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The accuracy of the “projected surgical transepicondylar axis” relative to the “true surgical transepicondylar axis” in total knee arthroplasty
THEKNE-02511; No of Pages 7 The Knee xxx (2017) xxx–xxx
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The accuracy of the “projected surgical transepicondylar axis” relative to the “true surgical transepicondylar axis” in total knee arthroplasty Takaaki Ohmori a, Tamon Kabata a,⁎, Yoshitomo Kajino a, Tadashi Taga a, Kazuhiro Hasegawa a, Daisuke Inoue a, Takashi Yamamoto a, Tomoharu Takagi a, Junya Yoshitani a, Takuro Ueno a, Tomohiro Ojima b, Hiroyuki Tsuchiya a
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Department of Orthopedic Surgery, Graduate School of Medical Science, Kanazawa University, Takaramachi13-1, Kanazawa, Ishikawa, Japan Department of Orthopedic Surgery, Fukui General Hospital, 58-16-1 Egami, Fukui, Fukui 910-8561, Japan
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Background: In TKA, we have used the “projected SEA”, which is obtained by projecting the “true SEA” on the distal femoral cutting plane in clinical practice to determine the femoral component rotation. There are no reports examining the accuracy of the “projected SEA”. In this study, we investigated the difference between the “true SEA” and “projected SEA”. Methods: The present study was a CT-based computer-simulated case series. We evaluated 34 knees without osteoarthritis changes. These patients were selected from the operative schedule prior to THA. We defined the “true SEA” on the 3D model and the “projected SEA” on the cutting plane parallel to the distal femoral axis obtained based on the “true SEA”. We changed the cutting angles from 20° flexion to 20° extension, and from five degrees varus to five degrees valgus. We measured the “true SEA angle” and “projected SEA angle” regarding the posterior condylar axis (PCA). Results: The mean “true SEA angle” was 3.04° ± 1.34° (0.6–5.0°). The mean “projected SEA angle” was 3.43° ± 1.58° at 20° flexion, 3.42° ± 1.56° at 0° flexion, 3.43° ± 1.52° at 20° extension, 3.39° ± 1.59° at five degrees valgus, and 3.39° ± 1.50° at five degrees varus. At each cutting angle, the “projected SEA angle” was significantly larger than the “true SEA angle” (p b 0.001). There was no significant difference between any of the “projected SEA angles” (p N 0.001). Conclusions: We found that the true SEA and projected SEA do not differ significantly (0.39° ± 0.29° [range 0–1.0°]). © 2017 Published by Elsevier B.V.
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Article history: Received 11 February 2017 Received in revised form 9 June 2017 Accepted 29 July 2017 Available online xxxx
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1. Introduction
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Keywords: TKA True SEA Projected SEA
Patient satisfaction after total knee arthroplasty (TKA) is lower compared to that of total hip arthroplasty (THA) [1]. Malalignment of the implant is one of the reasons for this dissatisfaction [2]. Some reports correlate anterior knee pain [3,4] and patellofemoral maltracking [5] with malalignment of the femoral component during TKA. The clinical transepicondylar axis (CEA), surgical transepicondylar axis (SEA), anteroposterior axis, posterior condylar axis (PCA), cylindrical axis (CA), and Whiteside line have been used as benchmarks of femoral component rotation [6–10]. Among these, Berger et al. identified SEA as the most reliable indicator [11]. SEA is defined as a line passing through the lateral epicondyle and the medial sulcus of the
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⁎ Corresponding author. E-mail address: [email protected] (T. Kabata).
http://dx.doi.org/10.1016/j.knee.2017.07.017 0968-0160/© 2017 Published by Elsevier B.V.
Please cite this article as: Ohmori T, et al, The accuracy of the “projected surgical transepicondylar axis” relative to the “true surgical transepicondylar axis” in total knee arthroplasty, Knee (2017), http://dx.doi.org/10.1016/j.knee.2017.07.017
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All primary TKA in our hospital (n=162)
Records excluded (n=124) OA of the bilateral hip
Records screened (n=38)
In apposite images (n=4)
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Analyzed data (n=34)
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2. Materials and methods
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femur on a three-dimensional (3D) model (“true SEA”). However, these two points are located proximal to the cutting plane of the femur. Therefore, some orthopedic surgeons have used the “projected SEA”, which is a line passing through these projected points on the distal femoral cutting plane parallel to the distal femoral axis. Thus far, the differences between these two SEA types have not been evaluated. There are no reports that examine the accuracy of the “projected SEA”. In this study, we investigated the difference between “true SEA” and “projected SEA”. We asked the following questions: (1) Is the “projected SEA” different from the “true SEA” when we cut the femur in flexion or extension? (2) Is the “projected SEA” different from the “true SEA” when we cut the femur in varus or valgus? (3) Is the “true SEA” angle affected based on the extent of discrepancy between the true and projected SEA angles?
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This is a computed tomography (CT) based computer simulated case series study.
