Erratum to The change in length of the medial and lateral collateral ligaments during in vivo knee flexion

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1 The Knee 13 (2006) Erratum to The change in length of the medial and lateral collateral ligaments during in vivo knee flexion Sang Eun Park a, Louis E. DeFrate a,b, Jeremy F. Suggs a,b, Thomas J. Gill a, Harry E. Rubash a, Guoan Li a, * a Bioengineering Laboratory, Department of Orthopaedic Surgery, Massachusetts General Hospital/Harvard Medical School, 55 Fruit Street, GRJ 1215 Boston, MA 02114, United States b Department of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, MA 02114, United States Received 3 November 2004; received in revised form 18 November 2004; accepted 30 December 2004 Abstract The collateral ligaments of the knee are important in maintaining knee stability. However, little data has been reported on the in vivo function of the collateral ligaments. The objective of this study was to investigate the change in length of different fiber bundles of the medial collateral ligament (MCL), deep fibers of the MCL (DMCL) and the lateral collateral ligament (LCL) during in vivo knee flexion. The knees of five healthy subjects were scanned using magnetic resonance imaging. These images were used to create three-dimensional models of the tibia and femur, including the insertions of the collateral ligaments. The MCL, DMCL, and LCL were each divided into three equal portions: an anterior bundle, a middle bundle and a posterior bundle. Next, the subjects were imaged from two orthogonal directions using fluoroscopy while performing a quasi-static lunge from 0- to 90- of flexion. The models and fluoroscopic images were then used to reproduce the in vivo motion of the knee. From these models, the length of each bundle of each ligament was measured as a function of flexion. The length of the anterior bundle of the MCL did not change significantly with flexion. The length of the posterior bundle of the MCL consistently decreased with flexion ( p <0.05). The change in length of the DMCL with flexion was similar to the trend observed for the MCL. The length of the anterior bundle of the LCL increased with flexion and the length of the posterior bundle decreased with flexion. These data indicate that the collateral ligaments do not elongate uniformly as the knee is flexed, with different bundles becoming taut and slack. These data may help to provide a better understanding of the in vivo function of the collateral ligaments and be used to improve surgical reconstructions of the collateral ligaments. Furthermore, the data suggest that the different roles of various portions of the collateral ligaments along the flexion path should be considered before releasing the collateral ligaments during knee arthroplasty. D 2005 Elsevier B.V. All rights reserved. Keywords: MCL; LCL; Deep MCL; MRI; 3D Modeling 1. Introduction The collateral ligaments play an important role in maintaining knee stability. The medial collateral ligament (MCL) may be injured alone or in combination with the anterior cruciate ligament (ACL) [1 5]. Injury of the lateral collateral ligament (LCL) often accompanies rupture of the DOIs of original articles: /j.knee , / j.knee * Corresponding author. Tel.: ; fax: address: gli1@partners.org (G. Li). posterior cruciate ligament (PCL) [6,7]. During total knee arthroplasty (TKA), the collateral ligaments may be partially sectioned in order to balance the tension in the two ligaments [8 10]. For example, in varus knees, the MCL and deep MCL fibers may be released, while in valgus knees the LCL may be released [8]. Previous studies have investigated the biomechanics of the MCL and LCL under in vitro conditions [4,11 23]. In these studies, the MCL has been shown to resist valgus moments [15,19] and anterior loads beyond 60- of flexion [20]. The MCL has also been thought to stabilize the knee in response to anterior tibial loads after ACL injury [18,20] /$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi: /j.knee

2 78 S.E. Park et al. / The Knee 13 (2006) The LCL is considered a component of the posterolateral complex and has been shown to restrain posterior tibial translation and external and varus tibial rotations especially after PCL injury [16,24]. The function of the MCL and LCL is usually investigated in separate studies [6,12,14,16,21,25 27]. Few studies have divided the MCL into superficial and deep fibers [3,28]. Furthermore, no data has been reported on the biomechanical role of the collateral ligaments during in vivo activities. Whether in vitro experimental data accurately represents in vivo collateral ligament function is unknown. Therefore, the objective of this paper was to investigate the in vivo elongation of the MCL, the deep fibers of the MCL, and the LCL as a function of flexion during a single leg lunge. 