Direct signs of ACL rupture:
- lack of continuity of the ligament,
- absence of the ligament in its anatomical position in the lateral intercondylar space,
- a wavy contour of the ligament,
- and displacement of tibial or femoral portions of the ligament from a course parallel to the line of Blumensaat.
In femoral-sided ruptures, the tibial portion of the ACL takes a more horizontal course.
Indirect signs of an ACL tear:
- the increased curvature of the PCL
- the posterior displacement of the lateral meniscus relative to the posterior border of the lateral tibia.
Both signs result from the anterior displacement of the tibia in relation to the femur resulting from anterior instability of the knee due to the ACL rupture.
Anterior subluxation of the tibia relative to a vertical tangent to the dorsal boundary of the lateral femoral condyle of more than 7 mm is strongly predictive of a complete tear of the ACL (Chan et a. 1994).
In acute ACL injury, subchondral contusional bone marrow edema-like signal alterations (BMEP) often are very conspicuous in the intermediate portion of the lateral femoral condyle and in the posterolateral tibia.
Partial tears of the ACL may progress to complete tears and subsequent instability. This can be avoided by early surgery.
In partial ACL tears, focal or diffuse signal intensity increase on T2-weighted images are depicted along with intact portions of the ligament, which can be followed from the femoral through to the tibial insertion site. Usually the contour of the ligament shows some bowing or a wavy configuration. Diagnostic accuracy is lower for partial ruptures than for complete ruptures (Umans et al. 1995).
ACL repair can depict the position of the transplant, rerupture, and impingement of the transplant, focal anterior fibrosis (“Cyclops lesion”) as well as patellar fractures and alterations in the patellar tendon (Recht et al. 1996) in case of bone patellar tendon bone reconstruction.
Knowledge of the operational technique is important for the radiologist because in hamstring grafts the excised tendon is doubled, sutured, and redoubled again, so that the ACL is reconstructed by a bundle of up to 4 separate tendon strands which normally exhibit linear strands of fluid in between the separate tendon strands.
The signal intensity of the transplant varies with time. In the first 3 to 4 months after the operation, it exhibits low signal intensity in all sequences. Within 4 to 8 months, and in some cases even 10 to 12 months, months SI is increased, presumably due to resynovialization and revascularization of the graft. After 1–2 years, the SI of the graft should become low in all sequences again. Usually, a complete rerupture of the graft can be confidently identified by depicting the loose ends of both remnants.
Arthrofibrosis may lead to persistent pain after ACL repair. Its focal form in the anterior intercondylar compartment gives rise to the term Cyclops lesion, which exhibits low SI. In the first months after ACL repair, pronounced inflammatory reactions may be present in the patellar tendon or the hamstring insertion site. With the course of time (up to 3 years) defects in the patellar tendon may fill with repair tissue in cases of bone patellar tendon bone grafts, whereas a so-called neo-tendon may form in the hamstring grafts growing from proximal to distal within the former tendon track.