Tension Changes Within the Bundles of Anatomic Double-Bundle Anterior Cruciate Ligament Reconstruction at Different Knee Flexion Angles: A Study Using a 3-Dimensional Finite Element Model Heon Young Kim, Ph.D., Young-Jin Seo, M.D., Hak-Jin Kim, M.S., Trung Nguyenn, Ph.D., Nagraj S. Shetty, M.S., and Yon-Sik Yoo, M.D., Ph.D. Purpose: The aim of this study was to determine the change in length and tension of the reconstructed anterior cruciate ligament (ACL) double bundles at different knee flexion angles by use of a 3-dimensional finite element model. Methods: The right knees of 12 living subjects were scanned with a high-resolution computed tomography scanner at 0°, 45°, 90°, and 135° of knee flexion. Several modeling programs were used to simulate double-bundle ACL reconstruction. A finite element model of each bundle with a tension of 20 N was put into each tunnel followed by fixation of the bundles. The tension and length changes of each bundle at different knee flexion angles were assessed. Results: For the anteromedial bundle, the length decreased gradually between 45° and 90° of knee flexion and then reached a plateau, whereas the length of the posterolateral bundle significantly decreased at 45° and 90° of flexion but then increased at full flexion. The reaction force of the anteromedial graft slightly decreased at 45° and then remained constant between 90° and 135° of knee flexion. The reaction force of the posterolateral bundle at full extension slightly decreased at 45° and 90° of flexion, followed by a rebound increase at 135°. Conclusions: We found that both bundles functioned throughout the arc of flexion with consistency in tension, although their lengths decreased. The 2 ACL grafts did not function in a reciprocal manner, unlike previous descriptions. Clinical Relevance: The data obtained for length and tension versus flexion angle have the potential to suggest the appropriate knee position for graft fixation and tensioning to be near extension in clinical situations. T From the Department of Mechanical and Biomedical Engineering, Kangwon National University, Chuncheon, Republic of Korea; and (H.Y.K., H-J.K., T.N.), Department of Orthopaedic Surgery, Hallym University (Y-J.S., N.S.S., Y-S.Y.), Chuncheon, Republic of Korea. Presented at the 2010 Congress of the Arthroscopy Association of North America, May 2010, Hollywood, FL. Supported by the National Research Foundation of Korea (Grant #2010-0005967). The authors report no conflict of interest. Received June 7, 2010; accepted May 13, 2011. Address correspondence to Yon-Sik Yoo, M.D., Department of Orthopaedic Surgery, College of Medicine, Hallym University, 153 Gyo-Dongl, Chuncheon, Gangwon-do, Republic of Korea. E-mail: [email protected] © 2011 by the Arthroscopy Association of North America 0749-8063/10345/$36.00 doi:10.1016/j.arthro.2011.05.012 he anterior cruciate ligament (ACL) is one of the most frequently injured ligaments of the knee. The ACL consists of 2 bundles: a slightly larger anteromedial (AM) bundle and a posterolateral (PL) bundle, named according to their relative tibial insertion sites. Recently, reconstruction techniques have focused on the anatomic and double-bundle method because each bundle (AM and PL) has been found to have a different length, width, area, and function.1,2 Knowledge of tension changes within the ligaments over the knee range of motion will contribute to a better understanding of knee function and serve as a useful basis for an improved anatomic ACL reconstruction. In general, the studies regarding ACL tension change have been performed using cadavers.3,4 However, recent in vivo data counter the notion about the reciprocal relation between the bundles and advo- Arthroscopy: The Journal of Arthroscopic and Related Surgery, Vol xx, No x (Month), 2011: pp xxx 1 2 H. Y. KIM ET AL. cate that both bundles shorten with flexion.5,6 A few studies have reported on the ligament tension that might be generated as a result of the interaction and contact between the 2 bundles.7-11 Therefore we conducted a finite element analysis of a 3-dimensional (3D) knee model to measure the tension changes within the 2 grafts created after the standard doublebundle ACL reconstruction procedure. The computational procedures were used to build a full-form computer-aided design (CAD) model from computed tomography (CT) images, to simulate tensioning in the initial extension state, and then to analyze the behavior of the ligaments at different knee flexion angles. The purpose of this study was to determine the change in length and tension of the reconstructed ACL double bundles at different knee flexion angles using a 3D finite element model. The hypothesis to be tested is that the tension of the AM and PL grafts may remain relatively consistent, despite substantial