Tibia

The tibia is a long bone which transmits the body weight from the medial and lateral condyles of the femur to the foot. It is by far the larger of the two bones of the leg, being situated medial to the fibula. It consists of a shaft and two extremities, the upper extremity being much larger than the lower.

The upper end is expanded in all directions, but particularly in its posterior dimension where it projects backwards beyond the line of the shaft. It consists of two condyles having between them anteriorly a large, truncated area elongated in its vertical axis, roughened in its upper and smooth in its lower parts. This is the tibial tuberosity, the roughened area of which gives attachment to the ligamentum patella. The lateral condyle projects further laterally than the shaft and has a round articular facet on its posterolateral part for articulation with the head of the fibula. Posteriorly, the space between the condyles is smooth. The two concyles have on their upper surfaces areas for articulation with the condyles of the femur. These superior surfaces are divided by two raised tubercles, the medial and lateral intercondylar tubercles, which are close together and termed the intercondylar eminence. In front of and behind the eminence there is an uneven non-articular area which is narrower close to the eminence and which becomes wider as it passes forwards and backwards. This area gives attachment to some important structures of the knee joint. Anterior to the intercondylar eminence three structures are attached: most anteriorly is the anterior horn of the medial meniscus, whilst closest to the eminence is the anterior horn of the lateral meniscus, and between the two the anterior cruciate ligament. The area behind the intercondylar eminence also gives attachment to three structures: most posteriorly is the posterior cruciate ligament, whilst closest to the eminence is the posterior horn of the lateral meniscus, between the two is the posterior horn of the medial meniscus.

The shaft of the tibia is triangular in cross-section tapering slightly from the condyles for about two-thirds of its length, widening again at its lower end. It has an anterior border which runs from the lower part of the tuberosity downwards to the anterior part of the medial malleolus. The medial border begins just below the back of the medial condyle, and although not always easy to see, can be traced to the posterior part of the medial malleolus. The interosseus border begins just below the articular facet on the lateral condyle and runs in a curved line with its concavity forwards, down to the roughened triangular area on the lateral side at the lower end of the bone.

The shaft therefore has three surfaces: medial, posterior and lateral. The smooth medial surface, sloping posteriorly from the anterior border, is subcutaneous in the whole of its extent, from the medial condyle above to the medial malleolus below and is commonly called the shin. The lateral surface between the anterior and interosseus borders is slightly concave, particularly in its upper two-thirds, and gives attachment to tibialis anterior. Inferiorly, it becomes continuous with the anterior surface of the lower end of the bone. The posterior surface between the interosseus and medial borders is crossed by two raised lines, one running obliquely from just below the lateral condyle downwards and medially to join the posterior border about half way down; this is the soleal line. The area above it is roughened for the attachment of the popliteus muscle. Below the soleal line is a vertical line to which the fascia covering tibialis posterior is attached. It divides the lower part of the posterior surface into two, both being roughened for the attachment of muscle, laterally tibialis posterior and medially flexor digitorum longus.

The lower end of the tibia is expanded, but to a lesser extent than the upper. It has a prominent medial malleolus which is continuous with the medial surface of the shaft projecting downwards from its medial side. The inferior surface is smooth for articulation with the superior surface of the body of the talus. Medially it is continuous with the malleolar articular surface. It usually turns upwards on the lateral surface where it becomes concave anteriorly for articulation with the fibula. It continues up as a rough triangular area for the attachment of the interosseus ligament. The posterior surface is coarse and grooved by tendons passing into the foot. The anterior surface is smooth and slightly convex.

Ossification

The primary ossification centre of the tibia appears in the shaft during the seventh week in utero and spreads so that at birth only the ends are cartilaginous. The secondary centre for the proximal end, which includes the tibial tuberosity, appears at birth, spreading down to the tuberosity after the tenth year. An independent centre for the tuberosity may appear; if it does so, this appears at 11 years. The secondary centre for the distal end appears during the second year. Fusion of the proximal epiphysis with the shaft occurs between 19 and 21 years, and of the distal epiphysis with the shaft a few years earlier, between 17 and 19.

Palpation

The tubercle of the tibia is easily recognizable at the upper end of the anterior border of the tibia(the shin), with the ligamentum patellae attaching to its upper portion. The medial and lateral condyles can be palpated about 2cm higher, as they are subcutaneous as far as the hamstring muscles on either side. The upper edge indicates the line of the knee joint.

Just below and behind the mid-point on the lateral side, the head of the fibula stands out clearly. Running down the whole length of the bone from the medial surface of the medial condyle is the inner surface of the shaft, being subcutaneous as far as the medial malleolus. Both anterior and posterior borders are palpable at its edges. The medial malleolus is subcutaneous and its medial surface, borders and tip are easily palpable.

Applied anatomy

Due to the fact that the medial surface of the tibia is subcutaneous, the risks of damage and fracture of this bone are increased. In addition, the likelihood of infection, and delayed/non-union of the bone is very high and a common complication in this region. The most common area of damage is at the junction between the upper two-thirds and the lower third of the shaft, this being its thinnest part, and unfortunately the area with the poorest blood supply.