What is the biomechanics of the knee joint?

Biomechanics is the study of human motion. To understand the knee joint’s biomechanics, you’ll have to study the knee joint’s motion. The knee is the mechanism of 4 bones, two joints, and multiple ligaments along with many muscles crossing the joint.

Therefore, the knee joint has biomechanical functions in permitting gait by flexing, rotating, and providing stability at the same time throughout daily life activities. During the static vertical posture, the knee joint functions in conjunction with the hip & ankle joints to assist your body’s weight.

To study the biomechanics of knee joint, you’ll have to go through the biomechanics of joints, weight-bearing forces, bones, menisci, ligaments, etc. of the knee joint. The knee joint contains two articulations: Tibiofemoral and patellofemoral joints.

Tibiofemoral joint – Articulation between the distal femur and the proximal tibia.

Patellofemoral joint – Articulation between the femur and the posterior patella.

The attributes, answers, and issues of the patellofemoral joint are different from the tibiofemoral joint. So they want separate attention. At first, let us now study the biomechanics of the tibiofemoral joint (knee joint).

Tibiofemoral joint biomechanics 

Tibiofemoral joint forms between the distal femur and the proximal tibia. The tibiofemoral articulation occurs between the tibial condyles and medial and lateral femoral condyles. It is a double condyloid and a hinge synovial joint with three degrees of freedom. Example includes – Flexion/extension, medial/lateral (internal/external) and abduction and adduction. The double condyloid joint means articulation between each femoral condyle and his corresponding meniscus and each tibial condyles. 

For the complete understanding of the biomechanics of knee joint (functions and dysfunctions), careful evaluation of the articular surfaces and complete understanding of the surface connections to each other becomes necessary.

Femur  

The distal end of the femur constitutes the proximal articulating surface of the knee joint, which consists of 2 femoral condyles. The medial and lateral. The lateral condyle is more prominent while the medial condyle is larger.

When the femur and its shaft are held perpendicularly, the medial condyle extends further distally (to a lower level) than does the lateral condyle. However, both the femoral condyles lower surface lies in the same horizontal plane when the femur is in its natural oblique position.

The lateral condyle of the femur lies more directly in line with the shaft than does the medial condyle, because of the obliquity of the shaft of the femur. Both the femoral condyles aren’t very parallel with each other. the axis of lateral condyles is directed in anteroposterior direction while the axis of medial condyles runs medialward & backward.

The medial femoral condyle is larger than the lateral femoral condyle as you can see in the below diagram. Therefore, the medial femoral condyle’s tibial articular surface is larger than the lateral femoral condyle’s tibial articular surface. Both these femoral condyles are separated inferiorly by the intercondylar notch. However, they are still connected anteriorly by a shallow groove called either the femoral sulcus or the patellar surface or patellar groove that engages the patella in ancient flexion.

Tibia 

The femoral condyles sit on the relatively flat tibial condyles. The tibia has two asymmetrical condyles, which are relatively flat, known as medial and lateral tibial condyles. They are also known as tibia plateau. These asymmetrical, relatively flat lateral and medial tibial condyles or plateaus create the tibiofemoral joint’s distal articular surface.

The lateral tibial plateau is shorter, or you can say the medial tibial plateau is much larger. Nevertheless, the lateral tibial articular cartilage is thicker as compared to the medial side articular cartilage.

The posterior proximal tibia’s diameter is much more than the tibial shaft diameter, which, consequently, overhangs the tibial shaft posteriorly. Along with this posterior overhang, the tibial plateau slopes posteriorly approx 7° to 10°, which helps bend or flex the femoral condyles on the tibia.

Both the condyles are separated by the two bony spines known as intercondylar tubercles. These intercondylar tubercles play a significant role during knee extension. During knee extension, these intercondylar tubercles become lodged in the intercondylar notch of the femur. Thus, adding a little bit of stability to the joint.

In contrast, the tibiofemoral joint is relatively unstable because the tibial plateaus are primarily flat and have a little convexity at both anterior and posterior margins.Therefore, to improve this lack of boney stability, menisci’s introduction become essential for enhancing joint stability.

Tibiofemoral Alignments & Weight – Bearing Forces on tibiofemoral joint

It is very important to study the biomechanics of weight-bearing forces and the effect of abnormal alignments on the knee joint. There are generally two methods of measuring tibiofemoral alignment. One is by longitudinal axis and another is by the mechanical axis.

Longitudinal Axis Method 

The femur’s longitudinal axis is oblique and directed medially and inferiorly from its proximal to the distal end. On the other hand, the tibia’s longitudinal axis is straighty pointed in a vertical direction. Therefore, both the femoral and tibial longitudinal axes form an angle of 185°-190° medially at the knee joint. Hence constructing a normal 5°-10° physiological valgus at the knee.

