Patient-specific design for articular surface conformity to preserve normal knee mechanics in posterior stabilized total knee arthroplasty

Abstract

Background:

Contemporary total knee arthroplasty (TKA) provides remarkable clinical benefits. However, the normal function of the knee is not fully restored. Recent improvements in imaging and manufacturing have utilized the development of customized design to fit the unique shape of individual patients.

Objective:

The purpose of the present study is to investigate the preservation of normal knee biomechanics by using specific articular surface conformity in customized posterior stabilized (PS)-TKA.

Methods:

This includes customized PS-TKA, PS-TKA with conforming conformity (CPS-TKA), medial pivot conformity with PS-TKA (MPS-TKA), and PS-TKA with mimetic anatomy femoral and tibial articular surface (APS-TKA). In this study, kinematics, collateral ligament force and quadriceps force were evaluated using a computational simulation under a deep knee bend condition.

Results:

A conventional TKA did not provide the normal internal tibial rotation with flexion leading to abnormal femoral rollback. The APS-TKA exhibited normal-like femoral rollback kinematics but did not exhibit normal internal tibial rotation. However, APS-TKA exhibited the most normal-like collateral ligament and quadriceps forces.

Conclusions:

Although the APS-TKA exhibited more normal-like biomechanics, it did not restore normal knee biomechanics owing to the absence of the cruciate ligament and post-cam mechanism.

Keywords

Patient-specific implant total knee arthroplasty conformity finite element analysis

1 Introduction

Conventional total knee arthroplasty (TKA) constitutes an effective surgical treatment for the restoration of basic function in late-stage arthritis patients and substantially increases their quality of life [1,2]. However, a common problem associated with TKA is the post-operative decrease in knee strength [3]. Traditionally, TKA has been available in a limited number of sizes, and this has led to a debate on poor patient outcomes in which implant sizing is a contributing factor. Oversized/undersized TKAs can cause patients discomfort during daily activities. Oversized TKA can lead to soft tissue impingement and knee pain [4,5]. Undersized TKA can lead to joint laxity and instability during weight-bearing activities [6]. Additionally, variations exist between each subject in the groups, and this suggests that a customized TKA may have advantages compared to ethnic and/or gender-specific TKA [7].

Initially, customized TKA is subject to a computerized tomography scan that is used to design an individualized implant that matches the anatomy of the tibial plateau and femoral condylar geometries for each patient while simultaneously achieving a neutral mechanical axis [8,9]. The implants have shown considerable success in the replication of normal knee kinematics in-vitro [9]. Nevertheless, there has been no study that evaluated this in terms of the posterior-stabilized (PS)-TKA [10].

Currently, two major design types of TKA are used: cruciate-retaining (CR) in which the anterior cruciate ligament is resected but the posterior cruciate ligament is preserved, and PS in which both cruciates are resected but the function of the posterior cruciate ligament is substituted by implant design elements [11]. In the PS type, the tibial insert surfaces provides a certain extent of anterior-posterior (AP) and rotational stability throughout the flexion range, whereas the intercondylar post-cam mechanism engages at approximately 60° to 90° of flexion and leads to the posterior displacement of the femur on the tibia while preventing anterior femoral displacement [11]. The PS with an intercondylar post-cam is treated as a generic style of TKA. Nevertheless, numerous designs are available that vary both in the frontal and sagittal radii of the tibial insert surfaces and in the configuration of the post-cam mechanism [11]. However, there has been no study on the radii of tibial insert corresponding to femoral component and conformity in customized PS-TKA.

The goal of the present study is to investigate the conformity of tibial insert and femoral component in customized PS-TKA. Customized PS-TKA was categorized into conforming design PS-TKA (CPS-TKA), medial pivot design PS-TKA (MPS-TKA) and anatomic design PS-TKA (APS-TKA) with tibial and femoral articular surface based on the anatomy. We compared the conventional PS-TKA and customized PS-TKA with three different tibial insert designs to evaluate the biomechanical effect using a computational simulation under a deep knee bend condition. We hypothesized that APS-TKA exhibits the most normal-like kinematics and biomechanical effect.

