It is important to consider the gait cycle as it pertains to femoral alignment in total knee arthroplasty. Two discrete stages are encountered. The stance phase is defined by weight bearing, and the swing phase is defined by advancement of the limb. During stance phase, a period of double-limb support is transitioned to single-limb support as the gait cycle progresses. Further division of the single-limb segment encompasses the first heel strike and progresses from a flat foot through heel-off and finally to toe-off and the swing phase. Following the initial heel rise of the incident limb, the contralateral limb enters heel strike. Stance phase accounts for the majority of the gait cycle, on average 62%, whereas the swing phase is attributed 38% [6]. During stance phase, the medial compartment of the knee experiences a significantly disproportionate weight-bearing load of approximately 60–70% of the sum total. Therefore, any disruption in axial or rotational alignment may significantly alter force distribution and portend degenerative changes of the articular surfaces [7–10]. Therefore, reestablishing normal force distribution during total knee arthroplasty is essential to a long-standing and successful intervention in terms of clinical results and component survivorship , especially in patients with higher body mass index and subsequent increased forces through the knee [11]. Studies have shown that deviation in axial alignment by as little as 5⁰ of tibial varus may alter the load-bearing mechanics of the knee by up to 40% and lead to early failure [12]. In particular, internal rotation of the tibia or femur may result in patellofemoral maltracking. Instances of internal rotation from 1 to 4⁰ result in lateral tracking and increased patellar tilt, whereas 3–8⁰ of internal rotation has been observed to result in patellar subluxation. In cases of exaggerated internal rotation from 7 to 17⁰, dislocation and ultimate failure of the patellar component have been routinely documented [13]. Additionally, the altered force distribution caused by malalignment may place undue stress on the bone-cement interface and lead to bone loss and failure secondary to loosening.

During primary and revision surgery, it is important to factor in the patients’ level of activity and the stresses that will be placed on the implant. In 2005, Colwell et al. began investigating in vivo knee forces utilizing an instrumented tibial component equipped with force transducers, affording a direct measure of intra-articular load distributions following total knee arthroplasty [14]. The initial assessment was limited to quantifying total axial load and localizing the center of pressure between the tibial and femoral components. In 2008, the same group utilized a more refined, second-generation implant capable of measuring all components of tibial force to analyze forces during activities of daily living and exercise. Measurements were obtained during walking, jogging, rowing, stair-climbing trainer use, elliptical trainer use, leg press/extension, stationary biking, tennis, and golfing. Stationary biking generated the least force. Interestingly, exercising on a so-called low impact elliptical trainer generated lower forces than jogging, but not lower forces than treadmill walking. Swinging a golf club generated relatively high forces, especially in the leading knee [15].

Restoration of a neutral coronal mechanical axis is essential to successful primary and revision total knee arthroplasty. Any deviation may ultimately result in failure and devastating consequence for the patient. Therefore, it is essential that the reconstruction surgeon places emphasis on accurate bony resections. A distal femoral resection in 5–7⁰ of valgus is most commonly employed, effectively restoring the anatomic tibiofemoral angle to 6⁰ (± 1–2⁰) of valgus [16, 17]. Bearing in mind the average 3⁰ varus alignment of the native tibia, a cut perpendicular to the long axis is generally performed. However, adjustments may be made depending upon such variables as preoperative alignment, integrity of the collateral ligaments, and patient phenotype. Proper bony resections of the distal femur and proximal tibia as well as appropriate soft tissue releases are needed to establish a rectangular extension gap.

The “kinematic approach” to alignment in primary total knee arthroplasty, as described by Stephen Howell, M.D., focuses on aligning the transverse axis of the femoral component with the primary axis of the femur in regards to tibial flexion and extension, removal of osteophytes to restore unimpeded native soft tissue tension, and placing a tibial component with longitudinal axis perpendicular to the transverse femoral axis of tibial flexion and extension [18]. This approach considers three axes between the femur, tibia, and patella. The primary transverse axis of the femur, in relation to tibial flexion-extension, is defined as a line that passes through the center of two best-fit circles drawn in the medial and lateral femoral condyles and equidistant from the articular surface of the femur from 10 to 160⁰ of flexion. The patellar axis exists in the transverse plane parallel to the primary axis. The third axis is the perpendicular relationship between the longitudinal axis of the tibia and that of the femur and patella. Selection and placement of the femoral component are based upon aligning the transverse axis of the component with the primary transverse axis of the femur, thereby shape matching the component to the femurs pre-arthritic geometry. Although positioning of the femoral component is rather straightforward, tibial component coronal positioning requires several more steps. Additionally, the utilization of a kinematically aligned total knee arthroplasty requires a preoperative MRI . Although functional results of primary operations utilizing a kinematic construct are encouraging at short-term follow-up, the approach for establishing correct femoral rotation is currently not utilized in the revision setting [19].

While many experienced, high-volume surgeons often use a “freehand” technique for performing bony cuts for its speed and practicality, we do not recommend this technique for less-experienced surgeons, especially in the revision setting where extensive bone loss may be encountered and the usual anatomic landmarks are deformed or absent. Instead, both intramedullary and extramedullary cutting guides are available to assist in measured resections. Multiple investigative series have revealed the accuracy of intramedullary guides in establishing a distal femoral resection in the desired 5–7⁰ of valgus. One series by Teter et al. reviewed radiographic analysis of 201 knee arthroplasties conducted with a standard intramedullary guide and revealed distal femoral cuts were accurate 92% of the time [20]. Errors in resection were encountered in scenarios of a capacious tibial canals and bowed femora . Additionally, intramedullary guides have been shown to be superior to extramedullary guides in establishing accurate alignment and joint line orientation. The same group, reviewing 352 total knees, found 94% of cuts performed with an intramedullary device were within ±4⁰ from the ideal 90⁰ cut (perpendicular to the mechanical axis) compared to 92% performed with a standard extramedullary guide [21].

