Acta Radiol OnlineFirst, published on July 15, 2015 as doi:10.1177/0284185115595656
Original Article
Assessment of implant position after total knee arthroplasty by dual-energy computed tomography
Acta Radiologica 0(0) 1–8 ! The Foundation Acta Radiologica 2015 Reprints and permissions: sagepub.co.uk/journalsPermissions.nav DOI: 10.1177/0284185115595656 acr.sagepub.com
Ferdinando Ferrara, Antonio Cipriani, Santi Rapisarda, Marta Iacobucci, Nicola Magarelli, Antonello Leone and Lorenzo Bonomo
Abstract Background: Correct alignment of prosthetic components is the most important factor for the success of total knee arthroplasty (TKA). Dual-energy computed tomography (DECT) may be a reliable method in determining implant position after TKA. Purpose: To evaluate the accuracy and reproducibility of DECT in determining implant position after TKA. Material and Methods: Institutional review board approval was obtained. Forty-five patients (age 75.2 6.4 years) prospectively underwent TKA at our institution between May and December 2012. DECT was performed 1 year after surgery, using an alignment similar to a standing position and generating images at an extrapolated energy of 120 kVp, in order to reduce metal artifacts. Implant position was evaluated by two independent readers. Intra- and inter-observer agreements were calculated. DECT measurements on implant position were compared with the preoperative planning based on radiographs. Additional clinical and DECT findings were taken into account. Results: Metal artifact reduction was judged satisfactory in all cases. Regarding implant position assessed with DECT, good to excellent intra-observer (k: 0.74–0.87 and k: 0.75–0.88, respectively), and inter-observer agreement (k: 0.72–0.82) were found. In the comparison with preoperative planning, the widest limits of agreement were within 3.9 for the sagittal orientation of tibial component. A single patient with postoperative knee pain and stiffness had periprosthetic osteopenia, quadriceps femoris tendon calcifications, articular effusion, and excessive intrarotation of the femoral component. Conclusion: DECT is an accurate and reproducible tool for determining implant position after TKA.
Keywords Knee arthroplasty, implant position control, dual-energy computed tomography Date received: 26 August 2014; accepted: 19 June 2015
Introduction Total knee arthroplasty (TKA) is a reliable treatment for providing pain relief and increased quality of life to patients with severe osteoarthritis (1). The increasing age of population in large parts of the world, coupled with improvements in surgical techniques and implant manufacturing, have increased the rate of TKA in industrialized countries by 60% in the last 10 years, and this may double by 2020 (2). Correct alignment of prosthetic components has been considered as the most important factor for the success of TKA (3).
Thus it is important to develop ways for improving surgical accuracy in TKA, and correct radiological diagnosis may become a valuable strategy (4).
Department of Radiological Sciences, Catholic University of the Sacred Heart, ‘‘A. Gemelli’’ Hospital, Rome, Italy Corresponding author: Ferdinando Ferrara, Department of Radiological Sciences, Catholic University of the Sacred Heart, ‘‘A. Gemelli’’ Hospital, Largo AgostinoGemelli 8, 00168, Rome, Italy. Email: [email protected]
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Although clinical examination may suggest the need for revision surgery, in some cases the results are inconclusive. Also, conventional radiographs cannot always identify the malpositioning of TKA components, especially regarding their rotational alignment. Computed tomography (CT) plays a role in the postoperative evaluation of implanted knees (5), allowing the identification of implant loosening or dislocation, periprosthetic bone resorption, fracture, or infection. However, there are still few data in the literature about the accuracy of CT in assessing implant position (6). To date, metal artifacts have represented a significant limitation to the diagnostic value of CT. Among the interesting applications of dual-energy CT (DECT) in musculoskeletal imaging, there is the possibility of reducing metal artifacts (7). In this study, we wanted to investigate the accuracy and reproducibility of DECT in determining implant position after TKA.
Material and Methods Forty-five patients (29 women, 16 men; age 75.2 6.4 years; range, 65–85 years; body mass index, 28.2 3.9 kg/m2) suffering from osteoarthritis, prospectively underwent TKA (24 right and 21 left knees) at our institution between May and December 2012. Patient inclusion criteria were: age between 50 and 85 years, failure of non-operative treatments, and no contraindication to surgery. Exclusion criteria were: previous orthopedic surgery, trauma and severe varus/valgus malalignment of lower limbs, considered as a hip-knee-ankle angle over 10 in absolute value. Our institutional review board approved the study protocol and written informed consent was obtained from all patients. Preoperative assessment included clinical examination and full-length weight-bearing radiographs of both lower limbs in the postero-anterior projection, plus a medio-lateral view of the knee to be implanted. Relying on this information, the size of implants and the depth of bone cuts were chosen for each patient by the same orthopedic surgeon. All patients were implanted with a cemented posterior stabilized prosthesis, sacrificing the cruciate ligaments (NexGen LPS, Zimmer, Warsaw, IN, USA), following a standardized technique with the assistance of positioning guides. Components were made of a non-porous cast cobaltchromium-molybdenum alloy, with tibial inserts covered by compression molded ultra-high molecular weight polyethylene. For each implanted knee, control radiographs in the two orthogonal projections were performed within 4 weeks after surgery. The same rehabilitation program was scheduled for all patients.
