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A Clinically Practical Method of Manually Assessing Polyethylene Liner Thickness

Authors David Pollock*, MD, Christi J. Sychterz**, MS, Charles A. Engh, MD** *Wake Forest University School of Medicine, Winston-Salem, NC **Anderson Orthopaedic Research Institute, Alexandria, VA Abstract Introduction: Many orthopaedists do not have access to contemporary computer-assisted radiographic techniques and rely upon manual radiographic methods to assess polyethylene (PE) wear. However, the accuracy of most manual methods has not been verified on clinical radiographs. In this study, we introduced a new method of assessing liner thickness and compared it to two other commonly used manual radiographic methods. Our purposes were to quantify the accuracy of the three techniques on clinical radiographs and to determine their usefulness in assessing component wear-through for patients. Materials and Methods: We analyzed 17 liners retrieved at revision operations after a mean 12.0 years in situ. Using digital calipers, we measured true polyethylene deformation directly from the explanted liner at the minimum thickness. We then measured polyethylene deformation radiographically from prerevision anteroposterior pelvic radiographs using three different manual techniques: the Dorr method, the Livermore method, and a new wear template method. The new method involved placing a manufacturer-developed templates over a follow-up radiograph and measuring the distance between the outer edge of the femoral head and the inner edge of the metal shell to determine the remaining liner thickness. The minimum polyethylene thickness calculated from each method was compared to the direct measurement from the explanted liner. Results: The mean error for the Dorr method (1.54 ± 1.21 mm) was significantly greater than the mean error for both the Livermore method (0.07 ± 0.62 mm) and the template method (-0.04 ± 0.28 mm). Whereas the Dorr method demonstrated a significant bias to overestimate the true amount of polyethylene thickness remaining in the liner, the mean errors for the other techniques were not significantly different from zero, indicating that they did not consistently under or overestimate true measurements. However, the large standard deviations of all three methods implied wide ranges in error. The limits of agreement for error with the Livermore technique ranged from a 1.17-mm underestimation to a 1.31-mm overestimation of the true PE thickness remaining. The limits of agreement for error with the template technique were smaller, indicating that for 95% of all hips, error would fall somewhere between a 0.60-millimeter underestimation and a 0.52-millimeter overestimation of the true polyethylene thickness remaining. Discussion: The strongest relationship between true polyethylene thickness and radiographically measured thickness was found with the wear template method. Although the limits for this method may be greater than with computer-assisted techniques, the technique was simple, quick, required only one radiograph, and necessitated no additional information from the manufacturer. The other manual methods were more time consuming, required imanufacturer information, and were less easily demonstrated to the patient. Conclusions: The template method of determining PE thickness of worn cementless acetabular cups was more accurate and simpler to use than the other manual techniques reviewed. We recommend the method for use by clinicians because of its ease of use and accuracy.

 

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