Abstract
Unlike conventional radiographic methods, the newly introduced EOS system provides simultaneously-synchronized anteroposterior (AP) and true-lateral (LAT) x-ray images. EOS offers considerable potential for calculating parameters such as true femoral and acetabular angular positioning, impingement sites, and also for measuring wear in polyethylene cups. In this study we used THA wear-simulation fixtures to assess 3D-wear in polyethylene cups using EOS algorithms.
Material and methods
A validated phantom apparatus was used to simulate values of three-dimensional wear, controlled in the 3 directions (antero-posterior, medio-lateral, cranio-caudal) using micrometers. (Figure 1)
24 simulations of wear with controlled amplitudes and directions were imaged using the biplane EOS slot-scanning system. Wear amplitudes were between 0 and 3464 μm. Using dedicated software, wear was measured by a 2D/3D matching of 3D spheres onto the 2D frontal and lateral radiographs, allowing the determination of the 3D coordinates of both the cup and femoral head centers and thus the calculation of a 3D wear vector. (Figure 2)
Measured wear vector were compared to real wear vectors in terms of amplitude and direction.3D wear vectors were measured twice by 3 independent observers (for a total of 144 measurements) in order to evaluate intra- and inter-observer reliability.
Results
There was a strong correlation between the measured wear amplitude and the real wear amplitude (Pearson's r = 0,99). Mean error when comparing wear measurement amplitude with real wear amplitude was 356 μm (SD = 127 μm). None of the 144 measurements presented an error over 1 mm.
The accuracy of wear direction evaluation was highly correlated with wear amplitude (Spearman's rho = 0,98), the measurement of 3D wear direction presenting an accuracy better than 15° for wear amplitudes over 1,5 mm. Intra-observer errors for wear amplitude were between 138 μm and 221 μm depending on the observer. Inter-observer error for wear amplitude was 333 μm.
Conclusion
EOS imaging is a promising technology for measuring 3D-wear in polyethylene cups. Further works are underway to compare the EOS algorithms with conventional methods.
