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Femoral Anterior Tangent Line of the Osteoarthritic Knee for Determining Rotational Alignment of the Femoral Component in Total Knee Arthroplasty
The Journal of Arthroplasty Vol. 26 No. 2 2011
Femoral Anterior Tangent Line of the Osteoarthritic Knee for Determining Rotational Alignment of the Femoral Component in Total Knee Arthroplasty Hiroki Watanabe, MD, PhD,* Ryuichi Gejo, MD, PhD,* Yoshikazu Matsuda, MD,y Ichiro Tatsumi, MD,y Kazuo Hirakawa, MD, PhD,y and Tomoatsu Kimura, MD, PhD*
Abstract: Several reference axes have been used to establish femoral rotational alignment during total knee arthroplasty. The current study examined the configuration of the anterior surface of the femur immediately proximal to the trochlea as an alternative rotational landmark. An analysis of computed tomographic images of 150 knees with osteoarthritis indicated that the configuration of the surface is mostly flat or slightly depressed, and the line tangential to the surface (femoral anterior tangent line; FAT line) was consistently determined to be 12.2° ± 3.6° internally rotated to the transepicondylar axis. This value was relatively constant and as reliable as the femoral anteroposterior axis for determining rotational alignment. In addition, the FAT line was not affected by the degree of the varus-valgus deformity of the osteoarthritic knees. Keywords: femoral anterior tangent line, rotational alignment, total knee arthroplasty, transepicondylar axis. © 2011 Elsevier Inc. All rights reserved.
The rotational alignment of the femoral component affects patellofemoral and tibiofemoral kinematics and influences the clinical outcome of total knee arthroplasty (TKA). It has been shown that femoral rotational malalignment is a major cause of petellofemoral complications such as anterior knee pain [1] and poor patellar tracking [2-5]. Femoral malalignment also causes flexion gap instability [6], a loss of motion, and early failure of the implant [7]. Several reference axes have been proposed to establish the appropriate rotational alignment of the femoral component, including the transepicondylar axis (TEA), the posterior condylar axis (PCA), and the anteroposterior (AP) axis (Whiteside's line). Anatomical and biomechanical studies have suggested that
From the *Department of Orthopedic Surgery, Faculty of Medicine, University of Toyama, Toyama City, Toyama, Japan; and yDepartment of Orthopedic Surgery, Shonan Kamakura Joint Reconstruction Center, Kamakura City, Kanagawa, Japan. Submitted April 29, 2009; accepted December 9, 2009. All authors declare that they have not received from any organization any personal or financial benefit that could influence to the work published in this article. Reprint requests: Hiroki Watanabe, MD, PhD, Department of Orthopedic Surgery, Faculty of Medicine, University of Toyama, 2630, Sugitani, Toyama City, Toyama, 930-0194, Japan. © 2011 Elsevier Inc. All rights reserved. 0883-5403/2602-0017$36.00/0 doi:10.1016/j.arth.2009.12.011
the TEA approximates the flexion-extension axis of the knee, and it is the most optimal landmark for the femoral component rotation [8-13]. However, the practical use of this axis is sometimes hampered by difficulty in the precise localization of the medial epicondyle and the sulcus during surgery [14]. Limited surgical exposure of minimally invasive TKA further disturbs such identification of the epicondyles and TEA. The PCA and AP axis could be used as an alternative to the TEA. The PCA has been often used as the landmark for the femoral rotational alignment, and many studies have demonstrated that the angle between the TEA and PCA is 3° to 5° [15-19]. The PCA, however, is not a reliable axis in arthritic knees, particularly in valgus knees with bony destruction. Whiteside et al [20,21] proposed the AP axis that was almost perpendicular to the TEA and demonstrated that the axis was not influenced by valgus deformity. Although there are arguments with regard to which axis is most reliable and easily identified during surgery, a single-axis measurement might result in a highly variable rotational alignment in TKA [22]. As an additional intraoperative bony landmark of the distal femur, anterior surface of the femur immediately proximal to the trochlea is observed to be flat in most of the patients during TKA and the surface may be preserved even in severely destroyed knees. We therefore hypothesize that the anterior surface of the
268
Femoral Anterior Tangent Line for Determining Rotational Alignment in TKA Watanabe et al
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Fig. 1. Three types of anterior femoral surface configuration and FAT line. The black lines indicate the FAT line.
