The anatomy of the knee is reflective of its function in ambulation. Knee stability and pain-free range of motion are important in maintaining daily function. Most commonly, overuse, age, and traumatic injuries cause structural damage to the knee that may limit its function. Therefore, a thorough understanding of the anatomy of the knee is essential to properly diagnosing and treating knee pathology. [1, 2]
An image depicting the knee joint can be seen below.
Femur, tibia, fibula, and patella
The knee is composed of 4 bones: the femur, tibia, fibula and patella. All these bones are functional in the knee joint, except for the fibula.
The femur is the longest and strongest bone in the human body. The proximal end forms the head of the femur, which projects anterosuperomedially to articulate with the acetabulum. The distal end is wider and forms a double condyle that articulates with the tibia and patella. The tibia articulates with the distal lateral and medial femoral condyles. The patella articulates anteriorly to the femoral condyles in the region of the intercondylar fossa (trochlear groove).
The tibia lies distal to the femur and medial to the fibula. The proximal end consists of medial and lateral condyles, an intercondylar area, and the tibial tuberosity that articulates with the medial and lateral condyles of the femur. Distally, the tibia articulates with the ankle. The distal and proximal ends of the tibia articulate with the fibula. In addition, the shaft of the tibia and fibula are connected with an interosseous membrane to form a syndesmosis joint.
The fibula does not articulate with the femur or patella. Furthermore, the fibula is not directly involved in weight transmission.
The patella is the largest sesamoid bone in the human body. This bone is flat, proximally curved, and distally tapered; however, the shape can vary. The posterior patella articulates with the femur, but the apex sits proximal to the line of the knee joint. The tendon of the quadriceps femoris completely encompasses the patella.
The paired menisci are crescentic, fibrocartilaginous pads that attach to the intercondylar area and periphery of the tibial plateau. This cartilage serves to widen and deepen the articulating surface between the femoral condyles and the tibia. To deepen the surface, the peripheral borders are thick and convex, and the inner borders are thin and concave.
The outer region is well-vascularized from capillaries branching off the fibrous capsule and synovial membrane. The inner regions, however, are avascular. Therefore, tears of the peripheral meniscus typically heal well, whereas tears of the inner meniscus do not. A discoid lateral meniscus occurs in up to 5% of the population. The lateral meniscus is generally wider and gets caught between the femur and tibia, which can cause "clunking" in some patients.
The ligaments of the knee joint can be divided into the extracapsular ligaments and intra-articular ligaments. The extracapsular ligaments or external ligaments are the patellar ligament, medial collateral ligament (MCL), lateral collateral ligament (LCLs), oblique popliteal ligament, and arcuate popliteal ligament. The intra-articular ligaments are the anterior cruciate ligament (ACL), posterior cruciate ligament (PCL), and the posterior meniscofemoral ligament. (See the image below.)
The patellar ligament is the anterior ligament of the knee joint. It is the distal part of the quadriceps tendon and attaches to the tibial tuberosity. The vastus medialis and lateralis contribute to the patellar ligament medially and laterally through the medial and lateral retinacula, which make up the joint capsule of the knee on either side of the patella. The retinacula also maintain alignment of the patella relative to the patellar surface of the femur.
The LCL extends from the lateral epicondyle of the femur to the lateral surface of the fibular head. The LCL is separated from the lateral meniscus by the popliteus tendon. The LCL also splits the tendon of the biceps femoris into 2 parts.
The MCL extends from the medial epicondyle of the femur to the medial condyle and superior part of the medial surface of the tibia. The MCL is firmly attached to the medial meniscus, which is why these are commonly torn at the same time in contact sports. The oblique popliteal ligament and arcuate popliteal ligament reinforce the joint capsule on the posterior aspect. The oblique popliteal ligament is a recurrent expansion of the tendon of the semimembranosus, and it arises from the medial tibial condyle and passes toward the lateral femoral condyle, where it blends in with the rest of the joint capsule. The arcuate popliteal ligament arises from the posterior fibular head and passes over the tendon of the popliteus and spreads over the posterior surface of the knee.
The ACL attaches posterior to the attachment of the medical meniscus on the anterior intercondylar area of the tibia and passes superior, posterior, and lateral, where it attaches to the posterior part of the medial side of the lateral condyle of the femur. The PCL arises from the posterior intercondylar area and passes on the medial side of the ACL to attach to the anterior part of the lateral surface of the medial condyle of the femur. The menisci are wedge shaped and attach at their ends to the intercondylar area of the tibia. The medial meniscus is C shaped and firmly adheres to the deep surface of the MCL medially, the ACL anteriorly, and the PCL posteriorly. Because of these attachments, the medial meniscus is less mobile than the lateral meniscus.