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2.2. Patients
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2.1. Study design
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This retrospective study included patients who underwent primary THA at our hospital between 2012 and 2013. We used the healthy side. Cases with bilateral hip osteoarthritis were excluded. We reviewed 34 patients (34 knees; six men and 28 women) that did not have any osteoarthritis changes in their knees and hips (Table 1). The mean patient age was 60.0 ± 10.3 years (range, 41–86 years) (Table 2).
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We constructed a 3D model of the femur from CT images (LightSpeed VCT Series/Discovery CT750; GE Healthcare, Tokyo, Japan). All imaging data were transferred to a CT-based 3D templating system (Zed Knee Lexi Co., Ltd., Tokyo, Japan). For all measurements and simulations, we defined the coordinate system of the femur according to the International Society of Biomechanics (ISB). In the ISB system, the X-axis was defined as a line passing through the lateral and medial epicondyle of the femur. The Z-axis was defined as a line connecting the femoral head and the middle point between lateral and medial epicondyle of the femur (Figure 1).
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2.4. Cutting plane of the distal femur
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We simulated the distal femoral osteotomy eight millimeters proximal to the lateral joint surface. Osteotomies were performed in flexion–extension 0° and varus–valgus 0°, with respect to the ISB coordinate system, to define the reference position. Subsequently, we changed the cutting angle from flexion 20° to extension 20°, and from varus five degrees to valgus five degrees. The cutting plane was defined as the plane of the distal femoral osteotomy in the above positions.
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2.3. Anatomical model and reference planes
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Table 2 Patient characteristics.
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N = 34 (male = 6, female = 28)
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Age Height (cm) Weight (kg) BMI (kg/m2)
60.4 155.4 54.3 22.0
± ± ± ±
10.3 6.6 8.1 2.6
Please cite this article as: Ohmori T, et al, The accuracy of the “projected surgical transepicondylar axis” relative to the “true surgical transepicondylar axis” in total knee arthroplasty, Knee (2017), http://dx.doi.org/10.1016/j.knee.2017.07.017
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Figure 1. Definition of axes in the femoral reference plane. The point of origin is defined as the midpoint between the medial and lateral epicondyles of the femur. The XZ plane is shown in the middle panel, with the X-axis defined as a line connecting the medial and lateral epicondyle of the femur and the Z-axis defined as a line connecting the origin point and center of the femoral head. The YZ plane is shown in the right panel, with the Y-axis defined as a line perpendicular to the reference plane, and bisecting the Z-axis.
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We defined the “true SEA” as a line passing through the lateral epicondyle and the medial sulcus of the femur on the 3D model (Figure 2a). We defined PCA as a line passing through the lateral and medial posterior condyle (Figure 2b). The “projected SEA” on the cutting plane was defined as a line passing through the projected points of the lateral epicondyle and the medial sulcus of the femur, which were projected parallel to the femoral distal bone axis (Figure 2c). We defined the distal femur axis as a line passing through the midpoint of the femoral shaft, five centimeters and 10 cm, proximal to the distal femur surface, respectively (Figure 3). These three lines, “true SEA”, “projected SEA”, and PCA, are not on the same plane. We defined “p true SEA”, “p PCA” and “p projected SEA” as the projections of these lines on the XY plane of the ISB coordinate system. The “true SEA angle” was defined as the angle between the “p true SEA”, and the “p PCA”. The “projected SEA angle” was defined as the angle between the “p projected SEA” and the “p PCA”. We measured the “true SEA angle” and the “projected SEA angle” on each of the cutting planes (Figure 4).
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2.5. Statistical analysis
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All data were expressed as mean ± standard deviation (SD) and analyzed by using statistical software (SPSS version 23). The 77 paired t-test was used to analyze the differences between the “true SEA angles” and the “projected SEA angles”. ANOVA was used Q9 78 to analyze the differences among the different “projected SEA angles”. p b 0.01 was considered statistically significant. 79
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3. Results
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Based on a prior power analysis, for an alpha level of 0.05, and a power of 0.8, a sample size of 34 was calculated. 81 Patients provided informed consent for their data use in this publication. All procedures were approved by the Institutional 82 Review Board of our university. 83 84
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The “projected SEA angles” were larger than the “true SEA angles” at all cutting angles when PCA was defined as a reference 85 line. 86 (1) Is the “projected SEA” different from the “true SEA” when we cut the femur in flexion or extension? The results are summarized in Table 3. The “projected SEA angles” were larger than the “true SEA angles” at all cutting angles. The mean “true SEA angle” was 3.04° ± 1.34° (0.6–5.0°). The mean “projected SEA angle” was 3.43° ± 1.58° (1.0–6.2°) at 20° flexion, 3.42° ± 1.54° (0.8–5.7°) at 10° flexion, 3.42° ± 1.56° (0.6–5.8°) at 0° flexion, 3.50° ± 1.50° (0.9–5.6°) at 10° extension, and 3.43° ± 1.52° (0.7–5.7°) at 20° extension. The mean “projected SEA angle” was significantly greater than the “true SEA angle” at all cutting angles (p b 0.001). On the other hand, there was no significant difference between any of the “projected SEA angles” (p N 0.05). (2) Is the “projected SEA” different from the “true SEA” when we cut the femur in varus or valgus? The results are summarized in Table 4. The “projected SEA angles” were larger than the “true SEA angles” at all cutting angles. The mean “projected SEA angle” was 3.39° ± 1.59° (0.6–6.0°) at five degrees valgus, and 3.39° ± 1.50° (0.6–5.8°) at five degrees varus. The mean “projected SEA angle” was significantly greater than the mean “true SEA Please cite this article as: Ohmori T, et al, The accuracy of the “projected surgical transepicondylar axis” relative to the “true surgical transepicondylar axis” in total knee arthroplasty, Knee (2017), http://dx.doi.org/10.1016/j.knee.2017.07.017
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Figure 2. a. Definition of true SEA. The “true SEA” is defined as a line connecting the medial epicondylar sulcus and the most prominent point of the lateral epicondyle. The “true SEA” is shown when viewed from the front (left panel) and the oblique view (right panel). b. Definition of the PCA. PCA is defined as a line connecting the medial and lateral posterior condyles of the femur. c. Definition of the “projected SEA”. The “projected SEA” is shown on the 3D model.
