The knee joint is a synovial joint that connects three bones; the femur, tibia and patella. It is a complex hinge joint composed of two articulations; the tibiofemoral joint and patellofemoral joint. The tibiofemoral joint is an articulation between the tibia and the femur, while the patellofemoral joint is an articulation between the patella and the femur.
The knee joint is the largest and arguably the most stressed joint in the body. The arrangement of the bones in the joint provides a fulcrum that translates the actions of the flexor and extensor muscles of the knee. The arrangement of the extracapsular and intracapsular and ligaments, as well as extensions of muscles that cross the joint, provide the much needed stability that counters the considerable biomechanical stress brought upon the joint. As a hinged joint, the knee joint mostly allows movement along one axis in terms of flexion and extension of the knee in the sagittal plane. It also allows slight medial rotation during flexion and the last stage of extension of the knee, as well as lateral rotation when “unlocking” the knee.
Tibiofemoral joint: Synovial hinge joint; uniaxial
Patellofemoral joint: Plane joint
Tibiofemoral joint: lateral and medial condyles of femur, tibial plateaus
Patellofemoral joint: patellar surface of femur, posterior surface of patella
|Ligaments and Menisci||
Extracapsular ligaments: patellar ligament, medial and lateral patellar retinacula, tibial (medial) collateral ligament, fibular (lateral) collateral ligament, oblique popliteal ligament, arcuate popliteal ligament, anterolateral ligament (ALL)
Intracapsular ligaments: anterior cruciate ligament (ACL), posterior cruciate ligament (PCL), medial meniscus, lateral meniscus
|Innervation||Femoral nerve (nerve to vastus medialis, saphenous nerve) tibial and common fibular (peroneal) nerves, posterior division of the obturator nerve|
|Blood supply||Genicular branches of lateral circumflex femoral artery, femoral artery, posterior tibial artery, anterior tibial artery and popliteal artery|
|Movements||Extension, flexion, medial rotation, lateral rotation|
This article will discuss the anatomy and function of the knee joint.
- Articular surfaces
- Joint capsule
- Ligaments and menisci
- Blood supply
- Muscles acting on the knee joint
- Clinical significance
The tibiofemoral joint
The tibiofemoral joint is an articulation between the lateral and medial condyles of the distal end of the femur and the tibial plateaus, both of which are covered by a thick layer of hyaline cartilage.
The lateral and medial condyles are two bony projections located at the distal end of the femur, which have a smooth convex surface, and are separated posteriorly by a deep groove known as the intercondylar fossa. The medial condyle is larger, more narrow and further projected than its lateral counterpart, which accounts for the angle between the femur and the tibia. The roughened outer surfaces of the medial and lateral condyles are defined as medial and lateral epicondyles, respectively. Along the posterior aspect of the distal femur, there are paired rough elevations above the medial and lateral epicondyles known as the medial and lateral supracondylar ridges.
The tibial plateaus are the two slightly concave superior surfaces of the condyles located at the proximal end of the tibia, and are separated by a bony protuberance known as the intercondylar eminence. The medial tibial articular surface is somewhat oval shaped along its anteroposterior length, while the lateral articular surface is more circular in shape
The articular surfaces of the tibiofemoral joint are generally incongruent, so compatibility is provided by the medial and lateral meniscus. These are crescent-shaped fibrocartilaginous structures that allow a more even distribution of the femoral pressure on the tibia.
The patellofemoral joint
The patellofemoral joint is a saddle joint formed by the articulation of the patellar surface of femur (also known as the trochlear groove of femur) and the posterior surface of patella. The patellar surface of femur is a groove on the anterior side of the distal femur, which extends posteriorly into the intercondylar fossa.
The patella is a triangular shaped bone, with a curved proximal base and a pointed distal apex. Its articular surface is defined by medial and lateral facets which are concave articular surfaces covered with a thick layer of hyaline cartilage and separated by a vertical ridge. Medial to the medial facet is a third minor facet, known as the ‘odd’ facet which lacks hyaline cartilage.
