In large part, the complex movement of the TF joint is a function of its articular surfaces. By synthesizing the content discussed in Module 1 & Module 2, one can begin to appreciate the structures of the knee and their role in sustaining normal knee function. 

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The surface area of the femoral condyles is larger than the tibial side which indicates that a pure rolling motion would be impossible (as this would require equal articular surfaces). If pure rolling were to occur, the femur would simply roll off of the tibia (Figure 3.1). Therefore, as the knee is flexed past 20°, a combination of rolling & gliding occurs at the TF joint. During this time, the point of contact between the tibia and femur moves anteriorly during extension and posteriorly during flexion; a lower level of congruence in the lateral compartment of the knee leads to increased translation on the lateral side (see Figure 2.7). The rolling & gliding motion also causes the instant centre of rotation to move posteriorly during flexion and anteriorly during extension. Use the interactive slider below (Figure 3.1) to learn more about rolling & gliding; the slider to the right illustrates a hypothetical scenario where only rolling occurs at the knee joint (ultimately causing failure).

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The nature of rolling and gliding changes between closed kinetic chain (CKC) and open kinetic chain (OKC) movements:

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• In a CKC knee flexion, the femur rolls posteriorly and glides anteriorly on the tibia. For knee extension, the femur rolls anteriorly and glides posteriorly. 

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• In an OKC knee flexion, the tibia rolls and glides posteriorly. As for knee extension, the roll and glide are anterior. 

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The interactive visualization below (Figure 3.1) illustrates rolling and gliding in a CKC scenario (where the femur is the mobile bone).