The Journal of Arthroplasty xxx (2015) xxx–xxx
Contents lists available at ScienceDirect
The Journal of Arthroplasty journal homepage: www.arthroplastyjournal.org
Effect of Tibial Posterior Slope on Knee Kinematics, Quadriceps Force, and Patellofemoral Contact Force After Posterior-Stabilized Total Knee Arthroplasty Shigetoshi Okamoto, MD, Hideki Mizu-uchi, MD, PhD , Ken Okazaki, MD, PhD, Satoshi Hamai, MD, PhD, Hiroyuki Nakahara, MD, PhD, Yukihide Iwamoto, MD, PhD Department of Orthopaedic Surgery, Graduate School of Medical Sciences, Kyushu University, Fukuoka, Japan
a r t i c l e
i n f o
Article history: Received 4 December 2014 Accepted 27 February 2015 Available online xxxx Keywords: total knee arthroplasty tibial posterior slope computer simulation knee instability patellofemoral contact force quadriceps force
a b s t r a c t We used a musculoskeletal model validated with in vivo data to evaluate the effect of tibial posterior slope on knee kinematics, quadriceps force, and patellofemoral contact force after posterior-stabilized total knee arthroplasty. The maximum quadriceps force and patellofemoral contact force decreased with increasing posterior slope. Anterior sliding of the tibial component and anterior impingement of the anterior aspect of the tibial post were observed with tibial posterior slopes of at least 5° and 10°, respectively. Increased tibial posterior slope contributes to improved exercise efficiency during knee extension, however excessive tibial posterior slope should be avoided to prevent knee instability. Based on our computer simulation we recommend tibial posterior slopes of less than 5° in posterior-stabilized total knee arthroplasty. © 2015 Elsevier Inc. All rights reserved.
Total knee arthroplasty (TKA) has become one of the most successful orthopedic procedures for providing pain relief and improving knee function, with reported survival rates of greater than 90% after 15 years [1,2]. Nonetheless, revision surgeries are sometimes required due to poor early results after surgery or premature implant failure. Sharkey et al reported that more than 20% of TKA failures were due to malalignment or malposition that can potentially be avoided with meticulous surgical technique [3]. Previous clinical studies demonstrated that patient-perceived outcomes were affected by malalignment of the components. These clinical results were supported by kinematic data in many biomechanical studies [4–6]. Regarding coronal alignment, Matsuda et al reported that varus postoperative alignment negatively correlated with patient satisfaction [4]. A finite element model showed that tibial coronal malalignment was associated with increased subsidence of the tibial tray [5]. As for rotational alignment, Barrack et al reported that the presence and severity of femoral component internal rotation correlated with patellofemoral complications [6]. In a finite element–rigid body model, patellar maltracking occurred with internal femoral component malrotation [7]. One or more of the authors of this paper have disclosed potential or pertinent conflicts of interest, which may include receipt of payment, either direct or indirect, institutional support, or association with an entity in the biomedical field which may be perceived to have potential conflict of interest with this work. For full disclosure statements refer to http://dx.doi.org/10.1016/j.arth.2015.02.042. Reprint requests: Hideki Mizu-uchi, MD, PhD, Department of Orthopaedic Surgery, Graduate School of Medical Sciences, Kyushu University, 3-1-1 Maidashi, Higashi-ku, Fukuoka 812-8582, Japan.
Even though many papers have recommended surgical techniques to avoid coronal and rotational malalignment, there have been few studies concerning sagittal alignment. In particular, the acceptable range of the tibial posterior slope is still controversial. Tibial posterior slope variations can have both positive and negative effects on knee function and kinematics. Increases in the posterior slope can contribute to improvement of knee flexion and reduce the required quadriceps force for knee motion [8–11], but were also shown to result in anterior tibial translation and posterior articular wear of the insert [12,13]. Anterior impingement of the tibial post is one risk associated with the use of posterior-stabilized TKA [14–16]. Many clinical and cadaveric experimental studies have sought to determine the optimal angle of the tibial posterior slope, however wide variations in cutting errors have led to difficult evaluations. Barrett et al reported that in one-third of cases there was a difference of ±2° between planned and actual posterior tibial slopes when both computer navigation and conventional extramedullary alignment guides were used by high-volume surgeons [17]. In addition, interindividual differences in muscular strength and soft tissue conditions usually affect patellofemoral contact force and quadriceps force. At the present time, the optimal angle of the tibial posterior slope for postoperative knee function is still controversial. Recently, three-dimensional computer models have been developed to predict postoperative knee condition after TKA (e.g., knee kinematics after TKA and mechanical behavior of the prosthesis). Computer simulations can apply large loads and duplicate weight-bearing activities using musculoskeletal models that can diminish the limitations of clinical and
http://dx.doi.org/10.1016/j.arth.2015.02.042 0883-5403/© 2015 Elsevier Inc. All rights reserved.