2. Materials and methods Five knees (two left and three right) from five young and healthy volunteers (25T5 years old) were imaged using a 1.5 Tesla magnet (GE, Milwaukee, WI) with a surface coil and a FIESTA (Fast Image Employing Steady-state Acquisition) sequence. The magnetic resonance (MR) scan spanned the medial and lateral extremes of the knee and enclosed a cubic volume of approximately 14 cm in each direction. Sagittal plane images at 0.7 mm intervals were acquired with a resolution of pixels. A typical MR image is shown in Fig. 1a. These MR images were used to construct a 3D model of each knee using solid modeling software (Rhinoceros\, McNeel and Associates, Seattle, WA). These models included the bony geometry of the femur and tibia as well as the insertion areas of the collateral Fig. 1. A 3D knee model of a typical specimen (b) created from sagittal plane MR images (a). The insertion areas of the MCL are shown by the shaded regions on the tibia and femur. Fig. 2. Medial (a) and lateral (b) views of a typical knee model at different flexion angles. The lengths of the MCL and LCL bundles changed with flexion. ligaments. Fig. 1b shows the tibiofemoral joint of one subject, including the insertion areas of the MCL, deep fibers of the MCL (DMCL), and LCL. These ligament insertion areas were further verified by comparing them to a classic anatomic study [29]. Next, each subject performed a quasi-static single-leg lunge to 0-, 30-, 60- and 90- of flexion as a 3D fluoroscope (SIREMOBIL Iso-C 3D, Siemens, Germany) was used to acquire two images of the knee from orthogonal directions. These images were recorded from the anteromedial and anterolateral directions. Flexion angle was verified with a goniometer as subjects stood upright on a platform with the C-arm of the fluoroscope positioned in the horizontal plane. Capturing the two orthogonal images at each flexion angle took less than 4 s [30]. This protocol was approved by the Institutional Review Board at Massachusetts General Hospital. These orthogonal images were used to recreate the in vivo knee positions at each of the targeted flexion angles. The fluoroscopic images were imported into the solid modeling software and placed in two orthogonal planes, based on the positions of the C-arm during image acquisition. Two virtual cameras were created to simulate the position of the X-ray source from these two orthogonal directions. The 3D knee model was then imported into the

3 S.E. Park et al. / The Knee 13 (2006) Length (mm) MCL Flexion Angle ( ) Anterior Middle Posterior Fig. 4. The in vivo lengths of the anterior, middle, and posterior fibers of the MCL as a function of flexion during the single leg lunge. Fig. 3. An anterior view of a typical knee model. The MCL length was calculated using two line segments that intersect on the edge of the tibial plateau. The bundle lengths of the LCL were calculated using straight lines. software and viewed from the two cameras. The model was manually manipulated in 6 degrees of freedom until its projection matched that of the two orthogonal X-ray images in order to reproduce the in vivo kinematics of the knee. This technique employing orthogonal images and computer models to measure knee kinematics has been described in detail in previous publications [30 32] and was shown to have an accuracy to within 0.1 mm and 0.1- in determining the relative position and orientation of regularly shaped solid objects [30]. Knee models reproducing the in vivo knee motion of a typical subject are shown in Fig. 2. From these knee models, the relative positions of the MCL and LCL insertion areas on the femur and tibia were determined. The MCL, DMCL and LCL were each separated into three equal portions [28,33]: the anterior bundle, middle bundle, and the posterior bundle. The length of each bundle was defined as the distances between the insertions on the tibia and femur. Since the MCL wraps along the surfaces of the tibia and femur, the length of the bundles of the MCL was defined by two straight lines: one from the femoral insertion to a point on the most prominent edge of the tibial plateau and the other from that point to the tibial insertion (Fig. 3). The point on the edge of the tibial plateau for each MCL bundle was defined as the point resulting in the shortest bundle length [28,33]. A repeated measures ANOVA and the Student Newman Kuels test were used to detect statistically significant differences between bundle lengths at different flexion angles. Statistical significance was set at p < Results 3.1. MCL The length of the anterior bundle of the MCL did not change significantly during flexion from 0- to 90- (Fig. 4). The length of the anterior bundle was 66.8 T4.9 mm at