Module 2 • 50 min
Structure
and function
In this module, we will build a deeper understanding of the knee joint's structure and function.
Learning Objectives
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Understand the structure and function of the distal femur and proximal tibia.
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Understand the structure and function of the patella.
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Understand the structure and function of the cruciate and collateral ligaments.
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Understand the structure and function of the menisci.
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Note: it is recommended that you complete Modules 1 before diving into more complex topics discussed below.
Section 2.1
Femur and tibia
Distal femur structure
The distal femur consists of two large medial & lateral femoral condyles that are separated posteriorly by the intercondylar fossa (Figure 2.2). The condyles become continuous anteriorly, which serves as the articular surface of the patella. The anterior surface of the condyles are concave in the medial-lateral direction and convex in the superior-inferior direction. Additionally, the medial and lateral surface of the intercondylar fossa provides attachments for the PCL and ACL, respectively.
Figure 2.1 - Knee joint rotation with highlighted femur. Click the video to pause/play. CC BY-NC-ND 4.0
Figure 2.2 - Femur anatomy. Left: anterior view of distal femur. Right: posterior view of distal femur. CC BY-NC-ND 4.0
Femoral condyles
The surfaces of the medial and lateral condyles are significantly different, and contribute to the complex movement of the TF joint.
The lateral femoral condyle prevents patellar dislocation — The lateral condyle extends farther anteriorly to restrain the patella in the lateral direction and holds it in normal alignment to prevent dislocation (Figure 2.3).
Figure 2.3 - Lateral femoral condyle extends farther anteriorly than medial femoral condyle. CC BY-NC-ND 4.0
The femoral condyles have a variable radius of curvature — As illustrated in Figure 2.4, the radius of curvature of the medial condyle varies highly in the anterior-posterior direction; it is flattest in its most distal surface (radius of curvature is highest), and curves more posteriorly (with a smaller radius of curvature). Likewise, the lateral condyle follows a similar pattern, but is flatter distally compared to the medial condyle. The variability in the radius of curvature adds to the complexity of the movement between the femur and tibia.
Medial femoral condyle is curved anteroposteriorly — The medial condyle has a curved articular surface in the anterior-posterior direction, thereby giving it a larger articular surface than the lateral condyle (Figure 2.4). This curvature contributes to the internal and external rotation of the femur and tibia, which is an important part of locking & unlocking the knee joint, otherwise known as the Screw Home Mechanism (see Module 3 - Section 3.2 for further details). During knee flexion-extension, the lateral condyle’s smaller surface is "used up" prior to the medial condyle. As a result, a pure flexion-extension movement would fail to accommodate the articular surface of the medial condyle. Therefore, a rotational movement must occur to accommodate the larger surface of the medial condyle during this movement. Later in this module, Figure 2.7 includes an animation of this movement and the corresponding points of contact between the femoral and tibial condyles.
Figure 2.4 - Left figure: variable radius of curvature in lateral and medial femoral condyles. Right figure: inferior view of femur depicting anteroposterior shapes of the medial and lateral femoral condyle. CC BY-NC-ND 4.0
Structure of the proximal tibia
The proximal tibia consists of medial and lateral tibial plateaus that are separated by an intercondylar region that is non-articular (Figure 2.6). The surface of the intercondylar region provides attachments for the PCL and ACL, and the medial and lateral menisci. The articulating surfaces of the tibia are significantly smaller than the corresponding femur.
Figure 2.5 - Knee joint rotation with highlighted tibia. Click the video to pause/play. CC BY-NC-ND 4.0
Figure 2.6 - Tibial anatomy. Left: anterior view of proximal tibia. Right: posterior view of proximal tibia. CC BY-NC-ND 4.0
Tibial plateau function
The medial tibial condyle bears a higher force — The medial tibial condyle has a larger articular surface compared to the lateral condyle by ≈ 60%. As a result, the medial condyle bears more force than the lateral side, but the stress on the medial side is lowered due to its larger area. The thicker articular cartilage on the medial side also helps reduce stress and protects the joint from wear.
The lateral tibial plateau is more mobile — The shapes of the medial and lateral tibial plateaus vary significantly and contribute to differences in movement between the medial and lateral sides of the femur and tibia. The medial tibial condyle is concave, while the lateral surface is more flat-to-convex. As a result, the medial surface fits more closely with the femur than the lateral side (please note: the joint congruence is also increased by the menisci, see Section 2.4). Therefore, there is a higher degree of translational and rotational movement between the lateral femur and tibia compared to the more congruent or snug medial side. Use the interactive slider below (Figure 2.7) to learn more about the tibiofemoral point of contact.
Figure 2.7 - Tibiofemoral point of contact. Left: superior view of tibia. Right: 3/4 view of femur (faded) and tibia. CC BY-NC-ND 4.0
Note: This is an interactive visual. Drag the knob of the slider below to interact with the visualization.
