Accurate measurement of the MFT angle requires long-leg radiographs and is traditionally thought to vary from 5° to 7° of valgus. Long-leg views are not routinely available, and short-leg views may be used [11, 12], but it must be remembered that the MFT may vary up to 1.4° (SD −3° to 5°) when short-leg views are used instead of long-leg radiographs [13, 14].

The femoral component can be aligned intraoperatively using intra- or extramedullary alignment rods that follow the anatomical axis of the femur. As the aim is to restore the mechanical alignment, the femoral component is aligned at 6° (FMA) to the femoral axis in the coronal plane.

On the tibial side, the mechanical and anatomical axes coincide, and hence the aim is to resect the tibia perpendicular to the neutral axis between the knee and ankle joint. The anatomical axis of the tibia can be determined by intramedullary or extramedullary alignment jigs.

Conventionally balancing the soft tissues in the coronal plane involves releasing the concave side of the deformity and balancing it to the convex side. This may be achieved by a combination of bone resection and soft tissue release. It should be noted here that a well-balanced knee could be difficult to achieve in the presence of severe varus or valgus deformity [15–34].

The sagittal mechanical axis is the line drawn from the center of the femoral head to the center of the talocrural joint. Optimal prosthetic alignment for a TKA in the sagittal plane is not well known and more difficult to achieve [35–37].

With conventional techniques, sagittal prosthetic alignment is based on limited anatomic features that are palpable during surgery and determined intraoperatively with intramedullary or extramedullary rods. The targeted sagittal prosthetic alignment toward femoral and tibial axes also differs according to the prosthetic design, because some prostheses are designed to be implanted with a posterior slope toward these axes.

In the sagittal plane, a posterior entry point of the intramedullary alignment rod may cause the distal femoral cut to be relatively flexed, while for an anterior entry point, the end result may be a distal femoral cut that is relatively extended. This has implications for component size, a flexed position undersizing, while an extended position may oversize.

In the flexed knee, moving the center of rotation of the femoral component by either displacing the prosthesis anterior or posterior or by changing its size will affect the flexion but not the extension gap. Movement of the femoral component proximally or distally will either increase or decrease the extension gap without changing the flexion gap.

The slope of the tibial component affects the balance between flexion and extension gaps. Increasing the posterior slope increases the flexion gap. Decreasing or reversing the slope produces tight flexion.

The proximal-distal positioning of the femorotibial articulation in relation to the patellofemoral articulation has three determining parameters: the rotary (axial) position of the femorotibial complex (discussed in the next section), the relative balance of the actions of muscles and the tightness of ligaments acting across the patella, and the position of the joint line. This defines the relative positions of the joint line and the patella.

With navigation systems, the surgeon has the opportunity to achieve sagittal prosthetic alignment based on the mechanical axis of the entire femur and tibia. The surgical technique of sagittal balancing a knee according to the kinematic principles is detailed below.

Few studies have evaluated the effects of sagittal alignment on function and survival. However, sagittal instability does occur due to malalignment [38, 39]. Kim et al. [36] found that flexion of the femoral component of >3° and sagittal alignment of the tibial component of <0° or a tibial slope of >7° were risk factors for failure. Several studies have described intentional sagittal flexion of the femoral component by several degrees during TKA as a useful downsizing technique for the femoral component without excessively increasing the flexion gap [40].

The best method of obtaining rotational alignment of the femoral component in flexion remains controversial. Some investigators favor a measured resection technique in which bony landmarks (femoral epicondyles, posterior femoral condyles, or the anterior-posterior axis) are the primary determinants of femoral component rotation [41]. The transepicondylar axis of the femur is used to assess rotation of the femoral component because it is the functional axis of flexion. The intercondylar groove of the femur is at right angles to the transepicondylar axis of the femur. This is the anterior-posterior axis. There is good, but not perfect, agreement between the two. The posterior condylar axis should be used with care as it has high variability, especially in valgus knees.

Alternatively, the gap-balancing technique may be employed, in which the femoral component is positioned parallel to the resected proximal tibia with each collateral ligament equally tensioned [42].

