Original Article
Influence of Posterior Cruciate Ligament Tension on Knee Kinematics and Kinetics Muhammad Shoifi Abubakar, MD1 Shinichiro Nakamura, MD, PhD1 Shinichi Kuriyama, MD, PhD1 Hiromu Ito, MD, PhD1 Masahiro Ishikawa, MD, PhD1 Moritoshi Furu, MD, PhD1 Yoshihisa Tanaka, MD1 Shuichi Matsuda, MD, PhD1
J Knee Surg
Abstract
Keywords
► posterior cruciate ligament ► total knee arthroplasty ► cruciate retaining ► knee kinematics ► knee kinetics
Address for correspondence Shinichiro Nakamura, MD, PhD, Department of Orthopaedic Surgery, Kyoto University, Shogoin Kawaharacho 54 Sakyo-ku, Kyoto 6068507, Japan (e-mail: [email protected]).
The posterior cruciate ligament (PCL) has an important role in cruciate-retaining total knee arthroplasty to achieve good clinical results. The purpose of the study was to examine the influence of PCL tension on knee kinematics and kinetics and to propose an indicator for proper PCL tension during surgery. A squatting activity was simulated in a weight-bearing deep knee bend using a musculoskeletal computer simulation knee model. The length of the PCL was changed to represent different PCL tension models. The amount of PCL tension significantly influenced knee kinematics and kinetics. In the normal PCL model, the facet center positions at 90 degrees of knee flexion were positioned at almost the same position as in full extension. A loose PCL-induced paradoxical anterior movement and greater patellofemoral forces, whereas a tight PCL was related to excessive rollback and increased tibiofemoral forces. This study suggested ideal knee kinematics with proper PCL tension, in which the medial contact position at full flexion was almost similar to the position at 90 degrees of knee flexion.
The posterior cruciate ligament (PCL) has an important role in cruciate-retaining (CR) total knee arthroplasty (TKA) to achieve good clinical results. The condition of the PCL affects postoperative range of motion, stability, forces at the bone– prostheses interface, femoral rollback phenomenon, gait pattern, wear, and proprioception.1 Several studies found that the PCL has important functions and that surgeons have difficulties in adequately adjusting the PCL tension in CR TKA.2,3 A loose PCL might result in knee instability and pain, whereas excessive PCL tension might be associated with restriction of flexion and could lead to high stress and subsequent polyethylene wear.2,3 Reproduction of normal knee kinematics after TKA is important for better range of motion and knee function.4,5 In previous kinematic studies during a deep knee bend, paradoxical anterior femoral translation was frequently observed in CR TKA, compared with posterior stabilizing TKA.6,7 The PCL acts as the primary restraint against poste-
rior translation of the tibia and induces posterior femoral rollback, particularly at 90 degrees and more of knee flexion.2 The femoral rollback mechanism is an important phenomenon during deep knee flexion. In a previous study, using an electromagnetic device to track knee motion, implanted knees with PCL retention showed similar motion to normal knees.8 This result would imply that rollback does occur with a CR implant, and PCL tension during CR TKA will influence the femoral rollback mechanism. The previous studies have shown that femoral rollback has significant influences on patellofemoral (PF) contact forces and tibiofemoral contact forces.1 Increasing femoral rollback will have two primary effects. First, the angle between the quadriceps and patellar tendon lines will decrease because of the posterior translation of the femoral component. Second, the force to the patellar tendon will decrease owing to a decreased angle between the quadriceps and patellar tendon. In contrast,
received August 31, 2015 accepted after revision December 27, 2015
Copyright © by Thieme Medical Publishers, Inc., 333 Seventh Avenue, New York, NY 10001, USA. Tel: +1(212) 584-4662.
DOI http://dx.doi.org/ 10.1055/s-0036-1571803. ISSN 1538-8506.
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1 Department of Orthopaedic Surgery, Kyoto University, Kyoto, Japan
decreasing femoral rollback will lead to higher force of the patellar tendon and higher PF contact force. In the current study, PCL tension was changed in the computer simulation model to analyze its effects on knee kinematics and kinetics. The purpose of the study was to examine the influence of PCL tension on knee kinematics and tibiofemoral and PF contact forces, and to propose an indicator for proper PCL tension during surgery. We hypothesize that a loose PCL will cause the femur to slide anteriorly, which might increase PF contact forces. Likewise, we think a tight PCL will induce posterior femoral rollback, which might increase tibiofemoral contact forces and PCL tension.
