THEKNE-01999; No of Pages 6 The Knee xxx (2014) xxx–xxx
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The Knee
Variations in morphological characteristics of prostheses for total knee arthroplasty leading to kinematic differences Xiaojun Shi, Zongke Zhou, Bin Shen, Jing Yang, Pengde Kang, Fuxing Pei ⁎ Orthopaedic Department, West China Hospital, Sichuan University, 37 Guo-xue Lane, Wu-hou District, Chengdu, China
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Article history: Received 31 May 2014 Received in revised form 10 October 2014 Accepted 30 October 2014 Available online xxxx Keywords: Total knee arthroplasty Kinematics Fluoroscopic analysis Fixed bearing
a b s t r a c t Background and purpose: The aim of this study is to compare kinematics during weight-bearing deep kneebending motion in patients after bilateral total knee arthroplasty (TKA) of two types: 1) a conventional ScorpioFlex prosthesis and 2) a contemporary redesigned non-restrictive-geometry (NRG) prosthesis installed by the same surgeon. Methods: We enrolled 15 patients who underwent conventional ScorpioFlex posterior-stabilised TKA in one knee and contemporary NRG TKA on the contralateral side (the same surgeon). During fluoroscopic examination, each patient performed weight-bearing deep knee bending. Motions among all components were analysed using a two- to three-dimensional registration technique. Results: The mean maximum flexion was 108° (SD 8) and 120° (SD 9) after ScorpioFlex and NRG TKAs, respectively; there were statistically significant differences between the groups. From extension to maximal flexion, the medial condyle translated by 4.8 mm (SD 1.2) and 5.4 mm (SD 2.4) posteriorly after ScorpioFlex TKA and NRG TKA, respectively. The lateral femoral condyle moved 8.4 mm (SD 1.5) and 12.2 mm (SD 2.1) posteriorly after ScorpioFlex TKA and NRG TKA, respectively. There were no significant differences in medial condyle translation between the groups except for the lateral condyle. The total amount of tibial axial rotation during extension to flexion was 5.1° (SD 1.8) after ScorpioFlex and 13.2° (SD 3.4) after NRG TKAs; there were statistically significant differences between the groups. Conclusions: NRG resulted in much better maximum flexion, lateral condyle movement and tibial internal rotation than did ScorpioFlex TKAs. The observed kinematic differences are most likely caused by variations in the morphological characteristics of the two implants. Level of evidence: Level I, Prospective randomed comparative study. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Total knee arthroplasty (TKA) has been proven to be an efficient surgical treatment of knee joint disorders because of excellent survival rates and long-term results, but patients' satisfaction after TKA is still ≤70–75% [27,28,33]. The range of motion is one important factor affecting a patient's satisfaction with the outcome of TKA, especially in Asian countries; cultural factors cause patients in Asia to frequently perform activities involving strong knee flexion such as squatting, kneeling or sitting cross-legged [26,30]. Patients' functional abilities seem strongly linked to successful restoration of normal motion in the replaced joint [2,3,23,32,40]. To perform deep knee bending after TKA, a bicondylar roll-back motion pattern such as that in a healthy knee is desirable [17,25]. Posterior translation of the femoral condyle and internal rotation of the tibia appear to be two important factors related to good knee flexion [3]. Nevertheless, in vivo fluoroscopic analyses of various
⁎ Corresponding author. E-mail address: [email protected] (F. Pei).
types of prostheses for total knee arthroplasty have shown numerous kinematic abnormalities: paradoxical anterior–femoral translation, reverse axial rotational patterns and abnormal femoral condylar lift-off are commonly present [5,9,35]. Although some studies reported kinematics after TKA procedures that is similar to that of a healthy knee, posterior roll-back of the femoral condyle and tibial axial rotation are still much worse than those in a healthy knee [31]. How to improve the femoral condylar roll-back and tibial internal rotation during knee flexion is still a challenging question for surgeons and prosthesis manufacturers. More recently, a novel rotationally unconstrained and fixed-bearing posterior-stabilised prosthesis for TKA, Scorpio Non-Restrictive Geometry (NRG) implant (Stryker Orthopedics, Mahwah, NJ) came on the market. This prosthesis is a recent product incorporating various modifications of the previous Scorpio Knee System (Stryker Orthopedics) and is advertised as a system improving the femoral condyle translation in the sagittal plane and reducing the restriction of tibial axial rotation in the transverse plane (Figs. 1 and 2). Although several studies have reported that an NRG implant improves TKA kinematics [4,39], no study to date has compared kinematics after the contemporary NRG
http://dx.doi.org/10.1016/j.knee.2014.10.013 0968-0160/© 2014 Elsevier B.V. All rights reserved.
Please cite this article as: Shi X, et al, Variations in morphological characteristics of prostheses for total knee arthroplasty leading to kinematic differences, Knee (2014), http://dx.doi.org/10.1016/j.knee.2014.10.013
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X. Shi et al. / The Knee xxx (2014) xxx–xxx
Fig. 1. A projection drawing showing the difference between the non-restrictive geometry (NRG; left) and ScorpioFlex (right) femoral components in their frontal (top) and sagittal (bottom) views; both exhibit medial–lateral symmetry in the condyles and are used in single-radius TKA. The anterior flange of the NRG design was reduced by 2 mm (d2 − d1 = 2) to decrease stress on the extensor mechanism, retinaculum and widened intercondylar notch (L1 N L2). Meanwhile, the NRG implant shows a much smaller deep-flexion radius from 95° to 155° compared to the ScorpioFlex implant (black arrow) and thereby ensures a relatively relaxed arrangement of the posterior structure.
