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Insert conformity variation affects kinematics and wear performance of total knee replacements
Clinical Biomechanics 65 (2019) 19–25
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Clinical Biomechanics journal homepage: www.elsevier.com/locate/clinbiomech
Insert conformity variation affects kinematics and wear performance of total knee replacements
T
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Qida Zhanga, Zhenxian Chenb, , Jing Zhanga, Jiayu Hua, Yinghu Penga, Xunjian Fana, Zhongmin Jina,c,d a
State Key Laboratory for Manufacturing System Engineering, School of Mechanical Engineering, Xi'an Jiaotong University, No.28, Xianning West Road, Xi'an, 710049, China b Key Laboratory of Road Construction Technology and Equipment (Ministry of Education), School of Mechanical Engineering, Chang'an University, Middle-section of Nan'er Huan Road, Xi'an 710064, China c Tribology Research Institute, School of Mechanical Engineering, Southwest Jiaotong University, Xibu Yuanqu, Gaoxin District, Chengdu 610031, China d Institute of Medical and Biological Engineering, School of Mechanical Engineering, University of Leeds, Leeds LS2 9JT, UK
A R T I C LE I N FO
A B S T R A C T
Keywords: Total knee replacement Insert conformity Contact mechanics Kinematics Wear
Background: The insert conformity is a critical factor for successful total knee replacement which must be considered in design of the implant. However, the effects of conformity on knee kinematics and wear under physiological environment are often neglected in previous studies. The present study involved evaluating the biomechanics and wear performance with regard to different insert conformity in total knee replacement. Methods: Different tibial inserts with different sagittal and coronal conformity levels were created and analyzed using a previously developed wear prediction framework, coupling a patient-specific musculoskeletal multibody dynamics simulation, finite element and wear analysis. The contact mechanics, kinematics, and wear performance were compared during 10 million cycles of wear simulation. Findings: The findings revealed that the knee kinematics was affected by sagittal conformity design variables, which further influenced the wear of insert bearing surface. Additionally, kinematics and wear of artificial knee joint were much more sensitive to sagittal than coronal conformity of tibial insert. The lower sagittal conformity designs had lower wear rates, worn area and contact area. In turn, the wear of insert bearing surface also changed insert conformity, and further impacted on knee kinematics. Interpretation: The present study indicated that the sagittal conformity design of insert surface played a crucial role to improve contact mechanics and kinematics performance and minimize wear of total knee replacement. The optimization of insert conformity should be considered carefully in implant design and surgical procedures.
1. Introduction Total knee replacement (TKR) is an effective way to relieve pain and restore daily activities for patients with severe knee osteoarthritis (Ardestani et al., 2015a, 2015b). Conformity, defined according to the different femoral and tibial curvature in the sagittal and coronal plane (Ardestani et al., 2015a, 2015b), is one important design parameter that may affect the biomechanics and wear of TKR. When the sagittal conformity is changed, the range of AP translation and IE rotation between the femoral component and tibial insert are changed as well (Fregly et al., 2010). Variation in knee kinematics is further associated with the wear performance and expected lifetime of TKR. Many computational and experimental studies also have shown that higher sagittal insert
conformity had higher predicted wear rates in TKR wear (Abdelgaied et al., 2014; Fregly et al., 2010; Galvin et al., 2009). Currently, the insert conformity has become one of the main focuses of research in artificial knee joint designs. The impact of insert conformity on biomechanics and wear performance of TKR has been regarded as an important design principle in optimizing the geometry of artificial knee joint (Ardestani et al., 2015a, 2015b; Koh et al., 2019). However, on the one hand, higher conformity designs decrease the contact stress and structural wear (Sathasivam and Walker, 1999), and improve the stability of the implants (Luger et al., 1997). On the other hand, lower conformity designs, which avoid the contact stress to exceed the fatigue limit of the material, may reduce delamination wear on the insert bearing surface (Abdelgaied et al.,
⁎ Corresponding author at: Key Laboratory of Road Construction Technology and Equipment, School of Mechanical Engineering, Chang'an University, Middlesection of Nan'er Huan Road, Xi'an 710064, China. E-mail address: [email protected] (Z. Chen).
https://doi.org/10.1016/j.clinbiomech.2019.03.016 Received 12 October 2018; Accepted 25 March 2019 0268-0033/ © 2019 Elsevier Ltd. All rights reserved.
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2014; Essner et al., 2003; Fisher et al., 2009; Galvin et al., 2009; Willing and Kim, 2009). Therefore, conformity demonstrates a conflicting influence on wear in previous studies. Understanding how the insert conformity impacts the biomechanics and wear in TKR is urgent for TKR design. Most previous computational studies (Abdelgaied et al., 2014; Ardestani et al., 2015a, 2015b; Fregly et al., 2010; Willing and Kim, 2009) have been performed to investigated the effects of insert conformity on wear in TKR using the International Standards Organization (ISO) defined motion and load inputs. Nevertheless, the effects of insert conformity on knee load and motion were always neglected. Musculoskeletal multibody dynamics modelling, where the body is modeled as a multibody mechanical system connected by joints and actuated by muscles, provided a strong platform for investigating the knee loading and kinematics. Compared with finite element models, musculoskeletal multibody dynamic models are capable of predicting muscle forces, joint contact forces and motion simultaneously under different physiological activity. The predicted knee load and motion profiles under in vivo musculoskeletal physiological environment can be used as boundary conditions for finite element analysis of TKR. Recently, the wear performance of an artificial knee joint was investigated using a novel patient-specific wear prediction framework coupling a patientspecific lower extremity musculoskeletal multibody dynamics model with the finite element contact mechanics and wear model of TKR (Zhang et al., 2017), and the changes in kinematics and loading due to tibial insert geometry variation caused by wear were taken into consideration. Compared to the traditional wear prediction using fixed load/motions, the novel wear prediction framework showed the tibialfemoral contact forces and kinematics were influenced by articular surface wear, and in turn, the variations from the knee dynamics resulted in increases in the volumetric wear (Zhang et al., 2017). However, using the multibody dynamics modelling of TKR, which takes the patient-specific lower extremity musculoskeletal system into consideration, to investigate the effects of insert conformity on biomechanics and wear in TKR, has not been assessed. The objective of the present study was to use a previously developed wear prediction framework coupling a patient-specific musculoskeletal (MSK) multibody dynamics (MBD), finite element analysis (FEA) and wear analysis of TKR to investigate the effects of insert conformity on biomechanics and wear in TKR. Quantifying the influence of variability of insert conformity will aid in determining how large an influence these parameters can have on joint forces, kinematics and wear, and hence must be accounted for in implant design and surgical procedures. It was hypothesized that higher insert conformity had lower amplitude of the femoral internal-external rotation, which further lead to higher wear rates and wear area.
