The tibia or shin-bone (French: le tibia)is situated at the front and medial side of the leg and nearly parallel with the fibula. Excepting the femur, it is the largest bone in the skeleton, and alone transmits the weight of the trunk to the foot. It articulates above with the femur, below with the tarsus, and laterally with the fibula. It is divisible into two extremities and a shaft.

The upper extremity (or head) of the tibia

The upper extremity (or head) consists of two lateral eminences, or condyles. Their superior articular surfaces receive the condyles of the femur, the articular parts being separated by a non-articular interval, to which ligaments are attached. The medial articular surface is oval in shape and concave for the medial condyle of the femur. The lateral articular surface is smaller, somewhat circular in shape, and presents an almost plane surface for the lateral condyle. The peripheral portion of each articular surface is overlaid by a fibro-cartilaginous meniscus of semilunar shape, connected with the margins of the condyles by bands of fibrous tissue termed coronary ligaments. Each semilunar meniscus is attached firmly to the rough interval separating the articular surfaces. This interval is broad and depressed in front, the anterior intercondyloid fossa, where it affords attachment to the anterior extremities of the medial and lateral menisci and the anterior crucial ligament; elevated in the middle to form the intercondyloid eminence or spine of the tibia, a prominent eminence, presenting at its summit two compressed intercondyloid tubercles, on to which the condylar articular surfaces are prolonged ; the posterior aspect of the base of the eminence affords attachment to the posterior extremities of the lateral and medial semilunar menisci, and limits a deep notch, inclined toward the medial condyle, known as the posterior intercondyloid fossa or popliteal notch. It separates the condyles on the posterior aspect of the head and gives attachment to the posterior crucial ligament, and part of the posterior ligament of the knee-joint. Anteriorly, the two condyles are confluent, and form a somewhat flattened surface of triangular outline, the apex of which forms the tuberosity of the tibia. The tuberosity is divisible into two parts. The upper part, rounded and smooth, receives the attachment of the ligamentum patellae. The lower part is rough, and into its lateral edges prolongations of the ligamentum patella are inserted. A prominent bursa intervenes between the ligament and the anterior aspect of the upper extremity of the bone.

The medial condyle is less prominent though more extensive than the lateral, and near the posterior part of its circumference is a deep horizontal groove for the attachment of the central portion of the semimembranosus tendon. The margins of this groove, and the surface of bone below, give attachment to the tibial (internal) lateral ligament of the knee. On the under aspect of the lateral condyle is a rounded articular facet for the head of the fibula, flat and nearly circular in outline, directed downward, backward, and laterally. The circumference of the facet is rough and gives attachment to the ligaments of the superior tibio-fibular joint, while above and in front of the facet, at the junction of the anterior and lateral surfaces of the condyle, is a ridge for the ilio-tibial band. A slip from the tendon of the biceps and parts of the extensor longus digitorum and peroneus longus muscles are attached to the head below the ilio-tibial band.

The shaft or body of the tibia

The shaft or body [corpus] of the tibia, thick and prismatic above, becomes thinner as it descends for about two-thirds of its length, and then gradually expands toward its lower extremity. It presents for examination three borders and three surfaces. The anterior border is very prominent and known as the anterior crest of the tibia. It commences above on the lateral edge of the tuberosity and terminates below at the anterior margin of the medial malleolus. It runs a some- what sinuous course, and gives attachment to the deep fascia of the leg. The medial border extends from the back of the medial condyle to the posterior margin of the medial malleolus, and affords attachment above, for about three inches, to the tibial (internal) lateral ligament of the knee-joint and in the middle third, to the soleus. The interosseous crest or lateral border, thin and prominent, gives attachment to the interosseous membrane. It commences in front of the fibular facet, on the upper extremity, and toward its termination bifurcates to enclose a triangular area for the attachment of the interosseous ligament uniting the lower ends of the tibia and fibula.

The medial surface is bounded by the medial margin and the anterior crest; it is broad above, where it receives the insertions of the sartorius, gracilis, and semi-tendinous; convex and subcutaneous in the remainder of its extent. The lateral surface lies between the crest of the tibia and the interosseous crest. The upper two-thirds presents a hollow for the origin of the tibialis anterior; the rest of the surface is convex and covered by the extensor tendons and the anterior tibial vessels. The posterior surface is limited by the interosseous crest and the medial border. The upper part is crossed obliquely by a rough popliteal line, extending from the fibular facet on the lateral condyle to the medial border, a little above the middle of the bone.

The popliteal line gives origin to the soleus and attachment to the popliteal fascia, while the triangular surface above is occupied by the popliteus muscle. Descending along the posterior surface from near the middle of the popliteal line is a vertical ridge, well-marked at its commencement, but gradually becoming indistinct below. The portion of the surface between the ridge and the medial border gives origin to the flexor digitorum longus; the lateral and narrower part, between the ridge and the interosseous border, to fibers of the tibialis posterior. The lower third of the posterior surface is covered by flexor tendons and the posterior tibial vessels. Immediately below the popliteal line and near the interosseous border is the large medullary foramen directed obliquely downward.

The lower extremity

The lower extremity, much smaller than the upper, is quadrilateral in shape and presents a strong process called the medial malleolus, projecting downward from its medial side. The anterior surface of the lower extremity is smooth and rounded above, where it is covered by the extensor tendons, rough and depressed below for the attachment of the anterior ligament of the ankle-joint. It some- times bears a facet for articulation with the neck of the talus (astragalus). (A. Thomson.) The posterior surface is rough and is marked by two grooves. The medial and deeper of the two encroaches on the malleolus, and receives the tendons of the tibialis posterior and flexor digitorum longus; the lateral, very shallow and sometimes indistinct, is for the tendon of the flexor hallucis longus. The lateral sur- face is triangular and hollowed for the reception of the lower end of the fibula and rough for the interosseous ligament which unites the two bones, except near the lower border, where there is usually a narrow surface, elongated from before back- ward, covered with cartilage in the recent state for articulation with the fibula. The lines in front of and behind the triangular surface afford attachment to the anterior and posterior ligaments of the inferior tibio-fibular articulation. The medial surface, prolonged downward on the medial malleolus, is rough, convex, and subcutaneous. The lateral surface of this process is smooth and articulates with the facet on the medial side of the talus (astragalus). Its lower border is notched, and from the notch, as well as from the tip and anterior border, the fibers of the deltoid ligament of the ankle-joint descend. The inferior or terminal surface, by which the tibia articulates with the talus, is of quadrilateral form, concave from before backward, wider in front than behind, and laterally than medially where it is continuous with the lateral surface of the malleolus.

The occasional facet on the anterior surface of the lower extremity of the tibia is a pressure facet, produced by extreme flexion of the ankle joint. It is therefore sometimes designated as the squatting facet."

Blood-supply of the tibia

The tibia is a very vascular bone. The nutrient artery of the shaft is furnished by the posterior tibial, and is the largest of its kind in the body. The head of the bone receives numerous branches from the inferior articular arteries of the popliteal and the recurrent branches of the anterior and posterior tibial. The lower extremity receives twigs from the posterior and anterior tibial, the peroneal, and the medial malleolar arteries.

Ossification of the tibia

The tibia is ossified from one principal center for the shaft, which appears in the eighth week of intra-uterine life, and two epiphyses, the centers for which appear in the cartilaginous head of the bone toward the end of the ninth month, and in the lower extremity during the second year. The latter unites with the shaft at eighteen, but the union of the head with the shaft does not take place until the twenty-first year, and it may even be delayed until twenty-five. The upper part of the tubercle of the tibia is ossified from the upper epiphysis, and the lower part from the diaphysis.