The Journal of Bone and Joint Surgery (American) 83:1803-1809 (2001)
© 2001 The Journal of Bone and Joint Surgery, Inc.
A Clinically Practical Method of Manually Assessing Polyethylene Liner Thickness
David Pollock, MD,
Christi J. Sychterz, MS and
Charles A. Engh, Sr., MD
Investigation performed at the Anderson Orthopaedic Research
Institute, Alexandria, Virginia
David Pollock, MD
Department of Orthopaedic Surgery, Wake Forest University School
of Medicine, Medical Center Boulevard, Winston-Salem, NC 27157-1070
Christi J. Sychterz, MS
Charles A. Engh Sr., MD
Anderson Orthopaedic Research Institute, P.O. Box 7088, Alexandria,
VA 22307. E-mail address for C.J. Sychterz: christi{at}aori.org
The authors did not receive grants or outside funding in support
of their research or preparation of this manuscript. They did not
receive payments or other benefits or a commitment or agreement
to provide such benefits from a commercial entity. No commercial
entity paid or directed, or agreed to pay or direct, any benefits
to any research fund, foundation, educational institution, or other
charitable or nonprofit organization with which the authors are
affiliated or associated.
A commentary is available with the electronic versions of this
article, on our web site (www.jbjs.org) and on our quarterly CD-ROM
(call our subscription department, at 781-449-9780, to order the
CD-ROM).
Background: Most orthopaedists do not have access
to contemporary computer-assisted radiographic techniques and therefore
must use manual radiographic methods to assess polyethylene wear.
The accuracy of most manual methods, however, has not been verified
on clinical radiographs. In this study, we used manufacturer-developed wear
templates to measure polyethylene wear and compared that technique
with two other commonly used manual radiographic methods. Our purpose
was to compare the accuracy of these techniques and thus determine
their usefulness in assessing polyethylene wear in individual patients.
Methods: We analyzed seventeen polyethylene liners
that were retrieved during revision operations after a mean of 12.0
years in situ. With use of digital calipers, we
directly measured the true minimum polyethylene thickness of the
explanted liner. We then measured the polyethylene thickness on anteroposterior
pelvic radiographs that had been made before the revision. Three
different manual techniques were used to evaluate the radiographs:
the Dorr method, the Livermore method, and the newly described wear-template
method. The minimum polyethylene thickness that was calculated with
use of each of these methods was compared with the thickness as
determined by direct measurement of the explanted liner.
Results: The mean error for the Dorr method (1.54 ± 1.21
mm) was significantly greater than that for both the Livermore method
(0.07 ± 0.62 mm) and the wear-template method (-0.04 ±
0.28 mm) (p < 0.01). The mean error for the Dorr method
was significantly greater than zero (p < 0.01), indicating
that this method consistently overestimated the true measurement.
In contrast, the mean errors for the Livermore and wear-template
methods were not significantly different from zero (p = 0.64
and 0.58, respectively), indicating that these methods did not consistently
underestimate or overestimate the true measurement. However, the
large standard deviations for all three methods suggest that the
ranges in error are wide. The limits of agreement for the Livermore
technique ranged from a 1.17-mm underestimation to a 1.31-mm overestimation
of the true thickness of the polyethylene; those of the wear-template technique
were smaller, ranging from a 0.60-mm underestimation to a 0.52-mm
overestimation.
Conclusion: The practicing orthopaedist needs an
accurate and efficient method for determining the polyethylene thickness of
modular acetabular components. We found that the template method
best met this requirement because it involved a simple procedure:
placing the template over the radiograph and measuring the distance
between the femoral head and the inner surface of the metal shell. The
other methods were less accurate, required additional information
from the manufacturer, were more time-consuming, and were not as
easy to demonstrate to the patient.
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