Section 2.2
Patella
Patella structure
The patella (or kneecap) is the largest sesamoid bone in the body positioned anteriorly relative to the patellofemoral joint. The patella is embedded within the quadriceps tendon. Distally, the patellar tendon extends from the apex of the patella and attaches to the tibial tuberosity (Figure 2.9). The posterior surface of the patella articulates with the femur and contains the thickest cartilage of any joint in the body. The posterior surface has a vertical ridge that forms the medial & lateral facets, which can be further subdivided into superior, middle and inferior facets. The medial and lateral facets articulate with the medial and lateral femoral condyles. A seventh facet - the odd facet - can be found on the far medial side of the patella.
Figure 2.8 - Knee joint rotation with highlighted patella. Click the video to pause/play. CC BY-NC-ND 4.0
Figure 2.9 - Patella anatomy. SMF = Superior medial facet, MMF = Medial medial facet, IMF = Inferior medial facet, SLF = Superior lateral facet, MLF = Medial lateral facet, ILF = Inferior lateral facet. CC BY-NC-ND 4.0
The patellofemoral joint is a planar synovial joint that has no rotational DOFs. The primary movement of the patella includes 5-7 cm of distal and proximal gliding on the femur during flexion and extension, respectively. This motion is also paired with lateral translation and minimal medial translation. As the patella glides on the femur during flexion-extension, the articulation or area of contact between the femur and patella is highly variable.
The articular surface of the patella makes minimal contact with the femur between 0 - 20° of flexion. However, once 20° of flexion is reached the patella is drawn into the trochlear groove, where the femur makes contact with the inferior facet. While conforming to the groove, the patella begins to shift laterally when flexion reaches 30 - 45°. The patella continues to follow the groove up until 90° of flexion, at which point it is in contact with the superior facet, and achieves peak femoral contact. If the knee reaches deep flexion at 130°, the patella falls into the femoral notch, where the medial femoral condyle makes contact with the odd facet and the contact area is greatly reduced. Use the interactive slider below (Figure 2.10) to learn more about the change in the area of contact between the patella and femur.
Patellofemoral area of contact
Figure 2.10 - Patellofemoral area of contact. CC BY-NC-ND 4.0
Note: This is an interactive visual. Drag the knob of the slider below to interact with the visualization.
Patella and quadriceps efficiency
The PF joint is subject to high compressive loads; for instance, normal day-to-day activities such as climbing and descending stairs can introduce compressive forces of up to 3 times bodyweight. Although the patella decreases the friction of the quadriceps tendon on the femur, the primary function of the patella is to increase the lever arm of the extensor mechanism and ultimately improve quadriceps efficiency. During knee flexion, the patella displaces the force vectors of the quadriceps and patellar tendon; this increases the moment arm of the quadriceps tendon. The removal of the patella, or a patellectomy, can result in a reduction of extensor forces by 30-70%. Use the interactive slider below (Figure 2.11) to learn how the patella increases quadriceps efficiency (and what these forces look like when there is no patella).
Figure 2.11 - Patella and quadriceps efficiency. ⬤ Quadriceps force, ⬤ Patellar force, ⬤ Patella-femoral reaction force. Left: knee joint with patella. Right: hypothetical situation where knee joint does not have a patella. CC BY-NC-ND 4.0
Note: This is an interactive visual. Drag the knob of the slider below to interact with the visualization.
Section 2.3
Ligaments
The cruciate and collateral ligaments work together to provide anterior-posterior, medial-lateral, and rotational stability and restraint to the knee joint.
Cruciate ligament structure
The cruciate ligaments - anterior cruciate ligament (ACL) and the posterior cruciate ligaments (PCL) - are incredibly important intrinsic structures of the knee joint that aid in mobility and stability. The ACL and PCL attach to the anterior and posterior aspects of the intercondylar area of the tibia, respectively, for which they are named (Figure 2.13). On the femur, the ACL attaches to the posterior region of the medial side of the lateral condyle while the PCL attaches to the anterior region of the medial side of the medial femoral condyle.
Figure 2.12 - Knee joint rotation with highlighted cruciate ligaments. Click the video to pause/play. CC BY-NC-ND 4.0
Figure 2.13 - Cruciate ligament anatomy. Left: anterior view of knee joint. Right: posterior view of knee joint. CC BY-NC-ND 4.0
Cruciate ligament function
The primary function of the cruciate ligaments is to limit the anterior-posterior sliding of the femur on the tibia during knee flexion and extension. More specifically, the ACL provides the primary restraint for anterior translation of the tibia relative to the femur. Conversely, the stronger PCL limits the posterior translation of the tibia with respect to the femur. These ligaments also provide secondary support to the knee in hyperextension, hyperflexion, and rotational movements. Use the interactive slider below (Figure 2.14) to see the ACL and PCL in action.
