Knee Surg Sports Traumatol Arthrosc DOI 10.1007/s00167-013-2819-y
KNEE
Patellofemoral kinematics during deep knee flexion after total knee replacement: a computational simulation Chang-Hung Huang • Lin-I Hsu • Kun-Jhih Lin • Ting-Kuo Chang Cheng-Kung Cheng • Yung-Chang Lu • Chen-Sheng Chen • Chun-Hsiung Huang
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Received: 5 February 2013 / Accepted: 19 December 2013 Ó Springer-Verlag Berlin Heidelberg 2013
Abstract Purpose Actions requiring deep knee flexion, such as kneeling and squatting, are challenging to perform after total knee replacement (TKR), though many manufactures emphasize that their knee prostheses could safely achieve high flexion. Little is known about the patellofemoral kinematics during deep flexion. This study aimed to track the movement of the patella during kneeling and squatting through dynamic computational simulation. Methods A validated knee model was used to analyse the patellar kinematics after TKR, including shifting, tilting
Kun-Jhih Lin and Lin-I Hsu have contributed equally to this article. C.-H. Huang L.-I. Hsu T.-K. Chang Y.-C. Lu C.-H. Huang Biomechanics Research Laboratory, Mackay Memorial Hospital, New Taipei City, Taiwan C.-H. Huang K.-J. Lin C.-K. Cheng C.-H. Huang Institute of Biomedical Engineering, National Yang-Ming University, Taipei, Taiwan L.-I. Hsu C.-S. Chen (&) Department of Physical Therapy and Assistive Technology, National Yang-Ming University, No. 155, Sec. 2, Li-Nung St., Taipei 112, Taiwan e-mail: [email protected] T.-K. Chang Y.-C. Lu (&) C.-H. Huang Department of Orthopaedic Surgery, Mackay Memorial Hospital, No. 45, Minsheng Rd., Tamshui District, New Taipei City 25160, Taiwan e-mail: [email protected] Y.-C. Lu Department of Cosmetic Application and Management, Mackay Junior College of Medicine, Nursing, and Management, Taipei, Taiwan
and rotation. The data were captured from full extension to 135° of knee flexion. For kneeling, an anterior force of 500 N was applied perpendicularly on the tibial tubercle as the knee flexed from 90° to 135°. For squatting, a ground reaction force was applied through the tibia from full extension to 135° of flexion. Results This study found that patellar shifting and rotation in kneeling were similar to those while squatting. However, during kneeling, the patella had a greater medial tilt and showed signs of abrupt patellar tilt owning to an external force being concentrated on the tibial tubercle. Conclusions In terms of squatting and kneeling movements, the latter is a more strenuous action for the patellofemoral joint after TKR due to the high forces acting on the tibial tubercle. It is suggested that overweight patients or those requiring high flexion should try to avoid kneeling to reduce the risk of the polyethylene wear. Further modification of trochlear geometry may be required to accommodate abrupt changes in patellar tilting. Level of evidence II. Keywords Total knee replacement Kinematics Patellofemoral joint High knee flexion Patellar tracking
Introduction Total knee replacement (TKR) has been reported with excellent survivorship in mid- to long-term follow-ups [1, 4, 26]. It is a highly successful and reliable surgical treatment for patients suffering from knee problems. Due to advances in medical care and quality of life, most patients expect to achieve high knee flexion for continuing their active lifestyle after TKR. Recently, several contemporary knee prostheses have been introduced that permit deep knee flexion [2, 27];
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clinical performance and range of motion (ROM) of these designs varies. Studies [14, 18, 32, 34] have shown the postoperative ROM of patients with these knees to be about 110°–139°, and weight bearing during deep knee flexion activities was achievable in about 40–80 % of patients [14, 27, 34]. Analysis of kinematics data during deep knee flexion would be beneficial for the development of advanced knee designs and with helping to restore normal kinematical patterns, thus reducing the risk of complications. Studies on such high-flexion TKRs primarily focused on investigating the kinematic behaviours of the tibiofemoral joint [2, 13, 21, 37]. Results have shown that some knees undergoing high flexion appeared to have paradoxical anterioposterior translation of the femoral component or reversed tibial rotation with increasing flexion angles. On the other hand, kinematical studies on patellar movements during high flexion have not provided sufficient data to clearly define such movement patterns. Most kinematics studies have analysed the effects of component position, prosthetic design features and surgical procedures on patellar kinematics [17, 20, 28, 29, 31, 36]. It has been shown that malrotation of the femoral component can alter patellar tracking and elevate pressure on the patellofemoral joint. The rotating platform does not appear to adjust the malalignment of the extensor mechanism resulting from femoral component malrotation [20]. Similar patellar kinematics has been found in fixed and mobile-bearing TKRs [17]. Repositioning the femoral component rotation alone could not restore all aspects of both patellar and tibial kinematics to normal patterns [29]. Asymmetrical femoral components do not seem to provide more anatomical patellar kinematics and stability when compared with symmetrical designs [36]. Greater thickness on the patella (overstuffing) increases lateral patellar tilt after surgery [28]. In addition to the factors discussed above, patellar tracking can also be affected by physical loading on the knee joint. Abnormal movements [29] and greater loads [35] leave the patellofemoral joint more susceptible to complications. It has been shown that a load approximating the full body weight is transmitted to the anterior aspect of the knee during kneeling [16]. Little is known about how external forces affect knee kinematics. It is hypothesized that the ground reaction force concentrated on the knee during kneeling could alter the patellar kinematics. This study attempted to analyse the patellar movements for determining the role of ground reaction forces in kneeling and compares these to squatting without an additional force on the knee.