TKA (has features designed to promote good flexion) with kinematics after the conventional ScorpioFlex TKA in patients with bilateral TKA of these two types. The goal of this study was to compare kinematics under weightbearing deep knee-bending motion in patients with bilateral TKA of two types: 1) involving a conventional ScorpioFlex prosthesis and 2) based on the contemporary redesigned NRG prosthesis installed by the same surgeon. We hypothesised that the knee that receives the
NRG prosthesis would show better posterior roll-back and axial rotation than would the ScorpioFlex implant. 2. Materials and methods Prior to the study, the Institutional Review Board of West China Hospital of Sichuan University approved the study protocol, and written informed consent was obtained from the patients. We enrolled the
Fig. 2. A projection drawing showing the difference between the non-restrictive geometry (NRG; left) and ScorpioFlex (right) polyethylene insert in their sagittal (above) and transverse (below) views. In the sagittal view, the post of the NRG implant is much closer to the posterior edge of the insert compared to the ScorpioFlex implant (d3 b d4); the NRG has a lower posterior lip (dashed line). In the transverse view, the spherical arc articulating surface (dashed line) and the round post (black arrow) are the most characteristic differences between the two designs.
Please cite this article as: Shi X, et al, Variations in morphological characteristics of prostheses for total knee arthroplasty leading to kinematic differences, Knee (2014), http://dx.doi.org/10.1016/j.knee.2014.10.013
X. Shi et al. / The Knee xxx (2014) xxx–xxx
patients with a primary diagnosis of knee osteoarthritis, who were prepared to undergo staged total knee arthroplasty on both sides and willing to participate in the study. From February to December 2011, 15 patients (5 males) underwent conventional ScorpioFlex posterior-stabilised (PS) TKA in one knee and contemporary redesigned NRG posterior-stabilised TKA on the contralateral knee within three months (the same surgeon using the same surgical technique). The choice of the implant type for the first operation was randomised by date of the operation: an odd date corresponded to the ScorpioFlex implant and an even date to the NRG implant. Eight patients received the ScorpioFlex implant and seven patients the NRG implant during the first operation. The mean duration of follow-up and the interval between the bilateral operations was 27 months (range 24–31) and 1.5 months (range 1–3), respectively. The average perioperative age of the patients was 65 years (range 53–75). The average height, weight and body–mass index were 166 cm (range 154–176), 71 kg (range 54–80) and 26 kg/m2 (range 22–29). These patients were analysed post-operatively using video-fluoroscopic analysis. Clinical assessment was performed using the Hospital for Special Surgery (HSS) score pre-operatively and at follow-up (Table 1) [12]. All operations were performed by the same surgeon using the standard surgical technique, including an anterior midline skin incision and the medial parapatellar approach. Femoral bone cuts of both prostheses were of anterior referencing design. Distal femora were cut nearly 8 mm away from the most distal normal surface according to the intramedullary alignment system with 5° valgus. Rotational alignment of the femoral component was parallel to the transepicondylar axis. After that, tibiae were cut perpendicularly to the long axis of tibia with a 5° posterior slope through the extramedullary alignment system. The flexion and extension spaces were equalised, and soft tissues were balanced. A tourniquet was used for all patients, and all patellae were not resurfaced. All components were fixed with cement. Weightbearing and knee flexion exercises were started on postoperative day 1. The postoperative rehabilitation programme was the same for all patients. The three-dimensional (3D) position and orientation of the tibial and femoral components were determined using model-based image registration techniques as described previously [34]. Each patient was asked to perform sequential deep knee bending under weight-bearing conditions from extension to maximum flexion during fluoroscopic examination in the sagittal plane. Knee motions were recorded as serial digital X-ray images (20 frames per second, 1024 × 1024 × 12 bits/ pixels) using a 12-in digital image intensifier system (AXIOM Artis VB31, Siemens, Germany). The images were analysed at 15° intervals according to fluoroscopically measured flexion angles. Parameters for calibration and distortion correction were determined using a calibration image of radiopaque beads with known patterns (Camera Calibration Toolbox for Matlab; The Mathworks Inc., Natick, Massachusetts, USA) and were used to correct all images. The corrected images and CAD models were imported into open-source shape-matching software (JointTrack, University of Florida) to complete the shape-matching process. The automated local optimisation algorithm was based on the method described by Mahfouz et al. [19], and a previous validation study demonstrated that the 2D–3D registration technique results in acceptable root-mean-square errors and good test–retest reliability [34]. Articular surface separation was assumed for any measure N2.4 mm described by Mahoney et al. [20].
