General features. The tibia is the medial and much the stronger of the two bones of the leg, and, excepting the femur, is the longest bone of the skeleton. It is prismoid in form, and possesses a shaft and two ends. Its lower end, though large, is smaller than the upper end, and on its medial side a stout process, termed the medial malleolus, projects downwards beyond the rest of the bone. The anterior border of the shaft forms a conspicuous sharp crest, which curves medially at the lower end towards the medial malleolus ; it is the most prominent of the three borders. The student should now be able to refer a given tibia to its correct side.
The upper end of the tibia is expanded, especially in its transverse axis, to provide a good bearing surface for the lower end of the femur. It comprises two prominent masses, named the medial and lateral condyles, and a smaller projection, termed the tubercle of the tibia. The condyles project backwards a little, so as to overhang the upper part of the posterior surface of the shaft, and superiorly each is covered by an articular surface, the two being separated by an irregularly roughened intercondylar area.
The medial condyle is the larger but does not project so much as the lateral condyle. Its upper articular surface (fig. 469), oval in outline, is concave in both diameters, and its lateral border projects upwards, deepening the concavity and covering an elevation, termed the medial intercondylar tubercle. The posterior surface of the condyle is marked, immediately below the articular margin, by a transverse roughened groove. Its medial and anterior aspects form a rough strip, separated from the anterior surface of the shaft by an inconspicuous ridge.
The anterior surfaces of the two condyles become continuous in front with a large triangular area, the apex of which is directed downwards and is formed by the tubercle of the tibia. The lateral edge of this area forms a sharp ridge which separates the lateral condyle from the lateral surface of the shaft.
The intercondylar area (fig. 469) forms a roughened strip on the superior surface which intervenes between the articular surfaces of the two condyles. It is narrowest at its central part, where it is marked by an elevation, termed the intercondylar eminence. The lateral and medial parts of the eminence project slightly upwards, and constitute the lateral and medial intercondylar tubercles. Both behind and in front of the eminence the intercondylar area becomes wider, as the curved margins of the articular surfaces recede from each other.
The tibial tubercle is placed at the upper end of the anterior border of the shaft, and at the truncated apex of the triangular area on the front of the bone where the anterior surfaces of the two condyles become continuous. It forms a low eminence, divided into a lower roughened and an upper smooth portion. The lower part can be felt through the skin, from which it is separated only by a bursa, termed the sub-cutaneous infrapatellar bursa ; the upper part gives attachment to the ligamentum patella.
Particular features.-The articular surface of the medial condyle is oval in shape, with its long axis anteroposterior. It is related around its anterior, medial and posterior margins to the medial semilunar cartilage (medial meniscus)- and the area of contact is flattened. The imprint of the cartilage, which is widest behind and narrower at the medial side and in front, can often be recognized on the bone. The rest of the surface is concave, and its raised lateral margin covers the medial intercondylar tubercle. The articular surface of the lateral condyle is more nearly circular in shape. Like the medial articular surface it is related to, and bears the flattened imprint of, the corresponding semilunar cartilage. Elsewhere the surface is very slightly concave to adapt it to the surface of the corresponding femoral condyle. and its raised medial margin is continued on to the lateral aspect of the lateral intercondylar tubercle. The edges of the two articular surfaces are sharp except at the posterior part of the lateral surface, where the margin is smooth and rounded ; in this situation the tendon of the popliteus is intimately related to the bone. The anterior and lateral margins of the lateral surface and the anterior, medial and posterior margins of the medial surface give attachment to the coronary ligaments.
The intercondylar area (fig. 470) is widest anteriorly. In its anteromedial part, just in front of the medial articular surface, it bears a slight depression which gives attachment to the anterior horn of the medial semilunar cartilage. Behind that depression a relatively smooth area affords attachment to the lower end of the anterior cruciate ligament. The anterior horn of the lateral semilunar cartilage is attached to the bone in front of the intercondylar eminence and lies lateral to the anterior cruciate ligament. The intercondylar eminence occupies the narrow central part of the area, and is surmounted by two tubercles, of which the medial is slightly the more prominent. The posterior slope of the eminence gives attachment to the posterior horn of the lateral semilunar cartilage, and behind that the intercondylar area inclines downwards and backwards. A depression behind the base of the medial intercondylar tubercle gives attachment to the posterior horn of the medial semilunar cartilage. The rest of the area is smooth and affords attachment to the lower end of the posterior cruciate ligament, as far back as the ridge to which the capsular ligament is attached.
