How the knee joint enables free movement

Have you ever wondered why your knee gets hurt so easily, yet it can carry you all day and has the potential to produce explosive force and power you. It is a pivotal joint crucial for mobility and weight-bearing activities, but its intricate anatomy facilitates both its functional significance and vulnerability to injury.  

“The knee joint is one of the most versatile joints in the human body from the locomotion and the musculoskeletal aspect, it is not only important for walking but also ensuring that humans perform various sports and various complex activities of locomotion,” says Dr Vaibhav Bagaria, director, department of orthopedics, Sir HN Reliance Foundation Hospital, Mumbai. 

Anatomy of the knee joint 

1. The bones: “Knee joint is a hinge joint where two surfaces articulate. One is the lower end of the femur and other is the upper end of tibia,” says Dr Dipchand Bhandare, senior consultant, Orthopedics, Manipal Hospitals, Goa.  

• Femur: The thigh bone, which has two rounded condyles (round prominence at the end of a bone) at its lower end. 
• Tibia: The shinbone, forming the larger part of the lower leg. 
• Patella: Also called the kneecap, is a small bone that sits in front of the knee joint. 

2.  Menisci: In between the femur and tibia are two wedge-like cartilages called menisci. These function as shock absorbers, load distributors, and joint stabilizers.  

3. Ligaments: “There are four primary groups of ligaments, the representative in knee joint, two on the outer side and two in the linear joint,” says Dr Bagaria.  

• Anterior Cruciate Ligament (ACL): This prevents the tibia from moving too far forward from the femur. 
• Posterior Cruciate Ligament (PCL): This resists the tibia from moving too far backward from the femur.
• Medial Collateral Ligament (MCL) and Lateral Collateral Ligament (LCL): These are located on either side of the knee and ensure side-to-side stability. 

4. Muscles and tendons: The knee joint is enclosed in a network of muscles that are at the crux of its mobility and stability. “When we look at the knee joint, the tendons in front of the knee are called extensor or quadricep tendons. They are a set of four muscles that converge to form a single tendon that helps to keep the knee straight while walking while doing activities. The tendons at the back of the knee are called hamstring tendons and they help bend or flex the knee,” says Dr Bagaria.  

5. Synovial Membrane and Fluid: The knee joint contains a synovial membrane that secretes synovial fluid, a lubricating substance that reduces friction within the joint, aiding in smooth motion. 

6. Bursae: Small sacs filled with synovial fluid located around the knee joint that cushion the tendons and reduce friction during movement and load.  

7. Fat Pads: These provide cushioning and support to the joint. 

Biomechanical strengths of the knee 

• Mobility: The design of the knee allows for a wide range of motion. “There are several other tendons surrounding the knee joint and which are responsible for not only the knee movement but also contribute to the hip movement and these movements are complex in nature, meaning that they allow the knee to travel back and forth, pivot or rotate on a certain axis,” says Dr Bagaria. This mobility is essential for activities such as walking, running, and jumping. 
• Stability: While the knee allows for movement, it also possesses inherent stability. Ligaments like the ACL, PCL, MCL, and LCL provide stability by preventing excessive motion in different directions. 
• Load-Bearing capacity: The knee can bear a significant amount of weight and force, making it crucial for supporting the body during activities like standing and carrying heavy loads. 
• Shock absorption: The menisci and articular cartilage within the knee absorb shock and distribute forces evenly, reducing wear and tear on the joint. 

 Biomechanical weaknesses of the knee 

1. Limited Rotation: Unlike ball-and-socket joints like the hips, the knee has limited rotational capacity. “The rotational moments – twisting or turning – lead to maximum injuries to the knee,” says Dr Bhandare. This limitation makes it susceptible to twisting injuries during activities that involve sudden changes in direction. 
2. Complexity: The knee’s complexity, with its various ligaments, tendons, and cartilaginous structures, increases the risk of injuries. “There are several reasons why certain people are more susceptible to such injuries. And one of the reasons is also the shape or the geometry of the knee joint. People who have got narrow notches, people at a certain age, and people who are not optimized for performing a certain set of exercises, when they do it uninitiated, are susceptible to such injuries,” says Dr Bagaria. Nothing can be done about the shape of one’s knee. Appropriate training with the physio and good sports instructor help prevent such injuries.”
3.  Vulnerability to injuries: Ligament sprains, meniscal tears, and cartilage damage are just some of the common injuries the knee is prone to. “The most common injuries are of anterior cruciate ligament. These injuries often happen among people who play football or are involved in sporting activities that require pivoting. The primary function of this ligament is to prevent the knee from slipping forward,” says Dr Bagaria.
4. Degeneration: A slave to time, the knee joint experiences repetitive stresses throughout life and can undergo degenerative changes that lead to conditions like osteoarthritis.
5. Weight distribution: The alignment of the knee can influence its stability. “Although muscles and ligaments help in stabilizing the knee, the knee is always subjected to rotational stress or side to side sprains, which can cause injury,” says Dr Bhandare. Poor alignment or muscle imbalances and uneven rotational forces can lead to uneven pressure on the joint, potentially resulting in conditions like patellofemoral pain syndrome.
6. Lack of self-repair: Unlike some tissues in the body, cartilage within the knee has limited regenerative capacity. Injuries to articular cartilage can be challenging to heal naturally.

Takeaways

• Bones (Femur, tibia, and patella), Menisci, Ligaments (ACL, PCL, MCL, and LCL), muscles and tendons (quadricep tendon and hamstring tendon), synovial membrane and fluid, bursae and the fat pad make up the knee joint.  
• The biomechanical strengths of the knee include a wide range of motion, inherent stability, load-bearing capacity, and shock absorption.  
• Its biomechanical weaknesses include limited rotational ability, complexity of the joint, vulnerability to injuries, prone to degeneration, and lack of self-repair.