femur immediately proximal to the trochlea may be another reliable landmark for determining the proper rotational alignment of the femoral component. The purpose of this study was to analyze the configuration of the anterior surface of the femur immediately proximal to the trochlea using computed tomographic (CT) images and to determine whether the line tangential to the surface (femoral anterior tangent line; FAT line) was comparatively consistent against the reference TEA, irrespective of the varus-valgus deformity.
wear and destruction. The “perpendicular” AP axis was then defined as a line perpendicular to the AP axis. In cases with osteophyte formation in the intercondylar notch, the center of the notch was determined after elimination of these osteophytes on
Materials and Methods The patient population included 135 patients (150 knees) who had unilateral knee arthroplasty or TKA because of osteoarthritis of the knee. The subjects included 19 males and 116 females, and the mean age was 71.9 ± 9.1 years old (mean ± SD; range, 3787 years). The CT data used for this study were taken before unilateral knee arthroplasty or TKA at 2 facilities. A CT scan of the lower extremity was performed in full knee extension at 2-mm or 5-mm intervals, 111 knees for 2-mm (Robusto-Ei, Hitachi Medico, Tokyo, Japan) and 39 knees for 5-mm intervals (Somatom Sensation 16, SIEMENS, München, Germany). Using the lateral scout view, the scans were taken perpendicular to the long axis of the femur. The anterior femoral surface around the upper pole level of patella was examined on the serial CT slices. Next, a CT slice immediately proximal to the femoral trochlea, which showed widest anterior width without trochleal prominence, was selected. The configuration of the anterior femoral surface was examined, and a line tangential to the anterior femoral surface (FAT line) was determined (Fig. 1). The CT slices on which the medial or lateral epicondyles were most prominently detectable were also selected to measure the clinical TEA, the AP axis, and the PCA (Fig. 2). The clinical TEA was defined as a line connecting the lateral and medial epicondylar prominences. The AP axis was defined as a line connecting the deepest point of the patellar groove anteriorly and the center of the intercondylar notch posteriorly and measured in the knees except for 6 knees that showed patellar groove
Fig. 2. Reference axes defined by bony landmarks of the distal femoral condyle and transverse CT slice. The lines indicate the clinical TEA, perpendicular AP axis, and PCA. (A) The anterior femoral surface immediately proximal to the trochlea with the FAT line is superimposed. (B) An image of corresponding position of FAT line and surgical TEA that could be determined at the time of surgery is also shown.
270 The Journal of Arthroplasty Vol. 26 No. 2 February 2011 the images. A line connecting the most posterior points of the medial and lateral posterior condyles was defined as the PCA. The selection of CT slices and the measurement was performed by one of the authors (HW) at 2 independent time points. Axis measurements on the CT images were performed using computer software (Photoshop; Adobe Systems, San Jose, Calif), and absolute values of each axis (FAT line, TEA, AP axis, PCA) were recorded. The intraobserver variation of the measurements was evaluated with intraclass correlation method. Namely, correlation was examined between 2 absolute values provided by 2 independent measurements on the CT slices at different time points. Thereafter, the angles between the FAT line and the TEA, perpendicular AP axis and the TEA, and the PCA and the TEA were calculated. The rotational angle to the TEA was arbitrarily expressed as a positive value for the external rotation and as a negative value for the external rotation. The data are presented as the mean ± SD. In addition, we investigated whether the preoperative femorotibial angle (FTA) correlated with each of the angles and affected their reliability. F tests were used to identify any significant differences in the variability of the respective axis to the referential clinical TEA. The Pearson correlation test was used to analyze the correlation between each angle and the FTA (Stat View 5.0; Abacus Concepts, Berkley, Calif). The level of significance was set at a value of 0.05.
histograms of these 2 angles showed some scattering of these angles, and if a deviation more than 4° from the mean value is defined as an outlier, 28% in FAT line and 22% in perpendicular AP axis would fall in this zone (Fig. 3A, B). The perpendicular AP axis showed less outlier but with more deviation as large as 15°. The F test showed no significant difference in variability between the FAT line and the perpendicular AP axis (P = .85). Therefore, the FAT line seemed to be as reliable as perpendicular AP axis for the femoral rotation in relation to the clinical TEA.
