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Module 1 • 20 min

Introduction to the knee joint

In this introductory module, we will explore the basic anatomy and functions of the knee joint.

Learning Objectives

  • 1.2 - Knee joint function

    • Describe the key functions of the knee joint and appreciate the complexity of its role in the body.

  • 1.3 - Degrees of freedom

    • Understand the translational and rotational degrees of freedom of the knee joint (both qualitatively and quantitatively).

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Section 1.1 - what is the kne joint?

Section 1.1

What is the knee joint?

The knee joint is composed of three bones (distal femur, proximal tibia, and patella) and several other specialized soft tissue structures (such as the cruciate & collateral ligaments, menisci and articular cartilage). Use the interactive 3D model viewer below (Figure 1.1) to review the anatomy of the knee joint.


The knee joint consists of two separate articulations; the tibiofemoral (TF) joint and the patellofemoral (PF) joint. The TF joint is the articulation of the femur with the tibia, and the PF joint is the articulation of the patella with the femur. The knee joint is classified as a double condyloid or modified hinge joint. The forces that act on and through the knee joint are primarily compressive (but can also include tension, shear, bending and torsion) and can range up to 4 times body weight during level walking and 8 times during downhill walking.

Figure 1.1

Figure 1.1 - Knee joint anatomy 3D model viewer. CC BY-NC-ND 4.0



Note: This is an interactive visual. Launch the model viewer to rotate, zoom and interact with the 3D knee model.

Knee joint anatomy 3D model viewer

Note: this application will open in a new tab and may take up to 1 minute to load on your device.

Secton 1.2 - Knee joint function

Section 1.2

Knee joint function

Main roles of the knee joint include the following points:


Stabilizes the body and facilitates movement by lengthening and shortening the lower limb.


Transmits, absorbs and distributes forces and moments that result from activities of daily living and sports.


Maximizes energy efficiency required during static and dynamic movement.

While the knee joint supports the weight of the body, it must also enable a wide range of motion during day-to-day activities. Therefore, the common misconception that the knee acts as a simple hinge joint is a gross oversimplification; rather, it requires complex 3D motion to properly react to and sustain external and internal loads. For instance, the impact of the foot on the ground causes a variety of complex forces to act on the knee joint, leading to translation and rotation in several planes.
In order to withstand such complex loads, the knee joint is supported by many structures including ligaments, tendons, and cartilage that provide
both active and passive stability and restraint.

Secton 1.3 - degreesof freedom

Section 1.3

Degrees of freedom

From the description above, it is evident that the knee joint must possess several degrees of freedom (DOF) to satisfy the complex functions of this joint. The knee joint is regarded as a 6 DOF joint as it allows motion in 3 rotational axes (flexion-extension, varus-valgus rotation & internal-external rotation) and 3 translational axes (proximal-distal translation, medial-lateral translation & anterior-posterior translation). Figure 1.2 and Figure 1.3 include animations of each rotational & translation DOF, and the respective range of motion (ROM). Although the knee’s primary movement occurs in the sagittal plane during flexion-extension, all of these movements play an important role in the function of the joint.

Rotational Range of Motion (ROM)

Varus-valgus rotation

Internal-external rotation

130 - 150° of flexion, and up to 5° of hyperextension.
within a 5 - 8° range for varus and valgus rotation.

10 - 20° of internal rotation, and 30 - 45° of external rotation

Translational Range of Motion (ROM)

Proximal-distal translation

Medial-lateral translation

Anterior-posterior translation

2 - 5 mm (articular cartilage and menisci compression)

1 - 2 mm

5 - 10 mm

Figure 1.2

Figure 1.2 - Rotational Range of Motion (ROM). CC BY-NC-ND 4.0



Note: This is an interactive visual. Click on the videos to play/pause the animations. 


Look closely! Some movements are very subtle.

Figure 1.3

Figure 1.3 - Translational Range of Motion (ROM). CC BY-NC-ND 4.0



Note: This is an interactive visual. Click on the videos to play/pause the animations. 


Look closely! Some movements are very subtle.


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Citing this page (APA) - Module 1 - Introduction to the knee joint. kneeMo.

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