The groove on the posterior aspect of the medial condyle receives the insertion of the semimembranosus muscle ; its upper border receives the capsular, and its lower border the posterior and shorter fibers of the medial ligament of the knee-joint. The medial and anterior surfaces of the condyle, which are pitted by numerous vascular foramina, give attachment to the medial patellar retinaculum.
The fibular facet on the lateral condyle is directed downwards and slightly backwards and laterally. Above and to its medial side the posterior aspect of the condyle is grooved by the tendon of the popliteus. but a bursa intervenes between the tendon and the bone. The lateral and anterior aspects of the condyle are separated from the lateral surface of the shaft by a sharp margin, which gives attachment to the deep fascia of the leg. An impression on the anterior surface, often well-marked though flattened affords attachment for the iliotibial tract. Near the fibulas facet the uppermost fibers of the extensor digitorum longus and peroneus longus arise from the lateral surface.
The tibial tubercle is subcutaneous in its lower part only ; its upper part receives the attachment of the ligamentum patellae. The two areas are sometimes separated by a rough crest, which receives the superficial fibers of the ligament. Above the tubercle the bone is related to the deep surface of the ligament, but the deep infrapatellar bursa and some fabrofatty tissue intervene.
The shaft of the tibia (figs. 471, 473) is triangular on section, and possesses medial, lateral and posterior surfaces, separated by anterior, interosseous and medial borders. It is thinnest at the junction of its middle and lower thirds, but expands considerably towards its upper and lower ends.
The anterior border commences at the tibial tubercle and runs downwards to the medial malleolus. It is subcutaneous throughout its whole length, and, except in its lower fourth, where it is rounded and indistinct, forms a sharp crest, which is familiarly known as the ‘shin bone.’ It is not straight, but follows a slightly sinuous course and its lower fourth diverges towards the medial side. The interosseous border commences below and a little in front of the fibular facet on the lateral condyle and descends to reach the anterior border of the fibular notch, which marks the lateral aspect of the lower end of the tibia. In nearly the whole of its length it affords attachment to the interosseous membrane which connects the tibia to the fibula, As a rule it is poorly defined at its uppermost end, but is easily identified in the rest of its extent. The medial border commences below the anterior end of the groove on the medial condyle and runs downwards to the posterior margin of the medial malleolus. Its upper and lower fourths are rounded and ill-defined, but its middle third is sharper and can be recognized without difficulty.
Particular features.-The anterior border provides attachment for the deep fascia of the leg throughout its whole extent. Just above the medial malleolus it receives the medial end of the superior extensor retinaculum (transverse ligament of the leg). Above the soleal line the medial border gives attachment to subcutaneous the fascia covering the popliteus muscle and to the posterior fibers of the medial (tibial collateral) ligament of the knee-joint; below the soleal line it gives origin for a short distance to fibers of the soleus muscle and attachment to the fascia which covers the deep muscles of the leg. At its lower end it becomes continuous with the medial border of the groove which lodges the tendon of the tibialis posterior muscle. The interosseous border gives attachment to the interosseous membrane of the leg, except at its upper and lower ends. Its upper end is scarcely recognizable, and in this situation there is a large gap in the interosseous membrane for the passage of the anterior tibial vessels. Its lower end forms the anterior boundary of the fibular notch and gives attachment to the anterior inferior tibiofibular ligament.
The lateral surface gives origin in its upper two-thirds, or less, to the tibialis anterior muscle. Its lower part is devoid of muscular attachments, but is crossed by the tendon of the tibialis anterior (which lies along the lateral side of the anterior border), the extensor hallucis longtus, the anterior tibial vessels and nerve (deep peroneal nerve), the extensor digitorum longus and peroneus tertius, in that order from the medial to the lateral side.