Results Configuration of the Anterior Femoral Surface and FAT Line The anterior femoral surface immediately proximal to the trochlea was readily observed in CT slices, and the configuration was classified into 3 types: type 1 (flat, 100 knees), type 2 (slight central depression, 32 knees), and type 3 (slight convex, 18 knees; Fig. 1). Therefore, most of the knees had flat or slightly depressed anterior surface at this axial section. In types 1 and 2 knees, a line tangential to the anterior femoral surface (FAT line) was easily determined in the CT slices, and therefore, we used 132 knees (88.0%) for the following analysis of axes and angle measurements. Angles of Each Axis in Relation to Clinical TEA The reproducibility of the measurement was evaluated using the intraclass correlation method. The measurements of each axis at 2 independent time points were closely correlated, and the correlation coefficients of the absolute values of each axis were 0.996 for the FAT line, 0.991 for the TEA, 0.997 for the PCA, and 0.973 for the AP axis. The calculated angle between the FAT and clinical TEA was −12.2° ± 3.6°. The angle between the perpendicular AP axis and the clinical TEA was −0.3° ± 3.6°. The
Fig. 3. Distribution of the angles between the FAT line and clinical TEA (A) and between the perpendicular AP axis and clinical TEA (B). The variability of the FAT line and perpendicular AP axis to the clinical TEA was similar (P = .85). The dotted line indicates a mean value.
Femoral Anterior Tangent Line for Determining Rotational Alignment in TKA Watanabe et al
Influence of FTA on the Reliability of the Axis Measurement The influence of varus-valgus deformity on the reliability of each axis for femoral rotation was analyzed. The FTA ranged from 160° to 211° (mean, 185.5° ± 7.4°) in the present patients with osteoarthritis. The angle
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between the FAT line and the clinical TEA showed no correlation to FTA (r = 0.17; P = .06; Fig. 4A), thus, indicating that the FAT line was not affected by the varus-valgus deformity of the knee. Similarly, the angle between the perpendicular AP axis and clinical TEA showed no correlation (r = 0.07; P = .45; Fig. 4B). On the other hand, the angle between the PCA and clinical TEA showed a significant correlation to the FTA (r = 0.31; P = .0001; Fig. 4C), thus, confirming that the PCA may not be a reliable rotational landmark in the knees with a varus-valgus deformity.
Discussion
Fig. 4. The relationships between each of the axes and the FTA. The angle between the FAT line and clinical TEA (A) and between the perpendicular AP axis and clinical TEA (B) have no correlations to the FTA (r = 0.17; P = .06 and r = 0.07; P = .45; respectively). The angle between the PCA and clinical TEA (C) is observed to correlate with the FTA (r = 0.31; P = .0001).
The rotational alignment of the femoral component of the TKA is essential for the optimal patellar tracking, ligament balancing, and functional outcome. The use of a reliable and reproducibly identifiable landmark and axis is important for accurately determining the proper rotational alignment. The current study indicated that the anterior surface of the femur immediately proximal to the trochlea and its tangent line (FAT line) could be used as a good index of the femoral rotation. The configuration of the surface is mostly constant, and the tangent line can be easily determined. Previous studies have demonstrated that the TEA and AP axis are reliable axes for the rotation of the femoral component, and TEA was the most reproducible landmark that produce the best balance [23], whereas the posterior femoral condyles were less reproducible in determining the rotation. In the present study, AP axis against clinical TEA was measured in knees with osteoarthritis. The perpendicular AP axis was relatively invariable and ran parallel (0.3° externally rotated) to the TEA. This is comparable to a previous report [20] and thus confirms the reliability of the AP axis for determining femoral rotation when the clinical TEA is defined as an absolute reference axis. The FAT line we demonstrated in the current study also showed relatively invariable internal rotation to the clinical TEA. Twelve degrees of external rotation to the FAT line during TKA would properly approximate the femoral rotational alignment to the TEA. In addition, FAT line, as well as AP axis, was not affected by the varus-valgus deformity of the osteoarthritic knee. It is possibly because these 2 axes avoid using a distorted condylar and cartilage anatomy for rotational alignment of the femoral component. Therefore, the FAT line located on the femoral surface immediately proximal to the trochlea could become useful even in knees with deformity. Various bony landmarks may not be always identified on x-ray images or at surgery. The clinical TEA could be easily obtained on CT images, whereas the surgical TEA is not readily determined. Previous reports indicated that the identification rate of the medial sulcus using CT ranges from 20% to 74% [15,19,24], which may further
