TIBIA

The tibia also known as the shin or shank bone which is a strong long bone of the leg which is medial to the fibula. It is larger, stronger and frontal to the fibula. It is the weight bearing bone of the leg due to which it is the 2^nd largest bone in the body after femur. It bears an axial force upto 4.7 bodyweight. Its bending moment in the sagittal plane in the late stance phase is upto 71.6 bodyweight times millimeter. The bone is named after flute. The tibia forms knee joint proximally with femur, connected with fibula laterally with interosseous membrane and forms a fibrous joint known as syndesmosis & distally tibia forms ankle joint with fibula and talus.

Tibia extension and flexion at the knee joint and dorsiflexion and plantarflexion at the ankle joint. Tibia is composed of a diaphysis and 2 epiphyses. The diaphysis is the midsection of the tibia also known as the shaft of tibia. The only thing epiphyses are the 2 rounded extremities of bone: an upper or proximal and lower or distal. The tibia is most contracted in lower third and distal extremity is smaller than proximal.

There are three parts of the tibia: proximal, shaft and distal.

PROXIMAL END

The proximal part of the tibia consists of medial and lateral condyle which together form the inferior portion of the knee joint. It is flattened in the horizontal plane. Both the condyles are separated by the intercondylar area. The proximal end includes Medial condyle, Lateral condyle, Intercondylar area and Tibial tuberosity.

Medial Condyle

The medial condyle is round. It is larger in comparison to the lateral condyle. The superior surface articulates with the medial condyle of the femur. The surface is oval central part is slightly concave which directly comes in contact with the femoral condyle and the peripheral part is flat and is separated from the femoral condyle by the medial meniscus. The lateral margin of the articular surface is raised to cover the medial intercondylar tubercle. The posterior surface of the medial condyle has a groove. The anterior and medial surfaces are marked by numerous vascular foramina.

Lateral Condyle

The lateral condyle is round, convex and articulates with the lateral condyle of the femur. It is smaller than the medial condyle. The lateral meniscus attaches to all its margins except for the elevated medial margin, which extends to the lateral intercondylar tubercle. The posteroinferior aspect of the lateral condyle articulates with the fibula. The fibula facet is flat, circular and is directed downwards, backwards and laterally. Superomedial to the fibular facet, the posterior surface of the condyle is marked by the groove. The anterior aspect of the condyle bears a flattened impression known as Gerdy’s tubercle.

Intercondylar Area

It is the irregular, non-articular, roughened area on the superior surface between both articular surfaces of the 2 condyles. The middle part is the narrowest. This part is elevated to form the intercondylar eminence (spine of tibia) which is flanked by the medial and lateral intercondylar tubercles. The area widens behind and in front of the eminence as the articular surfaces diverge. The eminence has attachments to the anterior and posterior cruciate ligaments and menisci.

The anterior intercondylar area is the widest anteriorly. Anterior horn of medial meniscus is attached to the medial articular surface anteriorly at the intercondylar area. At the back of the anterior horn of medial meniscus at the smooth area anterior cruciate ligament is there. The anterior horn of the lateral meniscus is attached anterior to the intercondylar eminence, lateral to the anterior cruciate ligament. The anterolateral region of the anterior intercondylar area is perforated by numerous small openings of the nutrient arteries.

The eminence, with medial and lateral tubercles, is the narrow central part of the area. The raised tubercles are thought to provide a slight stabilizing influence on the femur. It is believed that the eminence becomes prominent once walking commences and that the tibial condyles transmit the weight of the body through the tibia.

The posterior horn of the lateral meniscus is attached to the posterior slope of the intercondylar area. The posterior intercondylar area inclines down and backwards behind the posterior horn of the lateral meniscus. A depression behind the base of the medial intercondylar tubercle is for the attachment of the posterior horn of the medial meniscus. The rest of the area is smooth and provides attachment for the posterior cruciate ligament, spreading back to a ridge to which the capsule is attached.