commercially available posterior-stabilized (PS) TKA (U2 PS knee, United, Co., Hsinchu, Taiwan). The femoral component of the U2 PS knee prosthesis features a trochlea with a 4° valgus angle and extended deep patellar groove. The trochlear groove is oriented by approximately 4° from proximal-lateral to distal-medial across the anterior flange. The patella component is dome-shaped and symmetrical. The articular surface between the patellar component and femoral groove uses curve-on-curve contact on the transverse plane. Both femoral and tibial components were simulated in a neutral position and were well aligned without malrotation and maltranslation. The neutral position is defined with the femoral component contacting at the deepest point on the tibial articular surface [6]. In addition, the PS TKA was implanted parallel to the epicondylar axis of the natural knee with a 3° external rotation of the femoral component according to standard surgical protocols (Fig. 1). As for the passive stabilizers included in our model, only the collateral ligaments were preserved and the anterior and posterior cruciate ligaments were removed referring to the surgical procedure for the PS TKA. The medial collateral ligament was divided into anterior, oblique, and deep bundles, while the lateral collateral ligament was modelled as a single bundle. Each fibre bundle was modelled as a tensile-only spring with parabolic and linear regions [24]. In addition, quadriceps tendons (with a 5°
Materials and methods The computer simulations were carried out using a previously validated knee model [23, 24, 38] and a
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Fig. 1 Three-dimensional knee model
Knee Surg Sports Traumatol Arthrosc Fig. 2 a A simulated force (500 N) was applied perpendicularly to the tibial tubercle in single-stance kneeling, when the knee flexed over 90°. b A ground reaction force of 750 N was applied to the distal end of the tibiae throughout of ROM in squatting
valgus to the mechanical axis of the femur) and the patellar ligament were all considered as medial and lateral fibre bundles and simulated as tensile-only springs. The stiffness coefficients of the quadriceps tendon and patellar ligament were 2,000 [41] and 1,142 N/mm [15], respectively. In order to simulate the wrapping of quadriceps muscle around the trochlear groove during greater knee flexion, multiple beads connected by tensile-only springs were used to model the tendon [7]. Solid-to-solid contacts were then modelled between components. The friction coefficients of metal-to-polyethylene surfaces were designated as 0.04 [11]. Flexion of the femur was dependent on a linking line between flexion facet centres of the femoral condyles [19]. Knee flexion was then driven from the flexion axis. Patellar motion was measured every 15° from full extension to 135° flexion. For kneeling, knee flexion from 0° to 90° was a passive movement, and from 90° to 135°, a body weight of 50 kg (500 N) [16] was perpendicularly applied through the tubercle to simulate single-stance kneeling (Fig. 2). For squatting, a vertical ground reaction force of 750 N [8] was applied to the distal end of the tibia from 0° to 135° of knee flexion. The degree of patellar tilt, shift and rotation during kneeling and squatting was analysed for comparison [5]. In order to obtain the essential images of CT for reconstructing the 3-dimensional model, the study was approved by the ethics committee of the author’s institution (MMHI-S-687).
Fig. 3 Trend for patella shift in kneeling and squatting. The tilt started to shift laterally before knee flexion of 15°, and afterwards, the patella gradually shifted medially up to 135° knee flexion. Note that plus symbol indicates lateral shift
Fig. 4 Patellar tilt in kneeling and squatting. The trends for each were quite different. For kneeling, an abrupt change in tilting was found through knee flexion from 0° to 135°. Note that plus symbol indicates lateral tilt
Results The results showed that the patella shifted laterally in early flexion for both kneeling and squatting. The greatest lateral shift of 2.5 mm occurred at about 15° of knee flexion, and then, the patella shifted medially by 1.8 mm as the knee flexed to 90°. After 90°, the patella continued shifting medially, and at 135° of knee flexion, the kneeling movement had produced a slightly greater medial shift of 0.6 mm than squatting (Fig. 3).
