Figure 1. Patient sitting in office; observe the size difference between his lower legs.
Figure 2. Anteroposterior radiograph of the right knee.
Figure 3. Lateral view of the right knee.
A 65-year-old construction worker presents with complaints of right lower extremity edema. He describes how his knee occasionally swells, which coincided with his lower extremity edema over the past week. An emergency department visit a few days ago ruled out deep vein thrombosis. He reports that the ultrasound showed a Baker cyst in the back of his knee. He has a history of a right total knee replacement 13 years ago. He admits his left knee has bothered him occasionally over the past few years but he mostly ignores the pain because he is busy at work. On physical examination, knee effusion and pitting edema (1+, up to 2 mm of depression) edema is observed in the right lower extremity (Figure 1). He has no edema in his left leg and his ankle pulses are strong in bilateral lower extremities; radiographs are ordered (Figures 2 and 3).
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The patient has a Baker cyst that is compressing the blood vessels in the back of the knee resulting in venous obstruction and lower extremity edema. The most common cause of a Baker cyst is osteoarthritis of the knee; however, this patient has already had a total knee replacement. On careful inspection of the radiographs, wear on the medial tibial base plate from the femoral implant can be seen (Figure 2). This represents severe wear and failure of the polyethylene liner. Metal debris also can be seen on the lateral radiograph (Figure 3), which is evidence that the patient likely has synovitis from significant metallosis.
Metallosis is an uncommon complication of total joint surgery where metal debris from implant wear infiltrates the surrounding soft tissue causing chronic inflammation.1.2 The most common symptoms include pain and swelling in the involved joint. The treatment for metallosis includes nonurgent synovectomy and revision of the joint replacement.1 If left untreated, metallosis can cause progressive destruction of the involved joint. Metallosis can also cause systemic toxicity as cobalt and chromium can enter the bloodstream and accumulate in organs.2
Dagan Cloutier, MPAS, PA-C, practices in a multispecialty orthopedic group in the southern New Hampshire region and is editor in chief of the Journal of Orthopedics for Physician Assistants.
1. Vivegananthan B, Shah R, Karuppiah AS, Karuppiah SV. Metallosis in a total knee arthroplasty. BMJ Case Rep. 2014;2014:bcr2013202801. doi:10.1136/bcr-2013-202801
2. Bradberry SM, Wilkinson JM, Ferner RE. Systemic toxicity related to metal hip prostheses. Clin Toxicol (Phila). 2014;52(8):837-847. doi:10.3109/15563650.2014.944977.