Knee pain is an extremely common complaint among people at every stage of life.  The knee is a central pivot area of the leg and, therefore, it bears the brunt of force, especially with sporting activities.  As we age, mild degenerative changes may accentuate the normal process causing injuries to occur more easily.  It is important to keep your knee strong and flexible to maintain its overall health and avoid injury.

 

The knee joint is a very complex structure in which all the parts have to work together to maintain normal function.  It depends almost completely on its surrounding ligaments for stability.  It is more likely to be injured or affected by arthritis than any other joint in the body.  Understanding the anatomy of the knee joint will help you better understand what happens when problems occur.

 

Anatomy

 

The knee joint may look like a simple joint, but it is one of the most complex.  When we move our knee, it does not just bend and straighten, (flex and extend) but there is also a slight rotational motion.  The knee joint must support the body’s full weight when standing, and much more then that when walking or running.

 

The ends of the thighbone (the femur) and the shinbone (the tibia), two important bones in the leg, come together and form the knee joint.  The kneecap (the patella) sits in front of the knee.  Two rounded areas on the end of the femur, called the femoral condyles, form a groove (the trochlea) through which the patella glides with knee flexion and extension.  The smaller bone on the outside of the lower leg (the fibula) never enters the knee joint, but it does have a small joint that connects it to the tibia.  Normally, this joint has very little motion.  The knee joint is a synovial joint, enclosed by a capsule that contains a fluid, called synovial fluid, which lubricates the joint.

 

Articular cartilage covers the ends of bones of any joint.  It is a white, shiny material that is about a ¼-½ inch thick in most large joints.  It allows surfaces to glide over one another without damaging either surface.  Articular cartilage functions to absorb shock and provide a slippery smooth surface to facilitate motion.  In the knee, articular cartilage covers the ends of the femur, the back of the patella and the top of the tibia.

 

Ligaments are strong bands of tissue that connect ends of bones together.  The knee joint has four main ligaments.  Two are found on either side of the knee joint.  They are the medial collateral ligament (MCL) and the lateral collateral ligament (LCL).  Inside the knee joint, the other two important ligaments stretch between the tibia and femur:  the anterior cruciate ligament (ACL) in the front, and the posterior cruciate ligament (PCL) in the back.  The MCL and LCL prevent excessive side-to-side motion in the knee, while the ACL and PCL control the front-to-back motion of the knee joint.  The ACL stops the tibia from sliding too far forward from the femur.  The PCL keeps the tibia from sliding too far backward from the femur.  All the ligaments together are the most essential structures controlling knee stability.

 

The menisci, both medial and lateral, are two special types of cartilage that sit between the femur and tibia.  The two menisci of the knee are important because they distribute the force from the weight of the body over a larger area and help with the stability of the knee.  Without the menisci, any weight on the femur would concentrate to one area on the tibia.  But with the menisci, the weight is spread out across the entire tibial surface.  This is important because it protects the articular cartilage on the ends of the bones from high forces and damage, which lead to degeneration of the joint over time.

 

Tendons are similar to ligaments, except that tendons attach muscles to bone.  The patellar tendon connects the patella to the tibia.  This tendon runs over the patella and continues in the thigh as the quadriceps tendon, which attaches to the quadriceps muscles in the front of the thigh.  Together, these two tendons allow you to control bending and straightening your knee.  The hamstring muscles on the back of the thigh also have tendons that attach in different places around the knee joint.