As one of the largest and most advanced joints in the human body, the knee is critical to mobility. Its complex structure and constant utilization causes it to be particularly vulnerable to damage. Although the knee’s essential utility in the human body is universally known, many people are unaware of its complexity and the numerous pieces that converge to ensure it operates properly.
The anatomy of the knee extends well beyond the knee joint itself, as its constituents include bones, tendons, ligaments, muscles, and cartilage. Injuries to any of the following individual components will hinder the overall functionality of the knee:
Knee Joint Capsule
The joint capsule surrounding the entire knee contains the synovial membrane, which provides nourishment to all constructs surrounding the knee. Ligaments surrounding the capsule provide it with support.
There are four bones connecting at the knee:
- Femur: Also known as the thigh bone, the femur is the largest and strongest bone in the human body.
- Tibia: The tibia, or shin bone, extends to the ankle and has two shock-absorbing menisci attached to the top, where it meets the knee.
- Fibula: Extending to the ankle, the fibula is a thin bone on the lateral side of the leg that runs parallel with the tibia.
- Patella: Also known as the knee cap, the patella is located in front of the knee joint.
Ligaments present in and around the knee are essential in maintaining its stability as they connect the femur bone to the tibia bone and therefore, damage to them will almost certainly constrain an individual’s mobility. These four ligaments are as follows:
- Lateral Collateral Ligament (LCL): The LCL functions to withstand impact from the knee’s inner surface. These impacts are also known as varus forces. It extends from the head of the fibula bone to the external surface of the femur bone.
- Medial Collateral Ligament (MCL): Just as the LCL resists impact from the knee’s inner surface, the MCL resists forces stemming from the outer surface of the knee, which are known as valgus forces. The MCL ligament is located between the inner surfaces of the femur and tibia bones.
- Anterior Cruciate Ligament (ACL): As one of the most vital pieces of the knee, the ACL typically requires surgical repair and significant rehabilitation when injured. The ACL is a cruciate ligament, meaning it is located inside the knee, while the collateral ligaments are located on either side of the knee. Its primary functions are preventing the tibia from moving in front of the femur and providing rotational stability for the knee.
- Posterior Cruciate Ligament (PCL): Forming an X with the ACL in the middle of the knee joint, the PCL also works to connect the femur to the tibia. The PCL serves to prevent the backwards movement of the tibia and thus, an injury to it requires substantial force.
More commonly referred to as the knee’s cartilage, the menisci exist in two places: the outer edges and inner edges of the tibia bone. The role of the menisci is to absorb shock to the knee and evenly distribute weight between the tibia and femur bones.
The cartilage of the knee is an elastic tissue known as articular cartilage that serves the purpose of protecting the bones and allowing the joints to move smoothly. Like the menisci, the cartilage also absorbs shock.
There are two primary muscle groups surrounding the knee, the quadriceps and hamstrings, that both play an important role in the movement and stabilization of the knee.
Quadriceps: Four muscles comprise the quadriceps, which converge to form the quadriceps tendon. The quadriceps tendon is attached to the patella, which is attached to tibia by the patellar tendon.
Hamstrings: The hamstrings muscle group is comprised of three muscles and it enables flexibility of the knee. The hamstring muscles connect to both the fibula and tibia.
All the individual parts of the knee work together to enable its flexibility and maintenance of stability. The range of motion (ROM) of the knee is measured in flexion degrees; in men, the normal ROM is -6 to 140 degrees and in women, the normal ROM is -5 to 143 degrees. Different activities require varying degrees of movement, some of which are illustrated below:
|Activity||Knee Flexion (degrees)|
|Sitting down||83 -110|
|Tying a shoe||106|
Injuries to the knee typically occur as a result of movement beyond the normal range of motion, abnormal movement, or strenuous activity.
The following are typical injuries occurring in the knee, all of which are treated by Genesis Orthopedics & Sports Medicine:
Chondromalacia patella (Patellofemoral pain)
Ligament sprains and tears (ACL/PCL/MCL/LCL)
Patellar tracking disorder