The patellar tilt angle between squatting and kneeling was quite different. For squatting, the patella tilted medially as the knee flexed to 75° and then turned into lateral tilt up to 135° (Fig. 4). For kneeling, medial patellar tilt occurred during early flexion and showed a greater tilt than squatting. The maximal medial tilt angle was 5° and then suddenly turned into a lateral tilt. After 75° of knee flexion, the patella began to tilt medially again. During full flexion, the difference in patellar tilt angle between squatting and
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Fig. 5 Patella rotation in kneeling and squatting. The patella started to rotate internally before knee flexion of 15°, and afterwards, the patella gradually rotated externally up to 120° of knee flexion. The trend for squatting and kneeling is similar. Note that plus symbol indicates internal rotation
kneeling was 6.2°. The patella tilt in kneeling showed more abrupt changes throughout the motion. A similar patellar rotation pattern was seen for kneeling and squatting movements (Fig. 5). The internal rotation peaked at 15° of knee flexion. Thereafter, there was a progressive decrease in internal rotation with increasing knee flexion. Internal patellar rotation increased after 120°.
Discussion The most important finding of the present study was that patellar tilt during kneeling showed more abrupt changes in comparison with squatting. The patella tilting in kneeling, which changed abruptly from extension to 135°, was attributed to an external force applied to the tibial tubercle. In other words, the ground reaction force during kneeling not only resulted in a greater contact force on the anterior knee joint but also led to a drastic change in titling angle. In this situation, high shear stress and compressive pressure can be generated on the polyethylene patellar component, particularly when kneeling at 90°. Such alterations of patellar kinematics and the increased stress on the patellofemoral joint may be responsible for complications such as polyethylene wear [33], component loosening [22] and subluxation [30]. Computational simulation presents an efficient method for studying parametrical variables on patellofemoral mechanics [3, 10, 41]. Baldwin et al. [3] used an explicit finite element modelling approach to verify the accuracy of a natural and implanted knee. Their major finding focused on matching the predicted and experimental patellofemoral kinematics, and indicated that the computational model could be applied to further investigate the effects of component alignment or soft tissue variability on PF mechanics. Fitzpatrick et al. [10] used a finite element model for comparison of patellar bone strain in a natural and
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implanted knee. They found that the implanted specimens had significantly larger volumes of highly strained bone in the medial patella, whereas the strained bone was evenly distributed on the medial and lateral regions in the natural case. However, patellofemoral kinematics in kneeling has rarely been reported on. Wilson et al. [40] used a custommolded patellar clamp and an optoelectronic motion capture system to assess the characteristic motion patterns of the patella during squatting. Healthy subjects and subjects with patellofemoral pain were analysed. For their healthy subjects, the patella gradually shifted medially starting at about 2.8 mm and shifted to about 5 mm as the knee was flexed from 15° to deep flexion. The patella progressively tilted medially as the knee flexed to 45° and then tilted laterally. Also, the patella showed gradual medial rotation as the knee flexed to 90°. The patellar motion patterns of healthy subjects were different to those of the subjects with knee pain. Interestingly, the motion patterns in tilting, shifting and rotation were similar to current computational outcomes in squatting. On the other hand, the patellar tilting in kneeling showed more abrupt changes in comparison with squatting. It may be speculated that kneeling after TKA may be susceptive to patellar maltracking. In a cadaveric experimental study, Chew et al. [7] compared patellar motion between different brands of knee prostheses. They reported that there was no significant difference between the various implants regarding patellar lateral shift and rotation. All knees showed a slight lateral shift of the patella in early knee flexion, which reduced with increasing knee flexion. A progressive increase in internal rotation of the patella was seen in the implanted knees. However, significant lateral tilting was found when compared with the intact knee. An increasing lateral patellar tilt was seen during mid-flexion, but decreased following further knee flexion. When compared with our results regarding squatting, similar kinematics patterns were observed in patellar shift, tilt and rotation in the implanted knees before 120° knee flexion. This current study also found that there was no remarkable difference in patellar shifting and rotation. But a significant change in titling was observed between kneeling and squatting. The patellar components used in this study and in Chew et al.’s work (NexGen knee system) were dome-shaped, and the femoral component was asymmetrical with an elevated lateral flange. This might explain the similar kinematic patterns. Patellar maltracking is also associated with anterior knee pain, subluxation, wear and aseptic loosening [10, 22]. Wilkens et al. [39] performed TKR on cadaveric knees and tested the implants using a custom knee jig which permit the simulation of physiologic quadriceps loading as well as the application of an anterior force to simulate kneeling.