Being a sesamoid bone, the patella is tightly embedded and held in place by the tendon of the quadriceps femoris muscle. On the distal part of the patella, an extension of the quadriceps femoris tendon forms a central band called the patellar ligament. It is a strong, thick ligament that extends from the patellar apex to the superior area of the tibial tuberosity.
The joint capsule of the knee joint is one of a composite nature, mainly formed by muscle tendons and their expansions, forming a thick ligamentous sheath around the joint. The capsule is relatively weak and attached to the margins of the femoral and tibial articular surfaces. The anterior portion of the capsule features an opening, whose margins attach to the borders of the patella. A second gap is also present in the lateroposterior portion of the capsule to give passage to the tendon of the popliteus muscle.
The capsule is formed from an outer fibrous layer (which is continuous with adjacent tendons) and an inner synovial membrane that lubricates the articular surfaces, reducing friction in addition to providing nourishment to the cartilage. The joint capsule forms several fluid filled pouches called bursae, that reduce friction within the knee joint. Notable bursa of the knee joint include the:
- Suprapatellar bursa - Located superior to the patella between the femur and the tendon of the quadriceps muscle
- Prepatellar bursa- Lies in front of the patella between the patella and the skin
- Infrapatellar bursa - Located under the patella between the patellar ligament and the tibia
Ligaments and menisci
The ligaments of the knee joint can be divided into two groups; extracapsular ligaments and intracapsular ligaments. These ligaments connect the femur and tibia, holding them in place, providing stability, and preventing dislocation.
Extracapsular ligaments are found outside the joint capsule and include the patellar ligament, fibular (lateral) and tibial (medial) collateral ligaments, and oblique and arcuate popliteal ligaments. Intracapsular ligaments are found inside the joint capsule, with the cruciate ligaments being the most well known of this subgroup.
The patellar ligament is a strong, thick fibrous band that is a distal continuation of the quadriceps femoris tendon. It is found superficial/anterior to the infrapatellar bursa and extends from the apex of patella to the tibial tuberosity.
Along its outer margins, the patellar ligament blends with the medial and lateral patellar retinacula, which are extensions of the vastus medialis and lateralis muscles, respectively, as well as the overlying fascia. The patellar ligament plays a major role in stabilizing the patella and preventing its displacement.
Lateral (fibular) collateral ligament
The fibular collateral ligament is a strong ligament that originates from the lateral epicondyle of the femur, just posterior to the proximal attachment of the popliteus, and extends distally to attach on the lateral surface of the fibular head.
As it attaches to the fibular head, the ligament splits the tendon of biceps femoris muscle in two. The fibular collateral ligament is found deep to the lateral patellar retinaculum, and superficial to the tendon of popliteus muscle, which separates the ligament from the lateral meniscus.
Medial (tibial) collateral ligament
The tibial collateral ligament is the strong, flat ligament of the medial aspect of the knee joint. The tibial collateral ligament, in addition to its fibular counterpart, acts to secure the knee joint and prevent excessive sideways movement by restricting external and internal rotation of the extended knee. The tibial collateral ligament is sometimes divided the literature into superficial and deep parts:
- Superficial part: originates just proximal the medial epicondyle of the femur. This ligament has two attachment points; a proximal attachment on the medial condyle of the tibia, and a distal attachment on the medial shaft of the tibia. Anteriorly, the superficial part blends with the medial patellar retinaculum and the medial patellofemoral ligament, which courses from the medial femoral condyle to attach onto the medial border of the patella.
- Deep part: a vertical thickening of the knee joint capsule found underneath the superficial part of the tibial collateral ligament. It originates from the area of the distal femur, then attaches to the medial meniscus, and terminates on the proximal tibia. These two parts of the ligament are defined as meniscofemoral and meniscotibial ligaments (see below).
