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1 Department of Radiology and Imaging, Hospital for Special Surgery, 535 E. 70th
St., New York, NY 10021.
2 Department of Radiology, Weill Medical College of Cornell University, New
York, NY 10021.
3 Department of Orthopaedic Surgery, Hospital for Special Surgery, New York, NY
10021.
Received March 28, 2002;
accepted after revision October 22, 2002.
Presented at the annual meeting of the American Roentgen Ray Society,
Atlanta, April–May 2002.
Abstract
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MATERIALS AND METHODS. Twenty-four consecutive patients who were referred for Doppler screening for deep venous thrombosis after total knee arthroplasty were evaluated. The polyethylene liner was identified on sonography, and three measurements were obtained from four locations: anteromedial joint line (just medial to the midline incision), along the medial joint line, anterolateral joint line (just lateral to the midline incision), and along the lateral joint line. These sonographic measurements were compared with radiographic measurements of the radiolucent polyethylene liner and with the manufacturers' stated size of the polyethylene liner. Linear regression analyses were then performed.
RESULTS. The polyethylene liner is seen on sonography as a strong linear echogenic interface with posterior acoustic shadowing. Linear regression analyses showed a high correlation (r = 0.8) between the sonographic measurements and the radiographic measurements. A relatively poor correlation (r = 0.2) was noted between the manufacturers' stated size of the liner and the sonographic measurements.
CONCLUSION. We found that the polyethylene liner used in total knee arthroplasty can be clearly identified during sonographic evaluation of the knee. We also found a high correlation between the longitudinal measurement of the polyethylene liner with the thickness of the radiolucent polyethylene as measured on conventional radiographs. We propose that sonography could potentially be a useful noninvasive imaging modality to screen for subtle cases of polyethylene wear.
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Currently, conventional radiography is the imaging modality most commonly used to evaluate patients with knee pain after having undergone total knee arthroplasty. The polyethylene liner is visualized as a radiolucent band adjacent to the tibial tray (Fig. 1). The liner is composed of one piece of polyethylene that is nonuniform thickness. In general, the liner is thicker anteriorly and centrally. Standardizing radiographic imaging of the polyethylene liner is difficult. The appearance of the polyethylene can change depending on whether the radiographs are obtained while the patient is supine or weight-bearing. In addition, slight differences in obliquity can result in measurements that differ by several millimeters from one series of radiographs to the next.
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A tomographic imaging method that accurately images the polyethylene liner with high interobserver uniformity and uniformity across a series of examinations is needed to evaluate whether subtle changes in the thickness of the liner are present. The ability of sonography to image the polyethylene liner has been shown in a cadaver study [2]. To our knowledge, no in vivo studies evaluating the polyethylene liner used in total knee arthroplasty with sonography have been performed. We evaluated a series of patients who had undergone total knee arthroplasty with sonography to evaluate sonography's ability to enable the thickness of the polyethylene liner to be accurately quantified and to reveal the relationship of the liner visualized on sonography to the radiolucent polyethylene seen on conventional radiography.
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7 weeks]). All scans were obtained on a Logic 700
system (General Electric Medical Systems, Milwaukee, WI) with a medium
frequency (5.0–7.5 MHz) linear probe. Our hospital's institutional
review board approved this study. Four sonographic measurements were obtained. All measurements were obtained from a longitudinal approach with digital calipers. All patients were examined in the supine position with the knee fully extended. The polyethylene liner was measured from the interface between the tibial tray and the polyethylene liner to the superior border of the polyethylene to determine its thickness (Fig. 2). The polyethylene liner is seen on sonography as a linear echogenic interface with posterior acoustic shadowing. This liner can be contrasted with the adjacent tibial tray, which also exhibits a strong linear echogenic interface but shows posterior reverberation artifacts.
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The anteromedial measurement was obtained with the sonography probe
longitudinally oriented along the anteromedial aspect of the knee
(parasagittal), just medial to the midline incision. The direct medial
measurement was obtained directly along the medial aspect of the knee
(anatomic coronal plane). The anterolateral measurement was obtained just
lateral to the midline incision, and the lateral measurement was obtained
directly along the lateral joint line. Direct anterior (midline) sonographic
measurements could not be obtained given the subacute midline incision;
therefore, the paramedian sonographic measurements of the polyethylene liner
were correlated with the anterior measurements as seen on the lateral
radiographs. Three sonographic measurements were obtained from each location.
These measurements of the polyethylene liner were correlated with those
obtained from standard anteroposterior and lateral radiographs in 15 patients
(the interval between radiographic and sonographic examinations ranged from
the same day to 3 months (average, 33 days [5 weeks]). To reduce the
potential for measurement errors, we digitized all radiographs so we could use
the edge enhancement feature and obtained measurements with digital calipers.
Measurements of the medial and lateral thicknesses of the polyethylene liner
were obtained from anteroposterior radiographs obtained while patients stood,
and measurements of the anterior and posterior thicknesses were obtained from
lateral radiographs. In addition, when available, correlation was made between
the sonographic measurements and the stated manufacturers' size of the
polyethylene liner (n = 15).