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Pressure-sensitive film was used to gauge patellofemoral contact. Their results demonstrated that the contact area of the patellar component in kneeling and squatting was point contact, and the peak patellofemoral joint contact pressures exceeded the yield strength of ultra-high molecular weight polyethylene (UHMWPE), which has been reported to be 14.4 MPa [9]. This may have implications with respect to increased polyethylene wear with high knee flexion. In addition, their findings also suggested that squatting after TKR had a smaller effect on patellofemoral joint contact area and pressure than kneeling. A recent clinical study [12] reported an acute post-traumatic catastrophic failure of a second-generation, highly cross-linked UHMWPE patellar component when exercised. It is speculated that kneeling could increase the risk of polyethylene delamination wear. Therefore, overweight patients and patients requiring high flexion should avoid kneeling to reduce wear of the polyethylene patellar component. The geometry of the anterior flange of a femoral component plays an important role in determining patellar motion. The simulated femoral component was asymmetrical in design with an elevated lateral flange. The patellar insert is an axisymmetric design with a curve-on-curve contact with the patellofemoral joint. The lateral shift of the patella may be attributed to the force vector that is orientated in a valgus direction. Additional lateral displacement of the patella is restricted by the soft tissue restraints and the prominence of lateral facet of the trochlea. As the knee flexes to 60° and over, the patella may become trapped in the deep femoral groove resulting in a decreasing amount of lateral displacement with increasing knee flexion. Patellar shift in kneeling and squatting was similar in the current study. However, patella tilt may not be restricted by the trochlear groove in the femoral design, and an external force in kneeling can easily alter the angle of patella tilt. Furthermore, a more pronounced change in angle was found in kneeling when compared with squatting. In our coauthors’ previous study [25], the authors reconstructed and assessed three-dimensional models of the prostheses and patella. The results suggested that the geometry of the anterior flange of a femoral component affects the conformity of the patellofemoral joint when articulating with the native patella and different patellar implants. The dynamic kinematics in this study suggests that consideration of patellofemoral congruency and stability in high knee flexion is important, especially during kneeling when most of the body weight is loaded at the tibial tubercle and may result in changes of patellar tracking. The geometry of the femoral flange should be modified to ease patellar tilt. Some inherent limitations of the computational simulation should be mentioned. Firstly, the complexity of the
musculoskeletal system in the knee joint is difficult to simulate. Patellar movements may be impossible to realistically predict after TKR for both in vivo image measurements and the in vitro cadaveric experiments. This current study used a simplified computer model to analyse patellar tracking. Simplifying assumptions included in the simulation makes it difficult to reflect real physical systems. Instead, the strength of computer modelling lies in its ability to reduce the amount of individual differences and intersample variability. All other variables can be held constant, making it easy to identify the effect of loading conditions. Secondly, the geometry of the bones and the attachment sites of the ligament and tendon were obtained from one subject. The results of this study should be considered as a patient-specific analysis. Thirdly, one knee prosthesis was simulated, which cannot represent all design features of contemporary knee prostheses. Obviating these limitations, this study may provide an objective method to compare patellar tracking between different loading conditions after TKR and to understand the effect of ground reaction forces during kneeling on patellar tilt. This work provides important information for clinicians regarding patients with contemporary high-flexion knee prostheses. Knee postures such as kneeling could dramatically change the patellar movement. Abrupt changes of patellar angle along with high contact forces on the knee during kneeling could be associated with patellofemoral complications. This is of particular importance to patients that kneel frequently in their daily life, lifestyle or religious reasons. Furthermore, novel polymers, such as highly cross-linked polyethylene or vitamin E doping polyethylene material, may be alternatives for solving the issue of polyethylene damage due to their high wear resistance.
Conclusion Patellar shift and rotation in kneeling were similar to those in squatting, but more abrupt changes in tilt were found in kneeling. Such dramatic changes of tilting angle could be associated with an increase in shear stress on the patella. Therefore, it is suggested that overweight patients or those requiring high flexion should avoid kneeling in order to eliminate the potential for anterior knee pain or polyethylene wear. Further modifications to the trochlear geometry may be required to accommodate abrupt changes in patellar tilt. Acknowledgments The authors are pleased to acknowledge the financial supports of the National Science Council (NSC 100-2221-E195-001-MY2, NSC 102-2221-E-195-001) and computational support by the Institute of Biomedical Engineering, National Yang-Ming University. And we wish to thank Colin J. McClean for his assistance in language editing and proofreading of this manuscript.