Table 1 Different conformities of different implants in the sagittal and coronal plane used for biomechanics and wear prediction. Model Sagittal Plane
Coronal Plane
S-Model-1 S-Model-2 S-Model-3 C-Model-1 C-Model-2 C-Model-3 C-Model-4
Femoral radius (mm)
Tibial insert radius (mm)
36 36 36 45 45 45 45
36 50 64 45 60 75 90
2. Methods Different tibial inserts, with different sagittal (denoted by S) and coronal (denoted by C) conformity levels (Fig. 1), were considered and created using SolidWorks Software (Dassault Systems SolidWorks Corp., Waltham MA). These different tibial inserts with the same dimensions except for the sagittal and coronal radii were analyzed against a same femoral component (Table 1). Tibial coronal radius for S-Models with different sagittal radii and tibial sagittal radius for C-models with different coronal radii are 45 mm and 65 mm, respectively. Standard materials combinations were assumed for the femoral (CoCrMo) and the tibial (conventional ultra-high molecular weight polyethylene, UHMWPE) components. A previously developed wear prediction framework (Zhang et al., 2017), coupling a patient-specific MSK MBD model, FE model and wear model of a TKR, was used in the present study. Each tibial insert design was separately imported into the developed patient-specific wear prediction framework of the TKR. And then, the effects of insert conformity on biomechanics and wear in TKR were quantified during gait simulation. The patient-specific MSK MBD model of TKR was developed in the commercially software Anybody (version6.0; AnyBody Technology, Aalborg, Denmark) using an advanced bone morphing method (Pellikaan et al., 2014). A new knee joint model with 11 degrees of freedoms was defined using the force-dependent kinematics (FDK) method (Andersen and Rasmussen, 2011), which included deformable contact models of the artificial knee joint. The patellar ligament was assumed to be rigid, and therefore, there are 6 degrees of freedom in the tibial-femoral joint and 5 degrees of freedom in the patellar-femoral joint (Chen et al., 2016). For different insert conformities, each insert design was separately imported into Anybody in STL file. Three deformable contact models were defined between the femoral component and the medial/lateral compartment of tibial insert and between the femoral component and the patellar button based on the elastic foundation theory (Fregly et al., 2003). UHMWPE was considered as a nonlinear material (Chen et al., 2016). The contact forces for all contact pairs were calculated using a linear force-penetration volume law with a contact Pressure Module of 1.24 × 1011 N/m3 in this study (Chen
Fig. 1. Schematic diagram for different insert types with different conformities in the sagittal and coronal planes. S-Model-1(2,3): tibial inserts with different sagittal radii; C-Model-1(2,3, 4): tibial inserts with different coronal radii. 20
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Fig. 2. Total contact forces (a)(d), medial contact forces (b)(e), lateral contact forces (c)(f) of knee joint predicted from MSK MBD models with different sagittal radii (S-Model-1, S-Model-2 and S-Model-3) and coronal radii (C-Model-1, C-Model-2, C-Model-3 and C-Model-4).
Fig. 3. Femoral flexion-extension angle (a)(d), internal-external rotation (b)(e), anterior-posterior translation (c)(f) of knee joint from MSK MBD model in the sagittal plane and in the coronal plane.
kneeloads) were imported into the developed MSK model to calculate the medial, lateral and total tibial-femoral contact forces, patellar-femoral contact force and ligament forces, as well as the knee joint motions using the FDK method (Chen et al., 2016; Zhang et al., 2017). The average medial, lateral and total tibial-femoral contact forces were calculated to investigate the effects of insert conformity on biomechanics and wear in TKR. Further details on MSK modelling of TKA can be found in the previous studies (Chen et al., 2016; Zhang et al., 2017).
et al., 2014). The ligaments including the medial collateral ligament (MCL), the lateral collateral ligament (LCL), the posterior cruciate ligament (PCL), the posterior-medial capsule (PMC), and the medial and lateral patellar-femoral ligaments (MPFL, LPFL) are wrapped around the knee joint model (Chen et al., 2016; Zhang et al., 2017). Ligaments were modeled as non-linear spring elements with a piecewise forcedisplacement relationship (Blankevoort et al., 1991). Six patient-specific walking gait trials with experimental ground reaction forces obtained from the publically available data (https://simtk.org/home/ 21
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conditions for FEA to calculate the contact pressure and sliding distances. Custom Python scripts were used to extract the results from Abaqus output database, and a Matlab script was developed to calculate wear and creep for each node of the contact surfaces and output the worn insert geometry. The geometry of the contact surface was updated using a total linear damage depth at each node. The updated worn insert geometry was subsequently imported into the MSK MBD model of TKR to further calculate new knee contact forces and motions for the new FEA. Wear iterations were performed for the first 1 million cycles to account for the large creep at the early stage, and then were considered every 1 million cycles, and end till 10 million cycles.
