HIP
An analysis of the best method for evaluating anteversion of the acetabular component after total hip replacement on plain radiographs T. Nomura, M. Naito, Y. Nakamura, T. Ida, D. Kuroda, T. Kobayashi, T. Sakamoto, H. Seo From Department of Orthopaedic Surgery, Faculty of Medicine, Fukuoka University, Fukuoka, Japan
Several radiological methods of measuring anteversion of the acetabular component after total hip replacement (THR) have been described. These studies used different definitions and reference planes to compare methods, allowing for misinterpretation of the results. We compared the reliability and accuracy of five current methods using plain radiographs (those of Lewinnek, Widmer, Liaw, Pradhan, and Woo and Morrey) with CT measurements, using the same definition and reference plane. We retrospectively studied the plain radiographs and CT scans in 84 hips of 84 patients who underwent primary THR. Intra- and interobserver reliability were high for the measurement of inclination and anteversion with all methods on plain radiographs and CT scans. The measurements of inclination on plain radiographs were similar to the measurements using CT (p = 0.043). The mean difference between CT measurements was 0.6° (-5.9° to 6.8°). Measurements using Widmer’s method were the most similar to those using CT (p = 0.088), with a mean difference between CT measurements of -0.9° (-10.4° to 9.1°), whereas the other four methods differed significantly from those using CT (p < 0.001). This study has shown that Widmer’s method is the best for evaluating the anteversion of the acetabular component on plain radiographs. Cite this article: Bone Joint J 2014; 96-B:597–603.
T. Nomura, MD, Orthopaedic Surgeon M. Naito, MD, PhD, Orthopaedic Surgeon, Professor Y. Nakamura, MD, PhD, Orthopaedic Surgeon T. Ida, MD, PhD, Orthopaedic Surgeon D. Kuroda, MD, PhD, Orthopaedic Surgeon T. Kobayashi, MD, Orthopaedic Surgeon T. Sakamoto, MD, Orthopaedic Surgeon H. Seo, MD, Orthopaedic Surgeon Department of Orthopaedic Surgery, Faculty of Medicine, Fukuoka University, 7-45-1 Nanakuma, Jonan-ku, Fukuoka, 814-0180, Japan. Correspondence should be sent to Dr T. Nomura; e-mail: [email protected] ©2014 The British Editorial Society of Bone & Joint Surgery doi:10.1302/0301-620X.96B. 33013 $2.00 Bone Joint J 2014;96-B:597–603 Received 23 August 2013; Accepted after revision 31 January 2014
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Accurate measurement of the position of the acetabular component is important in the assessment of outcome after total hip replacement (THR).1-5 Both plain radiographs and CT scans have been used to evaluate its position. It has been shown that the position of the component can be evaluated accurately using CT 6-9; however, plain radiographs continue to be widely used as CT scans are expensive and expose patients to considerable radiation. The position of the acetabular component has been defined by radiographic, operative, and anatomical inclination and anteversion.10 The two reference planes for measuring its position are the anterior pelvic plane (APP)11 and the functional coronal plane (FCP)12 (Fig. 1). Inclination can easily be measured on anteroposterior (AP) radiographs, whereas anteversion is more difficult to measure. Several radiological methods have been described for recording anteversion of the acetabular component on plain radiographs,1,13-19 but few studies have compared the reliability and accuracy of these methods.20,21 Our purpose in this study was to evaluate the reliability of five methods of assessing anteversion (Lewinnek,13 Widmer,18 Liaw,16 Pradhan,17 and Woo and Morrey1) and to compare the
accuracy of these methods with those of CT measurements using the same definition and reference plane.
Patients and Methods Between August 2010 and December 2012, 268 patients underwent primary unilateral THR. We excluded 163 with a history of lumbar disease and/or contralateral hip disease, to prevent errors in measurement on crosstable lateral radiographs. We also excluded 21 patients with no post-operative CT scans, and we retrospectively studied AP and crosstable lateral radiographs and CT scans in the remaining 84 hips (84 patients). The study had ethical approval and all patients gave informed consent. There were 63 women and 21 men, with a mean age of 66.3 years (36 to 84) at the time of surgery. The pre-operative diagnosis was osteoarthritis in 79 hips and osteonecrosis of the femoral head in five. All THRs were performed by two experienced surgeons (MN and YN) through a posterolateral approach. A cementless acetabular component was used in all patients (44 Trilogy (Zimmer, Warsaw, Indiana), 31 TNH (Nakashima Medical, Okayama, Japan) and nine Pinnacle components (DePuy, Warsaw, Indiana)). Their 597
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Widmer’s method18: S is the short axis of the ellipse and TL is the total length of the projected cross-section of the component along the short axis. This method shows linear correlation for values of S/TL between 0.2 and 0.6 (Fig. 2b). Liaw’s method16: β is the angle formed by the long axis of the component (the line from point A to B) and the line connecting the top point of the ellipse and the endpoint of the long axis (the line from point A to C) (Fig. 2c). Pradhan’s method17: D is the maximum distance across the long axis of the ellipse of the component. A line is drawn perpendicular to the long axis and intersecting the rim of the component, beginning at a point one-fifth of the total distance of the longitudinal line. P is the distance along this perpendicular line from the longitudinal line to the rim (Fig. 2d). Fig. 1 CT scan showing reference planes for measuring the position of the acetabular component. (APP, anterior pelvic plane; FCP, functional coronal plane)
Measurement of anteversion of the acetabular component on cross-table lateral radiographs. Woo and Morrey’s method1:
Anteversion is measured as the angle formed by the long axis of the ellipsoid projection of the base of the component and a vertical line (Fig. 2e). Measurement of inclination and anteversion of the acetabular component on CT scans. We used the methods described in
mean size was 52 mm (48 to 54). The articulation was metal-on-polyethylene in all patients. One week post-operatively, the same group of radiology technicians performed AP and cross-table lateral radiographs and CT scans of the pelvis. AP radiographs were obtained in the supine position with the hips in a neutral position. The radiation beam was centred on the superior aspect of the pubic symphysis and was perpendicular to the patient. The film–focus distance was 110 cm and no extra positioning guide was used. Cross-table lateral radiographs were taken with the contralateral hip flexed to 90°. The direction of the beam was parallel to the examination table and 45° cephalad to the long axis of the body. The radiograph was held perpendicular to the examination table using a cassette holder. CT scans were obtained in the supine position with the hip in a neutral position. We used a 64-channel multidetector CT system (Aquilion 64-slice CT scanner; Toshiba, Tokyo, Japan), and the scan protocol had a slice distance of 0.5 mm. All images were digitally acquired using the Picture Archiving and Communication System (PACS). The volume data were stored on a server in Digital Imaging and Communications in Medicine (DICOM) format for later analysis. Measurement of inclination of the acetabular component on AP radiographs. The inclination of the component is the
angle between a line through the long axis of its ellipse and the inter-teardrop line on AP radiographs. Measurement of anteversion of the acetabular component on AP radiographs. Lewinnek’s method13: D1 is the dis-
tance across the short axis of an ellipse drawn perpendicular to the long axis of the acetabular component. D2 is its maximum diameter (Fig. 2a).
