For primary and revision surgery, numerous component designs have been developed, the posterior cruciate retaining, the posterior stabilised, the total stabilised and the rotating hinged knee (Fig. 1.4).

In the cruciate-retaining design (CR), the PCL is preserved. With an insufficient or absent ACL, the tibia is put at risk of abnormal anterior translation. In order to counter that tendency, the tibial PE bearing surface retains a concavity, which stabilises the femoral condyles against excessive anteroposterior translation during flexion–extension. An unfortunate side effect is that the physiological rollback of the femur over the tibial plateau during knee flexion is inhibited by the upward slope of the posterior edge of the bearing surface. This deviation from normal kinematics leads to impingement of the posterior aspect of the femur against the rim of the tibial tray, which limits the range of knee flexion and can lead to paradoxical anterior translation of the femur on the tibial bearing surface. Fluoroscopy studies have revealed that most posterior stabilised configurations feature a paradoxical slide forward motion when knee flexion starts, instead of the intended mimicking of the natural knee rolling back on the tibia with increasing flexion [9–13]. Taking this into consideration Bellemans et al. [14] suggest that the posterior offset (distance which the femoral component protrudes posteriorly from the femur) should be increased, to delay this impingement and allow more knee flexion, offering an increase in posterior offset.

In the posterior stabilised (PS) design, both ligaments are resected. The function of the PCL is mimicked using a vertical post in the centre of the tibial component and a crossbar (CAM) in between the posterior condyles of the femoral component (Fig. 1.5).

The post acts as a passive restraint to tibial anteroposterior motion, because the femoral component in this type of TKR has a central ‘box’, which must be inserted deeply into the centre of the distal femur to accommodate the tibial post. At the posterior outlet of the intercondylar notch, this type of femoral component has a cam acting against the posterior face of the tibial post. This mechanism forces the femur to roll/glide posteriorly during knee flexion, but it does not accommodate the remaining lack of ACL stabilising restraint. In addition, the fit of the post inside the femoral box reduces freedom of IE rotation. Clinical follow-up studies have shown that the PS design allows increased flexion in comparison to the CR design.

The total stabilised design is primarily used in knees presenting insufficient collateral ligament support, such as when either collateral ligaments are damaged, or in severe varus or valgus deformity with significant elongation of either one of the collateral ligaments (see Chap.​ 54). The knee stability is recouped by the central post of the PE inlay. Due to limited knee mobility in the sagittal plane, the bone–component interface is subject to increased stresses, which may cause early aseptic loosening.

To avoid such stress increases, the rotating hinged knee was developed in order to allow combined free rotation in both flexion and extension (see Chap.​ 55).

The tibial component is affected by large cyclic loading during gait. This repetitive strain may lead to micromotion at the bone–implant interface. The peak forces during walking are 3–4 times body weight (BW) [15]. These forces can increase up to more than 4 times body weight when descending downstairs or 8 times BW when tripping [16].

During flexion–extension of the knee, resulting joint forces in AP direction can lead to a rocking motion of the components. The resulting micromotion at the interface may subsequently cause loosening.

Increased fixation stability may be achieved using a larger-sized fixation stem. However, reverting to a larger-sized component should only occur when a regular size would be insufficiently supported by the prevalent posterior tibial bone stock. A long fixation stem may cause ‘stress shielding’, which triggers resorption of bone from the area below the proximal tray of the tibial component due to the load passing along the stem into the tip [17].

Offset stems are very common based on the centre of the diaphysis not coinciding with the centre of the metaphysis. The centre of the diaphysis lies approximately 4.1 mm more anteromedially at an average angle of 105° [18]. The usage of cement is a common point of discussion for revision TKR. Where the cement undoubtedly offers to fill gaps where bone stock is lost it also typically does not encounter any bone ingrowth. Moreover, using an increased volume to fill any gaps at the bone–implant interface, the additional temperature increase caused by the polymerisation process can harm the surrounding cells significantly.

There are two types of inlay, the fixed and mobile bearing. The fixed-bearing inlay can be flat or dished design (Fig. 1.6.). The flat design does not restrict femoral motion and thus decreases the stress between the tibial component and underlying bone. However, high-load transmission occurs due to the small contact area between the inlay and femoral component. The dished design increases the contact area but reduces AP translation, which in turn increases the bone-implant interface stress of the tibial component. As mentioned earlier, a lack of femoral rollback may limit greater knee flexion. The use of mobile bearings has been proposed to overcome this limitation. The PE bearing has a range of freedom in moving in the AP direction and/or allowing for rotation of the tibial plateau, compensating for a limited femoral rollback. The intact collateral ligaments and muscles around the knee joint thereby have greater freedom to allow a more natural kinematic knee flexion. In addition, it has also been postulated to be more forgiving in terms of femoral rotational malpositioning. A comparison of the in vivo kinematic performance of fixed- and mobile-bearing TKRs has found that the mobile-bearing implants were closer to the kinematic behaviour of the intact knee [19], which could explain clinical differences in patient outcomes [20]. However, when implanting a TKR with a freely rotating PE inlay, Walker et al. [6] demonstrated experimentally that static friction would inhibit the inlay from rotating: friction was only overcome at a level of torque higher than that at which the body would normally prevent any further rotation through soft tissues and muscle action.