change in length, at different knee flexion angles after ACL reconstruction. METHODS Computational Procedures A 3D CT image– based analysis study was conducted in 12 subjects (all men) with no history of knee pathology or previous injury. We excluded subjects who had any form of arthritis, infection, meniscal injury, or previous surgery on the same knee. The median age of the subjects at the time of the study was 29.4 ⫾ 5.3 years (range, 21 to 39 years). The study was approved by our institutional review board, and informed consent was obtained from all subjects. The right knee of each subject was scanned with a highresolution CT scanner (SOMATOM Sensation; Siemens, Erlangen, Germany) with 1-mm slices taken at 0° and 3 different knee flexion angles (45°, 90°, and 135°) in the lateral decubitus position. The DICOM (Digital Imaging and Communications in Medicine) files obtained were imported into visualization software (Amira R 4.0; Mercury Computer Systems, Chelmsford, MA) to construct virtual 3D models of 12 knees in total. These 3D images were then imported to validated customized software (Rapidform 2006; Rapidform, Seoul, South Korea) to analyze spatial relations between anatomic structures followed by export of the 3D images to CAD software (Catia V5; Dassault Systemes, Vélizy-Villacoublay, France). The final CAD model was constructed by drilling tunnels for the grafts. This converted CAD model was then exported to the special-purpose finite element preprocessor HyperWorks (Altair Engineering, Troy, MI), in which the final computational model was constructed for virtual analysis. Finite element analysis was performed with Abaqus/Explicit code (Simulia, Providence, RI) in which the femur and tibia were modeled as rigid bodies because the bony structure stiffness is much higher than that of the soft tissues. Insertion Site Identification The anatomy of the ACL insertion site has been extensively studied, and the lateral intercondylar and bifurcate ridges have been highlighted as key structures to delineate the ACL footprint.12,13 The tibial footprint has also been investigated, and the bony geometry has been described.14 These geometric data were usually achieved by use of CT because it includes irrefutable bony landmarks and reduces subjective evaluation.12,14 On the basis of these observations, the centers of the AM and PL footprint were established on the 3D model of the tibia. On the femoral side, these 2 points were also established. In some subjects the identification of the tibial bony landmarks was not possible; in such cases the subject’s knee model was matched to available cadaveric knee models that were similar in size to the subject’s knee, and the tibial points were obtained in the same manner as described earlier. Femoral bony landmarks were identifiable in all knees. Material Modeling of Reconstructed ACL The femur and tibia were assumed to be rigid, whereas the reconstructed ACL comprised hyperelastic rubberlike material. A hyperelastic model is generally used in engineering to represent large, incompressible deformation. The model is characterized by a strain energy potential function, represented as equations.15 Simulation of Knee Flexion To simulate a double-bundle ACL reconstruction, in the 0° analytic model, four 7-mm-diameter tunnels were drilled at the center of each AM and PL footprint on the femur and tibia, leaving a bone bridge that is 1.5 mm thick on average between the 2 tunnels. The simulation process was performed with Catia V5 (Fig 1). The AM and PL grafts under tension of 20 N were put in each tunnel, and the grafts were fixed at the middle of each tunnel. The grafts were bonded to the tunnel by use of mesh tie kinematic constraints. The TENSION CHANGES IN RECONSTRUCTED ACL 3 FIGURE 1. Construction of tunnels and ligaments. Seven-millimeter-diameter bone tunnels were created at the center of each AM and PL footprint (left, center), and the 2 grafts corresponding to the diameter of each tunnel were inserted (right). bone-ligament and ligament-ligament contacts were modeled through the penalty formulation assuming frictional coefficients of 0.1 and 0.001, respectively.16 Flexion in the reconstructed knee was simulated in 2 steps. (1) The tibia of the reconstructed knee is superimposed at 0° onto a discrete tibia at 45°, 90°, and 135° of knee flexion by use of the positional information of the coordinates. (2) The accuracy of the position and orientation of the femur is determined in space at 45°, 90°, and 135° (Fig 2). As the software creates a 3D coordinate system specific for each knee with 6 dof (the x-, y-, and z-axes and pitch, roll, and yaw), the precise position and orientation of the femur at 45°, 90°, and 135° can be achieved by translating and rotating the femur along the axes. The position