272 The Journal of Arthroplasty Vol. 26 No. 2 February 2011 decline in knees with advanced osteoarthritis. In addition, the debate continues with regard to how accurately the TEA can be located intraoperatively to obtain the best results [14]. The accuracy of the TEA measurement may be hampered by soft tissue coverage or difficulty in accessing the lateral condyle and also by surgeon factors [22,25]. As for the FAT line, it could be easily determined on the CT images in most of the cases. Although our current study focused on the analysis of the presurgical CT images, the FAT line should also be easily located intraoperatively because the anterior femoral surface immediately proximal to the trochlea is seen and palpated without difficulty. Therefore, a surgical jig that uses anterior femoral surface and the FAT line as the reference should become helpful for proper determination of the femoral rotational alignment during TKA. It should be noted, however, measurements of any reliable reference axes, including FAT line, inevitably vary to some extent among individuals. In addition, the FAT line may not be used as a single rotational reference axis in certain cases with slightly convex anterior femoral surface. The composite measurement of plural axes, TEA, AP axis, or FAT line should warrant more appropriate rotational alignment of the femoral component during TKA. Recently, Talbot and Bartlett [26] reported the use of the anterior femoral surface for anatomical reference during navigation surgery. Although the definition of the axis based on the femoral surface was not clearly indicated, they suggested that the axis, 2.9° internally rotated relative to the perpendicular AP axis (Whiteside's line), may be a useful landmark. In our current measurement, the FAT line was 12° internally rotated relative to the clinical TEA and to the perpendicular AP axis. Therefore, the FAT line, a line clearly defined as a tangent line to the femoral anterior surface immediately above the trochlea, was further internally rotated to the axis described by Talbot and Bartlett [26]. This difference may be due to the use of different landmarks of the femoral surface, a different definition of the axis or it may even reflect a racial difference. In either case, the distal anterior surface of the femur is nevertheless considered to be a new and useful landmark. In conclusion, there are various landmarks for the femoral rotational alignment, but a single axis is not always sufficient to precisely determine the rotation due to anatomical variations, poor reproducibility of the axis, and poor visibility of the landmark at surgery. In addition, the loss of bone stock observed at revision surgery further hinders the determination of rotational alignment. The use of an additional landmark of the anterior femoral surface and its tangent line would therefore help to properly determine the femoral rotational alignment both before and during surgery.
References 1. 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. 2. 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. 3. Berger RA, Crossett LS, Jacobs JJ, et al. Malrotation causing patellofemoral complications after total knee arthroplasty. Clin Orthop Relat Res 1998;356:144. 4. Kienapfel H, Springorum HP, Ziegler A, et al. Effect of rotation of the femoral and tibial components on patellofemoral malalignment in knee arthroplasty. Orthopade 2003;32:312. 5. Matsuda S, Miura H, Nagamine R, et al. Effect of femoral and tibial component position on patellar tracking following total knee arthroplasty: 10-year follow-up of Miller-Galante I knees. Am J Knee Surg 2001;14:152. 6. Scuderi GR, Komistek RD, Dennis DA, et al. The impact of femoral component rotational alignment on condylar liftoff. Clin Orthop Relat Res 2003;410:148. 7. D'Lima DD, Chen PC, Colwell Jr CW. Polyethylene contact stresses, articular congruity, and knee alignment. Clin Orthop Relat Res 2001;392:232. 