2 Methods

2.1 Design of customized PS-TKA

An existing three-dimensional (3D) model was used to develop customized PS-TKA [12–14]. The 3D reconstruction and editing of the models were performed in Mimics and 3-Matic software (Materialise, Leuven, Belgium). Planes were introduced by the intersection of the condyles in both the sagittal and coronal planes. Intersection curves were used to extract the articulating surface geometry in both planes. The three patient-specific ‘J’ curves for the trochlear grooves and the medial and lateral condyles from the patients’ normal articular anatomy were developed in the Unigraphics NX software (Version 7.0; Siemens PLM Software, Torrance, CA, USA) (Fig. 1).

The natural condylar offset is defined as the coronal offset. The customized femoral component uses these patient-specific differences and is designed with the patient’s exact coronal offset. The coronal offset is defined as the height difference between the medial and lateral femoral condyles in the coronal extension plane. Typically, this leads to asymmetry in the extension gap that should be considered at the tibial articular surface in a manner that is identical to the functions of a normal human knee. The same applies for the posterior condyles of the femur. Typically, the lateral posterior condyle is shorter than the medial condyle, and this creates a unique asymmetry in the flexion space. These are the customized design elements along with the patient’s unique ‘J’ curvatures that are incorporated into the customized femoral component [15–19].

On the tibial plateau, the profile of the patient’s tibia defines the geometry of the tibial implant. There is less opportunity to conserve bone stock on the tibial plateau. In this method, the patient receives an implant that is optimized for fit and also involves reduced contact stress in the polyethylene (PE) insert [15–19]. Generally, articular geometry in customized tibial insert design is derived from the femoral component. The medial insert geometry is slightly more conforming than that in the lateral insert. The coronal geometry utilizes a broad radius on both condyles, and thereby employs the round on round principle that was shown to reduce contact stress [19].

Three different tibial inserts were developed to investigate tibiofemoral articular surface conformity in this study. Furthermore, identical femoral component designs were designed, and the post-cam design was also the same to eliminate the effect of post-cam mechanism. We applied tibiofemoral conformity of conventional PS-TKA into patient-specific TKA conformity. In order to archive this, conforming design TKAs, such as Genesis II Total Knee System (Smith & Nephew Inc., Memphis, TN, USA) and medial pivot design Evolution Total Knee Arthroplasty (Wright Medical Technology, Arlington, TN, USA), were selected.

The conformity of TKA was investigated by scanning them with a non-contact 3D laser scanner (COMET VZ; Steinbichler Optotechnik GmbH, Neubeuern, Germany) with an accuracy of 50 μm. Scanned point data were converted to 3D models, and scanning was repeated until the 3D model dimensions had geometrical errors of <100 μm [20].

The ratio of the curvature radius for tibial insert to the total curvature radius for femoral component was investigated for conformity in the coronal and sagittal planes. Tibial insert with conventional PS-TKA conformity (Genesis II) and medial pivot tibial insert with medial pivot conformity (Evolution) were developed by applying the curvature radius ratio in coronal and sagittal planes for patient-specific femoral components (Fig. 2). Additionally, anatomy mimetic patient-specific TKA was developed in which the femoral as well as the tibial articular surface followed patient’s geometry. In general, the tibial geometry has a shallow medial plateau and a convex lateral side which makes the knee anatomy asymmetric [21]. The medial and lateral menisci provide differential medial and lateral constraint leading to the differential anterior-posterior kinematics of femoral condyle during flexion [21,22]. Therefore, anatomy mimetic tibial insert design represent subject’s anatomy based on 3D image data similar to femoral component. The anatomy mimetic tibial insert developed through this process has an anatomic geometry with a moderately dished medial and a convex lateral plateaus (Fig. 2).

There are three different patient-specific TKA designs that were categorized into anatomic articular surface customized PS-TKA (APS-TKA), and this conformed that customized PS-TKA (CPS-TKA) and medial pivot customized PS-TKA (MPS-TKA).

2.2 Development of the normal knee FE model

An existing validated normal knee joint model was used in this study [12–14,23,24]. A 3D non-linear finite element (FE) model of a normal knee joint was developed by using data from the medical image of a healthy 37-year-old male subject. The model includes the femur, tibial and fibular bones, cartilage layers, ligaments and meniscus (Fig. 3).