Murray’s concept10 to measure the position of the component. We used a three-dimensional (3D) workstation (Ziostation, version 1.17t; ZAIOSOFT Inc, Tokyo, Japan) to reformat the image. We corrected malposition of the pelvis in rotation and abduction/adduction, but maintained pelvic tilt relative to the CT table. Pelvic images were resliced parallel to the FCP. In this plane, the angle between the line connecting the two points of the edge of the acetabular component and the inter-teardrop line was measured as the radiographic inclination. In order to evaluate anteversion, we created a plane which was orthogonal to a line drawn from the medialmost point of the component to its lateral-most point. In this plane, the angle between a line perpendicular to the FCP and a tangential line across the open face of the component was measured as the radiographic anteversion (Fig. 3). Measurements of anteversion and inclination on CT scans were regarded as the reference standard. Assessment of reliability and accuracy. Reliability was defined as consistency of the measurements, and accuracy was defined as proximity to CT measurements. Four examiners (TN, TK, TS and HS) performed all measurements independently, using the same protocols. In order to assess the intra-observer reliability of each method, one examiner (TN) measured all hips three times, with a three-week interval between measurements, without comparison to the previous measurements. The inter-observer reliability of each method was assessed by comparing the results of the four examiners. All measurements were made without knowledge of the patient’s clinical status or of the measurements assigned by the other examiners. Plain radiographs and CT scans were presented to each examiner in random order by a research assistant who did not participate in the reliability and accuracy sessions. THE BONE & JOINT JOURNAL
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Fig. 2a
Fig. 2c
Fig. 2b
Fig. 2d
Radiographs demonstrating the methods for measuring anteversion on plain radiographs showing a) Lewinnek13 (anteversion = arcsin [D1/D2]); b) Widmer18 (anteversion = arcsin (short axis [S])/total length [TL]) = 48.05 × (S/TL) 0.3, if 0.2 < S/TL < 0.6; c) Liaw16 (anteversion = sin-1 tan β; d) Pradhan17) anteversion = arcsin (P/0.4D); and e) Woo and Morrey.1
Fig. 2e
In order to assess the accuracy of measurements on plain radiographs, we compared the mean value of the measurements on plain radiographs with the mean of CT measurements. We calculated the difference between measurements on plain radiographs and CT measurements. VOL. 96-B, No. 5, MAY 2014
Statistical analysis. The intra-class correlation coefficient (ICC) and 95% confidence interval (CI) were calculated for intra-and inter-observer reliability. We used the two-way random-effects intra-class correlation model and absolute agreement to calculate the ICC: an ICC of 1 means perfect reliability, and an ICC of 0 means the opposite.
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Fig. 3 CT scan demonstrating the method for measuring anteversion.
Table I. Intra- and inter-observer reliability of measurements on plain radiographs and CT. (ICC, intraclass correlation coefficient; CI, confidence interval)
Methods Inclination CT AP radiograph Anteversion CT Lewinnek13 Widmer18 Liaw16 Pradhan17 Woo and Morrey1
Intra-observer reliability
Inter-observer reliability
ICC
95% CI
ICC
95% CI
0.983 0.973
0.976 to 0.988 0.961 to 0.982
0.985 0.982
0.979 to 0.990 0.975 to 0.987
0.971 0.930 0.925 0.922 0.913 0.976
0.959 to 0.980 0.899 to 0.952 0.873 to 0.953 0.890 to 0.946 0.879 to 0.940 0.966 to 0.984
0.975 0.946 0.944 0.929 0.923 0.981
0.966 to 0.983 0.926 to 0.962 0.921 to 0.962 0.903 to 0.950 0.894 to 0.945 0.973 to 0.987
Paired t-tests were used to compare the accuracy of each method on plain radiography relative to CT measurements. Statistical analyses were conducted using SPSS for Windows, version 20.0 (SPSS Inc., Chicago, Illinois) and p < 0.01 was deemed to be statistically significant.
Results Reliability. All measurements on plain radiographs and CT scans had nearly perfect intra- and inter-observer reliability. The ICCs for measurements on CT scans ranged from 0.971 (95% CI 0.959 to 0.980) for the intra-observer reliability of anteversion to 0.985 (95% CI 0.979 to 0.990) for the inter-observer reliability of inclination. The ICCs for measurements on plain radiographs ranged from 0.913 (95% CI 0.879 to 0.940) for the intra-observer reliability of Pradhan’s methods17 to 0.981 (95% CI 0.973 to 0.987) for the inter-observer reliability of Woo and Morrey’s methods1 (Table I). Accuracy. The accuracy of the measurements of inclination on AP radiographs was similar to that on CT measurements (p = 0.043). The mean difference in inclination between
measurements on AP radiographs and CT measurements was 0.6° (-5.9° to 6.8°). In terms of the measurements of anteversion, those using Widmer’s method18 were not significantly different from CT measurements (p = 0.088), and the mean difference with Widmer’s method18 was -10.9° (-10.4° to 9.1°), whereas the other four methods were significantly different from CT measurements (p < 0.001) (Table II).
