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Aseptic loosening is the failure of the bond between an implant and bone in the absence of infection. The risk of loosening in patients who have total knee replacement surgery in their sixth decade of life is relatively low using modern techniques and prosthetics, but the risk increases as the age of patient at time of implant decreases.13
As pain and disability due to loosening become severe enough to require revision arthroplasty, abnormalities in the binding of the cement to the bone or prosthesis are almost always visible radiographically. Loosening is more commonly associated with the tibial than femoral component.
Radiographic abnormalities include:
- Progressive and extensive widening of interfaces between bone cement, bone prosthesis or cement prosthesis
- Fragmentation or fracture of cement
- Migration/subsidence of prosthetic components
- Bead shedding in porous coated prostheses
Ideally there is no interface lucency. However, frequently thin interfaces less than 2 mm wide are identified, particularly about the tibial component. These may develop about the bone-cement interfaces in cemented components or about the bone-component interface in non-cemented components and occur with equal frequency in cement and non-cemented arthroplasties. Sclerotic line at the interface edge may be present. If these interfaces are stable, and do not progress, they are usually considered normal.
Interface may be inadequately viewed on standard radiographs, and may require dedicated interface views obtained with fluoroscopic guidance. Using fluoroscopy, the patient can be optimally positioned so the x-ray beam is tangential to the interface, at which time a fluoroscopic spot film or overhead film can be obtained.
Progressively widened interfaces greater than 2 mm suggest loosening and/or infection. The tibial component loosens the most frequently. As the tibial component loosens, it may subside into the tibia and tilt. Documentation of movement of a prosthetic component is diagnostic for loosening.
Bead shedding from porous coated prostheses may be seen with loosening.
Baseline and 4-month follow-up radiographs with interval development of subtle, less than 2 mm interface about the medial tibial component (arrows). This was a normal finding and has been stable on follow-up radiographs.
Progressive interface widening about tibial component (arrows) with subsidence and tilting.
Marked interface widening, especially about femoral component with posterior tilting. Note cement fragmentation (arrows).
Loose non-cemented porous coated prosthesis with bead shedding (arrow).
Chronically loose long-stemmed revised femoral component with lateral migration of stem and subsidence. "Windshield wiper" lucency is present about the stem (arrows), and the lateral femoral cortex is remodeled.