General features.—The fibula is the lateral bone of the leg, and is very slender as compared with the tibia, for it is not called upon to share in the transmission of the body weight. It possesses an upper end or head, a shaft and a lower end, which constitutes the lateral malleolus. The shaft shows considerable variation in its form, for it is moulded by the muscles to which it gives attachment; and these variations may prove confusing to the junior student. The lower end should be identified first. It is expanded from before backwards but is somewhat flattened from side to side. It bears a triangular articular facet on the anterior part of its medial surface, which is directed medially to articulate with the talus. A well-marked depression, termed the malleolar fossa, lies posterior to the articular facet. The student has now been provided with sufficient information to enable him to determine the side to which a given fibula belongs.
The head of the fibula is slightly expanded in all its diameters, and projects beyond the shaft in front, behind and on the lateral side. It bears on its upper surface a somewhat circular articular facet, which articulates with the inferior surface of the lateral condyle of the tibia ; it is directed upwards, and slightly forwards and medially. A blunt elevation, termed the styloid process, projects upwards from the lateral part of its posterior aspect. The head of the fibula can be felt through the skin on the posterolateral aspect of the knee, nearly 2 cm. below the level of the knee-joint. Immediately below the head a large nerve, termed the lateral popliteal nerve, crosses the posterolateral aspect of the constricted upper end of the shaft and can
be rolled against the bone in the living subject. If sufficient pressure is exerted, tingling sensations will be experienced on the dorsum of the foot, radiating to the toes, and especially to the medial side of the great toe.
The lateral surface is bounded in front by the anterior and behind by the posterior border. It is associated with the peroneal muscles, and is directed laterally in its upper three-fourths. Its lower fourth inclines backwards and becomes continuous with the groove on the back of the lateral malleolus. The anterior surface is placed between the anterior and the interosseous borders. It is usually directed forwards and medially, but frequently faces directly forwards. Wider below, it becomes very narrow in its upper half, and may be reduced to little more than a rounded ridge on the upper part of the shaft. It is associated with the extensor muscles of the leg. The posterior surface is the largest of the three and is placed between the interosseous and the posterior borders. It is associated with the flexor muscles of the leg. In its upper two-thirds it is divided into two areas by a longitudinal ridge, termed the medial crest, which is separated from the interosseous border by a grooved surface, directed medially. The rest of the posterior surface faces backwards in its upper half or more, but its lower part, which inclines medially and forwards, is directed medially. The lower part of this area fits into the fibular notch on the tibia and is roughened for the attachment of the interosseous tibiofibular ligament.
Particular features. The head of the fibula affords origin to fibers of the extensor digitorum longus in front, peroneus longus anterolaterally, and soleus behind. The tendon of biceps femoris receives its principal insertion into the anterolateral sloping surface of the styloid process, but it is split near its insertion by the lower end of the lateral ligament of the knee-joint, and its smaller anterior part passes into the lateral aspect of the head, above the origin of the peroneus longus. A flattened impression on the lateral aspect of the head receives the lower attachment of the lateral ligament of the knee-joint. The margins of the articular facet provide attachment for the capsular ligament of the superior tibiofbular joint.
The anterior surface of the fibula is often termed the extensor surface, for it gives origin to the extensor digitorum longus and the extensor hallucis longus in addition to the peroneus tertius. The extensor digitorum longus arises from the whole breadth of the upper fourth of the surface and from the anterior part of the succeeding two-fourths ; the extensor hallucis longus arises from its middle two-fourths behind the extensor digitorum longus ; the peroneus tertius arises from its lower fourth or more, and is directly continuous with the lower part of the extensor digitorum longus. The lateral surface is frequently termed the peroneal surface, because it gives origin to the peroneus longus and the peroneus brevis. The former arises from the whole extent of the upper third of the surface and from the posterior part of the middle third. The peroneus brevis arises in front of the lower half of the peroneus longus and extends downwards beyond it almost to the looter end of the bone. On account of the relative attachments of their fleshy bellies the tendon of the peroneus brevis is closely applied to the bone below and separates it from the tendon of the peroneus longus. The posterior surface, which is divided longitudinally into two parts by the medial crest, is often termed the flexor surface. The portion which lies between the crest and the interosseous border is slightly hollowed out and gives origin to the tibialis posterior; it is often crossed by an oblique ridge, which corresponds to the attachment of an intramuscular tendon. This part of the surface is usually confined to the tipper three-fourths of the shaft, and at its lower end the medial crest becomes confluent with the interosseous border. The portion of the posterior surface which lies between the medial crest and the posterior border gives origin in its upper fourth to the soleus, which extends upwards on to the posterior aspect of the head ; near the upper end of the medial part of this origin a roughened tubercle marks the lateral end of the tendinous arch which is thrown across the posterior tibial vessels and nerve by the soleus muscle.
Ossification.-The fibula is ossified from three centers (fig. 478) : one for the shaft, and one for each end. Ossification begins in the shaft: about the eighth week of fetal life, in the lower end during the second year, and in the upper about the fourth year. The lower epiphysis, the first to ossify, unites with the shaft about the twentieth year ; the upper about the twenty-fifth year.
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