Oblique popliteal ligament
The oblique popliteal ligament (Bourgery ligament) is an expansion of the semimembranosus tendon which originates posterior to the medial tibial condyle and reflects superiorly and laterally to attach on the lateral condyle of the femur.
As it spans the intercondylar fossa, the oblique popliteal ligament reinforces the posterior part of the joint capsule and blends with its central portion.
Arcuate popliteal ligament
Arcuate popliteal ligament is a thick, fibrous band that arises on the posterior aspect of the fibular head and arches superiorly and medially to attach on the posterior side of the joint capsule of the knee. The arcuate popliteal ligament reinforces the posterolateral part of the joint capsule, and together with the oblique popliteal ligament, prevents overextension of the knee joint.
The paired cruciate ligaments got their name due to the fact that they cross each other obliquely within the joint in a way that resembles a cross (latin = crux), or a letter X. They cross within the joint capsule, however remain external to the synovial cavity. The cruciate ligaments are divided as follows:
- Anterior cruciate ligament: arises from the anterior intercondylar area of the tibia just behind the attachment of the medial meniscus, and extends posterolaterally and proximally to attach on the posterior part of the medial surface of the lateral femoral condyle. As it crosses to the other side of the knee joint, the ligament passes underneath the transverse ligament (see below) and blends with the anterior horn of the lateral meniscus. The anterior cruciate ligament is important to prevent posterior rolling and displacement of the femoral condyle during flexion, as well as to prevent hyperextension of the knee joint.
- Posterior cruciate ligament: arises from the posterior intercondylar area of the tibia and extends anteromedially and proximally to attach on the anterior part of the lateral surface of the medial femoral condyle. This ligament is almost twice as strong and has better blood supply than the anterior cruciate ligament. The posterior cruciate ligament has the opposite function of the anterior cruciate ligament, serving to prevent anterior rolling and displacement of the femoral condyle during extension, as well as to prevent hyperflexion of the knee joint.
The menisci are fibrocartilaginous crescent-shaped plates found between the articular surfaces of the femur and tibia and serve to provide their congruence and shock absorption. The menisci are thick and vascularized in their outer one third, while their inner two thirds are thinner and avascular. Additionally, the inner two thirds contain radially organized collagen bundles, whereas the outer third contains larger circumferentially arranged bundles. Thus, it is believed that the inner portion is more adapted for weight-bearing and resisting compressive forces, while the outer portions are suited for resisting tensional forces. The menisci are divided as follows:
- Medial meniscus: a C-shaped, almost semicircular fibrocartilaginous plate that overlies the surface of the medial tibial plateau. Its anterior horn attaches on the anterior intercondylar area of tibia and blends with the anterior cruciate ligament. Its posterior horn is attached to the posterior intercondylar area of the tibia, between the attachments of the lateral meniscus and the posterior cruciate ligament.
- Lateral meniscus: an almost circular fibrocartilaginous plate that overlies the surface of the lateral tibial plateau. Its anterior horn also attaches to the anterior intercondylar area of tibia and partially blends with the anterior cruciate ligament. Similarly, its posterior horn attaches to the posterior intercondylar areas anterior to the posterior horn of the medial meniscus.
The menisci are held in place by several ligaments, including the transverse ligament, meniscofemoral ligaments and meniscotibial (coronary) ligaments. By stabilizing the menisci, these ligaments are also indirectly involved in preventing displacement of the knee joint.
- Transverse ligament: connects the menisci anteriorly extending from the anterior margin of the lateral meniscus to the anterior horn of the medial meniscus. Its exact role is uncertain but it is thought that this ligaments stabilizes the menisci during knee movements and decrease tension generated in the longitudinal circumferential fibres.
- Meniscofemoral ligaments: are the superior portions of the distal tibial collateral ligament extending from the posterior horn of the lateral meniscus to the lateral aspect of the medial femoral condyle. They are divided into two ligaments; an anterior meniscofemoral ligament (of Humphry) that courses anterior to the posterior cruciate ligament; and a posterior meniscofemoral ligament (of Wrisberg), that runs posterior to the posterior cruciate ligament.
