Bony erosion/ skull defects
irstly it is important to make sure that the defect is really an area of bone erosion. Certain areas of the skull appear darker than others on plain films and it is important not to mistake them for an abnormality.
Also, patients who had previous surgery may have very dark well -defined holes in the bone due to burr holes.
Vault – there may be a solitary lytic area of bone erosion or there may be multiple lucencies
Multiple Myeloma – usually shows as multiple well defined punched out lesions
Histiocytosis – large irregular defect in a children resembling a country on a map (“Geographical skull”)
Eosinophilic granuloma – well defined lucency, may be multiple
Metastasis – irregular area of bony erosion. May be solitary or multiple
Tumour of the scalp may erode the underlying bone
Hyperparathyroidism – causes ill defined tiny areas of bone resorption – “pepper pot skull”
Osteomyelitis - often has a smooth dense edge
Multiple lytic defects are usually due to metastases or myeloma. Infection rarely.
Solitary well- defined lesion is usually a benign tumour
Solitary with smooth dense edges is usually a slow growing tumour or infection
Irregular ill defined edges is usually malignancy, or more rarely infection
Pituitary Fossa – erosion of the dorsum sella occurs with a local tumour or raised intracranial pressure. It shows firstly as loss of the white line of the lamina dura lining the floor of the fossa.
Base of skull - tumour, primary or secondary, eosinophilic granuloma
Sinuses - tumour, mucocele
Optic canal - glioma of the optic nerve
Petrous bone – cholesteatoma, tumour
Acoustic (auditory canal) tumour of the 8th nerve.
Congenital – well- defined bony defects may be due to a dermoid, epidermoid (well -defined lesion in the skull vault with bone expansion) or encephalocele (central defect).
Abnormal sutures – fusion or widening
At birth there may be accessory sutures present, the commonest being persistence of the metopic suture which runs vertically up the centre of the frontal bone. It usually disappears by 9 months. Sutures in babies have a smoother margin and are wider than those in older children. The outer suture margin becomes serrated and sutures fuse normally between the ages of 20-30 years. The normal suture width should not exceed 3 mm by the age of 3 months although it can be as wide as 1 cm in the neonate.
Craniostenosis occurs with premature fusion of the sutures.
The posterior fontanelle normally closes at 2-3 months of age & the anterior fontanelle between 18- 24 months.
The skull sutures should all be firmly closed by age 30yrs.
In Craniostenosis some sutures fuse early, while the brain is still growing, which results in an abnormal shape to the skull. There are different names attached to the abnormality according to the shape of the skull caused by fusion of a specific suture.
Fusion of the sagittal suture causes the skull to be narrow and boat- shaped, longer than wide. This is called doliochocephaly or scaphocephaly
Premature fusion of the coronals gives the skull a broader than long shape with a flat occiput. This is called brachycephaly or oxycephaly
Asymmetrical growth due to unilateral fusion of the lambdoid or coronal sutures is called plagiocephaly.
Generalised premature fusion causes microcephaly – a generally small skull.
Premature fusion may cause compression of cranial nerves or mental deficiency, especially microcephaly where the children are
invariably mentally retarded.
On plain films there is loss of definition of the suture margins and the suture lines are difficult to see but often the diagnosis is difficult to make on plain films and computed tomography is used.
Widening of the sutures - this may be due to:
Suture diastasis, which may be seen up to the age of 10 years. It may take just a few days to appear. This is seen in hydrocephalus and a space- occupying lesion within the skull, which may be a tumour or haematoma.
Infiltration of the sutures by metastases from neuroblastom or leukaemia and lymphoma
Defective ossification in bone dysplasia, renal osteodystrophy & rickets.
Fracture through the suture line