Effect of Radiographic Quality on Computer-Assisted Head Penetration Measurements

Christi J. Sycterz, MS, Anthony M. Young, BS, and Charles A. Engh, MD*

*Anderson Orthopaedic Research Institute, Alexandria, VA 22307

Published in Clinical Orthopaedics and Related Research, Number 386, 2001.

ABSTRACT

Background: Even the most sophisticated computer-assisted radiographic techniques of measuring polyethylene wear depend upon quality of the radiograph being evaluated, which varies greatly in clinical settings. The authors of this study sought to determine how the accuracy and reproducibility of three commercially available computer-assisted measurement systems differed when measuring optimal radiographs (with sharply defined component edges) and suboptimal radiographs (with less well-defined edges).

Methods: Using three computer-assisted measurement systems, the authors measured 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, two methods used digital edge-detection algorithms, whereas the other 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 mm. No statistical difference existed among the mean error for the three systems (p>0.17).
• For suboptimal radiographs, the mean error of the systems increased to 0.31 ± 0.28 mm.
  For edge-detection methods measuring suboptimal radiographs, error fluctuated greatly as evidenced by increased standard deviations.
• With one edge-detection method, error and standard deviation increased from 0.15 ± 0.08 mm for optimal radiographs to 0.54 ± 0.32 mm for suboptimal radiographs.
• For the digitizer-tablet method, error remained similar between optimal (0.15 ± 0.07 mm) and suboptimal (0.13 ± 0.06 mm) radiographs.

Summary: 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 research was funded, in part, by Inova Health Care Services, Virginia.