Knee Joint

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Knee Joint
Primary Type	Saddle joint between patella and femur"Saddle joint between patella and femur" is not in the list (Synovial Joint, Cartilaginous Joint, Fibrous Joint, Compound Joint) of allowed values for the "Has joint type" property.
Secondary Type
Bones	Tibia, femur, patella (tibiofemoral and patellofemoral joints)"Patella (tibiofemoral and patellofemoral joints)" is not in the list (Vertebra, Sacrum, Coccyx, Scapula, Clavicle, Humerus, Radius, Ulna, Scaphoid, Lunate, ...) of allowed values for the "Has joint bones" property.
Ligaments
Muscles
Innervation	Popliteal nerve, tibial nerve
Vasculature	Genicular branches of the popliteal artery
ROM	Mainly sagittal plane: 3° of hyperextension to 155° of flexion
Volume
Conditions

The knee is a two joint structure composed of the tibiofemoral joint and the patellofemoral joint and is the largest joint in the body. It sits between the body's two longest lever arms namely the tibia and the femur which lends it to injury.

This article discusses knee joint anatomy. For a discussion on biomechanics of the knee see Knee Biomechanics.

Bones and Articulations

The knee joint contains four bones: the femur, tibia, patella, and fibula.

It has three compartments: the medial tibiofemoral, lateral tibiofemoral, and patellofemoral compartments. The three compartments all share a common synovial cavity.

The knee has three articulations: medial and lateral tibiofemoral and patellofemoral joints. The tibiofemoral articulations connect the distal femur to the tibia. The distal femur broadens to form the medial and lateral femoral condyles. The tibia is relatively flat, but the sloped menisci permit a tight articulation with the convexity of the femoral condyles. The femoral condyles are separated by an intercondylar fossa, also known as the femoral groove, or femoral trochlea. The joint lines are formed by the femoral condyles and tibial plateaus.

The patella is sesamoid shaped and is embedded within the quadriceps tendon that articulates with the trochlear grove of the femur. It increases the mechanical advantage of the quadriceps. The fibula head is located within the knee capsule but isn't normally involved as a weight-bearing surface.

Structures Involved in Support and Stability

There are several structures that provide stability and cushioning to the knee joint.

• Hyaline cartilage lining the tibia and femur provide shock absorption
• Medial and lateral menisci provide shock absorption and distribute forces as well as play a role in joint lubrication.
• Anterior and posterior cruciate ligaments provide stability with anterior and posterior movements with flexion and extension
• Medial and lateral collateral ligaments provide support in their respective planes.
• Iliotibial band
• Posterolateral corner

Menisci

Both femoral condyles have a crescent-shaped meniscus. This structure deepens the fossa where the condyles articulate with the tibia. They are thickest peripherally and attach to the rim of the tibia here. They taper down in thickness towards the inner aspect of the knee where they are unattached.

The lateral meniscus has a more circular shape and is more mobile anteriorly and posteriorly compared to the medial meniscus. The medial meniscus is attached to the posterior oblique ligaments and the medial collateral ligament and is less mobile. This reduced mobility may be a factor in why it is more commonly injured than the lateral meniscus.

There are three vascular zones in the meniscus. The peripheral one third is directly supplied by blood vessels and so there is a reducing blood supply the more centrally located the tissue is. From outer to inner they are termed the red-red zone, the red-white zone, and the white-white zone. The inner part of the meniscus is supplied by diffusion from synovial fluid and so has a reduced healing capacity.

Anterior Cruciate Ligament

The ACL arises from the posteromedial aspect of the lateral femoral condyle, runs distally in an anterior and medial direction, and inserts on the anteromedial aspect of the tibia in the intercondylar region. It has two bundles: an anteromedial bundle and posterolateral bundle. The anteromedial bundle is tight in flexion. The posterolateral bundle is tight in extension.

The ACL provides the primary resistance to anterior translation of the tibia on the femur. It also provides secondary restraint against tibial rotation, varus, and valgus forces.

