Radiophosphate uptake in asymptomatic knee arthroplasty

The utility of radiophosphate bone scanning in the detection of complications following total knee replacement (TKR) is not yet fully established. A difficulty associated with the use of bone scanning is the persistent increased uptake seen around the prosthetic joint long after surgery, despite the absence of symptoms. In order to better characterize the time course of radiophosphate uptake, bone scans obtained 1 mo-12 yr after surgery were analyzed in 30 asymptomatic patients with 37 TKR. Uptake was graded 0-4+ in the femoral and tibial components. Scans of 18 implants were obtained 1 yr or less after surgery (Group 1), and 19 were obtained greater than 1 yr after surgery (Group 2). Mean uptake scores were as follows: femoral component Group 1 = 3.0 +/- 1.1; Group 2 = 1.8 +/- 0.9 (p less than 0.05); tibial component Group 1 = 3.2 +/- 0.8; Group 2 = 2.6 +/- 1.1 (not significant). Persistent increased uptake, particularly in the tibial component, reflects mechanical stresses peculiar to knee prostheses, and tends to undermine confidence in diagnosing loosening on the basis of a single study.