The knee is the largest joint in the body and the one most frequently injured. In fact, a knee problem is the most common reason patients have for visiting an orthopaedic surgeon.
The knee is a hinge joint. Unlike the ball joint of the shoulder or hip which rotates in a variety of directions, a hinge joint primarily moves only from bent to straight with very little pivoting motion. This is one reason the knee is vulnerable to traumatic injury.
The knee is comprised of bones, cartilage, ligaments, tendons and muscles that work together to keep the knee functioning correctly. A problem with any one of these parts can cause pain or knee function problems.
The knee joint provides the connection between the upper and lower bones of the leg. The primary bones that are part of the knee joint are the thighbone or femur, the two bones of the lower leg (the larger tibia shinbone on the inside of the leg and the smaller fibula bone on the outside of the leg) and the kneecap or patella.
There are two types of cartilage in the knees: fibrocartilage and articular cartilage.
Articular cartilage covers the end of the femur, the top of the tibia and the back of the patella. This smooth, lubricated joint surface helps reduce friction between the bones during movement.
The fibrocartilage in the middle of the knee is the meniscus. There are two types of menisci: the medial and lateral. Both menisci are crescent-shaped pads of gristle-like material located between the tibia and femur on the outer and inner sides of each knee. They help absorb shock during motion and cushion the knee.
The bones in the knee are joined to the other bones by short bands of tough fibrous connective tissue called ligaments. They connect the femur and tibia and give the joint strength and stability.
There are four major ligaments in the knee:
- The medial collateral ligament (MCL) runs alongside the inner part of the knee and limits side-to-side movement.
- The lateral collateral ligament (LCL) runs alongside the outer part of the knee and limits side-to-side movement.
- The anterior cruciate ligament (ACL) weaves inside the knee joint and limits rotation and forward movement of the tibia.
- The posterior cruciate ligament (PCL) weaves inside the knee joint and limits backward movement of the tibia.
Muscles and Tendons
There are quite a few muscles in the leg and knee that both support and stabilize the joint and cause the bending and straightening movements. Tendons connect muscles to bones. All muscles have tendons at the end where they meet the bone.
There are many muscles that are part of the knee joint, but the two main ones are the front and the back thigh muscles. The quadriceps starts at the hip and goes along the front of the thigh. The quadriceps are responsible for allowing the leg to extend or straighten. Hamstrings are the muscles that run along the back of the thigh. They allow the leg to flex or bend.
The two most important tendons in the knee are the patellar tendon and the iliotibial band. The patellar tendon attaches the quadriceps to the tibia. The iliotibial band runs along the outside of the thigh and upper leg connecting the muscle between the fibular and tibia.