Please cite this article as: Okamoto S, et al, Effect of Tibial Posterior Slope on Knee Kinematics, Quadriceps Force, and Patellofemoral Contact Force After Posterior-Stabilized Total Knee Arthroplasty, J Arthroplasty (2015), http://dx.doi.org/10.1016/j.arth.2015.02.042
2
S. Okamoto et al. / The Journal of Arthroplasty xxx (2015) xxx–xxx
cadaveric experimental studies. Several previous papers used simulations to predict postoperative flexion angle, knee kinematics, joint load, and damage to the insert [18–21]. These results were validated by clinical data [20] and implant retrieval findings [21]. Computer simulation can compare various factors while holding stiffness and soft tissue position constant, which should reduce the effects of interindividual differences. The purpose of this study was to determine the acceptable range of the tibial posterior slope by simulating quadriceps force, patellofemoral contact force, and knee kinematics.
constrained in the mediolateral and anteroposterior directions but was free to translate vertically under the axial force (in the direction of gravity) that generated a flexion moment at the knee (Fig. 1). Quadriceps force, patellofemoral contact force, and knee kinematics were computed during stair climbing (from 87° to 6° of flexion). Sixteen different angles of the tibial posterior slope were simulated in this study. A posterior slope of zero degrees was defined as perpendicular to the tibial mechanical axis. We changed the posterior angle at 1° intervals ranging from 0° to 15° based on the origin of the coordinates (the center of the tibial insert) in the sagittal alignment.
Materials and Methods Validation of the Computer Model Computer Simulation Weight-bearing stair climbing was simulated in this study, with posterior-stabilized components (NexGen LPS-Flex, Zimmer, Warsaw, Indiana) appropriate for a female (162 cm, 58 kg). Initial coordinates were determined using a computer-assisted design software program (Rhinoceros; Robert McNeel and Associates, Seattle, Washington) as reported in our previous studies [20]. The origin of the initial coordinates was the center of the tibial symmetrical insert in both the anteriorposterior dimension and the medial-lateral dimension. The most distal condylar points of the femoral component were set on the surface of the tibial insert in the superior-inferior dimension. The implant geometry was imported into a dynamic musculoskeletal modeling program (LifeMOD/KneeSIM 2010; LifeModeler, Inc, San Clemente, California). KneeSIM uses rigid body dynamics to simulate weight-bearing stair climbing. The masses of the limb segments and the body weight generate a flexion moment on the knee, whereas the quadriceps muscle exerts an extension moment. This musculoskeletal model of the knee included the medial collateral and lateral collateral ligaments, the quadriceps muscle and tendon, the patellar tendon, and the hamstring muscles. The proximal attachment points of the medial collateral ligament and lateral collateral ligament were defined as the most prominent epicondyles of the femur. Collateral ligaments were modeled as nonlinear springs with material properties obtained from a published report [22]. Contact was simulated between the tibiofemoral and patellofemoral articular surfaces. The hip and ankle joints had all three rotational degrees of freedom. The “ankle” section had no translational degrees of freedom. The “hip” section was
Clinical (in vivo) data were used to validate the computational model. Five female patients (mean age, 73 years (range, 66–78 years); mean tibial posterior slope, 5.0°; range, 2.6–6.7°; mean postoperative follow-up, 26.6 months (range, 13–42 months)) received the same posterior-stabilized TKA as used in our computer simulation (NexGen LPS-Flex, Zimmer Inc., Warsaw, Indiana). Continuous sagittal radiological images were obtained in each patient during stair climbing using a flat-panel detector (Hitachi, Clavis, Tokyo, Japan), and analyzed using a 2D-3D image-matching technique [23]. Antero-posterior translation of the femoral component relative to the tibial tray was compared between the computer simulation and the clinical data. Antero-posterior tibiofemoral position value was defined as anterior (positive) or posterior (negative) to the midline of the tibial tray. This study was approved by the institutional review board (Kyushu University, 3-1-1 Maidashi, Higashi-ku, Fukuoka 812-8582, Japan, No. 25–74). To participate in this study, informed consent was obtained from all the patients. Results Simulation of Quadriceps Force and Patellofemoral Contact Force Quadriceps force and patellofemoral contact force remained between 1500 N and 2000 N from 87° to 65° of knee flexion for all angles of tibial posterior slope (Figs. 2 and 3). Both forces increased rapidly at 65° of knee flexion, at which point maximum vertical load was placed on the knee joint. After the peak force, the forces decreased gradually along with knee extension. Maximum quadriceps forces were 2989 N
Fig. 1. The knee simulator model used in the present study; LifeMOD/KneeSIM 2010.