Suppose if there is a deviation in the tibiofemoral angle. In that case, it will lead to the development of two abnormal conditions known as genu valgum (knock knees) or genu varum (bow legs).

Genu valgum – If the medial tibiofemoral angle is greater than 190°, there will be genu valgum (knock knees).

Genu varum – If the medial tibiofemoral angle is 180° or less, there will be genu varum (bow legs).

Both conditions will alter the compressive and tensile stresses on the knee joint’s lateral and medial compartment.

Mechanical Axis Method 

Another way of measuring tibiofemoral alignment is by the mechanical axis method. In this, a line is drawn on the radiograph from the middle of the femoral head to the middle of the head of the talus. This line is known as a mechanical axis or weight-bearing line.

In a normally aligned knee, this line passes through the middle of the joint between the intercondylar tubercles, and it travels up to the lower extremity. Therefore, it can be used to calculate the ground reaction force.

Therefore, the weight-bearing forces on the knee joint are equally distributed between the medial and lateral condyles in a bilateral stance. But in a unilateral stance, when the foot is on the ground (stance phase), the weight-bearing line shifts towards the medial condyle (medial compartment). However, which raises the compressive forces on the medial condyle.

In case of directly measuring (by using prostheses) the mechanical loading across the lateral and medial compartment. Reports signaled that many daily living tasks place a larger load on the medial compartment in comparison to the lateral compartment.

For the calculation of compressive forces on the knee joint specifically, the external forces throughout the leg. These external forces can be measured with the help of force plates. Force plate is a measuring tool that measures the ground reaction forces generated when someone stands or moves across them. Therefore, this will help the researchers in measuring the ground reaction forces during daily living activities (e.g., running, walking, and stair climbing). In conclusion, these external forces can be used to calculate the torque around the knee joint.

Here is an example explaining this 

To study the biomechanics of compressive forces on the knee joint, it is essential to go through the knee joint’s adduction motion. If the force from the floor extends medially to the knee joint center during walking, it will tend to rotate the knee into a greater varus (adduction) position. Therefore, an adduction moment is created around the knee joint.

Therefore, for measuring the knee’s medial compartment loading during gait and everyday living tasks, the magnitude of knee adduction moment can be used as a substitute. Thus, the development of medial knee osteoarthritis is directly associated with abnormal high knee adduction moments.

The medial compartment is most commonly affected in knee osteoarthritis. Therefore, during walking, almost 50 to 60% of the weight-bearing forces pass-through this medial compartment of the knee.

There is a high connection between the knee malalignment (genu varum or genu valgum) and the progression of knee OA. For examples

In genu varum, the weight-bearing line shifts towards the medial side, increasing the medial condyle’s compressive force. Therefore, this also contributes to the development as well as progression of medial compartment knee osteoarthritis. While

In genu valgum, the weight-bearing line shifts toward the lateral side, increasing the lateral condyle’s compressive force. Therefore, this may lead to development as well as the progression of lateral compartment knee osteoarthritis.

These abnormal anatomical alignments create a constant overload on the medial and lateral articular cartilage. This constant overloading may result in damaging of the knee articular cartilage.

Menisci Biomechanics 

It is essential to study the biomechanics of knee menisci. The knee has two menisci known as medial and lateral menisci. Meniscus fixes the tibiofemoral joint’s lack of congruence and converts the convex tibial plateau into concavities for the femoral condyles.

They play a significant role in increasing the area of contact and improving the distribution of the weight-bearing forces. Meniscus also acts as a shock absorber and also reduces the friction between the femur and the tibia.

The structural arrangement of meniscal fibers enables axial loads to be dispersed radially (in a downward direction), diminishing the wear on the hyaline articular cartilage. The menisci are fibrocartilagenous discs and semicircular.

Difference between medial and lateral meniscus

Lateral meniscus	Medial meniscus
The lateral meniscus forms four-fifth of the circle and is more circular in shape and is located on the outside of the knee.	The medial meniscus is C-shaped and it is located more on the inside of the knee.
The lateral meniscus covers a higer percentage of the anterior smaller lateral tibial surface.	The medial meniscus covers less percentage of the larger medial tibial surface, as an outcome of its larger exposed surface, the medial condyles is far more prone to injury from the comparatively greater compressive loads that pass through the medial condyle during regular daily activities
Since it doesn't have any attatchemnt to the lateral collateral ligament therefore, it is more moveable and is less prone to injuries.	The medial meniscus is far more prone to injury because it is strongly attached to the medial collateral ligament.

However, during gait and stair climbing, the knee’s compressive forces may reach up to one to two times the body’s weight. And during running, it may reach up to 3 to 4 times the body’s weight.