Materials and Methods The musculoskeletal computer simulation knee model (LifeMOD/KneeSIM 2010; LifeModeler Inc., San Clemente, CA) was used in the current study. This simulation model consisted of a dynamic musculoskeletal modeling program of the knee (►Fig. 1). In previous biomechanical studies, this simulation program has been validated to secure the appropriate estimation of contact points and contact forces.9,10 A squatting activity in a weight-bearing deep knee bend was simulated using an Oxfordtype knee rig. In this activity, the knee model was flexed from full extension to 120 degrees of knee flexion and then extended to full extension. This musculoskeletal model included the tibiofemoral and PF contact, PCL, fibular collateral ligament (FCL), medial collateral ligament (MCL), elements of the knee capsule, quadriceps muscle and tendon, patellar tendon, and hamstring muscles. The PCL and MCL comprised two bundles.11–15 All ligament bundles were modeled as nonlinear springs and simulated as nonlinear force elements. The hip joint was modeled as a revolute joint parallel to the flexion axis of the knee and was allowed to slide vertically. The ankle joint was modeled as a combination of several joints that combine to allow free translation in the medial–lateral direction and free rotation in flexion, axial, and varus–valgus directions.
Shoifi Abubakar et al. The origins of the insertion points and stiffness were determined from the relevant anatomical studies.12,13,16–18 The anterolateral (AL) bundle of the PCL attached on the roof and the posteromedial (PM) bundle attached on the wall of the intercondylar notch for the femoral side. The tibial attachment of the AL and PM bundles was located at the anterior and posterior portion of the posterior intercondylar fossa, respectively. Based on previous studies, stiffness coefficients of the PCL (AL bundle), PCL (PM bundle), FCL, MCL anterior, and MCL posterior were determined as 102, 102, 59, 63, and 63 N/mm, respectively.11,13,19–21 Parasolid models of a fixed-bearing, CR total knee prosthesis (NexGen CR-Flex; Zimmer, Inc., Warsaw, IN) were imported into the KneeSIM program, and analysis was performed of the geometry of the femoral, tibial, and patellar components, as well as the tibial insert. Anteroposterior (AP) and mediolateral (ML) lengths of the femoral condyle of the model bone were 61.7 and 68.4 mm, respectively. Medial and lateral AP lengths and ML length were 52.6, 45.8, and 73.7 mm, respectively. For simulation, the following sizes were chosen: size E for the femoral component (AP length: 61.5 mm; ML length: 68.0 mm), size 6 for the tibial component (AP length: 46.0 mm; ML length: 74.0 mm), 29 mm for the patellar component, and 10 mm thickness for the polyethylene insert. Although aligning the component in the coronal plane, the femoral and tibial components were set perpendicular to the mechanical axis of the femur and the tibia, respectively. For the sagittal alignment of the components, the femoral component was aligned to the distal anatomical axis of the femur, and the tibial component was aligned to the proximal anatomical axis of the tibia, with a 7-degree posterior slope. In the setting for neutral rotational alignments, the femoral and tibial components were aligned according to the femoral epicondylar axis and the tibial AP axis. The tibial AP axis was defined as the line connecting the middle of the PCL and the medial border of the patellar tendon at the tibial attachment.22 Therefore, an
Fig. 1 Ligament and muscle models in a musculoskeletal simulation (LifeMOD/KneeSIM 2010; LifeModeler Inc., San Clemente, CA). (A) Anterolateral view. (B) Posteromedial view. The Journal of Knee Surgery
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Influence of PCL Tension on Knee Kinematics and Kinetics
Influence of PCL Tension on Knee Kinematics and Kinetics
Results The PCL tension of the base model was 1,260 N. With increased PCL tightness, the PCL tension increased, up to 2,145 N in the 6 mm tight model. The slack of the PCL
Fig. 2 PCL tension in each model. Maximum PCL tension was recorded at 120 degrees of knee flexion for all models. PCL, posterior cruciate ligament.
decreased PCL tension, to a maximum of approximately 50% decrease in the 6 mm slack model (►Fig. 2). The amount of PCL tension significantly influenced knee kinematics in CR TKA (►Fig. 3). In the base model, both facet center positions translated anteriorly from full extension to 60 degrees of knee flexion by 4 mm and then moving posteriorly by approximately 8 mm. The facet center positions at 90 degrees of knee flexion were positioned at almost the same position as in full extension. The medial facet center at 90 degrees of knee flexion was positioned 10 mm anterior from the posterior edge of the tibial component and 16.6 mm anterior from the posterior edge of the medial tibial plateau. Comparing different PCL models, the medial and lateral facet centers were positioned at almost similar locations at full extension, although the facet centers translated posteriorly with increased PCL tightness. The discrepancy in the facet center positions was more apparent with knee flexion.
