic radiotherapy, steroids, RA, renal osteodystrophy
Presentation & imaging
- Elderly, insidious back/buttock/groin pain, hard to weight-bear, often no injury - the 'great mimicker'
- Radiographs usually normal; MRI most sensitive (~99%, marrow oedema); CT morphology / exclude lytic lesion
- Bone scan Honda/H sign (full H ~4%, uncommon); EXCLUDE malignancy
Classification
- Pattern: bilateral vertical alar + transverse = H (Honda); variants common
- Rommens-Hofmann FFP (I anterior → IV bilateral/unstable) guides treatment
- Progression to instability is common
Management
- Most conservative: analgesia, early mobilisation, falls prevention
- ALWAYS: exclude malignancy + treat osteoporosis (secondary prevention) + orthogeriatrics
- Sacroplasty for refractory pain; iliosacral/transsacral screws or triangular lumbopelvic fixation for unstable/progressive
Evidence Base
Comprehensive classification of fragility fractures of the pelvic ring: recommendations for surgical treatment
- Based on 245 consecutive patients, proposes the FFP classification graded by morphology and degree of instability (FFP I anterior only to FFP IV bilateral/displaced posterior).
- Most FFPs are minimally displaced and do NOT need surgery, but insidious progression to displacement, nonunion and instability occurs in some.
- The grade corresponds to instability and guides treatment, from minimally invasive techniques to complex reconstruction.
Chronic pelvic insufficiency fractures and their treatment
- Fragility/insufficiency fractures of the pelvis and sacrum are increasingly prevalent with high morbidity (loss of independence 64-89%, mortality 13-27%); 71% of patients do not receive adequate secondary fracture prevention.
- Radiography often misses sacral fractures; CT is the gold standard for morphology and MRI has the highest sensitivity (~99%) for bone oedema; the Rommens-Hofmann FFP classification guides treatment.
- Conservative treatment is possible; unstable/progressive fractures are treated surgically (sacroplasty, iliosacral screws, triangular lumbopelvic fixation, transsacral bars; cement augmentation improves osteoporotic fixation) within a multidisciplinary approach.
Inter- and intraobserver reliability and critical analysis of the FFP classification of osteoporotic pelvic ring injuries
- 100 pelvic CTs classified by 4 observers: overall interobserver reliability was MODERATE (kappa 0.42-0.59) and intraobserver reliability SUBSTANTIAL (kappa 0.68-0.72).
- Reliability was poorest for the subtypes involving a complete non-displaced or displaced sacral fracture (FFP IIc/IIIc/IVb).
- The acceptable-but-imperfect reliability, especially for the complex sacral subtypes, limits the FFP system's precision for clinical and research use.
Clinical and scintigraphic evaluation of insufficiency fractures in the elderly
- Of 194 elderly patients with scintigraphic insufficiency fractures, the typical H-shaped (Honda) sacral pattern was present in only 17 (the paper reports this as 4%) - its absence should not exclude the diagnosis; fractures were often at multiple sites (commonest the vertebrae).
- Of those with bone-density measured, 38 of 48 had osteoporosis/osteopenia.
- These patients had markedly poorer survival than the age-matched population, underscoring that making the diagnosis (and treating the bone) matters.
The FFP classification and its conservative-vs-operative logic come from Rommens & Hofmann 2013 (DOI); the imaging hierarchy, morbidity/under-treatment figures and treatment ladder from Gewiess et al. 2024 (DOI); the FFP reliability caveat from Krappinger et al. 2019 (DOI); and the point that the full Honda 'H' is uncommon plus the poor-survival association from Wat et al. 2007 (DOI). The insufficiency-vs-fatigue definition is standard teaching. For the high-energy counterpart — Denis zones, spinopelvic dissociation and lumbopelvic fixation — see sacral fractures and pelvic ring injuries; for the underlying bone disease, osteoporosis and glucocorticoid-induced osteoporosis.