What is the pedestal sign on radiography of a hip prosthesis?
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Answer:
The pedestal sign is a transverse shelf of dense new bone bridging the medullary canal at or just beyond the tip of a cementless femoral stem, so that the stem appears to stand on a plinth. It reflects distal load transfer: the femur has responded to stress concentrated at the stem tip by laying down bone there, which happens when proximal fixation of the implant has not been achieved or has been lost. A pedestal by itself is not proof of failure, and it may be seen with a stem that is stable by fibrous fixation and asymptomatic. It becomes evidence of a loose stem when combined with the other criteria: a progressive or complete radiolucent line wider than 2 mm around the porous-coated portion, subsidence of more than 3 to 5 mm or any change in stem position or version on serial films, a divergent pedestal separated from the stem tip, and cortical hypertrophy at the tip with pain on weight-bearing. The opposite finding, indicating successful bone ingrowth, is the spot weld, a small area of endosteal new bone bridging to the porous surface, usually with an absence of reactive lines and with proximal stress shielding and calcar rounding from the transfer of load away from the proximal femur. Serial comparison with the immediate postoperative radiograph is essential, and where the appearances remain equivocal, metal artefact reduction CT, nuclear medicine studies or joint aspiration are used to separate aseptic loosening from infection.
Why is it called so?
The plate of new bone across the canal beneath the stem tip resembles a pedestal on which the implant is standing.
Pathophysiology
An uncemented stem is designed to achieve fixation proximally through bone ingrowth into a porous or hydroxyapatite-coated surface. If the proximal fit is inadequate or micromotion exceeds roughly 150 micrometres, fibrous tissue rather than bone forms at the interface and load is transmitted instead through the distal stem to the endosteal cortex. Bone remodels according to the stresses it experiences, so the concentrated stress at the tip stimulates new bone formation across the canal, while the unloaded proximal femur undergoes stress-shielding resorption. Continued micromotion also generates particulate debris that provokes a macrophage-driven osteolytic response, which widens the interface, produces the expanding radiolucent lines and ultimately allows subsidence.
Alternative names: Pedestal formation; distal pedestal
Other associated named signs: Spot welds of bone ingrowth, cement mantle radiolucency in cemented stems, and the bubble sign of periprosthetic osteolysis
References
Access all radiology signs posted so far: https://radiogyan.com/radiology-signs/
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