2025 ISAKOS Congress in Munich, Germany

2025 ISAKOS Biennial Congress Paper


Restriction Boundaries for Robotic Assisted Kinematic Total Knee Arthroplasty, the Stressed HKA and the Neutral–Normal–Aberrant–Deviant Morphotype Classification

Peter James McEwen, MBBS FRACS(Orth) FAOrthA DIpModLang , Pimlico, QLD AUSTRALIA
Orthopaedic Research Institute of Queensland, Townsville, QLD, AUSTRALIA

FDA Status Cleared

Summary

When using a restricted Kinematic technique, the Stressed HKA as an alignment target, results in fewer alignment outliers without collateral ligament release than the Arithmetic HKA.

Abstract

Introduction

The explosion of techniques and technologies around personalised alignment in total knee arthroplasty has spawned multiple morphotype classification systems and restriction boundaries matrices, all of which are based on coronal limb, femoral and tibial angles Functional adjustment based on the stressed HKA as a starting point, is a robotic–-assisted technique of making small adjustments in implant position from a Kinematic starting point to produce optimal and, if desired, uniform ligamentous balance. The recently published Neutral–Normal–Aberrant–Deviant (NNAD) morphotype classification defines femoral and tibial component restriction boundaries based on 2 standard deviations from the mean. The aim of this study was to examine preoperative and post-operative outlier percentages using a functional robotic technique from a restricted Kinematic baseline against the NNAD matrix.

Method

Preoperative and intraoperative data from 351 consecutive robotic assisted total knee arthroplasties performed by a single surgeon was collected prospectively and examined retrospectively. Preoperative data included coronal femoral and tibial angles from long leg standing radiographs. Intraoperative data included coronal femoral and tibial implant angles, releases, and final laxity measurements. In all cases, the patella was resurfaced, the posterior cruciate was retained and a medially congruent bearing was used. Restriction boundaries were 6 varus to 3 valgus for the HKA, 6 varus to 3 valgus for the tibia and 3 varus to 6 valgus for the femur. Target HKA was defined by the stressed HKA if within the limb restriction boundaries. Functional adjustments were made within these restriction boundaries to produce uniform ligament balance. The natural laxity of the lateral flexion gap was preserved. Central tendencies were analysed using the appropriate non-parametric test. The Chi-square test was used for categorical analysis

Results

Based on the preoperative arithmetic HKA and the defined restriction boundaries, 6% of cases were varus HKA outliers, 11% of cases were valgus HKA outliers, 17% were varus TMA outliers and 7% were valgus FMA outliers. Based on the intraoperative stressed HKA fewer than 1% were varus HKA outliers (p<001) and no valgus outliers. In 99.5% of cases prosthetic HKA was within restriction boundaries. In all cases tibial component alignment was in the recommended range on the NNAD matrix. In 11% of cases femoral component alignment was outside the varus limit of the recommended range. Medial extension, lateral extension and medial flexion laxities were similar across alignment groups. Lateral flexion laxity was significantly greater than all other laxity parameters.

Conclusions

The described technique allows precise and consistent management of limb and component restriction boundaries and consistent ligamentous balance. When using the stressed HKA to set target alignment significantly fewer alignment outliers were identified than the bony alignment alone would suggest. Widening the varus limit for the femoral component on the matrix by 1 degree would leave 99.5% of cases within the recommended range.