Discussion The position of the acetabular component is a predictor of outcome after THR. Malpositioning is associated with impingement, dislocation, limitation of movement and increased polyethylene wear.1-5 Inclination can be directly and easily measured on AP radiographs. Our study shows that these measurements of inclination are reliable and accurate compared with CT measurements. Anteversion is more difficult to measure and few studies have compared the reliability and accuracy of different methods of measuring it using plain radiographs.20,21 We THE BONE & JOINT JOURNAL
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Table II. Accuracy of measurements on plain radiographs compared with CT measurements using paired t-tests Difference Methods Inclination (°) CT AP radiograph Anteversion (°) CT Lewinnek13 Widmer18 Liaw16 Pradhan17 Woo and Morrey1
Mean (range)
Mean (range)
p-value (t-test)
38.4 (25.1 to 54.3) 39.0 (24.3 to 55.1)
0.6 (-5.9 to 6.8)
0.043
23.8 (8.6 to 38.7) 19.2 (7.3 to 32.8) 22.9 (10.1 to 40.4) 19.7 (7.6 to 33.9) 18.5 (6.6 to 37.0) 27.7 (8.8 to 43.7)
-4.6 (-14.9 to 5.4) -0.9 (-10.4 to 9.1) -4.1 (-14.5 to 4.1) -5.3 (-15.3 to 5.2) 3.9 (-4.9 to 15.2)
< 0.001 0.088 < 0.001 < 0.001 < 0.001
Table III. Reported accuracy of plain radiographic measurements compared with CT measurements for anteversion using paired t-tests Study Marx et al20
Nho et al21
SD,
Methods used to compare the measurement of anteversion
Definition of anteversion/ reference plane of CT Anteversion (SD) (°)
CT McLaren26 Ackland14 Pradhan17 Widmer18 Hassan15 CT Lewinnek13 Widmer18 Hassan15 Ackland14 Liaw16 Woo and Morrey1
Radiographic / APP
Anatomical / FCP
29.9 (SD 8.7) 15.4 (SD 7.7) 15.6 (SD 8.2) 15.4 (SD 8.4) 23.5 (SD 10.5) 15.5 (SD 8.3) 26.8 (SD 7.9) 26.9 (SD 6.2) 19.1 (SD 3.3) 26.1 (SD 5.3) 15.7 (SD 3.5) 28.5 (SD 6.3) 28.1 (SD 6.3)
Mean error (SD) (°)
p-value (t-test)
–14.5 (SD 10.5) –14.3 (SD 10.3) –14.5 (SD 10.2) –6.4 (SD 10.8) –14.4 (SD 10.2)
< 0.0001 < 0.0001 < 0.0001 < 0.0001 < 0.0001
Not Known
0.901 < 0.001 0.425 < 0.001 0.058 0.171
standard deviation
have analysed five plain radiographic methods and confirmed that the methods of Lewinnek,13 Widmer,18 Liaw,16 Pradhan,17 and Woo and Morrey1 were reliable. Our findings are similar to those of previous studies.21-24 We found that only Widmer’s method was accurate when compared with CT. However, previous studies reported different results. Marx et al20 compared the accuracy of five plain radiographic methods (Pradhan,17 Mclaren,25 Hassan et al,15 Ackland et al14 and Widmer18) with CT measurements and concluded that Widmer’s method had a smaller rate of errors than the others but that measurement of anteversion using plain radiographs was inaccurate because of a variety of errors. Nho et al21 compared the accuracy of six radiographic methods (Lewinnek,13 Widmer,18 Hassan,15 Ackland,14 Liaw,16 and Woo and Morrey1) with CT measurements and reported that measurements obtained using Widmer’s18 and Ackland’s14 methods were significantly different from CT measurements, whereas the other methods were not. They concluded that the methods of Lewinnek,13 Hassan,15 Liaw,16 and Woo and Morrey1 were accurate. Previous studies20,21 used different definitions and reference planes, resulting in different results from ours (Table III). The APP was determined by three reference points: the two anterosuperior iliac spines and the anterior VOL. 96-B, No. 5, MAY 2014
surface of the pubic symphysis.11 The FCP was based on the longitudinal axis of the body. In most patients the APP deviates from the FCP owing to pelvic tilt, and varies by up to 20° more from the FCP in some patients.26 Malik et al27 reported that an AP pelvic tilt of 1° led to a change of approximately 0.8° in anteversion. Thus, a difference of > 20° in pelvic tilt can cause a difference in anteversion of > 16°. Because of pelvic tilt, measurements of the position of the actebular component on the FCP cannot be compared with those on the APP.12,22,27 Recent studies have highlighted the concept of the functional orientation of the acetabular component, which is based on the FCP.12,27-29 These studies reported that the APP is not useful when it is used without considering the pelvic tilt, and could lead to complications. Miki et al28 reported that the FCP is adequate for planning the position of the acetabular component, whether the pelvic tilt is small or large. In general, on either the FCP or the APP, anatomical inclination and anteversion are always greater than radiographic inclination and anteversion. Anatomical inclination is always greater than operative inclination, and operative anteversion is always greater than radiographic anteversion. If a difference exists, depending on the
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combination of inclination and anteversion, the difference in inclination is small, whereas the difference in anteversion may be large.10 The position of the acetabular component on plain radiographs has been defined as radiographic inclination and anteversion relative to the FCP. Its position on CT scans must be measured using the same definition and reference plane to compare its accuracy with radiographic methods. Previous studies recommended using the radiographic definitions and the FCP12,22,27 when comparing measurements of position on radiographs with those on CT scans. Mixed use of definitions and reference planes prevents comparison of reported methods. Only a few studies have compared the accuracy of radiographic methods of measurement with CT methods using the same radiographic definition on the same FCP.22,23 Lu et al23 evaluated Lewinnek’s method13 and concluded that it was accurate and reliable. Nishino et al23 reported that Lewinnek’s method was more accurate than that of Woo and Morrey1 for measuring anteversion. To our knowledge, this is the first study comparing the accuracy of several radiographic methods with CT measurements using the same definition and reference plane. Our study has several limitations. First, there is no gold standard for the accuracy of radiographic- or CT-based values in vivo. However, the measurement of the position of the acetabular component using CT scans has been shown to be satisfactory.6-9 The principles of our methods are similar to those reported in the literature. Secondly, retroversion cannot be identified on AP radiographs and oblique or cross-table lateral radiographs are required.14 In this study there were no