Table 2 Computational volumetric wear rates (mm3/106cycles) for different bearing inserts with different sagittal radii (S-Model-1, S-Model-2 and S-Model-3) and coronal radii (C-Model-1, C-Model-2, C-Model-3 and C-Model-4). Volumetric wear rates (mm3/106 cycles)
Model S-Model-1 S-Model-2 S-Model-3 C-Model-1 C-Model-2 C-Model-3 C-Model-4
8.5389 8.5230 8.4251 8.4251 8.4195 8.0828 7.8443
3. Results The FE models of TKR with different insert conformities were separately established using Abaqus Software (Abaqus 6.12, Simulia Inc., Providence, RI). The conventional UHMWPE tibial insert was modeled as a non-linear material with a modulus of elasticity of 463 MPa and a Poisson's ratio of 0.46 (Abdelgaied et al., 2011). A coefficient of friction of 0.04 (Godest et al., 2002; Jin and Dowson, 2013; O'Brien et al., 2013; Sathasivam and Walker, 1997; Zhang et al., 2017) was defined in a penalty contact formulation to describe the contact between the tibial and the femoral contact surfaces. The calculated knee contact forces and motions during the walking simulation using the patient-specific MSK MBD model of TKR were used as boundary conditions. The total contact force was applied on the reference node of the femoral component, defined at the axis through the center of the femoral contact surface and offset by 5 mm in the medial direction according to ISO 14243 (International organization for standardization, 2009). The flexion-extension of the femoral component was also prescribed through the reference node. Meanwhile, anterior-posterior displacement and internal-external rotation of the tibial insert were applied on the tibial reference node. The wear model, which developed and validated by Abdelgaied et al. (Abdelgaied et al., 2011), was adopted to predict the articular surface wear of the UHMWPE tibial insert. The wear law was based on the idea that wear volume (V) is proportional to the contact area (A) and sliding distance (S): V = CAS. The non-dimensional wear coefficient (C) dependents on a cross-shear ratio (CS), determined from the experimental measurements of a multi-directional pin-on-disk (POD) wear test in the presence of bovine serum lubricant to achieve the same boundary and mixed lubrication condition as in artificial joints (Kang et al., 2008; Kang et al., 2009; Kennedy et al., 2013; Wang et al., 1998). The knee contact forces, motions and ligament forces during gait simulation were calculated using the patient-specific MSK MBD model of TKR, and then these results were input as the essential boundary
Effects of insert sagittal and coronal conformity on contact forces and kinematics in TKR are shown in Figs. 2 and 3. Overall, slight differences of the knee joint contact forces were observed in the sagittal plane and in the coronal plane (Fig. 2). The joint kinematics was less sensitive to coronal than sagittal conformity changes (Fig. 3). However, relatively larger changes were found in the internal-external rotation (Fig. 3b) and the anterior-posterior translation (Fig. 3c) in the sagittal plane. Higher insert conformity had lower amplitude of the femoral internal-external rotation (the amplitude values decreased by 22.6% from S-Model-3 to S-Model-1) (Fig. 3b). Higher insert conformity reduced the tibial external rotation (the maximum values decreased from −5.2 degrees to −4.8 degrees) (Fig. 3b). The primary flexion-extension angle (Figs. 3a and d) was not sensitive to sagittal and coronal conformity. Furthermore, the effects of insert sagittal and coronal conformity on the contact forces and kinematics of the knee joint from MSK MBD model from 0 million cycles to 10 million cycles were shown in Appendix. The predicted wear rates for different sagittal and coronal inserts are shown in Table 2. In the sagittal plane, the predicted wear rates for the three different inserts with different radii were 8.5389, 8.5230 and 8.4251 mm3/million cycles, respectively. Specifically, higher sagittal conformity inserts exhibited a higher predicted wear rates. In the coronal plane, the predicted wear rates for the four different inserts with different radii were 8.4795, 8.4251, 8.0828 and 7.8443 mm3/million cycles, respectively. Figs. 4 and 5 shows the wear area and the contact area of bearing surface for different insert types in the sagittal plane and in the coronal plane after 10 million cycles. It is clear that the wear area and the contact area are much more sensitive to the sagittal than coronal conformity changes and both increased with higher conformity. Furthermore, higher conformity had higher wear area (the wear area of the SModel-1 increased from 620mm2 to 960 mm2 after 10 million cycles,
Fig. 4. The wear area of bearing surface for different insert types in the sagittal plane (a) and in the coronal plane (b) after 10 million cycles. 22
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Fig. 5. The contact area of bearing surface for different insert types in the sagittal plane (a) and in the coronal plane (b) after 10 million cycles.
Fig. 6. The accumulated sliding distance distribution (mm) (a) and linear wear distribution (mm) (b) of bearing surface for different insert types in the sagittal plane after 10 million cycles.
the knee kinematics was influenced by the sagittal conformity design of the bearing surface, which further affected the wear of the bearing surface. In addition, wear and kinematics of the artificial knee joint are much more sensitive to the sagittal than coronal conformity variables, and the lower sagittal conformity designs had the lower predicted wear rates. Our studies showed that slight differences of the knee contact forces were observed among different sagittal and coronal conformity designs. However, the knee kinematics was markedly affected by the sagittal conformity design variables. Our studies were consistent with Fitzpatrick et al.'s studies (Fitzpatrick et al., 2012a, 2012b) in which design factors (including conformity design) were the primary contributors to tibiofemoral kinematics and the joint loads were dependent on patient-specific factors. Meanwhile, Ardestani et al. (2015a, 2015b) also found that conformity directly affected the reliability of the artificial knee joint, and insert sagittal conformity affected kinematics. These results indicated that the sagittal conformity design has a direct impact on determining the knee kinematics of TKR.
compared to the wear area of the S-Model-3) (Fig. 4). Figs. 6 and 7 shows the accumulated sliding distance distribution and the linear wear distribution of the bearing surface for different sagittal insert types and coronal insert types, respectively. The maximum values of the accumulated sliding distance and the linear wear were both increased with lower sagittal conformity while the distribution areas decreased after 10 million cycles. However, slight differences of the accumulated sliding distance distribution and the linear wear distribution for different coronal conformity were observed. 4. Discussion The most important findings of the present study involves the effects of variability of insert conformity on joint forces, kinematics and wear in TKR under the musculoskeletal physiological environment using a previously developed wear prediction framework coupling a patientspecific musculoskeletal multibody dynamics, finite element analysis and wear analysis. The study hypothesis was accepted. We found that 23
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Fig. 7. The accumulated sliding distance distribution (mm) (a) and linear wear distribution (mm) (b) of bearing surface for different insert types in the coronal plane after 10 million cycles.
