Design, Shape, and Materials of Total Knee Replacement

Fig. 8.1

Tantalum can be used as a backside surface in TKR. (a) Tantalum patella-backside surface, (b) with cemented PE-patella surface but also for augmentation in very severe bone defects of the distal femur or proximal tibia

Cobalt-chromium alloys are still considered the gold standard. Titanium is biologically inert but shows corrosion and fretting. Oxidized zirconium, a ceramic surface lining, has been available as a new material for femoral TKR components. Tantalum was introduced more recently and shares a number of characteristics with titanium, including high flexibility, high resistance to corrosion, and excellent biocompatibility.

8.2 Component Design

Component design affects interface stress and determines knee kinematics directly. High congruency between the components was decreased, as it was recognized that the shearing forces in highly congruent designs may led to premature aseptic loosening of the tibial component. Design modification also showed direct impact on femorotibial and patellofemoral knee kinematics.

There are two concepts followed in femoral component design: the single radius design and the dual or multiple radius design (Fig. 8.2).

Fig. 8.2

Different concepts: multiple radius, single radius, and gradually changing radius femoral curvature design

The multiple radius design is based on the natural anatomy of the femoral condyle. The model of the four-bar linkage system introduced by Kapandji, Hughston, and Menschik explains the basic principle of the knee joint. Presuming that the ACL and PCL are rigid bars, both ligaments are fixed on a line 40° to the perpendicular line of the femur. Both bars are connected on the tibial plateau. Drawing a tangent at each flexion angle reveals the shape of the condyle, which is helical (Fig. 8.2). The helical shape of the femoral condyle causes a larger distal radius and a smaller posterior radius. The “J curve” design respects the anatomical shape of the femur. It is a well-recognized design used for many years in a variety of different femoral component designs. In extension, the broad distal femoral radius comes in contact with a conforming and relatively congruent tibial surface, providing stability to the knee. The posterior femoral radius is much smaller. It is designed to allow femoral rollback and rotation when moving from extension to deeper flexion.

Fluoroscopic evaluation of TKR kinematics has shown a sudden anterior slide of the femur related to the tibia during flexion, especially in the posterior cruciate-retaining design [17]. This may cause a limitation in maximal knee flexion. Increased anterior paradoxical femoral sliding potentially increases polyethylene wear. In addition, it decreases the quadriceps moment arm, which could lead to decreased muscle efficiency. This phenomenon has been found in 83 % of all cruciate-retaining implants but not when the posterior stabilized design was used in conjunction with a multiple radius femoral component [18]. Comparing the multiple radius with the single radius design, more compensatory adaptations were required, due to increased performance time such as trunk flexion velocity and extension velocity of the hip and knee joints [19].

In order to minimize the paradoxical anterior sliding, which may occur due to the sudden change of the radius, a new design concept has been introduced. The femoral sagittal radius is gradually reduced from extension to flexion in order to avoid the sudden jump. The radius changes from 5° to 65° flexion for the CR design and from 5° to 70° in the PS design. This design may offer a better stability throughout the entire range of motion, and midflexion instability becomes unlikely. However, these benefits have not been proven in clinical studies so far.

The single radius design is based on the premise that for knee motion only one axis is located and subsequently one single radius of rotation. The axis of rotation is fixed to the femur and more posterior in comparison to the multiple radius design. Amore posterior axis of rotation increases the quadriceps moment arm. Consequently, less quadriceps muscle force is required during knee extension [20]. However, the clinical outcome and range of motion after TKR did not find any difference between the single and multiple radius femoral designs yet [21].

Different implant designs, such as non-constrained, semi-constrained, and fully constrained TKR with single radius, multiple radius, and gradually changing radius designs of the femoral components are used.

8.3 Gender-Specific Design

In the late 1990s, the concept of gender-specific TKR emerged. The reason for this concept was the fact that there is a high variability between males and females in terms of mediolateral and anteroposterior distance of the condyles. Anatomically, the female knee is more narrow than the male knee and shorter indimension [22]. Furthermore, the Q angle seems to be larger in females than in males. Based on these findings, some companies identified the demand for a “female” or “gender” knee.

However, despite these anatomical facts, there has never been any evidence that the outcome after TKR using solely standard implants is inferior in female patients compared to male patients [23]. No differences according to WOMAC, KSS, HSS, or range of motion and rate of loosening were reported by others after 2–7 years of follow-up [5, 24–26].

Gait analysis was also performed comparing unisex and gender-specific high-flex PS total knees in 24 females with bilateral TKR [27]. Again no significant differences in gait were found.

In summary, according to the current literature, there is no evidence that the gender concept is of any clinical relevance. One of the reasons might be the fact that not only the gender but also the morphotype of the patients seems to show an impact on anteroposterior and mediolateral dimension [28].

