The knee is the middle of three main joints of the leg. The function of the knee is to help place the foot in space and propel the body during ambulatory activities. There are four directions of movement in the knee, but by far the most important movements are flexion (bending) and extension (straightening). The knee is a mobile and strong hinge joint that connects the upper leg to the lower leg.
The knee consists of two joints. A joint is the coming together of bones that allows them to move over one another. The primary knee joint is made up of three compartments and is the coming together of three bones. It is a complex hinge joint since it is not just the simple coming together of two bones. The other is much smaller and is located just below the main knee joint.
Femur: This is the thighbone. It runs from the hip joint to the knee joint. It is the longest and strongest bone in the body, forming the upper part of the leg. At the knee this bone has three articulating surfaces. Two of them are the medial and lateral condyles; the third is the trochlea. The trochlea connects the two condyles and is the surface that the patella glides over during joint function. The condyles of the femur glides over the top part of the tibia bone.
Tibia: This is the bigger of the two bones that make up the lower leg. It runs from the knee joint to the ankle joint. At the knee joint, the bone forms a table-like structure called the tibial plateau. It is not flat, but rather has two separate areas for contact with the condyles. The medial side is slightly concave and the lateral side is slightly convex.
Fibula: This is the smaller of the two bones that make up the lower leg. It does not make up part of the knee joint, but supports the tibia by the knee. It is a supporting joint that serves as an attachment for the soft tissue structures around the knee. The lower end of the fibula bone forms the outside of the ankle bone and carries about 16% of a person’s weight when they stand on one leg.
Patella: This is the kneecap. It facilitates leg straightening by forming a gliding surface against the end of the thigh bone (the trochlea). It is the largest sesamoid bone in the body and it helps increase the lever arm for the quadriceps muscle. It is an odd-shaped bone at the articulating surface with facets and a ridge. The facets allow the patella to contour with the trochlea, and the ridge is the coming together of the facets toward the center of the patella, but not exactly at the center.
Articular cartilage is a specialized soft tissue that covers the bone ends. It is a delicate structure and has no blood supply or nerves. In a healthy joint it is smooth, spongy, and slippery, and allows the bones to glide easily over one another during motion.
A muscle connects to a bone via a tendon, which is a rope-like structure. A muscle crosses the joint to attach to the bone and allow for movement. Tendons are made up of many fibers that are interconnected and intertwined to remain flexible and pliable. Muscles and tendons have nerves within them and a blood supply to keep them alive.
Quadriceps: This is a long muscle that has four (quad) heads (ceps). It sits on the front of the femur bone and runs from the front of the pelvis and upper thigh to the knee cap. It then continues to the upper front of the tibia bone by attaching to and passing through the patella. The quadriceps tendon attaches the muscle to the top of the patella. The lower aspect of the tendon that runs from the patella to the upper part of the tibia (tibial tubercle) is called the patella tendon. The quadriceps muscle makes the lower leg straight and helps bend the thigh at the hip
Hamstrings: This is a group of long muscles that run from the back of the pelvis and upper thigh to the lower thigh and the upper leg. They end by attaching to the back of the tibia and fibula bones. The hamstrings help straighten the thigh bone at the hip and also bend the lower leg at the knee.
Adductors: This group of long muscles runs from the upper thigh and inner pelvis to the lower thigh and upper-inner tibia bone. They help bring the upper legs towards each other.
Abductors: This is group of long muscles is located along the outside aspect of the upper femur and the back of the pelvis. They are an extension of the buttocks muscles, running down the outside of the thigh to attach at the upper-outer tibia and fibula bones. In the thigh they form a long, wide tendon called the iliotibial band. The abductors bring the legs away from each other at the hip.
Calf: This is a group of muscles made up of the Gastrocnemius, Plantaris, and Soleus muscles. They sit along the back of the upper part of the lower leg. They attach to the back of the end of the femur, the back of the top of the tibia, and the back of the fibula bones. They travel down the back of the lower leg to attach to the back of the ankle at the calcaneus bone. The calf muscles help bend the knee by moving the lower leg backwards.
Ligaments are like small ropes, washers, or rubber bands that connect bone to bone. They are made up of many fibers that are interconnected and intertwined to remain flexible and pliable. They have nerves within them and a blood supply to keep them alive. The ligaments are static stabilizers, as they stabilize joints based on their connection. They prevent excessive motion in a joint.
There are four main ligaments in the knee joint: the ACL, PCL, MCL, and LCL.
Anterior cruciate ligament (ACL): This ligament is closely related to the PCL. They cross within the center of the knee joint. It is fan-like at the end and becomes more oval in the central aspect. It connects the back, outer aspect of the end of the femur to the front, inner aspect of the top of the tibia. It helps stabilize the knee during forward and external rotation stress.
Posterior cruciate ligament (PCL): This ligament is closely related to the ACL. They cross within the center of the knee joint. It is a fan-like structure at the ends and becomes more oval in the central aspect. It connects the front, inner aspect of the end of the femur to the back, outer aspect of the top of the tibia. It helps stabilize the knee during backward and internal rotation stress.
Medial collateral ligament (MCL): This ligament runs along the inside aspect of the knee. It has two separate yet connected layers. One is deep and close to the joint, and the other is more superficial or closer to the skin. The MCL is flatter than the ACL and PCL. It connects the inner end of the femur to the inner top of the tibia. It helps stabilize the knee to outward (valgus) stress.
Lateral collateral ligament (LCL): This ligament runs along the outside aspect of the knee. It is outside (superficial to) the actual knee joint. This ligament is more tubular or rounder than the ACL and PCL. It connects the outer end of the femur to the inner top of the fibula. It helps stabilize the knee to inward (varus) stress.Medial patellofemoral ligament (MPFL): This ligament is a flat, fan-shaped structure that runs along the inside aspect of the patella to the inside aspect of an outcropping of the inner end of the femur (medial epicondyle). It helps keep the patella gliding within the trochlea and stabilize the patella to lateral (sheer) and outward (valgus) stress.
Meniscus: This is a special type of cartilage: fibrocartilage. It is more durable, pliable, and rigid than articular cartilage. There are two within the knee. One sits on top of the lateral tibial plateau (lateral meniscus) and the other sits on top of the medial tibial plateau (medial meniscus). They help cushion the ends of the bones during weight bearing activities. They also help stabilize the knee, provide a means for nutrition for the articular cartilage, and spread out the stresses across the knee joint during activity.
Capsule: This tissue helps form the sack that surrounds and contains the joint. It is made up of stringy connective tissue that keeps the normal fluid within the joint. Joint capsules are made up of many fibers that are interconnected and intertwined to remain flexible and pliable. They have nerves within them and a blood supply to keep them alive.Synovial lining: This soft tissue lines the capsule of the joint. The lining is important for producing the lubricating joint fluid, which allows the joint to function well. This fluid allows the ends of the bones to glide over one another with very little resistance or friction. The synovial lining has nerves within it and a blood supply to keep it alive.