Please cite this article as: Okamoto S, et al, Effect of Tibial Posterior Slope on Knee Kinematics, Quadriceps Force, and Patellofemoral Contact Force After Posterior-Stabilized Total Knee Arthroplasty, J Arthroplasty (2015), http://dx.doi.org/10.1016/j.arth.2015.02.042
S. Okamoto et al. / The Journal of Arthroplasty xxx (2015) xxx–xxx
Fig. 2. Simulated quadriceps force from 87° to 6° of knee flexion during stair-ascending.
(slope: 15°) to 3171 N (slope: 0°), and maximum patellofemoral contact forces were 2598 N (slope: 15°) to 3061 N (slope: 0°). Increasing the posterior slope decreased the both maximum force. An increase of 15° in the posterior slope (relative to 0°) led to decreases of 6% in the maximum quadriceps force and 15% in the maximum patellofemoral contact force. Simulation of Knee Kinematics The femoral components translated anteriorly with knee extension for all angles of tibial posterior slope (Fig. 4). Increases in the tibial posterior slope resulted in more posterior positioning of the femoral component and reduced the amount of antero-posterior translation. Anterior sliding of the tibial component occurred with 65° of knee flexion if the tibial posterior slope was 5° or more. Anterior impingement between the anterior aspect of the tibial post and the femoral component was observed with near-full knee extension if the tibial posterior slope was 10° or more.
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Fig. 4. Simulated AP translation of the femoral component relative to the tibial insert from 87° to 6° of knee flexion on during stair-ascending. : Anterior sliding of the tibial component happened on 65° of knee flexion if the tibial posterior slope were 5° and more. : Anterior impingement between the anterior aspect of the tibial post and the femoral component was observed on almost full knee extension if the tibial posterior slope were 10° and more.
knees demonstrated femoral rollback with knee flexion, although there was some interspecimen variability in the magnitudes. The predicted rollback was also almost within the interspecimen variability. Discussion
Anteroposterior translation of the femoral component was simulated to closely approximate the measured in vivo data (Fig. 5). Simulated
The useful information of the present study was that increasing the tibial slope posed a risk of inducing knee instability, which caused anterior sliding of the tibial component and anterior impingement between the components. In contrast, quadriceps force and patellofemoral contact force decreased to some extent. These results supported the previous studies more clearly with little effect of the soft tissue condition and difficulty of bone cutting [10,12,14,16]. We used KneeSIM as a modeling program, the usefulness of which has been reported in several papers [20,21,24,25]. Colwell et al reported that a rotating bearing–type implant could not improve patellar kinematics and forces in the presence of femoral component malrotation using a model validated with experimental
Fig. 3. Simulated patellofemoral contact force from 87° to 6° of knee flexion during stair-ascending.
Fig. 5. Femoral component antero-posterior translation relative to the tibial insert. Solid line: computer model. Dot line: in-vivo data.
Model Validation With Clinical Data
Please cite this article as: Okamoto S, et al, Effect of Tibial Posterior Slope on Knee Kinematics, Quadriceps Force, and Patellofemoral Contact Force After Posterior-Stabilized Total Knee Arthroplasty, J Arthroplasty (2015), http://dx.doi.org/10.1016/j.arth.2015.02.042
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S. Okamoto et al. / The Journal of Arthroplasty xxx (2015) xxx–xxx
data from a cadaver study [24]. Mihalko et al validated their model using in vivo fluoroscopic data with anteroposterior translation of the femoral component [25]. Based on our validation and theirs, we considered our computer model to be appropriate although there were abnormal values from the range of experimental data at some angles of knee flexion because the data showed a similar trend and was almost within the interspecimen variability of the clinical data. Our study also validated the computer simulation data with clinical data on postoperative knee kinematics using the same implant. In the present study, increase of 15° in the posterior slope (relative to 0°) led to decreases of 6% in the maximum quadriceps force and 15% in the maximum patellofemoral contact force. Increasing the tibial posterior slope induced a more posterior position of the femoral component. A more posterior contact position between the femorotibial components leads to an increase in the quadriceps lever arm, which improve the movement efficiency which contributes to reduced quadriceps force and patellofemoral contact force [10,26–28]. Therefore, increasing the tibial posterior slope can be expected to reduce the quadriceps force and the patellofemoral contact force required to climb stairs to some extent. Anterior sliding of the tibial component and anterior impingement of the post in posterior-stabilized TKA should be avoided for longterm TKA success [29]. Anterior sliding of the tibial component occurred in the present