Bone scan of the lower extremities.
An 80-year-old woman presents with 3 months of right shin pain. She had a revision total knee replacement 5 years ago for aseptic loosening and had been doing well. Her pain is made worse with weight bearing and is relieved with rest. She has tried non-steroidal anti-inflammatory drugs (NSAIDs) and 4 weeks of touch-down weight bearing with little improvement. On examination, she has no effusion and full passive motion without pain. Anteroposterior and lateral X-rays taken in the office show no abnormalities (Figures 1 and 2). A bone scan of the lower extremities shows increased uptake of the right tibial shaft consistent with a tibial stress reaction (Figure 3). What is the most likely diagnosis?
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The most common complications following total knee arthroplasty (TKA) include aseptic loosening, infection, instability, and periprosthetic fracture. The most common cause of early TKA failure (within 2 years) is infection, while the most common cause of late failure is aseptic loosening. Approximately 40% of TKA failures overall result from aseptic loosening, and 27% are due to septic failure. Aseptic prosthetic loosening can be caused by a variety of factors, including implant malalignment, malpositioning, instability, and poor bone quality. This patient has a long, uncemented revision metal stem, and the implant is press fit into the tibial canal. The press fit metal component has no bony ingrowth potential at the implant-bone interface, so any excess motion of the stem can cause the surrounding bone to fatigue. Pain made worse with weight bearing is a sign that the metal implant is starting to cause fatigue, or a stress reaction, at the tibial diaphysis. This is confirmed on the patient’s bone scan.
In the work-up of aseptic loosening, X-rays are often negative early in the presentation. With normal X-rays, a bone scan is the next best study of choice. A bone scan can be positive up to 2 years after TKA, so results should be correlated with the clinical scenario. This patient has a positive bone scan 5 years from revision TKA, so the changes in the tibial stem can be ruled pathological. The patient’s bone scan shows increased uptake at the implant-bone interface of the tibial stem, suggesting aseptic loosening. Unfortunately, when stress reactions occur, as did in this patient, conservative treatment with a period of non-weight bearing is often unsuccessful, and a revision surgery is generally required.1,2
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 (JOPA).
- Sharkey PF, Lichstein PM, Shen C, Tokarski AT, Parvizi J. Why are total knee arthroplasties failing today—has anything changed after 10 years? J Arthroplasty. 2014;29:1774-1778.
- Lonner JH, Lotke PA. Aseptic complications after total knee arthroplasty. J Am Acad Orthop Surg. 1999;7:311-324.