updated worn insert geometry were subsequently imported into the MSK MBD model of TKR to further calculate new knee contact forces and motions for the new FEA from 0 million cycles to 10 million cycles. Meanwhile, the wear area increased by 35.4% from 620 mm2 to 960 mm2 after 10 million cycles (Fig. 4a), while the tibial insert radius decreased by 43.8% from 64 mm (S-Model-3) to 36 mm (S-Model-1) (Table 1). Therefore, the insert conformity, kinematics and wear, like a closed loop system, were actually affected by each other under the human musculoskeletal physiological environment, instead of the traditional wear prediction using ISO fixed load/motions. Because the traditional wear prediction always neglect the kinematics changes. It mean that investigating the effect of insert conformity on wear should take the patient-specific joint motion and loading into consideration. When the tibial insert radius decreased by 43.8% (Table 1), which means the insert conformity is increased, the wear area increased by 35.4% after 10 million cycles (Fig. 4a) but the kinematics of knee joint tends to more stable. Therefore, the results of present study indicated that the optimization of insert conformity may have a significant impact on restoring daily activities for TKR patients. The present study showed that higher sagittal conformity designs had higher wear rates (Table 2), wear areas (Fig. 4) and contact areas (Fig. 5). In contrast, the maximum value of accumulated depth and accumulated sliding distance showed lower values with higher sagittal conformity (Fig. 6) after 10 million cycles. Abdelgaied et al. (Abdelgaied et al., 2014) found that the predicted wear rates for the curved insert (most conformed) were more than three times those for the flat insert (least conformed). Meanwhile, Galvin et al. (Galvin et al., 2009) also found that the wear rates for the flat UHMWPE insert were significantly lower than those for the curved UHMWPE insert under same kinematic conditions. And Willing et al. (Willing and Kim, 2009) suggested that the tibial insert wear was reduced by modifying the contact geometry of both components in the sagittal and coronal planes. Our findings were consistent with previous studies (Abdelgaied et al., 2014; Essner et al., 2003; Fregly et al., 2010; Galvin et al., 2009), although the values of volumetric wear rates in similar tibia insert models were slightly different. Previous computational wear prediction models did not take musculoskeletal physiological environment into consideration to investigate the effect of insert conformity on wear in TKR in previous studies. Furthermore, different tibial inserts with the same dimensions except for the sagittal and coronal radii were analyzed against the same femoral in this study, whilst commercial knee implants were used in previous studies (Abdelgaied et al., 2014; Galvin et al., 2009). Therefore, it is difficult to investigate and isolate the effect of the insert conformity alone. In contrast, the present results provided more
Moreover, the present study investigated the changes in contact forces and kinematics of the artificial knee joint resulting from different sagittal and coronal insert conformity after a 10 million cycle wear simulation, compared with the initial stage. The present results suggested that smaller changes were found for knee contact forces before and after wear for different sagittal insert conformity, and higher changes were found for knee kinematics (range of anterior-posterior translation and range of internal-external rotation). Therefore, the insert conformity design influenced knee kinematics, which further influenced the wear of the bearing surface. Meanwhile, wear of the bearing surface also changed the knee kinematics under different insert sagittal conformity designs. Especially, the ranges of internal-external rotation and anterior-posterior displacement were influenced by the sagittal conformity designs as the wear processed. Our studies were consistent with Williams et al.'s study (Williams et al., 2010) in which there were large increases in the contact stress and measureable changes in the kinematics of the knee joint as wear increases in the polyethylene insert. These issues should be taken into consideration when designing total knee implants in order to minimize these long-term effects (Williams et al., 2010). Meanwhile, Fregly et al. (2010) also found that when the sagittal conformity was changed, the relative kinematics between the femoral component and tibial insert were changed as well. The most obvious changes were located in the range of anterior-posterior translation and internal-external rotation (Fregly et al., 2010). Therefore, the present results highlighted that the optimization of insert sagittal conformity should be considered carefully in implant design and surgical procedures. Interestingly, One of the most important findings of the present study was that the knee kinematics were influenced by the insert conformity design, which further affected the wear of the bearing surface (Koh et al., 2019; Zhou and Jin, 2015). Specifically, higher insert conformity had lower amplitude of the femoral internal-external rotation and higher anterior-posterior translation (Fig. 3b). When the tibial insert radius decreased by 43.8% from 64 mm (S-Model-3) to 36 mm (SModel-1) (Table 1), which means the insert conformity is increased, the maximum values of the tibial external rotation decreased by 26.9% from −5.2 degrees to −3.8 degrees (Fig. 3b). Therefore, the insert conformity obviously affects the knee joint motions. And then these different knee joint contact forces and motions (especially the internalexternal rotation and anterior-posterior translation) caused by the different insert conformity were input as the essential boundary conditions for FEA to calculate the different contact pressure and sliding distances, which further lead different wear and output the different worn insert geometry in current computational model. Furthermore, the different 24
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reliable information about how insert conformity affects the knee contact forces, kinematics and wear performance of TKR. Some limitations of this study should be discussed. First, only three different tibial insert models in the sagittal plane and four different tibial insert models in the coronal plane were investigated in this study, and wear simulations were performed exclusively for a walking gait. Future studies will create parametric knee models to test more different design variables and materials during different kinds of gait simulations. Second, our studies do not take the femoral sagittal and coronal radii as design variables into consideration. The effects of femoral conformity on biomechanics and insert wear in TKR would be investigated in the future work. Third, although the present results were in good agreement with the results obtained in previous studies, further clinical investigations are required. Despite these limitations, the present study still provide an insight into insert conformity variables, which benefits to improve the design of the artificial knee joint for reducing wear in TKRs.
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5. Conclusions The results showed that higher insert conformity had lower amplitude of the femoral internal-external rotation, lower maximum values of the tibial internal rotation, and higher anterior-posterior translation, which further lead to higher wear rates and wear area. Additionally, wear and kinematics of the artificial knee joint are much more sensitive to the sagittal than coronal conformity variables. Therefore, these results indicated that insert sagittal conformity design played an essential role to improve kinematics performance and minimize wear of TKR. Acknowledgments This study was supported by “National Natural Science Foundation of China” [Grants number: 51323007]. The authors thank Professor B.J. Fregly and colleagues for providing publicly accessible data. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.clinbiomech.2019.03.016. References Abdelgaied, A., Liu, F., Brockett, C., Jennings, L., Fisher, J., Jin, Z., 2011. Computational wear prediction of artificial knee joints based on a new wear law and formulation. J. Biomech. 44, 1108–1116. Abdelgaied, A., Brockett, C.L., Liu, F., Jennings, L.M., Jin, Z., Fisher, J., 2014. The effect of insert conformity and material on total knee replacement wear. Proc. Inst. Mech. Eng. H J. Eng. Med. 228, 98–106. Andersen, M.S., Rasmussen, J., 2011. Total knee replacement musculoskeletal model using a novel simulation method for non-conforming joints. In: Proceedings of the International Society of Biomechanics Conference International Society of Biomechanics. ISB. Ardestani, M.M., Moazen, M., Jin, Z., 2015a. Contribution of geometric design parameters to knee implant performance: conflicting impact of conformity on kinematics and contact mechanics. Knee 22, 217–224. Ardestani, M.M., Moazen, M., Maniei, E., Jin, Z., 2015b. Posterior stabilized versus cruciate retaining total knee arthroplasty designs: conformity affects the performance reliability of the design over the patient population. Med. Eng. Phys. 37, 350–360. Blankevoort, L., Kuiper, J.H., Huiskes, R., Grootenboer, H.J., 1991. Articular contact in a 3-dimensional model of the knee. J. Biomech. 24, 1019–1031. Chen, Z., Zhang, X., Ardestani, M.M., Wang, L., Liu, Y., Lian, Q., He, J., Li, D., Jin, Z.,