8. Berger RA, Rubash HE, Seel MJ, et al. Determining the rotational alignment of the femoral component in total knee arthroplasty using the epicondylar axis. Clin Orthop Relat Res 1993;286:40. 9. Elias SG, Freeman MA, Gokcay EI. A correlative study of the geometry and anatomy of the distal femur. Clin Orthop Relat Res 1990;260:98. 10. Hollister AM, Jatana S, Singh AK, et al. The axes of rotation of the knee. Clin Orthop Relat Res 1993;290: 259. 11. Katz MA, Beck TD, Silber JS, et al. Determining femoral rotational alignment in total knee arthroplasty: reliability of techniques. J Arthroplasty 2001;16:301. 12. Kurosawa H, Walker PS, Abe S, et al. Geometry and motion of the knee for implant and orthotic design. J Biomech 1985;18:487. 13. Miller MC, Berger RA, Petrella AJ, et al. Optimizing femoral component rotation in total knee arthroplasty. Clin Orthop Relat Res 2001;392:38. 14. Poilvache PL, Insall JN, Scuderi GR, et al. Rotational landmarks and sizing of the distal femur in total knee arthroplasty. Clin Orthop Relat Res 1996;331:35. 15. Akagi M, Yamashita E, Nakagawa T, et al. Relationship between frontal knee alignment and reference axes in the distal femur. Clin Orthop Relat Res 2001;388:147. 16. Griffin FM, Insall JN, Scuderi GR. The posterior condylar angle in osteoarthritic knees. J Arthroplasty 1998; 13:812. 17. Griffin FM, Math K, Scuderi GR, et al. Anatomy of the epicondyles of the distal femur: MRI analysis of normal knees. J Arthroplasty 2000;15:354. 18. Matsuda S, Miura H, Nagamine R, et al. Anatomical analysis of the femoral condyle in normal and osteoarthritic knees. J Orthop Res 2004;22:104. 19. Yoshino N, Takai S, Ohtsuki Y, et al. Computed tomography measurement of the surgical and clinical
Femoral Anterior Tangent Line for Determining Rotational Alignment in TKA Watanabe et al
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transepicondylar axis of the distal femur in osteoarthritic knees. J Arthroplasty 2001;16:493. Arima J, Whiteside LA, McCarthy DS, et al. Femoral rotational alignment, based on the anteroposterior axis, in total knee arthroplasty in a valgus knee. A technical note. J Bone Joint Surg Am 1995;77:1331. Whiteside LA, Arima J. The anteroposterior axis for femoral rotational alignment in valgus total knee arthroplasty. Clin Orthop Relat Res 1995;321:168. Siston RA, Patel JJ, Goodman SB, et al. The variability of femoral rotational alignment in total knee arthroplasty. J Bone Joint Surg Am 2005;2276:87. Olcott CW, Scott RD. A comparison of 4 intraoperative methods to determine femoral component rota-
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tion during total knee arthroplasty. J Arthroplasty 2000; 15:22. 24. Uehara K, Kadoya Y, Kobayashi A, et al. Bone anatomy and rotational alignment in total knee arthroplasty. Clin Orthop Relat Res 2002;402:196. 25. Jerosch J, Peuker E, Philipps B, et al. Interindividual reproducibility in perioperative rotational alignment of femoral components in knee prosthetic surgery using the transepicondylar axis. Knee Surg Sports Traumatol Arthrosc 2002;10:194. 26. Talbot S, Bartlett J. The anterior surface of the femur as a new landmark for femoral component rotation in total knee arthroplasty. Knee Surg Sports Traumatol Arthrosc 2008;16:258.
Femoral Anterior Tangent Line of the Osteoarthritic Knee for Determining Rotational Alignment of the Femoral Component in Total Knee Arthroplasty Hiroki Watanabe, MD, PhD,* Ryuichi Gejo, MD, PhD,* Yoshikazu Matsuda, MD,y Ichiro Tatsumi, MD,y Kazuo Hirakawa, MD, PhD,y and Tomoatsu Kimura, MD, PhD*
Abstract: Several reference axes have been used to establish femoral rotational alignment during total knee arthroplasty. The current study examined the configuration of the anterior surface of the femur immediately proximal to the trochlea as an alternative rotational landmark. An analysis of computed tomographic images of 150 knees with osteoarthritis indicated that the configuration of the surface is mostly flat or slightly depressed, and the line tangential to the surface (femoral anterior tangent line; FAT line) was consistently determined to be 12.2° ± 3.6° internally rotated to the transepicondylar axis. This value was relatively constant and as reliable as the femoral anteroposterior axis for determining rotational alignment. In addition, the FAT line was not affected by the degree of the varus-valgus deformity of the osteoarthritic knees. Keywords: femoral anterior tangent line, rotational alignment, total knee arthroplasty, transepicondylar axis. © 2011 Elsevier Inc. All rights reserved.
