Better known as the shin or shin bone, the tibia is the larger and stronger of the two lower leg bones. Together with the femur, it forms the knee joint. It also forms the ankle joint with the fibula and tarsal bones. Though both the tibia and the fibula occupy a similar area, the tibia sits closer to the middle and center of the leg. Additionally, the tibia acts as the anchor point for many powerful muscles, tendons, and ligaments that help perform activities such as walking and running.
The tibia is a long bone, a classification that describes its long and narrow shape. All long bones are hollow in the middle with ends that contain spongy bone consisting of many small columns that help protect the bones from outside stress and pressure. Holes between the columns contain red bone marrow that produces red blood cells. Within the hollow section or the medullary cavity is a layer of yellow bone marrow that can store energy for the body. The thick substance that surrounds this layer is compact bone containing a matrix of hard calcium mineral and collagen protein that provides strength and flexibility.
In anatomy, proximal refers to the area closest to the center of the body. In the case of the tibia, the proximal end forms the knee joint with the femur. The top widens into a flat area with two round condyles that help create the knee joint. Between these condyles is the intercondylar fossa, a small groove containing two intercondylar tubercles. On the front side of the tibia, just under the condyles, is the tibial tuberosity. The tubercles and the tuberosity connect to important ligaments.
The shaft of a bone is its long middle section. The tibial shaft is triangular and has a long, thin ridge running down it. This ridge begins at the tibial tuberosity and is known as the anterior crest. As the shaft approaches the ankle joint, it widens again. This occurs both from front to back and from side to side. As a whole, the shaft is essential both for absorbing stressors and providing attachment sites for many of the leg muscles.
Just as proximal refers to something close to the body’s center, distal refers to something further away. The distal end of the tibia is the section that forms the ankle joint. It is composed of two main features, the first being the medial malleolus, a bony projection that forms the ankle joint with the tarsal bones of the foot. The other key feature, a small, concave area on the outside of the tibia, is the fibular notch, which interacts with the fibula.
Most people know the patella as the kneecap. This bone assists with many movements of the knee, though it must act on the tibia and its muscles to do so. The patella connects to the tibial tuberosity via the patellar ligament. When a person extends their lower leg, a muscle pulls on the patella, which in turn pulls on the patellar ligament, which pulls on the tibial tuberosity. The result is leg extension.
The fibula is an interesting bone, particularly in the way it interacts with the tibia, which is significantly larger and stronger. The proximal end of the fibula connects to the bottom section of one of the condyles of the tibia. The distal end of the fibula inserts into the fibular notch on the tibia. A strong ligament connects the lower part of the fibula to the tibia. This end also acts with the tarsal bones of the foot to form the ankle joint.
The tibia helps form four joints: the ankle, the knee, the proximal tibiofibular joint, and the distal tibiofibular joint. At each of these joints, a thin layer of hyaline cartilage covers the tibia and helps the joints move more smoothly.
One of the tibia’s most important functions is serving as the attachment site for leg muscles. The tendons of three muscles attach to the front and inside section of the tibia; these are the sartorius, gracilis, and semitendinosus. The condyles of the tibia attach to the extensor digitorum longus muscle and the horizontal section of the semimembranosus muscle. The quadriceps femoris connect to the tibial tuberosity. Other muscles that attach to the tibia at various sites include the tensor fasciae latae, popliteus, tibialis anterior, soleus, and flexor digitorum longus.
When a child is first born, their tibia consists of two separate bones -- the central shaft or diaphysis and a thin cap known as the proximal epiphysis. A layer of hyaline cartilage separates these bones and provides a slight amount of movement. The distal section of the tibia also consists of hyaline cartilage, though it eventually hardens into bone to form the distal epiphysis. The hardening process usually occurs around two years old.
Throughout development, a thin layer of hyaline cartilage -- the epiphyseal or growth plate -- continues to separate the diaphysis from both epiphyses. As a child ages, the growth plate grows as bone slowly replaces it. This is what provides the tibia with its length. Eventually, the diaphysis and the epiphyses will grow into the growth plate and fuse to form the tibia. The area where the bones fuse is the metaphysis.
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