The Knee – Anatomy, Nerve Supply, Functions

The knee is the largest joint in the body. It is a compound synovial joint that consists of the tibiofemoral joint and the patellofemoral joint. It primarily serves as a hinge joint which allows flexion and extension as well as various other movements. It joins the lower leg and thigh bilaterally and is an essential component of efficient bipedal movements such as walking, running, and jumping. The anatomical function and stability of the knee depend on muscles, bones, ligaments, cartilage, synovial tissue, synovial fluid, and other connective tissues. The 4 main stabilizing ligaments of the knee are the anterior cruciate (ACL), posterior cruciate (PCL), medial collateral (MCL), and lateral collateral  (LCL). The ACL attaches at the lateral condyle of the femur and the intercondyloid eminence of the tibia, and functions to prevent anterior translation of the tibia on the femur. The PCL attaches at the medial condyle of the femur and the posterior intercondylar area of the tibia, and functions to prevent forward displacement of the femur on the tibia.[rx][rx][rx][rx]

Anatomy and Physiology

The knee is a synovial hinge joint with minimal rotational motion. It is comprised of the distal femur, proximal tibia, and the patella. There are 3 separate articulations and compartments: medial femorotibial, lateral femorotibial, and patellofemoral. The stability of the knee joint is provided by the congruity of the joint as well as by the collateral ligaments. The capsule surrounds the entire joint and extends proximally into the suprapatellar pouch. Articular cartilage covers the femoral condyles, tibial plateaus, trochlear groove, and patellar facets. Menisci are interposed in the medial and lateral compartments between the femur and tibia which act to protect the articular cartilage and support the knee.

The mechanical axis of the femur, defined by a line drawn from the center of the femoral head to the center of the knee, is 3 degrees valgus to the vertical axis. The anatomic axis of the femur, defined by a line bisecting the femoral shaft, is 6 degrees valgus to the mechanical axis of the femur and 9 degrees valgus to the vertical axis.[rx] The proximal tibia is oriented to 3 degrees of varus. The varus position of the proximal tibia, along with the offset of the hip center of rotation, results in the weight-bearing surface of the tibia being parallel to the ground. The sagittal alignment of the proximal tibia is sloped posteriorly approximately 5 to 7 degrees. The asymmetry of the natural bony anatomy maintains the alignment of the joint and ligamentous tension.

Structure of The Knee

The knee is a weight-bearing joint that serves to allow flexion and extension of the lower leg around a transverse axis in a sagittal plane. During this motion, the tibial condyles articulate with the femoral condyles as well as the medial and lateral menisci. Also, the patella articulates with the femoral trochlear groove. The knee secondarily allows for internal and external rotation, compression and distraction, anterior and posterior translation, medial and lateral translation, and varus and valgus movements. The tendonous and ligamentous structures of the knee are lubricated by bursae. The names of the bursae correspond with their location within the knee. The prepatellar bursa is located between the patella and the overlying subcutaneous tissue. The infrapatellar bursa has a superficial and deep component. The superficial infrapatellar bursa is located between the tibial tubercle and the overlying skin. The deep infrapatellar bursa is located between the posterior aspect of the patellar tendon and the tibia. The suprapatellar bursa is located between the quadriceps tendon and the femur. The pes anserinus bursa is located on the anteromedial aspect of the tibia, on the medial tibial epicondyle. Within the layers of the medial collateral ligament is the medial collateral ligament bursa. The iliotibial bursa is located on the distal iliotibial band by its insertion on the Gerdy tubercle. The popliteal bursa is located in the popliteal hiatus, by the proximal popliteal tendon. Although the knee is an inherently unstable joint, it has many dynamic stabilizers (muscles) and static stabilizers (ligaments).[rx][rx]

Blood Supply of The Knee

The structures of the knee receive much of their blood supply from a plexus of arteries with branches from the popliteal artery and femoral artery. The popliteal artery branches off the superficial femoral artery and runs posteriorly across the knee joint. The superior medial, inferior medial, superior lateral, and inferior lateral genicular arteries branch off the popliteal artery and travel anteriorly to anastomose with other parts of the plexus. Also, a descending genicular artery branches off the superficial femoral artery and anastomoses anteriorly with the other genicular arteries. The anterior and posterior tibial recurrent arteries travel laterally from the anterior tibial artery and also contribute to the plexus. The middle genicular artery travels directly into the joint. In addition, the sural arteries branch off the popliteal artery and travel inferiorly away from the midline. The flexor muscles are supplied mainly by the inferior gluteal, perforating, popliteal, deep femoral, and sural arteries. The femoral artery primarily supplies the extensor muscles.

Much of the lymphatic drainage from the knee and lower leg travel to the popliteal lymph nodes which are located in the popliteal fossa. The popliteal nodes along with other knee and lower limb lymphatics drain into the deep inguinal and sub inguinal nodes. The lymphatic system primarily follows vasculature.

Nerves

The structures of the knee and most of the flexor muscles receive innervation from branches of the femoral nerve (L1, L2, L3). The extensor muscles receive innervation from the sciatic nerve (L4, L5, S1, S2, S3) which branches into the tibial nerve and common peroneal nerve.

Muscles

Flexion is predominately accomplished by the articularis genus, rectus femoris, vastus lateralis, vastus intermedius, and vastus medialis. These muscles originate from various locations on the femur and anterior inferior iliac spine. The latter 4 conjoin to form the patellar tendon/ligament which crosses the knee anteriorly and inserts on the patella and tibial tuberosity.

The extension is predominately accomplished by the biceps femoris, semitendinosus, semimembranosus, gastrocnemius, plantaris, gracilis, and popliteus. These muscles originate from the ischial tuberosity, inferior pubic ramus, and different locations on the femur. They insert on various locations of the tibia, fibula, and calcaneus.

The Function of The Knee

The MCL attaches at the medial epicondyle of the femur and the medial condyle of the tibia, and functions to prevent valgus stress on the knee. The LCL attaches at the lateral epicondyle of the femur and the head of the fibula, and functions to prevent varus stress on the knee. The medial and lateral menisci are 2 separate fibrocartilage structures that are located between the articular surfaces of the tibia and femur. They function as shock absorbers, static stabilizers, and friction reducers during articulation. The knee’s bony structures include the distal end of the femur, proximal end of the tibia, and patella. The patella is the largest sesamoid bone in the body and functions as an attachment point for the quadriceps tendon and patellar ligament. It also protects the anterior articular surface of the femoral portion of the knee. The knee contains multiple bursas which serve to reduce friction between structures of the knee. Bursas are small sacs made up of synovial membranes and contain synovial fluid. Many of these structures mentioned above are part of the articular capsule which serves to stabilize the knee further and contain synovial fluid. Synovial fluid is made by synovial membranes and serves to reduce friction between articular surfaces of the knee.

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