One year after surgery, each patient underwent a clinical examination and a DECT performed with a Somatom Definition Flash scanner (Siemens Healthcare, Erlangen, Germany) in the supine position, with knees extended, in order to reproduce weightbearing conditions as closely as possible. Correct alignment of the lower limbs was checked on coronal and lateral scout views. If adequate, a helical acquisition of the implanted limb was performed from the acetabular roof to the talar dome. Technical parameters were: pitch, 0.5; rotation time, 0.5 s; collimation, 32 0.6 mm; matrix size, 512 512; and no gantry tilting. The field of view (FOV) was adjusted (range, 20–35 cm) in order to include the femoral head, the knee, and the talar dome in a single acquisition. Filtered 100 kVp and 140 kVp tube currents were employed with a 3:1 ratio in favor of the higher energy, keeping the lower energy at a level sufficient for beam hardening correction. Tube current was modulated so as to keep the estimated CT dose index (CTDI) below 20.0 mGy. The dose-length product (DLP) was recorded and the estimated dose (ED) was calculated, using a conversion factor of 0.018 mSv/ mGy for the hip and 0.001 mSv/mGy for distal extremities (8). Images were reconstructed through a relatively sharp iterative reconstruction algorithm (I50, Siemens Healthcare), with a 2 mm slice thickness and 30% overlap. Postprocessing was performed on a MMWP Somaris workstation (version CT2008G), through the ‘‘monoenergetic’’ application of Syngo Dual-energy software (version VE32B, Siemens Healthcare), decomposing the density of each voxel into a ‘‘water-like’’ and ‘‘iodine-like’’ component. According to Meinel et al. (9), extrapolation to a predetermined energy of 120 kVp was accomplished, in order to reduce metal artifacts (Fig. 1a and b). Multiplanar reconstructions (MPRs), maximum intensity projections (MIPs) and axial superimposed images were obtained. The position of prosthetic components was evaluated with DECT. The presumed mechanical axes of the femur and tibia were drawn by connecting the center of femoral head with the middle point of the intercondylar notch, and the talar dome with the intercondylar eminence, respectively (Fig. 2a). Taking each mechanical axis as a reference, the coronal orientation of both components was assessed on coronal MIPs, by using a line tangential to the distal surface of prosthetic condyles and another tangential to the prosthetic tibial tray, respectively (Fig. 2b). The coronal orientation of the tibial component with respect to the femoral was measured as the deviation between these two lines. Positive values of measured angles were attributed to valgus deviation, negative values to varus deviation. The sagittal orientation of both components was
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Fig. 1. Extrapolation of DECT source images (a) to a predetermined energy of 120 kVp (b), in order to reduce metal artifacts. DECT, dual-energy computed tomography.
Fig. 2. The presumed mechanical axes of the femur and tibia were drawn (red lines) on DECT coronal and sagittal MIPs (a). Taking these axes as a reference, the coronal (b) and sagittal orientation (c) of both components were assessed by using lines tangential to each prosthetic component (yellow lines). The rotational alignment was evaluated on axial superimposed images, using the posterior condylar line (red line) and the STEA (yellow line) for the femoral component (d). A line, connecting the most prominent point of anterior tuberosity with the geometric center of tibia (red line), and the antero-posterior diameter of prosthetic tibial tray (yellow line) was used for the tibial component (e). The TEA is also represented (orange line). DECT, dual-energy computed tomography; MIPs, maximum intensity projections; STEA, surgical transepicondylar axis; TEA, transepicondylar axis.
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assessed on sagittal MIPs, taking as a reference a line tangential to the distal bone cut of femur (bone-implant interface) and another tangential to the prosthetic tibial tray, respectively (Fig. 2c). Positive values of measured angles were attributed to flexion, negative values to extension. The rotational alignment of both components was evaluated on axial superimposed images. In particular, the angle formed between the surgical transepicondylar axis (STEA), connecting the lateral epicondyle with the deepest point of the medial sulcus, and the posterior condylar line, tangential to the posterior surface of prosthetic condyles, was considered for the femoral component (Fig. 2d). Conversely, the angle formed between a line, connecting the most prominent point of the anterior tuberosity with the geometric center of tibia, and the antero-posterior diameter of prosthetic tibial tray, was used for this component (Fig. 2e). The rotational alignment of the tibial component with respect to the femoral was measured as the angle formed between the posterior condylar line and the medio-lateral diameter of the prosthetic tibial tray. Positive values of the measured angles indicated extrarotation, negative values intrarotation. Two resident radiologists (AC, FF), blinded to each other, judged the quality of metal artifact reduction as satisfactory or unsatisfactory, and performed the measurements twice for each patient on randomly presented anonymized images. The same knee was assessed by each reader with an interval of at least 2 weeks, in order to avoid recall bias. At a later time, each reader reviewed all the images again and chose, between the two measurements previously assigned to each angle, the one he considered as the most correct. In a second session, a senior radiologist (NM, with more than 15 years of experience in musculoskeletal imaging) reviewed all the images and assigned a definitive measurement to each angle, by choosing, between the two measurements provided by the two readers, the one he considered as the most correct. Except for the rotational alignment, which is not predictable basing on radiographs, the definitive DECT measurements on the orientation of components were compared with the values planned preoperatively. Additional DECT findings, such as the presence of quadriceps femoris tendon calcifications (Fig. 3a), periprosthetic osteopenia (Fig. 3b), and the amount of articular effusion, were evaluated. In particular, joint effusion was classified as grade 0 if absent, grade 1 if distension of the supra-patellar recess, less than 5 mm in thickness, was present, grade 2 if it exceeded 5 mm, and grade 3 if fluid was also evident in the gastrocnemiussemimembranosus recess (Fig. 3c–e). Clinical and additional DECT findings were correlated with implant position.