study with tibial posterior slopes of 5° or more. Shimizu et al using the same implant as our simulation, reported that tibial anterior sliding was observed while shifting weight to the leg during stair climbing [30]. They concluded that knee instability was caused by several factors: absence of the anterior cruciate ligament, implant design, and tibial posterior slope. However, it is difficult to evaluate the effect of these factors on knee instability in the context of this complicated condition. Our study evaluated the actual effect of the posterior slope while controlling for the other factors. Anterior impingement between the tibial post and the femoral component was observed at near-full extension with 10° or more of tibial posterior slope. Anterior impingement on knee extension with the NexGen LPS prosthesis has been reported by several authors [14–16]. Banks et al reported that 75% of cases showed anterior impingement, 41% of knees demonstrated hyperextension during gait, and the hyperextension between the femoral and tibial component averaged 6°. They noted that a 5° anterior bow and 5° posterior slope led to approximately 10° of relative hyperextension of the component when the knee was in full extension. In the present study, the femoral component was positioned neutrally, so if the femoral component flexed, anterior impingement might occur with less than 10° of tibial posterior slope. In terms of clinical relevance, identifying the optimal tibial posterior slope angle should help surgeons cut the proximal tibia properly on the sagittal alignment. In the present study, excessive tibial posterior slope caused knee instability. Bai et al reported that an increased posterior slope cut angle significantly decreased tibial anterior compressive strains and significantly increased tibial posterior compressive strains; the authors recommended cutting the articular surface of the tibia at a 0° or 3° posterior slope to provide the greatest tibial component stability [31]. Based on their results and our own, surgeons should avoid cutting the proximal tibia with greater than 5° of tibial posterior slope in posterior-stabilized TKA. There are several limitations to this study. First, only one prosthesis was used for the simulation. Recently, some prostheses have demonstrated conformity and stability on guiding the medial pivot of the femoral component. The NexGen prosthesis has no inherent built in slope and has a flatter geometry of the insert surface compared with these guided-motion-type designs, which means that the NexGen can more easily be affected by ligaments and/or bone cutting conditions. Therefore, we should evaluate different types of implants in the future and compare the results to our present
findings. Second, only stair climbing was analyzed because we validated the simulation with clinical data based on the same activity. Other actions such as stair descending and sit-stand-sit movements should also be examined to assess the effect of the posterior slope in detail. Even with these limitations, our computer simulation clearly showed the effects of the tibial posterior slope on knee kinematics, patellofemoral contact force, and quadriceps force with accurate bone cutting and constant soft tissue parameters. In conclusion, increases in the tibial posterior slope contributed to improved exercise efficiency of the quadriceps muscle and to reduced contact force of the patellofemoral joint. However, excessive tibial posterior slope should be avoided to prevent anterior sliding of the tibial component and anterior impingement of the post/cam mechanism. Based on the results of our computer simulation, surgeons should maintain the tibial posterior slope at 5° or less when cutting the proximal tibia in posterior-stabilized TKA.
Acknowledgment No benefits in any form have been received or will be received from a commercial party related directly or in directly to the subject of this article.
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Please cite this article as: Okamoto S, et al, Effect of Tibial Posterior Slope on Knee Kinematics, Quadriceps Force, and Patellofemoral Contact Force After Posterior-Stabilized Total Knee Arthroplasty, J Arthroplasty (2015), http://dx.doi.org/10.1016/j.arth.2015.02.042
S. Okamoto et al. / The Journal of Arthroplasty xxx (2015) xxx–xxx 21. Morra EA, Heim CS, Greenwald AS. Preclinical computational models: predictors of tibial insert damage patterns in total knee arthroplasty: AAOS exhibit selection. J Bone Joint Surg Am 2012;94:1 [e137]. 22. Blankevoort L, Kuiper JH, Huiskes R, et al. Articular contact in a three-dimensional model of the knee. J Biomech 1991;24:1019. 23. Hamai S, Miura H, Higaki H, et al. Kinematic analysis of kneeling in cruciate-retaining and posterior-stabilized total knee arthroplasties. J Orthop Res 2008;26:435. 24. Colwell CW, Chen PC, D’Lima D. Extensor malalignment arising from femoral component malrotation in knee arthroplasty: effect of rotating-bearing. Clin Biomech (Bristol, Avon) 2011;26:52. 25. Mihalko WM, Conner DJ, Benner R, et al. How does TKA kinematics vary with transverse plane alignment changes in a contemporary implant? Clin Orthop Relat Res 2012;470:186.