- Meniscotibial (coronary) ligaments: are the inferior portions of the distal tibial collateral ligament, extend between the margin of the lateral meniscus and the peripheral area of the tibial condyles. It is comprised of a medial and lateral meniscotibial (coronary) ligament.
- Patellomeniscal ligament: comprised of a medial and lateral patellomeniscal ligament, often described as simply medial and lateral ligaments. These ligaments run from the inferior third of the patella to insert on the anterior portion of the medial and lateral meniscus, respectively.
Some other notable ligaments of the knee joint include ligamentum mucosum, popliteofibular and fabellofibular ligament.
- Ligamentum mucosum: consists of two alar folds that attach onto the infrapatellar fat pad, holding it in position. This structure is an embryonic remnant that separates the medial and lateral compartments of the knee.
- Popliteofibular ligament: located on the posterolateral aspect of the knee joint, extending from the popliteus muscle to the medial aspect of the fibula.
- Fabellofibular ligament: arises from a small sesamoid bone on the posterior aspect of the lateral supracondylar ridge of the femur and inserts distally on the posterolateral edge of the styloid process of the fibula.
The knee joint receives innervation from the femoral nerve, via the saphenous nerve and muscular branches. The joint also receives contributions from the tibial and common fibular (peroneal) nerves, and the posterior division of the obturator nerve.
The knee joint has a rich vascularization stemming from the genicular anastomosis formed by several arterial blood vessels. There are approximately ten arteries involved in the formation of the genicular anastomosis:
- Descending branches: Descending branch of the lateral circumflex femoral artery, descending genicular branch of the femoral artery.
- Ascending branches: circumflex fibular branch of the posterior tibial artery, anterior and posterior tibial recurrent branches of the anterior tibial artery.
- Branches of the popliteal artery: lateral superior and inferior genicular arteries, the medial superior and inferior genicular arteries, and the middle genicular arteries.
In recent years however, a study by Sabalbal et al. has rejected the traditional textbook representation of the genicular anastomosis described above. During their study of ten cadaver lower limbs, they found that “robust direct communications between branches of the femoral and popliteal arteries did not exist”, and that vasculature of this region was subject to a interindividual variation.
Being a hinge joint, the main movements in the knee joint are flexion and extension of the knee in the sagittal plane. It also allows limited medial rotation in a flexed position and in the last stage of extension, as well as lateral rotation when “unlocking” and flexing the knee. Unlike the elbow joint, the knee joint is not a true hinge since it has a rotational component, an accessory motion that accompanies flexion and extension, hence it is termed as a modified hinge joint.
The degree of possible knee flexion depends on the position of the hip joint and whether the movement is active or passive. When the hip is flexed, a maximum degree of flexion of 140° is achievable in the knee joint, whereas an extended hip allows for only 120°. This is due to the fact that the hamstrings are both extensors of the hip and flexors of the knee, so they lose some of their efficiency to flex the knee if the hip is extended, and vice versa. In addition, a wider range of motion in the knee joint is achieved with passive flexion of the knee, increasing it to 160°. The contact of the posterior leg (calf) with the thigh is the major limiting factor of flexion of the knee. In addition, the capsular pattern of the knee joint, in terms of most restriction, is flexion and extension to a lesser degree.
As you can see, the knee joint is deceptively simple but full of complex anatomy. Instead of passively reading and re-reading about it, start using active recall to cement the details and review it more efficiently!
During movement of the knee from flexion to extension, the femoral condyles roll and glide posteriorly over the tibial plateaus owing to their greater articular surface area. The posterior gliding motion is important because without it, the femur would simply roll off the tibia before full extension is complete. Additionally, as the articular surface of the lateral femoral condyle is less than its medial counterpart, the posterior gliding of the medial condyle during the last degrees of extension results in medial rotation of the femur on the tibia.