and orientation were both recorded to the nearest 0.01 mm. were determined alternatively through reaction force calculations at both ends of the bundle insertion sites by the finite element models. By balancing with the reaction forces acting at the constrained locations, the tensions in grafts were determined for 2 different Measurement Method The digital length of the 2 virtual bundles (AM and PL) was measured from the center of the AM and PL footprints of the femur to the center of the AM and PL footprints of the tibia with validated customized software (Rapidform 2006); these measurements had an accuracy of 0.1 mm. To minimize technical error of measurement, the center of the footprints at 0° of knee flexion was premarked before the superimposing process at 45°, 90°, and 135° of knee flexion. To decrease the error, the tunnels in the tibia and femur were placed by an expert knee arthroscopy surgeon. The digital length was then measured based on pre-marking on the footprint with 0°, superimposed 45°, 90°, and 135° of the knee model (Fig 3A). The tensions within the ACL grafts at different knee flexion angles FIGURE 2. Reconstructed knee models at different angles (0°, 45°, 90°, and 135°). The simulation of the knee flexion was conducted by superimposing the tibia of the model with 0° of flexion onto a tibia of the reconstructed model with 45°, 90°, and 135° of flexion, then moving the femur from the model with 0° of flexion to a discrete flexed position. This process enabled us to obtain more accurate length data than those of our previous method, which had been substantially affected by interobserver or intraobserver variability. AQ: 1 4 H. Y. KIM ET AL. FIGURE 3. (A) Length change (in centimeters) of virtual straight AM and PL grafts at knee flexion angles of 0°, 45°, 90°, and 135° (mean ⫾ standard error). The AM and PL length decreased with flexion (#P ⫽ .061, *P ⫽ .001). Also shown is an illustration of measuring the digital length of the virtual straight grafts. (B) Patterns of reaction force changes (in newtons) of the AM and PL grafts at knee flexion angles of 0°, 45°, 90°, and 135° (mean ⫾ standard error). The measuring point of the reaction forces at the tibial fixation site is also illustrated. The patterns of the reaction forces of both the AM and PL grafts were relatively constant compared with length patterns between full extension and discrete knee flexion angles (#P ⫽ .36, *P ⫽ .013, and *P ⫽ .012 for AM graft; †P ⫽ .08, *P ⫽ .044, and ‡P ⫽ .28 for PL graft). Furthermore, both the AM and PL grafts have their greatest length and reaction force at full extension. P values represent level of significance between 0° and each angle of flexion. *Statistically significant. segments. Tension in the upper segment was in equilibrium with the reaction force at the insertion site on the femur, whereas the reaction force on the tibia gave information on tension in the lower segments of grafts. The reaction force of grafts at different knee flexion angles was measured at each tibial fixation point (Fig 3B). The reaction forces can be used to evaluate the amount of graft tension. The contacts between the AM and PL bundles and between the ligaments and surrounding bony structure were also examined. Statistical Analysis The differences in length and tension within each bundle at the 4 different angles were analyzed statis- tically by 1-way analysis of variance, with the Tukey honestly significant difference test for pair-wise comparisons. The significance level was set at P ⬍ .05. RESULTS Both the AM and PL bundles were longest at full extension (3.82 ⫾ 0.18 cm and 2.89 ⫾ 0.14 cm, respectively). For the AM bundle, the length decreased gradually between 45° and 90° of knee flexion (3.50 ⫾ 0.14 cm and 3.39 ⫾ 0.13 cm, respectively) and then reached a plateau (3.40 ⫾ 0.13 cm); while, the length of the PL bundle significantly decreased at 45° and 90° of flexion (2.32 ⫾ 0.12 cm and 2.18 ⫾ 0.16 cm, respectively) but then increased at full flex- TENSION CHANGES IN RECONSTRUCTED ACL ion (2.48 ⫾ 0.12 cm). A statistical difference was found in the AM bundle between full extension and more than 90° of knee flexion (P ⬍ .001) and in the PL bundle between full extension and more than 45° of knee flexion (P ⬍ .001). The reaction force of the AM graft with a tension of 20 N at full extension slightly decreased at 45° and then remained constant between 90° and 135° of knee flexion. A statistical difference was detected in the AM bundle between full extension and more than 90° of knee flexion (P ⫽ .013 and P ⫽ .012, respectively); the reaction force of the PL bundle at full extension slightly decreased at 45° and 90° of flexion, followed by a rebound increase at 135°. A statistical difference was observed in the PL bundle between full extension and 90° of