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26. Wachowski MM, Walde TA, Balcarek P, et al. Total knee replacement with natural rollback. Ann Anat 2012;194:195. 27. Browne C, Hermida JC, Bergula A, et al. Patellofemoral forces after total knee arthroplasty: effect of extensor moment arm. Knee 2005;12:81. 28. D’Lima DD, Poole C, Chadha H, et al. Quadriceps moment arm and quadriceps forces after total knee arthroplasty. Clin Orthop Relat Res 2001;213–20. 29. Li G, Papannagari R, Most E, et al. Anterior tibial post impingement in a posterior stabilized tot a1 knee arthroplasty. J Orthop Res 2005;23:536. 30. Shimizu N, Tomita T, Yamazaki T, et al. Posterior sliding of the femur during stair ascending and descending in a high-flex posterior stabilized total knee arthroplasty. J Arthroplasty 2013;28:1707. 31. Bai B, Baez J, Testa NN, et al. Effect of posterior cut angle on tibial component loading. J Arthroplasty 2000;15:916.
Please cite this article as: Okamoto S, et al, Effect of Tibial Posterior Slope on Knee Kinematics, Quadriceps Force, and Patellofemoral Contact Force After Posterior-Stabilized Total Knee Arthroplasty, J Arthroplasty (2015), http://dx.doi.org/10.1016/j.arth.2015.02.042
Contents lists available at ScienceDirect
The Journal of Arthroplasty journal homepage: www.arthroplastyjournal.org
Effect of Tibial Posterior Slope on Knee Kinematics, Quadriceps Force, and Patellofemoral Contact Force After Posterior-Stabilized Total Knee Arthroplasty Shigetoshi Okamoto, MD, Hideki Mizu-uchi, MD, PhD , Ken Okazaki, MD, PhD, Satoshi Hamai, MD, PhD, Hiroyuki Nakahara, MD, PhD, Yukihide Iwamoto, MD, PhD Department of Orthopaedic Surgery, Graduate School of Medical Sciences, Kyushu University, Fukuoka, Japan
a r t i c l e
i n f o
Article history: Received 4 December 2014 Accepted 27 February 2015 Available online xxxx Keywords: total knee arthroplasty tibial posterior slope computer simulation knee instability patellofemoral contact force quadriceps force
a b s t r a c t We used a musculoskeletal model validated with in vivo data to evaluate the effect of tibial posterior slope on knee kinematics, quadriceps force, and patellofemoral contact force after posterior-stabilized total knee arthroplasty. The maximum quadriceps force and patellofemoral contact force decreased with increasing posterior slope. Anterior sliding of the tibial component and anterior impingement of the anterior aspect of the tibial post were observed with tibial posterior slopes of at least 5° and 10°, respectively. Increased tibial posterior slope contributes to improved exercise efficiency during knee extension, however excessive tibial posterior slope should be avoided to prevent knee instability. Based on our computer simulation we recommend tibial posterior slopes of less than 5° in posterior-stabilized total knee arthroplasty. © 2015 Elsevier Inc. All rights reserved.
Total knee arthroplasty (TKA) has become one of the most successful orthopedic procedures for providing pain relief and improving knee function, with reported survival rates of greater than 90% after 15 years [1,2]. Nonetheless, revision surgeries are sometimes required due to poor early results after surgery or premature implant failure. Sharkey et al reported that more than 20% of TKA failures were due to malalignment or malposition that can potentially be avoided with meticulous surgical technique [3]. Previous clinical studies demonstrated that patient-perceived outcomes were affected by malalignment of the components. These clinical results were supported by kinematic data in many biomechanical studies [4–6]. Regarding coronal alignment, Matsuda et al reported that varus postoperative alignment negatively correlated with patient satisfaction [4]. A finite element model showed that tibial coronal malalignment was associated with increased subsidence of the tibial tray [5]. As for rotational alignment, Barrack et al reported that the presence and severity of femoral component internal rotation correlated with patellofemoral complications [6]. In a finite element–rigid body model, patellar maltracking occurred with internal femoral component malrotation [7]. One or more of the authors of this paper have disclosed potential or pertinent conflicts of interest, which may include receipt of payment, either direct or indirect, institutional support, or association with an entity in the biomedical field which may be perceived to have potential conflict of interest with this work. For full disclosure statements refer to http://dx.doi.org/10.1016/j.arth.2015.02.042. Reprint requests: Hideki Mizu-uchi, MD, PhD, Department of Orthopaedic Surgery, Graduate School of Medical Sciences, Kyushu University, 3-1-1 Maidashi, Higashi-ku, Fukuoka 812-8582, Japan.