Fig. 3 Facet center position relative to tibial component in each model (blue dots, lateral; red dots, medial). In the loose PCL models, paradoxical anterior translation was observed. In the tight PCL models, facet center position was located around the posterior edge of polyethylene insert at 120 degrees of knee flexion. PCL, posterior cruciate ligament. The Journal of Knee Surgery
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uncovered bony surface of 6.6 mm was left at the posterior portion of the medial tibial plateau of the tibia. In this study, a constant vertical force of 4,000 N was applied at the hip and loaded on the knee joint. The quadriceps and hamstring loads were adjusted to obtain the prescribed flexion angle at each time point. First, the length of the PCL at the base model was adjusted to represent normal PCL tension; this adjustment was based on a previous cadaver study, in which maximum PCL tension reached up to 1,200 to 1,400 N in deep knee bend activities.23 This computer simulation model can decrease or increase the tension of the ligament by lengthening or shortening it, maintaining the same ligamentous attachment position. To investigate the effect of PCL tension, the length of the PCL was changed from þ 6 to 6 mm in 3 mm increments compared with the base model, so five models with different PCL lengths were made (i.e., 6 mm slack, 3 mm slack, base, 3 mm tight, and 6 mm tight models). In addition, a “no PCL” model was made, and analysis was performed for these six PCL tension models. The medial and lateral facet centers of the femoral condyles were used as geometric reference points.24 AP positions of the medial and lateral facet centers were measured using the coordinate system of the tibial component. Each facet center was determined by making a circle that approximated the articulating surface of the posterior condyle. For the PF and tibiofemoral forces and PCL tension, the maximum force was measured during the entire process for each model. To measure the tension applied to the PCL, the forces applied to the AL bundle and PM bundle were combined.
Shoifi Abubakar et al.
Paradoxical anterior translation was observed for all models. In tight PCL models, paradoxical anterior translation was limited until 30 degrees of knee flexion, and the amount of anterior translation was less than 4 mm. In loose PCL models, paradoxical anterior translation continued to 60 degrees of knee flexion, and the amount of paradoxical anterior translation was larger than in the tight PCL models. In no PCL model, the amount of paradoxical anterior translation was 4.6 and 4.9 mm for the medial and lateral condyles, respectively, whereas the corresponding values were 1.9 and 2.7 mm, respectively, in 6 mm tight model. Tibiofemoral contact forces were distributed almost equally to medial and lateral tibial plateaus. The medial and lateral contact forces in the base model were 1,743 and 1,725 N, respectively. With more PCL tightness, the tibiofemoral contact force became larger, increasing by approximately 15% in the 6 mm tight model. Slack PCLs resulted in decreased tibiofemoral contact forces, which showed an approximately 20% decrease in the no PCL model (►Fig. 4). Patellofemoral contact force at the base model was 2,830 N. If the PCL was tight, the PF contact force decreased by 15% in the 6 mm tight model. With a slack PCL model, PF contact forces increased by 15% in the no PCL model (►Fig. 5).
Fig. 4 TF contact force at the medial and lateral sides in each model. Maximum TF contact force was recorded at 120 degrees of knee flexion except lateral contact force of no PCL model. Maximum lateral contact force of no PCL model was recorded at 113 degrees of knee flexion. PCL, posterior cruciate ligament; TF, tibiofemoral.
Fig. 5 PF contact force in each model. Maximum PF contact force was recorded at 120 degrees of knee flexion for all models. PCL, posterior cruciate ligament; PF, patellofemoral. The Journal of Knee Surgery
Shoifi Abubakar et al.
Discussion The most important findings of this study were that paradoxical anterior movement and increased PF forces were observed in loose PCL models, and that excessive rollback, increased tibiofemoral forces and PCL tension were detected in tight PCL models. The hypotheses that loose PCLs might induce paradoxical anterior movement and greater PF forces, and that tight PCLs might be related to increased tibiofemoral forces and PCL tension, were confirmed. Obtaining correct PCL balance is important after CR TKA to achieve optimal knee flexion and good clinical results. Clinically, loose PCLs were reported to be associated with knee instability and pain.2 The anterior translation of the femur on the tibia was reported to have several potential negative consequences such as posterior impingement and reduced quadriceps efficiency.1,25 In the current simulation model with a loose PCL, the anterior translation of the femoral condyle and the increase of the PF force was confirmed, which might be related to such clinical problems as posterior impingement and anterior knee pain. A tight PCL could be associated with such problems as restriction of range of flexion, excessive stress to the polyethylene, and subsequent polyethylene wear.2 In the current study, PCL tension in the 6 mm tight model was significantly increased, up to 1.7 times (2,145 N) that of the base model (1,260 N), which might be the reason for restricted range of flexion. In knee kinematics, medial and lateral flexion facet centers in the 6 mm tight model were positioned almost at the posterior edge of the polyethylene, which might result in edge loading and consequent polyethylene wear. Moreover, tibiofemoral contact forces were increased by 15% compared with the base model. This simulation study showed that excessive force could be applied around the posterior edge during deep knee flexion if the PCL is too tight, which might cause polyethylene wear at the posterior side and early component loosening. Excessive polyethylene wear in tight PCL cases could be explained by this simulation study. The appropriate tension of the PCL must be determined during CR TKA surgery. A previous study reported that physiologic tensioning of the PCL can be achieved in only 10% patients with CR TKA.26 So far, however, the proper indicator for appropriate PCL tension during TKA remains unknown. When the PCL tension was close to normal in the current kinematic simulation (base model), the medial facet center at 90 degrees of knee flexion was located at almost the same position as at full extension. The previous kinematic study showed that geometric centers can represent lowest points of the femoral component,27 so our study based on the facet center can be applied to the lowest points. As a benchmark during the surgery, the medial contact point at 90 degrees of knee flexion in the base model was approximately at the posterior one-fourth of the AP dimension of the implant and at the posterior one-third of the medial tibial plateau (►Fig. 6). In the previous normal knee analysis, the lowest point of the medial condyle was approximately one-third from the posterior edge of the tibial plateau,28 which was the same as in our study. During
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Influence of PCL Tension on Knee Kinematics and Kinetics
Influence of PCL Tension on Knee Kinematics and Kinetics
Shoifi Abubakar et al.
