Controversy surrounds the use of weight-bearing hip-to-ankle radiographs in planning TKA. The use of these radiographs in cases with extra-articular deformities is generally accepted. Although their use in uncomplicated cases has been disputed [10–12], several reports have advocated their use routinely [13–16]. The authors obtain a weight-bearing long hip-to-ankle radiograph in all cases for TKA routinely not only to rule out extra-articular pathologies or deformities but also since most of the planning and radiographic assessment such as degree of deformity, plane of bone resection, amount of joint laxity and position of the femoral and tibial components in the coronal plane are done with respect to the mechanical axes of the femur and tibia. The following angles and radiographic features can be determined with this radiograph:

Frontal or Coronal Alignment – Relying on standard knee anteroposterior radiographs may grossly underestimate the knee deformity when compared to long hip-to-ankle radiographs especially in cases with severe coronal bowing of the femur (Fig. 1.2) which is common in patients with knee osteoarthritis [16–18]. The long hip-to-ankle radiograph helps in accurate estimation of preoperative knee deformity measured as the hip-knee-ankle (HKA) angle. This is defined as the angle between the mechanical axis of the femur (centre of the femoral head to the centre of the knee) and mechanical axis of the tibia (centre of the knee to the centre of the tibial plafond).

Distal Femoral Resection – The distal femoral valgus correction angle (VCA) decides the valgus angle at which the distal femur needs to be cut in the coronal plane in order to align the femoral component perpendicular to the mechanical axis of the femur. This is calculated as the angle between the mechanical axis of the femur and the distal anatomic axis of the femur (Fig. 1.3). Although the VCA is commonly believed to be between 5° and 7°, a recent study by the authors involving 503 arthritic limbs has shown wide variation in its value among patients with knee arthritis [18]. It may vary between 2.6° and 11.4° with 56 % of the limbs having a VCA outside the 5–7° range [18]. The percentage of limbs with VCA >7° was significantly more in varus knees and with VCA <5° more in valgus knees. Preoperative deformity showed significant correlation with VCA. The VCA can also be affected by excessive coronal bowing of the femoral shaft or variation in the femoral neck shaft angle. Excessive coxa vara and lateral femoral bowing both increase the VCA, and excessive coxa valga and medial femoral bowing have the reverse effect [18]. During conventional TKA, extremes of VCA will render use of an intramedullary femoral guide rod difficult due to distortion of the femoral canal and may lead to error in placement of the distal femoral cutting block. Hence, in these cases, a shorter guide rod needs to be used, and owing to such great variability among limbs, the VCA should be selected in each limb based on measurements done on preoperative long films. Although extremes of VCA or extra-articular deformity are irrelevant when placing the femoral component in computer-assisted TKA (as the software uses only the centre of the femoral head and centre of distal femur to plan the distal cut), these findings on the long weight-bearing radiograph may indicate the need for extensive soft-tissue releases for restoration of limb alignment and optimum gap balancing during TKA irrespective of the technique used (Fig. 1.4). When knee varus is associated with minimal osteophytes and a high VCA due to excessive femoral bowing, extensive soft-tissue release may have to be combined with a sliding medial condylar osteotomy during TKA [19].

Preoperative Joint Divergence Angle (JDA) – Varus knee deformities are associated with tight medial and lax lateral and soft-tissue structures. A fair indication of these soft-tissue changes can be obtained on a weight-bearing full-length radiograph. The proposed distal femoral and proximal tibial cuts are drawn perpendicular to the mechanical axes of the femur and the tibia. The angle formed by the proposed tibial and femoral resections (preoperative JDA) will give a fair estimation of the site (medial or lateral) and degree of soft-tissue contracture and the extent of soft-tissue release that may be required. Similarly, it will also give a fair estimation of the degree of laxity in the opposite compartment and how much bone resection may be required. Lesser the angle or more parallel the lines, lesser will be the amount of release required for correction (Fig. 1.5a, b). Similarly greater the angle or more divergent the lines, greater will be the amount of release required and more conservative the bone cuts (Fig. 1.5c).

Extra-Articular Deformities or Implant In Situ – A full-length radiograph will also reveal any extra-articular deformity (EAD) in the limb, hip pathology, prior trauma/surgery, associated stress fractures or an implant in situ. Excessive coronal bowing of the femur is a common cause of extra-articular deformity in arthritic knees undergoing TKA [16, 17]. Mullaji et al. [16] have reported that up to 20 % of varus arthritic knees have significant femoral bowing in the coronal plane in Asian patients. Stress fractures although uncommon are an important cause of extra-articular deformity [20]. Depending on the type seen on preoperative radiographs, TKA may have to be combined with long tibial stem extenders, metal augments or corrective osteotomies [20]. Similarly, in the tibia, extra-articular deformity due to malunited or ununited fractures, stress fractures, post-high tibial osteotomy or excessive tibial bowing may be present. Although most of these cases of knee arthritis with extra-articular deformities can be managed by extensive intra-articular soft-tissue releases, a corrective osteotomy may be rarely required [17]. This is discussed in detail in Chap.​ 8 on EADs.

The presence of implants will warrant the need to use computer navigation which will bypass the hardware or extramedullary cutting guides as use of intramedullary jigs may not be possible (Fig. 1.6a). However, in some cases where the implant may be close to the knee joint, they may have to be removed before implantation of the prosthesis especially the tibial component (Fig. 1.6b). We have shown that navigation is useful in cases of altered hip centre [21].

The knee AP view is probably the most commonly used radiograph for diagnosis of knee arthritis and planning for TKA. It is important that the knee AP view is obtained while the patient is weight-bearing as a supine view may underestimate the degree of arthritis, deformity and instability. This view allows a closer look at the knee in the coronal plane and is an important part of the preoperative planning process for TKA. As previously mentioned, although this view may underestimate the degree of knee deformity and will not show extra-articular bony deformity, it is invaluable in assessing certain features around the knee joint which gives an idea of what to expect during TKA.

Tibial and Femoral Resection – Tibial and femoral resections are performed perpendicular to their respective mechanical axes. Traditionally, the amount of tibial resection is primarily based on the actual thickness of the thinnest tibial component. If the composite thickness is 8 mm, then generally 8 mm of the less affected tibial plateau should be resected. However, both tibial and femoral resections need to be conservative in cases with severe bone loss, severe lateral or medial laxity in varus or valgus, recurvatum deformity and in knees with gross instability. More than usual distal femur may have to be occasionally resected in knees with fixed flexion deformity to achieve correction. By marking out these resections, the surgeon can get a good idea of the amount of bone being resected, and if the relative thickness of the medial and lateral tibial plateau and distal femur condyles matches the preoperative markings.