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Postoperative maximal flexion, anterior–posterior (AP) condylar positions and axial rotation between femoral and tibial components were quantified. The locations of medial and lateral condylar contacts were estimated as the lowest point in each femoral condyle relative to the transverse plane of the tibial baseplate. The distance between the contact point and the AP centre of the tibial baseplate was measured, and AP positions of the femoral component anterior to the tibial insert were recorded as positive, whereas the posterior positions as negative. Tibial internal rotation relative to the femur was defined as negative rotation. 3. Statistical analysis All data were expressed as mean ± standard deviation. Paired Student's t test was used for analysis of differences, including those in post-operative maximal flexion, HSS score, AP displacement of the nearest points and contact points on the medial and lateral sides and axial rotational angles between the femoral component and the tibial component after bilateral TKA. Statistical significance was assumed as p b 0.05. 4. Results The mean range of motion at the latest follow-up was 108° (SD 8°) after ScorpioFlex TKA and 120° (SD 9°) after NRG TKA; there were statistically significant differences between the bilateral TKAs (p b 0.05; Table 2). The mean postoperative HSS score was 89 (SD 5) after ScorpioFlex TKA and 91 (SD 7) after NRG TKA, without statistically significant differences between the bilateral TKAs (p = 0.31). From extension to maximal flexion, the medial condyle translated by 4.8 mm (SD 1.2) and 5.4 mm (SD 2.4) posteriorly after ScorpioFlex TKA and NRG TKA, respectively. There were no differences between the two designs in the deep knee-bending activity for the AP translation. Both designs showed that the medial condyle translated posteriorly with flexion, although it translated anteriorly at mid-flexion (p = 0.58; Table 1, Fig. 3a). The lateral femoral condyle moved by 8.4 mm (SD 1.5) and 12.2 mm (SD 2.1) posteriorly after ScorpioFlex TKA and NRG TKA, respectively. There were statistically significant differences between the bilateral TKAs (p b 0.05; Table 1, Fig. 3b). The total extent of tibial axial rotation during extension to flexion was 5.1° (SD 1.8°) after ScorpioFlex TKA and 13.2° (SD 3.4°) after NRG TKA (Table 2). The absolute extent of tibial axial rotation was significantly greater with the contemporary design than with the traditional design (p b 0.05; Table 2, Fig. 4). Separation of articular surfaces was not detected in any case in any position.
5. Discussion Multiple factors can affect postoperative knee flexion, for example, a preoperative range of motion, operative technique, and prosthesis design [8,18,21]. The NRG design is a new type of prosthesis for TKA and was derived from the previous ScorpioFlex models with the aim of supporting deep knee flexion by increasing posterior condylar translation and by reducing the rotation restraint during knee flexion. Morphological modifications (new features) in the NRG model include a spherical arc in the transverse plane of the insert articulating surface, the widened intercondylar notch with a modified round tibial post, and a reduction of the posterior radius of curvature. Compared to the ScorpioFlex implant, the above characteristics of the NRG implant cause the femoral component to rotate more freely around the tibia (Figs. 1 and 2). In addition, the anterior flange of the femoral component is reduced by 2 mm to decrease the stress on the extensor mechanism and retinaculum [16,38]. In our study, we compared knee kinematics during deep knee bending in patients with bilateral TKAs involving traditional and contemporary
Table 1 Comparsion of preoperative demographic data.
ScropioFlex TKA Scorpio NRG TKA p
Range of motion(°)
Valgus/varus defromtiy(°)
Flexion deformity(°)
HSS score
102 ± 10 106 ± 8 0.54
8±2 7±3 0.75
20 ± 5 16 ± 4 0.58
42 ± 3 45 ± 4 0.69
Values are expressed as mean ± SD.
Please cite this article as: Shi X, et al, Variations in morphological characteristics of prostheses for total knee arthroplasty leading to kinematic differences, Knee (2014), http://dx.doi.org/10.1016/j.knee.2014.10.013
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Table 2 Knee pose during maximum deep bending.
ScropioFlex TKA Scorpio NRG TKA p
Maximal flexion (°)
Tibial internal rotation (°)
Medial AP translation (mm)
Lateral AP translation (mm)
108 ± 8 120 ± 9 b0.05
5.1 ± 1.8 13.2 ± 3.4 b0.05
4.8 ± 1.2 5.4 ± 2.4 0.58
8.4 ± 1.5 12.2 ± 2.1 b0.05
Values are expressed as mean ± SD.
prostheses implanted by the same surgeon using the same operative technique, with the exception of an apparent tibial internal rotation and femoral roll-back bias in knees undergoing traditional TKA. We did observe knee flexion and kinematics that are functionally different between the two designs: the NRG prosthesis shows much better maximal knee flexion, lateral condyle translation and tibial internal rotation than does the ScorpioFlex implant. Nevertheless, there is no significant difference in medial femoral condylar posterior translation between the two designs. In the present study, the average range of motion after NRG TKA was much greater compared to the ScorpioFlex TKA design; these data are indicative of good postoperative results for the NRG design. Although the sample size is relatively small in this study, the above result seems plausible and is consistent with other studies of the same implants. Klein et al. [16] compared the intraoperative range of motion of the NRG and ScorpioFlex prostheses using a computerised navigation system under the same conditions of an extension/flexion gap and softtissue balance; the researchers found that the NRG design results in a significantly greater range of motion than does the ScorpioFlex implant. Nonetheless, there is no consensus on whether high-flexion TKA
ensures much better postoperative range of motion than does traditional TKA; differences among implants used in different studies seem to be the main reason for the discrepancies between the two types of prosthesis in the post-operative range of motion [13,14,41]. Previous kinematic analyses of healthy knees showed that during knee flexion, steady internal rotation of the tibia relative to the femur takes place, and the extent of internal rotation reaches 20° and more [1,10]. On the other hand, in vivo kinematic analysis of various TKA prostheses demonstrated that the tibial rotation during knee flexion includes not only medial pivot internal rotation similar to that in a healthy knee but also non-physiological lateral pivot reverse rotation; the extent of rotation is limited and much smaller than that of a healthy knee [31]. In the present study, the mean tibial axial rotation is 13.2° after NRG TKA during deep knee bending, and the mean tibial axial rotation after ScorpioFlex TKA is 5.1°; there is a statistically significant difference between the two designs (p b 0.05). Meanwhile, posterior translation of the lateral condyle with the NRG implant is much better than that of ScorpioFlex TKAs, but there is no significant difference in the medial condyle. These findings may be explained by the fact that the NRG design has a more relaxed geometrical arrangement in the
Fig. 3. The kinematic pathway of the medial (a) and lateral (b) nearest points between the femoral and tibial component during deep knee bending after either non-restrictive geometry (NRG) or ScorpioFlex total knee arthroplasty (TKA). The solid line corresponds to NRG TKA, and the dotted line corresponds to ScorpioFlex TKA. Positive values indicate the anterior direction.
