Paget disease: with blade of grass sign

Case contributed by Dr Andrew Dixon

Presentation

Planning for left total hip replacement

Patient Data

Age: 60 years
Gender: Female

Left hip

x_ray

Region of lysis in the left femoral shaft involving medullary cavity and cortex with a well-defined inverted V-shaped proximal margin. The distal extent of the lesion is not visualized. The appearance is characteristic of the lytic phase of Paget disease; the so-called blade of grass sign or candle flame appearance

Left femur

x_ray

Remainder of the femur was imaged to assess the distal extent of the lesion. As is typical of Paget disease, the active margin of lysis (in this case the proximal margin) is well-defined however the remainder of the bone is diffusely involved without a discrete margin. Coarsening of trabeculae throughout the distal femur is evident including the bone surrounding the femoral component on the total knee prosthesis.    

Left femur

ct

CT confirms the plain film findings. Note how the fat density of the marrow is maintained. If this lytic process in the bone was due to malignant infiltration such as by metastasis or myeloma, the marrow fat would be replaced by soft-tissue density similar to that of muscle. 

Bilateral femur

mri

MRI confirms the plain film findings and like the CT it shows nicely how the fat signal within the marrow is maintained. This would not be the case if the lytic process was due to metastasis or myeloma where the marrow signal on T1 would be expected to be very low. During the later phases of Paget disease the marrow signal typically become altered due to a fibrous process, however during the early lytic phase seen here the marrow fat signal is preserved.   

Bone scan

Nuclear medicine

Bone scan confirms blood pool and delayed phase increased tracer uptake throughout the distal femur with well-defined proximal margin in the mid-shaft consistent with active Paget disease

Annotated image

Blade of grass sign (candle flame appearance) in lytic phase Paget disease of bone

Case Discussion

This case shows the typical appearance of lytic phase Paget disease of the femur on x-ray, CT, MRI and bone scan. This lesion was incidentally detected as part of a work-up for total hip replacement. The classic blade of grass sign (candle flame appearance) was not appreciated by the initial reader of the x-ray and instead an aggressive process such as metastasis or myeloma was suggested. The initial reader of the CT also suggested metastasis or myeloma as the likely diagnosis despite the fat density of the marrow being preserved. It was not until the MRI study was read that the correct diagnosis of lytic phase Paget disease was made.

General teaching points

  • bone lesions can often have distinguishing diagnostic features on x-ray
  • don't forget that CT can be useful in assessing the presence or absence of fat density within marrow
  • T1 weighted MRI and recognition of preserved fat signal is critical in the assessment of bone lesions 

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