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Patellofemoral kinematics during deep knee flexion after total knee replacement: a computational simulation Chang-Hung Huang • Lin-I Hsu • Kun-Jhih Lin • Ting-Kuo Chang Cheng-Kung Cheng • Yung-Chang Lu • Chen-Sheng Chen • Chun-Hsiung Huang
•
Received: 5 February 2013 / Accepted: 19 December 2013 Ó Springer-Verlag Berlin Heidelberg 2013
Abstract Purpose Actions requiring deep knee flexion, such as kneeling and squatting, are challenging to perform after total knee replacement (TKR), though many manufactures emphasize that their knee prostheses could safely achieve high flexion. Little is known about the patellofemoral kinematics during deep flexion. This study aimed to track the movement of the patella during kneeling and squatting through dynamic computational simulation. Methods A validated knee model was used to analyse the patellar kinematics after TKR, including shifting, tilting
Kun-Jhih Lin and Lin-I Hsu have contributed equally to this article. C.-H. Huang L.-I. Hsu T.-K. Chang Y.-C. Lu C.-H. Huang Biomechanics Research Laboratory, Mackay Memorial Hospital, New Taipei City, Taiwan C.-H. Huang K.-J. Lin C.-K. Cheng C.-H. Huang Institute of Biomedical Engineering, National Yang-Ming University, Taipei, Taiwan L.-I. Hsu C.-S. Chen (&) Department of Physical Therapy and Assistive Technology, National Yang-Ming University, No. 155, Sec. 2, Li-Nung St., Taipei 112, Taiwan e-mail: [email protected] T.-K. Chang Y.-C. Lu (&) C.-H. Huang Department of Orthopaedic Surgery, Mackay Memorial Hospital, No. 45, Minsheng Rd., Tamshui District, New Taipei City 25160, Taiwan e-mail: [email protected] Y.-C. Lu Department of Cosmetic Application and Management, Mackay Junior College of Medicine, Nursing, and Management, Taipei, Taiwan
and rotation. The data were captured from full extension to 135° of knee flexion. For kneeling, an anterior force of 500 N was applied perpendicularly on the tibial tubercle as the knee flexed from 90° to 135°. For squatting, a ground reaction force was applied through the tibia from full extension to 135° of flexion. Results This study found that patellar shifting and rotation in kneeling were similar to those while squatting. However, during kneeling, the patella had a greater medial tilt and showed signs of abrupt patellar tilt owning to an external force being concentrated on the tibial tubercle. Conclusions In terms of squatting and kneeling movements, the latter is a more strenuous action for the patellofemoral joint after TKR due to the high forces acting on the tibial tubercle. It is suggested that overweight patients or those requiring high flexion should try to avoid kneeling to reduce the risk of the polyethylene wear. Further modification of trochlear geometry may be required to accommodate abrupt changes in patellar tilting. Level of evidence II. Keywords Total knee replacement Kinematics Patellofemoral joint High knee flexion Patellar tracking
Introduction Total knee replacement (TKR) has been reported with excellent survivorship in mid- to long-term follow-ups [1, 4, 26]. It is a highly successful and reliable surgical treatment for patients suffering from knee problems. Due to advances in medical care and quality of life, most patients expect to achieve high knee flexion for continuing their active lifestyle after TKR. Recently, several contemporary knee prostheses have been introduced that permit deep knee flexion [2, 27];
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clinical performance and range of motion (ROM) of these designs varies. Studies [14, 18, 32, 34] have shown the postoperative ROM of patients with these knees to be about 110°–139°, and weight bearing during deep knee flexion activities was achievable in about 40–80 % of patients [14, 27, 34]. Analysis of kinematics data during deep knee flexion would be beneficial for the development of advanced knee designs and with helping to restore normal kinematical patterns, thus reducing the risk of complications. Studies on such high-flexion TKRs primarily focused on investigating the kinematic behaviours of the tibiofemoral joint [2, 13, 21, 37]. Results have shown that some knees undergoing high flexion appeared to have paradoxical anterioposterior translation of the femoral component or reversed tibial rotation with increasing flexion angles. On the other hand, kinematical studies on patellar movements during high flexion have not provided sufficient data to clearly define such movement patterns. Most kinematics studies have analysed the effects of component position, prosthetic design features and surgical procedures on patellar kinematics [17, 20, 28, 29, 31, 36]. It has been shown that malrotation of the femoral component can alter patellar tracking and elevate pressure on the patellofemoral joint. The rotating platform does not appear to adjust the malalignment of the extensor mechanism resulting from femoral component malrotation [20]. Similar patellar kinematics has been found in fixed and mobile-bearing TKRs [17]. Repositioning the femoral component rotation alone could not restore all aspects of both patellar and tibial kinematics to normal patterns [29]. Asymmetrical femoral components do not seem to provide more anatomical patellar kinematics and stability when compared with symmetrical designs [36]. Greater thickness on the patella (overstuffing) increases lateral patellar tilt after surgery [28]. In addition to the factors discussed above, patellar tracking can also be affected by physical loading on the knee joint. Abnormal movements [29] and greater loads [35] leave the patellofemoral joint more susceptible to complications. It has been shown that a load approximating the full body weight is transmitted to the anterior aspect of the knee during kneeling [16]. Little is known about how external forces affect knee kinematics. It is hypothesized that the ground reaction force concentrated on the knee during kneeling could alter the patellar kinematics. This study attempted to analyse the patellar movements for determining the role of ground reaction forces in kneeling and compares these to squatting without an additional force on the knee.