angle” (p b 0.001). On the other hand, there was no significant difference between any of the “projected SEA angles” (p N 0.05). (3) Is the “true SEA” angle affected based on the extent of discrepancy between the true and projected SEA angles? We divided all included patients into two groups, one consisting of patients with a difference of b0.5° between the true and projected SEA angle (low discrepancy), and the other consisting of patients with a difference of ≥0.5° between the two angles (high discrepancy). The comparison between these two groups is summarized in Table 5. The “true SEA angle” did not vary significantly based on the extent of difference between the “true SEA angle” and the “projected SEA angle” (p = 0.65). Please cite this article as: Ohmori T, et al, The accuracy of the “projected surgical transepicondylar axis” relative to the “true surgical transepicondylar axis” in total knee arthroplasty, Knee (2017), http://dx.doi.org/10.1016/j.knee.2017.07.017
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Figure 3. Definition of the axis of the distal femur. The axis of the distal femur is defined as a line passing through the midpoint of the femoral shaft, five centimeters and 10 cm, proximal to the distal femur surface, respectively. The axis is shown in the coronal plane (left panel) and the sagittal plane (right panel).
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4. Discussion
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Figure 4. Definition of the “p PCA”, “p true SEA”, and “p projected SEA”. Definition of the “true SEA angle” and the “projected SEA angle”.
The most important finding of this study was that the difference between the “projected SEA” and the “true SEA” was very 108 small. Additionally, the projected SEA was not influenced by the distal femoral osteotomy angle. 109 t3:1 t3:2 t3:3
Table 3 Differences between the “projected SEA angles” in the flexion/extension cutting plane. True SEA angle
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Flexion 20° Flexion 10° 0° Extension 10° Extension 20°
3.04 ± 1.34° Projected SEA angle
Compared to true SEA angle
3.43 3.42 3.42 3.50 3.43
p p p p p
± ± ± ± ±
1.58° 1.54° 1.56° 1.50° 1.52°
b b b b b
0.001 0.001 0.001 0.001 0.001
Please cite this article as: Ohmori T, et al, The accuracy of the “projected surgical transepicondylar axis” relative to the “true surgical transepicondylar axis” in total knee arthroplasty, Knee (2017), http://dx.doi.org/10.1016/j.knee.2017.07.017
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Table 4 Differences between the “projected SEA angles” in the varus/valgus cutting plane. True SEA angle
3.04 ± 1.34°
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Varus 5° 0° Valgus 5°
Projected SEA angle
Compared to true SEA angle
3.39 ± 1.50° 3.42 ± 1.56° 3.39 ± 1.59°
p b 0.001 p b 0.001 p b 0.001
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The rotational alignment of the femoral component in TKA is usually based on the following anatomical landmarks: the CEA, SEA, Whiteside line, and PCA. Recently the concept of the cylindrical axis was proposed, which is a line equidistant from the articular surface of each femoral condyle [12]. However, it is difficult to detect this axis because it does not utilize anatomical landmarks. In clinical practice, many orthopedic surgeons use a rotational reference axis defined by anatomical landmarks. The Whiteside line passes through between the center of the patellar groove and intercondylar notch [13]. The vertical line of the Whiteside line was externally rotated approximately three degrees to four degrees relative to the posterior condylar line [14,15]. Hanada et al. stated that the Whiteside line is a good reference to obtain normal stability in flexion and extension [13]. The SEA and CEA are lines connecting the medial collateral ligament and the lateral collateral ligament. The CEA is located externally about three degrees relative to the SEA [16]. Kobayashi et al. stated that these axes were suitable rotational axes of the femur [17]. Additionally, they reported that the SEA is a logical and appropriate axis. The PCA, which passes through the most posterior points on each femoral condyle, is another anatomical reference line [18]. Some surgeons define rotational axis as three degrees externally rotated relative to the PCA. One of the reasons is the fact that the most of the SEA are located about three degrees externally rotated relative to the PCA [16,19]. Olcott et al. compared the Whiteside line, PCA, and transepicondylar axis. They reported on the effectiveness of the transepicondylar axis relative to the PCA and Whiteside line [20]. Considering these aspects, the SEA can be considered one of the most reliable rotational axes and a useful guide to determine the femoral component rotation during TKA, although further validation is needed. The “true SEA” is determined either based on the lateral epicondyle and the medial sulcus of the femur or from the PCA. The lateral epicondyle and the medial sulcus of the femur are the two commonly used reference points for determining the “true SEA”. There are some reports that