full extension and increased slightly to 68.9T3.3 mm at 90- of flexion. The maximal length of the middle bundle was 76.0 T 4.1 mm at full extension, which was statistically greater than the length measured at all other flexion angles. At 90- of flexion, the middle bundle was 71.1T3.4 mm long, a decrease of 7% relative to its length at full extension. The length of the posterior bundle consistently decreased with increasing flexion ( p <0.05). At full extension, its length was 87.9 T 4.9 mm. The length of the posterior bundle decreased significantly ( p < 0.05) to 80.4T3.8 mm at 30- of flexion, a 10% decrease in length. At 90- of flexion, the length of the posterior bundle decreased to 74.0T4.4 mm, which was 16% less than the length of full extension DMCL No statistically significant change in the length of the anterior bundle of the DMCL was observed with flexion (Fig. 5). The anterior bundle was 24.9T4.8 mm long at full extension and 26.3T4.7 mm at 60- of flexion. The maximum length of 29.2T4.4 mm was observed at 90- of flexion. The middle bundle had a maximal length of 30.1T3.7 mm at full extension. Its length decreased with flexion to 25.0T3.8 mm at 60- of flexion and then increased Length (mm) DMCL Flexion Angle ( ) Anterior Middle Posterior Fig. 5. The in vivo lengths of the anterior, middle, and posterior fibers of the DMCL as a function of flexion during the single leg lunge.

4 80 S.E. Park et al. / The Knee 13 (2006) Length (mm) to 26.3T3.6 mm at 90- of flexion. The posterior bundle had a maximal length of 34.2T3.9 mm at full extension. Its length statistically decreased ( p <0.05) to 26.5 T2.7 mm at 30- of flexion. Beyond 30-, the bundle length did not change significantly. The length was 22.9 T 3.9 mm and 22.8T3.1 mm at 60- and 90- of flexion, respectively LCL At full extension, the anterior bundle of the LCL was 54.1 T 6.6 mm (Fig. 6). At 90- of flexion, the length statistically increased ( p < 0.05) to 57.9 T 6.9 mm, a 7% increase compared to its length at full extension. The length of the middle bundle was relatively constant along the flexion path. At full extension, the bundle was 53.6 T7.1 mm long and was 52.7T6.7 mm long at 90- of flexion. The posterior bundle had a maximum length of 53.1T6.6 mm at full extension. It significantly decreased to 48.8 T 5.8 mm at 30- of flexion ( p <0.05). The length did not change significantly with further flexion of the knee. At 90- of flexion, the posterior bundle had a length of 47.7T6.1 mm. 4. Discussion LCL Flexion Angle ( ) Anterior Middle Posterior Fig. 6. The in vivo lengths of the anterior, middle, and posterior fibers of the LCL as a function of flexion during the single leg lunge. The collateral ligaments are frequently injured in sports activities [1,3 7]. In total knee arthroplasty, the collateral ligaments are partially sectioned in an effort to restore normal ligament tension. Engh reported that deep and superficial medial collateral ligament releases have been necessary in more than 50% of TKA patients with angular deformity [8]. Mihalko and Whiteside suggested that releasing a contracted collateral ligament is likely to improve function in patients with flexion contractures [9]. Many studies measured strains in the MCL using cadaveric knee specimens [4,11 14,26]. In vitro MCL tension has also been measured under anterior posterior tibial loads and tibial torques[2,18-20]. Fewer studies have investigated the function of the LCL [2,13,16,18,20,24]. However, the in vivo response of the collateral ligaments during active knee motion has not been quantified. This information is critical to the treatment of collateral ligament injuries and may provide important information for the balancing of soft tissues during total knee arthroplasty. This paper reported the in vivo elongation of the MCL and LCL during a single leg lunge in five living knees. The length of three functional bundles of the collateral ligaments was quantified from 0- to 90- of flexion. In the MCL, the length of the anterior bundle was relatively constant, increasing only slightly with flexion (Fig. 4). However, the length of the middle and posterior bundles were maximal at low flexion angles. The length of posterior bundle decreased by 16% when flexed to 90- from full extension, suggesting that the posterior bundle of the MCL might be tight in full extension. In vitro experiments using cadaveric knees have shown that the in situ forces in the MCL in response to the 134-N anterior tibial load did not change dramatically from 0- to 90- of flexion [18]. In vitro strain measurement also found that the MCL was strained similarly at all of the tested flexion angles [13]. Other studies have shown that the strain in the posterior and central portions of the medial collateral ligament generally decreased