In the late 1990s, the concept of gender-specific TKR emerged. The reason for this concept was the high variability between males and females in terms of mediolateral and anteroposterior distance of the condyles. Anatomically, the female knee is narrower than the male knee and shorter in anteroposterior dimension. However, there is little evidence that the gender concept is of any clinical relevance.

8.4 Component Design with Respect to the Cruciate Ligaments

8.4.1 Cruciate Ligament-Sacrificing Design

This concept goes back to the 1970s. When sacrificing the cruciate ligaments, adequate balancing of the collateral ligament appears to be easier. After sacrificing the cruciate ligaments, the stability has to be provided by the TKR itself. Typically a post-cam mechanism or an ultracongruent inlay is used.

Fluoroscopy studies of PS TKR have found a medial center of rotation during stair stepping in contrast to the cruciate-retaining design where the center of rotation moves into the lateral compartment [29].

A meta-analysis comparing posterior stabilized versus posterior cruciate-retaining TKR revealed a slightly better range of motion in favor of the PS design [30]. No difference was observed in terms of clinical outcome [31]. There are significant differences between the models in terms of the post-cam design, and thus it is not surprising that the moment of engagement of the post ranges from 23° (Journey, Smith & Nephew) to 89° (NexGen, Zimmer). It has also been reported that a smaller distance between the center of femoral condyle radius and the point of first contact of the post to the cam and a smaller initial post-cam distance cause lower engagement velocity. Lower velocity resulted in smoother anteroposterior kinematics and eliminates the potential source of instability.

The concept of cruciate-substituting TKR goes back to the 1970s. After sacrificing the cruciate ligaments, the stability has to be provided by the TKR itself. Typically a post-cam mechanism or an ultracongruent inlay is used.

8.4.2 Posterior Cruciate-Retaining Design

The cruciate ligaments are essential for natural knee kinematics. The anterior cruciate ligament is generally resected. The normal interaction between the four-bar link and the femorotibial contact is lost, which causes significant change of the natural rollback. By preserving the PCL, posterior translation of the tibia in regard to the femur will be prevented. However, appropriate tension of the PCL requires the restoration of the anatomical joint line, the shape of the femoral condyles, and the correct posterior slope of the tibial plateau. If the PCL is too lax, it will lose function, and if it is too tight, it could limit flexion and may elevate the tibiofemoral pressure and edge loading. A systematic literature review within the Cochrane framework compared the PCL-preserving and PCL-sacrificing design in TKR: solely the range of motion seems to show some difference favoring the posterior stabilized design [32].

The center of rotation of the posterior stabilized design remains at the medial compartment similar to the natural knee. This is different from the cruciate-retaining design where the center of rotation tends to move into the lateral compartment based on fluoroscopic motion analysis in patients [29]. The authors came to the conclusion that the center of rotation tends to migrate from medial to lateral with decrease in anteroposterior stability.

8.4.3 Bicruciate-Retaining TKR Design

Although the important role of the anterior cruciate ligament (ACL) regarding knee stability, physiologic kinematics, and proprioception is well recognized, to this point no bicruciate-retaining prosthesis has achieved general acceptance [33].

Numerous studies have shown that after artificial knee replacement, nonphysiologic knee kinematics prevail [34–39]. Implantation of bicruciate-retaining TKR is considered to be technically difficult. Also, osteoarthritic changes can lead to ACL insufficiency. However, according to Lee et al. [40], 60 % of ACLs may function normally, even when they are altered [1].

In the past, two different approaches have been implemented to retain the ACL during TKR [1]. One solution was the use of a modified PCR prosthesis, in which the recess for the PCL was extended anteriorly (Fig. 8.3a). Because of the enlarged recess, the implant bridge anteriorly across the tibial plateau is relatively narrow. Implant failure can result from the torsion loading in this region [33]. In addition, the short anchoring elements cannot prevent increased implant loosening. Fixation is not as good as that in traditional PCR prostheses because of the lack of a central stem [41].

Fig. 8.3

Two different design types: (a) Modified PCR prosthesis, in which the recess for the PCL is extended anteriorly. Because of the enlarged recess, the implant bridge anteriorly across the tibial plateau is relatively narrow. Implant failure can result from the torsion loading in this region. In addition, the short anchoring elements cannot prevent increased implant loosening. Fixation is not as good as that in traditional PCR prostheses, because of the lack of a central stem. (b) Implantation of two unicondylar knee prostheses. Even when optimal alignment of both plateaus is attained intraoperatively, implant subsidence can lead to asymmetry of the joint levels, causing malalignment of the components and unfavorable loading. (c) Such unfavorable loading can lead to increased wear and excessive erosion [33]. Reprinted by courtesy of the Medical Literary Publication Society (Medizinisch Literarische Verlagsgesellschaft mbH), Uelzen, Germany

Another solution to preserve the ACL is the implantation of two unicondylar knee prostheses. This procedure was reported in 1984 by Goodfellow and O’Connor [42]. However, even when optimal alignment of both plateaus is achieved intraoperatively, implant subsidence can lead to asymmetry of the joint levels, causing malalignment of the components and unfavorable loading (Fig. 8.3b). Such unfavorable loading can lead to increased wear and excessive erosion (Fig. 8.3c) [1].

