# Cartilage thickness at the apex of femoral resections in kinematically aligned total knee arthroplasty is close to 2.5 millimeters

## TL;DR

This study finds that adjusting femoral resection thickness to about 2.5 mm improves knee implant alignment, better than the previous 2 mm standard.

## Contribution

The study introduces a new method to measure cartilage thickness at femoral resection apices and suggests a refined 2.5 mm adjustment for kinematically aligned knee surgery.

## Key findings

- Apex cartilage thickness measured via MR images was closer to 2.5 mm than 2 mm for all four resections.
- Calibrated photographs showed slightly higher thickness than MR images but were consistent across resections.
- A 2.5 mm adjustment is proposed as a refinement to improve surgical outcomes.

## Abstract

Determining the correct resection thickness on worn femoral surfaces in kinematically aligned (KA) total knee arthroplasty (TKA) is uncertain since cartilage thickness is unknown. In the past, a 2 mm fixed adjustment has been used, but this may not reflect cartilage thickness at the apices of resections. Our objectives were to determine apex thickness for each resection based on a new method involving MR images, compare this thickness to direct measurements using calibrated photographs, and determine whether an increase in the 2 mm fixed adjustment is a refinement of interest to an already highly successful surgical procedure.

MR images from 100 knees without evidence of OA were accessed in the Osteoarthritis Initiative (OAI) database. After aligning images in kinematic planes, the apices of the distal and posterior femoral resections were identified, and the apex cartilage thickness was computed.

From MR images and calibrated photographs, respectively, apex cartilage thickness was 2.3 mm ± 0.5 mm vs. 2.6 mm ± 0.7 mm for the distal medial resection, 2.3 mm ± 0.4 mm vs. 2.7 mm ± 0.6 mm for the distal lateral resection, 2.4 mm ± 0.5 mm vs. 2.7 mm ± 0.6 mm for the posterior medial resection, and 2.4 mm ± 0.6 mm vs 2.5 mm ± 0.5 mm for the posterior lateral resection. Mean differences were statistically significant (p ≤ 0.0035) except for the posterior lateral cartilage thickness (p = 0.1498).

Apex cartilage thickness for MR image measurements was closer to 2.5 mm than 2 mm for all four resections. Apex cartilage thickness for calibrated photographs was greater than 2.5 mm and was comparable for all four resections. Although unlikely to affect patient‐reported outcome measures, a fixed adjustment for worn cartilage on the femur in KA TKA of 2.5 mm is a refinement of interest to bring more patients closer to the ideal alignment.

N/A.

Histograms showing cartilage thickness at the apices of four femoral resections based on analysis of MR image. Mean thickness was closer to 2.5 mm than 2 mm in all resections.