Even though many papers have recommended surgical techniques to avoid coronal and rotational malalignment, there have been few studies concerning sagittal alignment. In particular, the acceptable range of the tibial posterior slope is still controversial. Tibial posterior slope variations can have both positive and negative effects on knee function and kinematics. Increases in the posterior slope can contribute to improvement of knee flexion and reduce the required quadriceps force for knee motion [8–11], but were also shown to result in anterior tibial translation and posterior articular wear of the insert [12,13]. Anterior impingement of the tibial post is one risk associated with the use of posterior-stabilized TKA [14–16]. Many clinical and cadaveric experimental studies have sought to determine the optimal angle of the tibial posterior slope, however wide variations in cutting errors have led to difficult evaluations. Barrett et al reported that in one-third of cases there was a difference of ±2° between planned and actual posterior tibial slopes when both computer navigation and conventional extramedullary alignment guides were used by high-volume surgeons [17]. In addition, interindividual differences in muscular strength and soft tissue conditions usually affect patellofemoral contact force and quadriceps force. At the present time, the optimal angle of the tibial posterior slope for postoperative knee function is still controversial. Recently, three-dimensional computer models have been developed to predict postoperative knee condition after TKA (e.g., knee kinematics after TKA and mechanical behavior of the prosthesis). Computer simulations can apply large loads and duplicate weight-bearing activities using musculoskeletal models that can diminish the limitations of clinical and
http://dx.doi.org/10.1016/j.arth.2015.02.042 0883-5403/© 2015 Elsevier Inc. All rights reserved.
Please cite this article as: Okamoto S, et al, Effect of Tibial Posterior Slope on Knee Kinematics, Quadriceps Force, and Patellofemoral Contact Force After Posterior-Stabilized Total Knee Arthroplasty, J Arthroplasty (2015), http://dx.doi.org/10.1016/j.arth.2015.02.042
2
S. Okamoto et al. / The Journal of Arthroplasty xxx (2015) xxx–xxx
cadaveric experimental studies. Several previous papers used simulations to predict postoperative flexion angle, knee kinematics, joint load, and damage to the insert [18–21]. These results were validated by clinical data [20] and implant retrieval findings [21]. Computer simulation can compare various factors while holding stiffness and soft tissue position constant, which should reduce the effects of interindividual differences. The purpose of this study was to determine the acceptable range of the tibial posterior slope by simulating quadriceps force, patellofemoral contact force, and knee kinematics.
constrained in the mediolateral and anteroposterior directions but was free to translate vertically under the axial force (in the direction of gravity) that generated a flexion moment at the knee (Fig. 1). Quadriceps force, patellofemoral contact force, and knee kinematics were computed during stair climbing (from 87° to 6° of flexion). Sixteen different angles of the tibial posterior slope were simulated in this study. A posterior slope of zero degrees was defined as perpendicular to the tibial mechanical axis. We changed the posterior angle at 1° intervals ranging from 0° to 15° based on the origin of the coordinates (the center of the tibial insert) in the sagittal alignment.
Materials and Methods Validation of the Computer Model Computer Simulation Weight-bearing stair climbing was simulated in this study, with posterior-stabilized components (NexGen LPS-Flex, Zimmer, Warsaw, Indiana) appropriate for a female (162 cm, 58 kg). Initial coordinates were determined using a computer-assisted design software program (Rhinoceros; Robert McNeel and Associates, Seattle, Washington) as reported in our previous studies [20]. The origin of the initial coordinates was the center of the tibial symmetrical insert in both the anteriorposterior dimension and the medial-lateral dimension. The most distal condylar points of the femoral component were set on the surface of the tibial insert in the superior-inferior dimension. The implant geometry was imported into a dynamic musculoskeletal modeling program (LifeMOD/KneeSIM 2010; LifeModeler, Inc, San Clemente, California). KneeSIM uses rigid body dynamics to simulate weight-bearing stair climbing. The masses of the limb segments and the body weight generate a flexion moment on the knee, whereas the quadriceps muscle exerts an extension moment. This musculoskeletal model of the knee included the medial collateral and lateral collateral ligaments, the quadriceps muscle and tendon, the patellar tendon, and the hamstring muscles. The proximal attachment points of the medial collateral ligament and lateral collateral ligament were defined as the most prominent epicondyles of the femur. Collateral ligaments were modeled as nonlinear springs with material properties obtained from a published report [22]. Contact was simulated between the tibiofemoral and patellofemoral articular surfaces. The hip and ankle joints had all three rotational degrees of freedom. The “ankle” section had no translational degrees of freedom. The “hip” section was
