Indications and imaging modalities
Acute knee trauma is common, especially in pediatric practice. To avoid unjustified radiation exposure, strictly validated rules determine when an X-ray is truly necessary. Radiography is indicated if the patient experiences isolated tenderness strictly in the area of the patella or the fibular head. The examination is also prescribed for functional limitations: the inability to flex the knee to a 90-degree angle or the inability to bear weight on the leg.
The standard examination includes two main projections: the anteroposterior (AP) and lateral. At the specialist's discretion, an axial projection of the patella (skyline view) may be additionally used. Soft tissue evaluation on a radiograph is limited, but joint effusion can be detected: fluid accumulation outlines the suprapatellar region and obliterates normal fat planes.
Osteochondral lesions and fractures
Radiography is the primary, but not the most sensitive, method for evaluating articular cartilage and subchondral bone structures. For example, osteochondral lesions (osteochondral lesions, formerly osteochondritis dissecans) develop in children and adolescents due to chronic microtrauma, ischemia, and local growth disturbances. A common location is the lateral surface of the medial femoral condyle. On radiographs, they appear as a linear subcortical lucency with adjacent sclerosis.
In direct trauma, simple osteochondral fractures can occur. In addition, subchondral insufficiency fractures are encountered—they develop against the background of osteoporosis with minimal or no trauma. They are frequently preceded by bone marrow oedema syndrome, also known as transient osteoporosis.
In dislocations, a "kissing contusions" pattern is often observed. In the patellofemoral compartment, this manifests as synchronous injury to the medial facet of the patella and the lateral femoral condyle. Such injuries are frequently accompanied by a tear of the medial patellofemoral ligament or the medial patellar retinaculum.
Tendon and ligament pathology
Although tendons are not directly visible on X-rays, secondary bony and contour changes suggest pathology, which is subsequently confirmed by ultrasound or MRI.
- Osgood-Schlatter disease (Osgood-Schlatter disease): a frequent cause of pain in children aged 8–13 years. The condition arises from sports-related traction stress. It is characterized by fragmentation or enlargement of the tibial tuberosity, distal patellar tendinopathy, and soft tissue thickening.
- Jumper's knee (jumper's knee): tendinopathy affecting the proximal attachment site of the patellar tendon. Tears of the patellar tendon itself or the quadriceps femoris usually occur on the background of this chronic pathology.
- Chondromalacia patellae (chondromalacia patellae): damage to the patellofemoral cartilage in adolescents and young adults caused by microtrauma or joint malalignment.
- Bursitis: inflammation of the prepatellar or infrapatellar bursae due to friction from repetitive movements or in the setting of systemic diseases (e.g., rheumatoid arthritis).
To remember the pediatric indications for a knee X-ray in trauma, use the rule of three "N"s: No flexion (to 90 degrees), No weight-bearing (inability to bear weight), Not localized (isolated pain in the patella or fibula).
Class and exam questions
Which projections are mandatory for knee radiography?
The mandatory minimum is considered to be the anteroposterior (AP) and lateral projections. An axial view of the patella is prescribed additionally at the physician's discretion.
Is a classical X-ray sufficient for diagnosing osteochondral lesions in adolescents?
No, radiography has low sensitivity (showing only subcortical lucency with sclerosis). The imaging modality of choice for accurately measuring the lesion and assessing its stability is MRI.
How does knee effusion manifest on a plain radiograph?
The accumulation of intra-articular fluid effaces (blurs) normal fat planes and clearly outlines the suprapatellar region.
What else to study on this topic
- MRI semiotics of unstable osteochondral lesions (evaluation of the fluid rim)
- Stages of patellofemoral cartilage degeneration on MRI (from signal alteration to full-thickness defect)
- Ultrasound diagnostics and the role of Doppler sonography in jumper's knee
- Differential diagnosis of tumors of the infrapatellar fat pad (Hoffa's fat pad)
- MRI findings of transient osteoporosis and subchondral insufficiency fractures
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