hips with retroversion according to CT measurements. Thirdly, it is difficult to identify the apex of the ellipse of the acetabular component on AP radiographs when a ceramic or metal liner is used.21 Finally, the patient’s position during radiography influences the measurements.20 These limitations must be taken into consideration when our results are applied in clinical practice. We found that Woo and Morrey’s method1 was inaccurate. However, this method directly evaluates anteversion without complicated calculations and identifies ante- or retroversion. Cross-table lateral radiographs were taken with the contralateral hip flexed to 90°. Stiffness of the lumbar spine and/or contralateral hip joint might cause errors in the measurement of anteversion on cross-table lateral radiographs because they influence the pelvic tilt when the contralateral hip is flexed.30 Therefore, we excluded patients with lumbar disease and/or contralateral hip disease. If the angle of flexion of the contralateral hip was < 90°, measurements of anteversion using cross-table lateral radiographs might be more accurate.23 McArthur et al31 reported that modifying the cross-table lateral radiograph by adjusting the angle of the beam to match the angle of inclination of the acetabular component improves accuracy and reliability. If the contralateral hip is flexed to < 90° and the angle of the x-ray beam matches the angle of inclination of the
component, the measurement of anteversion using crosstable lateral radiographs may be more accurate and reliable. We plan to study this possibility in the future. We found that Widmer’s method18 had some small errors in analysing anteversion. Considering the advantages of plain radiographs, including low cost, low levels of radiation, convenience for follow-up and assessment of the position of the acetabular component, the errors were within a clinically acceptable range. Our study showed that Widmer’s method was the best for assessing anteversion on plain radiographs. No benefits in any form have been received or will be received from a commercial party related directly or indirectly to the subject of this article. This article was primary edited by D. Johnstone and first proof edited by J. Scott.
References 1. Woo RY, Morrey BF. Dislocations after total hip arthroplasty. J Bone Joint Surg [Am] 1982;64-A:1295–1306. 2. Widmer KH, Zurfluh B. Compliant positioning of total hip components for optimal range of motion. J Orthop Res 2004;22:815–821. 3. Kennedy JG, Rogers WB, Soffe KE, et al. Effect of acetabular component orientation on recurrent dislocation, pelvic osteolysis, polyethylene wear, and component migration. J Arthroplasty 1998;13:530–534. 4. Jolles BM, Zangger P, Leyvraz PF. Factors predisposing to dislocation after primary total hip arthroplasty: a multivariate analysis. J Arthroplasty 2002;17:282–288. 5. Wan Z, Boutary M, Dorr LD. The influence of acetabular component position on wear in total hip arthroplasty. J Arthroplasty 2008;23:51–56. 6. Mian SW, Truchly G, Pflum FA. Computed tomography measurement of acetabular cup anteversion and retroversion in total hip arthroplasty. Clin Orthop Relat Res 1992;276:206–209. 7. Olivecrona H, Weidenhielm L, Olivecrona L, et al. A new CT method for measuring cup orientation after total hip arthroplasty: a study of 10 patients. Acta Orthop Scand 2004;75:252–260. 8. Ybinger T, Kumpan W, Hoffart HE, et al. Accuracy of navigation-assisted acetabular component positioning studied by computed tomography measurements: methods and results. J Arthroplasty 2007;22:812–817. 9. Arai N, Nakamura S, Matsushita T, Suzuki S. Minimal radiation dose computed tomography for measurement of cup orientation in total hip arthroplasty. J Arthroplasty 2010;25:263–267. 10. Murray DW. The definition and measurement of acetabular orientation. J Bone Joint Surg [Br] 1993;75-B:228–232. 11. McKibbin B. Anatomical factors in the stability of the hip joint in the newborn. J Bone Joint Surg [Br] 1970;52-B:148–159. 12. Wan Z, Malik A, Jaramaz B, Chao L, Dorr LD. Imaging and navigation measurement of acetabular component position in THA. Clin Orthop Relat Res 2009;467:32– 42. 13. Lewinnek GE, Lewis JL, Tarr R, Compere CL, Zimmerman JR. Dislocations after total hip-replacement arthroplasties. J Bone Joint Surg [Am] 1978;60-A:217–220. 14. Ackland MK, Bourne WB, Uhthoff HK. Ante-version of the acetabular cup. Measurement of angle after total hip replacement. J Bone Joint Surg [Br] 1986;68-B:409– 413. 15. Hassan DM, Johnston GH, Dust WN, Watson LG, Cassidy D. Radiographic calculation of anteversion in acetabular prostheses. J Arthroplasty 1995;10:369–372. 16. Liaw CK, Hou SM, Yang RS, Wu TY, Fuh CS. A new tool for measuring cup orientation in total hip arthroplasties from plain radiographs. Clin Orthop Relat Res 2006;451:134–139. 17. Pradhan R. Planar anteversion of the acetabular cup as determined from plain antero-posterior radiographs. J Bone Joint Surg [Br] 1999;81-B:431–435. 18. Widmer KH. A simplified method to determine acetabular cup anteversion from plain radiographs. J Arthroplasty 2004;19:387–390. 19. Visser JD, Konings JG. A new method for measuring angles after total hip arthroplasty. A study of the acetabular cup and femoral component. J Bone Joint Surg [Br] 1981;63-B:556–559. 20. Marx A, von Knoch M, Pförtner J, Wiese M, Saxler G. Misinterpretation of cup anteversion in total hip arthroplasty using planar radiography. Arch Orthop Trauma Surg 2006;126:487–492. THE BONE & JOINT JOURNAL
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21. Nho JH, Lee YK, Kim HJ, et al. Reliability and validity of measuring version of the acetabular component. J Bone Joint Surg [Br] 2012;94-B:32–36. 22. Lu M, Zhou YX, Du H, Zhang J, Liu J. Reliability and validity of measuring acetabular component orientation by plain antero-posterior radiographs. Clin Orthop Relat Res 2013;471:2987–2994. 23. Nishino H, Nakamura S, Arai N, Matsushita T. Accuracy and precision of version angle measurements of the acetabular component after total hip arthroplasty. J Arthroplasty 2013;28:1644–1647. 24. Haenle M, Mittelmeier W, Barbano R, et al. Accuracy and reliability of different methods to evaluate the acetabular cup version from plain radiographs. Surg Radiol Anat 2010;32:725–730. 25. McLaren RH. Prosthetic hip angulation. Radiology 1973;107:705–706. 26. Babisch JW, Layher F, Amiot LP. The rationale for tilt-adjusted acetabular cup navigation. J Bone Joint Surg [Am] 2008;90-A:357–365.
