The tibia, also called the shinbone or shankbone, is the larger and stronger of the two bones in the lower leg, located below the knee in animals with backbones. The other bone in this area is the fibula, which is positioned behind and to the outside of the tibia. The tibia connects the knee to the ankle and is found on the inner side of the leg, near the middle part of the body. The tibia and fibula are joined by a fibrous tissue called the interosseous membrane, forming a type of joint with very little movement. The name "tibia" comes from the Greek word for a type of flute. It is the second largest bone in the human body, after the femur. The bones in the legs are the strongest long bones because they support the rest of the body.

Structure

In human anatomy, the tibia is the second largest bone in the body after the femur. Like other vertebrates, the tibia is one of two bones in the lower leg, the other being the fibula. It is a part of the knee and ankle joints. The tibia and fibula together form the front part of the leg, between the knee and the ankle, known as the shin.

The formation of the tibia begins from three centers: one in the middle of the bone and one at each end. The tibia is classified as a long bone, meaning it has a shaft (called the diaphysis) and two rounded ends (called the epiphyses). The shaft is the middle section of the tibia, while the epiphyses are the rounded ends. The upper end (called the proximal epiphysis) is closer to the thigh, and the lower end (called the distal epiphysis) is closer to the foot. The tibia is narrower in the lower third, and the distal end is smaller than the proximal end.

The proximal end of the tibia is wider and has two rounded parts called the medial and lateral condyles. These condyles are flat on the top and bottom. The medial condyle is larger and sits above the shaft. The top surfaces of the condyles connect with the femur to form the tibiofemoral joint, which supports the weight of the body in the knee.

The medial and lateral condyles are separated by a region called the intercondylar area. This area has bumps called the medial and lateral intercondylar tubercles, which form the intercondylar eminence. Together with the condyles, this region creates the tibial plateau, which connects with the lower end of the femur. The intercondylar eminence divides the intercondylar area into front and back sections. Small openings in the front part of the intercondylar area allow blood vessels to pass through.

The top surfaces of the condyles are curved inward, especially in the center. The flatter edges of the condyles touch the menisci, which are cartilage pads in the knee. The top surface of the medial condyle is oval-shaped and extends onto the side of the medial intercondylar tubercle. The top surface of the lateral condyle is more circular and extends onto the side of the lateral intercondylar tubercle. The back of the medial condyle has a groove where a muscle called the semimembranosus attaches. The lateral condyle has a rounded surface that connects with the head of the fibula.

Below the condyles is the tibial tuberosity, where the patellar ligament attaches. This ligament is part of the quadriceps muscle.

The top surface of the tibia has two smooth areas called articular facets. The medial facet is oval-shaped and slightly curved from side to side and front to back. The lateral facet is nearly circular and curved from side to side but slightly curved the other way at the back. These facets connect with the condyles of the femur and support the menisci in the knee.

In the intercondylar area, between the articular facets, is the intercondyloid eminence (also called the spine of the tibia). This area has bumps on either side where the articular facets extend. In front of and behind the eminence are rough areas where ligaments and menisci attach.

The front surfaces of the condyles form a flat, triangular area. This area has large openings for blood vessels and ends in the tibial tuberosity, where the patellar ligament attaches. A bursa (a fluid-filled sac) lies between the ligament and the bone above the tuberosity.

The back of the condyles is separated by a shallow depression called the posterior intercondyloid fossa, where part of the posterior cruciate ligament attaches. The back of the medial condyle has a deep groove for the tendon of the semimembranosus muscle.

The medial surface of the tibia is curved outward and rough. It connects with the medial collateral ligament of the knee. The lateral surface is also curved outward and rough. It has a bump where the iliotibial band attaches. Below this bump, part of the extensor digitorum longus muscle begins, and a part of the biceps femoris tendon attaches.

The shaft of the tibia is triangular in shape and has three borders: the front (anterior), inner (medial), and outer (lateral or interosseous). These borders form three surfaces: the medial, lateral, and posterior.

The front border is the most prominent and starts at the tibial tuberosity and ends at the front edge of the medial malleolus. It is rough and connects with the deep fascia of the leg.

The medial border is smooth and rounded at the top and bottom but more prominent in the middle. It starts at the back of the medial condyle and ends at the back of the medial malleolus. It connects with the tibial collateral ligament and some fibers of the popliteus muscle. Some fibers of the soleus and flexor digitorum longus muscles also begin here.

The lateral border is thin and prominent in the center. It connects with the interosseous membrane, which joins the tibia and fibula. It starts in front of the fibular articular facet and splits below to form a triangular area for the interosseous ligament.

The medial surface is smooth and curved outward. Its upper part is covered by tendons from the sartorius, gracilis, and semitendinosus muscles. The rest of the surface is under the skin.

The lateral surface is narrower than the medial surface. Its upper two-thirds have a groove where the tibialis anterior muscle begins. Its lower third is smooth and curved forward, covered by tendons from the tibialis anterior, extensor hallucis longus, and extensor digitorum longus muscles.

The back surface of the tibia has a ridge called the popliteal line, which runs from the back of the fibular articular facet to the medial border. This ridge marks the lower edge of the popliteus muscle and connects with the soleus, flexor digitorum longus, and tibialis posterior muscles. The area above the popliteal line is where the popliteus muscle attaches. The middle third of the back surface is divided by a vertical ridge into two parts.

Function

The tibia has been shown to experience an axial force during walking that is up to 4.7 times a person's bodyweight. In the sagittal plane during the late stance phase, its bending moment reaches up to 71.6 times the person's bodyweight multiplied by a millimetre.

Clinical significance

Fractures of the tibia can be broken into two main groups: those that affect only the tibia, such as bumper fracture, Segond fracture, Gosselin fracture, toddler's fracture, and those that involve both the tibia and fibula, such as trimalleolar fracture, bimalleolar fracture, and Pott's fracture.

The tibial shaft is where stress fractures most often happen in athletes.

Other animals

The structure of the tibia in most other tetrapods is very similar to that in humans. In mammals, the tuberosity of the tibia is a crest where the patellar ligament attaches. In reptiles, birds, and amphibians, this same area is where the tendon of the quadriceps muscle attaches. These animals do not have a patella.

Additional images

• Shape of right tibia
• 3D image of tibia
• Lengthwise view of the tibia showing inside parts
• Right knee joint viewed from the front
• Right knee joint from the front, showing inside ligaments
• Left knee joint viewed from the back, showing inside ligaments
• Left talocrural joint
• Front-to-back cut through the right talocrural and talocalcaneal joints
• Top of the foot. Ankle joint. Detailed dissection
• Top of the foot. Ankle joint. Detailed dissection
• Ankle joint. Detailed dissection. Viewed from the front
• Bones of the right leg. Front side
• Bones of the right leg. Back side
• Top of the foot. Ankle joint. Detailed dissection
• Ankle joint. Detailed dissection
• Ankle joint. Detailed dissection
• Ankle joint. Detailed dissection
• Ankle joint. Detailed dissection
• Tibia anatomy overview