The knee joint (latin: articulatio genus) is a hinge joint that is formed between three bones: the femur, the tibia and the patella. The knee joint is the most complicated and one of the strongest joints in the human body.
The knee joint is composed of two articulations:
- Tibiofemoral articulation, where the articular surface of the medial and lateral condyles of the femur articulate with the articular surface of the superior medial and lateral condyles of the tibia.
- Patellofemoral articulation, where the patellar surface of the femur articulates with the articular surface of the patella.
The surfaces forming the knee joint are covered with a thin layer of forbrocartilage, each known as the meniscus. There are two menisci in the knee joint - the medial meniscus that is semicircular in shape, and the lateral meniscus that is almost circular. The menisci provide a smooth surface and protect the bones from damage.
The knee joint is surrounded by the articular capsule that provides strength and lubrication. The synovial membrane fills the hollow space between the bones, it produces an oily synovial fluid that lubricates the knee to reduce friction. The synovial membrane forms several bursa around the knee joint. A bursa is a synovial fluid filled sac between moving structures in a joint. There are four bursae that communicate with the cavity of the knee joint, which are the following:
- The anserine bursa - separates the pes anserinus from the tiba and the tibial collateral ligament.
- The suprapatellar bursa - between the femur and the tendon of the quadriceps femoris muscle
- The gastrocnemius bursa - anteriorly of the tendon of the medial head of the gastrocnemius.
- The popliteus bursa - between the tendon of the popliteus muscle and the lateral condyle of the tibia.
Oher bursae of the knee joint are the following:
- The subcutaneous prepatellar bursa - between the skin and anterior surface of the patella.
- The subtendinous prepatellar bursa - situated deep to the tendon.
- The subfascial prepatellar bursa - situated deep to the fascia.
- The subcutaneous infrapatellar bursa - between the skin and the tibial tuberosity.
- The deep infrapatellar bursa - between the patellar ligament and the anterior surface of the tibia.
The knee joint is fastened by fibrous connective tissue, ligaments that hold the joint in place. There are two groups of ligaments stabilizing the knee joint: the intracapsular and extracapsular ligaments.
There are three intracapsular ligaments:
- The transverse ligament of the knee (latin: lig.transversum genus) is anteriorly between the medial and lateral menisci.
- The anterior cruciate ligament (latin: lig. cruciatum anterius) extends from the inner surface of the lateral condyle of the femur to the anterior intercondylar area of the tibia.
- The posterior cruciate ligament (latin: lig. cruciatum posterius) extends from the lateral surface of the medial condyle of the femur to the posterior intercondylar area of the tibia.
The extracapsular ligaments are continuous with the outer surface of the knee joint capsule. There are seven extracapsular ligaments in the knee joint:
- The tibial collateral ligament (latin: lig. collaterale tibiale) passes from the medial epicondyle of the femur to the medial condyle and surface of the body of the tibia.
- The fibular collateral ligament (latin: lig. collaterale fibulare) passes from the lateral epicondyle of the femur to the lateral side of the head of the fibula.
- The oblique popliteal ligament (latin: lig.popliteum obliquum) is on the posterior surface and extends from the lateral condyle of the femur to the medial condyle of the tibia.
- The arcuate popliteal ligament (latin: lig. popliteum arcuatum) is on the posterior surface and passes from the head of the fibula to the oblique popliteal ligament.
- The medial patellar retinaculum (latin: retinaculum patellae mediale) is made up by the fibers of the vastus medialis.
- The lateral patellar retinaculum (latin: retinaculum patellae laterale) is formed by the fibers of the vastus lateralis.
- The patellar ligament (lig.patellae) is the continuation of the tendon of the quadriceps femoris muscle.
The knee joint permits four movements: flexion and extension, external and internal rotation of the leg.