Tibial Tuberosity

The tibial tuberosity is a prominence which is a triangular, broad above, narrow below & located on the anterior aspect of the upper end of the tibia. It limits intercondylar area anteriorly. Inferiorly its continuous with anterior border of the shaft of the tibia. The tuberosity is divided into upper smooth area and a lower rough area. The lower rough area is palpable and is separated from the skin by the subcutaneous infrapatellar bursa. The patellar ligament is attached to the upper smooth surface.

SHAFT

The shaft of the tibia is prismoid in shape. The forward flat part is known as Fibia which is often confused with the fibula. It has three borders—anterior, medial and interosseous; and three surfaces—lateral, medial and posterior.

Borders

Anterior Border

It is the most prominent border which is above the tibial tuberosity and ends below at the anterior margin of the medial malleolus like S-shaped. Its sinuous and prominent in the upper 2/3^rd of its extent but smooth and rounded below and gives attachment to the deep fascia of the leg which forms the shin. It divides medial and lateral surface.

Medial Border

The medial border descends from the anterior end of the groove on the medial condyle to the posterior border of the medial malleolus. The medial border is the most prominent at the medial aspect of the middle third of the tibia. The border in the anteromedial side is smooth and rounded above and below.

Lateral or Interosseous Border

The interosseous or lateral border is thin and prominent. The lateral border begins from the distal and anterior to the fibular articular facet and descends to the anterior border of the fibular notch and is indistinct proximally. The lateral border connects to the interosseous border of the fibula by the interosseous membrane.

Surfaces

Lateral Surface

The lateral surface is between the anterior and lateral borders of tibia which is broad and smooth. The surface in its upper 3/4^th is concave and directed laterally and in the lower 1/4^th is smooth, convex & is directed forwards. The surface is covered by the muscles of the anterior leg compartment.

Medial Surface

The medial surface is smooth, convex and broader above than below. The surface is bounded by anterior and medial borders. The lower 2/3^rd surface is commonly known as the shin of tibia as it is subcutaneous & having only little amount of fat between skin and bone and has no muscle attachments along the most of it. Hence this surface is palpable in the anteromedial aspect of the leg entirely. The upper 1/3^rd is directed forwards and medially which is covered by the aponeurosis derived from the sartorius, gracilis and semitendinosus which are inserted far forward at the anterior crest.

Posterior Surface

The posterior surface is between the medial and lateral borders of the tibia. It is widest in the upper part which is crossed obliquely by the rough line known as the soleal line. The soleal line is beginning behind the fibular facet which descends downwards and medially and ends by joining the medial border at the junction of its upper and middle thirds. The area below the soleal line is elongated. It is divided into medial and lateral parts by a vertical ridge. A nutrient foramen is situated near the upper end of this ridge. It is directed downwards and transmits the nutrient artery which is a branch of the posterior tibial artery.

LOWER OR DISTAL END

The distal end of the tibia changes the shape from triangular in cross section to rectangular like a box in the cross section. The distal end is smaller than the proximal end of the tibia. it is prolonged downwards on its medial side as a strong pyramidal process known as the medial malleolus. The lower extremity of the tibia forms ankle joint with the fibula and talus. The lower end has 5 surfaces: Anterior, Posterior, Lateral, Distal and Medial & Medial malleolus.

Surfaces

Anterior Surface

The surface is smooth and rounded above; covered by the tendons of the extensor muscles. The lower margin of the anterior surface presents a rough transverse depression for the attachment of the articular capsule of ankle joint.

Posterior Surface

The posterior surface is a smooth surface and a shallow, vertical groove which is directed obliquely downward & medially and is continuous with the posterior groove of the talus. The posterior surface is smaller than the anterior surface. The groove serves as a passage for the tendon of tibialis posterior muscle which usually separates the tendon of the flexor digitorum longus from the bone. And laterally to that, the surface has contact of posterior tibial vessels, tibial nerve and flexor hallucis longus.

Lateral Surface

The lateral surface has a triangular fibular notch which is rough on the upper side and on the lower side has a smooth depression. The upper part gives attachment to the inferior interosseous ligament which connects with the fibula. The lower part is covered with cartilage and articulates with fibula and forms the distal tibiofibular joint.