Figure 2.14 - Cruciate ligament function. CC BY-NC-ND 4.0
Note: This is an interactive visual. Drag the knob of the slider to interact with the visualization.
4-bar linkage mechanism
A 4-bar linkage mechanism can be a useful model to understand the movement of the ACL & PCL relative to the femur and tibia in the sagittal plane (although the movement is very much multidimensional). Use the interactive slider to the left (Figure 2.15) to visualize the 4-bar linkage mechanism during knee flexion-extension.
Figure 2.15 - 4-bar linkage mechanism. CC BY-NC-ND 4.0
Note: This is an interactive visual. Drag the knob of the slider to interact with the visualization.
Collateral ligament structure
The collateral ligaments - lateral collateral ligament (LCL) and medial collateral ligament (MCL) - are situated on the lateral and medial side of the knee joint, respectively. The LCL is a cord-like structure that extends from the lateral epicondyle of the femur to the head of the fibula. The more extensive MCL is a flat band that extends from the medial epicondyle of the femur to the medial condyle of the tibia & medial surface of its body (Figure 2.17).
Figure 2.16 - Knee joint rotation with highlighted collateral ligaments. Click the video to pause/play. CC BY-NC-ND 4.0
Figure 2.17 - Collateral ligament anatomy. Left: lateral view of knee joint. Right: medial view of knee joint. CC BY-NC-ND 4.0
Collateral ligament function
The collateral ligaments provide the primary support for mediolateral stability and restraint. The MCL and LCL sustain knee stability against valgus stress (medially directed force acting on the lateral side) and varus stress (laterally directed force acting on the medial side), respectively. The collateral ligaments also provide additional support against excessive rotation at the knee joint. Use the interactive slider to the left (Figure 2.18) to see the MCL and LCL in action.
Figure 2.18 - Collateral ligament function. CC BY-NC-ND 4.0
Note: This is an interactive visual. Drag the knob of the slider to interact with the visualization.
Section 2.4
Menisci
Menisci structure
The two menisci – medial meniscus & lateral meniscus – are half-moon-shaped fibrocartilaginous discs situated on the medial and lateral tibial plateaus. The menisci are wedge-shaped in the transverse section, tapering from a thick external margin to a thin unattached interior margin; their concave shape reflects the structure of the femoral condyles (Figure 2.20). The menisci are attached securely at their ends to the intercondylar region of the tibia, while the external margins merge with the joint capsule. The medial meniscus also attaches to the medial collateral ligament discussed in Section 2.3.
Figure 2.19 - Knee joint rotation with highlighted menisci. Click the video to pause/play. CC BY-NC-ND 4.0
Figure 2.20 - Menisci anatomy. CC BY-NC-ND 4.0
Menisci function
Enhances joint congruence — The relatively incongruent surfaces of the femoral condyles and tibial plateaus are significantly enhanced by the menisci by deepening the articulation at the TF joint. These structures improve the concavity of the articular surface of the tibia to better reflect the shapes of the femoral condyles (thereby improving stability at the joint).
Decrease friction — The menisci act as space-filling structures that enable a higher degree of synovial fluid dispersion on the TF joint surface, thereby decreasing friction by approximately 20%.
Load transmission — By increasing the contact area at the TF joint by approximately 2 times, the menisci significantly reduce joint pressures. In load-bearing scenarios, axial forces are introduced across the knee which lead to circumferential stresses on the meniscal surfaces. This stress is then converted to tensile forces by the circumferential collagen fibres and distributed across the joint surface. It is estimated that 50% of the load in the medial compartment and 70% of the load in the lateral compartment are transferred through the menisci. In the absence of these structures, the stress levels at the tibial plateaus and femoral condyles would be approximately 3 times higher.
Meniscal movement
The complex movement between the tibia and femur results in equally complex loads on the meniscal surfaces; as one would expect, the menisci shift and deform under these circumstances. As the knee undergoes flexion and extension, the menisci act as a space-filling buffer by deforming and gliding ahead of the movement. While the medial meniscus is more strongly tethered to the joint, the lateral meniscus is able to deform and move to a much higher extent; while the medial meniscus translates between 3 - 5 mm, the lateral meniscus can reach between 9 - 11 mm. Use the interactive slider below (Figure 2.21) to learn how the menisci move during knee flexion-extension.
Figure 2.21 - Meniscal movement. Left: superior view of tibia with menisci. Right: lateral view of knee joint. CC BY-NC-ND 4.0
Note: This is an interactive visual. Drag the knob of the slider below to interact with the visualization.
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Citing this page (APA) - Module 2 - Structure and function. kneeMo. https://www.kneemo.ca/module-two