indicate that these landmarks may be difficult to identify intraoperatively owing to the presence of overlying soft tissue [21,22]. Studies have also demonstrated variations in the relationship between the PCA and SEA that may result in component malrotation [23–25]. In addition, the PCA is found to be particularly unreliable in valgus knees owing to the presence of lateral posterior condylar hypoplasia, which results in the SEA being located at three degrees or more on external rotation in these patients [26]. Franceschini et al. observed that the SEA was more reliable than the PCA as an intraoperative rotational reference [27]. Therefore, we should reconstruct the “true SEA”. However, a difference is expected between the “true SEA” and the “projected SEA” angle. Many surgeons have used the projected SEA during surgeries. Therefore, if there are large differences between the “true SEA” and the “projected SEA”, it is possible that the current placement of the femoral implants is based on an incorrect rotational axis. This is the first report examining the accuracy of the “projected SEA” as an indicator of the rotational axis of the femur. This study has four limitations. First, this is a simulation study wherein clinical outcomes were not evaluated. Second, among the different coordinate systems available for the femur, we used only the ISB coordinate system [28]. The results may vary with the use of another coordinate system. Third, we evaluated only one pattern of projection by using the distal femoral axis. Although this is the most commonly identified axis intraoperatively, the result may vary with the use of other projection axes. Fourth, we did not evaluate the relationship of the “projected SEA” with the mediolateral tibial axis. The distal femoral axis is at an angle to its mechanical axis because of femoral bowing [29–31]. Therefore, when we cut the distal femur in a plane vertical to its distal axis, the cutting plane would be in a flexed position relative to its mechanical axis. This results in the osteotomy in TKA to be in a flexed position with respect to the mechanical axis of the femur. In this study, we found that there were no significant differences between any of the “projected SEA angles” when the distal femur was cut from a 20° flexed position to a 20° extended position. However, the “projected SEA angle” was always larger than the “true SEA angle” for every position that was tested. An internal rotation error of the femoral component would result in a poorer outcome compared to that of an external rotation error [2]. In this study, we found that the “projected SEA” was always positioned in external rotation compared to the “true SEA”. This implies that the use of the “projected SEA” in the place of the “true SEA” for TKA will avoid an internal rotation setting. There are two major ways of cutting the distal femur. The first is based on the mechanical alignment; the cut is placed vertical to the mechanical axis of the femur. The second is based on the kinematic alignment; the cut is placed on the distal femur at three degrees valgus relative to its mechanical axis. Recently, Howell et al. reported good clinical outcomes with the use of the kinematics alignment, hence popularizing the valgus cutting of the distal femur [32]. Furthermore, the cutting error of the distal femur is t5:1 t5:2
Table 5 Relation between the “true SEA angle” and the discrepancy between the projected and true SEA angles.
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Difference between “true SEA angle” and “projected SEA angle” True SEA angle
b0.5 (n: 19)
0.5≤ (n: 15)
0.16 ± 0.14° 2.94 ± 1.42°
0.67 ± 0.14° 3.16 ± 1.23°
p b 0.001 p = 0.65
Please cite this article as: Ohmori T, et al, The accuracy of the “projected surgical transepicondylar axis” relative to the “true surgical transepicondylar axis” in total knee arthroplasty, Knee (2017), http://dx.doi.org/10.1016/j.knee.2017.07.017
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reported to occur to an extent of about three degrees in varus–valgus [33,34]. In this study, we found that distal femur cuts placed within an arc of five degrees varus–valgus did not result in a significant variation in the “projected SEA angle”. There was a discrepancy between the projected and true SEA of more than 0.5° in about 40% of cases in this study. We found that there was no significant difference between the “true SEA angle” between the two groups of patients with either a low (b 0.5°) or a high (N 0.5°) discrepancy between the two angles (p = 0.65) (Table 5). However, the difference was about 0.5° on average, and we believe that there is no clinically meaningful relevance. In conclusion, our study demonstrated differences between the “projected SEA” and “true SEA”. The difference was small (0.39° ± 0.29° [range 0–1.0°]), showing that the “true SEA” and “projected SEA” do not differ significantly.
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Conflict of interest statement
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We declare that there is no relationship between the organization and finance related to their work improperly.
