Neutral mechanical alignment prioritises the relationship between the prosthesis and the mechanical axis of the limb.1 Implanting both the femoral and tibial components at 90◦ to the mechanical axis may improve the survivorship of the construct by reducing the sheer and bending forces acting at the interface with the implant. 1 However, in this approach the anatomy of the native knee and its relationship with the soft tissues are not considered, and the large variations in knee anatomy create an increased reliance on soft tissue releases to achieve a well-balanced knee. 1 According to the authors, this “clash between prosthetic and native joint kinematics has been blamed for persistent pain, stiffness, instability and dissatisfaction following TKR.”2
By contrast, Kinematic Alignment as described by Stephen Howell1 aims to recreate the anatomy of the knee, and in so doing respects the soft tissue envelope but ignores the overall limb alignment.1 The authors suggest that “over-reliance on the diseased joint surfaces to guide bone resections can result in prosthetic and limb alignment that fall outside the purported safe range, and this may prejudice longevity.”1
The proposed solution – a hybrid technique that is both mechanically sound and soft tissue friendly.
The technique described has been developed using surgery-assisting technology like robotics and navigation. The additional precision of which allows non-mechanical limb alignment targets to be achieved more reproducibly, reducing the risk of significant alignment outliers.1
Through this technique the gaps can be balanced by adapting the resections in all three planes.1 As described by the authors, “a smaller medial extension gap can be balanced by placing the tibial component in up to 3◦ of varus… (while) a tighter lateral flexion gap can be balanced by internally rotating the femoral component.” 1 The natural obliquity of the joint line can be safely restored by applying a valgus correction to the distal femoral resection and varus correction to the tibial resection.1 Although the ‘targets’ are personalized to the individual patient’s knee; the resections and overall limb alignment are maintained within the currently accepted safe zones.1 While the article focuses primarily on coronal alignment, the authors state that “the goal of functional alignment is to position the components in the position that least compromises the soft tissue envelop of the knee, and hence to restore the plane and obliquity of the joint to that which the soft tissues dictate.”1 They conclude that this technique, which provides “a balanced knee that remains within the safe limits of limb alignment may be a promising compromise, allowing patients to achieve improved satisfaction without prejudicing longevity.” 1