and Claudio Chillemi2
(1)
Sapienza University of Rome, Latina, Italy
(2)
Latina, Italy
Ahlbäck Classification System
Classification for grading knee osteoarthritis. It must be performed on plain weight-bearing AP radiographs.
It is possible to divide the alterations in five grades according to severity: (1) joint rim space <3 mm, (2) rim disappearance, (3) subchondral sclerosis and remodeling (0–5 mm), (4) subchondral sclerosis and remodeling (5–10 mm), and (5) severe subchondral sclerosis and remodeling (>10 mm) with joint axis alterations.
AIIS Avulsion Injury
The AIIS represents the proximal attachment of the extensor mechanism. It is important to know that the rectus femoris tendon is the most involved tendon on quadriceps injuries, both by flexion of the hip and extension of the knee, and has two branches of insertion, first on AIIS and a second on the hip capsule called indirect or reflected tendon. On MR images, attention must be reserved also on the hip when AIIS and rectus femoris edema is seen.
Anterior Cruciate Ligament, Anatomy
The anterior cruciate ligament (ACL) is an oblique extra-synovial ligament that courses in the intercondylar groove, from the anterior area of the proximal tibial plate to the medial facet of the lateral femoral condyle. It has two fiber bundles, the anteromedial and the posteromedial, and, occasionally, a third bundle between these two bundles.
The ACL measures more or less 40 mm in length and 13 mm in width.
Anterior Cruciate Ligament, Acute Tear MR Imaging
MRI
ACL tear must be evaluated both with primary signs and secondary or associated signs on MR images.
Primary signs: ligament rupture involves a change of the signal intensity of the fibers and their morphological and anatomical course.
Although the oblique sagittal plane is considered as the most helpful in diagnosis, we prefer to use the axial plane as the main plane, avoiding magic angle artifact, with the support of coronal and sagittal imaging. In the acute phase, the ligament appears thickened and fibers show high signal intensity in T2- or intermediate-weighted sequences. As the blood vessels of ACL are located between longitudinal fiber bundles, high signal on T1-weighted images has to be interpreted as a tear of almost one-half of the ligament, resulting in blood infarction (ill-defined area of focal edema and hemorrhage) (Fig. 1). The aspects must be distinguished from mucoid degeneration of the intact ACL (see relative lemma). The ACL ligament should be considered disrupted when its course has the long axis more horizontal than Blumensaat line: a line projected along the intercondylar roof (see lemma) on sagittal images.
Figure 1
ACL complete disruption: axial T2-weighted image (a) shows fibers interruption with fluid on the ligament (arrows in a); on (b) sagittal PD FS image, the ligament appears deflected with enlarged shape (arrows in b)
Secondary signs: several signs associated with articular lesions probably correlated with the injure mechanism. These have low sensitivity, but the presence of secondary signs should lead us to seek ACL abnormalities or associated crucial meniscal or capsular lesions. The main classified signs are (see lemmas) (1) bone bruise, pivot shift, (2) femorotibial translation, (3) Segond fracture, (4) arched-appearing PCL, and (5) deep lateral femoral notch sign.
Anterior Cruciate Ligament, Ganglion
Anterior cruciate ligament cysts may be differentiated in intraligamentous and extraligamentous ganglions.
The most common is the extraligamentous location and appears like a synovial recess but well defined, often septated near the ACL. Fibers or intrasubstance ganglions of ACL are less common; their identification becomes crucial as ACL ganglions can be difficult to appreciate on standard arthroscopy; therefore, on the basis of MRI diagnosis, the arthroscopist may decide to probe the ACL or add a posterior portal. On MRI, cyst appears similar to a partial tear, but unlike the latter, it is oriented parallel to the long axis of the ligament with a concomitant normal-appearing ACL (Figs. 2 and 3).
Figure 2
ACL ganglia: axial T2-weighted image (a) shows peripheral fibers interruption (arrows) with fluid on the ligament (arrowhead); on (b) sagittal PD FS, cystic fluid collection with intermediate signal may be seen on the ligament (arrows)
Figure 3
Intra-articular ganglion cysts: axial T2-weighted image shows capsulated cystic lesion within the infrapatellar Hoffa’s fat pad. Note the displacement of the anterior intermeniscal ligament (lateral aspect of the cyst) and the absent communication with ACL or menisci (arrows)
Anterior Cruciate Ligament, Imaging Technical Features
Radiography: Radiographs have a limited significance in the evaluation of bone indirect signs of ACL injury. On radiographs or on computed tomography (CT), avulsion bone fractures at the tibial or femoral insertion have to be reported. Segond fracture is a type of avulsion fracture of the iliotibial band on the lateral tibial plate (see lemma Segond fracture) and is commonly related with an ACL tear. As result of osteochondral impaction, a fracture may be appreciated on the lateral femoral condyle (lateral femoral notch sign) as sulcus deeper than 1.5 mm. Opacity on suprapatellar recess may be a sign of hemarthrosis that is very common in ACL injuries.
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