Please cite this article as: Shi X, et al, Variations in morphological characteristics of prostheses for total knee arthroplasty leading to kinematic differences, Knee (2014), http://dx.doi.org/10.1016/j.knee.2014.10.013
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Fig. 4. Axial rotation of the femoral component relative to the tibial tray. The horizontal axis shows the flexion angle between the components, and the vertical axis shows axial rotation. Positive values represent internal rotation.
posterior aspect, including a spherical arc in the transverse plane of the insert's articulating surface, a much smaller deep-flexion radius from 95° to 155° compared to the larger single AP radius from 0° to 95° and the round-on-round design of the post-cam mechanism. These characteristics ensure a relatively relaxed arrangement in the posterior structure resulting in a lesser posterior constraint on rotation and translation during knee flexion. Furthermore, the surrounding ligaments perform a much better restricting function and induce femoral condyle movement, which may be beneficial for posterior translation of the lateral condyle looking like a medial pivot motion of a healthy knee. Several studies have shown that NRG TKA results in much better tibial rotation during various activities. Tamaki et al. [38] reported that tibial internal rotation after NRG TKA (as used in our study) is 13.5° during deep knee bending, and the internal rotation is increased up to the maximal flexion. Catani et al. [4] reported that after NRG TKA, the average tibial internal rotation during various activities is nearly 11°. These data are similar to our results on NRG TKA. In one study, tibial internal rotation was found to be associated with maximum knee flexion both in healthy and post-operative knees [6]. Although axial rotation of the tibia after NRG TKA is much better than that after ScorpioFlex TKA, it is still limited compared to a healthy knee. On the other hand, greater posterior translation of the lateral condyle may be the main reason for the greater range of motion after NRG TKA [7,36]. There are several limitations to our study. First, the key limitation is the relatively small sample size. Characteristics of a kinematic study necessitate a large sample size, although the majority of similar kinematic studies also include only 10–20 knees [11,15,22,24,29,37,42]. Second, the staged rather than simultaneous bilateral total knee arthroplasty may affect the postoperative functional outcome of the second TKA. Nevertheless, the randomised choice of the implant type for the first operation may have reduced this bias. In conclusion, we examined kinematics of weight-bearing maximalflexion activity of knees in subjects with bilateral TKA of two types. We demonstrate that the NRG TKA results in much better maximal flexion, lateral condyle AP translation and tibial internal rotation than does the traditional ScorpioFlex TKA (there are no significant differences in medial condyle AP translation between the two designs). The observed kinematic differences are most likely caused by variations in the morphological characteristics of the two prostheses.
6. Conflict of interest statement No benefit in any form has been or will be received from any commercial party related either directly or indirectly to the subject of this manuscript.
Source of funding No funds were received in support of this work. No benefits in any form have been or will be received from any commercial party related either directly or indirectly to the subject of this manuscript. References [1] Asano T, Akagi M, Tanaka K, Tamura J, Nakamura T. In vivo three-dimensional knee kinematics using a biplanar image-matching technique. Clin Orthop Relat Res 2001; 388:157–66. [2] Banks SA, Hodge WA. 2003 Hap Paul Award Paper of the International Society for Technology in Arthroplasty. Design and activity dependence of kinematics in fixed and mobile-bearing knee arthroplasties. J Arthroplasty 2004;19:809–16. [3] Banks S, Bellemans J, Nozaki H, Whiteside LA, Harman M, Hodge WA. Knee motions during maximum flexion in fixed and mobile-bearing arthroplasties. Clin Orthop Relat Res 2003;410:131–8. [4] Catani F, Belvedere C, Ensini A, Feliciangeli A, Giannini S, Leardini A. In-vivo knee kinematics in rotationally unconstrained total knee arthroplasty. J Orthop Res 2011; 29:1484–90. [5] Dennis DA, Komistek RD, Colwell Jr CE, Ranawat CS, Scott RD, Thornhill TS, et al. In vivo anteroposterior femorotibial translation of total knee arthroplasty: a multicenter analysis. Clin Orthop Relat Res 1998;356:47–57. [6] Dennis DA, Komistek RD, Mahfouz MR. In vivo fluoroscopic analysis of fixed-bearing total knee replacements. Clin Orthop Relat Res 2003;410:114–30. [7] Dennis DA, Komistek RD, Mahfouz MR, Walker SA, Tucker A. A multicenter analysis of axial femotibial rotation after total knee arthroplasty. Clin Orthop Relat Res 2004; 428:180–9. [8] Dennis DA, Komistek RD, Scuderi GR, Zingde S. Factors affecting flexion after total knee arthroplasty. Clin Orthop Relat Res 2007;464:53–60. [9] Dennis DA, Komistek RD, Walker SA, Cheal EJ, Stiehl JB. Femoral condylar lift-off in vivo in total knee arthroplasty. J Bone Joint Surg Br 2001;83:33–9. [10] Dennis DA, Mahfouz MR, Komistek RD, Hoff W. In vivo determination normal and anterior cruciate ligament-deficient knee kinematics. J Biomech 2005;38:241–53. [11] Futai K, Tomita T, Yamazaki