Extensor Mechanism

The extensor mechanism is composed of complex osseous and soft tissue structures such as the patella, patellar tendon, quadriceps muscle and tendons, and the patellofemoral joint, which altogether provide delicate synergism for the extension–flexion motion of the knee joint. Anterior knee pain is a common clinical problem, especially in young patients with underlying abnormal patellofemoral morphology, whereas other more serious and less common disorders also involve all components of the extensor mechanism. There have been tremendous advances in the diagnosis and characterization of these disorders using various imaging techniques such as radiography, ultrasound, computed tomography (CT) (including advanced four-dimensional CT i.e., 4D CT), and magnetic resonance imaging (MRI) (including conventional and novel compositional sequences). Abnormal patellofemoral morphology measurements such as trochlear groove depth (in trochlear dysplasia), patellar height ratio (in patella alta or patella baja), abnormal tibial tubercle–trochlear groove distance (in abnormal lateralization of the tibial tubercle), and lateral patellar tilt may cause pathologies such as patellofemoral pain syndrome and patellofemoral osteoarthritis (OA). Common patellofemoral injuries include patellar tendinosis, patellar tendon ruptures, Osgood–Schlatter disease, Sinding–Larsen–Johansson syndrome, patellar fractures, patellar sleeve fractures, and acute dislocations. In addition to the patellofemoral joint, other injuries to the extensor mechanism are also seen in clinical practice such as partial or complete quadriceps tendon tears and quadriceps muscle injuries. The extensor mechanism may also be affected by soft tissue disorders such as bone and soft tissue tumors, Morel–Lavallee lesions, prepatellar bursitis, superolateral infrapatellar fat pad edema, quadriceps fat pad edema, infrapatellar fat pad abnormalities, and patellar tendon-lateral femoral condyle friction syndrome. In this chapter, we will cover clinical and imaging manifestations of common knee extensor mechanism disorders, with a special emphasis on advanced imaging modalities.