Statistical analysis Continuous data were presented as mean standard deviation. Cohen’s weighted kappa statistic was used to evaluate intra-observer and inter-observer agreement. Bland-Altman plots were constructed in order to compare between them the two measurements assigned by each reader to each angle, as well as the two measurements chosen by the two readers. The differences between each pair of measurements were plotted against their averages. Bland-Altman plots were used also to compare the definitive DECT measurements on implant position with the values planned preoperatively, taking these latter as a reference. In all cases, outliers were defined as measurements differing from their corresponding preoperative values beyond the limits of agreement, that is 1.96 times the standard deviation of differences. The Mann-Whitney U test was adopted to assess differences between the DECT measurements on implant position, grouped depending on dichotomous additional findings (periprosthetic osteopenia and quadriceps femoris tendon calcifications). The Spearman’s rho correlation coefficient was used to verify eventual relationships between the DECT measurements and the amount of articular effusion, providing 95% confidence intervals (CI). Applying the Bonferroni correction, a P value less than 0.017 was considered as significant. The MedCalc software, version 12.5.0.0 (MedCalc Software, Mariakierke, Belgium) was employed for inferential statistics.
Results The quality of metal artifact reduction was judged adequate by both readers in all examinations. As regards the DECT measurements on implant position, a good to excellent intra-observer agreement was found for both readers with k in the range of 0.74–0.87 for the first, and 0.75–0.88 for the second. A good interobserver agreement was also found with k in the range of 0.70–0.84. In the comparison between the two measurements assigned by each reader to each angle, the limits of agreement always fell within 1.7 from the averages, whereas they did not exceed 2.0 in the comparison between the two measurements chosen by the two readers. Five outliers were found in the case of each reader, and there were seven outliers in the comparison between the two readers. Comparing the definitive DECT measurements on implant position with the values planned preoperatively, the widest limits of agreement fell within 3.9 in the sagittal orientation of tibial component. Three outliers were found in this comparison (Table 1). Regarding the DECT additional findings, periprosthetic osteopenia was found in 14 cases (31%), quadriceps femoris tendon calcifications in five (11%).
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Fig. 3. DECT images were evaluated for the presence of quadriceps femoris tendon calcifications (a), periprosthetic osteopenia (b), and the amount of articular effusion. This latter was classified as grade 1 if distension of the supra-patellar recess, less than 5 mm in thickness, was present (c), grade 2 if it exceeded 5 mm (d), and grade 3 if fluid was also evident in the gastrocnemius-semimembranosus recess (e). DECT, dual-energy computed tomography.
No articular effusion was reported in 12 knees (27%), grade 1 effusion in 16 (36%), grade 2 in 15 (33%), and grade 3 in the remaining two (4%). Knees presenting with periprosthetic osteopenia had, compared with those without, lower degree of flexion (1.0 2.1 vs. 2.8 2.3 , P ¼ 0.005) and extrarotation of the femoral component (0.9 2.2 vs. 1.0 2.4 , P < 0.001), and greater flexion of the tibial component (7.5 2.0 vs. 5.9 2.2 , P ¼ 0.007). The presence of quadriceps femoris tendon calcifications was not influenced by implant position. The amount of articular effusion inversely correlated with the degree of flexion
(k ¼ 0.41, 95% CI: 0.64 to 0.12, P ¼ 0.008) and extrarotation of the femoral component (k ¼ 0.48, 95% CI: 0.68 to 0.19, P ¼ 0.002), as well as with the degree of extension of the tibial component (k ¼ 0.45, 95% CI: 0.16 to 0.66, P ¼ 0.004) and extrarotation of this component with respect to the femoral (k ¼ 0.54, 95% CI: 0.73 to 0.27, P < 0.001, Table 2). At 1-year postoperative clinical examination, a single patient suffered from pain and decreased range of movement on passive knee mobilization. In this case, DECT showed periprosthetic osteopenia, quadriceps
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Table 1. Implant position: comparison between the definitive DECT measurements and the values planned preoperatively.
Component
Orientation
Planned value
Femoral
Coronal* Sagittaly Axialz Coronal* Sagittaly Axialz Coronal* Axialz
00 3.0 1.4 NA 00 7.0 1.3 NA 00 NA
Tibial
Tibial to femoral
DECT measurement
Mean difference
Limits of agreement (1.6 SD)
Outliers
0.2 1.4 2.6 2.4 0.3 2.8 0.0 1.3 6.5 2.5 6.7 3.4 0.4 1.5 3.7 3.6
0.2 0.4 NA 0.0 0.5 NA 0.4 NA
2.5–2.9 4.2–3.4 NA 2.5–2.5 3.4–4.4 NA 3.3–2.5 NA
0 1 NA 0 1 NA 1 NA
Unless otherwise indicated, data are expressed in degrees and presented as mean standard deviation. Outliers are defined as measurements differing from their corresponding preoperative values beyond the limits of agreement. *Positive values for valgus deviation, negatives for varus. y Positive values for flexion, negatives for extension. z Positive values for extrarotation, negatives for intrarotation. DECT, dual-energy computed tomography; NA, not applicable; SD, standard deviation.
