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Biomechanics of the Knee
At first glance the knee may appear to be a simple hinge joint, but in fact there are many different movement occurring within the joint every time you bend and straighten your knee. Let’s review how the tibia (shin bone) and femur (thigh bone) move together as well as how the patella (knee cap) moves in relation to these two bones.
What does “arthrokinematics” mean?
Arthrokinematics of a joint explains the subtle movement that occurs between the joint surfaces. These movements are necessary for the movement of the joint, but are not consciously controlled by the individual
Tibiofemoral arthrokinematics: Screw Home Mechanism
When you flex (bend) and extend (straighten) your knee, the tibia and femur move in relation to eachother and rotation occurs within the knee joint. The movement of these two bones depends on your body position; closed kinetic chain or open kinetic chain.
Open kinetic chain (OKC): This is any position where your body and thigh are fixed and your lower leg and foot move freely. For example, when you are seated with your foot hanging off a table or chair. In this position, your femur remains stationary and your tibia is free to move (see below).
Closed kinetic chain (CKC): This is any position when your foot is fixed and your body and thigh are able to move. For example, when you are standing on the ground and perform a squat. In this position, your femur must move around your tibia
The Screw Home Mechanism: This term refers to the rotation that occurs between the tiba and femur when the knee is coming in and out of full knee extension.
In the OKC position, the tibia must slide and roll forward on the base of the femur to create knee extension and at the same time rotated laterally or away from midline to lock out the knee joint upon full extension. This rotation of the tibial is extremely subtle, but also necessary to attain full knee extension. When you flex your knee in this position, the opposite occurs. First, the tibia rotates medially or towards midline in order to initiate the movement, then it slide and rolls backward to bend the knee.
In the CKC position, the femur is our primary mover. When you move from a squat to standing with knees locked out, the base of the femur rolls forward while sliding backward in order to remain on top of the tibia. This creates the extension moment at the knee, but in order to fully lock out the knee the femur must rotate medially. Just as with the OKC position, knee flexion is created in the opposite manner. First the femur rotates laterally to “unlock” the knee, then the femur rolls backward while sliding forward on the top of the tibia to bend the knee.
Your patella is a flat, round bone that floats within the Patello Femoral tendon (PFT) that attaches your quadriceps muscles to your tibia.
As you bend and straighten your knee, the tension on this tendon changes, pulling on and moving your patella up and down. The patella moves over the end of your femur along a smooth path called the trochlear groove (see picture below).
As your knee extends, your quadriceps muscles contract and thus pull on the PFT and the patella. This causes the patella to move in a superior or upwards direction.
As the patella moves superiorly and the knee begins to lock out into extension, the pull from the quads becomes asymmetrical and the muscle on the outside of your leg become dominant. This causes the Patella to move laterally as the knee extends, creating an upside down “J” path. This “Inverted J” represent normal patellar movement. Pathology can occur when the line of pull on the patella changes, causing the patella to deviate from its healthy path and out of the trochlear groove. This may cause the undersurface of the patella to rub on the femur, creating increased friction and leading to pain and arthritis down the road.
The Knee: Relevant Anatomy
The Anatomy of the knee is complex, with multiple layers of both inert and contractile tissue. Inert tissues are structures, which move passively as the knee joint moves such as ligaments, cartilage, bones, and bursa. Contractile tissues are the muscles and tendons (tendons attach muscles to bones), which actively move the knee joint.
There are four bones that make up the knee joint:
1. Femur - The long bone in your thigh that runs from your hip to your knee.
2. Tibia - Your shin bone, running from the bottom of your knee joint to your ankle.
3. Fibula - A long thin bone that runs on the outside of your leg from your knee down to the bony bump on the outside of your ankle (lateral malleoli).
4. Knee cap or Patella - Sits on the front of your knee. It is suspended in the center of your femur by the tendons of your quadriceps muscle (see below), which allow it to move up and down as your knee moves. If your knee is relaxed and in a straight position you can easily move your patella from side to side because it is a floating bone or Sesamoid bone.
While there are many muscles crossing the knee joint, the primary movers can be divided into two categories: knee flexors and knee extensors.
1. Knee Extensors - The quadriceps is one of the most powerful muscle groups. As the name implies, this muscle group is made up of four separate muscles: Vastus Medialis, Vastus Intermedius, Vastus Lateralis, and Rectus Femoris. These muscles all connect to the same tendon at the front of your knee. This tendon, the Patello Femoral Tendon (PFT), holds your Patella in place and attaches to your Tibia (see above).
2. Knee Flexors – The hamstrings are the primary knee flexors, made up of the Biceps Femoris, Semitendinosis, and Semimembranosis. These muscles run from the bottom of your pelvis (you can feel there attachment on your “sit-bones”) to the top of your Tibia and Fibula just behind your knee. You can feel these tendons “pop-out” when you bend your knee.
These are the support structures of your joints. Ligaments connect bone to bone and stabilize the joint as it moves.
As you look at the picture above, notice that there are four main ligaments in the knee: The Anterior Cruciate Ligament (ACL), Posterior Cruciate Ligament (PCL), Medial Cruciate Ligament (MCL), and Lateral Cruciate Ligament (LCL).
Other Supporting Structures
1. Meniscus – This is a fibrocartilaginous structure that sits on top of your tibia and acts as a “cup” for the femur to glide over. The Menisci (plural) also provide shock absorption during weight bearing activities such as walking and running. Every knee has two menisci: one on the medial side and one on the lateral side.
2. Bursa – Bursae (plural) are little fluid filled pads that lie wherever skin, muscle, or tendon rub against bone. They provide lubrication and allow for smooth movement between all the moving parts of your knee. There are many bursae associated with each joint in your body.