Sacral Insufficiency / Stress Fracture

• Demographics: elderly, predominantly postmenopausal osteoporotic women; also patients after pelvic radiotherapy, on chronic steroids, or with rheumatoid/renal/metabolic bone disease.
• Rising incidence with an ageing population; commonly under-diagnosed because radiographs look normal.
• High morbidity: loss of independence in ~64-89% and reported mortality of ~13-27% - comparable to other major fragility fractures.
• A care-gap condition: ~71% of patients are NOT started on secondary fracture prevention - the examiner's governance angle.

A stress fracture is a fracture from repetitive sub-maximal load, and it has two types:

• Insufficiency fracture — abnormal (weakened) bone fails under normal physiological load. The sacral insufficiency fracture is the classic example: an osteoporotic sacrum in an elderly patient, often after minimal or no trauma.
• Fatigue fracture — normal bone fails under abnormal, repetitive load (runner, dancer, military recruit).

Beyond osteoporosis, weakened-bone causes include pelvic radiotherapy, chronic glucocorticoids, rheumatoid arthritis, renal osteodystrophy, Paget disease, hyperparathyroidism and osteomalacia. The sacral insufficiency fracture sits within the fragility fracture of the pelvis (FFP) spectrum and signals under-treated bone fragility.

The FFP system grades the pelvic ring/stability; the Denis classification grades the sacral fracture itself by its medial–lateral position and predicts the neurological risk — and a sacral fracture topic that skips the nerve-root exam 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 (it runs over the ala).
• Zone 2 (transforaminal): through the foramina. Intermediate risk — 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 neurological risk — bilateral deficits and bowel/bladder/sexual dysfunction (cauda equina). The transverse component of the H/U pattern, i.e. 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).
• So examine the sacral roots in every case: 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/H (zone-3) pattern can still cause cauda equina and is a surgical red flag.

Plain radiographs are insensitive and frequently normal (overlying bowel gas, osteopenia and the sacral curvature obscure the fracture). CT is excellent for fracture morphology (a sclerotic fracture line/cortical break) and helps exclude a lytic/destructive lesion. MRI is the most sensitive test (around 99%), showing bone-marrow oedema in the sacral ala and the fracture line even when other imaging is normal — and it best distinguishes fracture from tumour. A bone scan classically shows the 'Honda sign' (an H-shaped pattern of increased uptake), though the full H is actually uncommon (~4% in one scintigraphy series) — so a missing H does not exclude the diagnosis.

"Exclude malignancy" is the refrain — but the examiner wants the discriminating features of a benign insufficiency fracture versus a metastasis/myeloma in the same elderly sacrum:

• Benign insufficiency fracture: a linear fracture line (often vertical in the ala, parallel to the SI joint); marrow signal change that is band-like and tracks ALONG the fracture line; the cortex is preserved; 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/geographic destructive lytic lesion with cortical destruction/breakthrough; mass-like marrow replacement (a nodular low-T1 focus, not a thin band along a line); an associated soft-tissue / epidural mass; often multiple lesions elsewhere.
• When it is genuinely uncertain (atypical/asymmetric pattern, a focal mass, marrow change that does NOT track a fracture line, or a known primary): CT-guided biopsy, with myeloma/metastasis bloods (serum/urine electrophoresis and free light chains, calcium, PSA, the relevant tumour work-up). The bilateral-H morphology and oedema-along-the-line are what let you accept a benign fracture with confidence.

The majority of sacral insufficiency fractures are managed conservatively: analgesia, early mobilisation (prolonged bed rest is harmful in the elderly), physiotherapy and fall prevention. Two things are non-negotiable: exclude malignancy, and investigate and treat the underlying osteoporosis (calcium/vitamin D, bone-protective therapy) — secondary fracture prevention is widely neglected (about 71% of patients do not receive it), yet the fracture is a sentinel event for further fragility fractures.

• Insufficiency = weak (osteoporotic) bone + normal load; fatigue = normal bone + abnormal load.
• The 'great mimicker' — elderly, insidious back/buttock pain, often no injury, normal radiographs.
• MRI is most sensitive (~99%, marrow oedema); CT for morphology / exclude a lytic lesion.
• Honda sign = H-shaped bone-scan uptake — but the full H is the minority (~4%), so its absence doesn't exclude it.
• Always exclude malignancy and treat the osteoporosis (secondary prevention; ~71% currently miss it).
• Most conservative; sacroplasty for refractory pain, fixation (iliosacral/transsacral, triangular lumbopelvic) for unstable/progressive (Rommens-Hofmann FFP grade guides it).