knee flexion (P ⫽ .044) (Fig 3). The pattern of contact stress in the different angles of the knee is shown in Fig 4. The contact stresses between 2 bundles were close to 0 MPa at extension and 45° of knee flexion but then increased at 90° and 135° of knee flexion (Fig 4A). The maximum stresses 5 between bone and bundles at full extension were monitored in the lateral portion of the bundles near the femoral tunnel, where the measured stress in the AM bundle was 11.3 ⫾ 2.2 MPa and the measured stress in the PL bundle was 7.0 ⫾ 3.1 MPa. With the flexion progressed, the portion of highest contact stresses was moved to the distal end of the bundles near the orifice of the tibial tunnels by way of the midportion of the bundle around the lateral intercondylar tubercle, where the measured stresses in the AM and PL bundles were 3.5 ⫾ 3.6 MPa and 9.6 ⫾ 4.1 MPa, respectively (Fig 4B). The fringes of distributed contact sites and contact stresses at each knee flexion angle are given in Fig 5. DISCUSSION We plotted the length-versus-flexion and tensionversus-flexion curves between 0°, 45°, 90°, and 135° of flexion using a 3D finite element knee model and generating virtual grafts. The most important findings FIGURE 4. (A) Contact stress pattern between bundle and bundle. Interbundle impingement just occurred at 90° and 135° with the change in alignment of the AM and PL femoral tunnel, which allows AM and PL grafts to form a crossing pattern. (B) Contact stress pattern between bundle and bone. The contact stresses of the femur/AM bundle and femur/PL bundle were highest at full extension and gradually decreased with increasing flexion, reaching 0 MPa at 135° because of increments in the femoral tunnel– graft angle. At 135°, the contact stresses of both bundles were mainly localized on the tibial side. At 90°, the contact stress of the tibia/PL bundle was significantly higher than that of the femur/PL bundle, which indicated that regaining the tension within the PL bundle at this flexion range was largely due to interaction between the PL bundle and the tibial bone. Meanwhile, the contact stresses of the tibia/AM bundle were relatively constant throughout the entire arc of flexion because of minimal change in tibial tunnel– graft angle. The contact stresses of the tibia/PL bundle were slightly lower than those of the femur/AM bundle at 90°, implying that both the femoral and tibial contact stresses could contribute to regaining the tension within the AM bundle at this angle. 6 H. Y. KIM ET AL. FIGURE 5. Stress distribution within AM and PL bundles at different knee angles in a representative case. The contour shows the level of stress on the grafts. At 0° and 45° of flexion (left 2 figures, posterior view), most of the contact force was generated at the sharp edge of the anterior margin of the femoral tunnel. At 90° and 135° of flexion (right 2 figures, anterior view), contact force caused by both grafts being deformed and rerouted by the lateral intercondylar tubercle was noted, which may be an important factor in maintaining tension within both grafts despite a decrease in direct length at deep flexion. of this study were that both grafts achieved their greatest length and tension at full extension. Being greatest at full extension, the length of the AM graft was decreased gradually with knee flexion between 45° and 90° and increased thereafter. Being also greatest at full extension, the length of the PL graft dropped suddenly with 45° of knee flexion and reached a plateau between 45° and 90°; a rebound increase in length was seen thereafter. These results corroborate the findings of our previous in vivo study,17 showing a significant reduction in the length of both the AM and PL bundles with increasing knee flexion. In fact, the measurement process in this study enabled us to obtain more accurate length data than those from our previous method, which had been substantially affected by interobserver or intraobserver variability. To enhance the accuracy of the length data in this study, we established the center of the footprint only on the 0° analytic model. By superimposing the tibia of the model with 0° of flexion onto the tibia of the model with a different degree of flexion, followed by moving the femur from the model with 0° of flexion to a discrete flexed position as per the coordinates, the digital length can be obtained with high reliability based on the marking on the center of the footprint with 0° of knee flexion; one may then proceed with measurements at each superimposed 45°, 90° and 135° of knee flexion model. The tension of both bundles, which is represented as the reaction force, was relatively consistent at all flexion angles, although the patterns of the curves were similar to those of the length curves. Being 20 N at full extension, the