Table 2. Correlation between DECT additional findings and implant position. Component
Orientation
Periprosthetic osteopenia
Articular effusion
Femoral
Flexion/extension Intra-/extrarotation Flexion/extension Intra-/extrarotation Intra-/extrarotation
/þ þ/ þ/ NC NC
/þ þ/ þ/ NC þ/
Tibial Tibial to femoral
þ, positive influence, , negative influence. DECT, dual-energy computed tomography; NC, no correlation.
femoris tendon calcifications, a grade 2 articular effusion, and excessive intrarotation (3.5 ) of the femoral component. The CTDI was 11.8 3.7 mGy, the DLP 944 315 mGy*cm, and the ED 4.0 1.2 mSv.
Discussion The main mechanisms responsible for metal artifacts in CT are beam hardening and photon starvation (10), the first referring to the absorption by metal of low energy photons, the second to the unfavorable signal-to-noise ratio produced by low photon counts. Factors influencing the amount of metal artifacts include the CT acquisition parameters, as well as the composition and orientation of prosthetic components (11). A number of approaches have been developed to reduce metal artifacts, the more recent based on
iterative algebraic reconstruction (12), beam-hardening correction, or filtered back projections (13). Nevertheless, none of these can eliminate all sources of artifacts. DECT allows characterization of the chemical composition of materials according to the energy-dependent photon attenuation of elements. In a pilot study of 31 patients with metallic implants, Bamberg et al. (7) compared different monoenergetic reconstructions, showing high energy (105–120 kVp) to be superior to low energy, both in image quality and diagnostic value. In our study, the quality of metal artifact reduction was judged to be good by both readers in all examinations. Among the several factors influencing the success of TKA, the positioning of components, especially as regards their rotational alignment, plays a key role (14). In fact, implant malpositioning may lead to knee instability or stiffness, patello-femoral maltracking, early wear of the polyethylene inlay or loosening of components. Malrotation of the femoral component in particular is an important element potentially causing joint instability, patellar subluxation, notching of the anterior cortex, or periprosthetic fractures (15,16). In a recent study, Konigsberg et al. (6) found a good inter- and intra-observer agreement in measuring the rotational alignment of the femoral component on single axial CT images. Hirschmann et al. (17) evaluated 30 knees with both single axial CT images and volumetric reconstructions, finding more accurate measurements with this latter method. The main advantage of CT over radiographs in TKA is the possibility of assessing the rotational alignment of components. Berger et al. (18) first used the STEA for measuring the rotational alignment of the
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femoral component. Due to difficulties in visualizing the medial sulcus in severely arthritic knees, Suter et al. (19) identified it on single axial CT images in 53% of cases, Yoshino et al. (20) only in 30%. In our study, perfect identification of this bony landmark was obtained in all implanted knees and, according to the literature (21), the STEA was considered as the best representation of flexion-extension fulcrum of the knee. In our study, the DECT measurements on the orientation of components presented a strong intra-observer agreement, for both readers and for all the angles assessed, with small differences between repeated measurements. Also the agreement between the two readers was good. Therefore, DECT was considered as a reproducible method for evaluating the spatial position of prosthetic components. On this basis, the definitive DECT measurements were compared with their corresponding preoperative values, with no significant difference between them. The relatively tight limits of agreement (never exceeding 2.9 ) found in this comparison for the coronal alignment of components in particular, indicated that surgery was performed with sufficient accuracy and that DECT, although not performed in weight-bearing conditions, was capable of providing a reliable representation of the loading axis of lower limbs (Table 1). In our prospective study, DECT of TKA was performed 1 year after surgery for evaluating the orientation of components, without a necessary clinical indication. Interestingly, lower degree of flexion and extrarotation of the femoral component, together with excessive flexion of the tibial component, correlated with the presence of periprosthetic osteopenia and the amount of articular effusion (Table 2). In the only patient with problems at 1-year postoperative clinical examination, DECT revealed periprosthetic osteopenia, quadriceps femoris tendon calcifications, articular effusion, and excessive intrarotation of the femoral component. In this case, surgical revision of TKA was performed. DECT of TKA is currently requested at our institution when implant malposition is suspected clinically, since it proved effective in providing the orthopedic surgeon with useful information on the alignment of components and the presence of complications. Certainly, larger clinical studies are needed to confirm the role of DECT in correctly determining the postoperative alignment of components, in predicting the occurrence of complications and the need for revision surgery. An increased radiation dose remains the main disadvantage of DECT. Bamberg et al. (7) found an average CTDI of 11.0 mGy for distal extremities and 15.4 mGy for the trunk, whereas, according to Mettler et al. (22), the total ED for a CT of lower limbs was 2.5 mSv.
The ED of our DECT examinations was in agreement with the values reported. Furthermore, older individuals, constituting most of the TKA population, are at lower risk from radiation exposure. This study has some limitations. First, the number of patients is relatively small. However, the design of the study partially compensates for this bias. Second, only 1-year postoperative DECT and clinical findings were taken into account, although there is no study in the literature investigating CT in the long-term follow-up of TKA. In conclusion, we believe DECT is an accurate and reproducible tool in determining implant position after TKA, with potential clinical implications. Conflict of interest None declared.
Funding This research received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors.