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27. Malik A, Wan Z, Jaramaz B, Bowman G, Dorr LD. A validation model for measurement of acetabular component position. J Arthroplasty 2010;25:812–819. 28. Miki H, Kyo T, Sugano N. Anatomical hip range of motion after implantation during total hip arthroplasty with a large change in pelvic inclination. J Arthroplasty 2012;27:1641–1650. 29. Eilander W, Harris SJ, Henkus HE, Cobb JP, Hogervorst T. Functional acetabular component position with supine total hip replacement. Bone Joint J 2013;95B:1326–1331. 30. Arai N, Nakamura S, Matsushita T. Difference between 2 measurement methods of version angles of the acetabular component. J Arthroplasty 2007;22:715–720. 31. McArthur B, Cross M, Geatrakas C, Mayman D, Ghelman B. Measuring acetabular component version after THA: CT or plain radiograph? Clin Orthop Relat Res 2012;470:2810–2818.
An analysis of the best method for evaluating anteversion of the acetabular component after total hip replacement on plain radiographs T. Nomura, M. Naito, Y. Nakamura, T. Ida, D. Kuroda, T. Kobayashi, T. Sakamoto, H. Seo From Department of Orthopaedic Surgery, Faculty of Medicine, Fukuoka University, Fukuoka, Japan
Several radiological methods of measuring anteversion of the acetabular component after total hip replacement (THR) have been described. These studies used different definitions and reference planes to compare methods, allowing for misinterpretation of the results. We compared the reliability and accuracy of five current methods using plain radiographs (those of Lewinnek, Widmer, Liaw, Pradhan, and Woo and Morrey) with CT measurements, using the same definition and reference plane. We retrospectively studied the plain radiographs and CT scans in 84 hips of 84 patients who underwent primary THR. Intra- and interobserver reliability were high for the measurement of inclination and anteversion with all methods on plain radiographs and CT scans. The measurements of inclination on plain radiographs were similar to the measurements using CT (p = 0.043). The mean difference between CT measurements was 0.6° (-5.9° to 6.8°). Measurements using Widmer’s method were the most similar to those using CT (p = 0.088), with a mean difference between CT measurements of -0.9° (-10.4° to 9.1°), whereas the other four methods differed significantly from those using CT (p < 0.001). This study has shown that Widmer’s method is the best for evaluating the anteversion of the acetabular component on plain radiographs. Cite this article: Bone Joint J 2014; 96-B:597–603.
T. Nomura, MD, Orthopaedic Surgeon M. Naito, MD, PhD, Orthopaedic Surgeon, Professor Y. Nakamura, MD, PhD, Orthopaedic Surgeon T. Ida, MD, PhD, Orthopaedic Surgeon D. Kuroda, MD, PhD, Orthopaedic Surgeon T. Kobayashi, MD, Orthopaedic Surgeon T. Sakamoto, MD, Orthopaedic Surgeon H. Seo, MD, Orthopaedic Surgeon Department of Orthopaedic Surgery, Faculty of Medicine, Fukuoka University, 7-45-1 Nanakuma, Jonan-ku, Fukuoka, 814-0180, Japan. Correspondence should be sent to Dr T. Nomura; e-mail: [email protected] ©2014 The British Editorial Society of Bone & Joint Surgery doi:10.1302/0301-620X.96B. 33013 $2.00 Bone Joint J 2014;96-B:597–603 Received 23 August 2013; Accepted after revision 31 January 2014
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Accurate measurement of the position of the acetabular component is important in the assessment of outcome after total hip replacement (THR).1-5 Both plain radiographs and CT scans have been used to evaluate its position. It has been shown that the position of the component can be evaluated accurately using CT 6-9; however, plain radiographs continue to be widely used as CT scans are expensive and expose patients to considerable radiation. The position of the acetabular component has been defined by radiographic, operative, and anatomical inclination and anteversion.10 The two reference planes for measuring its position are the anterior pelvic plane (APP)11 and the functional coronal plane (FCP)12 (Fig. 1). Inclination can easily be measured on anteroposterior (AP) radiographs, whereas anteversion is more difficult to measure. Several radiological methods have been described for recording anteversion of the acetabular component on plain radiographs,1,13-19 but few studies have compared the reliability and accuracy of these methods.20,21 Our purpose in this study was to evaluate the reliability of five methods of assessing anteversion (Lewinnek,13 Widmer,18 Liaw,16 Pradhan,17 and Woo and Morrey1) and to compare the
accuracy of these methods with those of CT measurements using the same definition and reference plane.