The bony structures were modeled as rigid bodies [25]. The articular cartilage and menisci were modeled as isotropic and transversely isotropic, respectively, with linear elastic material properties [26]. Additionally, the major ligaments were modeled with nonlinear and tension-only spring elements [27,28]. The force-displacement relationship based on the functional bundles in the actual ligament anatomy refers to the following: $\begin{eqnarray}\displaystyle f\left({\varepsilon}\right)&=&\left\{\begin{array}{@{}ll@{}}\frac{k{\varepsilon}^{2}}{4{\varepsilon}_{1}}, & 0\leq {\varepsilon}\leq 2{\varepsilon}_{1}\\ k\left({\varepsilon}-{\varepsilon}_{1}\right), & {\varepsilon}>2{\varepsilon}_{1}\\ 0, & {\varepsilon}<0\end{array}\right. & & \displaystyle \nonumber\\ \displaystyle {\varepsilon}&=&\frac{l-l_{0}}{l_{0}} & & \displaystyle \nonumber\\ \displaystyle l_{0}&=&\frac{l_{r}}{{\varepsilon}_{r}+1} & & \displaystyle \nonumber\end{eqnarray}$ where f (𝜀) is the current force, k is the stiffness, 𝜀 is the strain, and 𝜀₁ is assumed to be constant at 0.03. The ligament bundle slack length l ₀ can be calculated using the reference bundle length l _r and reference strain 𝜀_r in the upright reference position.

The interfaces between the articular cartilage and the bones were assumed as fully bonded. Six pairs of tibiofemoral contacts between the femoral cartilage and the meniscus, meniscus and tibial cartilage, and femoral cartilage and tibial cartilage were modeled for both the medial and lateral sides [23].

2.3 Development of the conventional PS-TKA and customized PS-TKA FE models

Genesis II was used for the conventional TKA FE model. Computer-assisted design models of a PS design from the Genesis II Total Knee System (Smith & Nephew Inc.) were virtually implanted in the bone geometry. Based on the dimensions of the femur and tibia, devices with sizes of 7 and 5–6 were selected for the femoral component and tibial insert, respectively. Conventional and customized TKA models were implanted as shown below (Fig. 3).

In the neutral position, the femoral component was aligned such that the distal bone resection was perpendicular to the mechanical axis of the femur, and the anterior and posterior resections were parallel to the clinical epicondylar axis in the transverse plane. The tibial default alignment was rotated by 0° relative to the AP axis, and the coronal alignment corresponded to 90° relative to the mechanical axis. Similarly, three different patient-specific TKA were virtually implanted in the bone geometry.

Contact conditions were applied between the femoral component, tibial insert and patellar button in TKA. The coefficient of friction between the polyethylene and metal materials was assumed as 0.04 for consistency with previous explicit FE models [29]. The materials for the femoral component, PE insert and tibial base plate were described in previous studies [14,29].

2.4 Boundary and loading conditions

The FE investigation included two types of loading conditions corresponding to the loads used in the experiments in existing studies that focused on TKA model validation and model predictions under deep-knee-bend loading conditions. The intact model was validated in previous studies [12–14,23,24], and the TKA model was validated by comparing it with the models used in a previous study [30].

A conservative ankle force of 50 N and hamstring forces of 10 N were constantly exerted with respect to a linearly increasing force, and a maximum of approximately 600 N was applied at a 90° flexion of the quadriceps actuators for the TKA model under the first loading condition [30]. The second loading conditions corresponded to deep-knee-bend loading that was applied to evaluate the effects of conformity in customized PS-TKA on normal knee mechanics generation [10,31–33]. A computational analysis was performed with the application of the anterior-posterior force to the femur with respect to the compressive load applied to the hip [31–33]. A proportional-integral-derivative controller was incorporated into the computational model to control the quadriceps in a manner similar to that performed in a previous experiment [34]. A control system was used to calculate the instantaneous quadriceps displacement required to match a target flexion profile, and this was the same as that in the experiment. Internal-external and varus-valgus torques were applied to the tibia [31–33].

The FE model was analyzed by using the ABAQUS software (version 6.11; Simulia, Providence, RI, USA). Specifically, kinematics, collateral ligament force and quadriceps force were examined to evaluate the achievement of the restoration of close normal knee mechanics in customized PS-TKA for different conformity when compared to conventional TKA. A three-cylindrical knee joint model was developed in six degrees-of-freedom for the relative kinematics of the tibiofemoral and patellofemoral articulations [35]. Embedded coordinate frames in the femur, tibia and patella were considered by using nodes, and their positions were evaluated throughout the loading conditions.

3 Results

3.1 Validation of the conventional TKA FE model

The conventional TKA FE model for the tibia was rotated by 0.57°, −0.88°, −0.71°, −0.11° and 0.83° in the internal rotation under 20°, 40°, 60°, 80° and 100° flexions, respectively (Fig. 4). The simulation exhibited good agreement with those observed in previous experiments in which a range of values under the loading conditions applied to a prosthetic implant [30].