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Please cite this article as: Ohmori T, et al, The accuracy of the “projected surgical transepicondylar axis” relative to the “true surgical transepicondylar axis” in total knee arthroplasty, Knee (2017), http://dx.doi.org/10.1016/j.knee.2017.07.017
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The accuracy of the “projected surgical transepicondylar axis” relative to the “true surgical transepicondylar axis” in total knee arthroplasty Takaaki Ohmori a, Tamon Kabata a,⁎, Yoshitomo Kajino a, Tadashi Taga a, Kazuhiro Hasegawa a, Daisuke Inoue a, Takashi Yamamoto a, Tomoharu Takagi a, Junya Yoshitani a, Takuro Ueno a, Tomohiro Ojima b, Hiroyuki Tsuchiya a
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Department of Orthopedic Surgery, Graduate School of Medical Science, Kanazawa University, Takaramachi13-1, Kanazawa, Ishikawa, Japan Department of Orthopedic Surgery, Fukui General Hospital, 58-16-1 Egami, Fukui, Fukui 910-8561, Japan
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Background: In TKA, we have used the “projected SEA”, which is obtained by projecting the “true SEA” on the distal femoral cutting plane in clinical practice to determine the femoral component rotation. There are no reports examining the accuracy of the “projected SEA”. In this study, we investigated the difference between the “true SEA” and “projected SEA”. Methods: The present study was a CT-based computer-simulated case series. We evaluated 34 knees without osteoarthritis changes. These patients were selected from the operative schedule prior to THA. We defined the “true SEA” on the 3D model and the “projected SEA” on the cutting plane parallel to the distal femoral axis obtained based on the “true SEA”. We changed the cutting angles from 20° flexion to 20° extension, and from five degrees varus to five degrees valgus. We measured the “true SEA angle” and “projected SEA angle” regarding the posterior condylar axis (PCA). Results: The mean “true SEA angle” was 3.04° ± 1.34° (0.6–5.0°). The mean “projected SEA angle” was 3.43° ± 1.58° at 20° flexion, 3.42° ± 1.56° at 0° flexion, 3.43° ± 1.52° at 20° extension, 3.39° ± 1.59° at five degrees valgus, and 3.39° ± 1.50° at five degrees varus. At each cutting angle, the “projected SEA angle” was significantly larger than the “true SEA angle” (p b 0.001). There was no significant difference between any of the “projected SEA angles” (p N 0.001). Conclusions: We found that the true SEA and projected SEA do not differ significantly (0.39° ± 0.29° [range 0–1.0°]). © 2017 Published by Elsevier B.V.
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Article history: Received 11 February 2017 Received in revised form 9 June 2017 Accepted 29 July 2017 Available online xxxx
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1. Introduction
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Keywords: TKA True SEA Projected SEA
Patient satisfaction after total knee arthroplasty (TKA) is lower compared to that of total hip arthroplasty (THA) [1]. Malalignment of the implant is one of the reasons for this dissatisfaction [2]. Some reports correlate anterior knee pain [3,4] and patellofemoral maltracking [5] with malalignment of the femoral component during TKA. The clinical transepicondylar axis (CEA), surgical transepicondylar axis (SEA), anteroposterior axis, posterior condylar axis (PCA), cylindrical axis (CA), and Whiteside line have been used as benchmarks of femoral component rotation [6–10]. Among these, Berger et al. identified SEA as the most reliable indicator [11]. SEA is defined as a line passing through the lateral epicondyle and the medial sulcus of the
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⁎ Corresponding author. E-mail address: [email protected] (T. Kabata).
http://dx.doi.org/10.1016/j.knee.2017.07.017 0968-0160/© 2017 Published by Elsevier B.V.
Please cite this article as: Ohmori T, et al, The accuracy of the “projected surgical transepicondylar axis” relative to the “true surgical transepicondylar axis” in total knee arthroplasty, Knee (2017), http://dx.doi.org/10.1016/j.knee.2017.07.017
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All primary TKA in our hospital (n=162)
Records excluded (n=124) OA of the bilateral hip
Records screened (n=38)
In apposite images (n=4)
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2. Materials and methods
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femur on a three-dimensional (3D) model (“true SEA”). However, these two points are located proximal to the cutting plane of the femur. Therefore, some orthopedic surgeons have used the “projected SEA”, which is a line passing through these projected points on the distal femoral cutting plane parallel to the distal femoral axis. Thus far, the differences between these two SEA types have not been evaluated. There are no reports that examine the accuracy of the “projected SEA”. In this study, we investigated the difference between “true SEA” and “projected SEA”. We asked the following questions: (1) Is the “projected SEA” different from the “true SEA” when we cut the femur in flexion or extension? (2) Is the “projected SEA” different from the “true SEA” when we cut the femur in varus or valgus? (3) Is the “true SEA” angle affected based on the extent of discrepancy between the true and projected SEA angles?
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This is a computed tomography (CT) based computer simulated case series study.
