The Tibia

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The tibia, sometimes known as the shin bone, is the larger and stronger of the two lower leg bones. It forms the knee joint with the femur and the ankle joint with the fibula and tarsus. Many powerful muscles that move the foot and lower leg are anchored to the tibia. The support and movement of the tibia is essential to many activities performed by the legs, including standing, walking, running, jumping and supporting the body's weight.

The tibia is located in the lower leg medial to the fibula, distal to the femur and proximal to the talus of the foot. It is widest at its proximal end near the femur, where it forms the distal end of the knee joint before tapering along its length to a much narrower bone at the ankle joint. The proximal end is roughly flat with the smooth, concave medial and lateral condyles forming the knee joint with the femur. Between the condyles is the intercondylar region, which includes the tibial spine and provides attachment points for the meniscus and anterior and posterior cruciate ligaments (ACL and PCL) of the knee. At the inferior edge of the lateral condyle is a small facet where the tibia forms the proximal tibiofibular joint with the fibula. This joint is a planar joint, allowing the tibia and fibula to slightly glide past each other and adjust the position of the lower leg.

Just below the condyles on the anterior surface is the tibial tuberosity, a major bony ridge that provides an attachment point for the patella through the patellar ligament. Extension of the lower leg involves the contraction of the rectus femoris muscle to pull on the patella, which in turn pulls on the tibial tuberosity. A thin, bony ridge known as the anterior crest continues distally from the tibial tuberosity, giving the shaft of the tibia a triangular cross section. The tibial tuberosity and anterior crest are clearly identifiable landmarks of the shin as they can be easily palpated through the skin.

Approaching the ankle joint, the tibia widens slightly in both the medial-lateral and anterior-posterior planes. On the medial side, the tibia forms a rounded bony prominence known as the medial malleolus. The medial malleolus forms the medial side of the ankle joint with the talus of the foot; it can be easily located by palpation of the skin in this region. On the lateral side of the tibia is a small recess known as the fibular notch, which forms the distal tibiofibular joint with the fibula.

The tibia is classified as a long bone due to its long, narrow shape. Long bones are hollow in the middle, with regions of spongy bone filling each end and solid compact bone covering their entire structure. Spongy bone is made of tiny columns known as trabeculae that reinforce the ends of the bone against external stresses. Red bone marrow, which produces blood cells, is found in the holes in the spongy bone between the trabeculae.

The hollow middle of the bone, known as the medullary cavity, is filled with fat-rich yellow bone marrow that stores energy for the body. Surrounding the medullary cavity and spongy bone is a thick layer of compact bone that gives the bone most of its strength and mass. Compact bone is made of cells surrounded by a matrix of hard calcium mineral and collagen protein that is both extremely strong and flexible to resist stress.

Surrounding the compact bone is a thin, fibrous layer known as the periosteum. Periosteum is made of a dense, fibrous connective tissue, which is continuous with the ligaments that connect the tibia to the surrounding bones and the tendons that connect the muscles to the tibia. These connections prevent the separation of the muscles and bones from each other.

Finally, a thin layer of hyaline cartilage covers the ends of the tibia where it forms the knee and ankle joints. Hyaline is extremely smooth and slightly flexible, providing a smooth surface for the joint to slide across and a shock absorber to resist impacts.

At birth, the tibia consists of two bones: a central shaft known as the diaphysis, and a thin cap just below the knee known as the proximal epiphysis. A thin layer of hyaline cartilage separates these two bones and allows them to move slightly relative to each other. The distal end of the tibia at the ankle is made of hyaline cartilage at birth, but begins to ossify around age 2 to form the distal epiphysis. Throughout childhood the diaphysis and the two epiphyses remain separated by a thin layer of hyaline cartilage known as the epiphyseal plate or the growth plate. Cartilage in the epiphyseal plate grows throughout childhood and adolescence and is slowly replaced by bone. The net result of this growth is the lengthening of the tibia. At the end of adolescence, the diaphysis and epiphyses grow into the last of the cartilage and fuse to form a single tibia. The region where the diaphysis and epiphyses fuse is known as the metaphysis.