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10. Barret JF, Keat N. Artifacts in CT: recognition and avoidance. Radiographics 2004;24:1679–1691. 11. Lee MJ, Kim S, Lee SA, et al. Overcoming artifacts from metallic orthopedic implants at high-field-strength MR imaging and multi-detector CT. Radiographics 2007;27: 791–803. 12. Boas FE, Fleischmann D. Evaluation of two iterative techniques for reducing metal artifacts in computed tomography. Radiology 2011;259:894–902. 13. Prell D, Kyriakou Y, Beister M, et al. A novel forward projection-based metal artifact reduction method for flatdetector computed tomography. Phys Med Biol 2009;54: 6575–6591. 14. Barrack RL, Schrader T, Bertot AJ, et al. Component rotation and anterior knee pain after total knee arthroplasty. Clin Orthop Relat Res 2001;392:46–55. 15. Akagi M, Matsusue Y, Mata T, et al. Effect of rotational alignment on patellar tracking in total knee arthroplasty. Clin Orthop Relat Res 1999;366:155–163. 16. Victor J. Rotational alignment of the distal femur: a literature review. Orthop Traumatol Surg Res 2009;95: 365–372. 17. Hirschmann MT, Konala P, Amsler F, et al. The position and orientation of total knee replacement components:
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Original Article
Assessment of implant position after total knee arthroplasty by dual-energy computed tomography
Acta Radiologica 0(0) 1–8 ! The Foundation Acta Radiologica 2015 Reprints and permissions: sagepub.co.uk/journalsPermissions.nav DOI: 10.1177/0284185115595656 acr.sagepub.com
Ferdinando Ferrara, Antonio Cipriani, Santi Rapisarda, Marta Iacobucci, Nicola Magarelli, Antonello Leone and Lorenzo Bonomo
Abstract Background: Correct alignment of prosthetic components is the most important factor for the success of total knee arthroplasty (TKA). Dual-energy computed tomography (DECT) may be a reliable method in determining implant position after TKA. Purpose: To evaluate the accuracy and reproducibility of DECT in determining implant position after TKA. Material and Methods: Institutional review board approval was obtained. Forty-five patients (age 75.2 6.4 years) prospectively underwent TKA at our institution between May and December 2012. DECT was performed 1 year after surgery, using an alignment similar to a standing position and generating images at an extrapolated energy of 120 kVp, in order to reduce metal artifacts. Implant position was evaluated by two independent readers. Intra- and inter-observer agreements were calculated. DECT measurements on implant position were compared with the preoperative planning based on radiographs. Additional clinical and DECT findings were taken into account. Results: Metal artifact reduction was judged satisfactory in all cases. Regarding implant position assessed with DECT, good to excellent intra-observer (k: 0.74–0.87 and k: 0.75–0.88, respectively), and inter-observer agreement (k: 0.72–0.82) were found. In the comparison with preoperative planning, the widest limits of agreement were within 3.9 for the sagittal orientation of tibial component. A single patient with postoperative knee pain and stiffness had periprosthetic osteopenia, quadriceps femoris tendon calcifications, articular effusion, and excessive intrarotation of the femoral component. Conclusion: DECT is an accurate and reproducible tool for determining implant position after TKA.
Keywords Knee arthroplasty, implant position control, dual-energy computed tomography Date received: 26 August 2014; accepted: 19 June 2015
Introduction Total knee arthroplasty (TKA) is a reliable treatment for providing pain relief and increased quality of life to patients with severe osteoarthritis (1). The increasing age of population in large parts of the world, coupled with improvements in surgical techniques and implant manufacturing, have increased the rate of TKA in industrialized countries by 60% in the last 10 years, and this may double by 2020 (2). Correct alignment of prosthetic components has been considered as the most important factor for the success of TKA (3).
Thus it is important to develop ways for improving surgical accuracy in TKA, and correct radiological diagnosis may become a valuable strategy (4).
Department of Radiological Sciences, Catholic University of the Sacred Heart, ‘‘A. Gemelli’’ Hospital, Rome, Italy Corresponding author: Ferdinando Ferrara, Department of Radiological Sciences, Catholic University of the Sacred Heart, ‘‘A. Gemelli’’ Hospital, Largo AgostinoGemelli 8, 00168, Rome, Italy. Email: [email protected]
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Although clinical examination may suggest the need for revision surgery, in some cases the results are inconclusive. Also, conventional radiographs cannot always identify the malpositioning of TKA components, especially regarding their rotational alignment. Computed tomography (CT) plays a role in the postoperative evaluation of implanted knees (5), allowing the identification of implant loosening or dislocation, periprosthetic bone resorption, fracture, or infection. However, there are still few data in the literature about the accuracy of CT in assessing implant position (6). To date, metal artifacts have represented a significant limitation to the diagnostic value of CT. Among the interesting applications of dual-energy CT (DECT) in musculoskeletal imaging, there is the possibility of reducing metal artifacts (7). In this study, we wanted to investigate the accuracy and reproducibility of DECT in determining implant position after TKA.
Material and Methods Forty-five patients (29 women, 16 men; age 75.2 6.4 years; range, 65–85 years; body mass index, 28.2 3.9 kg/m2) suffering from osteoarthritis, prospectively underwent TKA (24 right and 21 left knees) at our institution between May and December 2012. Patient inclusion criteria were: age between 50 and 85 years, failure of non-operative treatments, and no contraindication to surgery. Exclusion criteria were: previous orthopedic surgery, trauma and severe varus/valgus malalignment of lower limbs, considered as a hip-knee-ankle angle over 10 in absolute value. Our institutional review board approved the study protocol and written informed consent was obtained from all patients. Preoperative assessment included clinical examination and full-length weight-bearing radiographs of both lower limbs in the postero-anterior projection, plus a medio-lateral view of the knee to be implanted. Relying on this information, the size of implants and the depth of bone cuts were chosen for each patient by the same orthopedic surgeon. All patients were implanted with a cemented posterior stabilized prosthesis, sacrificing the cruciate ligaments (NexGen LPS, Zimmer, Warsaw, IN, USA), following a standardized technique with the assistance of positioning guides. Components were made of a non-porous cast cobaltchromium-molybdenum alloy, with tibial inserts covered by compression molded ultra-high molecular weight polyethylene. For each implanted knee, control radiographs in the two orthogonal projections were performed within 4 weeks after surgery. The same rehabilitation program was scheduled for all patients.