Several studies have looked at methods of identifying the anatomical landmarks to aid placement of the femoral component such as matching femoral rotation to the TEA to replicate the mean flexion axis of the knee [43], using the centers of the spherical posterior condyles of the knee [44], and dividing the femoral condyles into three distinct regions, each with its own radius and center [45]. From 10 to 150 tibiofemoral flexion, the centers of rotation were found to lie on a line connecting the area of attachment of the medial collateral ligament and the lateral collateral ligament [45]. However, the latter model included out-of-plane movement of both centers, which could not be explained through a single, fixed axis of rotation [43]. Hollister et al. [4] used a mechanical device, the “axis finder,” to locate a flexion-extension axis and a longitudinal rotation axis, with all positions of knee flexion occurring through movement about these two axes in a compound hinge model. The flexion-extension axis passed through the origins of the medial and lateral collateral ligaments, while the longitudinal rotation axis passed through the insertion of the ACL on the tibia and the insertion of the PCL on the femur [4]. The helical axis model [46, 47] has a moving axis of rotation about which all tibiofemoral motion takes place.

For the tibia, the reliable landmarks for rotational alignment remain unresolved. It is however known that the posterior condyles, the tibial tubercle, and the tibial spines may not be the best guide for rotational alignment of the tibial component [48].

There is a direct correlation between internal rotation of the components (femur and tibia) and patellar maltracking [49]. Mild combined internal rotation of between 1° and 4° causes lateral patellar tracking and patellar tilting, whereas moderate internal rotation of between 5° and 8° causes subluxation, and marked internal rotation of between 7° and 17° causes patellar dislocation or failure.

An internally rotated femoral component in isolation has clear implications for patellar tracking. In addition, there will be relative laxity in flexion on the lateral side, but with tightness on the medial side. This can be difficult to balance by soft tissue release. In contrast, excessive external rotation may make the lateral side tight in flexion and the medial side relatively lax, which can cause a medial lift-off as the knee is flexed. In both situations, because of ligament tightness, there will be restriction in the range of flexion. The force through the extensor mechanism in both situations will be altered, causing abnormal loading of any patellar component and contributing to early wear and loosening.

Rotation of the tibia in isolation is also critical [50–52]. If the tibial component is internally rotated relative to the cut surface of the tibial plateau, the tibia will be externally rotated relative to the femur. The tibial tubercle will then be lateralized, and the Q angle increased, predisposing to lateral patellar subluxation [49].

There is little reliable evidence of the effect of rotational alignment on survival because the techniques for measuring it intra- and postoperatively are often inaccurate, and the optimal rotational alignment has not been defined [36, 53]. However, rotational alignment is probably critical to the outcome of TKA [54–56]. External rotation of the tibial component of <2° or >5° has been shown to increase the rate of failure significantly [36].

Kinematically aligned TKA sets the femoral component at the natural angle and level of the distal (0°) and posterior (90°) joint line. This technique applies a distal and posterior femoral referencing guide at 0° and 90° of flexion, which are adjusted to compensate for wear and kerf and perform resections equal in thickness to the condyles of the femoral component (Figs. 13.3 and 13.4).

The surgical technique begins by assessing the locations of cartilage wear on the distal femur and removing worn cartilage to bone with a curette. The flexion-extension position of the femoral component is set by drilling a positioning rod 10 cm into the diaphysis of the femur, perpendicular to the distal joint, and parallel to the anterior femoral cortex (Fig. 13.5). The varus-valgus and proximal-distal positions of the femoral component are set by placing an assembly of the offset distal femoral referencing guide and distal femoral resection guide over the positioning rod in contact with the distal femur, which compensates for 2 mm of cartilage wear on the worn condyle(s) and sets the thicknesses of the distal medial and lateral femoral resections equal to the condylar thickness of the femoral component after compensating for cartilage wear and kerf [3, 15]. The anterior-posterior and internal-external rotation positions of the femoral component are set by placing a 0° rotation posterior referencing guide in contact with the posterior femoral condyles (Fig. 13.6).