The rotational alignment of the femoral component affects patellofemoral and tibiofemoral kinematics and influences the clinical outcome of total knee arthroplasty (TKA). It has been shown that femoral rotational malalignment is a major cause of petellofemoral complications such as anterior knee pain [1] and poor patellar tracking [2-5]. Femoral malalignment also causes flexion gap instability [6], a loss of motion, and early failure of the implant [7]. Several reference axes have been proposed to establish the appropriate rotational alignment of the femoral component, including the transepicondylar axis (TEA), the posterior condylar axis (PCA), and the anteroposterior (AP) axis (Whiteside's line). Anatomical and biomechanical studies have suggested that
From the *Department of Orthopedic Surgery, Faculty of Medicine, University of Toyama, Toyama City, Toyama, Japan; and yDepartment of Orthopedic Surgery, Shonan Kamakura Joint Reconstruction Center, Kamakura City, Kanagawa, Japan. Submitted April 29, 2009; accepted December 9, 2009. All authors declare that they have not received from any organization any personal or financial benefit that could influence to the work published in this article. Reprint requests: Hiroki Watanabe, MD, PhD, Department of Orthopedic Surgery, Faculty of Medicine, University of Toyama, 2630, Sugitani, Toyama City, Toyama, 930-0194, Japan. © 2011 Elsevier Inc. All rights reserved. 0883-5403/2602-0017$36.00/0 doi:10.1016/j.arth.2009.12.011
the TEA approximates the flexion-extension axis of the knee, and it is the most optimal landmark for the femoral component rotation [8-13]. However, the practical use of this axis is sometimes hampered by difficulty in the precise localization of the medial epicondyle and the sulcus during surgery [14]. Limited surgical exposure of minimally invasive TKA further disturbs such identification of the epicondyles and TEA. The PCA and AP axis could be used as an alternative to the TEA. The PCA has been often used as the landmark for the femoral rotational alignment, and many studies have demonstrated that the angle between the TEA and PCA is 3° to 5° [15-19]. The PCA, however, is not a reliable axis in arthritic knees, particularly in valgus knees with bony destruction. Whiteside et al [20,21] proposed the AP axis that was almost perpendicular to the TEA and demonstrated that the axis was not influenced by valgus deformity. Although there are arguments with regard to which axis is most reliable and easily identified during surgery, a single-axis measurement might result in a highly variable rotational alignment in TKA [22]. As an additional intraoperative bony landmark of the distal femur, anterior surface of the femur immediately proximal to the trochlea is observed to be flat in most of the patients during TKA and the surface may be preserved even in severely destroyed knees. We therefore hypothesize that the anterior surface of the
268
Femoral Anterior Tangent Line for Determining Rotational Alignment in TKA Watanabe et al
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Fig. 1. Three types of anterior femoral surface configuration and FAT line. The black lines indicate the FAT line.
femur immediately proximal to the trochlea may be another reliable landmark for determining the proper rotational alignment of the femoral component. The purpose of this study was to analyze the configuration of the anterior surface of the femur immediately proximal to the trochlea using computed tomographic (CT) images and to determine whether the line tangential to the surface (femoral anterior tangent line; FAT line) was comparatively consistent against the reference TEA, irrespective of the varus-valgus deformity.
wear and destruction. The “perpendicular” AP axis was then defined as a line perpendicular to the AP axis. In cases with osteophyte formation in the intercondylar notch, the center of the notch was determined after elimination of these osteophytes on
Materials and Methods The patient population included 135 patients (150 knees) who had unilateral knee arthroplasty or TKA because of osteoarthritis of the knee. The subjects included 19 males and 116 females, and the mean age was 71.9 ± 9.1 years old (mean ± SD; range, 3787 years). The CT data used for this study were taken before unilateral knee arthroplasty or TKA at 2 facilities. A CT scan of the lower extremity was performed in full knee extension at 2-mm or 5-mm intervals, 111 knees for 2-mm (Robusto-Ei, Hitachi Medico, Tokyo, Japan) and 39 knees for 5-mm intervals (Somatom Sensation 16, SIEMENS, München, Germany). Using the lateral scout view, the scans were taken perpendicular to the long axis of the femur. The anterior femoral surface around the upper pole level of patella was examined on the serial CT slices. Next, a CT slice immediately proximal to the femoral trochlea, which showed widest anterior width without trochleal prominence, was selected. The configuration of the anterior femoral surface was examined, and a line tangential to the anterior femoral surface (FAT line) was determined (Fig. 1). The CT slices on which the medial or lateral epicondyles were most prominently detectable were also selected to measure the clinical TEA, the AP axis, and the PCA (Fig. 2). The clinical TEA was defined as a line connecting the lateral and medial epicondylar prominences. The AP axis was defined as a line connecting the deepest point of the patellar groove anteriorly and the center of the intercondylar notch posteriorly and measured in the knees except for 6 knees that showed patellar groove
Fig. 2. Reference axes defined by bony landmarks of the distal femoral condyle and transverse CT slice. The lines indicate the clinical TEA, perpendicular AP axis, and PCA. (A) The anterior femoral surface immediately proximal to the trochlea with the FAT line is superimposed. (B) An image of corresponding position of FAT line and surgical TEA that could be determined at the time of surgery is also shown.
