This 92 year-old male patient presented for assessment of acute hemiparesis (left or right sided weakness of half of the body). Skull X-Ray shows marked thickening of the calvarium; the upper (or superior) part of the skull commonly known as the skullcap. The diploic space shows marked widening. The classic cotton wool spots seen on this film result from thickened trabeculae (supporting strands of connective tissue) which lead to sclerotic (hardened) lucent (bright) areas. These areas are poorly defined and fluffy hence the classic name of cotton wool sign in Paget’s disease. There is also thickening and irregularity of the cortex (the outer layer of the cerebrum). In the skull the lytic defect (or destruction of an area of bone) is referred to as osteoporosis circumscripta and is most often seen in the frontal bone. Impression: Later stages of Paget’s disease.
Paget’s Disease (Osteitis Deformans)
Paget’s disease is a chronic bone disorder that is typified by enlarged and misshapen bones. It is caused by a dysfunction in the normal process of bone breakdown and formation. Bone is constantly renewed or remodelled by osteoclast and osteoblast cells. Osteoclasts are responsible for the breakdown of old damaged bone and osteoblasts for the laying down of new bone. In Paget’s disease, the osteoclasts malfunction and become larger than usual and start breaking down bone too rapidly. The osteoblasts react by depositing new bone too quickly; this new bone tends to be soft with a disorganized trabeculae pattern. The whole process results in an increase in the size of the bone and a corresponding misshapen and weakened structure which can result in fractures and arthritis. Paget’s disease can affect all the bones in the body but is often localized to a few. The most commonly affected areas tend to be the pelvis, lumbar and thoracic spine, femur, sacrum, skull, tibia and humerus.
Paget’s Disease has 3 phases of progression:-
Early or Lytic phase:- This is also sometimes called the ‘hot’ stage. Osteoclast cell activity is predominant in the lytic phase. The disease often begins at the end of long bones. A distinct area or zone of osteolysis (bone loss or destruction) may begin in the subcortical bone (or bone marrow) and advance along the diaphysis (the shaft or central part of a long bone). At this stage osteoblast cell activity is not quick enough to replace the loss and radiolucent fibrous tissue replaces normal bone.
X-Ray of long bone in the lytic phase of Paget’s disease. The blade of grass sign, also known as the candle flame sign, refers to the lucent (bright) leading edge as seen in this X-Ray and is characteristic of Paget’s disease.
The intermediate or mixed phase:- reveals evidence of osteoclastic activity and a corresponding increase in disorganized osteoblastic activity. New bone forms abnormally demonstrating characteristically thickened trabeculae bone which can have a ‘mosaic pattern’ rather than a normal linear patterning .
X-Ray of the lumbar spine demonstrates a mildly sclerotic and enlarged L3 vertebral body. The combination of trabecular bone hypertrophy and thickening at the end-plates with apposition/absorption on the bone and cartilage surfaces at the front and back of the vertebral borders leads to the picture frame sign.(1) This phenomena often occurs in the intermediate or mixed phase of Pagent’s Disease.
The final or ‘cold’ Stage:- This stage is sometimes called the Sclerotic phase. Previous laid down woven bone is replaced by normal lamellar bone, which is the rigid form of the connective tissue. Eventually there is minimal bone formation or destruction and the condition becomes dormant.
X-Ray of spine demonstrating an ivory vertebrae which is a radiological sign often found in the sclerotic stage of Paget’s Disease due to an increase in the density of the vertebra This can clearly be seen as a uniformly white vertebra with no contour abnormalities. This is a result of atrophy or shrinkage of the spongiosa (which is the spongy cancellous part of a bone) combined with thickening of the vertical trabeculae.
1. Graham TS. The ivory vertebra sign. Radiology 2005; 235 2: 614–615. [PubMed]