Along with the muscles of the lower leg and thigh, the knee joint is part of an intricate cable pulley system that is the basis for athletic locomotion. A well-defined axis of rotation allows the knee to bend and extend a full 150 degrees, a range of motion that doesn’t get exploited unless under extreme circumstances.
In sports, the knee is the most victimized joint. Biomechanically, muscle imbalances in the thigh can pull the kneecap one way or another; improper alignment in the foot or ankle can increase the rotation of the leg bone, placing additional strain on the knee. When it comes to injuries sustained from external forces, the knee is particularly unforgiving. Its important fibrocartilage discs, the menisci and the cartilage behind the kneecap itself are very difficult to replace. The ligaments and cartilage inside the knee are poorly nourished due to an inadequate blood supply and don’t regenerate or heal efficiently on their own.
The knee is essentially two joints, the patellofemoral joint and the tibiofemoral joint. The patellofemoral joint is where the patella, or knee cap, articulates with the femur, or thigh bone. The patella has a ridge along its undersurface which rides up and down, like a train on a track, in a groove in the femur called the trochlea as knee is bent and straightened. The function of the patella is to act as a pulley for the large, powerful quadriceps muscles of the thigh, increasing their mechanical advantage. The tibiofemoral joint is the larger joint of the knee. The articulation itself is not very stable, so the ligaments and tendons that run over, through, and around the knee perform the stabilizing work. Additionally, the menisci impart some stability to the joint as well.
Knee problems occur because of biomechanics, impact injuries, age and overuse. If the patella is misaligned, either as a result of a congenital problem, poor technique, muscle imbalance or poor mechanics in the foot it will cause chronic knee pain and inflammation. The knee is designed to follow a fairly rigid axis of rotation that allows it to bend and straighten, but not much else. The side-by-side rotation is minimal and any force, whether extrinsic, such as a tackle to the side of the leg, or intrinsic, such as twisting the knee while stepping into a ditch, may result in damage to the ligaments and cartilage. Both the back of the kneecap and the ends of the tibia and femur are lined with articular cartilage that can erode or wear away, allowing the exposed bone beneath to eventually become chipped, jagged and eroded. In addition, age affects the amount of synovial fluid, or joint lubricant, produced by the joint capsule of the knee.
A spare, ingenious design provides the healthy knee with the stability to channel great amounts of force through the lower extremity muscles. Named the cruciate ligaments because they cross each other in the knee joint, these are the most important stabilizers of the knee. The anterior cruciate ligament (ACL) crosses from the front of the tibia to the back of the femur and prevents the tibia from sliding forward on the femur. The posterior cruciate ligament (PCL) runs from the back of the tibia to the front of the femur and keeps the tibia from sliding backward on the femur. The medial and lateral collateral ligaments (MCL and LCL) add stability to the sides of the tibiofemoral joint.
The menisci are disc-like structures made of fibrocartilage that sit between the femur and tibia. There are two menisci in each knee, the medial and lateral, and they serve several important functions. First they decrease the compressive forces on the knee by spreading out on impact, thereby protecting the underlying articular cartilage. Second, they help stabilize the knee and lubricate the knee joint by spreading synovial fluid.
The quadriceps tendon connects the quadriceps muscle to the top of the patella. The patellar tendon is a strong strip of tissue that runs from the kneecap to the tibia. It feels like a strip of lead about three inches long and acts as the terminal connection between the thigh muscles and the tibia.
The cavity in the back of the knee contains numerous important nerves and blood vessels called the popliteal fossa. It’s the juncture of the hamstrings which control bending of the knee. The top portion of the gastrocnemius also attaches in this area. The knee is not a joint that recovers well or quickly. Prevention, conditioning, and adequate recovery time are all essential for the joint’s proper functioning.
Patellofemoral pain syndrome also known as kneecap pain, runner’s knee, chondromalacia, and maltracking patella is as threatening as it sounds. In most cases it’s not a major problem and can be controlled.
Wherever two bones come together to form a joint, the parts of the bones that contact each other are lined with articular, or hyaline, cartilage. This is the type of cartilage seen on the white, shiny ends of a drumstick. It’s different than the fibrocartilage that comprises the meniscus. The function of the articular cartilage is to absorb shock and protect the underlying bone, which, when damaged even slightly, can be very sensitive and painful. The good news about cartilage is that it can absorb force without hurting. The bad news is that cartilage cannot repair itself because it does not have a good blood supply so it slowly wears away and often with age and overuse. Major injuries to cartilage mean major problems with regard to movement. When it comes to the patella, however, injuries to the cartilage often present only a minor, nagging problem.