Alignment in TKA: what has been clear is not anymore!

The optimal implant orientation when performing a total knee arthroplasty (TKA) remains a timely, pertinent, and unanswered question. For many years, the alignment debate has filled journals and congresses, and monopolised discussions among knee surgeons all around the globe. In the development of TKA surgery, Michael Freeman introduced the concept of right-angled femoral and tibial bone cuts (mechanical alignment) and the idea of parallel and equal flexion and extension spaces. Using the mechanical alignment target, the knee surgeon strived to create a neutral lower limb alignment represented by a hip-knee-ankle angle target of 180° ± 3° [4, 12]. Although the mean hip-kneeankle angle (HKA) of patients scheduled for TKA is near neutral, there is a wide variation and only 0.1% have neutral femoral and tibial mechanical axes [1]. With a systematic approach, mechanical alignment introduces anatomic modifications for many individuals and results in unequal medial–lateral or flexion–extension bone resections. Multiple ligament release techniques and algorithms have been proposed to re-balance the unbalanced gaps created. In the past years we have seen increased questioning of the concept of mechanical alignment, which has been considered the gold standard for decades. Historically, the alignment philosophy for TKA was driven by the desire to maximize durability and relieve pain with less regard for restoring normal knee kinematics and function. However, several landmark studies have shown the concept of neutrally aligning every TKA is dogma and not true anymore [23]. Questioning such a dogma leads to a certain amount of uncertainty among knee surgeons and opens the door to non-scientific subjective definitions and personal opinions of what alignment targets are preferred and which ones to safely recommend. Hence, it is important to unambiguously describe and define the current implant orientation and alignment options [18]. Only when knee surgeons use the same definitions to discuss alignment progress can be made and misinterpretation be limited. Anatomical alignment was introduced in the 1980s by Hungerford and Krackow with the goal to improve functionality by closer mimicking the native knee alignment [18]. With a systematic approach, anatomical alignment still aims for a neutral HKA, but the bones are cut 3° oblique to their mechanical axes to reflect the population’s mean native joint line orientation (3° femoral valgus and 3° tibial varus) [18]. Kinematic alignment, first proposed in 2006 by Howell et al., is an ‘individualised’ or patient-specific technique, aiming to restore the pre-arthritic or native limb and joint line alignment of each patient [3, 8, 9, 11]. By resurfacing the knee joint, kinematic alignment technique aims to coalign the axes and joint lines of the components with the three ‘kinematic’ axes and joint lines of the pre-arthritic or native knee. Femoral and tibial bone resection thicknesses checked with caliper measurements should match the thickness of the components after compensating for wear and the kerf of the saw cut. Intrinsically, it preserves/restores native ligament laxities, does not create gap imbalance and thus minimises the need for ligament release [14]. In his protocol, Howell does not place restrictions on the patient’s anatomy and post-operative correction. Kinematic alignment requires a precise surgical technique which can be performed by different techniques: manual instruments, computer navigation, personalised instruments, and computer guidance, * Michael T. Hirschmann michael.hirschmann@ksbl.ch; michael.hirschmann@unibas.ch