Introduction
- The anterior cruciate ligament (ACL) is one of the main stabilizers of the knee, preventing anterior translation of the tibia relative to the femur and controlling rotational stability.
- MRI is the gold standard for non-invasive assessment of ACL injuries, providing high sensitivity and specificity.
Anatomy (MRI)
- Origin: Posteromedial aspect of lateral femoral condyle (intercondylar notch).
- Insertion: Anterior intercondylar area of the tibial plateau, just anterior to the tibial spine.
- Bundles: Anteromedial (AM) and posterolateral (PL) bundles.
- Normal MRI appearance: Low-signal intensity band on all sequences, oblique in sagittal plane, best seen on sagittal PD/T2-weighted images.
Mechanism of Injury
- Most common: Non-contact pivoting injury with valgus and rotational stress.
- Associated injuries: Meniscal tears (esp. lateral meniscus), bone contusions (lateral femoral condyle & posterolateral tibial plateau), collateral ligament injuries.
MRI Protocol
- Sequences:
- Sagittal PD/T2 FS – best for primary diagnosis.
- Coronal PD/T2 FS – for collateral ligaments and menisci.
- Axial T2 FS – for associated bone and cartilage injuries.
- Slice thickness: 3 mm or less.
MRI Findings in ACL Injury
A. Direct signs
- Fiber discontinuity – Complete tear shows discontinuous, wavy, or non-visualized fibers.
- Abnormal signal – Increased signal intensity on T2-weighted and PD FS images.
- Abnormal orientation – Loss of normal oblique course; more horizontal or absent.
- Empty notch sign – Ligament not seen in intercondylar notch.
- Midsubstance vs avulsion – Tear usually at midsubstance; avulsion less common.
B. Indirect signs
- Anterior tibial translation – >7 mm from posterior femoral condyle to posterior tibial plateau.
- Uncovered posterior horn of lateral meniscus – Secondary to anterior tibial subluxation.
- Deep lateral femoral notch sign – >1.5 mm depression at lateral femoral condyle.
- Bone bruises – Lateral femoral condyle & posterolateral tibial plateau (pivot shift injury pattern).
- Segond fracture – Avulsion of lateral tibial plateau margin (associated with ACL tears).
Grading of ACL injury
- Grade 1 (Sprain): Mild fiber stretching, intact low signal.
- Grade 2 (Partial tear): Increased signal within ligament, some fibers intact.
- Grade 3 (Complete tear): Discontinuous fibers, abnormal orientation, high T2 signal.
Advantages of MRI
- Direct visualization of ligament.
- Detection of associated injuries (menisci, cartilage, collateral ligaments).
- Assessment of acute vs chronic tears.
- Avoids radiation exposure.
Differential Diagnosis
- Mucoid degeneration of ACL (bulky ligament, increased signal, but intact fibers).
- ACL ganglion cyst (well-defined cystic lesion within ligament substance).
Summary Table – MRI Features of ACL Tear
| Feature | Acute Tear | Chronic Tear |
|---|---|---|
| Fiber continuity | Discontinuous | Thinned or absent |
| Signal | High T2 | Low T2 due to fibrosis |
| Orientation | Wavy or absent | Straight but thinned |
| Bone bruise | Common | Absent |
