Normal Posterior Cruciate Ligament Thickness on MRI
The posterior cruciate ligament (PCL) is the primary restraint to posterior tibial translation and one of the strongest ligaments in the knee. On MRI, accurate measurement of PCL thickness is important for distinguishing normal anatomy from tears, mucoid degeneration, and other pathological processes. Establishing a reliable normal threshold guides appropriate clinical management and avoids unnecessary intervention.
Normal Reference Values
| Measurement |
|---|
| <6 mm |
Clinical Significance
A PCL thickness of less than 6 mm is considered within normal limits on MRI. Focal or diffuse thickening beyond this threshold raises concern for pathology, including partial or complete tears, mucoid degeneration, or ganglion cyst formation within the ligament. PCL injuries are often associated with high-energy trauma, posterior knee pain, and posterior drawer test laxity on physical examination.
Mucoid degeneration, also known as myxoid degeneration, is a particularly important mimic of PCL tear; it produces diffuse ligament enlargement with intermediate-to-high T2 signal but preserved internal fiber continuity. Distinguishing this entity from a true tear has direct implications for surgical planning. Thickening in the absence of trauma should also prompt consideration of inflammatory or neoplastic etiologies.
- PCL partial or complete tear (acute or chronic)
- Mucoid (myxoid) degeneration of the PCL
- Intraligamentous ganglion cyst
- Ligament hypertrophy secondary to chronic ACL deficiency
- Rare: primary ligament neoplasm or synovial infiltration
Reference: Helms CA, Major NM, Anderson MW et al. Musculoskeletal MRI. Saunders. p. 370. (2009)
Imaging Notes
PCL thickness is best assessed on sagittal proton-density or T2-weighted MRI sequences with the knee in slight flexion. The measurement is taken at the midsubstance of the ligament, where it appears as a uniformly low-signal-intensity, gently curved band connecting the posterior tibial plateau to the medial femoral condyle. A dedicated field of view of 14–16 cm and slice thickness of 3–4 mm optimizes resolution for ligament evaluation.
Care should be taken not to mistake oblique partial-volume averaging of adjacent structures for true ligament thickening. Axial sequences can serve as a complementary plane to confirm anteroposterior dimensions when sagittal assessment is equivocal. Correlation with clinical history and physical examination remains essential when borderline measurements are encountered.