270 The Journal of Arthroplasty Vol. 26 No. 2 February 2011 the images. A line connecting the most posterior points of the medial and lateral posterior condyles was defined as the PCA. The selection of CT slices and the measurement was performed by one of the authors (HW) at 2 independent time points. Axis measurements on the CT images were performed using computer software (Photoshop; Adobe Systems, San Jose, Calif), and absolute values of each axis (FAT line, TEA, AP axis, PCA) were recorded. The intraobserver variation of the measurements was evaluated with intraclass correlation method. Namely, correlation was examined between 2 absolute values provided by 2 independent measurements on the CT slices at different time points. Thereafter, the angles between the FAT line and the TEA, perpendicular AP axis and the TEA, and the PCA and the TEA were calculated. The rotational angle to the TEA was arbitrarily expressed as a positive value for the external rotation and as a negative value for the external rotation. The data are presented as the mean ± SD. In addition, we investigated whether the preoperative femorotibial angle (FTA) correlated with each of the angles and affected their reliability. F tests were used to identify any significant differences in the variability of the respective axis to the referential clinical TEA. The Pearson correlation test was used to analyze the correlation between each angle and the FTA (Stat View 5.0; Abacus Concepts, Berkley, Calif). The level of significance was set at a value of 0.05.
histograms of these 2 angles showed some scattering of these angles, and if a deviation more than 4° from the mean value is defined as an outlier, 28% in FAT line and 22% in perpendicular AP axis would fall in this zone (Fig. 3A, B). The perpendicular AP axis showed less outlier but with more deviation as large as 15°. The F test showed no significant difference in variability between the FAT line and the perpendicular AP axis (P = .85). Therefore, the FAT line seemed to be as reliable as perpendicular AP axis for the femoral rotation in relation to the clinical TEA.
Results Configuration of the Anterior Femoral Surface and FAT Line The anterior femoral surface immediately proximal to the trochlea was readily observed in CT slices, and the configuration was classified into 3 types: type 1 (flat, 100 knees), type 2 (slight central depression, 32 knees), and type 3 (slight convex, 18 knees; Fig. 1). Therefore, most of the knees had flat or slightly depressed anterior surface at this axial section. In types 1 and 2 knees, a line tangential to the anterior femoral surface (FAT line) was easily determined in the CT slices, and therefore, we used 132 knees (88.0%) for the following analysis of axes and angle measurements. Angles of Each Axis in Relation to Clinical TEA The reproducibility of the measurement was evaluated using the intraclass correlation method. The measurements of each axis at 2 independent time points were closely correlated, and the correlation coefficients of the absolute values of each axis were 0.996 for the FAT line, 0.991 for the TEA, 0.997 for the PCA, and 0.973 for the AP axis. The calculated angle between the FAT and clinical TEA was −12.2° ± 3.6°. The angle between the perpendicular AP axis and the clinical TEA was −0.3° ± 3.6°. The
Fig. 3. Distribution of the angles between the FAT line and clinical TEA (A) and between the perpendicular AP axis and clinical TEA (B). The variability of the FAT line and perpendicular AP axis to the clinical TEA was similar (P = .85). The dotted line indicates a mean value.
Femoral Anterior Tangent Line for Determining Rotational Alignment in TKA Watanabe et al
Influence of FTA on the Reliability of the Axis Measurement The influence of varus-valgus deformity on the reliability of each axis for femoral rotation was analyzed. The FTA ranged from 160° to 211° (mean, 185.5° ± 7.4°) in the present patients with osteoarthritis. The angle
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between the FAT line and the clinical TEA showed no correlation to FTA (r = 0.17; P = .06; Fig. 4A), thus, indicating that the FAT line was not affected by the varus-valgus deformity of the knee. Similarly, the angle between the perpendicular AP axis and clinical TEA showed no correlation (r = 0.07; P = .45; Fig. 4B). On the other hand, the angle between the PCA and clinical TEA showed a significant correlation to the FTA (r = 0.31; P = .0001; Fig. 4C), thus, confirming that the PCA may not be a reliable rotational landmark in the knees with a varus-valgus deformity.