Posterior Cruciate Ligament

The PCL arises from the anterolateral aspect of the medial femoral condyle. Here it traverses distally in a posterior and lateral direction inserting behind the intercondylar eminence, medial to the ACL. It is the thickest and strongest of all the knee ligaments. It is the primary structure providing resistance to posterior translation and external rotation. Resistance to posterior translation is greatest with knee flexion greater than 30 degrees. Resistance to external rotation is greatest with knee flexion greater than 90 degrees. In the setting of a deranged posterolateral corner it has maximal control of external rotation at 90 degrees of knee flexion.

Medial Collateral Ligament

This structure has both superficial and deep components.

The superficial MCL arises just proximal and posterior to the medial femoral epicondyle. There are two locations for insertion on the tibia: 1cm below the joint line and 6cm below the joint line. The superficial MCL is the primary static stabiliser of the medial knee and provides resistance against valgus and external rotation forces.

The deep MCL is a thickening of the joint capsule. It originates 1cm distal to the origin of the superficial MCL on the femur. Its insertion is on the medial meniscus and on the medial tibia slightly distal to the joint line. The deep MCL is a secondary valgus stabiliser when the superficial MCL is injured.

Lateral Collateral Ligament

The LCL has its origin on the lateral femoral epicondyle. It inserts on the lateral head of the fibula. It is the principal stabiliser against varus forces.

The biceps femoris splits and runs along either side of the LCL along to its attachment on the fibula head.

The popliteus muscle originates slightly inferior to the lateral femoral condyle, runs deep to the LCL, next to the biceps femoris, and inserts onto the posterosuperior aspect of the medial tibial condyle. Its has a few functions. It is an internal rotator of the tibia or external rotator of the distal femur. It has a protective effect to the posterior horn of the lateral meniscus during flexion as it pulls the posterior horn posteriorly. It also reinforces the PCL in prevention of anterior translation of the femur during deceleration and downhill running. The popliteus is reinforced by several posterolateral structures.

Iliotibial Band

The ITB is a broad and thick fascial structure that runs from the tensor fascia lata, gluteus maximus, and other proximal muscles and attaches to the vastus lateralis anteriorly. It crosses the lateral femoral condyle prominence and attaches at Gerdy's tubercle which is found on the anterolateral aspect of the proximal tibia. It blends with the lateral patellar retinaculum close to the joint line.

Posterolateral Corner

The PLC is made up of several structures:

• Iliotibial tract
• Lateral collateral ligament
• Popliteus tendon
• Popliteofibular ligament,
• Popliteotibial and popliteomeniscal fascicles
• Middle third of the lateral capsular ligament
• Fabellofibular ligament
• Arcuate ligament
• Posterior horn of the lateral meniscus
• Lateral coronary ligament
• Posterolateral portion of the joint capsule

The PLC along with the PCL stabilise the knee against external rotation and posterior translation against a wide range of knee positions.

Plica

Plica are embryological tissue remnants. They are related to the synovium and are normally thin and narrow. If irritated they become thick and fibrotic. Several plica can be found in the knee, but the medial plica is the one that most commonly becomes painful. The medial plica arises from the medial wall of the knee, traverses inferiorly around the medial femoral epicondyle, and attaches to the synovium that surrounds the infrapatellar fat pad.

Anterolateral Ligament

The ALL arises from the lateral femoral epicondyle just anterior to the LCL, and inserts on the anterolateral region of the proximal tibia between Gerdy's tubercle and the fibula head. The precise role and clinical relevance of this structure are unknown, but it may help control internal tibial rotation and affect the pivot shift phenomenon.

Neurovascular Structures

There is a neurovascular bundles that contains the popliteal artery, popliteal nerve, and tibial nerve (continuation of sciatic nerve). It travels directly posterior to the knee joint. Injury to the vasculature can occur in severe trauma such as in tibiofemoral dislocation.

The peroneal nerve is found laterally and is a division of the sciatic nerve. It starts its course between the medial border of the biceps femoris muscle and lateral head of gastrocnemius. It then passes laterally around the biceps femoris muscle, and dives between the peroneus longus and fibular head called the peroneal tunnel. Finally it splits into three branches: the deep peroneal, superficial peroneal, and recurrent peroneal nerves. The peroneal nerve is susceptible to trauma due to its superficial location and course through narrow spaces.

See Also

Knee Biomechanics

References

• Basic biomechanics of the musculoskeletal system - Nordin 4th edition 2012.