Clinical (in vivo) data were used to validate the computational model. Five female patients (mean age, 73 years (range, 66–78 years); mean tibial posterior slope, 5.0°; range, 2.6–6.7°; mean postoperative follow-up, 26.6 months (range, 13–42 months)) received the same posterior-stabilized TKA as used in our computer simulation (NexGen LPS-Flex, Zimmer Inc., Warsaw, Indiana). Continuous sagittal radiological images were obtained in each patient during stair climbing using a flat-panel detector (Hitachi, Clavis, Tokyo, Japan), and analyzed using a 2D-3D image-matching technique [23]. Antero-posterior translation of the femoral component relative to the tibial tray was compared between the computer simulation and the clinical data. Antero-posterior tibiofemoral position value was defined as anterior (positive) or posterior (negative) to the midline of the tibial tray. This study was approved by the institutional review board (Kyushu University, 3-1-1 Maidashi, Higashi-ku, Fukuoka 812-8582, Japan, No. 25–74). To participate in this study, informed consent was obtained from all the patients. Results Simulation of Quadriceps Force and Patellofemoral Contact Force Quadriceps force and patellofemoral contact force remained between 1500 N and 2000 N from 87° to 65° of knee flexion for all angles of tibial posterior slope (Figs. 2 and 3). Both forces increased rapidly at 65° of knee flexion, at which point maximum vertical load was placed on the knee joint. After the peak force, the forces decreased gradually along with knee extension. Maximum quadriceps forces were 2989 N
Fig. 1. The knee simulator model used in the present study; LifeMOD/KneeSIM 2010.
Please cite this article as: Okamoto S, et al, Effect of Tibial Posterior Slope on Knee Kinematics, Quadriceps Force, and Patellofemoral Contact Force After Posterior-Stabilized Total Knee Arthroplasty, J Arthroplasty (2015), http://dx.doi.org/10.1016/j.arth.2015.02.042
S. Okamoto et al. / The Journal of Arthroplasty xxx (2015) xxx–xxx
Fig. 2. Simulated quadriceps force from 87° to 6° of knee flexion during stair-ascending.
(slope: 15°) to 3171 N (slope: 0°), and maximum patellofemoral contact forces were 2598 N (slope: 15°) to 3061 N (slope: 0°). Increasing the posterior slope decreased the both maximum force. An increase of 15° in the posterior slope (relative to 0°) led to decreases of 6% in the maximum quadriceps force and 15% in the maximum patellofemoral contact force. Simulation of Knee Kinematics The femoral components translated anteriorly with knee extension for all angles of tibial posterior slope (Fig. 4). Increases in the tibial posterior slope resulted in more posterior positioning of the femoral component and reduced the amount of antero-posterior translation. Anterior sliding of the tibial component occurred with 65° of knee flexion if the tibial posterior slope was 5° or more. Anterior impingement between the anterior aspect of the tibial post and the femoral component was observed with near-full knee extension if the tibial posterior slope was 10° or more.
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Fig. 4. Simulated AP translation of the femoral component relative to the tibial insert from 87° to 6° of knee flexion on during stair-ascending. : Anterior sliding of the tibial component happened on 65° of knee flexion if the tibial posterior slope were 5° and more. : Anterior impingement between the anterior aspect of the tibial post and the femoral component was observed on almost full knee extension if the tibial posterior slope were 10° and more.
knees demonstrated femoral rollback with knee flexion, although there was some interspecimen variability in the magnitudes. The predicted rollback was also almost within the interspecimen variability. Discussion
Anteroposterior translation of the femoral component was simulated to closely approximate the measured in vivo data (Fig. 5). Simulated
The useful information of the present study was that increasing the tibial slope posed a risk of inducing knee instability, which caused anterior sliding of the tibial component and anterior impingement between the components. In contrast, quadriceps force and patellofemoral contact force decreased to some extent. These results supported the previous studies more clearly with little effect of the soft tissue condition and difficulty of bone cutting [10,12,14,16]. We used KneeSIM as a modeling program, the usefulness of which has been reported in several papers [20,21,24,25]. Colwell et al reported that a rotating bearing–type implant could not improve patellar kinematics and forces in the presence of femoral component malrotation using a model validated with experimental
Fig. 3. Simulated patellofemoral contact force from 87° to 6° of knee flexion during stair-ascending.
Fig. 5. Femoral component antero-posterior translation relative to the tibial insert. Solid line: computer model. Dot line: in-vivo data.
