The knee is responsible for joining together the leg and the thigh. Like the shoulder, the knee is a joint. The knee is a huge joint that lets the leg and thigh extend and flex, though it is actually made of two different joints, the tibiofemoral joint and the patellofemoral joint. The tibiofemoral joint is located in between the tibia and femur while the patellofemoral joint is located in between the patella and the femur.
The femur, patella, and tibia are all bones within the knee. Meanwhile, the muscles found within the knee include a variety of flexors and extensors. Flexors include the biceps femoris, the plantaris, the semitendinosus and the semimembranosus. Extensors include the articularis genus, the rectus femoris, and the quadriceps femoris.
That’s the short version of what is found in the knee, but a more in-depth examination of the knee’s structure and function, as well as the bones and muscles that comprise it will help reveal the critical role the knee plays in the movement of the body.
Three Knee Bones
The femur is also known as the thigh bone, and it runs the length of the thigh ending at the knee. The distal portion of the femur interlocks with the kneecap and tibia creating the knee joint. The femur is one of the strongest and longest bones in the entire human body, but it is the only bone found within the upper leg. The upper portion of the femur is near the torso and it contains the neck, head, and trochanters of the femur. Meanwhile, the lower portion of the femur is referred to as the distal extremity, and it is linked to the tibia via the tibial collateral ligament of the knee joint.
The tibia is also referred to as the shank bone or shinbone. It is one of two bones within the lower leg, and it is the larger and stronger of the two bones (the other bone is the fibula). The tibia links the knee ankle bones with the knee. The upper end of the tibia is linked to the knee via number of ligaments, a bundle of fibrous connective tissue that joins bones together.
The patella Is also referred to as the kneecap, and it is a big piece of bone that fits over the other portions of the knee, protecting the anterior articular portion of the knee joint. The patella is triangular-shaped, and the tendon from the quadriceps femoris muscle is attached to the bottom of the patella. Other muscles, such as the vastus media lists and a vastus lateralis are bonded to the lateral and medial portions of the patella. The vastus intermedius muscle is attached to the patella’s base. The top 75% of the patella is articulated by the femur. The entire articular surface of the patella is covered by cartilage, which helps to protect the patellofemoral joint from the stress of movement.
The Knee Muscles
The muscles of the knee are either extensors or flexors, and they belong to one of three muscle groups. The three groups are the posterior compartment, the medial compartment and the anterior compartment of the thigh. In general, the flexors belong to the posterior compartment while the extensors belong to the anterior compartment, though there are some exceptions.
The extensor muscles include: the articularis genus, which is found at the distal end of the femoral shaft. The quadriceps femoris, which is made out of the combined vastus and rectus femoris muscles. The rectus femoris, which starts near the spine of the anterior inferior iliac. There are also the vastus media list, the vastus intermedius, and the vastus lateralis. These three vastus muscles originate at some point on the femur.
Knee joint pictured from behind. Condyles and interior ligaments shown. Photo: Henry Vandyke Carter – Henry Gray (1918) Anatomy of the Human Body, licensed under CC0
The flexors of the knee include: the biceps femoris, which originates from the femur, the Semitendinosus and the Semimembranosus, which both originate from the tuberosity of the ischium. The gastrocnemius, which originates from the lateral and medial condyle of the femur. The plantaris, which originates at the femur’s lateral supracondylar ridge. The popliteus which originates at the middle facet of the lateral surface. The gracilis, which starts at the inferior pubic ramus.
The exception to the rule that extensors belong to the anterior compartment and flexors to the posterior compartment are the gracilis and the sartorius. The gracilis is a flexor that belongs to both the medial compartment. The sartorius is a flexor that belongs to the anterior compartment.
Photo: By Henry Vandyke Carter – Henry Gray (1918) Anatomy of the Human Body (See “Book” section below)Bartleby.com: Gray’s Anatomy, Plate 552, Public Domain, https://commons.wikimedia.org/w/index.php?curid=541417
The muscles of the knee must receive a constant blood supply, which they receive through a network of arteries that runs through the knee. The popliteal artery and the femoral artery together make up the plexus, or the arterial network, which is a network of arteries that surround the knee. The arterial network has six branches. There are two inferior genicular arteries, two superior genicular arteries, a descending genicular artery, and an anterior tibial artery that has a recurrent branch. Most of these arteries surround the knee joint, but the medial genicular arteries actually run through the knee joint.
Format Of The Knee
The knee is a type of hinge joint referred to as a synovial joint, and it is made out of three different functional parts. These functional parts of the patellofemoral articulation, the patellar groove, and the lateral and medial tibiofemoral articulations. The patellofemoral articulation is made out of the patella, and the patellar groove is located on the front of the femur. Meanwhile, the lateral and medial tibiofemoral articulations join together the thigh bone with the tibia.
Knee bursae shown. Photo: Henry Vandyke Carter – Henry Gray (1918) Anatomy of the Human Body, Public Domain
The femur has two main articular bodies, the medial condyle and the lateral condyle, which diverge somewhat in both posterior and distal directions. There are transverse axes on the articular bodies that let the knee perform rolling and sliding motions from side to side, letting the knee rotate to a limited degree. The articular capsule is a fibrous membrane with fatty deposits in it that is located on the femur a little distance away from the cartilage. It helps prevent of the suprapatellar bursa, fluid-filled sacks around the knee joint, from being squeezed during extension of the knee by the articularis genus muscle.
Menisci are discs that partially divide up the joint space in the knee. There are two discs in the knee, the lateral meniscus and the medial meniscus. They are discs of connective tissue that have collagen fibers running through them. The weaker fibers link menisci to each other, while the stronger fibers link the menisci to surrounding structures. The two menisci are fused with the synovial membrane, but are capable of some movement as they can shift over the surface of the tibia. The function of the menisci is to protect the bones by preventing the two bones from rubbing against one another and being degraded. If the knee sustains a substantial, forceful bend or rotation the menisci may be torn or cracked.
Function Of The Knee
Photo: Mysid via Wikimedia Commons, Public Domain, CC0
To enable locomotion, the knee must be able to flex and extend and it must be able to rotate to a certain extent as well. Fortunately, the knee is designed in such a way that it can rotate slightly on medial and lateral axes. The design of the knee also enables extension and flexion on a transverse axis. While many portions of the knee don’t move, the lateral meniscus and femur slide over the tibia during the rotation of the knee, and during flexion or extension the femur slides over both of the menisci.
The transverse axis centers around where the cruciate ligaments and collateral ligaments intersect with one another. This position moves backwards and upwards during the flexion of the knee.
When the knee is in an extended position, the medial and lateral collateral ligaments and the anterior cruciate ligament are pulled tight. Meanwhile, in the flexed position the cruciate ligaments are pulled tight while the collateral ligaments are relaxed.
Damage To The Knee
Various traumatic injuries, degenerative conditions such as arthritis, and misalignments can cause pain in the knee.
Patellofemoral syndrome is one of the most common knee disorders, and it can often be treated with rest and ice and physical therapy, although more severe cases of patellofemoral syndrome may require surgery. Patellofemoral syndrome may manifest in different ways to, depending on the exact form of the damage the knee has sustained. Inflammation between the trochlea and the patella often causes pain, and it is one form of the syndrome. A dislocation, slipped joint, or tear in the knee can lead to problems maintaining balance. Patellofemoral syndrome can cause either balance issues, pain, or both.
Another common type of knee injury is a condition known as prepatellar bursitis or housemaids knee, frequently seen in individuals who have occupations involving frequent kneeling, such as roofing. Beyond this, degradation of the cartilage and other tissues around the knee frequently appears with age, leading to osteoarthritis.