The knee is the largest joint in the body. It is built for weight bearing, stability and mobility. The knee complex is composed of four bones and three osseous bone-to-bone joints. These are the patellafemoral joint, tibiofemoral joint, and the tibiofibular joint.
When the knee is in its fully extended position it depends on its ligaments, meniscus, and mechanical screw home mechanism for stability. When it is in the bent position it is unstable but allows for movement in several planes.
Bone and Joint
The tibiofemoral joint is a hinge joint, located between the largest bone in the body (the femur) and the larges bone in the lower leg (the tibia.) These two bones are joined together and form a medial and lateral compartment.
The second joint in the knee complex is located between the patella (knee cap) and the femur. The patella glides up and down a groove on the anterior distal aspect of the femur called the patellofemoral joint.
There is also a joint between the small lateral bone of the lower leg (fibula) and the larger tibia.
The osseous joint surface all have articular cartilage that covers the ends of the bones. The articular cartilage has a smooth and shiny surface, which allows the ends of the bones to slide freely over each other.
Common conditions of these joints include knee osteoarthritis, rheumatoid arthritis, tibial plateau fractures, patellofemoral syndrome, degenerative joint disease and chondromalacia.
The muscles surrounding the knee function to both move and stabilize the joint. The two main muscle groups are the quadriceps on the anterior side of the knee and femur, and the hamstrings on the posterior side.
The four muscles of the quadriceps: vastus lateralus, vastus medialus, vastus intermedius and rectus femoris function to extend the knee. The muscles join together to form the common quadriceps tendon. Tendons are part of the muscle, and attach muscle to bone. Within the quadriceps tendon is the patella (knee cap.) The patella is a sesamoid bone, which provides increased leverage to the quadriceps muscle to improve its efficiency.
The three posterior hamstring muscles: biceps femoris, semitendinosis, and semimembranosis function to decelerate, stabilize and bend the knee joint, and attach to the posterior part of the tibia and fibula.
There are two other important muscles of the knee complex. The gastrocnemius pushes the foot down (plantar flexes) and helps bend the knee. The popliteus helps unlock the knee from a straightened or extended position. Two adductor muscles, the adductor magnus and gracilis, cross the knee joint and help rotate the leg and can be a source of inflammation.
Common conditions of the muscles and tendon of the knee include patella tendinitis, ruptured quadriceps tendon, quadriceps strain, hamstring strain, torn hamstring, muscle strain, gastrocnemius tear and gastrocnemius strain.
Ligaments and Meniscus
Ligaments are soft tissue structures that attach bone to bone. The primary job of a ligament is to provide stability to a joint. There are four main ligaments of the knee.
Located on the inner aspect of the knee is the medial collateral (MCL) ligament, and on the outside is the lateral collateral ligament (LCL). These ligaments provide medial and lateral stability to the knee.
Located on the inside of the knee are the anterior cruciate ligament (ACL), and posterior cruciate ligament (PCL). They provide anterior, posterior and rotatory stability to the knee.
Two other soft tissue structures within the knee are the medial and lateral meniscus. These structures are attached to the tibia, and provide added stability and cushioning to the knee joint. They are cartilaginous in nature and when injured can be referred to as a “torn cartilage”.
Common conditions for this aspect of the knee include ACL tear, ACL insufficiency, PCL tear, medial meniscus tear, lateral meniscus tear, meniscus tear, unhappy triad, meniscectomy, ACL reconstruction and PCL reconstruction.
A bursa is a fluid filled sac that decreases friction between two tissues.The knee is surrounded by several bursa.The largest bursa in the body is the suprapatella bursa and will often fill with fluid when the knee is injured. This may be referred to as “water on the knee”.
The most commonly injured bursa are the prepatella bursa, which is in front of the patella and the pes anserinus bursa, which is located at insertion of the three medial muscles of the knee.
Common conditions of the bursa include prepatella bursitis, pes anserinus bursitis, “fluid on the knee”.
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