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The sonographic and radiographic measurements obtained are listed in Table 1. At least three measurements were obtained in all locations except in one patient whose clinical status required that he return to the nursing unit, so imaging was discontinued. The average measurements at each location are also outlined. Table 2 lists the average medial and lateral sonographic measurements and the medial and lateral measurements obtained from postoperative anteroposterior radiographs. Linear regression analyses were performed comparing the sonographic measurements at the individual locations with the measurements from the conventional radiographs (Figs. 4 and 5). We found a high correlation between the sonographic measurements and the radiographic measurements (r = 0.8).
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Table 3 lists the sonographic measurements compared with the stated manufacturers' size of the polyethylene liner. The nomenclature for the size of the liner varies among companies in that there is no industry standard for the stated size of the polyethylene liner [3]. Linear regression analyses showed a relatively poor correlation between the sonographic measurements and the minimum size of the polyethylene as stated by the manufacturers (Fig. 6).
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We also found a poor correlation between the stated manufacturers' size of the liner and the sonographic measurements.
This study has several limitations. The lack of uniformity between manufacturers' classification systems of the thickness of polyethylene liners hinders accurate direct correlation. It should be noted that there is an overall industry discrepancy between the stated size of the polyethylene liners and to what these numbers actually refer [3]. The manufacturers' stated size of the liner often includes the combined thickness of the metal and the polyethylene liner [7]. It is generally understood that the stated size of the liner represents the minimal thickness of polyethylene present; however, Edwards et al. [3] found that, despite recorded sizes ranging between 8 and 10 mm, the true minimal thickness of the polyethylene present ranged between approximately 5.5 and 6.2 mm. Therefore, it can be postulated that sonography, as a tomographic imaging modality, enables the circumference of the polyethylene liner to be seen so that the range of thicknesses present can be measured, likely accounting for this relatively poor correlation. In a cadaver study, Yashar et al. [2] ensured accurate correlative measurements by creating a scratch in the polyethylene, yielding an irregular interface, that appeared as an echogenic line during sonographic evaluation. Additional studies in which the liner is marked before it is implanted, as in the study conducted by Yashar et al., and the exact thickness of the polyethylene liner at that location is recorded would yield a more accurate evaluation of the sonographic depiction of the polyethylene thickness in vivo.
We reduced measurement errors when evaluating the conventional radiographs by digitizing the images and obtaining measurements with electronic calipers. Measurements were obtained in the midportion of the medial, lateral, anterior, and posterior aspects of the liner on the radiographs. The parasagittal sonographic measurements were averaged and correlated with direct anterior polyethylene thickness as measured on the radiographs; a true anterior midline sonographic measurement could not be obtained because the anterior midline incision was relatively recent in our patients.
Variations in measurements between sonography and radiography can possibly be accounted for by the fact that the polyethylene thickness is not uniform throughout and the position of the knee at the time the radiographs were obtained can vary. The r2 value (0.64) indicates that approximately 36% of the uncertainty in the measurements results from factors other than the sonographic measurement. This finding underscores the limitations of using radiographs to establish accuracy. Future work includes establishing true determination of sonographic accuracy in vivo by obtaining measurements of the liner before it is implanted.
The thickness of the polyethylene liner is an important determinant of overall functionality of the replacement joint. Wear of the polyethylene is a major source of particulate debris in the knees of patients who undergo total knee arthroplasty [1] and may lead to osteolysis [8]. The thickness of the polyethylene has been shown to be inversely proportional to the contact stresses sustained by the replacement joint [9]. In fact, Bartel et al. [9] have defined a threshold thickness of 8–10 mm of polyethylene; thinner liners are more susceptible to surface damage and wear. A noninvasive tomographic imaging modality, ideally one that does not expose patients to radiation if used for repetitive examinations, is necessary to function as a screening tool to detect subtle cases of polyethylene wear and delamination.
Wear patterns in total knee arthroplasty have been identified. Although Benjamin et al. [10] found central wear to be the overall most common pattern in patients who underwent knee arthroplasty but retained the posterior cruciate ligament, peripheral and asymmetric patterns of wear were found most commonly in liners with flat articular surfaces, as opposed to those with more curved anteroposterior alignment. These authors observed eccentric patterns of wear involving both the medial and lateral aspects of the polyethylene liner, primarily in the posterior compartment, to be the most common pattern in spacers with flat articular surfaces. It is these areas that are amenable to sonographic visualization.
We found that sonography can depict the polyethylene liner in patients who have undergone total knee arthroplasty. In addition, we found a high correlation between the in vivo sonographic measurements of the polyethylene liner and the radiographic measurements. The ability to tomographically image the liner and to obtain direct measurements of the liner with digital calipers avoids the problems inherent to conventional radiography including differences in projection and magnification. We propose that sonography, as a noninvasive imaging modality with no ionizing radiation, can be used as a method to screen patients who have undergone total knee arthroplasty for potential wear of the polyethylene liner, primarily involving the eccentric portion of the implant.
Acknowledgments
We thank Russell Windsor, Geoffrey Westrich, Thomas Sculco, and Answorth
Allen from the Department of Orthopaedic Surgery, Hospital for Special
Surgery, for supplying much of the clinical information.
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