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2.2. Patients
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2.1. Study design
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This retrospective study included patients who underwent primary THA at our hospital between 2012 and 2013. We used the healthy side. Cases with bilateral hip osteoarthritis were excluded. We reviewed 34 patients (34 knees; six men and 28 women) that did not have any osteoarthritis changes in their knees and hips (Table 1). The mean patient age was 60.0 ± 10.3 years (range, 41–86 years) (Table 2).
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We constructed a 3D model of the femur from CT images (LightSpeed VCT Series/Discovery CT750; GE Healthcare, Tokyo, Japan). All imaging data were transferred to a CT-based 3D templating system (Zed Knee Lexi Co., Ltd., Tokyo, Japan). For all measurements and simulations, we defined the coordinate system of the femur according to the International Society of Biomechanics (ISB). In the ISB system, the X-axis was defined as a line passing through the lateral and medial epicondyle of the femur. The Z-axis was defined as a line connecting the femoral head and the middle point between lateral and medial epicondyle of the femur (Figure 1).
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2.4. Cutting plane of the distal femur
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We simulated the distal femoral osteotomy eight millimeters proximal to the lateral joint surface. Osteotomies were performed in flexion–extension 0° and varus–valgus 0°, with respect to the ISB coordinate system, to define the reference position. Subsequently, we changed the cutting angle from flexion 20° to extension 20°, and from varus five degrees to valgus five degrees. The cutting plane was defined as the plane of the distal femoral osteotomy in the above positions.
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2.3. Anatomical model and reference planes
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Table 2 Patient characteristics.
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N = 34 (male = 6, female = 28)
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Age Height (cm) Weight (kg) BMI (kg/m2)
60.4 155.4 54.3 22.0
± ± ± ±
10.3 6.6 8.1 2.6
Please cite this article as: Ohmori T, et al, The accuracy of the “projected surgical transepicondylar axis” relative to the “true surgical transepicondylar axis” in total knee arthroplasty, Knee (2017), http://dx.doi.org/10.1016/j.knee.2017.07.017
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Figure 1. Definition of axes in the femoral reference plane. The point of origin is defined as the midpoint between the medial and lateral epicondyles of the femur. The XZ plane is shown in the middle panel, with the X-axis defined as a line connecting the medial and lateral epicondyle of the femur and the Z-axis defined as a line connecting the origin point and center of the femoral head. The YZ plane is shown in the right panel, with the Y-axis defined as a line perpendicular to the reference plane, and bisecting the Z-axis.
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We defined the “true SEA” as a line passing through the lateral epicondyle and the medial sulcus of the femur on the 3D model (Figure 2a). We defined PCA as a line passing through the lateral and medial posterior condyle (Figure 2b). The “projected SEA” on the cutting plane was defined as a line passing through the projected points of the lateral epicondyle and the medial sulcus of the femur, which were projected parallel to the femoral distal bone axis (Figure 2c). We defined the distal femur axis as a line passing through the midpoint of the femoral shaft, five centimeters and 10 cm, proximal to the distal femur surface, respectively (Figure 3). These three lines, “true SEA”, “projected SEA”, and PCA, are not on the same plane. We defined “p true SEA”, “p PCA” and “p projected SEA” as the projections of these lines on the XY plane of the ISB coordinate system. The “true SEA angle” was defined as the angle between the “p true SEA”, and the “p PCA”. The “projected SEA angle” was defined as the angle between the “p projected SEA” and the “p PCA”. We measured the “true SEA angle” and the “projected SEA angle” on each of the cutting planes (Figure 4).
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2.5. Statistical analysis
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All data were expressed as mean ± standard deviation (SD) and analyzed by using statistical software (SPSS version 23). The 77 paired t-test was used to analyze the differences between the “true SEA angles” and the “projected SEA angles”. ANOVA was used Q9 78 to analyze the differences among the different “projected SEA angles”. p b 0.01 was considered statistically significant. 79
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3. Results
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Based on a prior power analysis, for an alpha level of 0.05, and a power of 0.8, a sample size of 34 was calculated. 81 Patients provided informed consent for their data use in this publication. All procedures were approved by the Institutional 82 Review Board of our university. 83 84
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The “projected SEA angles” were larger than the “true SEA angles” at all cutting angles when PCA was defined as a reference 85 line. 86 (1) Is the “projected SEA” different from the “true SEA” when we cut the femur in flexion or extension? The results are summarized in Table 3. The “projected SEA angles” were larger than the “true SEA angles” at all cutting angles. The mean “true SEA angle” was 3.04° ± 1.34° (0.6–5.0°). The mean “projected SEA angle” was 3.43° ± 1.58° (1.0–6.2°) at 20° flexion, 3.42° ± 1.54° (0.8–5.7°) at 10° flexion, 3.42° ± 1.56° (0.6–5.8°) at 0° flexion, 3.50° ± 1.50° (0.9–5.6°) at 10° extension, and 3.43° ± 1.52° (0.7–5.7°) at 20° extension. The mean “projected SEA angle” was significantly greater than the “true SEA angle” at all cutting angles (p b 0.001). On the other hand, there was no significant difference between any of the “projected SEA angles” (p N 0.05). (2) Is the “projected SEA” different from the “true SEA” when we cut the femur in varus or valgus? The results are summarized in Table 4. The “projected SEA angles” were larger than the “true SEA angles” at all cutting angles. The mean “projected SEA angle” was 3.39° ± 1.59° (0.6–6.0°) at five degrees valgus, and 3.39° ± 1.50° (0.6–5.8°) at five degrees varus. The mean “projected SEA angle” was significantly greater than the mean “true SEA Please cite this article as: Ohmori T, et al, The accuracy of the “projected surgical transepicondylar axis” relative to the “true surgical transepicondylar axis” in total knee arthroplasty, Knee (2017), http://dx.doi.org/10.1016/j.knee.2017.07.017
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Figure 2. a. Definition of true SEA. The “true SEA” is defined as a line connecting the medial epicondylar sulcus and the most prominent point of the lateral epicondyle. The “true SEA” is shown when viewed from the front (left panel) and the oblique view (right panel). b. Definition of the PCA. PCA is defined as a line connecting the medial and lateral posterior condyles of the femur. c. Definition of the “projected SEA”. The “projected SEA” is shown on the 3D model.
