Chapter 78: Knee & Leg Anatomy

The provided chapter presents a detailed anatomical and biomechanical overview of the knee joint and the structures of the leg, emphasizing clinical relevance. The knee is classified as the body's largest synovial joint, a dynamic hinge relying on robust intra- and extracapsular ligaments to manage impressive weight-bearing demands. The leg skeleton comprises the strong, stress-bearing tibia and the more slender fibula, connected by the interosseous membrane. Structural integrity involves the patella, the largest sesamoid bone, which possesses the body's thickest articular cartilage and is stabilized against lateral displacement primarily by the medial patellofemoral ligament and the pull of the vastus medialis obliquus muscle. Tibiofemoral articulation stability is enhanced by the fibrocartilaginous menisci, which conform to the femoral condyles, increase joint contact area, and spread axial loads by converting them into circumferential hoop tension. The medial meniscus is relatively fixed, while the lateral meniscus is highly mobile, adapting to motion, and includes variants such as the potentially unstable Wrisberg type discoid meniscus. Key passive stabilizers include the cruciate ligaments, specifically the strong anterior and posterior cruciate ligaments (ACL and PCL), which cross internally to resist anterior and posterior tibial translation. Movement involves complex kinematics combining femoral rolling and sliding, notably including the obligatory rotational movement, known as the 'screw-home movement,' which locks the knee in full extension. The popliteus muscle performs the crucial function of initiating lateral femoral rotation to 'unlock' the joint at the start of flexion. The muscles of the leg are divided into three main fascial compartments—anterior (extensor), lateral (fibular), and posterior (flexor, subdivided into superficial and deep)—which are common sites for compartment syndromes. The muscle groups follow the ‘one compartment – one nerve’ principle, governed by the deep fibular nerve (anterior), superficial fibular nerve (lateral), and tibial nerve (posterior). The posterior superficial group, the triceps surae (gastrocnemius and soleus), are the primary plantar flexors, while the tibialis posterior, located deep, is the main invertor and essential arch supporter. Vascular supply originates from the popliteal artery, which forms an intricate genicular anastomosis around the knee and divides into the anterior and posterior tibial arteries and the fibular artery, whose perforating branches are vital for soft tissue flap surgery. Neurologically, the common fibular nerve is highly vulnerable as it wraps around the fibular neck, where injury can result in foot drop. Lymphatic drainage from the knee and leg primarily flows to the popliteal and inguinal nodes.