Effect of post-cam design on the kinematics and contact stress of posterior-stabilized total knee arthroplasty

Abstract

BACKGROUND:

The post-cam mechanism in the posterior-stabilized (PS) implant plays an important role, such as durability and kinematic performances, in total knee arthroplasty (TKA).

OBJECTIVE:

The purpose of this study was to evaluate the difference in the kinematics and contact stress of five post-cam designs, which are flat-and-flat, curve-and-curve (concave), curve-and-curve (concave and convex), helical, and asymmetrical post-cam designs, using three-dimensional finite element models.

METHODS:

We designed the post-cam model with five different geometries. The kinematics, contact stress, and contact area were evaluated in the five post-cam designs under gait cycle loading conditions using the finite element method.

RESULTS:

There were no differences in the contact stress and area on the tibial insert in all designs. The largest internal rotation was shown in the swing phase for the helical design, and the largest tibial posterior translation was observed for the curve-and-curve (concave) design. The curve-and-curve (concave) design showed the lowest contact stress and the largest posterior tibial translation during the gait cycle.

CONCLUSIONS:

Considering the kinematics and contact stress, we found that the curve-and-curve (concave) design was more stable than other designs. From the results, we found the important factors of TKA implant considering stability and kinematics.

Keywords

Total knee arthroplasty implant kinematics design gait

1. Introduction

Posterior-stabilized (PS) total knee arthroplasty (TKA) was introduced to prevent posterior subluxation of the tibia and to increase posterior femoral translation in femoral rollback [1]. In addition, PS-TKA shows satisfactory long-term survival rates and good functional performance [2,3]. However, complications from the post-cam mechanism, such as dislocation of the knee and fracture or severe wear of the post, have occurred [4,5]. In some retrieval analyses, post damages were found anteriorly and posteriorly, related to hyperextension and hyperflexion, respectively [6]. The tibial polyethylene post in the PS-TKA designs has shown substantial wear debris, and the retrieval analyses showed evidence of post wear on all implants examined [7,8]. In the severe cases, this wear can cause fracture of the tibial post, leading to instability and revision knee arthroplasty. Thus, wear on the posterior surface of the tibial post is expected from contact with the femoral cam [9]. Akasaki et al. evaluated the effects of the post-cam conformity on the contact area and stress at the post-cam mechanism using four different posterior stabilized TKAs [6]. In addition, Nakayama et al. evaluated the contact stress in three different designs, in which the apex of the cam was orientated distally, posteriorly, or posterodistally [10]. However, in the Nakayama study, the placement of the sensor could affect the stress fields, and the pressure sensor could only measure the contact pressure of the articular surface. Huang et al. investigated the stress on the tibial posts using a finite element method for the curve-and-curve and flat-and-flat designs [11]. Watanabe et al. showed the difference in the contact stress on the tibial post between a round post-cam and a square design during deep knee flexion and at hyperextension using finite element models [12]. However, this experiment was performed in a static condition, and only two post cam designs were examined. The contact stress on the posterior aspect of the tibial post can increase because of the reduced contact area rather than the increased contact force, and the contact area is dependent on the post-cam geometry.

Based on the above-mentioned background, the purpose of this study was to evaluate the difference in the kinematics and contact stress for five different post-cam designs using three-dimensional (3D) finite element (FE) models. The five designs were flat-and-flat, curve-and-curve (concave), curve-and-curve (concave and convex), helical, and asymmetrical post-cam designs. The tibiofemoral surface contained an identical design for the different post cam designs. The kinematics and contact stress were evaluated in the five different post-cam designs under gait cycle loading conditions. We hypothesized that the post-cam in the curve-on-curve (concave) design would provide a lower contact stress and higher kinematics than those of the other designs.

