Evolution of alignment concepts in total knee arthroplasty

Functional knee phenotypes

In recent years, surgeons have come to acknowledge that in the general nonarthritic population there is a vast variability in knee alignment, anatomy, laxity, and kinematics [1–3]. Functional knee phenotypes have been devised by analyzing coronal knee alignment parameters. In a study of 308 knees from 160 young nonarthritic participants, data showed an overall mean femoral mechanical angle (FMA) of 93.4° ± 2.0°and values ranged from 87.9° (distal femoral joint line slightly varus oriented) to 100° (distal femoral joint line significantly valgus oriented), and an overall mean tibial mechanical angle (TMA) of 87.2° ± 2.4° with a range of 81.3° (varus proximal tibial joint line) to 94.6° (valgus proximal tibial joint line) [4], demonstrating this variability in alignment in the general nonarthritic population. Gender differences could also be seen with women having more valgus alignment than men, but the most common femoral phenotype was neutral in both genders. Hirschmann and colleagues [5, 6] expanded on these findings and generated the novel classification for lower limb frontal alignment (Figure 1). If variability occurs in the general population, evidently, the arthritic knee population also shows highly variable coronal tibial and femoral alignment [7]. So, what can we do with this knowledge? Accepting this variability has created debate on how to deal with alignment in TKA for patients with arthritic knees, and clearly continuing to follow the one-size-fits-all approach will lead to further patient dissatisfaction secondary to persistent (or residual) knee symptoms [8]. Shifting from the concept of anatomical or mechanical alignment to kinematic or functional alignment for TKA is becoming the gold standard. But what do all these terms actually mean?

The timeline of alignment in total knee arthroplasty

The philosophy of alignment in TKA has come a long way since TKA was first described in the 1970s (Figure 2). Insall [9] introduced mechanical alignment (MA) in the 1980s to establish a reproducible approach in which implants were aligned the same in all patients to create a neutral limb axis. Although MA did overall generate acceptable clinical outcomes and implant survival, a significant proportion of patients still reported dissatisfaction with ongoing symptoms and functional issues [10, 11]. With frequent disappointing patient outcomes and alterations of native knee anatomy when using an MA TKA, Hungerford and Krackow [12] proposed in 1985 the concept of anatomical alignment (AA), which focused on a more physiological approach and aimed to systematically produce an oblique joint line while maintaining the lower limb neutrally aligned. This is achieved by making the lower limb straight, with a joint line at 3° relative to the axes of the two bones (3° varus for the tibial component and 3° valgus for the femoral component) so that the joint line is more parallel to the ground when standing. However, the joint line obliquity varies within the population, thus it is not natural to use AA techniques as the prosthetic joint line obliquity is set at 3° for everyone. Constitutional alignment, also known as 'adjusted MA', followed a similar principle and systematically produced a varus limb deformity of 2–3° for patients with medial osteoarthritis but again, issues arise when applying the same parameters to all patients. In the last decade, the term kinematic alignment (KA) has advanced and brings a more physiological approach to TKA alignment. It aims to recreate the native (or prearthritic) joint anatomy (three-dimensional) by aligning the femoral and tibial components to the normal or prearthritic knee, thus altering as little as possible the joint’s physiology. In 2016, the concept of ‘restricted KA’ was developed to ensure that extreme constitutional limb/knee anatomies would not be reproduced but rather attenuated: the tibial and distal femoral cuts do not deviate more than 5° from the bone’s mechanical axis and the resulting hip-knee-ankle (HKA) angle must not deviate more than 3° from a 180° alignment [13]. From this concept some surgeons developed a restricted inverse KA approach that also focuses on reducing the laxity on the lateral components. Functional alignment (FA) was recently first described by the group of Fares Haddad [14], which continues on the 'mechanical approach’ for arthroplasty by focusing on aligning the lower limb and reducing the occurrence of soft-tissue imbalance without aiming to reproduce the native knee anatomy. More recently, Lustig et al [15] have used the term ‘functional alignment’ to describe a surgical technique aiming to restore the tibiofemoral and patellofemoral joint anatomies (a compromise is made between the two), to reduce the physiological lateral knee laxity, and to keep the prosthetic limb neutrally aligned (max ± 3° frontal deformity); this technique is closed to the above-mentioned restricted inverse KA technique. Functional alignment has gained momentum by using navigational or robotic technology to adjust component position in all planes to better balance the knee while maintaining a negligible limb deformity (inferior to 3°). Rivière states that "for young surgeons today, it is important to recognize what options they have; they should decide to either follow a traditional approach, for example MA, or a more physiological approach that takes into account the native knee anatomy and laxity through a KA approach".

Understanding the types of alignment and when to use them

It is important for surgeons' training in this field to understand the alignment techniques that are available, how, and when to use them, especially in terms of availability of technologies, and how to plan the surgery accordingly. Here, we describe the different principles of alignment for TKA and offer some insights into studies that document the efficacy of the strategies.

Mechanical alignment. The aim is to create a neutral lower limb alignment which evens the loading forces between medial and lateral compartments. It is defined as both tibial and femoral components perpendicular to the limb's mechanical axis, achieving a neutral overall limb alignment [17]. Mechanical alignment considers the coronal, sagittal, and femoral targets. The coronal targets reference the bone resections to the femoral and tibial mechanical axes, the sagittal targets vary on implant design and constraint used, and the femoral rotation targets take into account the flexion gap (gap balancing technique) or femoral anatomy (measured resection) [17]. However, few patients have a neutral mechanical axis, so following a MA TKA will systematically alter knee anatomy, laxity, and kinematics, explaining frequent residual symptoms. Furthermore, poor soft tissue and ligament balancing can result in an unbalanced knee, which will increase the medial and lateral forces on the tibial components [18] and cause functional issues or early implant wear. Rivière states that "the knee joint is highly forgiving and may explain why not everyone has poor outcomes following MA TKA".

Functional alignment has evolved from MA with the advancement in surgical aids such as computer-aided surgery (CAS) and robotics. It can offer a personalized approach to TKA and high precision for the surgeon by allowing for intraoperative assessments in relation to resection thickness, joint gaps, and limb alignment (Sebastien Lustig goes on to explain more about the use of robotics in Part 2 of this series). Functional alignment allows for the gaps to be balanced by changing the implant orientation in all three planes [17]. Currently, the important thing is that these targets are individualized to the patient’s knee and if the patient does not have any extreme deformity, then the surgeon can apply the 'safe zone' values to develop the overall limb alignment within the 0° ± 3° of coronal alignment (Table 1).