References
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Fig. 6 Upper view for the base model at 90 degrees of knee flexion. The medial contact point was approximately at the posterior onefourth of the AP dimension of the implant and at the posterior onethird of the medial tibial plateau.
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surgery with CR implants, the medial contact position at 90 degrees of knee flexion might be considered a good indicator for adequate PCL tension, based on our simulation study analyzing the effects of PCL tension on tibiofemoral and PF forces. There are some limitations in the current study. First, computer knee simulation was used to analyze the knee kinematics and kinetics. As is common in most computational models, soft tissue material properties, such as the origins of the insertion points and stiffness of each ligament, were taken from previous relevant cadaveric studies, but it is still difficult to reproduce exactly in vivo mechanical loading. Second, a single implant design with a multiple radius femoral component was used in the current study. The result might be different in other CR TKAs. Third, in this simulation, analyses were performed with different PCL lengths and the same thickness of polyethylene; therefore, the original lengths of MCL and FCL were not changed. These conditions might not represent in vivo knee status.
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Conclusion The influences of PCL tension on knee kinematics and contact forces were analyzed in a computer simulation with different length PCL models. Loose PCLs induced paradoxical anterior movement and increased PF forces, whereas tight PCLs were related to increased tibiofemoral forces and PCL tension. The current study suggested that ideal knee kinematics would occur with proper PCL tension, in which the medial contact position at full flexion was almost similar to the position at 90 degrees of knee flexion. Surgeons are required to check the contact positions at full extension and 90 degrees of knee flexion to achieve proper PCL tension.
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femoral rollback on patellofemoral contact loads in total knee arthroplasty. J Arthroplasty 2001;16(7):909–918 Christen B, Heesterbeek P, Wymenga A, Wehrli U. Posterior cruciate ligament balancing in total knee replacement: the quantitative relationship between tightness of the flexion gap and tibial translation. J Bone Joint Surg Br 2007;89(8):1046–1050 Jacobs WC, Clement DJ, Wymenga AB. Retention versus removal of the posterior cruciate ligament in total knee replacement: a systematic literature review within the Cochrane framework. Acta Orthop 2005;76(6):757–768 Nakamura S, Ito H, Yoshitomi H, Kuriyama S, Komistek RD, Matsuda S. Analysis of the Flexion Gap on In Vivo Knee Kinematics Using Fluoroscopy. J Arthroplasty 2015;30(7):1237–1242 Nishio Y, Onodera T, Kasahara Y, Takahashi D, Iwasaki N, Majima T. Intraoperative medial pivot affects deep knee flexion angle and patient-reported outcomes after total knee arthroplasty. J Arthroplasty 2014;29(4):702–706 Dennis DA, Komistek RD, Colwell CE Jr, et al. In vivo anteroposterior femorotibial translation of total knee arthroplasty: a multicenter analysis. Clin Orthop Relat Res 1998;(356):47–57 Dennis DA, Komistek RD, Mahfouz MR, Haas BD, Stiehl JB. Multicenter determination of in vivo kinematics after total knee arthroplasty. Clin Orthop Relat Res 2003;(416):37–57 el Nahass B, Madson MM, Walker PS. Motion of the knee after condylar resurfacing—an in vivo study. J Biomech 1991;24(12): 1107–1117 Mizu-Uchi H, Colwell CW Jr, Flores-Hernandez C, Fregly BJ, Matsuda S, D’Lima DD. Patient-specific computer model of dynamic squatting after total knee arthroplasty. J Arthroplasty 2015;30(5): 870–874 Okamoto S, Mizu-uchi H, Okazaki K, Hamai S, Nakahara H, Iwamoto Y. Effect of tibial posterior slope on knee kinematics, quadriceps force, and patellofemoral contact force after posteriorstabilized total knee arthroplasty. J Arthroplasty 2015;30(8): 1439–1443 Anderson CJ, Ziegler CG, Wijdicks CA, Engebretsen L, LaPrade RF. Arthroscopically pertinent anatomy of the anterolateral and posteromedial bundles of the posterior cruciate ligament. J Bone Joint Surg Am 2012;94(21):1936–1945 Edwards A, Bull AM, Amis AA. The attachments of the fiber bundles of the posterior cruciate ligament: an anatomic study. Arthroscopy 2007;23(3):284–290 Harner CD, Xerogeanes JW, Livesay GA, et al. The human posterior cruciate ligament complex: an interdisciplinary study. Ligament morphology and biomechanical evaluation. Am J Sports Med 1995;23(6):736–745 