commercially available posterior-stabilized (PS) TKA (U2 PS knee, United, Co., Hsinchu, Taiwan). The femoral component of the U2 PS knee prosthesis features a trochlea with a 4° valgus angle and extended deep patellar groove. The trochlear groove is oriented by approximately 4° from proximal-lateral to distal-medial across the anterior flange. The patella component is dome-shaped and symmetrical. The articular surface between the patellar component and femoral groove uses curve-on-curve contact on the transverse plane. Both femoral and tibial components were simulated in a neutral position and were well aligned without malrotation and maltranslation. The neutral position is defined with the femoral component contacting at the deepest point on the tibial articular surface [6]. In addition, the PS TKA was implanted parallel to the epicondylar axis of the natural knee with a 3° external rotation of the femoral component according to standard surgical protocols (Fig. 1). As for the passive stabilizers included in our model, only the collateral ligaments were preserved and the anterior and posterior cruciate ligaments were removed referring to the surgical procedure for the PS TKA. The medial collateral ligament was divided into anterior, oblique, and deep bundles, while the lateral collateral ligament was modelled as a single bundle. Each fibre bundle was modelled as a tensile-only spring with parabolic and linear regions [24]. In addition, quadriceps tendons (with a 5°
Materials and methods The computer simulations were carried out using a previously validated knee model [23, 24, 38] and a
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Fig. 1 Three-dimensional knee model
Knee Surg Sports Traumatol Arthrosc Fig. 2 a A simulated force (500 N) was applied perpendicularly to the tibial tubercle in single-stance kneeling, when the knee flexed over 90°. b A ground reaction force of 750 N was applied to the distal end of the tibiae throughout of ROM in squatting
valgus to the mechanical axis of the femur) and the patellar ligament were all considered as medial and lateral fibre bundles and simulated as tensile-only springs. The stiffness coefficients of the quadriceps tendon and patellar ligament were 2,000 [41] and 1,142 N/mm [15], respectively. In order to simulate the wrapping of quadriceps muscle around the trochlear groove during greater knee flexion, multiple beads connected by tensile-only springs were used to model the tendon [7]. Solid-to-solid contacts were then modelled between components. The friction coefficients of metal-to-polyethylene surfaces were designated as 0.04 [11]. Flexion of the femur was dependent on a linking line between flexion facet centres of the femoral condyles [19]. Knee flexion was then driven from the flexion axis. Patellar motion was measured every 15° from full extension to 135° flexion. For kneeling, knee flexion from 0° to 90° was a passive movement, and from 90° to 135°, a body weight of 50 kg (500 N) [16] was perpendicularly applied through the tubercle to simulate single-stance kneeling (Fig. 2). For squatting, a vertical ground reaction force of 750 N [8] was applied to the distal end of the tibia from 0° to 135° of knee flexion. The degree of patellar tilt, shift and rotation during kneeling and squatting was analysed for comparison [5]. In order to obtain the essential images of CT for reconstructing the 3-dimensional model, the study was approved by the ethics committee of the author’s institution (MMHI-S-687).
Fig. 3 Trend for patella shift in kneeling and squatting. The tilt started to shift laterally before knee flexion of 15°, and afterwards, the patella gradually shifted medially up to 135° knee flexion. Note that plus symbol indicates lateral shift
Fig. 4 Patellar tilt in kneeling and squatting. The trends for each were quite different. For kneeling, an abrupt change in tilting was found through knee flexion from 0° to 135°. Note that plus symbol indicates lateral tilt
Results The results showed that the patella shifted laterally in early flexion for both kneeling and squatting. The greatest lateral shift of 2.5 mm occurred at about 15° of knee flexion, and then, the patella shifted medially by 1.8 mm as the knee flexed to 90°. After 90°, the patella continued shifting medially, and at 135° of knee flexion, the kneeling movement had produced a slightly greater medial shift of 0.6 mm than squatting (Fig. 3).
