Knee flexion influences periprosthetic BMD measurement in the tibia. Suggestions for a reproducible clinical scan protocol.

background and purposeThe quality and quantity of bone is important for the success of joint prostheses and may be monitored by dual energy X-ray absorptiometry (DXA). Available protocols suggest that the knee should be positioned in full extension. This is not possible for most patients in the first days after surgery; however, deficits in extension normalize with rehabilitation. Individual knee flexion between the baseline and follow-up investigations may therefore be different. We investigated the sensitivity of bone mineral density (BMD) measurements to knee flexion in a phantom study and in patients. We suggest a protocol for clinical use.

methods2 phantom tibial bones with tibia components were secured in a clamp and BMD measurements were repeated 5 times at every 5 degrees change in flexion from 0 degrees to 20 degrees. For clinical use, a soft foam positioner was produced, in which the lower leg could be placed in neutral rotation and with the knee in approximately 25 degrees of flexion. The clinical repeatability was tested with double examinations in 38 patients. We investigated 3 regions of interest (ROIs) below the tibial plateau.

resultsIn the phantom study, just 5 degrees of flexion was found to change the measured mean BMD. The reproducibility of clinical measurements (coefficient of variation) in the 3 ROIs assessed ranged from 1.8% to 3.7% for the anteroposterior scans, and from 3.4% to 6.2% for the lateral scans.

interpretationKnee flexion does affect the measured periprosthetic tibial BMD, and knee flexion should be the same at all clinical follow-ups. The protocol and soft foam positioner that we suggest permit precise and reliable assessment of BMD in the proximal tibia and they can be used in clinical work.