Appropriate employment of an intramedullary alignment guide relies on correctly identifying a femoral canal entry point just anterior to the insertion of the posterior cruciate ligament and medial to the center of the intercondylar notch. This may be challenging in the revision arthroplasty setting, especially in situations where the native anatomy is altered such as with post-traumatic arthritis following femur fracture, extensive bone loss with osteolysis, or bone and soft tissue compromise associated with infection. Excessive varus or valgus cuts may result if the starting point errs medially or laterally. Preoperatively, obtaining weight-bearing full-length anteroposterior and lateral plain radiographs of the limb allows for assessment of both the axial alignment of the limb and morphology of the intramedullary canal of the femur. One concern that arises from the use of an intramedullary guide is fat emboli syndrome [22]. Strategies such as overdrilling the starting point and utilizing fluted guide rods have been developed to reduce intramedullary pressures during insertion and the incidence of fat embolism [22]. New data suggest that the use of computer navigation, a technique where the intramedullary canal is not violated, reduces the embolic load compared to the use of traditional, intramedullary-based, mechanical cutting guides [23, 24].

Accurate restoration of femoral axial alignment is key to establishing not only a balanced flexion gap but also an acceptable Q-angle and patellofemoral kinematics. A large subset of complications following primary and revision total knee arthroplasty arise from malalignment of the patellofemoral joint [25, 26]. Problems such as poor patellar tracking, patellar subluxation, anterior knee pain, patellar clunk, and accelerated wear of the polyethylene patellar component may arise in a construct with improper axial alignment [27–30]. Slight external rotation may assist in producing a favorable relationship between the patellar and femoral components in terms of functional outcomes, but patella-associated problems leading to revision remain of serious concern [30–34].

Gap balancing and measured resection are the two primary strategies for producing proper femoral axial alignment. There may be wide variation in anatomy, bone stock, and soft tissue integrity encountered in each revision scenario, and thus certain strategies may offer more or less utility depending on the specific case. Careful physical examination, preoperative planning and templating, and intraoperative assessment are essential in choosing an appropriate strategy.

The gap balancing technique was originally described by Insall [5, 35, 32]. When surgeons use this method, an accurate femoral cut is dependent upon an initial tibial cut that is perpendicular to its long axis. Prior to distal femoral resection, osteophytes that may adversely influence alignment should be removed, and the soft tissues should be balanced in extension. Following the tibial cut, the limb is placed under extension, and tensioning instruments are inserted. Utilizing the cut tibial surface as a guide, resection of the distal femur is performed in parallel, establishing a rectangular extension space . The knee is then flexed to 90⁰, and the tensioning instruments (Figs. 11.3 and 11.4) are again inserted for cutting the posterior femur, again in parallel to the tibial cut, thus establishing a rectangular flexion space (Fig. 11.5). This technique may be challenging when performing revision arthroplasty, as distorted morphology secondary to bone and soft tissue compromise is encountered.

The measured resection technique may be employed, whereby defects created by degenerative changes in the distal femur and proximal tibia are supplemented by prosthetic components. With femoral resections, the chosen prosthesis must mimic the amount of condylar bone removed (Fig. 11.6). Measured resection relies on the use of bony landmarks to guide appropriate cuts. Femoral axial alignment is generally determined with reference to the posterior condylar line, the anteroposterior axis, and/or the transepicondylar axis. Tibial resection is similar in both the measured resection and gap balancing techniques, as the goal is to achieve a cut surface that is perpendicular to the mechanical axis of the tibia.

In our previous chapter, we highlighted the unique features of both the clinical and surgical epicondylar axes. For review, the clinical epicondylar axis, as defined by Yoshioka et al. in 1987, is a virtual line connecting the lateral epicondylar prominence and the most prominent part of the medial epicondyle [36]. The group employed this orientation to additionally describe the condylar twist, or the angle subtended by the posterior condylar line and the clinical epicondylar axis (Fig. 11.7). The prominent part of the medial and lateral epicondyles may usually be palpated below the skin and subcutaneous tissues except perhaps in the morbidly obese patient. The medial prominence lies on the crescent ridge and serves as the femoral attachment site for the superficial fibers of the medial collateral ligament. The lateral collateral ligament originates on the lateral prominence posterior and superior to the insertion of the popliteus. The posterior condylar line is another anatomical reference created by a line tangent to the most posterior aspects of the medial and lateral femoral condyles. Yoshioka et al. utilized the virtual angle created by the intersection of the posterior condylar line and the clinical epicondylar axis as the condylar twist angle. The posterior condylar line is frequently used by total knee arthroplasty systems to determine axial alignment for distal femoral cutting guides. Typically, the cutting guides reference to posterior condylar line to establish a prefixed cut in 3–5⁰ of external rotation (Fig. 11.8). The use of such posterior referencing systems may be limited by not only the deformities and/or defects in the articular surface encountered during primary surgery but by the potential absence of anatomic reference points during revision surgery [37].