The patellar tilt angle between squatting and kneeling was quite different. For squatting, the patella tilted medially as the knee flexed to 75° and then turned into lateral tilt up to 135° (Fig. 4). For kneeling, medial patellar tilt occurred during early flexion and showed a greater tilt than squatting. The maximal medial tilt angle was 5° and then suddenly turned into a lateral tilt. After 75° of knee flexion, the patella began to tilt medially again. During full flexion, the difference in patellar tilt angle between squatting and
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Fig. 5 Patella rotation in kneeling and squatting. The patella started to rotate internally before knee flexion of 15°, and afterwards, the patella gradually rotated externally up to 120° of knee flexion. The trend for squatting and kneeling is similar. Note that plus symbol indicates internal rotation
kneeling was 6.2°. The patella tilt in kneeling showed more abrupt changes throughout the motion. A similar patellar rotation pattern was seen for kneeling and squatting movements (Fig. 5). The internal rotation peaked at 15° of knee flexion. Thereafter, there was a progressive decrease in internal rotation with increasing knee flexion. Internal patellar rotation increased after 120°.
Discussion The most important finding of the present study was that patellar tilt during kneeling showed more abrupt changes in comparison with squatting. The patella tilting in kneeling, which changed abruptly from extension to 135°, was attributed to an external force applied to the tibial tubercle. In other words, the ground reaction force during kneeling not only resulted in a greater contact force on the anterior knee joint but also led to a drastic change in titling angle. In this situation, high shear stress and compressive pressure can be generated on the polyethylene patellar component, particularly when kneeling at 90°. Such alterations of patellar kinematics and the increased stress on the patellofemoral joint may be responsible for complications such as polyethylene wear [33], component loosening [22] and subluxation [30]. Computational simulation presents an efficient method for studying parametrical variables on patellofemoral mechanics [3, 10, 41]. Baldwin et al. [3] used an explicit finite element modelling approach to verify the accuracy of a natural and implanted knee. Their major finding focused on matching the predicted and experimental patellofemoral kinematics, and indicated that the computational model could be applied to further investigate the effects of component alignment or soft tissue variability on PF mechanics. Fitzpatrick et al. [10] used a finite element model for comparison of patellar bone strain in a natural and
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implanted knee. They found that the implanted specimens had significantly larger volumes of highly strained bone in the medial patella, whereas the strained bone was evenly distributed on the medial and lateral regions in the natural case. However, patellofemoral kinematics in kneeling has rarely been reported on. Wilson et al. [40] used a custommolded patellar clamp and an optoelectronic motion capture system to assess the characteristic motion patterns of the patella during squatting. Healthy subjects and subjects with patellofemoral pain were analysed. For their healthy subjects, the patella gradually shifted medially starting at about 2.8 mm and shifted to about 5 mm as the knee was flexed from 15° to deep flexion. The patella progressively tilted medially as the knee flexed to 45° and then tilted laterally. Also, the patella showed gradual medial rotation as the knee flexed to 90°. The patellar motion patterns of healthy subjects were different to those of the subjects with knee pain. Interestingly, the motion patterns in tilting, shifting and rotation were similar to current computational outcomes in squatting. On the other hand, the patellar tilting in kneeling showed more abrupt changes in comparison with squatting. It may be speculated that kneeling after TKA may be susceptive to patellar maltracking. In a cadaveric experimental study, Chew et al. [7] compared patellar motion between different brands of knee prostheses. They reported that there was no significant difference between the various implants regarding patellar lateral shift and rotation. All knees showed a slight lateral shift of the patella in early knee flexion, which reduced with increasing knee flexion. A progressive increase in internal rotation of the patella was seen in the implanted knees. However, significant lateral tilting was found when compared with the intact knee. An increasing lateral patellar tilt was seen during mid-flexion, but decreased following further knee flexion. When compared with our results regarding squatting, similar kinematics patterns were observed in patellar shift, tilt and rotation in the implanted knees before 120° knee flexion. This current study also found that there was no remarkable difference in patellar shifting and rotation. But a significant change in titling was observed between kneeling and squatting. The patellar components used in this study and in Chew et al.’s work (NexGen knee system) were dome-shaped, and the femoral component was asymmetrical with an elevated lateral flange. This might explain the similar kinematic patterns. Patellar maltracking is also associated with anterior knee pain, subluxation, wear and aseptic loosening [10, 22]. Wilkens et al. [39] performed TKR on cadaveric knees and tested the implants using a custom knee jig which permit the simulation of physiologic quadriceps loading as well as the application of an anterior force to simulate kneeling.
