Functional Anatomy of the Knee

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Anatomy Basic Vocabulary

Axes: lines around which an object rotates. The rotation axis is a line that passes through the centre of mass. There are three axes of rotation: sagittal passing from posterior to anterior, frontal passing from left to right, and vertical passing from inferior to superior. The rotation axes of the foot joints are perpendicular to the cardinal planes. Therefore, motion at these joints results in rotations within three planes. Example: supination involves inversion, internal rotation, and plantarflexion.

Bursae: reduces friction between the moving parts of the body joints. It is a fluid-filled sac. There are four types of bursae: adventitious, subcutaneous, synovial, and sub-muscular.

Capsule: one of the characteristics of the synovial joints. It is a fibrous connective tissue which forms a band that seals the joint space, provides passive and active stability and may even form articular surfaces for the joint.

Closed pack position: the position where there is the most congruency of the joint surfaces. In this position, joint stability increases. The closed pack position for interphalangeal joints is at full extension.

Degrees of freedom: the direction of joint movement or rotation; there is a maximum of six, including three translations and three rotations.

Ligament: fibrous connective tissue that holds the bones together.

Open (loose) pack position: position with least amount of joint congruency where joint stability is reduced.

Planes of movement: describe how the body moves. Up and down movements (flexion/extension) occur in the sagittal plane.Sideway movements (abduction/adduction) occur in the frontal plane. Movements in the transverse plane are rotational (internal and external rotation).

Knee Structure

The knee joint include four bones: distal end of the femur, proximal end of the tibia, patella and the proximal part of the fibula. Only the fibula is indirectly involved in the knee joint.

• Prominent lateral and medial condyles are found at the distal end of the femur. Projecting from each condyle is an epicondyle that act as attachment sites for the collateral ligaments. The lateral and medial condyles are separated by the intercondylar notch.
• The proximal end of the tibia terminates in a broad, flat region called the tibial plateau. The intercondylar eminence runs down the midline of the plateau, separating the medial and lateral condyles of the tibia. The two condyles form a flat, broad surface for articulation with the medial and lateral condyles of the femur.
• The patella articulates with the femur to form the patellofemoral joint. It functions as a fulcrum to increase extension power of the quadriceps and serves as a stabilising structure that reduces frictional forces placed on femoral condyles.
• A enlarged pointed head and small neck form the proximal part of the fibula.

Bones and Articulations of the Knee

Bones	Articulations	Characteristic	Key palpation
Tibia Femur	Tibiofemoral joint	Joint articulation occurs between the medial and lateral femoral and tibial condyles. The tibiofemoral joint has two crescent-shaped fibrocartilaginous structures, a lateral and a medial meniscus, attached to the tibial plateaus through coronary ligaments. Both menisci are concaved superiorly to accommodate the surfaces of the femoral condyles. Its role is to increase the contact area between the tibial and femoral articular surfaces, thus increasing the joint's stability.	Femoral epicondyle Tibial tubersity
Femur Patella	Patellofemoral joint	Joint articulation is located between the anterior aspect of the distal femur and the posterior surface of the patella.
Tibia Fibula	Superior Tibiofibular joint

Knee Kinematics

Joint	Type of Joint	Plane of Movement	Motion	Kinematics	Closed pack position	Open pack position
Tibiofemoral (TFJ)	Modified hinge	Saggital Transverse	Normal range of motion: Extension: 0 degree Flexion: 140 degrees Internal and External Rotation of the tibia in relation to the femur is possible only when the knee is flexed.		Full extension	25 degrees of flexion.
Patellofemoral Joint (PFJ)	Saddle		At full knee extension patella is resting on the suprapatellar fat pad. At 20-30 degrees of knee flexion, the patella moves inferiorly, lying in the shallow part of the intercondylar groove, the patella contacts the femur with its inferior pole. The patella occupies the intercondylar groove at 60-90 degrees of knee flexionhere is maximum contact between the patella and the intercondylar groove (1/3 of the posterior surface area of the patella). At 135 degrees of knee flexion (near full knee flexion), the patella contacts the femur with its superior pole and rests below the intercondylar groove.
Superior Talofibular Joint (STFJ)

Knee Bursae

There are four bursae located in the knee joint:

• Suprapatellar bursa between the quadriceps femoris and the femur.
• Prepatellar bursa between the apex of the patella and the skin.
• Deep and superficial infrapatellar bursa with deep bursa located between the tibia and the patella ligament, and the superficial found between the patella ligament and the skin.
• Semimembranosus bursa between the semimembranosus muscle and the medial head of the gastrocnemius.

Knee Joint Capsule

The knee joint capsule connects the margins of the femoral and tibial articular surfaces. The anterior portion of the capsule attaches to the borders of the patella. The lateroposterior portion of the capsule gives passage to the tendon of the popliteus muscle.

The capsular pattern is "the proportional motion restriction in range of motion during passive exercises due to tightness of the joint capsule."

Ligaments of the Knee

The ligaments of the knee joint can be divided into two groups; extracapsular ligaments and intracapsular ligaments.

Clinical Relevance

1. The most common pathology affecting the knee joint involves collateral ligaments due to force applied to the side of the knee with the foot pinned on the ground. You can learn more about this condition here.
2. The anterior cruciate ligament (ACL) can be injured by hyperextension of the knee joint. Another cause of the ACL pathology is the application of a large force to the back of the knee when the knee joint is partly flexed. Study the rehabilitation of the ACL through completion of this programme.
3. A housemaid’s knee (Prepatellar bursitis) is a condition involving the prepatellar bursa. The signs include inflammation and swelling on the anterior side of the knee.
4. The menisci are a common site of injuries in the knee.

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