Patients and Methods Between August 2010 and December 2012, 268 patients underwent primary unilateral THR. We excluded 163 with a history of lumbar disease and/or contralateral hip disease, to prevent errors in measurement on crosstable lateral radiographs. We also excluded 21 patients with no post-operative CT scans, and we retrospectively studied AP and crosstable lateral radiographs and CT scans in the remaining 84 hips (84 patients). The study had ethical approval and all patients gave informed consent. There were 63 women and 21 men, with a mean age of 66.3 years (36 to 84) at the time of surgery. The pre-operative diagnosis was osteoarthritis in 79 hips and osteonecrosis of the femoral head in five. All THRs were performed by two experienced surgeons (MN and YN) through a posterolateral approach. A cementless acetabular component was used in all patients (44 Trilogy (Zimmer, Warsaw, Indiana), 31 TNH (Nakashima Medical, Okayama, Japan) and nine Pinnacle components (DePuy, Warsaw, Indiana)). Their 597
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Widmer’s method18: S is the short axis of the ellipse and TL is the total length of the projected cross-section of the component along the short axis. This method shows linear correlation for values of S/TL between 0.2 and 0.6 (Fig. 2b). Liaw’s method16: β is the angle formed by the long axis of the component (the line from point A to B) and the line connecting the top point of the ellipse and the endpoint of the long axis (the line from point A to C) (Fig. 2c). Pradhan’s method17: D is the maximum distance across the long axis of the ellipse of the component. A line is drawn perpendicular to the long axis and intersecting the rim of the component, beginning at a point one-fifth of the total distance of the longitudinal line. P is the distance along this perpendicular line from the longitudinal line to the rim (Fig. 2d). Fig. 1 CT scan showing reference planes for measuring the position of the acetabular component. (APP, anterior pelvic plane; FCP, functional coronal plane)
Measurement of anteversion of the acetabular component on cross-table lateral radiographs. Woo and Morrey’s method1:
Anteversion is measured as the angle formed by the long axis of the ellipsoid projection of the base of the component and a vertical line (Fig. 2e). Measurement of inclination and anteversion of the acetabular component on CT scans. We used the methods described in
mean size was 52 mm (48 to 54). The articulation was metal-on-polyethylene in all patients. One week post-operatively, the same group of radiology technicians performed AP and cross-table lateral radiographs and CT scans of the pelvis. AP radiographs were obtained in the supine position with the hips in a neutral position. The radiation beam was centred on the superior aspect of the pubic symphysis and was perpendicular to the patient. The film–focus distance was 110 cm and no extra positioning guide was used. Cross-table lateral radiographs were taken with the contralateral hip flexed to 90°. The direction of the beam was parallel to the examination table and 45° cephalad to the long axis of the body. The radiograph was held perpendicular to the examination table using a cassette holder. CT scans were obtained in the supine position with the hip in a neutral position. We used a 64-channel multidetector CT system (Aquilion 64-slice CT scanner; Toshiba, Tokyo, Japan), and the scan protocol had a slice distance of 0.5 mm. All images were digitally acquired using the Picture Archiving and Communication System (PACS). The volume data were stored on a server in Digital Imaging and Communications in Medicine (DICOM) format for later analysis. Measurement of inclination of the acetabular component on AP radiographs. The inclination of the component is the
angle between a line through the long axis of its ellipse and the inter-teardrop line on AP radiographs. Measurement of anteversion of the acetabular component on AP radiographs. Lewinnek’s method13: D1 is the dis-
tance across the short axis of an ellipse drawn perpendicular to the long axis of the acetabular component. D2 is its maximum diameter (Fig. 2a).
Murray’s concept10 to measure the position of the component. We used a three-dimensional (3D) workstation (Ziostation, version 1.17t; ZAIOSOFT Inc, Tokyo, Japan) to reformat the image. We corrected malposition of the pelvis in rotation and abduction/adduction, but maintained pelvic tilt relative to the CT table. Pelvic images were resliced parallel to the FCP. In this plane, the angle between the line connecting the two points of the edge of the acetabular component and the inter-teardrop line was measured as the radiographic inclination. In order to evaluate anteversion, we created a plane which was orthogonal to a line drawn from the medialmost point of the component to its lateral-most point. In this plane, the angle between a line perpendicular to the FCP and a tangential line across the open face of the component was measured as the radiographic anteversion (Fig. 3). Measurements of anteversion and inclination on CT scans were regarded as the reference standard. Assessment of reliability and accuracy. Reliability was defined as consistency of the measurements, and accuracy was defined as proximity to CT measurements. Four examiners (TN, TK, TS and HS) performed all measurements independently, using the same protocols. In order to assess the intra-observer reliability of each method, one examiner (TN) measured all hips three times, with a three-week interval between measurements, without comparison to the previous measurements. The inter-observer reliability of each method was assessed by comparing the results of the four examiners. All measurements were made without knowledge of the patient’s clinical status or of the measurements assigned by the other examiners. Plain radiographs and CT scans were presented to each examiner in random order by a research assistant who did not participate in the reliability and accuracy sessions. THE BONE & JOINT JOURNAL
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Fig. 2a
Fig. 2c
Fig. 2b
Fig. 2d
Radiographs demonstrating the methods for measuring anteversion on plain radiographs showing a) Lewinnek13 (anteversion = arcsin [D1/D2]); b) Widmer18 (anteversion = arcsin (short axis [S])/total length [TL]) = 48.05 × (S/TL) 0.3, if 0.2 < S/TL < 0.6; c) Liaw16 (anteversion = sin-1 tan β; d) Pradhan17) anteversion = arcsin (P/0.4D); and e) Woo and Morrey.1
Fig. 2e
In order to assess the accuracy of measurements on plain radiographs, we compared the mean value of the measurements on plain radiographs with the mean of CT measurements. We calculated the difference between measurements on plain radiographs and CT measurements. VOL. 96-B, No. 5, MAY 2014
Statistical analysis. The intra-class correlation coefficient (ICC) and 95% confidence interval (CI) were calculated for intra-and inter-observer reliability. We used the two-way random-effects intra-class correlation model and absolute agreement to calculate the ICC: an ICC of 1 means perfect reliability, and an ICC of 0 means the opposite.
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Fig. 3 CT scan demonstrating the method for measuring anteversion.
