Choice of imaging modality and baseline signs
In acute knee trauma, standard radiography remains the primary diagnostic method. However, cross-sectional imaging methods are actively used to clarify the nature of injuries:
- Computed tomography (CT): a valuable tool for detecting detached bone fragments and accurately assessing displacement in intra-articular fractures (e.g., fractures of the medial tibial condyle).
- Magnetic resonance imaging (MRI): the method of choice for evaluating the joint outside the acute phase of injury. It allows for a detailed study of the menisci, ligaments, tendons, and articular cartilage.
One of the most important imaging symptoms is lipohaemarthrosis (lipohaemarthrosis)—the presence of fluid levels of different densities in the joint cavity due to the accumulation of blood and bone marrow-derived fat. Its presence on sagittal tomograms or radiographs serves as a definitive sign of intra-articular injury.
Specifics of cruciate ligament injuries
In adult patients and older adolescents, a typical consequence of forced hyperextension (hyperextension) with tibial rotation is an anterior cruciate ligament (ACL) tear. In younger children, the biomechanics are different: the tensile strength of the ligament itself significantly exceeds the strength of immature bone tissue.
As a result, an avulsion of the tibial spine (avulsion of the tibial spine) occurs. On standard anteroposterior and lateral radiographs, a detached bone fragment is visualized within the joint cavity. For a more thorough assessment of its displacement and rotation, CT or MRI is advisable.
Osteochondral fractures and osteochondritis dissecans
Under axial compressive load on the femur or in the setting of a traumatic lateral dislocation of the patella (lateral dislocation of the patella), children may sustain osteochondral fractures (osteochondral fractures). In this case, the fracture line passes through the subchondral bone of the lateral femoral condyle (lateral femoral condyle) or patella, forming a small loose body in the joint cavity.
A similar localization is seen in osteochondritis dissecans (osteochondritis dissecans), which is a defect of the subchondral region of the distal femur (most commonly on the postero-lateral surface of the medial condyle). It is believed to arise from subchondral bone necrosis due to chronic microtrauma and overload.
- On radiographs: the pathology appears as an oval lucency (lucency) directly adjacent to the articular margin.
- On MRI: the scan is performed routinely and dynamically to assess fragment stability, detail the defect, and identify underlying bone marrow oedema (bone oedema).
Patellar injuries and pathologies
Patellar (patella) fractures are rare in pediatric practice. When they do occur, they are usually transverse (transverse) or comminuted (comminuted). Avulsion fractures of the sleeve type—sleeve fractures—pose the greatest diagnostic challenge.
In this injury, the inferior pole of the patella is avulsed: a small portion of ossified periosteum (periosteum) separates from the bone, taking along a large volume of non-ossified cartilage and retinacular elements (retinaculum). On a standard X-ray, the physician may notice only a tiny bone fragment, falsely underestimating the severity of the injury. The true scale of the damage and soft tissue defect are well visualized only on MRI.
Additionally, recurrent patellar dislocations (recurrent patellar dislocations) are frequently diagnosed in adolescents, developing against the background of patellofemoral instability and accompanied by injury to the respective ligaments.
Briefly: answers to topical questions
What are the direct MRI criteria for instability of an osteochondral fragment in osteochondritis dissecans of the knee?
A direct MRI criterion for the instability of an osteochondral fragment in osteochondritis dissecans (osteochondritis dissecans) is the presence of fluid-equivalent signal intensity at its base. Additionally, during arthrography, contrast agent tracking around the fragment serves as a sign of instability. Conversely, a reliable sign of lesion stability is the preserved integrity of the overlying articular cartilage. To detect radiographically occult lesions and assess the grade of osteochondral defects, cross-sectional imaging methods, including standard MRI and MR arthrography, are used.
What special radiographic projections are used to visualize dislocations and vertical patellar fractures?
To visualize dislocations and vertical patellar fractures, an additional axial projection (skyline view) is used. It allows for the easy detection of vertical fractures that are poorly visualized on standard radiographs, as well as small osteochondral or avulsion fragments in dislocations. The axial patellar projection can be performed in the supine position or as weight-bearing functional radiography. Furthermore, in dislocations, a curvilinear bone fragment in the anterior compartment of the joint, not visible on the anteroposterior view, may be identified on the lateral view.
What is a Segond fracture, and of which intra-articular structures is it a marker of injury?
A Segond fracture (Segond fracture) is an avulsion fracture (avulsion fracture) of the lateral margin of the lateral tibial plateau at the attachment site of the joint capsule. It occurs during a forced varus trauma mechanism due to tension on the lateral capsular ligament. On the radiograph, it is visualized as a small bone fragment adjacent to the proximal lateral part of the tibia. This fracture is a pathognomonic sign of severe knee trauma and acts as a marker for the following injuries:
- Anterior cruciate ligament tear (ACL) — the probability ranges from 75% to 100%.
- Medial meniscus tear — frequently accompanies the injury.
Class and exam questions
Why are classic ACL tears rarely diagnosed in young children?
In children, the ligamentous apparatus is stronger than the bone. During trauma, the ligament does not tear; instead, it avulses a bone fragment—resulting in an avulsion of the tibial intercondylar eminence.
What does the lipohaemarthrosis sign mean?
This is an accumulation of blood and fat in the joint cavity that separate based on density. Its detection is a definitive sign of an intra-articular injury.
What is the danger of a patellar sleeve fracture?
On the radiograph, only a small portion of the periosteum is avulsed and barely visible, whereas the main avulsed bulk consists of non-ossified cartilage that is invisible on X-rays. MRI is essential for an accurate diagnosis.
How does osteochondritis dissecans appear on a radiograph?
It is visualized as a characteristic oval lucency (subchondral bone defect) adjacent to the articular margin, most commonly in the region of the medial femoral condyle.
What else to study on this topic
- Proximal tibial stress fractures in adolescents
- Assessment of osteochondral defect stability on MRI
- Occult tibial shaft fractures (Toddler's fracture) and diagnostic strategies
- Anatomy and injuries of the patellar retinaculum
- Differential diagnosis of accessory ossification centers and fractures
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