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Findings On the radiographs there is an eccentric positioning of the femoral head within the cup as well as multiple subtle, amorphous densities within the soft tissue. On the frog leg view in particular the amorphous densities appear to extend into the acetabulum. On the CT study there is a striking degree of amorphous, density within the soft tissue extending into large areas of bone resorption of the acetabulum. Speckled areas of linear density are present along the wall of these soft tissue masses. Although not completely shown areas of incorporation of the acetabular component into the underlying bone were present. Also seen well is a displaced metal liner with asymmetric positioning of the femoral head within the acetabular cup. On the MRI, the corresponding soft tissue masses demonstrate a striking degree of low signal intensity of thickened walls of the masses as well as within the masses themselves. These masses produce osteolysis about the acetabular component and to a much less degree the femoral component.
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Diagnosis: Metallosis As has been discussed recently in the orthopedic and radiology literature is the concept of adverse local tissue reaction comprising the phenomenon that occur in the setting of metal on metal bearing interactions. This incorporates the issues of a lymphocytic or ALVAL response related to metal ions and metal debris engendering a histiocytic foreign body reaction and producing metallosis. Both of these processes are more typically seen to involve the soft tissue but as seen in this case can also destroy the underlying bone and may cause a loosening of the prosthesis.
In this case the implant had a metal cup with a composite metal and polyethyelene liner that became displaced allowing wear of the poly liner and the metal liner. The metal liner impinged upon the neck almost transecting the neck and in the process produced a huge amount of metallic debris. This metallic debris on radiographs can be seen as amorphously dense tissues often referred to as a cloud sign or may cause a dense, linear staining of the wall of the collections producing a metal line sign at the periphery of the mass. Similar findings are seen on CT examination but become more conspicuous and CT allows for better demonstration of the bony destruction. On MRI, metallosis typically produces markedly low signal intensity tissue. This can be seen in isolation or frequently with findings of an ALVAL response and a polymeric wear induced synovitis.
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References MR Imaging of Adverse Local Tissue Reactions around Rejuvenate Modular Dual-Taper Stems. Burge AJ, Gold SL, Lurie B, Nawabi DH, Fields KG, Koff MF, Westrich G, Potter HG1. Radiology. 2015 Oct;277(1):142-50. doi: 10.1148/radiol.2015141967. Epub 2015 Jun 1. Metallosis and Metal-Induced Synovitis Following Total Knee Arthroplasty: Review of Radiographic and CT Findings. Jason W. Romesburg, Paul L. Wasserman, and Candace H. Schoppe. J Radiol Case Rep. 2010; 4(9): 7–17. Results of total knee arthroplasty with a metal-backed patellar component. A 6-year follow-up study. Rader CP, Löhr J, Wittmann R, Eulert J. J Arthroplasty. 1996 Dec;11(8):923-30.
The imaging appearances of metallosis. Heffernan EJ, Alkubaidan FO, Nielsen TO, Munk PL. Skeletal Radiol. 2008 Jan;37(1):59-62. Epub 2007 Sep 18.
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