Commentary & Perspective on
"Use of Helical Computed Tomography for the Assessment of Acetabular Osteolysis After Total Hip Arthroplasty"
By Lalit Puri, MD, et al.

Commentary & Perspective by
Carl S. Winalski, MD*,
Department of Radiology, Brigham and Women's Hospital, Boston, MA

Osteolysis and loosening of joint prostheses after total hip arthroplasty remain perplexing problems. For the most part, radiographs have served as the primary radiological tool for the diagnosis and serial analysis of periprosthetic osteolysis. It even has been suggested that the main role of radiography in the management of patients following hip arthroplasty is to detect asymptomatic osteolysis¹. However, plain radiographs allow for analysis of only a limited portion of the interface between the prosthesis and the bone—the regions that are tangential to the x-ray beam. Therefore, radiographs are not sensitive diagnostic tools for periprosthetic osteolysis over a large percentage of the prosthesis-bone interface. While cross-sectional imaging techniques such as computed tomography (CT) and magnetic resonance imaging (MRI) have been employed with great success for the accurate quantification of other bone and soft-tissue lesions, the artifacts caused by the metal in joint prostheses have limited utilization of these methods.

Capitalizing on the advances in CT hardware, helical scanning, and software for metal artifact reduction, Puri et al. have shown in this study a large improvement in the sensitivity of detection and accuracy in measurement of osteolytic lesions around the acetabular components of total hip prostheses. In their study, fifty total hip arthroplasties in forty patients at risk for rapid polyethylene wear were evaluated with both conventional radiography and helical CT in combination with metal artifact reduction techniques. The CT scan revealed osteolysis in twenty-six (52%) of the hips while radiography showed osteolysis in only sixteen hips (32%). Additionally, in the sixteen hips in which osteolysis was detected with use of both methods, the radiographic method resulted in a systematic underestimation of the extent of osteolysis in over 80% of cases.

It is not surprising that after the elimination of the streak artifact, CT, a cross-sectional technique, performs better than radiography, a projectional technique. Improved visualization and accurate assessments have been demonstrated in an in vitro study of the cement mantle around plastic models of prosthetic components implanted in cadaveric femora². In this model of hip arthroplasty, there was no metal to cause artifact. CT proved much more accurate than plain radiography with regard to measurement of the thickness of the cement and the detection of cement deficiencies².

CT hardware technology has further improved with the development of multidetector-row computed tomography (MDCT)³. It is now possible to examine rapidly large areas of the body with very thin (£ 1 mm) image slices. With the ability to collect thin slices come a reduction in artifacts produced by averaging partial volume and the ability to obtain exquisite reformatted images in any image plane. Additionally, ongoing clinical investigations evaluating the bone/metal interface have shown that the MDCT technique produces a reduction in metal-induced artifacts⁴. Also, software for metal artifact reduction continues to improve⁵.

This work by Puri et al. represents an important contribution to the evaluation of periprosthetic osteolysis and will likely become the standard for research studies.

Should CT scans replace plain radiographs in the routine clinical evaluation of hip prostheses? Although the technique shows greater sensitivity and accuracy in the evaluation of periprosthetic osteolysis, there are several factors to consider before adopting CT as the standard diagnostic tool for clinical practice. CT delivers a higher dose of radiation to patients and is more costly than plain radiography. The radiation dose with helical CT and MDCT are often even higher than that with single detector non-helical CT. Assessment of the cost-benefit and risk-benefit ratios is needed before routine use of CT can be recommended. At present, these CT techniques would most likely benefit patients with osteolysis who are being considered as candidates for revision arthroplasty.

*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

1. Tigges S, Roberson JR, Cohen DE. Hip arthroplasty: the role of plain radiographs in outpatient management. Radiology. 1995;194:73-5.
2. Valdivia GG, Dunbar MJ, Parker DA, Woolfrey MR, MacDonald SJ, McCalden RW, Rorabeck CH, Bourne RB. The John Charnley Award: Three-dimensional analysis of the cement mantle in total hip arthroplasty. Clin Orthop. 2001;393:38-51.
3. Hu H, He HD, Foley WD, Fox SH. Four multidetector-row helical CT: image quality and volume coverage speed. Radiology. 2000;215:55-62.
4. Farber JM, Buckwalter KA. Multislice CT and the evaluation of osseous structures and joints in the presence of metal [abstract]. In: Society of Skeletal Radiology 24th annual meeting program; 2001 Feb 25-28; Phoenix, AZ. p 32.
5. Zhao S, Robertson DD, Wang G, Whiting B, Bae KT. X-ray CT metal artifact reduction using wavelets: an application for imaging total hip prostheses. IEEE Trans Med Imaging. 2000;19:1238-47.