T, Tamaki M, Yoshikawa H, Sugamoto K. In vivo kinematics of mobile-bearing total knee arthroplasty during deep knee bending under weight-bearing conditions. Knee Surg Sports Traumatol Arthrosc 2011;19:914–20. [12] Insall JN, Dorr LD, Scott RD, Scott WN. Rationale of the Knee Society clinical rating system. Clin Orthop Relat Res 1989;248:13–4. [13] Kim YH, Choi Y, Kim JS. Range of motion of standard and high-flexion posterior cruciate-retaining total knee prostheses a prospective randomized study. J Bone Joint Surg Am 2009;91:1874–81. [14] Kim YH, Sohn KS, Kim JS. Range of motion of standard and high-flexion posterior stabilized total knee prostheses. A prospective, randomized study. J Bone Joint Surg Am 2005;87:1470–5. [15] Kitagawa A, Tsumura N, Chin T, Gamada K, Banks SA, Kurosaka M. In vivo comparison of knee kinematics before and after high-flexion posterior cruciate-retaining total knee arthroplasty. J Arthroplasty 2010;25:964–9. [16] Klein GR, Restrepo C, Hozack WJ. The effect of knee component design changes on range of motion evaluation in vivo by a computerized navigation system. J Arthroplasty 2006;21:623–7. [17] Kurosawa H, Walker PS, Abe S, Garg A, Hunter T. Geometry and motion of the knee for implant design. J Biomech 1985;18:487–99. [18] Long WJ, Scuderi GR. High-flexion total knee arthroplasty. J Arthroplasty 2008;23: 6–10. [19] Mahfouz MR, Hoff WA, Komistek RD, Dennis DA. A robust method for registration of three-dimensional knee implant models to two-dimensional fluoroscopy images. IEEE Trans Med Imaging 2003;22:1561–74.
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Please cite this article as: Shi X, et al, Variations in morphological characteristics of prostheses for total knee arthroplasty leading to kinematic differences, Knee (2014), http://dx.doi.org/10.1016/j.knee.2014.10.013
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The Knee
Variations in morphological characteristics of prostheses for total knee arthroplasty leading to kinematic differences Xiaojun Shi, Zongke Zhou, Bin Shen, Jing Yang, Pengde Kang, Fuxing Pei ⁎ Orthopaedic Department, West China Hospital, Sichuan University, 37 Guo-xue Lane, Wu-hou District, Chengdu, China
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Article history: Received 31 May 2014 Received in revised form 10 October 2014 Accepted 30 October 2014 Available online xxxx Keywords: Total knee arthroplasty Kinematics Fluoroscopic analysis Fixed bearing
a b s t r a c t Background and purpose: The aim of this study is to compare kinematics during weight-bearing deep kneebending motion in patients after bilateral total knee arthroplasty (TKA) of two types: 1) a conventional ScorpioFlex prosthesis and 2) a contemporary redesigned non-restrictive-geometry (NRG) prosthesis installed by the same surgeon. Methods: We enrolled 15 patients who underwent conventional ScorpioFlex posterior-stabilised TKA in one knee and contemporary NRG TKA on the contralateral side (the same surgeon). During fluoroscopic examination, each patient performed weight-bearing deep knee bending. Motions among all components were analysed using a two- to three-dimensional registration technique. Results: The mean maximum flexion was 108° (SD 8) and 120° (SD 9) after ScorpioFlex and NRG TKAs, respectively; there were statistically significant differences between the groups. From extension to maximal flexion, the medial condyle translated by 4.8 mm (SD 1.2) and 5.4 mm (SD 2.4) posteriorly after ScorpioFlex TKA and NRG TKA, respectively. The lateral femoral condyle moved 8.4 mm (SD 1.5) and 12.2 mm (SD 2.1) posteriorly after ScorpioFlex TKA and NRG TKA, respectively. There were no significant differences in medial condyle translation between the groups except for the lateral condyle. The total amount of tibial axial rotation during extension to flexion was 5.1° (SD 1.8) after ScorpioFlex and 13.2° (SD 3.4) after NRG TKAs; there were statistically significant differences between the groups. Conclusions: NRG resulted in much better maximum flexion, lateral condyle movement and tibial internal rotation than did ScorpioFlex TKAs. The observed kinematic differences are most likely caused by variations in the morphological characteristics of the two implants. Level of evidence: Level I, Prospective randomed comparative study. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Total knee arthroplasty (TKA) has been proven to be an efficient surgical treatment of knee joint disorders because of excellent survival rates and long-term results, but patients' satisfaction after TKA is still ≤70–75% [27,28,33]. The range of motion is one important factor affecting a patient's satisfaction with the outcome of TKA, especially in Asian countries; cultural factors cause patients in Asia to frequently perform activities involving strong knee flexion such as squatting, kneeling or sitting cross-legged [26,30]. Patients' functional abilities seem strongly linked to successful restoration of normal motion in the replaced joint [2,3,23,32,40]. To perform deep knee bending after TKA, a bicondylar roll-back motion pattern such as that in a healthy knee is desirable [17,25]. Posterior translation of the femoral condyle and internal rotation of the tibia appear to be two important factors related to good knee flexion [3]. Nevertheless, in vivo fluoroscopic analyses of various
⁎ Corresponding author. E-mail address: [email protected] (F. Pei).
