The purpose of this trial is to define knee joint pathomechanics (KJPM) in the first 2 years after ACL reconstruction (ACLR) and relate these KJPM with changes in articular cartilage morphology and quality.

This research project is a longitudinal prospective study. The investigators aim to define the KJPM longitudinally in the first 2 years following ACLR. The investigators relate these KJPM, combined with the cumulative mechanical exposure, to changes in articular cartilage/joint surface morphology and quality. To investigate the loading pattern on the knee joint, daily activities like walking, running and climbing stairs will be performed by the participants.

The overall goal of this research project is to investigate whether KJPM are related with early cartilage degeneration. This is a first major step towards reducing the risk for post-ACLR onset of Post-Traumatic Knee Osteoarthritis (PTOA).

Two work packages (WP) can be separated, that each have a separate aim:

WP1: The first aim of this WP (WP1.A) is to profile longitudinal changes in knee contact forces (KCFs) and knee contact pressure (KCP) distribution of the articular cartilage in the first 2 years following ACLR. Second, in WP1.B, the investigators will identify multivariate proxies of KCFs (and KCP) to improve clinical translation (for example by facilitating the development of prognostic tools for verifying KJPM in real life).

WP2: the second aim is to establish the relationship between KJPM at different time points following ACLR and (changes in) articular cartilage morphology and quality (WP2.A). Furthermore, the investigators will compare the biomechanical profile of "fast PTOA progressors" with "fast non-traumatic early osteoarthritis progressors" (WP2.B).

Inclusion Criteria (healthy volunteers):

• Age: 18-35 years
• No previous anterior cruciate ligament (ACL) injury
• Body Mass Index (BMI) between 18-30
• Regular physical activity (active during work or practicing sports regularly)
• Signed informed consent to participate

Exclusion Criteria (healthy volunteers):

• Major injuries to the lower extremities with absence from sports or daily life activities longer than 2 weeks or that required surgery, less than 6 months before participation
• Pre-existing arthritic disease including inflammatory or infectious arthritis

Inclusion Criteria (ACL patients):

• Age: 18-35 years
• ACL tear which is recently reconstructed or will be reconstructed after enrollment in the current study (the reconstruction will take place within 6 months after injury) with a Hamstring autograft
• BMI between 18-30
• Regular physical activity (active during work or practicing sports regularly)
• Signed informed consent to participate

Exclusion Criteria (ACL patients):

• Major injuries to the lower extremities with absence from sports or daily life activities longer than 2 weeks or that required surgery, less than 6 months before participation
• Pre-existing arthritic disease including inflammatory or infectious arthritis
• Image based confirmation of local cartilage lesion grade 4 or major cartilage damage (Kellgren & Lawrence > 2)
• Medial collateral ligament damage (> grade 2)
• Intra-articular knee fractures
• Menisectomy with <80% remnant

Adult patients up until 35 years old with an ACL reconstruction (recruited at the outpatient clinic) Healthy controls (community sample recruited)

The full pseudonymized dataset will be made available in RDR (KU Leuven tool) and will be shared upon written request after approval of the PI, for clearly defined research purposes.

Pseudonymized data and documentation will be available on request in RDR.

Data sharing can only happen when it is not for commercial use or it doesn't reveal the identity of the participants. Data will always be pseudonimized.

Upon publication of the research results and for an undetermined period of time.

Data sharing can only happen when it is not for commercial use or it doesn't reveal the identity of the participants. Data will always be pseudonimized. A written request has to be submitted to the PI.