Branches of the obturator, femoral, tibial, and common fibular nerves contribute to innervation of the knee joint. The infrapatellar branch of the saphenous nerve provides cutaneous sensation to the medial anterior aspect of the knee. The peripatellar plexus is composed of branches from anterior cutaneous branches of the femoral nerve and the lateral femoral cutaneous nerve to provide cutaneous sensation to the rest of the knee.
Muscles and tendons
The main motion of the knee joint is flexion and extension, with limited medial and lateral rotation. The main muscle for extension is the quadriceps femoris, which is the most important muscle in stabilizing the knee joint. The quadriceps is made up of the vastus medialis and lateralis, rectus femoris, and vastus intermedius. The tensor fasciae latae is a weak extensor.
Flexion is produced by the hamstring muscles, which consist of the semitendinosus, semimembranosus, and long head of the biceps femoris along with the short head of the biceps and, weakly, the gracilis, sartorius, gastrocnemius, and popliteus. The medial rotators of the knee are the semitendinosus, semimembranosus, popliteus, gracilis, and sartorius. The lateral rotator of the knee is the biceps femoris.
Many vessels are involved in forming the arterial anastomosis around the knee joint. This anastomosis is formed by the superior, middle, and inferior genicular branches of the popliteal artery; the descending genicular branch of the femoral artery; and the descending branch of the lateral circumflex femoral artery, the circumflex fibular artery, and the anterior and posterior tibial recurrent arteries. The anastomosis supplies the patella, the femoral and tibial condyles, bone marrow, articular capsule, and synovial membrane.
The popliteal artery, however, is the major artery that crosses the popliteal fossa in the posterior aspect of the knee. Because of its location, this artery is susceptible to damage in knee injuries.
The venous system of the knee joint is primarily composed of the popliteal and femoral veins. These veins simply run with their corresponding arteries and drain deoxygenated blood from the arterial anastomosis. The popliteal veins contain 3-4 valves.
Many bursae surround the knee and the arrangement is complex and variable. However, the medial group, including the anserine bursa and the semimembranosus bursa, are clinically important. The anserine bursa is located 4-5 cm distal to the anteromedial joint line and deep to the pes anserinus, the conjoint tendon formed by the sartorius, gracilis, and semitendinosus tendons (see the following image). Deep to the anserine bursa is the insertion of the MCL. This bursa can become inflamed with excessive physical activity. The semimembranosus bursa is located in the popliteal fossa and commonly causes posterior swelling of the knee joint secondary to degeneration within the knee joint.
The joint capsule consists of the external fibrous layer and the internal synovial membrane. The fibrous layer is thin, except for the thickened parts that make up the intrinsic ligaments of the knee. On the posterior aspect, it encloses the condyles and the intercondylar fossa. Distally, it attaches to the margin of the superior articular surface of the tibia.
The synovial membrane lines all surfaces of the articular cavity not covered with articular cartilage. It attaches to the periphery of the articular cartilage, which covers the femoral and tibial condyles, posterior surface of the patella, and the popliteal surface of the femur.
All freely moveable joints have some synovial fluid in them. Synovial fluid originates from plasma that is filtered by the capillary net and diffuses into the knee along with hyaluronic acid, which is locally synthesized. Synovial fluid transports nutrients, assists in the joint's defense, and lubricates the joint.
In children, the appearance of bowlegged knees for the first 1-2 years of life right after the child is learning how to walk is common. "Knock knees" are commonly seen in the 2-4 – year-old age group. If these deformities persist into later childhood, the deformity may have a pathophysiologic cause.
Osgood-Schlatter is a disease that causes pain at the tibial tuberosity in young adolescents, which is exacerbated with activities and direct contact. Repetitive overload at the patellar ligament insertion can cause inflammation, irregularity, and partial avulsion of the secondary ossification center. Radiographs typically reveal irregularity and fragmentation.
Valgus and varus
Genu valgum or "knock knees" is a lateral angulation of the leg in relation to the thigh, which causes excess pressure to be placed on the outside or lateral aspect of the knee joint; this, in turn, can lead to joint-space narrowing and degenerative changes.
Genu varum or bowleg is a medial angulation of the leg in relation to the thigh, which causes excess pressure to be placed on the inside or medial aspect of the knee joint; this, in turn, can cause arthritis, which is destruction of the cartilage between the femur and tibia. This leads to joint-space narrowing and places stress on the fibular collateral ligament.