Composition of the knee

The knee is one of the important parts of the human body, located between the thigh and calf. The main components inside the knee are the menisci and ligaments. The knees are very familiar to us, but they are also very fragile. People often suffer from knee pain in life, which greatly affects their daily life and work. Therefore, the knees are very important to us.

Knee bone structure

The knee joint is composed of the distal femur, proximal tibia and patella. In terms of joint classification, the knee joint is a synovial joint.

The front part of the distal femur is called the trochlea, and the back part of the distal femur is the femoral condyle, which is divided into the medial femoral condyle and the lateral femoral condyle, which are respectively connected to the medial and lateral trochlea to form the convex femoral articular surface.

There are two intercondylar spines, one anterior and one posterior, in the center of the tibial plateau, surrounded by the attachment sites of the meniscus and cruciate ligaments.

Anatomy of the collateral ligaments

The medial and lateral collateral ligaments of the knee joint are also called the tibial collateral ligament and the fibular collateral ligament.

The medial collateral ligament is divided into two layers, shallow and deep. It starts from the medial epicondyle of the femur and ends at the medial side of the tibia downward and forward. The parallel fibers are about 1.5 cm wide and extend backward to form the oblique fibers of the superficial layer of the medial collateral ligament, interweaving with the straight head of the semimembranosus muscle. When the medial knee joint capsule runs deep to the superficial layer of the medial collateral ligament, it thickens to become the deep medial collateral ligament, and forms a bursa between it and the superficial layer to facilitate movement.

The lateral collateral ligament is located in the posterior 1/3 of the lateral side of the knee joint and can be divided into a long head and a short head. The long head originates from the lateral epicondyle of the femur, and the short head originates from the pisiform bone (fabella), and is located at the same level as the styloid process of the fibula. The lateral collateral ligament is taut when the knee is fully extended and tends to relax when the knee is flexed.

Three menisci

The meniscus is the only tissue in the joint that is not covered by synovium, and its coronal section is a triangular structure.

Histologically, the meniscus is a fibrous cartilage tissue composed of three groups of interwoven fibers: horizontal fibers run in an anterior-posterior direction and constitute the main body of the meniscus, straight fibers and oblique fibers connect the upper and lower surfaces, and radial fibers connect the meniscus wall and the free edge.

The blood supply of the meniscus mainly comes from the blood vessels at the junction of the edge and the periosteal joint capsule and from the blood vessels entering from the anterior and posterior horns. The outer 1/3 of the edge side has blood supply, which gradually decreases towards the free edge. The inner 1/3 has no blood supply, and nutrition comes from the synovial fluid.

The anterior and posterior ends of the meniscus are attached to the non-articular surface of the middle part of the tibial plateau, in front and behind the intercondylar spine. This area is also called the anterior and posterior horn of the meniscus. The meniscus is a semilunar cartilage between the femoral condyle and the tibial plateau. Its lateral edge is thicker and the medial edge is thinner. The medial meniscus is "c"-shaped, while the lateral meniscus is approximately "o"-shaped.

The meniscus itself has poor blood supply and weak repair ability. Once damaged, it is difficult to repair itself. If not treated in time, it can cause traumatic arthritis in the late stage. Therefore, patients diagnosed with meniscus injury, discoid meniscus, or meniscus cyst should undergo early resection. After resection, it will be repaired by fibrous tissue to form fibrocartilage to replace the function of the meniscus. If handled correctly, it generally does not affect the function of the knee joint.

Four anterior and posterior cruciate ligaments

The anterior and posterior cruciate ligaments between the medial and lateral condyles of the femur and the tibia are the most important and strongest ligament structures that maintain the stability of the knee joint.

The anterior cruciate ligament (ACL) is tense when the knee is fully extended and loose when the joint is flexed. Its function is to prevent the femur from dislocating posteriorly, the tibia from dislocating anteriorly, and the knee from overextension and overrotation.

The posterior cruciate ligament (PCL) becomes increasingly tight as the knee flexes, which helps prevent anterior dislocation of the femur, posterior dislocation of the tibia, and excessive flexion of the knee.

The anterior cruciate ligament originates from the articular surface in front of the medial intercondylar ridge of the tibial plateau and near the anterior horn of the medial meniscus, runs outward, upward, and backward, and ends at the medial surface of the lateral femoral condyle. The length of the anterior cruciate ligament in an adult is about 38 mm and the width is about 11 mm.

The posterior cruciate ligament is similar in length to the anterior cruciate ligament and is approximately 13 mm wide. It is the strongest ligament structure in the knee joint. The posterior cruciate ligament originates from the posterior part of the intercondylar area of ​​the tibial plateau near the tibial epiphysis, extends medially, superiorly, and anteriorly, and ends at the lateral surface of the medial femoral condyle. The infrapatellar fat pad and synovial branches are the main sources of blood supply to the anterior cruciate ligament. Protecting or anatomically repairing these tissues during surgery has important clinical significance.

Five fat pads

The fat pad, also known as the infrapatellar fat pad, is a mass of fat tissue limited to the bottom of the patella, the back of the patellar ligament, and the anterior part of the tibial plateau. Its surface is covered by synovium and isolated from the joint cavity.

6. Synovium and synovial bursa

The synovial cavity of the knee joint is the largest synovial cavity in the human body. Most of the avascular tissues in the joint rely on the synovial fluid secreted by the synovium for nutrition.

There are many large and small synovial bursae around the knee joint, including the suprapatellar bursa, prepatellar bursa and infrapatellar bursa.

Knee osteoarthritis

Knee osteoarthritis (OA) is the most common joint disease and a chronic joint disease characterized by degeneration, destruction and bone hyperplasia of articular cartilage.

OA is divided into primary and secondary types. The cause of primary OA is still unknown. It is generally believed to be related to genetic factors, environmental factors, age, gender, race, smoking, diet, obesity, etc.

Pain in the affected joints is the most common symptom in OA patients. Early pain occurs during activities, especially when going up and down stairs, and is relieved after rest. As the disease progresses, the pain becomes persistent, accompanied by clinical manifestations such as joint swelling, deformation, friction during activity, and limited joint movement. X-ray manifestations include narrowing of the joint space, osteophyte formation, subchondral bone cystic changes or sclerosis, etc.

OA is more common after middle age. After the age of 50, the prevalence of OA in women is significantly higher than that in men, and the prevalence of OA in postmenopausal women increases.