"A Clinically Practical Method of
Manually Assessing Polyethylene Liner Thickness"
By David Pollock, MD et al.

Commentary & Perspective by
Robert B. Bourne, MD, FRCSC*,
Division of Orthopaedic Surgery, London Health Sciences Centre,
University of Western Ontario, London, Ontario, Canada

The success of total hip arthroplasty has prompted its use in younger, more active patients. As a result, polyethylene wear and osteolysis have become major issues. Most contemporary long-term studies now include data on polyethylene wear^1-3. Charnley recognized the limitations of wear measurements made on plain radiographs and suggested an error of ± 0.5 mm⁴. To improve the accuracy of wear assessments, computer-assisted radiographic techniques incorporating edge detection or the use of roentgen stereophotogrammetric analysis (RSA) have been advocated^3,5.

In this study, Pollock et al. contend that most orthopaedic surgeons do not have access to computer-assisted radiographic techniques or other sophisticated systems such as RSA. Yet, assessment of polyethylene wear is important in scheduling polyethylene liner exchange before complete wear-through of the polyethylene liner and the associated complications occur. To help surgeons who do not have access to sophisticated radiographic techniques for assessing wear, the authors have developed a new manual technique whereby a clinician can use a wear template to measure the minimum polyethylene thickness on an anteroposterior pelvic radiograph. By placing the template over a follow-up radiograph, the surgeon can simply measure the distance between the outer edge of the femoral head and the inner edge of the metal shell to determine the thickness of the remaining polyethylene. In this study, the authors compared the Dorr and Livermore techniques with their new template technique in the wear analysis of seventeen polyethylene liners that had been retrieved during revision procedures and for which pre-revision clinical radiographs had been made^2,6. A single observer measured the true minimum polyethylene thickness of the explanted liners by probing the articular surface of the liner with digital calipers. A second observer blinded to the results of the direct measurements then assessed polyethylene thickness of the liners with use of all three manual radiographic methods.

Minimum polyethylene thickness averaged 1.37 ± 1.06 mm with direct measurement, 2.91 ± 1.27 mm with the Dorr radiographic technique, 1.44 ± 1.44 mm with the Livermore radiographic technique, and 1.33 ± 1.14 mm with the wear-template technique. Linear regression analysis demonstrated a poor relationship between thickness measured directly and thickness measured with the Dorr technique (r² = 0.22). The mean difference between the Dorr radiographic measurements and the direct measurements (i.e., the error of the Dorr radiographic technique) was 1.54 ± 1.21 mm; this was significantly greater than 0 (p < 0.01), indicating a bias for this method of manual radiographic measurement to consistently overestimate the true amount of polyethylene remaining in the liner.

A stronger relationship was found between thickness measured directly and that measured with use of the Livermore technique (r² = 0.85); the mean difference was 0.07 ± 0.62 mm. This value did not significantly differ from 0 (p = 0.64).

The strongest relationship was found between the measurements made with the authors' wear templates and those made directly (r² = 0.94). The mean difference, –0.04 ± 0.28 mm, was not significantly different from 0 (p = 0.58).

One-way analysis of variance demonstrated that the average error of the Dorr radiographic method was significantly larger than that of either the Livermore or the template method (p < 0.01). The average errors of the Livermore and template methods were not significantly different from each other (p = 0.97).

This paper is useful for clinicians who do not have computer-assisted radiographic methods or RSA techniques available for the assessment of polyethylene wear. Importantly, the authors acknowledge that manual radiographic methods are much less accurate than are contemporary computer-assisted techniques. It would have been interesting to have also assessed femoral head penetration with use of one of the available computer-assisted techniques. Nevertheless, this study validates the use of either the Livermore method or the authors' template technique over the Dorr technique for wear measurement of polyethylene liner thickness.

*The author did not receive grants or outside funding in support of his research or preparation of this manuscript. He 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 author is affiliated or associated.

References

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2. Dorr LD, Wan Z. Comparative results of a distal modular sleeve, circumferential coating, and stiffness relief using the Anatomic Porous Replacement II. J Arthroplasty. 1996;11:419-28.
3. Martel JM, Berdia S. Determination of polyethylene wear in total hip replacements with use of digital radiographs. J Bone Joint Surg Am. 1997;79:1635-41.
4. Charnley J, Halley DK. Rate of wear in total hip replacement. Clin Orthop. 1975;112:170-9.
5. Selvik G. Roentgen stereophotogrammetry. A method for the study of the kinematics of the skeletal system. Acta Orthop Scand Suppl. 1989; 232:1-51.
6. Livermore J, Ilstrup D, Morrey B. Effect of femoral head size on wear of the polyethylene acetabular component. J Bone Joint Surg Am. 1990;72:518-28.