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Clinical Biomechanics journal homepage: www.elsevier.com/locate/clinbiomech
Insert conformity variation affects kinematics and wear performance of total knee replacements
T
⁎
Qida Zhanga, Zhenxian Chenb, , Jing Zhanga, Jiayu Hua, Yinghu Penga, Xunjian Fana, Zhongmin Jina,c,d a
State Key Laboratory for Manufacturing System Engineering, School of Mechanical Engineering, Xi'an Jiaotong University, No.28, Xianning West Road, Xi'an, 710049, China b Key Laboratory of Road Construction Technology and Equipment (Ministry of Education), School of Mechanical Engineering, Chang'an University, Middle-section of Nan'er Huan Road, Xi'an 710064, China c Tribology Research Institute, School of Mechanical Engineering, Southwest Jiaotong University, Xibu Yuanqu, Gaoxin District, Chengdu 610031, China d Institute of Medical and Biological Engineering, School of Mechanical Engineering, University of Leeds, Leeds LS2 9JT, UK
A R T I C LE I N FO
A B S T R A C T
Keywords: Total knee replacement Insert conformity Contact mechanics Kinematics Wear
Background: The insert conformity is a critical factor for successful total knee replacement which must be considered in design of the implant. However, the effects of conformity on knee kinematics and wear under physiological environment are often neglected in previous studies. The present study involved evaluating the biomechanics and wear performance with regard to different insert conformity in total knee replacement. Methods: Different tibial inserts with different sagittal and coronal conformity levels were created and analyzed using a previously developed wear prediction framework, coupling a patient-specific musculoskeletal multibody dynamics simulation, finite element and wear analysis. The contact mechanics, kinematics, and wear performance were compared during 10 million cycles of wear simulation. Findings: The findings revealed that the knee kinematics was affected by sagittal conformity design variables, which further influenced the wear of insert bearing surface. Additionally, kinematics and wear of artificial knee joint were much more sensitive to sagittal than coronal conformity of tibial insert. The lower sagittal conformity designs had lower wear rates, worn area and contact area. In turn, the wear of insert bearing surface also changed insert conformity, and further impacted on knee kinematics. Interpretation: The present study indicated that the sagittal conformity design of insert surface played a crucial role to improve contact mechanics and kinematics performance and minimize wear of total knee replacement. The optimization of insert conformity should be considered carefully in implant design and surgical procedures.
1. Introduction Total knee replacement (TKR) is an effective way to relieve pain and restore daily activities for patients with severe knee osteoarthritis (Ardestani et al., 2015a, 2015b). Conformity, defined according to the different femoral and tibial curvature in the sagittal and coronal plane (Ardestani et al., 2015a, 2015b), is one important design parameter that may affect the biomechanics and wear of TKR. When the sagittal conformity is changed, the range of AP translation and IE rotation between the femoral component and tibial insert are changed as well (Fregly et al., 2010). Variation in knee kinematics is further associated with the wear performance and expected lifetime of TKR. Many computational and experimental studies also have shown that higher sagittal insert
conformity had higher predicted wear rates in TKR wear (Abdelgaied et al., 2014; Fregly et al., 2010; Galvin et al., 2009). Currently, the insert conformity has become one of the main focuses of research in artificial knee joint designs. The impact of insert conformity on biomechanics and wear performance of TKR has been regarded as an important design principle in optimizing the geometry of artificial knee joint (Ardestani et al., 2015a, 2015b; Koh et al., 2019). However, on the one hand, higher conformity designs decrease the contact stress and structural wear (Sathasivam and Walker, 1999), and improve the stability of the implants (Luger et al., 1997). On the other hand, lower conformity designs, which avoid the contact stress to exceed the fatigue limit of the material, may reduce delamination wear on the insert bearing surface (Abdelgaied et al.,
⁎ Corresponding author at: Key Laboratory of Road Construction Technology and Equipment, School of Mechanical Engineering, Chang'an University, Middlesection of Nan'er Huan Road, Xi'an 710064, China. E-mail address: [email protected] (Z. Chen).
https://doi.org/10.1016/j.clinbiomech.2019.03.016 Received 12 October 2018; Accepted 25 March 2019 0268-0033/ © 2019 Elsevier Ltd. All rights reserved.
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2014; Essner et al., 2003; Fisher et al., 2009; Galvin et al., 2009; Willing and Kim, 2009). Therefore, conformity demonstrates a conflicting influence on wear in previous studies. Understanding how the insert conformity impacts the biomechanics and wear in TKR is urgent for TKR design. Most previous computational studies (Abdelgaied et al., 2014; Ardestani et al., 2015a, 2015b; Fregly et al., 2010; Willing and Kim, 2009) have been performed to investigated the effects of insert conformity on wear in TKR using the International Standards Organization (ISO) defined motion and load inputs. Nevertheless, the effects of insert conformity on knee load and motion were always neglected. Musculoskeletal multibody dynamics modelling, where the body is modeled as a multibody mechanical system connected by joints and actuated by muscles, provided a strong platform for investigating the knee loading and kinematics. Compared with finite element models, musculoskeletal multibody dynamic models are capable of predicting muscle forces, joint contact forces and motion simultaneously under different physiological activity. The predicted knee load and motion profiles under in vivo musculoskeletal physiological environment can be used as boundary conditions for finite element analysis of TKR. Recently, the wear performance of an artificial knee joint was investigated using a novel patient-specific wear prediction framework coupling a patientspecific lower extremity musculoskeletal multibody dynamics model with the finite element contact mechanics and wear model of TKR (Zhang et al., 2017), and the changes in kinematics and loading due to tibial insert geometry variation caused by wear were taken into consideration. Compared to the traditional wear prediction using fixed load/motions, the novel wear prediction framework showed the tibialfemoral contact forces and kinematics were influenced by articular surface wear, and in turn, the variations from the knee dynamics resulted in increases in the volumetric wear (Zhang et al., 2017). However, using the multibody dynamics modelling of TKR, which takes the patient-specific lower extremity musculoskeletal system into consideration, to investigate the effects of insert conformity on biomechanics and wear in TKR, has not been assessed. The objective of the present study was to use a previously developed wear prediction framework coupling a patient-specific musculoskeletal (MSK) multibody dynamics (MBD), finite element analysis (FEA) and wear analysis of TKR to investigate the effects of insert conformity on biomechanics and wear in TKR. Quantifying the influence of variability of insert conformity will aid in determining how large an influence these parameters can have on joint forces, kinematics and wear, and hence must be accounted for in implant design and surgical procedures. It was hypothesized that higher insert conformity had lower amplitude of the femoral internal-external rotation, which further lead to higher wear rates and wear area.
