Arterial adventitial cystic disease is an uncommon type of non-atherosclerotic peripheral vessel disease of unknown etiology. Several theories about their pathogenesis have been proposed such as repetitive trauma, embryological origin, direct communication with the herniated synovial structures of the adjacent joint and connective tissue disease. It affects most commonly the popliteal artery (85 % of cases), typically in young to middle-aged men without significant vascular risk factors, with a male-to-female ratio of 15:1. However, involvement of other arteries like axillary, brachial, radial, ulnar, external iliac, and common femoral has also been reported. Rarely, it can also affect the veins.

Clinically this condition results in intermittent claudication due to peripheral vascular insufficiency caused by compression of the arterial lumen by a cystic collection of mucinous material in the adventitial layer of the artery.

Magnetic resonance imaging (MRI) with magnetic resonance angiography (MRA) is very helpful in the diagnosis, as it provides excellent anatomical detail with cysts appearing as hyperintense structure on T2WI images, and luminal compromise can also be assessed.

It is an important treatable cause of vascular disease with multiple therapeutic options, which include restore arterial flow, resection of the affected artery and interposition grafting, percutaneous aspiration of cystic contents under ultrasound, or CT guidance and resectional adventitial cystotomy.

Post-contrast coronal MIP (maximal intensity projection) image from upper extremity contrast-enhanced MR angiography (Fig. 5.5) demonstrates focal narrowing of the radial artery (long arrow). The remaining vasculature is normal.

Axial T2-weighted turbo spin-echo images without (Fig. 5.6) and with fat suppression (Fig. 5.7) demonstrate a well-defined, homogeneously hyperintense cystic lesion (short arrow) corresponding to mucinous material in the adventitia layer surrounding and compressing the lumen of the radial artery.

Acute lateral patellar dislocation (LPD) occurs often in children during sporting activities and is commonly transient with spontaneous reduction. It accounts for approximately 9–16 % of acute knee trauma in young athletes presenting with hemarthrosis. The mechanism of injury usually involves a twisting motion: fixed foot and internal rotation on a flexed knee with valgus stress. Spontaneous relocations occur in the majority of patients. Between 50 and 75 % of diagnosis go unsuspected after clinical exam and initial plain radiology evaluation. Differentiation from other knee injuries is often difficult as extensive tenderness, hemarthrosis, and pain are often present.

Radiographs may show hemarthrosis, and a minority of patients will have a patellar chip fracture. Concomitant injuries, including medial patellofemoral ligament injuries, are common, occurring in up to 90 % of patients.

Magnetic resonance (MR) imaging findings may include typical contusion patterns in the patellar inferomedial pole and the anterolateral aspect of the femur’s nonarticular portion (kissing sign) with injury to the medial patellar soft-tissue restraints. Up to one third of patients will also show concomitant injury to the major knee ligaments or menisci. These are frequently associated with osteochondral fractures, which may be an indication for surgery.

Trauma alone rarely causes patellar dislocations in patients without predisposing anatomical factors for patellar instability, such as trochlear dysplasia, dominant lateral patellar facet, high riding patella and tibial tuberosity lateralization. Trochlear dysplasia is found in up to 85 % of patients with LPD.

Nonsurgical management is generally recommended for first-time dislocations, with brace immobilization in extension for 3–6 weeks. Following initial conservative management, recurrent instability occurs in 15–50 % cases. Chronic instability and recurrent patellofemoral dislocations may cause cartilage damage with bone soft-tissue restraint abnormalities and eventual arthritic changes. Surgery may be indicated when associated injuries or abnormal patellofemoral relationships are identified.

Axial (Fig. 5.8) and sagittal (Fig. 5.9) proton density-weighted images with fat suppression reveal focal bone marrow edema involving the patellar medial aspect (arrow) and the lateral femoral condyle’s lateral aspect. A medial patellar retinaculum high signal is visible without tearing. Note the trochlear dysplasia and the minimal joint effusion. Coronal T1- (Fig. 5.10) and coronal T2-GE-weighted (Fig. 5.11) MRI demonstrate focal bone marrow edema involving the medial femoral condyle with a hyperintense signal along the superficial aspect of the medial collateral ligament (MCL) (sprain grade I).

