ACL and Lateral Meniscus old avulsion fractures without marrow edema

Clinical history: 40Y M patient with previous history of knee injury, now presenting with lateral joint pain, ROM restriction, locking and sounds. 

ACL tibial attachment appears avulsed and displaced postero-superiorly. No marrow edema seen in the tibial attachment site or in the fragment. Orange arrows point to the avulsed fragment from the tibial intercondylar region, which appears more rounded and corticated.

 

Attachment site of anterior root of lateral meniscus is seen avulsed from the tibia, and is seen displaced slightly laterally from the anterior intercondylar region. No marrow edema was seen. Meniscal avulsion to produce a bony detachment is unusual. The meniscus usually detaches from the cortical surface of bone and is then called as a 'floating meniscus'. (Differential consideration for this would be a meniscal ossicle. But this is commonly seen in posterior horn of medial meniscus and a donor site would not be present in the tibia).

 

Irregular near full thickness cartilage losses of the central portion of lateral tibio-femoral compartment is noted with subchondral marrow edema of both surfaces. Yellow arrow points to the cartilage loss, green arrow to the marrow edema. These in addition to the tiny tibio-femoral marginal osteophytes, favor the development of secondary osteoarthritis. Lateral meniscal extrusion is also noted in the coronal images. Blue arrow(s) shows absence of marrow edema in the tibial intercondylar region, indicating the avulsions are old ones. A small old Segond fracture is also seen in the lateral proximal tibia. 

Prominent horizontal tear (yellow arrow) of posterior horn of medial meniscus is noted extending into the tibial surface and the posterior free margin. A multiloculated cystic area (green arrows) is noted close to the posterior horn of medial meniscus, seen communicating with the tear (orange arrow), suggesting a parameniscal cyst.

A small T2 hypointense intra-articular loose body is noted posterior to the medial femoral condyle.

 T2 oblique sagittal images showing the bony avulsions, the meniscal tear and the parameniscal tear.

Patellar Dislocation and Relocation (PDR)

20Y old male presented with history of fall and direct injury to anterior knee.

The above axial PDFS images demonstrate the torn medial patello-femoral ligament (MPFL, blue arrow) and the medial patellar retinaculum (orange arrow) more distally. Marrow edema is noted in the medial portion of the patella (green arrow). Lateral patellar tilt, mild to moderate joint effusion and shallow trochlear sulcus angle (145°) ( suggesting trochlear dysplasia) were also noted. The lateral trochlear inclination angle measured approximately 10°.

The two axial and last coronal PDFS images shows the contusion in the lateral femoral condyle, caused due to the impaction by the dislocated patella.

Here the first image is showing the PDFS coronal image in the anterior aspect of the knee, with the arrow pointing to edema in the inferomedial anterior aspect of patella with small a avulsion fragment. Axial and coronal CT bone window images shows multiple small chip/avulsion fractures of the medial patella. In the CT sections of patella appear slightly laterally subluxed. (Note: The small fragment like appearance of the femoral condyles in CT axial section is actually due to the physeal plate, and not fractures).

The above PDFS sagittal image shows injury to the Hoffa's fat pad represented by the yellow arrow heads. The image on the right (T2 sagittal) shows increased Insall-Salvati Index, measuring 1.55, suggestive of Patella Alta. 

The risk factors of patellar dislocation include shallow patellar depth, shallow trochlear sulcus, dysplasia of the femoral condyle or patella, lateral position of the tibial tuberosity, patella alta, patellar dysplasia (nail patella syndrome) , ligamentous laxity (Marfan syndrome, Ehlers Danlos, Down's syndrome and polio) and tight lateral retinaculum.

The most common finding in the patellar dislocation is hemarthrosis or lipohemarthrosis. 

The contusion in the lateral femoral condyle which may be seen up to 80-100% of patients and is considered most specific MR imaging finding of a patellar dislocation. The contusion of the lateral femoral condyle seen in the patellar dislocation/relocation is located more anteriorly, laterally and superiorly when compared to the ACL injury contusion pattern.

Patellar contusion is seen in approximately 40% of the patients and is located in the medial and inferior aspect of the patella, in relation to the attachment of the medial retinacular complex.

The injury pattern characteristic of 'patellar dislocation/relocation' is the so called 'kissing contusions' because of the patella compressing on the lateral femoral condyle during dislocation.

Reference:

Thomas Lee Pope, MR imaging of patellar dislocation and relocation, Seminars in Ultrasound, CT and MRI, Volume 22, Issue 4, 2001,Pages 371-382, ISSN 0887-2171,
https://doi.org/10.1016/S0887-2171(01)90027-7.

Special Thanks to Prasad George, Senior MRI Technologist. 😆

Ring shaped lateral meniscus

Ring shaped lateral meniscus is a very rare anatomical variant which can easily mimic and make it difficult to distinguish from a bucket handle tear. Unlike the normal C-shape, this variant forms a complete ring.

Usually the lateral portion appears as in normal cases, with angular margins or sometimes can appear deficient anteriorly. The additional medial component appear like displaced torn fragment and easily gets mistaken for a bucket handle tear (where the body of meniscus will be truncated c.f. ring meniscus).

In differentiation from central perforation of a discoid lateral meniscus, the inner margins in central perforation will be irregular with degenerative changes. Associated osteophytes and chondral lesions may also be seen unlike a case of ring shaped lateral meniscus.

ACL avulsion fracture

ACL avulsion fracture is the fracture of the tibial attachment of ACL at the anterior intercondylar region.

Zaricznyj modification of Meyers and McKeevers classification is the most used system, which divides these fractures into 4 types, with type 3 further divided into 3a and 3b.

Type 1 : Minimally / Non-displaced fragment.

Type 2 : Anterior elevation of the fragment.

Type 3 : Complete separation of the fragment.
   
         3a - Involves small portion of the eminence.
         3b - Majority of eminence is involved.

Type 4 : Comminuted avulsion or rotation of the fragment.

T1 WI showing Type 4 (Comminuted) Avulsion fracture in a 21yr male.

Type 3b ACL Avulsion fracture in an 18y female patient.