Discussion
Fig. 4. The relationships between each of the axes and the FTA. The angle between the FAT line and clinical TEA (A) and between the perpendicular AP axis and clinical TEA (B) have no correlations to the FTA (r = 0.17; P = .06 and r = 0.07; P = .45; respectively). The angle between the PCA and clinical TEA (C) is observed to correlate with the FTA (r = 0.31; P = .0001).
The rotational alignment of the femoral component of the TKA is essential for the optimal patellar tracking, ligament balancing, and functional outcome. The use of a reliable and reproducibly identifiable landmark and axis is important for accurately determining the proper rotational alignment. The current study indicated that the anterior surface of the femur immediately proximal to the trochlea and its tangent line (FAT line) could be used as a good index of the femoral rotation. The configuration of the surface is mostly constant, and the tangent line can be easily determined. Previous studies have demonstrated that the TEA and AP axis are reliable axes for the rotation of the femoral component, and TEA was the most reproducible landmark that produce the best balance [23], whereas the posterior femoral condyles were less reproducible in determining the rotation. In the present study, AP axis against clinical TEA was measured in knees with osteoarthritis. The perpendicular AP axis was relatively invariable and ran parallel (0.3° externally rotated) to the TEA. This is comparable to a previous report [20] and thus confirms the reliability of the AP axis for determining femoral rotation when the clinical TEA is defined as an absolute reference axis. The FAT line we demonstrated in the current study also showed relatively invariable internal rotation to the clinical TEA. Twelve degrees of external rotation to the FAT line during TKA would properly approximate the femoral rotational alignment to the TEA. In addition, FAT line, as well as AP axis, was not affected by the varus-valgus deformity of the osteoarthritic knee. It is possibly because these 2 axes avoid using a distorted condylar and cartilage anatomy for rotational alignment of the femoral component. Therefore, the FAT line located on the femoral surface immediately proximal to the trochlea could become useful even in knees with deformity. Various bony landmarks may not be always identified on x-ray images or at surgery. The clinical TEA could be easily obtained on CT images, whereas the surgical TEA is not readily determined. Previous reports indicated that the identification rate of the medial sulcus using CT ranges from 20% to 74% [15,19,24], which may further
272 The Journal of Arthroplasty Vol. 26 No. 2 February 2011 decline in knees with advanced osteoarthritis. In addition, the debate continues with regard to how accurately the TEA can be located intraoperatively to obtain the best results [14]. The accuracy of the TEA measurement may be hampered by soft tissue coverage or difficulty in accessing the lateral condyle and also by surgeon factors [22,25]. As for the FAT line, it could be easily determined on the CT images in most of the cases. Although our current study focused on the analysis of the presurgical CT images, the FAT line should also be easily located intraoperatively because the anterior femoral surface immediately proximal to the trochlea is seen and palpated without difficulty. Therefore, a surgical jig that uses anterior femoral surface and the FAT line as the reference should become helpful for proper determination of the femoral rotational alignment during TKA. It should be noted, however, measurements of any reliable reference axes, including FAT line, inevitably vary to some extent among individuals. In addition, the FAT line may not be used as a single rotational reference axis in certain cases with slightly convex anterior femoral surface. The composite measurement of plural axes, TEA, AP axis, or FAT line should warrant more appropriate rotational alignment of the femoral component during TKA. Recently, Talbot and Bartlett [26] reported the use of the anterior femoral surface for anatomical reference during navigation surgery. Although the definition of the axis based on the femoral surface was not clearly indicated, they suggested that the axis, 2.9° internally rotated relative to the perpendicular AP axis (Whiteside's line), may be a useful landmark. In our current measurement, the FAT line was 12° internally rotated relative to the clinical TEA and to the perpendicular AP axis. Therefore, the FAT line, a line clearly defined as a tangent line to the femoral anterior surface immediately above the trochlea, was further internally rotated to the axis described by Talbot and Bartlett [26]. This difference may be due to the use of different landmarks of the femoral surface, a different definition of the axis or it may even reflect a racial difference. In either case, the distal anterior surface of the femur is nevertheless considered to be a new and useful landmark. In conclusion, there are various landmarks for the femoral rotational alignment, but a single axis is not always sufficient to precisely determine the rotation due to anatomical variations, poor reproducibility of the axis, and poor visibility of the landmark at surgery. In addition, the loss of bone stock observed at revision surgery further hinders the determination of rotational alignment. The use of an additional landmark of the anterior femoral surface and its tangent line would therefore help to properly determine the femoral rotational alignment both before and during surgery.
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