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Pressure-sensitive film was used to gauge patellofemoral contact. Their results demonstrated that the contact area of the patellar component in kneeling and squatting was point contact, and the peak patellofemoral joint contact pressures exceeded the yield strength of ultra-high molecular weight polyethylene (UHMWPE), which has been reported to be 14.4 MPa [9]. This may have implications with respect to increased polyethylene wear with high knee flexion. In addition, their findings also suggested that squatting after TKR had a smaller effect on patellofemoral joint contact area and pressure than kneeling. A recent clinical study [12] reported an acute post-traumatic catastrophic failure of a second-generation, highly cross-linked UHMWPE patellar component when exercised. It is speculated that kneeling could increase the risk of polyethylene delamination wear. Therefore, overweight patients and patients requiring high flexion should avoid kneeling to reduce wear of the polyethylene patellar component. The geometry of the anterior flange of a femoral component plays an important role in determining patellar motion. The simulated femoral component was asymmetrical in design with an elevated lateral flange. The patellar insert is an axisymmetric design with a curve-on-curve contact with the patellofemoral joint. The lateral shift of the patella may be attributed to the force vector that is orientated in a valgus direction. Additional lateral displacement of the patella is restricted by the soft tissue restraints and the prominence of lateral facet of the trochlea. As the knee flexes to 60° and over, the patella may become trapped in the deep femoral groove resulting in a decreasing amount of lateral displacement with increasing knee flexion. Patellar shift in kneeling and squatting was similar in the current study. However, patella tilt may not be restricted by the trochlear groove in the femoral design, and an external force in kneeling can easily alter the angle of patella tilt. Furthermore, a more pronounced change in angle was found in kneeling when compared with squatting. In our coauthors’ previous study [25], the authors reconstructed and assessed three-dimensional models of the prostheses and patella. The results suggested that the geometry of the anterior flange of a femoral component affects the conformity of the patellofemoral joint when articulating with the native patella and different patellar implants. The dynamic kinematics in this study suggests that consideration of patellofemoral congruency and stability in high knee flexion is important, especially during kneeling when most of the body weight is loaded at the tibial tubercle and may result in changes of patellar tracking. The geometry of the femoral flange should be modified to ease patellar tilt. Some inherent limitations of the computational simulation should be mentioned. Firstly, the complexity of the
musculoskeletal system in the knee joint is difficult to simulate. Patellar movements may be impossible to realistically predict after TKR for both in vivo image measurements and the in vitro cadaveric experiments. This current study used a simplified computer model to analyse patellar tracking. Simplifying assumptions included in the simulation makes it difficult to reflect real physical systems. Instead, the strength of computer modelling lies in its ability to reduce the amount of individual differences and intersample variability. All other variables can be held constant, making it easy to identify the effect of loading conditions. Secondly, the geometry of the bones and the attachment sites of the ligament and tendon were obtained from one subject. The results of this study should be considered as a patient-specific analysis. Thirdly, one knee prosthesis was simulated, which cannot represent all design features of contemporary knee prostheses. Obviating these limitations, this study may provide an objective method to compare patellar tracking between different loading conditions after TKR and to understand the effect of ground reaction forces during kneeling on patellar tilt. This work provides important information for clinicians regarding patients with contemporary high-flexion knee prostheses. Knee postures such as kneeling could dramatically change the patellar movement. Abrupt changes of patellar angle along with high contact forces on the knee during kneeling could be associated with patellofemoral complications. This is of particular importance to patients that kneel frequently in their daily life, lifestyle or religious reasons. Furthermore, novel polymers, such as highly cross-linked polyethylene or vitamin E doping polyethylene material, may be alternatives for solving the issue of polyethylene damage due to their high wear resistance.
Conclusion Patellar shift and rotation in kneeling were similar to those in squatting, but more abrupt changes in tilt were found in kneeling. Such dramatic changes of tilting angle could be associated with an increase in shear stress on the patella. Therefore, it is suggested that overweight patients or those requiring high flexion should avoid kneeling in order to eliminate the potential for anterior knee pain or polyethylene wear. Further modifications to the trochlear geometry may be required to accommodate abrupt changes in patellar tilt. Acknowledgments The authors are pleased to acknowledge the financial supports of the National Science Council (NSC 100-2221-E195-001-MY2, NSC 102-2221-E-195-001) and computational support by the Institute of Biomedical Engineering, National Yang-Ming University. And we wish to thank Colin J. McClean for his assistance in language editing and proofreading of this manuscript.
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