2. Material and methods

2.1. Design of the post-cam

Five different 3D models of PS-TKA were developed with identical surfaces, except for the post-cam geometry, including the intercondylar notch of the femoral component (Fig. 1). The different post-cam designs for PS-TKA were flat-and-flat PS-TKA (FF PS-TKA), curve-and-curve (cam: concave) PS-TKA (CC PS-TKA), curve-and-curve PS-TKA (cam: concave and convex) (CAC PS-TKA), helical PS-TKA (HC PS-TKA), and asymmetrical PS-TKA (AC PS-KTA). The PS-TKA model was developed using the geometrical dimensions of Genesis II (Smith & Nephew, Inc., Memphis, TN, USA). The post anterior–posterior position; post size, such as the height, width, and depth; and cam position, such as the distance from the posterior edge and height above the joint line, were controlled to evaluate the effect of the post-cam design [13].

Fig. 1.

Customized PS-TKAs with the five different post-cam designs (and conventional PS-TKA).

2.2. Development of the finite element model

A validated computational model for the knee joint was developed to evaluate the effect of the post-cam design on the knee kinematics and contact stress [14,15]. Each PS-TKA consisted of three parts: the femoral component, tibial component, and tibia insert. The femoral and tibial components with a high elastic modulus relative to tibia insert, which was 300 times higher, were developed as the rigid bodies for the surface representation [16,17]. Therefore, rigid body assumptions were considered for the femoral and tibial components. The tibial insert was modeled as an elastic-plastic material with a modulus of elasticity of 463 MPa and a Poisson’s ratio of 0.4 using eight-node hexahedral elements [16]. A friction coefficient of 0.04 was applied between the femoral and tibial insert [16]. Solid modeling and meshing were performed using Hypermesh 11.0 (Altair Engineering, Inc., Troy, MI, USA), and an analysis and post-processing were performed using ABAQUS 6.13 (Simulia, Providence, RI, USA). The kinematics, contact mechanics, and wear performance were investigated using a computational model based on a Stanmore knee simulator [14–18].

The Stanmore simulator is a well-established load-controlled knee simulator, in which an in vivo condition of the knee joint is represented by applying the appropriate forces and moments to the femoral and tibial components. The soft tissue constraints were modeled with a mechanical spring-based assembly of four linear springs (Fig. 2). For PS-TKA, the resected anterior cruciate ligament (ACL) as well as the posterior cruciate ligament (PCL) were simulated with a translational stiffness of 7.24 N/mm, in the anterior and posterior sides of the tibial component, while the medial lateral collateral ligaments were simulated by adding a rotational stiffness of 0.3 N/° to the springs [16,19]. A spring gap of 2.5 mm was used for each side to simulate the anatomical laxity, and the axial load was offset towards the medial condyle to reproduce a 60 to 40 proportion in the experiment [14,15]. The loading and boundary conditions were based on the force-controlled protocol, and the results were consistent with those of previous studies (Fig. 3) [14,15]. The femoral component and tibial insert were used in the testing conditions with the input profiles of an anterior-posterior (AP) load and internal-external (IE) torque applied to the insert and a flexion–extension angle and an axial force applied to the femoral component (Fig. 3). The femoral component was constrained in the IE, medial-lateral (ML), and AP directions, and it was free to translate in the inferior-superior direction and to rotate about the frontal and transverse axes to represent the varus-valgus (VV) rotation and flexion-extension, respectively. The tibial insert was allowed to translate in the AP direction and rotate about a fixed vertical axis located in the center of the tibial condyles to simulate IE rotation. The distal surface of the tibial insert was supported in the inferior-superior (IS) direction, while the insert tilt was constrained, and the VV and ML degrees of freedom were free. The center of rotation for the finite element model was determined between the medial and lateral condyles.

Fig. 2.

Finite element model of total knee arthroplasty used in this study.

Fig. 3.

Three loading conditions for gait cycle used in the study: (a) flexion angle; (b) axial load; (c) AP load; (d) IE torque.