Fig. 8.4

Novel transversal support tibial plateau (TSTP) concept [3]. Essentially, the TSTP consists of two individual joint surfaces reinforced beneath the joint line by joint surface supports and linked by a single transversal support. Reprinted by courtesy of Elsevier GmbH, Urban & Fischer, Munich, Germany

These considerations prompted us to develop the transversal support tibial plateau (TSTP) concept [33]. Essentially, the TSTP consists of two individual joint surfaces reinforced beneath the joint line by joint surface supports and linked by a single transversal support (Fig. 8.4).

This configuration should provide good bony fixation and ensure long-term alignment of the individual joint surfaces [33].

In addition, depending on the configuration and the fixation of the modular elements of the TSTP design to the joint surfaces, it is possible that additional pressure might be applied to the remaining bone stock. Whether primary stability would increase with the addition of “press fit” and whether this would have beneficial osseointegrative effects are questions to be answered in the future.

8.4.4 Fixed and Mobile Bearing Design

To avoid high peak contact pressure and minimize the resulting wear, mobile bearing inlays were developed as an alternative, reducing shearing forces while providing higher congruency. However, the use of mobile inlays decreases the stability and could lead to soft tissue impingement [33]. Thus, various guides or stops are generally used to limit mobility, which in turn also increase shearing forces [43, 44]. A meta-analysis of the literature showed a 15-year survival rate of 96.4 % in mobile bearing design [45]. There was no clinical difference between the fixed and mobile bearing design [46]. The literature has not proven yet that the mobile bearing design will decrease wear [47].

Mobile bearing inlays were developed as an alternative, reducing shearing forces while providing higher congruency. However, the use of mobile inlays decreases the stability and could lead to soft tissue impingement.

8.5 Component Design in Revision Total Knee Replacement

Higher constrained design is requested in general in revision total knee replacement. The degree of constraint depends on the functionality of the collateral ligaments.

8.5.1 Constrained Condylar TKR Design

Severe flexion contracture, intraarticular deformities, and collateral ligament imbalance or inability to balance the flexion and extension spaces during surgery are the indications for constrained condylar TKR design. The constrained condylar TKR design provides varus and valgus, anteroposterior, and rotational support due to the robust upper spine of the inlay, which forms a tight fit with the intercondylar box of the femoral component. Unlinked constrained TKR design has the advantage of allowing change of the center of rotation during knee flexion. Due to the constrained design, increased stress may occur, especially at the tibial component. Therefore, intramedullary stems are generally recommended. However, in cases showing well-preserved bone stock, a constrained design might be used even without stems. Kim et al. showed no difference in outcome and loosening with and without stems after a mean follow-up time of 4.2 years [48].

Studies have also shown good range of motion of 113° in a constrained condylar design after a mean follow-up time of 5 years [49].

The comparison of the outcome between the condylar constrained TKR design and the posterior stabilized design revealed no difference in clinical outcome and component survival rate. The femoral bone defect was the only factor that affected the choice of implant. Other studies have shown that the infection rate is higher in the condylar constrained TKR design compared to the posterior stabilized design [50].

Severe flexion contracture, intraarticular deformities, and collateral ligament imbalance or inability to balance the flexion and extension spaces are the indication for constrained condylar TKR design. It provides varus and valgus, anteroposterior, and rotational support.

8.5.2 Hinged TKR Design

Hinged TKR design was used at the very early stage of the development of joint replacement.

The first generation of hinged prostheses (Wallidus prosthesis, Guepar prosthesis, Shiers prosthesis, Stanmore prosthesis) was very highly constrained and allowed pure flexion and extension only. The highly constrained implants transferred high stress to the implant-cement-bone interface, causing early implant failure.

The second-generation hinged prostheses such as the Sheehan, Herbert, or Blauth prosthesis in the early 1970s showed decreased prosthetic constraint by allowing axial rotation and varus and valgus motion in the hinge. After early promising results, the midterm results were rather disappointing, and 15–40 % failure was reported. Despite the improved survivorship, the rate of aseptic loosening, deep infection, and other major complications was unacceptable.

The third-generation design (S-ROM, LINK, NexGen RHK) has a non-weight-bearing link design between the components. Weight is transferred via the femoral condyles to the tibial plateau, similar to the less constrained design.

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