Extensor apparatus of the knee: anatomy and injury patterns

The extensor mechanism of the knee is essential for maintaining normal daily function and for activities such as walking and athletic performance. It comprises the quadriceps muscles and tendon, patella and peripatellar fat pads, patellofemoral joint and retinacula, patellar tendon, and tibial tuberosity. Disruption of any component can result in pain, instability, or loss of active extension. Owing to its superficial location and continuous biomechanical loading, it is particularly vulnerable to acute trauma, repetitive microtrauma, chronic degeneration, and traction apophysitis in the pediatric population. Imaging is essential for accurately differentiating between diagnoses and guiding management. Radiographs remain indispensable for assessing patellar height, fracture morphology, and skeletal variants, while computed tomography (CT) is a reliable preoperative tool for defining fracture configuration, comminution, and articular involvement. Ultrasound (US) offers dynamic, high-resolution evaluation of tendons and retinacula, facilitating differentiation of partial from complete tears and assessment of postoperative integrity. Magnetic resonance imaging (MRI) provides comprehensive characterization of soft tissues, bone marrow, cartilage, and associated intra-articular pathology, and is central to preoperative planning in complex injuries and patellofemoral instability. In addition to diagnosis, imaging findings directly influence treatment decisions, including fracture fixation strategies, tendon repair or augmentation, and treatment selection. Postoperative imaging plays a vital role in monitoring healing, detecting complications such as construct elongation, re-tear, hardware irritation, and recurrent instability, and guiding rehabilitation. This review highlights the radiological anatomy and injury patterns of the knee extensor mechanism, emphasizing that integrated use of radiographs, ultrasound, and MRI enables accurate, clinically relevant assessment.