Marginal ossification center irregularities in the distal femoral epiphyses are extremely common in children during normal skeletal maturation, showing a high rate of bilaterality. There are four different knee ossification variants: puzzle piece (completely filled by bone), partial puzzle piece (partially filled by bone), spiculated configuration with an irregular subchondral bone plate, and extra ossification centers within the non-ossified physeal cartilage that remain separated from the condyle’s ossified part. These variants may occur in the posterior aspect of the maturing femoral condyle. A subchondral bone plate focal defect results if the variant is embedded within the epiphysis’ already ossified part.

Often the irregularities take a form roentgenographically indistinguishable from osteochondritis dissecans (OCD). OCD is an acquired, potentially reversible subchondral bone disorder that can secondarily affect the overlying articular cartilage and, in some cases, may lead to cartilage separation and fragmentation. The typical symptoms are pain, swelling, stiffness, locking, and clicking. Accessory ossification centers, on the other hand, do not cause any clinical symptoms and do not require treatment.

Magnetic resonance imaging provides several helpful key points distinguishing normal ossification variants from stage I OCD. Features of normal variants are infero-central defect location in the posterior femoral condyles with intact overlying articular cartilage, the presence of accessory ossification centers, lesion angle measurement <105º, spiculations, residual physeal cartilage, and lack of joint effusion and bone marrow edema. Osteochondritis dissecans MRI features have been discussed in Chap.​ 4 (Case 4.6).

Right knee sagittal T1-weighted MRI (Fig. 5.12) demonstrates an OCD-like lesion (arrow) in the femoral condyle’s posterolateral aspect. A corresponding sagittal fat-suppressed T2 (Fig. 5.13) view shows a normal overlying articular cartilage and lack of bone marrow edema.

A left knee coronal T2-weighted sequence (Fig. 5.14) shows a posterolateral femoral condyle irregularity (arrow). Sagittal fat-suppressed fast spin-echo T2-weighted MR image (Fig. 5.15) demonstrates the normal knee maturation anatomy: heterogeneous signal intensity (SI) of non-ossified posterior femoral condyle (long arrow), hyperintense signal of organized articular cartilage (short arrow), and relative hypointense signal in the less organized hyaline cartilage of the non-ossified epiphysis.

A discoid lateral meniscus (DLM) is a common variant of normal meniscal morphology, which is usually C shaped. The enlarged body of the meniscus extends medially toward the intercondylar notch and covers the articular surface of the tibial plateau, considering three types of DLM in arthroscopy: complete, incomplete (50–80 % of coverage), and the Wrisberg variant (complete or incomplete with no capsular attachments). Their prevalence is between 0.4 and 17 % depending on the authors. It occurs on the lateral side of the knee in about 1–3 % of the population and even less commonly on the medial side of the knee. It is bilateral in up to 20 % of cases.

Magnetic resonance imaging (MRI) provides a precise identification of the lesion. According to Silverman et al., criteria for adult knees are the presence of three or more 5-mm thick contiguous sagittal images showing continuity of the anterior, and posterior horns of the meniscus are suggestive for a discoid lateral meniscus. In children, particularly those younger than 10 years should be used a ratio of lateral tibiofemoral joint space coverage. If the meniscus occupies greater than 50 % of the lateral tibiofemoral joint space, a discoid lateral meniscus should be considered. Unlike adults, children frequently present symptoms: projection on bending the knee, pain, and reduction in the range of movement. The discoid meniscus is more susceptible than normal to mechanical forces due to its thickness, hypermobility, poor vascularization, and a weak attachment of the posterior area to the capsule.

Surgical treatment is necessary in symptomatic cases, with satisfactory results following arthroscopic partial resection.

Sagittal fat-suppressed T2-weighted image and coronal STIR image (Figs. 5.16 and 5.17) show an abnormally large and thick lateral meniscus (arrow) with internal increased signal due to mucinous degeneration. The abnormal signal does not reach the articular surface (Fig. 5.17). Coronal STIR (Fig. 5.17) also demonstrates the body of the lateral meniscus extending into the intercondylar notch, consistent with complete discoid meniscus. Note the normal shape and size of the medial meniscus. Incidental fibrous cortical defect is seen on the medial condyle with hyperintense signal (arrow) on sagittal fat-suppressed T2-weighted image (Fig. 5.18) and hypointense on axial T1-weighted image (Fig. 5.19) outlined by a thin rim of sclerosis (arrow). Fibrous cortical defect and non-ossifying fibroma are discussed in length within Case 5.7.