types of prostheses for total knee arthroplasty have shown numerous kinematic abnormalities: paradoxical anterior–femoral translation, reverse axial rotational patterns and abnormal femoral condylar lift-off are commonly present [5,9,35]. Although some studies reported kinematics after TKA procedures that is similar to that of a healthy knee, posterior roll-back of the femoral condyle and tibial axial rotation are still much worse than those in a healthy knee [31]. How to improve the femoral condylar roll-back and tibial internal rotation during knee flexion is still a challenging question for surgeons and prosthesis manufacturers. More recently, a novel rotationally unconstrained and fixed-bearing posterior-stabilised prosthesis for TKA, Scorpio Non-Restrictive Geometry (NRG) implant (Stryker Orthopedics, Mahwah, NJ) came on the market. This prosthesis is a recent product incorporating various modifications of the previous Scorpio Knee System (Stryker Orthopedics) and is advertised as a system improving the femoral condyle translation in the sagittal plane and reducing the restriction of tibial axial rotation in the transverse plane (Figs. 1 and 2). Although several studies have reported that an NRG implant improves TKA kinematics [4,39], no study to date has compared kinematics after the contemporary NRG
http://dx.doi.org/10.1016/j.knee.2014.10.013 0968-0160/© 2014 Elsevier B.V. All rights reserved.
Please cite this article as: Shi X, et al, Variations in morphological characteristics of prostheses for total knee arthroplasty leading to kinematic differences, Knee (2014), http://dx.doi.org/10.1016/j.knee.2014.10.013
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X. Shi et al. / The Knee xxx (2014) xxx–xxx
Fig. 1. A projection drawing showing the difference between the non-restrictive geometry (NRG; left) and ScorpioFlex (right) femoral components in their frontal (top) and sagittal (bottom) views; both exhibit medial–lateral symmetry in the condyles and are used in single-radius TKA. The anterior flange of the NRG design was reduced by 2 mm (d2 − d1 = 2) to decrease stress on the extensor mechanism, retinaculum and widened intercondylar notch (L1 N L2). Meanwhile, the NRG implant shows a much smaller deep-flexion radius from 95° to 155° compared to the ScorpioFlex implant (black arrow) and thereby ensures a relatively relaxed arrangement of the posterior structure.
TKA (has features designed to promote good flexion) with kinematics after the conventional ScorpioFlex TKA in patients with bilateral TKA of these two types. The goal of this study was to compare kinematics under weightbearing deep knee-bending motion in patients with bilateral TKA of two types: 1) involving a conventional ScorpioFlex prosthesis and 2) based on the contemporary redesigned NRG prosthesis installed by the same surgeon. We hypothesised that the knee that receives the
NRG prosthesis would show better posterior roll-back and axial rotation than would the ScorpioFlex implant. 2. Materials and methods Prior to the study, the Institutional Review Board of West China Hospital of Sichuan University approved the study protocol, and written informed consent was obtained from the patients. We enrolled the
Fig. 2. A projection drawing showing the difference between the non-restrictive geometry (NRG; left) and ScorpioFlex (right) polyethylene insert in their sagittal (above) and transverse (below) views. In the sagittal view, the post of the NRG implant is much closer to the posterior edge of the insert compared to the ScorpioFlex implant (d3 b d4); the NRG has a lower posterior lip (dashed line). In the transverse view, the spherical arc articulating surface (dashed line) and the round post (black arrow) are the most characteristic differences between the two designs.
Please cite this article as: Shi X, et al, Variations in morphological characteristics of prostheses for total knee arthroplasty leading to kinematic differences, Knee (2014), http://dx.doi.org/10.1016/j.knee.2014.10.013
X. Shi et al. / The Knee xxx (2014) xxx–xxx
patients with a primary diagnosis of knee osteoarthritis, who were prepared to undergo staged total knee arthroplasty on both sides and willing to participate in the study. From February to December 2011, 15 patients (5 males) underwent conventional ScorpioFlex posterior-stabilised (PS) TKA in one knee and contemporary redesigned NRG posterior-stabilised TKA on the contralateral knee within three months (the same surgeon using the same surgical technique). The choice of the implant type for the first operation was randomised by date of the operation: an odd date corresponded to the ScorpioFlex implant and an even date to the NRG implant. Eight patients received the ScorpioFlex implant and seven patients the NRG implant during the first operation. The mean duration of follow-up and the interval between the bilateral operations was 27 months (range 24–31) and 1.5 months (range 1–3), respectively. The average perioperative age of the patients was 65 years (range 53–75). The average height, weight and body–mass index were 166 cm (range 154–176), 71 kg (range 54–80) and 26 kg/m2 (range 22–29). These patients were analysed post-operatively using video-fluoroscopic analysis. Clinical assessment was performed using the Hospital for Special Surgery (HSS) score pre-operatively and at follow-up (Table 1) [12]. All operations were performed by the same surgeon using the standard surgical technique, including an anterior midline skin incision and the medial parapatellar approach. Femoral bone cuts of both prostheses were of anterior referencing design. Distal femora were cut nearly 8 mm away from the most distal normal surface according to the intramedullary alignment system with 5° valgus. Rotational alignment of the femoral component was parallel to the transepicondylar axis. After that, tibiae were cut perpendicularly to the long axis of tibia with a 5° posterior slope through the extramedullary alignment system. The flexion and extension spaces were equalised, and soft tissues were balanced. A tourniquet was used for all patients, and all patellae were not resurfaced. All components were fixed with cement. Weightbearing and knee flexion exercises were started on postoperative day 1. The postoperative rehabilitation programme was the same for all patients. The three-dimensional (3D) position and orientation of the tibial and femoral components were determined using model-based image registration techniques as described previously [34]. Each patient was asked to perform sequential deep knee bending under weight-bearing conditions from extension to maximum flexion during fluoroscopic examination in the sagittal plane. Knee motions were recorded as serial digital X-ray images (20 frames per second, 1024 × 1024 × 12 bits/ pixels) using a 12-in digital image intensifier system (AXIOM Artis VB31, Siemens, Germany). The images were analysed at 15° intervals according to fluoroscopically measured flexion angles. Parameters for calibration and distortion correction were determined using a calibration image of radiopaque beads with known patterns (Camera Calibration Toolbox for Matlab; The Mathworks Inc., Natick, Massachusetts, USA) and were used to correct all images. The corrected images and CAD models were imported into open-source shape-matching software (JointTrack, University of Florida) to complete the shape-matching process. The automated local optimisation algorithm was based on the method described by Mahfouz et al. [19], and a previous validation study demonstrated that the 2D–3D registration technique results in acceptable root-mean-square errors and good test–retest reliability [34]. Articular surface separation was assumed for any measure N2.4 mm described by Mahoney et al. [20].
