Comparison of Three Computer-Assisted Wear Measurement Systems on Optimal and Suboptimal Radiographs
Christi J. Sychterz, MSE Anthony M. Yang, BS Charles A. Engh, MD
Background: Although computer-assisted radiographic techniques measure polyethylene wear more accurately than previous manual methods, even the most sophisticated technique depends upon the quality of the radiograph being evaluated. Since the quality of clinical radiographs varies greatly, we determined how the accuracy and reproducibility of three commercially available computer-assisted measurement systems differed when measuring both on optimal radiographs (edges of the head and cup were sharply defined) and suboptimal radiographs (component edges not sharply defined).
Materials/Methods: Three dimensional head penetration was measured on simulated and clinical hip radiographs using three computer-assisted measurement systems. All systems calculated head penetration as the movement of the head center relative to the cup center. To define the periphery of the prosthetic head and cup, two methods used digital edge-detection algorithms, and one used the human eye and a digitizing tablet. For simulated hip radiographs, error was calculated as the absolute value of the difference between the known amount of head penetration (determined by a coordinate measuring machine) and the amount of penetration determined by the software.
Results: For optimal simulated hip radiographs, measurement error averaged 0.18 ± 0.11 millimeter; no statistical difference existed among the mean error for the three systems (p>0.17). For suboptimal radiographs, the systems' mean error increased to 0.31 ± 0.28 millimeter. For edge-detection methods, measurement error fluctuated greatly as evidenced by increased standard deviations. With one edge-detection method, error and standard deviation increased from 0.15 ± 0.08 millimeter for optimal radiographs to 0.54 ± 0.32 millimeter for suboptimal radiographs. For the digitizer-tablet method, error remained similar between optimal (0.15 ± 0.07 millimeter) and suboptimal (0.13 ± 0.06 millimeter) radiographs.
Conclusion: These data demonstrate the susceptibility of head penetration measurements to radiographic technique. When technique is optimal, the systems are accurate and reproducible; when suboptimal, system error can vary widely. This study underscores the importance of consistent quality radiographs for all wear analyses.