LaPrade RF, Engebretsen AH, Ly TV, Johansen S, Wentorf FA, Engebretsen L. The anatomy of the medial part of the knee. J Bone Joint Surg Am 2007;89(9):2000–2010 LaPrade CM, Civitarese DM, Rasmussen MT, LaPrade RF. Emerging updates on the posterior cruciate ligament: a review of the current literature. Am J Sports Med 2015;43(12):3077–3092 LaPrade RF, Ly TV, Wentorf FA, Engebretsen L. The posterolateral attachments of the knee: a qualitative and quantitative morphologic analysis of the fibular collateral ligament, popliteus tendon, popliteofibular ligament, and lateral gastrocnemius tendon. Am J Sports Med 2003;31(6):854–860 Kennedy NI, LaPrade RF, Goldsmith MT, et al. Posterior cruciate ligament graft fixation angles, part 1: biomechanical evaluation for anatomic single-bundle reconstruction. Am J Sports Med 2014; 42(10):2338–2345 Kennedy NI, LaPrade RF, Goldsmith MT, et al. Posterior cruciate ligament graft fixation angles, part 2: biomechanical evaluation for anatomic double-bundle reconstruction. Am J Sports Med 2014;42(10):2346–2355
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Influence of PCL Tension on Knee Kinematics and Kinetics
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analysis of the posterior cruciate ligament, part 1: the individual and collective function of the anterolateral and posteromedial bundles. Am J Sports Med 2013;41(12): 2828–2838 Sugita T, Amis AA. Anatomic and biomechanical study of the lateral collateral and popliteofibular ligaments. Am J Sports Med 2001; 29(4):466–472 Wijdicks CA, Ewart DT, Nuckley DJ, Johansen S, Engebretsen L, Laprade RF. Structural properties of the primary medial knee ligaments. Am J Sports Med 2010;38(8):1638–1646 Akagi M, Oh M, Nonaka T, Tsujimoto H, Asano T, Hamanishi C. An anteroposterior axis of the tibia for total knee arthroplasty. Clin Orthop Relat Res 2004;(420):213–219 Pandy MG, Shelburne KB. Dependence of cruciate-ligament loading on muscle forces and external load. J Biomech 1997;30(10): 1015–1024
contemporary knee design on high flexion: a kinematic comparison with the normal knee. J Bone Joint Surg Am 2008;90 (Suppl 4):195–201 Dennis DA, Komistek RD, Hoff WA, Gabriel SM. In vivo knee kinematics derived using an inverse perspective technique. Clin Orthop Relat Res 1996;(331):107–117 Lotke PA, Corces A, Williams J, Hirsch H. Strain characteristics of the posterior cruciate ligament after total knee arthroplasty. Am J Knee Surg 1993;6(3):104–107 Nakamura S, Sharma A, Ito H, Nakamura K, Zingde SM, Komistek RD. Kinematic difference between various geometric centers and contact points for tri-condylar bi-surface knee system. J Arthroplasty 2015;30(4):701–705 Fukagawa S, Matsuda S, Tashiro Y, Hashizume M, Iwamoto Y. Posterior displacement of the tibia increases in deep flexion of the knee. Clin Orthop Relat Res 2010;468(4):1107–1114
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The Journal of Knee Surgery
Influence of Posterior Cruciate Ligament Tension on Knee Kinematics and Kinetics Muhammad Shoifi Abubakar, MD1 Shinichiro Nakamura, MD, PhD1 Shinichi Kuriyama, MD, PhD1 Hiromu Ito, MD, PhD1 Masahiro Ishikawa, MD, PhD1 Moritoshi Furu, MD, PhD1 Yoshihisa Tanaka, MD1 Shuichi Matsuda, MD, PhD1
J Knee Surg
Abstract
Keywords
► posterior cruciate ligament ► total knee arthroplasty ► cruciate retaining ► knee kinematics ► knee kinetics
Address for correspondence Shinichiro Nakamura, MD, PhD, Department of Orthopaedic Surgery, Kyoto University, Shogoin Kawaharacho 54 Sakyo-ku, Kyoto 6068507, Japan (e-mail: [email protected]).
The posterior cruciate ligament (PCL) has an important role in cruciate-retaining total knee arthroplasty to achieve good clinical results. The purpose of the study was to examine the influence of PCL tension on knee kinematics and kinetics and to propose an indicator for proper PCL tension during surgery. A squatting activity was simulated in a weight-bearing deep knee bend using a musculoskeletal computer simulation knee model. The length of the PCL was changed to represent different PCL tension models. The amount of PCL tension significantly influenced knee kinematics and kinetics. In the normal PCL model, the facet center positions at 90 degrees of knee flexion were positioned at almost the same position as in full extension. A loose PCL-induced paradoxical anterior movement and greater patellofemoral forces, whereas a tight PCL was related to excessive rollback and increased tibiofemoral forces. This study suggested ideal knee kinematics with proper PCL tension, in which the medial contact position at full flexion was almost similar to the position at 90 degrees of knee flexion.