Biomechanical studies have revealed that out of these total imposed loads, menisci alone bear almost 50% to 60%. The medial meniscus absorbs up to 40% to 50% of the load, while the lateral meniscus bears up to 70% to 80% of the load.

However, these compressive loads can be influenced by abnormal anatomical alignments like genu varum or genu valgum. The larger the degree of genu varum, the higher the compression on the medial meniscus. And the larger the degree of genu valgum, the higher the compression on the lateral meniscus.

Menisci Attachments 

Both the menisci (lateral and medial) have anterior and posterior end known as anterior and posterior horns. The transverse ligament connects both the menisci to each other. Coronary ligaments connect both the menisci with the tibia. The Patellomeniscal ligament connects both the menisci to the patella.

There is a relative lack of mobility of the medial meniscus compared to the lateral meniscus due to its hard attachments to the medial collateral ligament and the knee joint capsule. In contrast, this relative lack of mobility of the medial meniscus contributes to restricted motion and more prone to injury. While the lateral meniscus is less tightly attached to the knee joint capsule and does not have any attachment to the lateral collateral ligament. That’s why the lateral meniscus is more mobile compared to the medial meniscus.

The anterior horn of the medial meniscus is attached to the anterior cruciate ligament and the posterior horn of the medial meniscus is attached to the posterior cruciate ligament. The anterior horn of the lateral meniscus combines/ blends into the attachment of the anterior cruciate ligament. While the posterior horn attaches just behind the intercondylar eminence, often combing/blending into the posterior part of the ACL.

Functions of menisci 

The menisci serve several significant biomechanical functions. There are numerous functions of menisci, which includes –

Shock Absorber

The meniscus enacts as a shock absorber by dispersing the compression forces coming from the femur over a larger (wider) area of the tibia. By research, the shocks absorbing ability of the menisci have been determined by measuring the vibrations in the proximal tibia coming from gait. By that, it’s been proven that shock absorption is roughly 20 percent less in knees without menisci.

Load Spreader (Force transmission)

The key function of the meniscus is in inducing force transmission. For example, If the round block (femoral condyles) sat directly on the flat block (the tibial plateau). Then there will be little contact area between the bony surfaces, and the joint stress will be high, which may lead to the damage of articular cartilage. With the menisci’s insertion, the contact area between the bony surfaces increases, and joint stress is decreased.

Joint Stability

There is a rigid attachment between the medial meniscus and tibia, leading to the knee’s anterior stability. But in the case of ACL deficient knee, the medial meniscus gets more frequently torn as it is less mobile.

The intact meniscus contributes to the knee joint’s stability as it restricts excess movements in all directions. Soft-tissue structures of the knee joint capsule also further facilitate knee joint stability.

The menisci’s joint stability function can best be shown in research exploring laxity in meniscectomized, ACL-deficient, or meniscus-torn knees. Research shows that in ACL sectioned and medial meniscectomy knees, greater anterior tibial translation is seen compared with only ACL sectioned knees. However, there is less anterior tibial translation seen in ACL sectioned and lateral meniscectomy knees.

In the ACL deficient knee, the medial meniscus’ posterior horn provides better knee stability and in resisting anterior tibial force.

Joint Lubrication and Nutrition

The menisci can also play a part in the lubrication and nourishment of their knee joints. Researchers reported the coefficient of friction of the knee joint is raised by 20% after meniscectomy.

The exact mechanism by which lubrication happens stays unknown. nonetheless, some researchers consider that during weight-bearing (when the knee is loaded), the menisci compress and circulates the synovial fluid into the articular cartilage, thus decreasing the frictional forces.

Loss of menisci

In eliminating the menisci, the forces are no more dispersed over a larger region of the tibia. The contact area between the bony surfaces is decreased, and joint stress will be increased (up to 200%), leading to damage to the underlying articular cartilage.

A meniscectomy (elimination of the menisci) multiplies the forces by seven or six times on the tibial plateau. It also increases cartilage to cartilage contact and doubles the femur’s articular cartilage stress (up to 200%).

Therefore, in contrast, total meniscectomies are rarely performed following a meniscal tear. Instead, care is taken to preserve as much of the remaining meniscus, either via elimination of the damaged meniscus or repairing the damaged meniscus.

Releated Article

Anterior Cruciate Ligament – Anatomy And Biomechanics

Posterior Cruciate Ligament – Anatomy And Biomechanics

Medial Collateral Ligament – Anatomy And Biomechanics

Lateral Collateral Ligament – Anatomy And Biomechanics

Payr Test – Examines the meniscus injury in the knee, mainly the posterior horn of the lateral or medial meniscu

Genu Recurvatum Test – Indicates a tear in the posterior cruciate ligament of the knee joint.