Model Validation With Clinical Data
Please cite this article as: Okamoto S, et al, Effect of Tibial Posterior Slope on Knee Kinematics, Quadriceps Force, and Patellofemoral Contact Force After Posterior-Stabilized Total Knee Arthroplasty, J Arthroplasty (2015), http://dx.doi.org/10.1016/j.arth.2015.02.042
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S. Okamoto et al. / The Journal of Arthroplasty xxx (2015) xxx–xxx
data from a cadaver study [24]. Mihalko et al validated their model using in vivo fluoroscopic data with anteroposterior translation of the femoral component [25]. Based on our validation and theirs, we considered our computer model to be appropriate although there were abnormal values from the range of experimental data at some angles of knee flexion because the data showed a similar trend and was almost within the interspecimen variability of the clinical data. Our study also validated the computer simulation data with clinical data on postoperative knee kinematics using the same implant. In the present study, increase of 15° in the posterior slope (relative to 0°) led to decreases of 6% in the maximum quadriceps force and 15% in the maximum patellofemoral contact force. Increasing the tibial posterior slope induced a more posterior position of the femoral component. A more posterior contact position between the femorotibial components leads to an increase in the quadriceps lever arm, which improve the movement efficiency which contributes to reduced quadriceps force and patellofemoral contact force [10,26–28]. Therefore, increasing the tibial posterior slope can be expected to reduce the quadriceps force and the patellofemoral contact force required to climb stairs to some extent. Anterior sliding of the tibial component and anterior impingement of the post in posterior-stabilized TKA should be avoided for longterm TKA success [29]. Anterior sliding of the tibial component occurred in the present study with tibial posterior slopes of 5° or more. Shimizu et al using the same implant as our simulation, reported that tibial anterior sliding was observed while shifting weight to the leg during stair climbing [30]. They concluded that knee instability was caused by several factors: absence of the anterior cruciate ligament, implant design, and tibial posterior slope. However, it is difficult to evaluate the effect of these factors on knee instability in the context of this complicated condition. Our study evaluated the actual effect of the posterior slope while controlling for the other factors. Anterior impingement between the tibial post and the femoral component was observed at near-full extension with 10° or more of tibial posterior slope. Anterior impingement on knee extension with the NexGen LPS prosthesis has been reported by several authors [14–16]. Banks et al reported that 75% of cases showed anterior impingement, 41% of knees demonstrated hyperextension during gait, and the hyperextension between the femoral and tibial component averaged 6°. They noted that a 5° anterior bow and 5° posterior slope led to approximately 10° of relative hyperextension of the component when the knee was in full extension. In the present study, the femoral component was positioned neutrally, so if the femoral component flexed, anterior impingement might occur with less than 10° of tibial posterior slope. In terms of clinical relevance, identifying the optimal tibial posterior slope angle should help surgeons cut the proximal tibia properly on the sagittal alignment. In the present study, excessive tibial posterior slope caused knee instability. Bai et al reported that an increased posterior slope cut angle significantly decreased tibial anterior compressive strains and significantly increased tibial posterior compressive strains; the authors recommended cutting the articular surface of the tibia at a 0° or 3° posterior slope to provide the greatest tibial component stability [31]. Based on their results and our own, surgeons should avoid cutting the proximal tibia with greater than 5° of tibial posterior slope in posterior-stabilized TKA. There are several limitations to this study. First, only one prosthesis was used for the simulation. Recently, some prostheses have demonstrated conformity and stability on guiding the medial pivot of the femoral component. The NexGen prosthesis has no inherent built in slope and has a flatter geometry of the insert surface compared with these guided-motion-type designs, which means that the NexGen can more easily be affected by ligaments and/or bone cutting conditions. Therefore, we should evaluate different types of implants in the future and compare the results to our present
findings. Second, only stair climbing was analyzed because we validated the simulation with clinical data based on the same activity. Other actions such as stair descending and sit-stand-sit movements should also be examined to assess the effect of the posterior slope in detail. Even with these limitations, our computer simulation clearly showed the effects of the tibial posterior slope on knee kinematics, patellofemoral contact force, and quadriceps force with accurate bone cutting and constant soft tissue parameters. In conclusion, increases in the tibial posterior slope contributed to improved exercise efficiency of the quadriceps muscle and to reduced contact force of the patellofemoral joint. However, excessive tibial posterior slope should be avoided to prevent anterior sliding of the tibial component and anterior impingement of the post/cam mechanism. Based on the results of our computer simulation, surgeons should maintain the tibial posterior slope at 5° or less when cutting the proximal tibia in posterior-stabilized TKA.
Acknowledgment No benefits in any form have been received or will be received from a commercial party related directly or in directly to the subject of this article.
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