The posterior cruciate ligament (PCL) has an important role in cruciate-retaining (CR) total knee arthroplasty (TKA) to achieve good clinical results. The condition of the PCL affects postoperative range of motion, stability, forces at the bone– prostheses interface, femoral rollback phenomenon, gait pattern, wear, and proprioception.1 Several studies found that the PCL has important functions and that surgeons have difficulties in adequately adjusting the PCL tension in CR TKA.2,3 A loose PCL might result in knee instability and pain, whereas excessive PCL tension might be associated with restriction of flexion and could lead to high stress and subsequent polyethylene wear.2,3 Reproduction of normal knee kinematics after TKA is important for better range of motion and knee function.4,5 In previous kinematic studies during a deep knee bend, paradoxical anterior femoral translation was frequently observed in CR TKA, compared with posterior stabilizing TKA.6,7 The PCL acts as the primary restraint against poste-
rior translation of the tibia and induces posterior femoral rollback, particularly at 90 degrees and more of knee flexion.2 The femoral rollback mechanism is an important phenomenon during deep knee flexion. In a previous study, using an electromagnetic device to track knee motion, implanted knees with PCL retention showed similar motion to normal knees.8 This result would imply that rollback does occur with a CR implant, and PCL tension during CR TKA will influence the femoral rollback mechanism. The previous studies have shown that femoral rollback has significant influences on patellofemoral (PF) contact forces and tibiofemoral contact forces.1 Increasing femoral rollback will have two primary effects. First, the angle between the quadriceps and patellar tendon lines will decrease because of the posterior translation of the femoral component. Second, the force to the patellar tendon will decrease owing to a decreased angle between the quadriceps and patellar tendon. In contrast,
received August 31, 2015 accepted after revision December 27, 2015
Copyright © by Thieme Medical Publishers, Inc., 333 Seventh Avenue, New York, NY 10001, USA. Tel: +1(212) 584-4662.
DOI http://dx.doi.org/ 10.1055/s-0036-1571803. ISSN 1538-8506.
Downloaded by: Universite Laval. Copyrighted material.
1 Department of Orthopaedic Surgery, Kyoto University, Kyoto, Japan
decreasing femoral rollback will lead to higher force of the patellar tendon and higher PF contact force. In the current study, PCL tension was changed in the computer simulation model to analyze its effects on knee kinematics and kinetics. The purpose of the study was to examine the influence of PCL tension on knee kinematics and tibiofemoral and PF contact forces, and to propose an indicator for proper PCL tension during surgery. We hypothesize that a loose PCL will cause the femur to slide anteriorly, which might increase PF contact forces. Likewise, we think a tight PCL will induce posterior femoral rollback, which might increase tibiofemoral contact forces and PCL tension.
Materials and Methods The musculoskeletal computer simulation knee model (LifeMOD/KneeSIM 2010; LifeModeler Inc., San Clemente, CA) was used in the current study. This simulation model consisted of a dynamic musculoskeletal modeling program of the knee (►Fig. 1). In previous biomechanical studies, this simulation program has been validated to secure the appropriate estimation of contact points and contact forces.9,10 A squatting activity in a weight-bearing deep knee bend was simulated using an Oxfordtype knee rig. In this activity, the knee model was flexed from full extension to 120 degrees of knee flexion and then extended to full extension. This musculoskeletal model included the tibiofemoral and PF contact, PCL, fibular collateral ligament (FCL), medial collateral ligament (MCL), elements of the knee capsule, quadriceps muscle and tendon, patellar tendon, and hamstring muscles. The PCL and MCL comprised two bundles.11–15 All ligament bundles were modeled as nonlinear springs and simulated as nonlinear force elements. The hip joint was modeled as a revolute joint parallel to the flexion axis of the knee and was allowed to slide vertically. The ankle joint was modeled as a combination of several joints that combine to allow free translation in the medial–lateral direction and free rotation in flexion, axial, and varus–valgus directions.
Shoifi Abubakar et al. The origins of the insertion points and stiffness were determined from the relevant anatomical studies.12,13,16–18 The anterolateral (AL) bundle of the PCL attached on the roof and the posteromedial (PM) bundle attached on the wall of the intercondylar notch for the femoral side. The tibial attachment of the AL and PM bundles was located at the anterior and posterior portion of the posterior intercondylar fossa, respectively. Based on previous studies, stiffness coefficients of the PCL (AL bundle), PCL (PM bundle), FCL, MCL anterior, and MCL posterior were determined as 102, 102, 59, 63, and 63 N/mm, respectively.11,13,19–21 Parasolid models of a fixed-bearing, CR total knee prosthesis (NexGen CR-Flex; Zimmer, Inc., Warsaw, IN) were imported into the KneeSIM program, and analysis was performed of the geometry of the femoral, tibial, and patellar components, as well as the tibial insert. Anteroposterior (AP) and mediolateral (ML) lengths of the femoral condyle of the model bone were 61.7 and 68.4 mm, respectively. Medial and lateral AP lengths and ML length were 52.6, 45.8, and 73.7 mm, respectively. For simulation, the following sizes were chosen: size E for the femoral component (AP length: 61.5 mm; ML length: 68.0 mm), size 6 for the tibial component (AP length: 46.0 mm; ML length: 74.0 mm), 29 mm for the patellar component, and 10 mm thickness for the polyethylene insert. Although aligning the component in the coronal plane, the femoral and tibial components were set perpendicular to the mechanical axis of the femur and the tibia, respectively. For the sagittal alignment of the components, the femoral component was aligned to the distal anatomical axis of the femur, and the tibial component was aligned to the proximal anatomical axis of the tibia, with a 7-degree posterior slope. In the setting for neutral rotational alignments, the femoral and tibial components were aligned according to the femoral epicondylar axis and the tibial AP axis. The tibial AP axis was defined as the line connecting the middle of the PCL and the medial border of the patellar tendon at the tibial attachment.22 Therefore, an
Fig. 1 Ligament and muscle models in a musculoskeletal simulation (LifeMOD/KneeSIM 2010; LifeModeler Inc., San Clemente, CA). (A) Anterolateral view. (B) Posteromedial view. The Journal of Knee Surgery
Downloaded by: Universite Laval. Copyrighted material.