3. Results

Figure 4 shows the AP and IE kinematics in the five post-cam designs during the gait cycle. Posterior translation and internal rotation were found in the swing phase for all five post-cam designs. The CC PS-TKA design showed the largest posterior translation in the swing phase at 6.3 mm. The HC PS-TKA design showed the largest internal rotation reaching 7.2° and 6.6° during the post-cam engagement for the HC PS-TKA and AC PS-TKA designs, respectively. There was a minimal difference between the AP and IE kinematics in the five post-cam designs.

Fig. 4.

Comparison of kinematics for the five different post-cam designs under the gait cycle condition: (a) AP translation; (b) IE rotation.

Figure 5 shows the contact stress and area on the surface and post in the tibial insert for the five different post-cam designs during the gait cycle. There was no difference in the contact stress and area on the surface and post in the tibial insert for the five different post-cam designs during the gait cycle. However, there was a difference in the contact stress on the post in the swing phase, where engagement of the post cam occurred. The lowest contact stress on the tibial post was observed in the CC PS-TKA model, followed by the CAC PS-TKA, HC PS-TKA, AC PS-TKA, and FF PS-TKA models. The higher contact stresses corresponded to the smaller contact areas in the tibial post. The contact stress on the post in the FF PS-TKA model was 63% higher than that of the CC PS-TKA model.

Fig. 5.

Comparison of (a) maximum contact stress, (b) contact stress and (c) contact area of the surface and post in the tibial insert for the five different post-cam designs under the gait cycle.

4. Discussion

In this study, the kinematics and contact stress on the tibial post were influenced by the post-cam design. PS-TKA is one of the most successful treatments in orthopedic surgery [20]. However, kinematics problems after TKA are of great concern. Numerous studies have investigated the effects of the tibiofemoral articular surface on knee kinematics to restore normal knee kinematics; however, the post-cam mechanism has not been well studied [2,21]. Post-cam designs have different sizes and shapes in various implants [6]. Despite the advances in the TKA design, many patients with TKA still experience limited knee flexion [22,23]. This is evident in the Asian population, which requires an average individual to adopt special posture positions in their daily activities [24]. To overcome this problem, several knee designs have incorporated new features, such as modification of the posterior sagittal femoral geometry to prevent the occurrence of polyethylene edge loading and increase the articulation curvature under deep knee flexion [23]. However, greater flexion angles have resulted in a high wear rate in the tibial insert, leading to an increased number of surgeries required for patients [25]. Lin et al. evaluated the effect of the post-cam design on femoral translation and axial tibial rotation [26]. They found that the post-cam engagement to extreme flexion in the curve-on-curve design had a greater internal tibial rotation [26]. Recently, Fallahiarezoodar et al. determined the most appropriate cam and post designs to produce normal femoral rollback of the knee [25]. They demonstrated that the use of a circle cam and convex post created the best femoral rollback effect, which produced the highest amount of knee flexion [25]. Li et al. stated that the post-cam mechanism is important for guiding the tibiofemoral motion in high knee flexion [27]. Therefore, it is necessary to evaluate the effect of different post-cam designs on the kinematics for dynamic activity. This study showed that the posterior tibial translation and internal tibial rotation increased with knee flexion in the five post-cam designs after post-cam engagement. When the femoral cam engaged with the tibial post, an increase of the tibial rotational angle against knee flexion occurred. Therefore, a reverse torsion was exerted on the tibia. Internal tibial rotation is a fundamental motion pattern during high flexion angles. Tsumori et al. developed a helical post-cam design to induce and accommodate internal tibial rotation with deep knee flexion, avoiding impingement at the post-cam interface [28]. Their results showed consistent internal rotation of the tibia in deep flexion with a wide contact area at the post-cam interface as intended by the original design concept of the TKA system [28]. This trend was observed in the HC PS-TKA model in this study. However, although the geometry of the post-cam contact surfaces of the HC PS-TKA and AC PS-TKA designs assisted knee motion, edge loading could occur during engagement and lead to a greater stress on the tibial post. The material wear and catastrophic fracture of the post with these types of implants should be evaluated. The CC PS-TKA design showed the largest posterior translation in the swing phase. Theoretically, posterior tibiofemoral translation is important in TKA as it allows more flexion prior to tibiofemoral impingement [29]. A more posterior contact position between the tibiofemoral components leads to an increase in the quadriceps lever arm, which improves the movement efficiency, thus contributing to better International Knee Society Function scores [29].