One year after surgery, each patient underwent a clinical examination and a DECT performed with a Somatom Definition Flash scanner (Siemens Healthcare, Erlangen, Germany) in the supine position, with knees extended, in order to reproduce weightbearing conditions as closely as possible. Correct alignment of the lower limbs was checked on coronal and lateral scout views. If adequate, a helical acquisition of the implanted limb was performed from the acetabular roof to the talar dome. Technical parameters were: pitch, 0.5; rotation time, 0.5 s; collimation, 32 0.6 mm; matrix size, 512 512; and no gantry tilting. The field of view (FOV) was adjusted (range, 20–35 cm) in order to include the femoral head, the knee, and the talar dome in a single acquisition. Filtered 100 kVp and 140 kVp tube currents were employed with a 3:1 ratio in favor of the higher energy, keeping the lower energy at a level sufficient for beam hardening correction. Tube current was modulated so as to keep the estimated CT dose index (CTDI) below 20.0 mGy. The dose-length product (DLP) was recorded and the estimated dose (ED) was calculated, using a conversion factor of 0.018 mSv/ mGy for the hip and 0.001 mSv/mGy for distal extremities (8). Images were reconstructed through a relatively sharp iterative reconstruction algorithm (I50, Siemens Healthcare), with a 2 mm slice thickness and 30% overlap. Postprocessing was performed on a MMWP Somaris workstation (version CT2008G), through the ‘‘monoenergetic’’ application of Syngo Dual-energy software (version VE32B, Siemens Healthcare), decomposing the density of each voxel into a ‘‘water-like’’ and ‘‘iodine-like’’ component. According to Meinel et al. (9), extrapolation to a predetermined energy of 120 kVp was accomplished, in order to reduce metal artifacts (Fig. 1a and b). Multiplanar reconstructions (MPRs), maximum intensity projections (MIPs) and axial superimposed images were obtained. The position of prosthetic components was evaluated with DECT. The presumed mechanical axes of the femur and tibia were drawn by connecting the center of femoral head with the middle point of the intercondylar notch, and the talar dome with the intercondylar eminence, respectively (Fig. 2a). Taking each mechanical axis as a reference, the coronal orientation of both components was assessed on coronal MIPs, by using a line tangential to the distal surface of prosthetic condyles and another tangential to the prosthetic tibial tray, respectively (Fig. 2b). The coronal orientation of the tibial component with respect to the femoral was measured as the deviation between these two lines. Positive values of measured angles were attributed to valgus deviation, negative values to varus deviation. The sagittal orientation of both components was
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Fig. 1. Extrapolation of DECT source images (a) to a predetermined energy of 120 kVp (b), in order to reduce metal artifacts. DECT, dual-energy computed tomography.
Fig. 2. The presumed mechanical axes of the femur and tibia were drawn (red lines) on DECT coronal and sagittal MIPs (a). Taking these axes as a reference, the coronal (b) and sagittal orientation (c) of both components were assessed by using lines tangential to each prosthetic component (yellow lines). The rotational alignment was evaluated on axial superimposed images, using the posterior condylar line (red line) and the STEA (yellow line) for the femoral component (d). A line, connecting the most prominent point of anterior tuberosity with the geometric center of tibia (red line), and the antero-posterior diameter of prosthetic tibial tray (yellow line) was used for the tibial component (e). The TEA is also represented (orange line). DECT, dual-energy computed tomography; MIPs, maximum intensity projections; STEA, surgical transepicondylar axis; TEA, transepicondylar axis.
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assessed on sagittal MIPs, taking as a reference a line tangential to the distal bone cut of femur (bone-implant interface) and another tangential to the prosthetic tibial tray, respectively (Fig. 2c). Positive values of measured angles were attributed to flexion, negative values to extension. The rotational alignment of both components was evaluated on axial superimposed images. In particular, the angle formed between the surgical transepicondylar axis (STEA), connecting the lateral epicondyle with the deepest point of the medial sulcus, and the posterior condylar line, tangential to the posterior surface of prosthetic condyles, was considered for the femoral component (Fig. 2d). Conversely, the angle formed between a line, connecting the most prominent point of the anterior tuberosity with the geometric center of tibia, and the antero-posterior diameter of prosthetic tibial tray, was used for this component (Fig. 2e). The rotational alignment of the tibial component with respect to the femoral was measured as the angle formed between the posterior condylar line and the medio-lateral diameter of the prosthetic tibial tray. Positive values of the measured angles indicated extrarotation, negative values intrarotation. Two resident radiologists (AC, FF), blinded to each other, judged the quality of metal artifact reduction as satisfactory or unsatisfactory, and performed the measurements twice for each patient on randomly presented anonymized images. The same knee was assessed by each reader with an interval of at least 2 weeks, in order to avoid recall bias. At a later time, each reader reviewed all the images again and chose, between the two measurements previously assigned to each angle, the one he considered as the most correct. In a second session, a senior radiologist (NM, with more than 15 years of experience in musculoskeletal imaging) reviewed all the images and assigned a definitive measurement to each angle, by choosing, between the two measurements provided by the two readers, the one he considered as the most correct. Except for the rotational alignment, which is not predictable basing on radiographs, the definitive DECT measurements on the orientation of components were compared with the values planned preoperatively. Additional DECT findings, such as the presence of quadriceps femoris tendon calcifications (Fig. 3a), periprosthetic osteopenia (Fig. 3b), and the amount of articular effusion, were evaluated. In particular, joint effusion was classified as grade 0 if absent, grade 1 if distension of the supra-patellar recess, less than 5 mm in thickness, was present, grade 2 if it exceeded 5 mm, and grade 3 if fluid was also evident in the gastrocnemiussemimembranosus recess (Fig. 3c–e). Clinical and additional DECT findings were correlated with implant position.