Table 1 Different conformities of different implants in the sagittal and coronal plane used for biomechanics and wear prediction. Model Sagittal Plane
Coronal Plane
S-Model-1 S-Model-2 S-Model-3 C-Model-1 C-Model-2 C-Model-3 C-Model-4
Femoral radius (mm)
Tibial insert radius (mm)
36 36 36 45 45 45 45
36 50 64 45 60 75 90
2. Methods Different tibial inserts, with different sagittal (denoted by S) and coronal (denoted by C) conformity levels (Fig. 1), were considered and created using SolidWorks Software (Dassault Systems SolidWorks Corp., Waltham MA). These different tibial inserts with the same dimensions except for the sagittal and coronal radii were analyzed against a same femoral component (Table 1). Tibial coronal radius for S-Models with different sagittal radii and tibial sagittal radius for C-models with different coronal radii are 45 mm and 65 mm, respectively. Standard materials combinations were assumed for the femoral (CoCrMo) and the tibial (conventional ultra-high molecular weight polyethylene, UHMWPE) components. A previously developed wear prediction framework (Zhang et al., 2017), coupling a patient-specific MSK MBD model, FE model and wear model of a TKR, was used in the present study. Each tibial insert design was separately imported into the developed patient-specific wear prediction framework of the TKR. And then, the effects of insert conformity on biomechanics and wear in TKR were quantified during gait simulation. The patient-specific MSK MBD model of TKR was developed in the commercially software Anybody (version6.0; AnyBody Technology, Aalborg, Denmark) using an advanced bone morphing method (Pellikaan et al., 2014). A new knee joint model with 11 degrees of freedoms was defined using the force-dependent kinematics (FDK) method (Andersen and Rasmussen, 2011), which included deformable contact models of the artificial knee joint. The patellar ligament was assumed to be rigid, and therefore, there are 6 degrees of freedom in the tibial-femoral joint and 5 degrees of freedom in the patellar-femoral joint (Chen et al., 2016). For different insert conformities, each insert design was separately imported into Anybody in STL file. Three deformable contact models were defined between the femoral component and the medial/lateral compartment of tibial insert and between the femoral component and the patellar button based on the elastic foundation theory (Fregly et al., 2003). UHMWPE was considered as a nonlinear material (Chen et al., 2016). The contact forces for all contact pairs were calculated using a linear force-penetration volume law with a contact Pressure Module of 1.24 × 1011 N/m3 in this study (Chen
Fig. 1. Schematic diagram for different insert types with different conformities in the sagittal and coronal planes. S-Model-1(2,3): tibial inserts with different sagittal radii; C-Model-1(2,3, 4): tibial inserts with different coronal radii. 20
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Fig. 2. Total contact forces (a)(d), medial contact forces (b)(e), lateral contact forces (c)(f) of knee joint predicted from MSK MBD models with different sagittal radii (S-Model-1, S-Model-2 and S-Model-3) and coronal radii (C-Model-1, C-Model-2, C-Model-3 and C-Model-4).
Fig. 3. Femoral flexion-extension angle (a)(d), internal-external rotation (b)(e), anterior-posterior translation (c)(f) of knee joint from MSK MBD model in the sagittal plane and in the coronal plane.
kneeloads) were imported into the developed MSK model to calculate the medial, lateral and total tibial-femoral contact forces, patellar-femoral contact force and ligament forces, as well as the knee joint motions using the FDK method (Chen et al., 2016; Zhang et al., 2017). The average medial, lateral and total tibial-femoral contact forces were calculated to investigate the effects of insert conformity on biomechanics and wear in TKR. Further details on MSK modelling of TKA can be found in the previous studies (Chen et al., 2016; Zhang et al., 2017).
et al., 2014). The ligaments including the medial collateral ligament (MCL), the lateral collateral ligament (LCL), the posterior cruciate ligament (PCL), the posterior-medial capsule (PMC), and the medial and lateral patellar-femoral ligaments (MPFL, LPFL) are wrapped around the knee joint model (Chen et al., 2016; Zhang et al., 2017). Ligaments were modeled as non-linear spring elements with a piecewise forcedisplacement relationship (Blankevoort et al., 1991). Six patient-specific walking gait trials with experimental ground reaction forces obtained from the publically available data (https://simtk.org/home/ 21
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conditions for FEA to calculate the contact pressure and sliding distances. Custom Python scripts were used to extract the results from Abaqus output database, and a Matlab script was developed to calculate wear and creep for each node of the contact surfaces and output the worn insert geometry. The geometry of the contact surface was updated using a total linear damage depth at each node. The updated worn insert geometry was subsequently imported into the MSK MBD model of TKR to further calculate new knee contact forces and motions for the new FEA. Wear iterations were performed for the first 1 million cycles to account for the large creep at the early stage, and then were considered every 1 million cycles, and end till 10 million cycles.