Table I. Intra- and inter-observer reliability of measurements on plain radiographs and CT. (ICC, intraclass correlation coefficient; CI, confidence interval)
Methods Inclination CT AP radiograph Anteversion CT Lewinnek13 Widmer18 Liaw16 Pradhan17 Woo and Morrey1
Intra-observer reliability
Inter-observer reliability
ICC
95% CI
ICC
95% CI
0.983 0.973
0.976 to 0.988 0.961 to 0.982
0.985 0.982
0.979 to 0.990 0.975 to 0.987
0.971 0.930 0.925 0.922 0.913 0.976
0.959 to 0.980 0.899 to 0.952 0.873 to 0.953 0.890 to 0.946 0.879 to 0.940 0.966 to 0.984
0.975 0.946 0.944 0.929 0.923 0.981
0.966 to 0.983 0.926 to 0.962 0.921 to 0.962 0.903 to 0.950 0.894 to 0.945 0.973 to 0.987
Paired t-tests were used to compare the accuracy of each method on plain radiography relative to CT measurements. Statistical analyses were conducted using SPSS for Windows, version 20.0 (SPSS Inc., Chicago, Illinois) and p < 0.01 was deemed to be statistically significant.
Results Reliability. All measurements on plain radiographs and CT scans had nearly perfect intra- and inter-observer reliability. The ICCs for measurements on CT scans ranged from 0.971 (95% CI 0.959 to 0.980) for the intra-observer reliability of anteversion to 0.985 (95% CI 0.979 to 0.990) for the inter-observer reliability of inclination. The ICCs for measurements on plain radiographs ranged from 0.913 (95% CI 0.879 to 0.940) for the intra-observer reliability of Pradhan’s methods17 to 0.981 (95% CI 0.973 to 0.987) for the inter-observer reliability of Woo and Morrey’s methods1 (Table I). Accuracy. The accuracy of the measurements of inclination on AP radiographs was similar to that on CT measurements (p = 0.043). The mean difference in inclination between
measurements on AP radiographs and CT measurements was 0.6° (-5.9° to 6.8°). In terms of the measurements of anteversion, those using Widmer’s method18 were not significantly different from CT measurements (p = 0.088), and the mean difference with Widmer’s method18 was -10.9° (-10.4° to 9.1°), whereas the other four methods were significantly different from CT measurements (p < 0.001) (Table II).
Discussion The position of the acetabular component is a predictor of outcome after THR. Malpositioning is associated with impingement, dislocation, limitation of movement and increased polyethylene wear.1-5 Inclination can be directly and easily measured on AP radiographs. Our study shows that these measurements of inclination are reliable and accurate compared with CT measurements. Anteversion is more difficult to measure and few studies have compared the reliability and accuracy of different methods of measuring it using plain radiographs.20,21 We THE BONE & JOINT JOURNAL
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Table II. Accuracy of measurements on plain radiographs compared with CT measurements using paired t-tests Difference Methods Inclination (°) CT AP radiograph Anteversion (°) CT Lewinnek13 Widmer18 Liaw16 Pradhan17 Woo and Morrey1
Mean (range)
Mean (range)
p-value (t-test)
38.4 (25.1 to 54.3) 39.0 (24.3 to 55.1)
0.6 (-5.9 to 6.8)
0.043
23.8 (8.6 to 38.7) 19.2 (7.3 to 32.8) 22.9 (10.1 to 40.4) 19.7 (7.6 to 33.9) 18.5 (6.6 to 37.0) 27.7 (8.8 to 43.7)
-4.6 (-14.9 to 5.4) -0.9 (-10.4 to 9.1) -4.1 (-14.5 to 4.1) -5.3 (-15.3 to 5.2) 3.9 (-4.9 to 15.2)
< 0.001 0.088 < 0.001 < 0.001 < 0.001
Table III. Reported accuracy of plain radiographic measurements compared with CT measurements for anteversion using paired t-tests Study Marx et al20
Nho et al21
SD,
Methods used to compare the measurement of anteversion
Definition of anteversion/ reference plane of CT Anteversion (SD) (°)
CT McLaren26 Ackland14 Pradhan17 Widmer18 Hassan15 CT Lewinnek13 Widmer18 Hassan15 Ackland14 Liaw16 Woo and Morrey1
Radiographic / APP
Anatomical / FCP
29.9 (SD 8.7) 15.4 (SD 7.7) 15.6 (SD 8.2) 15.4 (SD 8.4) 23.5 (SD 10.5) 15.5 (SD 8.3) 26.8 (SD 7.9) 26.9 (SD 6.2) 19.1 (SD 3.3) 26.1 (SD 5.3) 15.7 (SD 3.5) 28.5 (SD 6.3) 28.1 (SD 6.3)
Mean error (SD) (°)
p-value (t-test)
–14.5 (SD 10.5) –14.3 (SD 10.3) –14.5 (SD 10.2) –6.4 (SD 10.8) –14.4 (SD 10.2)
< 0.0001 < 0.0001 < 0.0001 < 0.0001 < 0.0001
Not Known
0.901 < 0.001 0.425 < 0.001 0.058 0.171
standard deviation
have analysed five plain radiographic methods and confirmed that the methods of Lewinnek,13 Widmer,18 Liaw,16 Pradhan,17 and Woo and Morrey1 were reliable. Our findings are similar to those of previous studies.21-24 We found that only Widmer’s method was accurate when compared with CT. However, previous studies reported different results. Marx et al20 compared the accuracy of five plain radiographic methods (Pradhan,17 Mclaren,25 Hassan et al,15 Ackland et al14 and Widmer18) with CT measurements and concluded that Widmer’s method had a smaller rate of errors than the others but that measurement of anteversion using plain radiographs was inaccurate because of a variety of errors. Nho et al21 compared the accuracy of six radiographic methods (Lewinnek,13 Widmer,18 Hassan,15 Ackland,14 Liaw,16 and Woo and Morrey1) with CT measurements and reported that measurements obtained using Widmer’s18 and Ackland’s14 methods were significantly different from CT measurements, whereas the other methods were not. They concluded that the methods of Lewinnek,13 Hassan,15 Liaw,16 and Woo and Morrey1 were accurate. Previous studies20,21 used different definitions and reference planes, resulting in different results from ours (Table III). The APP was determined by three reference points: the two anterosuperior iliac spines and the anterior VOL. 96-B, No. 5, MAY 2014
surface of the pubic symphysis.11 The FCP was based on the longitudinal axis of the body. In most patients the APP deviates from the FCP owing to pelvic tilt, and varies by up to 20° more from the FCP in some patients.26 Malik et al27 reported that an AP pelvic tilt of 1° led to a change of approximately 0.8° in anteversion. Thus, a difference of > 20° in pelvic tilt can cause a difference in anteversion of > 16°. Because of pelvic tilt, measurements of the position of the actebular component on the FCP cannot be compared with those on the APP.12,22,27 Recent studies have highlighted the concept of the functional orientation of the acetabular component, which is based on the FCP.12,27-29 These studies reported that the APP is not useful when it is used without considering the pelvic tilt, and could lead to complications. Miki et al28 reported that the FCP is adequate for planning the position of the acetabular component, whether the pelvic tilt is small or large. In general, on either the FCP or the APP, anatomical inclination and anteversion are always greater than radiographic inclination and anteversion. Anatomical inclination is always greater than operative inclination, and operative anteversion is always greater than radiographic anteversion. If a difference exists, depending on the