Influence of PCL Tension on Knee Kinematics and Kinetics
Influence of PCL Tension on Knee Kinematics and Kinetics
Results The PCL tension of the base model was 1,260 N. With increased PCL tightness, the PCL tension increased, up to 2,145 N in the 6 mm tight model. The slack of the PCL
Fig. 2 PCL tension in each model. Maximum PCL tension was recorded at 120 degrees of knee flexion for all models. PCL, posterior cruciate ligament.
decreased PCL tension, to a maximum of approximately 50% decrease in the 6 mm slack model (►Fig. 2). The amount of PCL tension significantly influenced knee kinematics in CR TKA (►Fig. 3). In the base model, both facet center positions translated anteriorly from full extension to 60 degrees of knee flexion by 4 mm and then moving posteriorly by approximately 8 mm. The facet center positions at 90 degrees of knee flexion were positioned at almost the same position as in full extension. The medial facet center at 90 degrees of knee flexion was positioned 10 mm anterior from the posterior edge of the tibial component and 16.6 mm anterior from the posterior edge of the medial tibial plateau. Comparing different PCL models, the medial and lateral facet centers were positioned at almost similar locations at full extension, although the facet centers translated posteriorly with increased PCL tightness. The discrepancy in the facet center positions was more apparent with knee flexion.
Fig. 3 Facet center position relative to tibial component in each model (blue dots, lateral; red dots, medial). In the loose PCL models, paradoxical anterior translation was observed. In the tight PCL models, facet center position was located around the posterior edge of polyethylene insert at 120 degrees of knee flexion. PCL, posterior cruciate ligament. The Journal of Knee Surgery
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uncovered bony surface of 6.6 mm was left at the posterior portion of the medial tibial plateau of the tibia. In this study, a constant vertical force of 4,000 N was applied at the hip and loaded on the knee joint. The quadriceps and hamstring loads were adjusted to obtain the prescribed flexion angle at each time point. First, the length of the PCL at the base model was adjusted to represent normal PCL tension; this adjustment was based on a previous cadaver study, in which maximum PCL tension reached up to 1,200 to 1,400 N in deep knee bend activities.23 This computer simulation model can decrease or increase the tension of the ligament by lengthening or shortening it, maintaining the same ligamentous attachment position. To investigate the effect of PCL tension, the length of the PCL was changed from þ 6 to 6 mm in 3 mm increments compared with the base model, so five models with different PCL lengths were made (i.e., 6 mm slack, 3 mm slack, base, 3 mm tight, and 6 mm tight models). In addition, a “no PCL” model was made, and analysis was performed for these six PCL tension models. The medial and lateral facet centers of the femoral condyles were used as geometric reference points.24 AP positions of the medial and lateral facet centers were measured using the coordinate system of the tibial component. Each facet center was determined by making a circle that approximated the articulating surface of the posterior condyle. For the PF and tibiofemoral forces and PCL tension, the maximum force was measured during the entire process for each model. To measure the tension applied to the PCL, the forces applied to the AL bundle and PM bundle were combined.
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Paradoxical anterior translation was observed for all models. In tight PCL models, paradoxical anterior translation was limited until 30 degrees of knee flexion, and the amount of anterior translation was less than 4 mm. In loose PCL models, paradoxical anterior translation continued to 60 degrees of knee flexion, and the amount of paradoxical anterior translation was larger than in the tight PCL models. In no PCL model, the amount of paradoxical anterior translation was 4.6 and 4.9 mm for the medial and lateral condyles, respectively, whereas the corresponding values were 1.9 and 2.7 mm, respectively, in 6 mm tight model. Tibiofemoral contact forces were distributed almost equally to medial and lateral tibial plateaus. The medial and lateral contact forces in the base model were 1,743 and 1,725 N, respectively. With more PCL tightness, the tibiofemoral contact force became larger, increasing by approximately 15% in the 6 mm tight model. Slack PCLs resulted in decreased tibiofemoral contact forces, which showed an approximately 20% decrease in the no PCL model (►Fig. 4). Patellofemoral contact force at the base model was 2,830 N. If the PCL was tight, the PF contact force decreased by 15% in the 6 mm tight model. With a slack PCL model, PF contact forces increased by 15% in the no PCL model (►Fig. 5).