reaction force of the AM graft decreased slightly at 45° and then continued to be maintained at 90° and 135° of knee flexion. Being greatest at full extension, the reaction force of the PL graft showed a pattern similar to that of the AM graft and it decreased slightly between 45° and 90° and then increased again at full flexion. These data would therefore suggest that although the direct length between the femoral and tibial footprint decreases with increasing flexion of the knee, both the AM and PL grafts maintain relative constant tensile properties throughout the range of movement. The reaction forces of the bundles seen in our study had patterns similar to those reported by Yasuda et al.,18 and Vercillo et al.19 constructed the tensionversus-flexion curves of the 2 bundles and showed that the tension of both bundles was greatest at full extension. However, the decrease in reaction force with flexion of the PL graft18 was not as marked as that observed in our study. Vercillo et al. reported that in TENSION CHANGES IN RECONSTRUCTED ACL situ forces of the AM and PL grafts were greatest at 45° and 15° of knee flexion, respectively. Contrary to current knowledge, our observation is that this disparity between the length-versus-flexion and tension-versus-flexion curves might be attributable to another factor that generates tension without an actual increase in length. The contact, friction, and deformation caused by the graft impingement against surrounding bone at different knee flexion angles might play the role of transferring the force from the graft to the bone and thus had a direct effect on the tension in grafts. In other words, the actual deformation caused by the interaction between the 2 grafts and the impingement between the grafts and surrounding bony structures may play an important role in maintaining the tension within the grafts without an actual increase in length. As flexion increases, there would be more contact between the grafts and the bone during knee flexion. The possible interaction between the AM and PL bundles, as well as the frictional contact caused by the grafts wrapping around the bone for different knee flexion angles, is shown in Fig 4. Depending on the knee flexion, several potential contact sites are triggered and thus the size of contact areas was determined numerically by the finite element tools. Potential contact sites include AM-to-PL, AM-to-femur, AM-to-tibia (at tunnel edges and at anterior intertubercular ridge), PL-to-femur, and PLto-tibia (at tunnel edges and at intercondylar tubercle) contact areas. Among different knee flexion angles, the local contact stress became highest when the graft wrapped over the sharp edges of the tunnels and was bent to make an acute angle. However, it is uncertain whether the intensity of contact stress measured in our experimental finite element graft could represent the absolute value occurring by real graft interaction because we adopted graft having hyperelastic, incompressible, and isotropic properties. In fact, the behavior of tissues can effectively be represented by a hyperelastic material with an assumption of a negligible time- and rate-dependent effect in the preconditioned state.20-22 Hyperelasticity has been suitably used as an ideal framework for numeric simulation of ligaments because of its capability of describing large deformation.22 Meanwhile, the anisotropy should be taken into account because the ACL is a dense connective tissue consisting of mainly parallel collagen fibers embedded in a ground substance matrix of proteoglycans, glycolipids, water, and so on. However, the anisotropic mechanical properties of the ACL grafts were not available because of difficulties in measurement. Hence, to accommodate and make use 7 of the available experimental data of tensile tests,6,20 we adopted an isotropic hyperelastic material model. Besides the merits of simplicity, selection of such a model was decided by consideration of the predominant tension loading condition of the grafts in our study. According to our simulation of knee flexion, the AM and PL bundles were compressed and rerouted by the surrounding bony structures as flexion progressed. At 0° of flexion, the AM and PL bundles were initially deformed with a tension of 20 N by being compressed against the inner portion of the lateral femoral condyle. When flexion increased to 45°, both bundles were still compressed by the lateral femoral condyle. At 90°, the PL bundle started to wrap around the lateral intercondylar tubercle of the tibia and bent over it, losing contact with the inner portion of the lateral femoral condyle. This phenomenon was seen more intensely with the progression of knee flexion. These findings would suggest that ligament tension is maintained despite a decrease in direct