Codfish Vertebrae: Causes, Fish Vertebra Sign and Grading

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What causes biconcave vertebral bodies (codfish vertebrae) on sagittal radiographs of the spine?

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Answer:

Codfish vertebra = both endplates concave, the body hourglass-shaped and the disc biconvex, because soft trabecular bone has yielded to the pressure of a normal nucleus pulposus. It is a shape, not a diagnosis. Three things decide the report: what softened the bone (osteoporosis in most adults), whether the endplate is depressed or merely low (fracture versus deformity), and whether the depression is a smooth arc (soft bone) or a squared central step (sickle cell, H-shaped vertebra).

Sagittal lumbar CT in renal osteodystrophy showing biconcave codfish vertebrae with dense endplate bands of the rugger jersey spine — Renal osteodystrophy: biconcave lumbar bodies with biconvex discs, plus dense endplate bands (rugger jersey spine) giving the cause.

Why is it called so?

Fish vertebrae are amphicoelous, hollowed at both ends, and a softened human body comes to match. Useful pedantry: fish vertebra is the bone, fish mouth is the biconvex disc space between two of them [1].

Pathophysiology

The endplate sits between trabecular support below and nucleus pulposus pressure above. Weaken the trabeculae and the endplate bows inwards. The corollary is the useful bit: the sign needs a healthy turgid disc, so it appears at levels with preserved disc height. A flattened or sclerotic endplate at a degenerate, narrowed level is degenerative change, not this sign.

Alternative names: Fish vertebra, fish mouth vertebra, fishmouthing, biconcave vertebra, hourglass vertebra

Other associated named signs: H-shaped or Lincoln log vertebra of sickle cell disease, rugger jersey spine of renal osteodystrophy, hair-on-end skull of chronic haemolysis, and the picture frame vertebra of Paget disease

Causes

Mechanism	Causes	Give-away elsewhere
Reduced bone quantity	Postmenopausal and senile osteoporosis, glucocorticoids, heparin, Cushing syndrome, hypogonadism, immobilisation, anorexia nervosa	Cortical thinning, coexisting wedge or crush deformities, normal biochemistry
Defective mineralisation	Osteomalacia, X-linked hypophosphataemia, hypophosphatasia	Looser zones, coarse indistinct trabeculae, blurred cortex
Hyperparathyroid and renal	Primary hyperparathyroidism, CKD mineral and bone disorder, dialysis	Rugger jersey spine, subperiosteal resorption of radial middle phalanges, brown tumours
Marrow expansion or replacement	Thalassaemia, chronic haemolysis, sickle cell disease, myeloma, leukaemia, metastases	Hair-on-end skull, widened diploe, lytic lesions, pedicle destruction if malignant
Inherited matrix disease	Osteogenesis imperfecta, homocystinuria, Marfan, Ehlers-Danlos	Blue sclerae, bowed long bones, ectopia lentis, dural ectasia, young patient

Osteoporosis, renal osteodystrophy, osteomalacia and the chronic anaemias cover almost all adult cases.

Two corrections to the usual list. Sickle cell disease classically gives the squared H-shaped vertebra, not this shape; sickle patients do get smooth biconcavity, but from chronic-disease osteoporosis. Paget disease does not belong: it squares and enlarges the body (picture frame vertebra), the opposite morphology.

Codfish or H-shaped: read the step-off

Feature	Codfish (fish) vertebra	H-shaped (Lincoln log)
Contour	Smooth curved arc	Squared step, sharp shoulders
Extent	Whole endplate, corner to corner	Central only; anterior and posterior thirds normal height
Mechanism	Global trabecular weakening	Infarction of the centrally supplied endplate
Cause	Osteoporosis, osteomalacia, renal osteodystrophy, marrow expansion	Sickle cell disease; less often Gaucher, thalassaemia
Specificity	Low	High for haemoglobinopathy

The step-off is the discriminator, not the depth.

Fracture or deformity?

Grade height loss with the Genant semiquantitative method on T4 to L4, typing each vertebra as wedge, biconcave or crush. The codfish vertebra is the biconcave type [2].

Genant grade	Height loss	Area loss	Report as
0	None	None	Normal
0.5	Under 20%	Not applicable	Borderline deformity
1	20% to 25%	10% to 20%	Mild fracture
2	25% to 40%	20% to 40%	Moderate fracture
3	Over 40%	Over 40%	Severe fracture

Height alone over-calls. The algorithm-based qualitative (ABQ) method calls a fracture only on endplate depression, buckling or cortical break; a merely short vertebra with an intact endplate is non-osteoporotic short vertebral height [3]. In MrOS (732 men aged 65+), fracture prevalence was 10% to 13% depending on method, but over 50% had short vertebral height without endplate depression, and their bone density matched men with normal vertebrae [4].

Sagittal and coronal thoracic CT showing osteoporotic vertebral compression fractures with endplate depression and intravertebral vacuum cleft sign — Genuine fractures: endplates depressed and buckled, sclerotic condensation, intravertebral vacuum cleft.

At the workstation: run the endplate, not the ruler. A smooth unbroken cortical line that sits low is a deformity. One that steps, buckles, condenses or disappears is a fracture, whatever the percentage says.

Mimics

Mimic	How it differs
Cupid’s bow	Normal variant. Two parasagittal concavities of the inferior endplate in the lower lumbar spine; the midline endplate is normal [5]
Schmorl node	Focal rounded disc herniation with a sclerotic rim, not a global concavity [6]
Scheuermann disease	Anterior wedging of three or more contiguous thoracic bodies with irregular endplates and kyphosis
Limbus vertebra	Corticated triangular anterosuperior fragment with a matching corticated defect
Degenerative endplate change	Flat or sclerotic endplates at a level with reduced disc height
Malignant collapse	Convex posterior cortical bulge, pedicle involvement, soft tissue mass, diffuse T1 marrow replacement

What to do next

Say fracture when the endplate is depressed. The IMPACT study found a 34% false-negative rate in local reports, two thirds missed outright and one third hedged into uselessness [7].
Give level, morphology, Genant grade and a count. Two or more changes risk category on its own.
If it is a short vertebra with an intact endplate, say so, and say it is not a fracture.
Recommend DXA with vertebral fracture assessment, plus calcium, phosphate, alkaline phosphatase, PTH, vitamin D and renal function. A vertebral fragility fracture is itself a treatment trigger under the 2024 UK NOGG guideline [8].
MRI only for a specific question: acute or old, benign or malignant.

Frequently asked questions

What causes codfish vertebrae?

Any cause of generalised soft trabecular bone: osteoporosis (commonest by far), osteomalacia, hyperparathyroidism and renal osteodystrophy, marrow expansion in thalassaemia and chronic haemolysis, marrow replacement by myeloma or leukaemia, and osteogenesis imperfecta or homocystinuria in the young.

Is a codfish vertebra a fracture?

Only if the endplate is depressed, buckled or broken. A vertebra that is merely short with an intact endplate is a deformity, not a fracture. In the MrOS study over 50% of men aged 65 and over had short vertebral height of that kind, with bone density no lower than normal.

How do you tell codfish from H-shaped vertebrae?

By the shape of the depression. A smooth corner to corner arc means soft bone. A squared central step with preserved anterior and posterior thirds is the H-shaped or Lincoln log vertebra of sickle cell disease.

Why is it called a codfish vertebra?

Fish vertebrae are amphicoelous, concave at both ends, so a softened human body comes to look like one. Strictly, fish vertebra names the bone and fish mouth names the biconvex disc between two of them.

References

Access all radiology signs posted so far: https://radiogyan.com/radiology-signs/