Table 2 Computational volumetric wear rates (mm3/106cycles) for different bearing inserts with different sagittal radii (S-Model-1, S-Model-2 and S-Model-3) and coronal radii (C-Model-1, C-Model-2, C-Model-3 and C-Model-4). Volumetric wear rates (mm3/106 cycles)
Model S-Model-1 S-Model-2 S-Model-3 C-Model-1 C-Model-2 C-Model-3 C-Model-4
8.5389 8.5230 8.4251 8.4251 8.4195 8.0828 7.8443
3. Results The FE models of TKR with different insert conformities were separately established using Abaqus Software (Abaqus 6.12, Simulia Inc., Providence, RI). The conventional UHMWPE tibial insert was modeled as a non-linear material with a modulus of elasticity of 463 MPa and a Poisson's ratio of 0.46 (Abdelgaied et al., 2011). A coefficient of friction of 0.04 (Godest et al., 2002; Jin and Dowson, 2013; O'Brien et al., 2013; Sathasivam and Walker, 1997; Zhang et al., 2017) was defined in a penalty contact formulation to describe the contact between the tibial and the femoral contact surfaces. The calculated knee contact forces and motions during the walking simulation using the patient-specific MSK MBD model of TKR were used as boundary conditions. The total contact force was applied on the reference node of the femoral component, defined at the axis through the center of the femoral contact surface and offset by 5 mm in the medial direction according to ISO 14243 (International organization for standardization, 2009). The flexion-extension of the femoral component was also prescribed through the reference node. Meanwhile, anterior-posterior displacement and internal-external rotation of the tibial insert were applied on the tibial reference node. The wear model, which developed and validated by Abdelgaied et al. (Abdelgaied et al., 2011), was adopted to predict the articular surface wear of the UHMWPE tibial insert. The wear law was based on the idea that wear volume (V) is proportional to the contact area (A) and sliding distance (S): V = CAS. The non-dimensional wear coefficient (C) dependents on a cross-shear ratio (CS), determined from the experimental measurements of a multi-directional pin-on-disk (POD) wear test in the presence of bovine serum lubricant to achieve the same boundary and mixed lubrication condition as in artificial joints (Kang et al., 2008; Kang et al., 2009; Kennedy et al., 2013; Wang et al., 1998). The knee contact forces, motions and ligament forces during gait simulation were calculated using the patient-specific MSK MBD model of TKR, and then these results were input as the essential boundary
Effects of insert sagittal and coronal conformity on contact forces and kinematics in TKR are shown in Figs. 2 and 3. Overall, slight differences of the knee joint contact forces were observed in the sagittal plane and in the coronal plane (Fig. 2). The joint kinematics was less sensitive to coronal than sagittal conformity changes (Fig. 3). However, relatively larger changes were found in the internal-external rotation (Fig. 3b) and the anterior-posterior translation (Fig. 3c) in the sagittal plane. Higher insert conformity had lower amplitude of the femoral internal-external rotation (the amplitude values decreased by 22.6% from S-Model-3 to S-Model-1) (Fig. 3b). Higher insert conformity reduced the tibial external rotation (the maximum values decreased from −5.2 degrees to −4.8 degrees) (Fig. 3b). The primary flexion-extension angle (Figs. 3a and d) was not sensitive to sagittal and coronal conformity. Furthermore, the effects of insert sagittal and coronal conformity on the contact forces and kinematics of the knee joint from MSK MBD model from 0 million cycles to 10 million cycles were shown in Appendix. The predicted wear rates for different sagittal and coronal inserts are shown in Table 2. In the sagittal plane, the predicted wear rates for the three different inserts with different radii were 8.5389, 8.5230 and 8.4251 mm3/million cycles, respectively. Specifically, higher sagittal conformity inserts exhibited a higher predicted wear rates. In the coronal plane, the predicted wear rates for the four different inserts with different radii were 8.4795, 8.4251, 8.0828 and 7.8443 mm3/million cycles, respectively. Figs. 4 and 5 shows the wear area and the contact area of bearing surface for different insert types in the sagittal plane and in the coronal plane after 10 million cycles. It is clear that the wear area and the contact area are much more sensitive to the sagittal than coronal conformity changes and both increased with higher conformity. Furthermore, higher conformity had higher wear area (the wear area of the SModel-1 increased from 620mm2 to 960 mm2 after 10 million cycles,
Fig. 4. The wear area of bearing surface for different insert types in the sagittal plane (a) and in the coronal plane (b) after 10 million cycles. 22
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Fig. 5. The contact area of bearing surface for different insert types in the sagittal plane (a) and in the coronal plane (b) after 10 million cycles.
Fig. 6. The accumulated sliding distance distribution (mm) (a) and linear wear distribution (mm) (b) of bearing surface for different insert types in the sagittal plane after 10 million cycles.
the knee kinematics was influenced by the sagittal conformity design of the bearing surface, which further affected the wear of the bearing surface. In addition, wear and kinematics of the artificial knee joint are much more sensitive to the sagittal than coronal conformity variables, and the lower sagittal conformity designs had the lower predicted wear rates. Our studies showed that slight differences of the knee contact forces were observed among different sagittal and coronal conformity designs. However, the knee kinematics was markedly affected by the sagittal conformity design variables. Our studies were consistent with Fitzpatrick et al.'s studies (Fitzpatrick et al., 2012a, 2012b) in which design factors (including conformity design) were the primary contributors to tibiofemoral kinematics and the joint loads were dependent on patient-specific factors. Meanwhile, Ardestani et al. (2015a, 2015b) also found that conformity directly affected the reliability of the artificial knee joint, and insert sagittal conformity affected kinematics. These results indicated that the sagittal conformity design has a direct impact on determining the knee kinematics of TKR.
compared to the wear area of the S-Model-3) (Fig. 4). Figs. 6 and 7 shows the accumulated sliding distance distribution and the linear wear distribution of the bearing surface for different sagittal insert types and coronal insert types, respectively. The maximum values of the accumulated sliding distance and the linear wear were both increased with lower sagittal conformity while the distribution areas decreased after 10 million cycles. However, slight differences of the accumulated sliding distance distribution and the linear wear distribution for different coronal conformity were observed. 4. Discussion The most important findings of the present study involves the effects of variability of insert conformity on joint forces, kinematics and wear in TKR under the musculoskeletal physiological environment using a previously developed wear prediction framework coupling a patientspecific musculoskeletal multibody dynamics, finite element analysis and wear analysis. The study hypothesis was accepted. We found that 23
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Fig. 7. The accumulated sliding distance distribution (mm) (a) and linear wear distribution (mm) (b) of bearing surface for different insert types in the coronal plane after 10 million cycles.
