The goal of this observational study is to learn whether the Robotic Knee Testing system can measure knee looseness in adults with an anterior cruciate ligament, or ACL, tear.

The main question it aims to answer is whether measurements from the Robotic Knee Testing system are related to the doctor's pivot shift exam grade.

Participants will complete one study visit before ACL reconstruction. During the visit, both knees will be tested in the Robotic Knee Testing system. Researchers may also review existing medical information and imaging.

This study includes two parts. In Phase 1, researchers will adjust the Robotic Knee Testing system settings to identify testing conditions that are safe, tolerable, and able to reproduce knee motion similar to the pivot shift exam.

During Phase 1, the device will test how knee motion changes when the leg is placed in different positions. Researchers will evaluate whether tibial internal rotation and knee flexion angle change varus-valgus knee motion. They will also evaluate whether varus-valgus knee position and knee flexion angle change internal-external tibial rotation. These tests are intended to help identify device settings that may reproduce the coupled motion seen during a clinical pivot shift exam.

The robotic torque-motion curves will also be reviewed for abrupt changes, discontinuities, or "jumps." These findings may represent the knee moving from a stable position into an unstable position.

In Phase 2, participants will be tested using the finalized device settings from Phase 1. The device will apply controlled anterior-posterior, internal-external rotation, and varus-valgus movements while measuring knee motion in six degrees of freedom. Researchers will compare the robotic measurements with the pivot shift grade documented by the treating orthopedic surgeon.

When available, existing MRI or X-ray images may also be reviewed to evaluate whether knee bone structure is related to robotic laxity measurements or pivot shift severity. No additional imaging will be performed for this study.

Inclusion Criteria:

• Adults aged 18-65 years
• Confirmed ACL tear by MRI or arthroscopic diagnosis
• Able to provide informed consent

Exclusion Criteria:

• Prior major surgery on the affected knee
• Multi-ligament knee injury
• Active infection or inflammatory arthropathy
• Inability to tolerate passive motion testing
• Pregnant or planning to become pregnant during the study period
• Unable to provide informed consent

Study participants will be selected from patients being treated for ACL injury by orthopedic providers in the Atlanta metropolitan area. Participants will generally be identified through orthopedic clinic schedules, clinical referrals, or patients scheduled for ACL reconstruction.