3.2 Comparison of kinematics in conventional PS-TKA and customized PS-TKA with three different designs

Figure 5 shows femoral rollback and internal-external rotation for PS-TKA with four different designs under the deep-knee-bend condition. In all four different TKA designs, they exhibited less femoral rollback when compared that of a normal knee. The highest difference was observed in conventional PS-TKA followed by CPS-TKA, MPS-TKA and APS-TKA. There were 7.1 mm, 5.9 mm, 5.1 mm and 4.2 mm less femoral rollback in conventional PS-, CPS-, MPS- and APS-TKA compared to normal knee. During the deep-knee-bend activities, knees of TKA exhibited a substantial reduction in the internal tibial rotation when compared with that of a normal knee. The trend in the difference was similar to that of femoral rollback. There were 8.8°, 7.6°, 6.9° and 5.6° less internal rotations in conventional PS-, CPS-, MPS- and APS-TKA compared to normal knee. Specifically, APS-TKA exhibited the most normal-like femoral rollback, screw home mechanism, and internal tibial rotation during deep-knee-bend condition.

3.3 Comparison of collateral ligament force and quadriceps force in conventional PS-TKA and customized PS-TKA with three different designs

Figure 6 shows collateral ligament force and quadriceps force in four different TKA designs during the deep knee bend condition. Forces on both medial collateral ligament (MCL) and lateral collateral ligament (LCL) increased and were higher than those in the normal condition during deep-knee-bend after TKA. However, APS-TKA exhibited the least increase in MCL and LCL forces when compared to other TKA designs. MCL forces increased by 19% and 74% in APS-TKA and conventional PS TKA, respectively, while LCL forces increased by 15% and 71%, respectively, compared to normal knee. All TKA required greater and lower quadriceps force when compared to the normal knee in low and high flexions, respectively. However, APS-TKS exhibited a quadriceps force with the most normal-like pattern.

4 Discussion

The most important finding of this study was that APS-TKA with perfectly mimetic tibiofemoral articular surface exhibited the closest normal-like biomechanical effect.

Although several TKAs display well-documented successful clinical results, it is possible to achieve improvements in restoring more normal kinematics and stability behavior. Additionally, functional performance has attracted increasing research attention as an important factor in TKA. A previous clinical follow up study indicated that with respect to patient satisfaction, certain design types were preferred to others, thereby indicating that design potentially plays an important role [36]. This issue is due to the altered geometry of the articular surfaces, variation in implant alignment and differences in soft-tissue balance after surgery. Patient-specific cutting guides demonstrated a reduction in the variation in implant alignment. However, the articular surfaces of a conventional implant do not correspond to those of a patient’s native anatomy. Thus, a customized approach to TKA was implemented to improve functional outcomes and satisfaction rates [37].

Patient-specific implants customized to the patient’s geometry attempt to better match the size and shape of the patient’s knee and to restore articular surface geometry [10,15,19]. A recent in-vitro cadaveric study revealed that customized TKA generated kinematics more closely resembled normal knee kinematics when compared to conventional TKA [10]. Wang et al. used motion capture to indicate that customized bi-compartmental knee arthroplasty corresponded to a viable treatment option when compared to conventional TKA and may contribute to superior mechanical advantages [38]. Recently, Zeller et al. performed an in-vivo kinematic study and suggested that customized TKA demonstrated kinematics that are more similar to that of a normal knee [39]. Therefore, the use of customized implant technology through customized TKA designs affords benefits including more normal motion when compared with that of a conventional CR-TKA [39]. However, previous customized TKA study with comparison to conventional TKA has focused on CR-TKA. Furthermore, tibial insert design was dependent on the femoral component in most previous studies, and there is a paucity of studies on tibiofemoral articular surface conformity. Preclinical laboratory methods enable the provision of direct comparisons between different designs independent of the numerous surgical and patient variables and can also be used at the preclinical design stage [40,41].

In the present study, we applied various conformities to customized PS-TKA by using computational simulation under the deep-knee-bend condition to investigate biomechanical effects.

The intact knee model was validated and the results indicated a good agreement with previous experimental data in terms of the kinematics and contact area as demonstrated by the FE model with same subject [13,23,24,42]. Moreover, the conventional TKA model was validated by using experimental and kinematics data [30]. Therefore, the TKA model developed in the study is considered reasonable.

