Neutral boundary alignment in total knee arthroplasty: a novel concept

Abstract

The goal of total knee arthroplasty (TKA) surgery is to provide a stable and functional knee joint using current implant designs. Several alignment philosophies and surgical techniques have been introduced to provide a stable and functional knee joint, such as mechanical alignment (MA), kinematic alignment (KA), and anatomical alignment (AA). Recently, functional alignment (FA) is proposed. In this concept article, we propose a TKA approach, which we termed "Neutral Boundary Alignment (NBA)." The proposed approach seeks to establish the overall limb alignment in the direction of gravity at the midstance of gait cycle; consequently, a potential native knee can be restored from an arthritic state by establishing the joint line parallel to the ground. Herein, the NBA approach is described, and an iterative algorithm of structural layout patterns of truss is developed. The following three hypotheses are proposed: 1) The joint line should be parallel to the ground during the midstance of gait as an important initial condition for stability when transitioning toward gait propulsion in the gait cycle; 2) The NBA stability criteria purports that the leg is stable when the direction of gravity is simultaneously situated within the hip, knee and ankle during the midstance of gait, which generally agrees with the Varus/Valgus 3 degree envelope of MA; 3) Femoral and tibial resections that are made parallel to the ground remain within 1.5 degrees of traditional mechanical alignment resections.

Keywords: Anatomical alignment; Functional alignment; Kinematic alignment; Knee alignment; Mechanical alignment; Neutral boundary alignment; Total knee Arthroplasty.

Fig. 1

a Hip, knee, ankle coronal radiograph taken in midstance of gait for the right leg, which was obtained with the patient taking one-step forward and pausing for X-ray (static capture). The contact boundaries are defined in this view. b Based on six points representing the contact boundaries of femur and the tibia, the iterative algorithm of structural layout patterns of truss is applied. c A truss model is generated for the femur and tibia. d The NB Axis is defined by a line L connecting the intersecting points P1 and P2 and the joint line J perpendicular to NB Axis

Fig. 2

Restored varus/valgus angles of 21 arthritis patients using MRI scans of the hip, knee and ankle obtained in a single global coordinate system and in the supine position. MA is determined by finding the femoral MA and tibial MA separately. Boundary conditions for the NBA approach are defined for the hip, knee and ankle and the NB axis is defined as described in the article. Angle Deviations of Femoral and Tibial MA to NBA Resections are reported. The NBA combined femoral and tibial cuts are oriented parallel to the ground and maintain global mechanical alignment stability