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Postoperative maximal flexion, anterior–posterior (AP) condylar positions and axial rotation between femoral and tibial components were quantified. The locations of medial and lateral condylar contacts were estimated as the lowest point in each femoral condyle relative to the transverse plane of the tibial baseplate. The distance between the contact point and the AP centre of the tibial baseplate was measured, and AP positions of the femoral component anterior to the tibial insert were recorded as positive, whereas the posterior positions as negative. Tibial internal rotation relative to the femur was defined as negative rotation. 3. Statistical analysis All data were expressed as mean ± standard deviation. Paired Student's t test was used for analysis of differences, including those in post-operative maximal flexion, HSS score, AP displacement of the nearest points and contact points on the medial and lateral sides and axial rotational angles between the femoral component and the tibial component after bilateral TKA. Statistical significance was assumed as p b 0.05. 4. Results The mean range of motion at the latest follow-up was 108° (SD 8°) after ScorpioFlex TKA and 120° (SD 9°) after NRG TKA; there were statistically significant differences between the bilateral TKAs (p b 0.05; Table 2). The mean postoperative HSS score was 89 (SD 5) after ScorpioFlex TKA and 91 (SD 7) after NRG TKA, without statistically significant differences between the bilateral TKAs (p = 0.31). From extension to maximal flexion, the medial condyle translated by 4.8 mm (SD 1.2) and 5.4 mm (SD 2.4) posteriorly after ScorpioFlex TKA and NRG TKA, respectively. There were no differences between the two designs in the deep knee-bending activity for the AP translation. Both designs showed that the medial condyle translated posteriorly with flexion, although it translated anteriorly at mid-flexion (p = 0.58; Table 1, Fig. 3a). The lateral femoral condyle moved by 8.4 mm (SD 1.5) and 12.2 mm (SD 2.1) posteriorly after ScorpioFlex TKA and NRG TKA, respectively. There were statistically significant differences between the bilateral TKAs (p b 0.05; Table 1, Fig. 3b). The total extent of tibial axial rotation during extension to flexion was 5.1° (SD 1.8°) after ScorpioFlex TKA and 13.2° (SD 3.4°) after NRG TKA (Table 2). The absolute extent of tibial axial rotation was significantly greater with the contemporary design than with the traditional design (p b 0.05; Table 2, Fig. 4). Separation of articular surfaces was not detected in any case in any position.
5. Discussion Multiple factors can affect postoperative knee flexion, for example, a preoperative range of motion, operative technique, and prosthesis design [8,18,21]. The NRG design is a new type of prosthesis for TKA and was derived from the previous ScorpioFlex models with the aim of supporting deep knee flexion by increasing posterior condylar translation and by reducing the rotation restraint during knee flexion. Morphological modifications (new features) in the NRG model include a spherical arc in the transverse plane of the insert articulating surface, the widened intercondylar notch with a modified round tibial post, and a reduction of the posterior radius of curvature. Compared to the ScorpioFlex implant, the above characteristics of the NRG implant cause the femoral component to rotate more freely around the tibia (Figs. 1 and 2). In addition, the anterior flange of the femoral component is reduced by 2 mm to decrease the stress on the extensor mechanism and retinaculum [16,38]. In our study, we compared knee kinematics during deep knee bending in patients with bilateral TKAs involving traditional and contemporary
Table 1 Comparsion of preoperative demographic data.
ScropioFlex TKA Scorpio NRG TKA p
Range of motion(°)
Valgus/varus defromtiy(°)
Flexion deformity(°)
HSS score
102 ± 10 106 ± 8 0.54
8±2 7±3 0.75
20 ± 5 16 ± 4 0.58
42 ± 3 45 ± 4 0.69
Values are expressed as mean ± SD.
Please cite this article as: Shi X, et al, Variations in morphological characteristics of prostheses for total knee arthroplasty leading to kinematic differences, Knee (2014), http://dx.doi.org/10.1016/j.knee.2014.10.013
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Table 2 Knee pose during maximum deep bending.