length with flexion by increasing the contact area, as well as contact force. These data further validate our observation that the role of both the PL and AM bundles is not diminished with increasing flexion. As evident from Fig 5, the regaining of tension in the PL bundle at high flexion may be attributable to contact, friction, and deformation caused by impingement mainly against the lateral intercondylar tubercle of the tibia, which enables it to restore practical bundle length. Song et al.23 noted the importance of contact and friction forces occurring as a result of the interaction between the grafts themselves and the bony structures. According to their study, the stress distribution of the ACL might be influenced by the intensity of contact and friction forces. They stated that the AM bundle had more contact with bone under an anterior tibial load at full extension. A similar contact pattern was observed in our extension knee models. At 0° and 135°, the contact stresses generated by interaction between grafts and bone were higher than those at 45° and 90°. Interbundle impingement just occurred at 90° and 135°, although the intensity of stresses was negligible. One interesting finding of our study is that the lateral intercondylar tubercle of the tibia plays a major role in preserving the tension of the PL bundle in mid to high flexion, as evident from the surface stress parameters and gross findings. In addition, the inner portion of the lateral femoral condyle has a similar role in maintaining the tension of both bundles at low 8 H. Y. KIM ET AL. flexion. In our opinion, our findings are highly reliable, because we simulated knee flexion models and grafts similar to an actual reconstructed joint by tensioning the ligaments in full extension and then moving the joint to a discrete location as per the coordinates. So the same tensioned ligament was studied at all positions of the joint, rather than by creating the ligament individually at each joint position. On the basis of the data obtained from our study, we postulate that both the bundles act in coordination rather than reciprocally and the knee position near full extension would be appropriate for graft fixation. On the other hand, if the graft is secured at a high flexion angle, the graft would be straight by tensioning during final fixation and remain straight throughout the entire flexion arc. For this reason, the fixation angle can be recommended at the extension position to accomplish the graft-bone relation during flexion progression. We believe graft impingement might be 1 of the important factors that can induce normal knee kinematics if the graft is strong enough. This study has some limitations. First, we considered both ligaments as incompressible hyperelastic and isotropic materials, which is not feasible in a normal knee. In particular, the incompressibility assumption may become imprecise when the water in tissues is expelled; however, there is actually no clear experimental evidence showing that the amount of expelled water during the traction test is important enough to jeopardize the incompressibility hypothesis.22,24-26 Second, the stress pattern was only considered at discrete flexion angles, rather than as a continuum. Third, we used just 1 tendon diameter regardless of the size of the knee, which can induce excess friction between bundles or between the bundle and the surrounding bony structure in case of a small knee. Proper matching of the size of the graft with knee size may be included in a future study. In our study no data were collected under anterior tibial loads or combined rotatory loads. A better understanding of the strain patterns and behavior of each graft might be obtained in the future if studied under the influence of an external force. Finally, the 3D modeling of the bone geometry from the CT scan taken at axial planes and 1.0-mm intervals does not express the cartilaginous components, which may induce measurement errors. Therefore the use of CT rather than magnetic resonance imaging in this study can be a limitation. However, CT is advantageous over magnetic resonance imaging in constructing the concise portion of the knee such as the surrounding ridges on the ACL footprint in both the femur and the tibia. Furthermore, in fact, the cartilage thickness would be negligible on the lateral wall of the femur or lateral intercondylar tubercle of the tibia. Thus we believe that the measurement errors originating from the use of CT are not crucial in our study. CONCLUSIONS We found that both bundles functioned with consistent tension throughout the arc of flexion, although their lengths decreased. The 2 ACL grafts did not function in a reciprocal manner, unlike previous studies. REFERENCES 1. 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