angle” (p b 0.001). On the other hand, there was no significant difference between any of the “projected SEA angles” (p N 0.05). (3) Is the “true SEA” angle affected based on the extent of discrepancy between the true and projected SEA angles? We divided all included patients into two groups, one consisting of patients with a difference of b0.5° between the true and projected SEA angle (low discrepancy), and the other consisting of patients with a difference of ≥0.5° between the two angles (high discrepancy). The comparison between these two groups is summarized in Table 5. The “true SEA angle” did not vary significantly based on the extent of difference between the “true SEA angle” and the “projected SEA angle” (p = 0.65). Please cite this article as: Ohmori T, et al, The accuracy of the “projected surgical transepicondylar axis” relative to the “true surgical transepicondylar axis” in total knee arthroplasty, Knee (2017), http://dx.doi.org/10.1016/j.knee.2017.07.017
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Figure 3. Definition of the axis of the distal femur. The axis of the distal femur is defined as a line passing through the midpoint of the femoral shaft, five centimeters and 10 cm, proximal to the distal femur surface, respectively. The axis is shown in the coronal plane (left panel) and the sagittal plane (right panel).
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4. Discussion
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Figure 4. Definition of the “p PCA”, “p true SEA”, and “p projected SEA”. Definition of the “true SEA angle” and the “projected SEA angle”.
The most important finding of this study was that the difference between the “projected SEA” and the “true SEA” was very 108 small. Additionally, the projected SEA was not influenced by the distal femoral osteotomy angle. 109 t3:1 t3:2 t3:3
Table 3 Differences between the “projected SEA angles” in the flexion/extension cutting plane. True SEA angle
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Flexion 20° Flexion 10° 0° Extension 10° Extension 20°
3.04 ± 1.34° Projected SEA angle
Compared to true SEA angle
3.43 3.42 3.42 3.50 3.43
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± ± ± ± ±
1.58° 1.54° 1.56° 1.50° 1.52°
b b b b b
0.001 0.001 0.001 0.001 0.001
Please cite this article as: Ohmori T, et al, The accuracy of the “projected surgical transepicondylar axis” relative to the “true surgical transepicondylar axis” in total knee arthroplasty, Knee (2017), http://dx.doi.org/10.1016/j.knee.2017.07.017
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Table 4 Differences between the “projected SEA angles” in the varus/valgus cutting plane. True SEA angle
3.04 ± 1.34°
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Varus 5° 0° Valgus 5°
Projected SEA angle
Compared to true SEA angle
3.39 ± 1.50° 3.42 ± 1.56° 3.39 ± 1.59°
p b 0.001 p b 0.001 p b 0.001
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The rotational alignment of the femoral component in TKA is usually based on the following anatomical landmarks: the CEA, SEA, Whiteside line, and PCA. Recently the concept of the cylindrical axis was proposed, which is a line equidistant from the articular surface of each femoral condyle [12]. However, it is difficult to detect this axis because it does not utilize anatomical landmarks. In clinical practice, many orthopedic surgeons use a rotational reference axis defined by anatomical landmarks. The Whiteside line passes through between the center of the patellar groove and intercondylar notch [13]. The vertical line of the Whiteside line was externally rotated approximately three degrees to four degrees relative to the posterior condylar line [14,15]. Hanada et al. stated that the Whiteside line is a good reference to obtain normal stability in flexion and extension [13]. The SEA and CEA are lines connecting the medial collateral ligament and the lateral collateral ligament. The CEA is located externally about three degrees relative to the SEA [16]. Kobayashi et al. stated that these axes were suitable rotational axes of the femur [17]. Additionally, they reported that the SEA is a logical and appropriate axis. The PCA, which passes through the most posterior points on each femoral condyle, is another anatomical reference line [18]. Some surgeons define rotational axis as three degrees externally rotated relative to the PCA. One of the reasons is the fact that the most of the SEA are located about three degrees externally rotated relative to the PCA [16,19]. Olcott et al. compared the Whiteside line, PCA, and transepicondylar axis. They reported on the effectiveness of the transepicondylar axis relative to the PCA and Whiteside line [20]. Considering these aspects, the SEA can be considered one of the most reliable rotational axes and a useful guide to determine the femoral component rotation during TKA, although further validation is needed. The “true SEA” is determined either based on the lateral epicondyle and the medial sulcus of the femur or from the PCA. The lateral epicondyle and the medial sulcus of the femur are the two commonly used reference points for determining the “true SEA”. There are some reports that indicate that these landmarks may be difficult to identify intraoperatively owing to the presence of overlying soft tissue [21,22]. Studies have also