Statistical analysis Continuous data were presented as mean standard deviation. Cohen’s weighted kappa statistic was used to evaluate intra-observer and inter-observer agreement. Bland-Altman plots were constructed in order to compare between them the two measurements assigned by each reader to each angle, as well as the two measurements chosen by the two readers. The differences between each pair of measurements were plotted against their averages. Bland-Altman plots were used also to compare the definitive DECT measurements on implant position with the values planned preoperatively, taking these latter as a reference. In all cases, outliers were defined as measurements differing from their corresponding preoperative values beyond the limits of agreement, that is 1.96 times the standard deviation of differences. The Mann-Whitney U test was adopted to assess differences between the DECT measurements on implant position, grouped depending on dichotomous additional findings (periprosthetic osteopenia and quadriceps femoris tendon calcifications). The Spearman’s rho correlation coefficient was used to verify eventual relationships between the DECT measurements and the amount of articular effusion, providing 95% confidence intervals (CI). Applying the Bonferroni correction, a P value less than 0.017 was considered as significant. The MedCalc software, version 12.5.0.0 (MedCalc Software, Mariakierke, Belgium) was employed for inferential statistics.
Results The quality of metal artifact reduction was judged adequate by both readers in all examinations. As regards the DECT measurements on implant position, a good to excellent intra-observer agreement was found for both readers with k in the range of 0.74–0.87 for the first, and 0.75–0.88 for the second. A good interobserver agreement was also found with k in the range of 0.70–0.84. In the comparison between the two measurements assigned by each reader to each angle, the limits of agreement always fell within 1.7 from the averages, whereas they did not exceed 2.0 in the comparison between the two measurements chosen by the two readers. Five outliers were found in the case of each reader, and there were seven outliers in the comparison between the two readers. Comparing the definitive DECT measurements on implant position with the values planned preoperatively, the widest limits of agreement fell within 3.9 in the sagittal orientation of tibial component. Three outliers were found in this comparison (Table 1). Regarding the DECT additional findings, periprosthetic osteopenia was found in 14 cases (31%), quadriceps femoris tendon calcifications in five (11%).
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Fig. 3. DECT images were evaluated for the presence of quadriceps femoris tendon calcifications (a), periprosthetic osteopenia (b), and the amount of articular effusion. This latter was classified as grade 1 if distension of the supra-patellar recess, less than 5 mm in thickness, was present (c), grade 2 if it exceeded 5 mm (d), and grade 3 if fluid was also evident in the gastrocnemius-semimembranosus recess (e). DECT, dual-energy computed tomography.
No articular effusion was reported in 12 knees (27%), grade 1 effusion in 16 (36%), grade 2 in 15 (33%), and grade 3 in the remaining two (4%). Knees presenting with periprosthetic osteopenia had, compared with those without, lower degree of flexion (1.0 2.1 vs. 2.8 2.3 , P ¼ 0.005) and extrarotation of the femoral component (0.9 2.2 vs. 1.0 2.4 , P < 0.001), and greater flexion of the tibial component (7.5 2.0 vs. 5.9 2.2 , P ¼ 0.007). The presence of quadriceps femoris tendon calcifications was not influenced by implant position. The amount of articular effusion inversely correlated with the degree of flexion
(k ¼ 0.41, 95% CI: 0.64 to 0.12, P ¼ 0.008) and extrarotation of the femoral component (k ¼ 0.48, 95% CI: 0.68 to 0.19, P ¼ 0.002), as well as with the degree of extension of the tibial component (k ¼ 0.45, 95% CI: 0.16 to 0.66, P ¼ 0.004) and extrarotation of this component with respect to the femoral (k ¼ 0.54, 95% CI: 0.73 to 0.27, P < 0.001, Table 2). At 1-year postoperative clinical examination, a single patient suffered from pain and decreased range of movement on passive knee mobilization. In this case, DECT showed periprosthetic osteopenia, quadriceps
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Table 1. Implant position: comparison between the definitive DECT measurements and the values planned preoperatively.
Component
Orientation
Planned value
Femoral
Coronal* Sagittaly Axialz Coronal* Sagittaly Axialz Coronal* Axialz
00 3.0 1.4 NA 00 7.0 1.3 NA 00 NA
Tibial
Tibial to femoral
DECT measurement
Mean difference
Limits of agreement (1.6 SD)
Outliers
0.2 1.4 2.6 2.4 0.3 2.8 0.0 1.3 6.5 2.5 6.7 3.4 0.4 1.5 3.7 3.6
0.2 0.4 NA 0.0 0.5 NA 0.4 NA
2.5–2.9 4.2–3.4 NA 2.5–2.5 3.4–4.4 NA 3.3–2.5 NA
0 1 NA 0 1 NA 1 NA
Unless otherwise indicated, data are expressed in degrees and presented as mean standard deviation. Outliers are defined as measurements differing from their corresponding preoperative values beyond the limits of agreement. *Positive values for valgus deviation, negatives for varus. y Positive values for flexion, negatives for extension. z Positive values for extrarotation, negatives for intrarotation. DECT, dual-energy computed tomography; NA, not applicable; SD, standard deviation.