with increasing flexion, whereas strain in the anterior fibers remained relatively constant [26]. Hull et al. found that the posterior superior region of the MCL experienced significantly greater strain than other regions at 0- of flexion. However, at 30- of flexion the anterior superior region experienced significantly greater strain than the other regions [25]. Even though a direct comparison of our data and the data in the literature is difficult due to the differences between in vivo and in vitro loading conditions, similar trends on the function of the MCL were observed. These data indicate that the different bundles of the MCL are not loaded uniformly throughout the flexion range considered in this study, with the length of the middle and posterior bundles decreasing and the anterior bundle increasing slightly with increasing flexion. Little data has been reported on the function of the deep fibers of the MCL. In biomechanical studies, the deep fibers are not usually considered separately from the overall function of the MCL [28,33,34]. Our data indicated that the length of the anterior bundle increased slightly as flexion increased, with maximal elongation at high flexion. The middle and posterior bundles decreased in length with flexion and were longest at full extension. The DMCL fiber bundles demonstrated a reciprocal function, with different regions of the DMCL functioning as the knee was flexed from 0- to 90-. Even though the DMCL is often sectioned during knee arthroplasty, few studies have considered its role after knee replacement. The data from this study suggest that cutting the DMCL might increase knee laxity from 0- to 90- of flexion because different regions of the DMCL are loaded throughout this flexion range. Current knee arthroplasty designs have not considered the function of the DMCL. Additional studies may be necessary in order to better understand the biomechanical role of the DMCL in the intact knee and the effects of

5 S.E. Park et al. / The Knee 13 (2006) sectioning the DMCL on knee biomechanics after arthroplasty. These data may be used in order to improve knee arthroplasty surgery and the design of prostheses. The change in length of the bundles of the LCL was somewhat different from the trends observed in both the MCL and DMCL. The anterior bundle consistently increased in length with flexion. The length of the middle bundle did not dramatically change with flexion, which may indicate that the insertions of the middle bundle are close to the isometric fiber location. The posterior bundle was longest at full extension and decreased by 8% at 30- of flexion, while the length of the anterior bundle increased with flexion. These data suggest a reciprocal function of the anterior and posterior bundles during flexion. Based on these data, the authors believe that different regions of the LCL are loaded throughout the flexion range considered in this study. Previous in vitro studies have measured the LCL function under anterior posterior tibial load and axial tibial rotation torques [2,13,16,18,20,24]. These in vitro data indicate that the LCL plays a significant role in restraining tibial rotation [2,13,24] and acts as a secondary support to the PCL under posterior tibial loads [16]. A relatively constant force in the LCL was measured under an anterior tibial load of 134 N from 0- to 90- of flexion, although the force was below 20 N [18]. Further investigation is needed to estimate the in vivo tension of the LCL. This study only examined the elongation of the bundles of the collateral ligaments and the deep fibers of the MCL. While these data may help to explain the biomechanical role of the ligaments during in vivo knee flexion, the actual magnitudes of the forces of the ligaments are still unknown. Future studies might also focus on studying the MCL deficient or LCL deficient knees, in order to provide a better understanding of the effects of collateral ligament injury on in vivo knee function. In summary, these data on the in vivo elongations of the MCL and LCL indicate that the collateral ligaments may be loaded non-uniformly throughout the flexion path of the knee, with the length of some regions increasing with flexion, while other regions decrease. The large variation in the length of the bundles of the DMCL indicated that the DMCL might also play an important role during in vivo knee flexion. These data might provide important insight for the surgical treatment of combined ligament injuries, such as combined ACL and MCL injuries and combined PCL and posterolateral structure injuries. The data also imply that the complex function of the collateral ligaments should be considered when releasing the collateral ligaments during knee arthroplasty. 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