ScropioFlex TKA Scorpio NRG TKA p
Maximal flexion (°)
Tibial internal rotation (°)
Medial AP translation (mm)
Lateral AP translation (mm)
108 ± 8 120 ± 9 b0.05
5.1 ± 1.8 13.2 ± 3.4 b0.05
4.8 ± 1.2 5.4 ± 2.4 0.58
8.4 ± 1.5 12.2 ± 2.1 b0.05
Values are expressed as mean ± SD.
prostheses implanted by the same surgeon using the same operative technique, with the exception of an apparent tibial internal rotation and femoral roll-back bias in knees undergoing traditional TKA. We did observe knee flexion and kinematics that are functionally different between the two designs: the NRG prosthesis shows much better maximal knee flexion, lateral condyle translation and tibial internal rotation than does the ScorpioFlex implant. Nevertheless, there is no significant difference in medial femoral condylar posterior translation between the two designs. In the present study, the average range of motion after NRG TKA was much greater compared to the ScorpioFlex TKA design; these data are indicative of good postoperative results for the NRG design. Although the sample size is relatively small in this study, the above result seems plausible and is consistent with other studies of the same implants. Klein et al. [16] compared the intraoperative range of motion of the NRG and ScorpioFlex prostheses using a computerised navigation system under the same conditions of an extension/flexion gap and softtissue balance; the researchers found that the NRG design results in a significantly greater range of motion than does the ScorpioFlex implant. Nonetheless, there is no consensus on whether high-flexion TKA
ensures much better postoperative range of motion than does traditional TKA; differences among implants used in different studies seem to be the main reason for the discrepancies between the two types of prosthesis in the post-operative range of motion [13,14,41]. Previous kinematic analyses of healthy knees showed that during knee flexion, steady internal rotation of the tibia relative to the femur takes place, and the extent of internal rotation reaches 20° and more [1,10]. On the other hand, in vivo kinematic analysis of various TKA prostheses demonstrated that the tibial rotation during knee flexion includes not only medial pivot internal rotation similar to that in a healthy knee but also non-physiological lateral pivot reverse rotation; the extent of rotation is limited and much smaller than that of a healthy knee [31]. In the present study, the mean tibial axial rotation is 13.2° after NRG TKA during deep knee bending, and the mean tibial axial rotation after ScorpioFlex TKA is 5.1°; there is a statistically significant difference between the two designs (p b 0.05). Meanwhile, posterior translation of the lateral condyle with the NRG implant is much better than that of ScorpioFlex TKAs, but there is no significant difference in the medial condyle. These findings may be explained by the fact that the NRG design has a more relaxed geometrical arrangement in the
Fig. 3. The kinematic pathway of the medial (a) and lateral (b) nearest points between the femoral and tibial component during deep knee bending after either non-restrictive geometry (NRG) or ScorpioFlex total knee arthroplasty (TKA). The solid line corresponds to NRG TKA, and the dotted line corresponds to ScorpioFlex TKA. Positive values indicate the anterior direction.
Please cite this article as: Shi X, et al, Variations in morphological characteristics of prostheses for total knee arthroplasty leading to kinematic differences, Knee (2014), http://dx.doi.org/10.1016/j.knee.2014.10.013
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Fig. 4. Axial rotation of the femoral component relative to the tibial tray. The horizontal axis shows the flexion angle between the components, and the vertical axis shows axial rotation. Positive values represent internal rotation.
posterior aspect, including a spherical arc in the transverse plane of the insert's articulating surface, a much smaller deep-flexion radius from 95° to 155° compared to the larger single AP radius from 0° to 95° and the round-on-round design of the post-cam mechanism. These characteristics ensure a relatively relaxed arrangement in the posterior structure resulting in a lesser posterior constraint on rotation and translation during knee flexion. Furthermore, the surrounding ligaments perform a much better restricting function and induce femoral condyle movement, which may be beneficial for posterior translation of the lateral condyle looking like a medial pivot motion of a healthy knee. Several studies have shown that NRG TKA results in much better tibial rotation during various activities. Tamaki et al. [38] reported that tibial internal rotation after NRG TKA (as used in our study) is 13.5° during deep knee bending, and the internal rotation is increased up to the maximal flexion. Catani et al. [4] reported that after NRG TKA, the average tibial internal rotation during various activities is nearly 11°. These data are similar to our results on NRG TKA. In one study, tibial internal rotation was found to be associated with maximum knee flexion both in healthy and post-operative knees [6]. Although axial rotation of the tibia after NRG TKA is much better than that after ScorpioFlex TKA, it is still limited compared to a healthy knee. On the other hand, greater posterior translation of the lateral condyle may be the main reason for the greater range of motion after NRG TKA [7,36]. There are several limitations to our study. First, the key limitation is the relatively small sample size. Characteristics of a kinematic study necessitate a large sample size, although the majority of similar kinematic studies also include only 10–20 knees [11,15,22,24,29,37,42]. Second, the staged rather than simultaneous bilateral total knee arthroplasty may affect the postoperative functional outcome of the second TKA. Nevertheless, the randomised choice of the implant type for the first operation may have reduced this bias. In conclusion, we examined kinematics of weight-bearing maximalflexion activity of knees in subjects with bilateral TKA of two types. We demonstrate that the NRG TKA results in much better maximal flexion, lateral condyle AP translation and tibial internal rotation than does the traditional ScorpioFlex TKA (there are no significant differences in medial condyle AP translation between the two designs). The observed kinematic differences are most likely caused by variations in the morphological characteristics of the two prostheses.
6. Conflict of interest statement No benefit in any form has been or will be received from any commercial party related either directly or indirectly to the subject of this manuscript.
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