Fig. 4 TF contact force at the medial and lateral sides in each model. Maximum TF contact force was recorded at 120 degrees of knee flexion except lateral contact force of no PCL model. Maximum lateral contact force of no PCL model was recorded at 113 degrees of knee flexion. PCL, posterior cruciate ligament; TF, tibiofemoral.
Fig. 5 PF contact force in each model. Maximum PF contact force was recorded at 120 degrees of knee flexion for all models. PCL, posterior cruciate ligament; PF, patellofemoral. The Journal of Knee Surgery
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Discussion The most important findings of this study were that paradoxical anterior movement and increased PF forces were observed in loose PCL models, and that excessive rollback, increased tibiofemoral forces and PCL tension were detected in tight PCL models. The hypotheses that loose PCLs might induce paradoxical anterior movement and greater PF forces, and that tight PCLs might be related to increased tibiofemoral forces and PCL tension, were confirmed. Obtaining correct PCL balance is important after CR TKA to achieve optimal knee flexion and good clinical results. Clinically, loose PCLs were reported to be associated with knee instability and pain.2 The anterior translation of the femur on the tibia was reported to have several potential negative consequences such as posterior impingement and reduced quadriceps efficiency.1,25 In the current simulation model with a loose PCL, the anterior translation of the femoral condyle and the increase of the PF force was confirmed, which might be related to such clinical problems as posterior impingement and anterior knee pain. A tight PCL could be associated with such problems as restriction of range of flexion, excessive stress to the polyethylene, and subsequent polyethylene wear.2 In the current study, PCL tension in the 6 mm tight model was significantly increased, up to 1.7 times (2,145 N) that of the base model (1,260 N), which might be the reason for restricted range of flexion. In knee kinematics, medial and lateral flexion facet centers in the 6 mm tight model were positioned almost at the posterior edge of the polyethylene, which might result in edge loading and consequent polyethylene wear. Moreover, tibiofemoral contact forces were increased by 15% compared with the base model. This simulation study showed that excessive force could be applied around the posterior edge during deep knee flexion if the PCL is too tight, which might cause polyethylene wear at the posterior side and early component loosening. Excessive polyethylene wear in tight PCL cases could be explained by this simulation study. The appropriate tension of the PCL must be determined during CR TKA surgery. A previous study reported that physiologic tensioning of the PCL can be achieved in only 10% patients with CR TKA.26 So far, however, the proper indicator for appropriate PCL tension during TKA remains unknown. When the PCL tension was close to normal in the current kinematic simulation (base model), the medial facet center at 90 degrees of knee flexion was located at almost the same position as at full extension. The previous kinematic study showed that geometric centers can represent lowest points of the femoral component,27 so our study based on the facet center can be applied to the lowest points. As a benchmark during the surgery, the medial contact point at 90 degrees of knee flexion in the base model was approximately at the posterior one-fourth of the AP dimension of the implant and at the posterior one-third of the medial tibial plateau (►Fig. 6). In the previous normal knee analysis, the lowest point of the medial condyle was approximately one-third from the posterior edge of the tibial plateau,28 which was the same as in our study. During
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Influence of PCL Tension on Knee Kinematics and Kinetics
Influence of PCL Tension on Knee Kinematics and Kinetics
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References
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Fig. 6 Upper view for the base model at 90 degrees of knee flexion. The medial contact point was approximately at the posterior onefourth of the AP dimension of the implant and at the posterior onethird of the medial tibial plateau.
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surgery with CR implants, the medial contact position at 90 degrees of knee flexion might be considered a good indicator for adequate PCL tension, based on our simulation study analyzing the effects of PCL tension on tibiofemoral and PF forces. There are some limitations in the current study. First, computer knee simulation was used to analyze the knee kinematics and kinetics. As is common in most computational models, soft tissue material properties, such as the origins of the insertion points and stiffness of each ligament, were taken from previous relevant cadaveric studies, but it is still difficult to reproduce exactly in vivo mechanical loading. Second, a single implant design with a multiple radius femoral component was used in the current study. The result might be different in other CR TKAs. Third, in this simulation, analyses were performed with different PCL lengths and the same thickness of polyethylene; therefore, the original lengths of MCL and FCL were not changed. These conditions might not represent in vivo knee status.
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Conclusion The influences of PCL tension on knee kinematics and contact forces were analyzed in a computer simulation with different length PCL models. Loose PCLs induced paradoxical anterior movement and increased PF forces, whereas tight PCLs were related to increased tibiofemoral forces and PCL tension. The current study suggested that ideal knee kinematics would occur with proper PCL tension, in which the medial contact position at full flexion was almost similar to the position at 90 degrees of knee flexion. Surgeons are required to check the contact positions at full extension and 90 degrees of knee flexion to achieve proper PCL tension.
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