# 3D geometric modelling and clinical validation of lower limb and preliminary 2‐year post‐operative clinical outcomes of neutral boundary alignment in total knee arthroplasty

## TL;DR

This paper introduces a new 3D modeling approach for knee replacement surgery that aligns the joint with gravity during walking, showing promising clinical results.

## Contribution

A novel physics-based 3D alignment method for TKA called neutral boundary alignment (NBA) is introduced and clinically validated.

## Key findings

- NBA achieved an average joint line alignment deviation of 0.1° from the ideal orientation.
- Surgeries using NBA did not require formal ligament balancing.
- Patients had a high mean FJS-12 score of 86 ± 11 at 2 years post-surgery.

## Abstract

To introduce a novel three‐dimensional (3D) modelling concept for lower limb alignment in total knee arthroplasty (TKA), termed neutral boundary alignment (NBA) and to present preliminary clinical data supporting this physics‐based custom alignment strategy.

NBA is defined as the alignment at which the knee joint line is parallel to the ground during the mid‐stance phase of gait, achieving static equilibrium with the NBA axis aligned to gravity. A theoretical geometric model of the lower limb was developed, integrating principles of physics to describe this condition. Preliminary clinical validation was performed using radiographic data from an initial cohort of 12 patients, followed by an additional 45 patients who underwent TKA planned and executed using the NBA concept. Postoperative alignment relative to the ideal parallel joint line, intraoperative need for ligament balancing and clinical outcomes were evaluated. Clinical function was assessed using the forgotten joint score (FJS‐12) at 2 years postoperatively.

Across the combined cohort, postoperative joint line alignment demonstrated an average deviation of 0.1° from the ideal parallel orientation, with a standard deviation of 0.9°. Surgeries performed using NBA consistently achieved stable knees without the need for formal ligament balancing. The mean 2‐year postoperative FJS‐12 was 86 ± 11, indicating a high level of joint ‘forgettability’ in daily activities.

Despite the relatively small sample size, this study supports NBA as a physics‐based, custom alignment method in TKA that closely reproduces a joint line parallel to the ground in mid‐stance, optimises knee stability and preserves soft tissue integrity. These preliminary findings justify further investigation of NBA in larger, prospective cohorts.

Level IV.