During the last few degrees of extension, the femoral condyle rotates medially on the tibial plateau “locking” the knee. This is called the the “screw-home mechanism”, which allows for prolonged weight-bearing without the help from the muscles of the knee. The full extension of the knee along with the medial rotation of the femoral condyle put the knee in a so called “close packed position”, in which there are no additional movements possible. In order to flex the knee again, the knee must be “unlocked” by the contraction of the popliteus, which causes lateral rotation of the femur on the tibia. The flexion of the knee puts it in the “open packed position”, which is less stable and has more laxity than the “close packed position”.
In the patellofemoral joint, the main motion is gliding of the posterior surface of the patella over the patellar surface of the femur as far as the intercondylar notch. The main function of the patella is to provide a larger moment arm for the quadriceps femoris muscle, which is the distance between the axis of the muscle and the center of the joint. It does so by acting like a pulley for the quadriceps femoris, increasing its mechanical advantage and providing greater angular force.
Some accessory movements in the knee joint are possible, but depend on the position of the knee. For example, adduction and abduction is prevented by the interlocking of the femoral and
tibial condyles, as well as collateral ligaments and cruciate ligaments when the knee is fully extended. On the other hand, when the knee is slightly flexed, limited adduction and abduction are possible. Motions in the longitudinal axis, as in medial and lateral rotation, are similarly affected by the amount of joint flexion, and are possible if the knee joint is slightly flexed.
For more videos, articles, illustrations and quizzes explaining the knee joint, take a look below:
Muscles acting on the knee joint
|Flexion||Biceps femoris, semitendinosus and semimembranosus; initiated by popliteus; assisted by gracilis and sartorius|
|Extension||Quadriceps femoris (rectus femoris, vastus lateralis, vastus medialis and vastus intermedius) assisted by tensor fasciae latae|
|Medial rotation||Popliteus, semimembranosus and semitendinosus, assisted by sartorius and gracilis.|
|Lateral rotation||Biceps femoris|
The primary extensor of the knee joint is quadriceps femoris, assisted by the tensor fasciae latae. Quadriceps femoris of four muscle bellies; rectus femoris, vastus lateralis, vastus medialis and vastus intermedius, all innervated by the femoral nerve.
Medial rotation, as discussed earlier, occurs when the knee is in the last stage of extension, with some also occurring when the knee is flexed. It is primarily produced by the actions of popliteus, semimembranosus and semitendinosus, which are assisted by sartorius and gracilis. Lateral rotation is produced by biceps femoris and also occurs when the knee is flexed.
As the knee joint is a complicated structure subjected to significant biomechanical stress every day, it is a common site of injury. As it is primarily stabilized by the ligaments mentioned above, any unnatural movement of the knee such as twisting, pivoting, sudden change of direction, or a forceful blow can cause injury to these structures. Common conditions include:
- Patellar tendinitis: is an inflammation of the patellar ligament due to overuse stress on the patellofemoral joint. It is also called “jumper’s knee” because it commonly occurs in sports such as basketball or volleyball, where the players exhibit a sudden impact on the joint when landing after a jump. It is also more common in overweight individuals, as the knee is subjected to more stress.
- Anterior cruciate ligament (ACL) injury: ranging from a mild sprain to complete tear of the ligament, this injury is common among athletes in contact sports, such as football or soccer occuring due to a sudden change of direction or improper landing after a jump. ACL injury causes pain, swelling and instability of the knee joint.
- Collateral ligament tears: these injuries occur most commonly in contact sports due to a blow on the side of the knee. The tibial collateral ligament can be injured by a direct blow to the lateral side of the knee that pushes the knee inwards. Conversely, a fibular collateral ligament tear occurs due to a direct blow to the medial side of the knee.
- Terrible triad: a knee injury characterized by tears of the cruciate ligament, the tibial collateral ligament, and the medial meniscus. It is commonly caused by a lateral blow to the knee, while the foot is fixed on the ground. This injury often occurs in sports such as rugby or football.