Femoral subsidence assessment after hip replacement: an experimental study.

Background: In an experimental set up information is to be gained on the error of measurement for subsidence assessment of the stem after hip replacement.

Methods: Subsidence was measured with a pencil and ruler for four different reference lines and with the computerized EBRA-FCA method. Hip flexion, rotation and abduction were simulated in a standardized way. In a second experimental set up, subsidence was simulated in defined steps.

Results: In the tilt study, the maximum error of measurement was 7 mm with standard methods and 1.7 mm with the EBRA-FCA method. In the subsidence study, there was a maximum error of measurement of 1.9 mm with the standard methods. With EBRA-FCA, the maximum error of measurement was 0.2 mm when taking all radiographs into account.

Conclusions: The main error of subsidence measurement is caused by tilt of the femur for standard methods and partly can be reduced by EBRA-FCA. EBRA-FCA is more reliable than standard methods but might underestimate the actual subsidence in a clinical situation.