Summary

Alignment technique contributes more towards laxity and balance variability than implant design.

Abstract

Introduction

Implant selection and alignment technique may affect balance in TKA. The relative contribution of each of these factors is unknown. This study investigates the relative impact of single vs multi-radius implant design and alignment technique on balance variability using intra-operative displacement and tensioning measurements through range of motion.

Methods

Ligament balance data was collected using a robotic ligament tensioner at a static tensioning force of 70-90N in 154 consecutive TKAs. A tibia-first resection technique and a single-radius cruciate-retaining implant was used for all cases. Simulation software was used to virtually replace the single-radius with a multi-radius design of equivalent size, by aligning the implants to have equivalent gaps at 10° and 90° flexion.

Two alignment techniques were assessed by aligning the implants in gap balanced (GB) and measured resection (MR) positions.

Differences in mediolateral (ML) balance and mean laxity between the two implants and two techniques were compared throughout the flexion range at 0°, 20°, 30°, 45°, and 60°.

To estimate the pre-resection state of the knee, gaps prior to making any bone resections were calculated using the ligament tension and bone morphology data acquired.

Results

Implant design had no effect on ML balance variability, while the effect due to technique ranged from 2±1.3 mm to 2.5±1.8 mm throughout the flexion range. Due to implant design having no effect on ML balance variability, the effects due to technique were significantly greater compared to implant design across all comparison angles (p<0.001).

Technique also had a greater contribution to mean laxity variability compared to the implant design at 0°(1.3±1 vs. 0.7±0.2 mm, p<0.001), 20°(1.5±1.2 vs. 0.4±0.2 mm, p<0.001), 30°(1.5±1.2 vs. 0.4±0.2 mm, p<0.001), 45°(1.5±1.1 vs. 0.2±0.2 mm, p<0.001), and 60°(1.5±1.1 vs. 0.2±0.2 mm, p<0.001).

Single-radius implant laxity was 0.7 mm looser at full extension and 0.4 mm tighter in midflexion compared to the multi-radius design.

MR had greater lateral ML imbalance compared to GB in extension (-0.7±3.1 vs 0.1±1 mm, p<0.01), midflexion (-1.4±2.7 vs -0.4±1 mm, p<0.001), and flexion (-2±2.9 vs -0.4±0.9 mm, p<0.001).

Pre-resection laxity had a standard deviation of 2.5 mm in extension and 2.5 mm in flexion.

Conclusions

Alignment technique contributes more towards laxity and balance variability than implant design. Implant geometry has no effect on ML balance variability. The laxity variability from alignment technique was similar to the variability measured from the arthritic knee population. Greater consideration should be given to alignment technique than implant geometry when achieving balance in TKA.