Effect of radiographic quality on computer-assisted head penetration measurements

Even the most sophisticated computer-assisted radiographic techniques of me asuring femoral head penetration into the polyethylene liner depend on the quality of the radiograph being evaluated, which varies greatly in clinical settings. The authors of this study sought to determine how the accuracy a nd reproducibility of three commercially available computer-assisted measur ement systems differed when measuring optimal radiographs (with sharply def ined component edges) and suboptimal radiographs (with less well defined ed ges). Using three computer-assisted measurement systems, the authors measur ed head penetration on simulated and clinical hip radiographs. 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, one method (System One) used the human eye and a digitizing tablet, whereas th e other two methods (System Two and System Three) used digital edge detecti on algorithms. For simulated hip radiographs, error was calculated as the a bsolute value of the difference between the known amount of head penetratio n, determined by a coordinate measuring machine, and the amount of penetrat ion determined by the software. Three way analysis of variance showed a sig nificant difference in absolute error among the three measurement technique s. System One had a significantly smaller absolute error (0.11 +/- 0.06 mm) than did System Two (0.25 +/- 0.25 mm) and System Three (0.19 +/- 0.13 mm) . In addition, three-way analysis of variance showed that optimal radiograp hs were associated with a significantly lower absolute error (0.14 +/- 0.09 mm) than were suboptimal radiographs (0.23 +/- 0.22 mm). For optimal radio graphs, there was no significant difference in error among the three measur ement methods; all systems were accurate and reproducible. However, for sub optimal radiographs absolute error increased and varied widely, and a signi ficant difference among the methods existed. These data show the susceptibi lity of head penetration measurements to radiographic technique and undersc ore the importance of good quality radiographs for all analyses of head pen etration.