The posterior surface gives insertion to the popliteus over the triangular area above the soleal line, with the exception of the area adjoining the fibular facet. The soleal line gives attachment to the strong fascia which covers the popliteus muscle, and to the soleus muscle, its covering fascia, and the fascia covering the deep muscles of the leg. The upper end of the line does not reach the interosseous border and is marked by a tubercle, which gives attachment to the medial end of the tendinous arch in the coleus. Lateral to that tubercle the posterior tibial vessels and nerve descend on the surface of the tibialis posterior. Below the soleal line, the vertical line separates the origin of the flexor digitorum longus on the medial side from the origin of the tibialis posterior (fig. 474). The lower fourth, more, of the posterior surface is devoid of muscular attachments, but is intimately related to the tendon of the tibialis posterior as it runs downwards and medially to reach the groove on the back of the medial malleolus. The flexor digitorum longus lies on the posterior surface of the tibialis posterior, crossing it obliquely from the medial to the lateral side, but the posterior tibial vessels and nerve and the flexor hallucis longus come into contact with the lateral part of this surface for a short distance above the lower end of the bone.
The anterior surface, is smooth and bulges forwards beyond the inferior surface, from which it is separated by a narrow groove. It is continuous above with the lateral surface of the shaft. The medial surface is also smooth and is continuous above with the medial surface of the shaft and below with the medial surface of the medial malleolus. It is subcutaneous and can easily be felt through the skin. The posterior surface is crossed at its medial end by a groove, which is usually conspicuous and can be traced down on to the posterior aspect of the medial malleolus. Elsewhere this aspect of the lower end is smooth and is continuous above with the posterior surface of the shaft. The Lateral surface is formed by a triangular notch, termed the fibular notch, which is intimately related to the lower end of the fibula. The anterior and posterior borders of the notch are salient and converge to meet above on the interosseous crest. The floor of the notch is roughened in its upper part for the attachment of the strong interosseous ligament which binds the lower ends of the two bones securely together. Its lower part is smooth and is sometimes covered with articular cartilage. The inferior surface is smooth for articulation with the body of the talus. Wider in front than behind, it is concave from before backwards and slightly convex from side to side. Medially it is uninterruptedly continuous with the articular surface of the medial malleolus.
Particular features.-The anterior surface of the lower end is related to the tendons, vessels and nerve which lie on the lower part of the lateral surface of the shaft and have already been enumerated. The narrow groove adjoining the anterior border of the inferior surface gives attachment to the anterior and capsular ligaments of the anklejoint. The groove on the posterior surface lodges the tendon of the tibialis posterior, which usually separates the flexor digitorum longus tendon from the bone. More medially the posterior tibial vessels and nerve and the flexor hallucis longus tendon are in contact with this surface. The floor of the fibular notch, especially in its rough upper part, gives attachment to the interosseous tibiofbular ligament; its lower smooth part may be covered with articular cartilage. The anterior and posterior borders of the notch give attachment respectively to the anterior and posterior inferior tibiofibular ligaments. The medial malleolus is shorter than the lateral malleolus and is placed somewhat anterior to it. Its anterior aspect gives attachment to the anterior and capsular ligaments of the ankle-joint. Its posterior aspect is grooved by the tibialis posterior tendon, and the raised medial margin of the groove gives attachment to the flexor retinaculum (laciniate ligament). The upper end of the deltoid ligament is attached to the lower border of the malleolus, both to the pointed anterior part and to the depression behind it.
Ossification.-The tibia is ossified from three centers (figs. 472, 474, 475) : one for the shaft and one for each end. Ossification begins in the middle of the shaft about the seventh week of feotal life. The center for the upper end appears before or shortly after birth, and from it a thin tongue-shaped process extends downwards in front, to form the tubercle (fig. 475); the center for the lower end appears in the second year. The lower end joins the shaft about the eighteenth year; the upper about the twentieth year. Two additional centers occasionally exist, one for the tongue-shaped process which forms the tubercle, and one for the medial malleolus.
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