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combination of inclination and anteversion, the difference in inclination is small, whereas the difference in anteversion may be large.10 The position of the acetabular component on plain radiographs has been defined as radiographic inclination and anteversion relative to the FCP. Its position on CT scans must be measured using the same definition and reference plane to compare its accuracy with radiographic methods. Previous studies recommended using the radiographic definitions and the FCP12,22,27 when comparing measurements of position on radiographs with those on CT scans. Mixed use of definitions and reference planes prevents comparison of reported methods. Only a few studies have compared the accuracy of radiographic methods of measurement with CT methods using the same radiographic definition on the same FCP.22,23 Lu et al23 evaluated Lewinnek’s method13 and concluded that it was accurate and reliable. Nishino et al23 reported that Lewinnek’s method was more accurate than that of Woo and Morrey1 for measuring anteversion. To our knowledge, this is the first study comparing the accuracy of several radiographic methods with CT measurements using the same definition and reference plane. Our study has several limitations. First, there is no gold standard for the accuracy of radiographic- or CT-based values in vivo. However, the measurement of the position of the acetabular component using CT scans has been shown to be satisfactory.6-9 The principles of our methods are similar to those reported in the literature. Secondly, retroversion cannot be identified on AP radiographs and oblique or cross-table lateral radiographs are required.14 In this study there were no hips with retroversion according to CT measurements. Thirdly, it is difficult to identify the apex of the ellipse of the acetabular component on AP radiographs when a ceramic or metal liner is used.21 Finally, the patient’s position during radiography influences the measurements.20 These limitations must be taken into consideration when our results are applied in clinical practice. We found that Woo and Morrey’s method1 was inaccurate. However, this method directly evaluates anteversion without complicated calculations and identifies ante- or retroversion. Cross-table lateral radiographs were taken with the contralateral hip flexed to 90°. Stiffness of the lumbar spine and/or contralateral hip joint might cause errors in the measurement of anteversion on cross-table lateral radiographs because they influence the pelvic tilt when the contralateral hip is flexed.30 Therefore, we excluded patients with lumbar disease and/or contralateral hip disease. If the angle of flexion of the contralateral hip was < 90°, measurements of anteversion using cross-table lateral radiographs might be more accurate.23 McArthur et al31 reported that modifying the cross-table lateral radiograph by adjusting the angle of the beam to match the angle of inclination of the acetabular component improves accuracy and reliability. If the contralateral hip is flexed to < 90° and the angle of the x-ray beam matches the angle of inclination of the
component, the measurement of anteversion using crosstable lateral radiographs may be more accurate and reliable. We plan to study this possibility in the future. We found that Widmer’s method18 had some small errors in analysing anteversion. Considering the advantages of plain radiographs, including low cost, low levels of radiation, convenience for follow-up and assessment of the position of the acetabular component, the errors were within a clinically acceptable range. Our study showed that Widmer’s method was the best for assessing anteversion on plain radiographs. No benefits in any form have been received or will be received from a commercial party related directly or indirectly to the subject of this article. This article was primary edited by D. Johnstone and first proof edited by J. Scott.
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21. Nho JH, Lee YK, Kim HJ, et al. Reliability and validity of measuring version of the acetabular component. J Bone Joint Surg [Br] 2012;94-B:32–36. 22. Lu M, Zhou YX, Du H, Zhang J, Liu J. Reliability and validity of measuring acetabular component orientation by plain antero-posterior radiographs. Clin Orthop Relat Res 2013;471:2987–2994. 23. Nishino H, Nakamura S, Arai N, Matsushita T. Accuracy and precision of version angle measurements of the acetabular component after total hip arthroplasty. J Arthroplasty 2013;28:1644–1647. 24. Haenle M, Mittelmeier W, Barbano R, et al. Accuracy and reliability of different methods to evaluate the acetabular cup version from plain radiographs. Surg Radiol Anat 2010;32:725–730. 25. McLaren RH. Prosthetic hip angulation. Radiology 1973;107:705–706. 26. Babisch JW, Layher F, Amiot LP. The rationale for tilt-adjusted acetabular cup navigation. J Bone Joint Surg [Am] 2008;90-A:357–365.
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27. Malik A, Wan Z, Jaramaz B, Bowman G, Dorr LD. A validation model for measurement of acetabular component position. J Arthroplasty 2010;25:812–819. 28. Miki H, Kyo T, Sugano N. Anatomical hip range of motion after implantation during total hip arthroplasty with a large change in pelvic inclination. J Arthroplasty 2012;27:1641–1650. 29. Eilander W, Harris SJ, Henkus HE, Cobb JP, Hogervorst T. Functional acetabular component position with supine total hip replacement. Bone Joint J 2013;95B:1326–1331. 30. Arai N, Nakamura S, Matsushita T. Difference between 2 measurement methods of version angles of the acetabular component. J Arthroplasty 2007;22:715–720. 31. McArthur B, Cross M, Geatrakas C, Mayman D, Ghelman B. Measuring acetabular component version after THA: CT or plain radiograph? Clin Orthop Relat Res 2012;470:2810–2818.