Distal Surface

The distal surface is quadrilateral that is wider in front, smooth, concave sagittally and slightly convex transversely which looks like a saddle shape and articulates with the talus and forms the ankle joint. On the medial side of the distal surface, it continues in the malleolar articular surface and extends into the groove which further separates it from the anterior surface of the shaft. Such extensions, medial or lateral or both, are squatting facets, and they articulate with reciprocal talar facets in extreme dorsiflexion. These features have been used in the field of forensic medicine to identify the race of skeletal material.

Medial Surface

The medial surface is smooth, subcutaneous and visible. The surface is continuous above and below with the medial surface of the shaft of tibia and medial malleolus.

Medial Malleolus

The medial malleolus is short, thick, strong and has a smooth surface. It has a crescentic facet which articulates with the medial surface of the talus. It forms a subcutaneous prominence on the medial side of the ankle. Its anterior aspect is rough, and its posterior aspect features the continuation of the groove from the posterior surface of the tibial shaft for the tendon of tibialis posterior. The distal border is pointed anteriorly, posteriorly depressed, and gives attachment to the deltoid ligament. The tip of the medial malleolus does not project as far distally as the tip of the lateral malleolus, the latter also being the more posteriorly located of the two malleoli. The capsule of the ankle joint is attached to the anterior surface of the medial malleolus, and the flexor retinaculum is attached to its prominent posterior border.

VASCULAR SUPPLY

The proximal end of tibia is supplied by the metaphyseal arteries deriving from the genicular anastomosis. The periosteal supply to the shaft arises from the anterior tibial artery and from muscular branches. The distal metaphysis is supplied by branches from the arterial anastomosis around the ankle. In the nutrient foramen, nutrient artery enters which is branch of posterior tibial artery. The nutrient artery of tibia is the largest nutrient artery of the body which is directed downwards.

NERVOUS SUPPLY

The nervous supply of tibia are all the branches of main nerves which innervates adjacent compartments. The posterior part of tibia is supplied by the posterior tibial nerves and in the anterior compartment is supplied by the deep fibular nerves.

MUSCLE ATTACHMENTS

·       Tensor fascia lata tendon inserts at the Gerdy’s tubercle which is at the lateral tubercle of tibia.

·       Tendon of the quadriceps femoris muscle also known as ligamentum patellae inserts anteriorly at the tibial tuberosity.

·       Tendons of Sartorius, gracilis and semitendinosus inserts on the upper part of the medial surface at the shaft from before backwards.

·       The tendon of horizontal head of semimembranosus tendon inserts into the groove of posterior surface of the medial condyle.

·       Tendon of the popliteus inserts on the groove of the posterior surface of the lateral condyle of the tibia that is on the soleal line.

·       Tibialis anterior originates from the upper 2/3^rd of the lateral side of the shaft of tibia.

·       Extensor digitorum longus originates from the lateral condyle of tibia

·       Soleus arises from the soleal line.

·       Flexor digitorum longus origins on the soleal line.

·       On the medial condyle, capsular ligament of knee joint attaches on the upper border and deeper fibres of tibial collateral ligament. Medial patellar retinaculum is attached on anterior surface.

·       At the lateral condyle capsular ligament of superior tibiofibular joint is attached around the margins of fibular facet.

OSSIFICATION

Tibia ossifies from 3 centres; 1 primary center at the diaphysis or the shaft of tibia and 2 secondary centres one at each epiphysis. Ossification begins at the centre of the shaft at around the 7^th week of fetal life. The centre of upper epiphysis appears before or immediately after birth close to 34^th week of gestation & fuses with the shaft around 16^th to 18^th year. the upper epiphysis forms a smooth surface at the tibial tuberosity projecting downwards. The centre for lower epiphysis appears during the 1st year forms medial malleolus by the 7^th year and fuses with the shaft by 15^th– 17^th year.

Written & Complied by Dr. Palak Shah