# Exploratory analysis of the 2-year changes in knee cartilage thickness and transverse relaxation time (T2) in ACL-injured versus healthy participants

## TL;DR

This study compares changes in knee cartilage thickness and T2 relaxation time over two years in ACL-injured knees, their uninjured counterparts, and healthy controls.

## Contribution

The study introduces location-independent measures to detect cartilage thinning in ACL-injured knees over two years.

## Key findings

- Cartilage thickness decreased in at least 50% of regions/subregions over two years.
- ACL-injured knees showed the most significant thinning in the lateral tibia and its subregions.
- T2 changes were limited and not specific to ACL injury.

## Abstract

To investigate the 2-year changes in cartilage thickness and transverse relaxation time (T2) in magnetic resonance images (MRI) of ACL-injured (ACL_in) and uninjured contralateral knees (ACL_unin), compared to healthy controls (HEA).

Baseline and 2-year follow-up MRIs were acquired in both knees of 78 participants (ACL-injured 2–10 years prior, 20–30 years, n = 20; 40–60 years, n = 14; healthy, 20–30 years, n = 23; 40–60 years, n = 21). Weight bearing femorotibial cartilages were manually segmented to determine cartilage thickness and laminar (deep and superficial) T2. 2-year changes were compared between ACL_in, ACL_unin and HEA knees using estimated marginal means (EMM) and group differences in femorotibial compartments, (sub-)regions, and location-independent ordered values (OV).

From baseline to 2-year follow-up, cartilage thickness decreased in at least 50 % of regions/subregions whereas significant T2 changes ([95 % CI] ∉ 0) were rare. In ACL_in, cartilage EMM thinning (adjusted: MRI acquisition time, participant age) was strongest in the lateral tibia (LT) and its subregions (∼50 μm). In the interior LT subregion, thinning was 46 [70, 22], 46 [70, 22] and 13 [34, 8]μm in ACL_in, ACL_unin and HEA, respectively. In OV1 and OV2 of ACL_in, thinning was 178 [207, 150] and 130 [148, 111] μm, respectively and greater than in HEA. No EMM group differences were found for 2-year change in T2.

ACL-injured knees displayed greater cartilage thinning, particularly using location-independent measures. Change in T2, in contrast, was limited and not specific to injury. Our findings underscore the need for relevant thresholds of cartilage thickness and T2 changes, to define early OA.