Current concept of kinematic alignment total knee arthroplasty and its derivatives

Abstract

The kinematic alignment (KA) approach to total knee arthroplasty (TKA) has recently increased in popularity. Accordingly, a number of derivatives have arisen and have caused confusion. Clarification is therefore needed for a better understanding of KA-TKA. Calipered (or true, pure) KA is performed by cutting the bone parallel to the articular surface, compensating for cartilage wear. In soft-tissue respecting KA, the tibial cutting surface is decided parallel to the femoral cutting surface (or trial component) with in-line traction. These approaches are categorized as unrestricted KA because there is no consideration of leg alignment or component orientation. Restricted KA is an approach where the periarthritic joint surface is replicated within a safe range, due to concerns about extreme alignments that have been considered 'alignment outliers' in the neutral mechanical alignment approach. More recently, functional alignment and inverse kinematic alignment have been advocated, where bone cuts are made following intraoperative planning, using intraoperative measurements acquired with computer assistance to fulfill good coordination of soft-tissue balance and alignment. The KA-TKA approach aims to restore the patients' own harmony of three knee elements (morphology, soft-tissue balance, and alignment) and eventually the patients' own kinematics. The respective approaches start from different points corresponding to one of the elements, yet each aim for the same goal, although the existing implants and techniques have not yet perfectly fulfilled that goal.

Keywords: Alignment; Arthroplasty; Kinematic; Knee; Mechanical; Soft-tissue; Treatment; cartilage; femur; kinematics; knees; soft-tissue; soft-tissue balancing; tibial bone; tibial components; tibial cutting; total knee arthroplasty (TKA).

Fig. 1

a) The mechanical bone cut. The cutting surface is perpendicular to the femoral and tibial mechanical axis (FMA and TMA) and parallel to the transepicondylar axis (TEA). b) Soft-tissue release. Leg alignment is not always neutral and constitutional varus is prevalent, so soft-tissue release is necessary to make the rectangular gap.

Fig. 2

The joint line alteration in mechanically aligned total knee arthroplasty. a) As the joint line inclines medially, if the implant is set at the lateral joint line, the medial joint line becomes higher than the native joint line and b) vice versa (right).

Fig. 3

The variation of deciding the tibial cutting surface in the kinematic alignment total knee arthroplasties (KA-TKAs). a) Calipered (or pure, true) KA technique. The tibia is cut parallel to the tibial articular surface, compensating for the cartilage wear, similar to the femoral side. b) Soft-tissue respecting technique. The tibia is cut parallel to the femoral cutting surface under proper traction, parallel to the distal cutting surface of the femur in extension with respect to the trial component, and parallel to the posterior cutting surface of the femur in flexion. The calipered and soft-tissue respecting approaches are categorized as unrestricted KA. c) Restricted KA technique. A similar bone cut is done within the safe range (e.g. < 5° varus); otherwise, the resection is performed at a defined angle. Intraoperative adjustment with computer assistance: d1) Functional alignment. The tibial cutting surface along with the femoral cutting surface is decided based on the intraoperative information, including alignment and gap under computer assistance. d2). Inverse kinematic alignment or tibia-based KA. In this technique, the alignment of tibial component is decided first, followed by that of femoral component. Note that the femoral cutting line can be altered in these techniques. HKA, hip-knee-ankle angle; LDFA, lateral distal femoral angle; MPTA, medial proximal tibial angle.

Fig. 4

Three fundamental knee elements; morphology, alignment, and soft-tissue. Each element determines and is determined by the other two elements, creating the best harmony on an individual basis. As a result of this harmony, the individualized kinematics are provided. The mechanical approach tends to change all, while the kinematic approach aims to reproduce all. However, the starting point is different among the respective kinematic alignment (KA) approaches. The calipered approach starts from ‘morphology’, focusing on replicating the original articular surface. The soft-tissue respecting approach starts from the soft-tissue balance, and the restricted approach starts from the control of the alignment. All other elements are expected to be eventually overcome and the patients’ original kinematics will be restored. The functional alignment, inverse KA, and other computer-assisted approaches adjust all elements based on intraoperative measurements.