Nakayama et al. reported that the contact stress of the post-cam increased with the internal tibial rotation [10]. In a previous study that compared two post-cam designs, there was a high contact stress on the post with tibial rotation, where the curve-on-curve design reduced the stress concentration [11]. These investigations were analyzed in a static condition, and only physiological loadings were considered. However, the post-cam mechanism should sustain the physiologic loading as well as the aforementioned rotational force under the dynamic post-cam interaction. The contact stress of the post-cam interface could be increased. In this condition, tibial post could be affected owing an increased risk of polyethylene wear. Therefore, the contact stress on the tibial post was investigated under the dynamic loading condition in this study. We think the contact stress of the post-cam interface may substantially be raised. With this condition, tibial post would be jeopardized at an increased risk of polyethylene wear.

In this study, the contact stress on the posterior aspect of the tibial post during flexion increased dramatically in the FF PS-TKA design. However, the contact stress increase was lower in the CC PS-TKA and CAC PS-TKA designs than that of the FF PS-TKA design when the post and cam were engaged.

A previous study reported high contact pressures on the tibial post using four different systems when a posterior force of 500 N was applied to the tibial components against the femoral components [10]. This value was greater than the contact stress on the tibial post in our study because their study was performed at a higher flexion angle.

However, the value of the contact stress was similar in other studies that used a similar flexion angle [11]. Damage of the tibial post was observed on each side of the post, and the implant design could affect to the tibial post wear. Dolan et al. evaluated retrieved inserts of three different TKAs, and the primary determinant of the post wear was the implant design [9]. Polyethylene wear at the tibial post is influenced by various factors, such as the post location and height, alignment of the tibiofemoral joint, and the post-cam shapes. The design that more effectively avoids excessive wear or tibial post fracture should be biomechanically evaluated and verified by retrieval studies [9]. A previous study on retrieval post-cam designs used flat-on-flat contact surfaces, and polyethylene wear and damage were observed on all tibial posts. Wear was primarily shown on the posterior side of the post [5]. Our results were consistent with the data from these retrieval studies, suggesting that the curve-on-curve post-cam designs reduced edge loading, therefore leading to lower stresses than those of the other designs. In particular, the maximum contact stress of CC PS-TKA was the lowest compared to FF PS-TKA.

Based on the biomechanics, our results showed that the kinematics and contact stress were influenced by the post-cam design. The CC PS-TKA design was recommended, although the HC PS-TKA or AC PS-TKA designs were more effective for tibial internal rotation because edge loading occurs during engagement and leads to greater stress on the tibial post in the HC PS-TKA and AC PS-TKA designs. In addition, in addition to the contact stress on the post, a positive effect on the tibial posterior translation was shown in the CC PS-TKA design.

There were two limitations to this study. First, a finite element analysis was performed, and a real-life situation was not reflected. Although our data were consistent with other previously reported studies, the results are not fully applicable to a clinical situation. Second, the five specific designs in this study did not represent all existing design features of contemporary PS-TKA. Despite some limitations, this study provided guidance for choosing a suitable PS-TKA design that could avoid wear on the tibial post in the post-cam design.

5. Conclusions

In conclusion, the curve-and-curve (concave) design showed the lowest contact stress and the highest posterior tibial translation during the gait cycle. Tibial internal rotation is inevitable, and thus a post-cam design that provides a constant contact area and stress during tibial rotation is required during the gait cycle. These results can be used as a guideline for the development of implant designs and the prediction of the wear on the tibial post in PS-TKA.

Footnotes

Conflict of interest

None to report.

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