Table 2. Correlation between DECT additional findings and implant position. Component
Orientation
Periprosthetic osteopenia
Articular effusion
Femoral
Flexion/extension Intra-/extrarotation Flexion/extension Intra-/extrarotation Intra-/extrarotation
/þ þ/ þ/ NC NC
/þ þ/ þ/ NC þ/
Tibial Tibial to femoral
þ, positive influence, , negative influence. DECT, dual-energy computed tomography; NC, no correlation.
femoris tendon calcifications, a grade 2 articular effusion, and excessive intrarotation (3.5 ) of the femoral component. The CTDI was 11.8 3.7 mGy, the DLP 944 315 mGy*cm, and the ED 4.0 1.2 mSv.
Discussion The main mechanisms responsible for metal artifacts in CT are beam hardening and photon starvation (10), the first referring to the absorption by metal of low energy photons, the second to the unfavorable signal-to-noise ratio produced by low photon counts. Factors influencing the amount of metal artifacts include the CT acquisition parameters, as well as the composition and orientation of prosthetic components (11). A number of approaches have been developed to reduce metal artifacts, the more recent based on
iterative algebraic reconstruction (12), beam-hardening correction, or filtered back projections (13). Nevertheless, none of these can eliminate all sources of artifacts. DECT allows characterization of the chemical composition of materials according to the energy-dependent photon attenuation of elements. In a pilot study of 31 patients with metallic implants, Bamberg et al. (7) compared different monoenergetic reconstructions, showing high energy (105–120 kVp) to be superior to low energy, both in image quality and diagnostic value. In our study, the quality of metal artifact reduction was judged to be good by both readers in all examinations. Among the several factors influencing the success of TKA, the positioning of components, especially as regards their rotational alignment, plays a key role (14). In fact, implant malpositioning may lead to knee instability or stiffness, patello-femoral maltracking, early wear of the polyethylene inlay or loosening of components. Malrotation of the femoral component in particular is an important element potentially causing joint instability, patellar subluxation, notching of the anterior cortex, or periprosthetic fractures (15,16). In a recent study, Konigsberg et al. (6) found a good inter- and intra-observer agreement in measuring the rotational alignment of the femoral component on single axial CT images. Hirschmann et al. (17) evaluated 30 knees with both single axial CT images and volumetric reconstructions, finding more accurate measurements with this latter method. The main advantage of CT over radiographs in TKA is the possibility of assessing the rotational alignment of components. Berger et al. (18) first used the STEA for measuring the rotational alignment of the
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femoral component. Due to difficulties in visualizing the medial sulcus in severely arthritic knees, Suter et al. (19) identified it on single axial CT images in 53% of cases, Yoshino et al. (20) only in 30%. In our study, perfect identification of this bony landmark was obtained in all implanted knees and, according to the literature (21), the STEA was considered as the best representation of flexion-extension fulcrum of the knee. In our study, the DECT measurements on the orientation of components presented a strong intra-observer agreement, for both readers and for all the angles assessed, with small differences between repeated measurements. Also the agreement between the two readers was good. Therefore, DECT was considered as a reproducible method for evaluating the spatial position of prosthetic components. On this basis, the definitive DECT measurements were compared with their corresponding preoperative values, with no significant difference between them. The relatively tight limits of agreement (never exceeding 2.9 ) found in this comparison for the coronal alignment of components in particular, indicated that surgery was performed with sufficient accuracy and that DECT, although not performed in weight-bearing conditions, was capable of providing a reliable representation of the loading axis of lower limbs (Table 1). In our prospective study, DECT of TKA was performed 1 year after surgery for evaluating the orientation of components, without a necessary clinical indication. Interestingly, lower degree of flexion and extrarotation of the femoral component, together with excessive flexion of the tibial component, correlated with the presence of periprosthetic osteopenia and the amount of articular effusion (Table 2). In the only patient with problems at 1-year postoperative clinical examination, DECT revealed periprosthetic osteopenia, quadriceps femoris tendon calcifications, articular effusion, and excessive intrarotation of the femoral component. In this case, surgical revision of TKA was performed. DECT of TKA is currently requested at our institution when implant malposition is suspected clinically, since it proved effective in providing the orthopedic surgeon with useful information on the alignment of components and the presence of complications. Certainly, larger clinical studies are needed to confirm the role of DECT in correctly determining the postoperative alignment of components, in predicting the occurrence of complications and the need for revision surgery. An increased radiation dose remains the main disadvantage of DECT. Bamberg et al. (7) found an average CTDI of 11.0 mGy for distal extremities and 15.4 mGy for the trunk, whereas, according to Mettler et al. (22), the total ED for a CT of lower limbs was 2.5 mSv.
The ED of our DECT examinations was in agreement with the values reported. Furthermore, older individuals, constituting most of the TKA population, are at lower risk from radiation exposure. This study has some limitations. First, the number of patients is relatively small. However, the design of the study partially compensates for this bias. Second, only 1-year postoperative DECT and clinical findings were taken into account, although there is no study in the literature investigating CT in the long-term follow-up of TKA. In conclusion, we believe DECT is an accurate and reproducible tool in determining implant position after TKA, with potential clinical implications. Conflict of interest None declared.
Funding This research received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors.
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