updated worn insert geometry were subsequently imported into the MSK MBD model of TKR to further calculate new knee contact forces and motions for the new FEA from 0 million cycles to 10 million cycles. Meanwhile, the wear area increased by 35.4% from 620 mm2 to 960 mm2 after 10 million cycles (Fig. 4a), while the tibial insert radius decreased by 43.8% from 64 mm (S-Model-3) to 36 mm (S-Model-1) (Table 1). Therefore, the insert conformity, kinematics and wear, like a closed loop system, were actually affected by each other under the human musculoskeletal physiological environment, instead of the traditional wear prediction using ISO fixed load/motions. Because the traditional wear prediction always neglect the kinematics changes. It mean that investigating the effect of insert conformity on wear should take the patient-specific joint motion and loading into consideration. When the tibial insert radius decreased by 43.8% (Table 1), which means the insert conformity is increased, the wear area increased by 35.4% after 10 million cycles (Fig. 4a) but the kinematics of knee joint tends to more stable. Therefore, the results of present study indicated that the optimization of insert conformity may have a significant impact on restoring daily activities for TKR patients. The present study showed that higher sagittal conformity designs had higher wear rates (Table 2), wear areas (Fig. 4) and contact areas (Fig. 5). In contrast, the maximum value of accumulated depth and accumulated sliding distance showed lower values with higher sagittal conformity (Fig. 6) after 10 million cycles. Abdelgaied et al. (Abdelgaied et al., 2014) found that the predicted wear rates for the curved insert (most conformed) were more than three times those for the flat insert (least conformed). Meanwhile, Galvin et al. (Galvin et al., 2009) also found that the wear rates for the flat UHMWPE insert were significantly lower than those for the curved UHMWPE insert under same kinematic conditions. And Willing et al. (Willing and Kim, 2009) suggested that the tibial insert wear was reduced by modifying the contact geometry of both components in the sagittal and coronal planes. Our findings were consistent with previous studies (Abdelgaied et al., 2014; Essner et al., 2003; Fregly et al., 2010; Galvin et al., 2009), although the values of volumetric wear rates in similar tibia insert models were slightly different. Previous computational wear prediction models did not take musculoskeletal physiological environment into consideration to investigate the effect of insert conformity on wear in TKR in previous studies. Furthermore, different tibial inserts with the same dimensions except for the sagittal and coronal radii were analyzed against the same femoral in this study, whilst commercial knee implants were used in previous studies (Abdelgaied et al., 2014; Galvin et al., 2009). Therefore, it is difficult to investigate and isolate the effect of the insert conformity alone. In contrast, the present results provided more
Moreover, the present study investigated the changes in contact forces and kinematics of the artificial knee joint resulting from different sagittal and coronal insert conformity after a 10 million cycle wear simulation, compared with the initial stage. The present results suggested that smaller changes were found for knee contact forces before and after wear for different sagittal insert conformity, and higher changes were found for knee kinematics (range of anterior-posterior translation and range of internal-external rotation). Therefore, the insert conformity design influenced knee kinematics, which further influenced the wear of the bearing surface. Meanwhile, wear of the bearing surface also changed the knee kinematics under different insert sagittal conformity designs. Especially, the ranges of internal-external rotation and anterior-posterior displacement were influenced by the sagittal conformity designs as the wear processed. Our studies were consistent with Williams et al.'s study (Williams et al., 2010) in which there were large increases in the contact stress and measureable changes in the kinematics of the knee joint as wear increases in the polyethylene insert. These issues should be taken into consideration when designing total knee implants in order to minimize these long-term effects (Williams et al., 2010). Meanwhile, Fregly et al. (2010) also found that when the sagittal conformity was changed, the relative kinematics between the femoral component and tibial insert were changed as well. The most obvious changes were located in the range of anterior-posterior translation and internal-external rotation (Fregly et al., 2010). Therefore, the present results highlighted that the optimization of insert sagittal conformity should be considered carefully in implant design and surgical procedures. Interestingly, One of the most important findings of the present study was that the knee kinematics were influenced by the insert conformity design, which further affected the wear of the bearing surface (Koh et al., 2019; Zhou and Jin, 2015). Specifically, higher insert conformity had lower amplitude of the femoral internal-external rotation and higher anterior-posterior translation (Fig. 3b). When the tibial insert radius decreased by 43.8% from 64 mm (S-Model-3) to 36 mm (SModel-1) (Table 1), which means the insert conformity is increased, the maximum values of the tibial external rotation decreased by 26.9% from −5.2 degrees to −3.8 degrees (Fig. 3b). Therefore, the insert conformity obviously affects the knee joint motions. And then these different knee joint contact forces and motions (especially the internalexternal rotation and anterior-posterior translation) caused by the different insert conformity were input as the essential boundary conditions for FEA to calculate the different contact pressure and sliding distances, which further lead different wear and output the different worn insert geometry in current computational model. Furthermore, the different 24
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reliable information about how insert conformity affects the knee contact forces, kinematics and wear performance of TKR. Some limitations of this study should be discussed. First, only three different tibial insert models in the sagittal plane and four different tibial insert models in the coronal plane were investigated in this study, and wear simulations were performed exclusively for a walking gait. Future studies will create parametric knee models to test more different design variables and materials during different kinds of gait simulations. Second, our studies do not take the femoral sagittal and coronal radii as design variables into consideration. The effects of femoral conformity on biomechanics and insert wear in TKR would be investigated in the future work. Third, although the present results were in good agreement with the results obtained in previous studies, further clinical investigations are required. Despite these limitations, the present study still provide an insight into insert conformity variables, which benefits to improve the design of the artificial knee joint for reducing wear in TKRs.
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5. Conclusions The results showed that higher insert conformity had lower amplitude of the femoral internal-external rotation, lower maximum values of the tibial internal rotation, and higher anterior-posterior translation, which further lead to higher wear rates and wear area. Additionally, wear and kinematics of the artificial knee joint are much more sensitive to the sagittal than coronal conformity variables. Therefore, these results indicated that insert sagittal conformity design played an essential role to improve kinematics performance and minimize wear of TKR. Acknowledgments This study was supported by “National Natural Science Foundation of China” [Grants number: 51323007]. The authors thank Professor B.J. Fregly and colleagues for providing publicly accessible data. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.clinbiomech.2019.03.016. References Abdelgaied, A., Liu, F., Brockett, C., Jennings, L., Fisher, J., Jin, Z., 2011. Computational wear prediction of artificial knee joints based on a new wear law and formulation. J. Biomech. 44, 1108–1116. Abdelgaied, A., Brockett, C.L., Liu, F., Jennings, L.M., Jin, Z., Fisher, J., 2014. The effect of insert conformity and material on total knee replacement wear. Proc. Inst. Mech. Eng. H J. Eng. Med. 228, 98–106. Andersen, M.S., Rasmussen, J., 2011. Total knee replacement musculoskeletal model using a novel simulation method for non-conforming joints. In: Proceedings of the International Society of Biomechanics Conference International Society of Biomechanics. ISB. Ardestani, M.M., Moazen, M., Jin, Z., 2015a. Contribution of geometric design parameters to knee implant performance: conflicting impact of conformity on kinematics and contact mechanics. Knee 22, 217–224. Ardestani, M.M., Moazen, M., Maniei, E., Jin, Z., 2015b. Posterior stabilized versus cruciate retaining total knee arthroplasty designs: conformity affects the performance reliability of the design over the patient population. Med. Eng. Phys. 37, 350–360. Blankevoort, L., Kuiper, J.H., Huiskes, R., Grootenboer, H.J., 1991. Articular contact in a 3-dimensional model of the knee. J. Biomech. 24, 1019–1031. Chen, Z., Zhang, X., Ardestani, M.M., Wang, L., Liu, Y., Lian, Q., He, J., Li, D., Jin, Z.,
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