The advantage of computational simulation involving the use a single subject was that the effects of conformity for tibiofemoral articular surface within the identical subject were determined without the effect of variables such as weight, height, bony geometry, ligament properties and component size [43].

The results indicated that APS-TKA with mimetic tibiofemoral articular surface displayed the most normal-like kinematics. These types of APS-TKA surfaces are compatible with normal knee kinematics and soft tissue. The geometric comparisons of the APS-TKA surface to contemporary designs indicated that articular surfaces of contemporary implants are incompatible with normal knee motion. Furthermore, kinematic simulations revealed that the anatomic geometry of the APS-TKA surface directly contributes to the restoration of normal knee kinematics. The kinematic simulations confirmed that both the medial and lateral geometry of the APS-TKA articular surface are responsible for the restoration of normal screw home mechanics during deep-knee-bend conditions.

The moderately dished medial compartment displays an advantage in terms of providing laxity to accommodate pivot center variations during limited flexion as well as accommodating intrasubject variations in kinematics of the knee joint [21,44,45]. The fore-mentioned variations may not be accommodated by the strict ball-in-socket articulation of traditional “medial-pivot” implants. Therefore, MPS-TKA exhibited a slightly higher level of normal-like kinematics when compared to that of CPS-TKA. However, changing the convex geometry of the lateral compartment in the APS TKA to a flat geometry in the MPS-TKA led to a decrease in the femoral rollback. Furthermore, any customized PS-TKA did not restore normal knee kinematics. The main reason for this corresponded to the post-cam mechanism and absence of cruciate ligament.

Recently, Koh et al. showed that customized bicruciate-retaining TKA and preservation of the anterior cruciate ligament can lead to improvements in kinematics when compared with the conventional CR-TKA and bicruciate-retaining TKA [12]. The abnormal posterior femoral location in CR-TKA is mainly caused by a missing anterior cruciate ligament, which is under tension in extension and holds the femur anteriorly on the tibia [46]. Additionally, a previous study revealed that ‘third condyle’ TKA provides similar anteroposterior and mediolateral stability to the normal knee [47]. This feature led to an adequate balance between laxity and constraint to restore normal kinematics including smooth femoral rollback without causing paradoxical external tibial rotation [47]. Therefore, it is necessary to improve post-cam design to restore more normal-like kinematics in customized PS-TKA. Moreover, previous study showed that kinematics in bicruciate substituting TKA were qualitatively more similar to natural knees than CR- or PS-TKA [48]. Significant findings were found in collateral ligament force and quadriceps force.

The anatomic geometry of the APS-TKA exhibited the most normal-like ligament forces and quadriceps forces. The patella functions as a spacer increasing extension moment arm in addition to acting as a lever arm and altering the magnitude and direction of the quadriceps tendon that increases the mechanical advantage in extension. A prosthetic design in which the center of flexion is more posteriorly located causes additional lengthening of the quadriceps lever arm [49]. The result revealed that anatomic geometry of the APS-TKA with a lever arm that was not lengthened provided a normal-like quadriceps force pattern. Furthermore, it matches with that in a previous study indicating that quadriceps force required an increase in low flexion and a decrease in high flexion after TKA [50]. However, the results showed that post-cam mechanism should also be enhanced in addition to the articular surface to apply customized TKA to PS-TKA.

The study includes several limitations. First, a deep-knee-bend simulation was performed although simulations related to more demanding activities (for e.g., chair rising, sitting, stair climbing and stair descending) are required in the future for a more reliable investigation. However, the simulation was performed under a deep-knee-bend motion since it included both a wide range of flexion-extension and a significant muscular endeavor around the knee joint. Second, implant kinematics and quadriceps force were evaluated by using computational simulations that did not fully represent in-vivo condition. Finally, the results were unable to substitute clinical results and consider patient satisfaction since they corresponded to the outcomes of computational analysis. However, the main factor analyzed in the present study corresponded to the investigation of the main components to evaluate a biomechanical effect in computational biomechanics [11–14,18,20,23–26,31,34,43,45].

5 Conclusion

We investigated a customized PS-TKA with tibiofemoral articular surface conformity using a computational simulation. Our results indicated that the anatomic geometry of the APS-TKA provided a normal-like biomechanical effect. However, post-cam design should be improved in order to the apply concept of customization with respect to PS-TKA.

Footnotes

Conflict of interest

None to report.

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