demonstrated variations in the relationship between the PCA and SEA that may result in component malrotation [23–25]. In addition, the PCA is found to be particularly unreliable in valgus knees owing to the presence of lateral posterior condylar hypoplasia, which results in the SEA being located at three degrees or more on external rotation in these patients [26]. Franceschini et al. observed that the SEA was more reliable than the PCA as an intraoperative rotational reference [27]. Therefore, we should reconstruct the “true SEA”. However, a difference is expected between the “true SEA” and the “projected SEA” angle. Many surgeons have used the projected SEA during surgeries. Therefore, if there are large differences between the “true SEA” and the “projected SEA”, it is possible that the current placement of the femoral implants is based on an incorrect rotational axis. This is the first report examining the accuracy of the “projected SEA” as an indicator of the rotational axis of the femur. This study has four limitations. First, this is a simulation study wherein clinical outcomes were not evaluated. Second, among the different coordinate systems available for the femur, we used only the ISB coordinate system [28]. The results may vary with the use of another coordinate system. Third, we evaluated only one pattern of projection by using the distal femoral axis. Although this is the most commonly identified axis intraoperatively, the result may vary with the use of other projection axes. Fourth, we did not evaluate the relationship of the “projected SEA” with the mediolateral tibial axis. The distal femoral axis is at an angle to its mechanical axis because of femoral bowing [29–31]. Therefore, when we cut the distal femur in a plane vertical to its distal axis, the cutting plane would be in a flexed position relative to its mechanical axis. This results in the osteotomy in TKA to be in a flexed position with respect to the mechanical axis of the femur. In this study, we found that there were no significant differences between any of the “projected SEA angles” when the distal femur was cut from a 20° flexed position to a 20° extended position. However, the “projected SEA angle” was always larger than the “true SEA angle” for every position that was tested. An internal rotation error of the femoral component would result in a poorer outcome compared to that of an external rotation error [2]. In this study, we found that the “projected SEA” was always positioned in external rotation compared to the “true SEA”. This implies that the use of the “projected SEA” in the place of the “true SEA” for TKA will avoid an internal rotation setting. There are two major ways of cutting the distal femur. The first is based on the mechanical alignment; the cut is placed vertical to the mechanical axis of the femur. The second is based on the kinematic alignment; the cut is placed on the distal femur at three degrees valgus relative to its mechanical axis. Recently, Howell et al. reported good clinical outcomes with the use of the kinematics alignment, hence popularizing the valgus cutting of the distal femur [32]. Furthermore, the cutting error of the distal femur is t5:1 t5:2
Table 5 Relation between the “true SEA angle” and the discrepancy between the projected and true SEA angles.
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Difference between “true SEA angle” and “projected SEA angle” True SEA angle
b0.5 (n: 19)
0.5≤ (n: 15)
0.16 ± 0.14° 2.94 ± 1.42°
0.67 ± 0.14° 3.16 ± 1.23°
p b 0.001 p = 0.65
Please cite this article as: Ohmori T, et al, The accuracy of the “projected surgical transepicondylar axis” relative to the “true surgical transepicondylar axis” in total knee arthroplasty, Knee (2017), http://dx.doi.org/10.1016/j.knee.2017.07.017
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reported to occur to an extent of about three degrees in varus–valgus [33,34]. In this study, we found that distal femur cuts placed within an arc of five degrees varus–valgus did not result in a significant variation in the “projected SEA angle”. There was a discrepancy between the projected and true SEA of more than 0.5° in about 40% of cases in this study. We found that there was no significant difference between the “true SEA angle” between the two groups of patients with either a low (b 0.5°) or a high (N 0.5°) discrepancy between the two angles (p = 0.65) (Table 5). However, the difference was about 0.5° on average, and we believe that there is no clinically meaningful relevance. In conclusion, our study demonstrated differences between the “projected SEA” and “true SEA”. The difference was small (0.39° ± 0.29° [range 0–1.0°]), showing that the “true SEA” and “projected SEA” do not differ significantly.
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Conflict of interest statement
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We declare that there is no relationship between the organization and finance related to their work improperly.
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Please cite this article as: Ohmori T, et al, The accuracy of the “projected surgical transepicondylar axis” relative to the “true surgical transepicondylar axis” in total knee arthroplasty, Knee (2017), http://dx.doi.org/10.1016/j.knee.2017.07.017
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