Fibula

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.

Figure 476
The lower end or lateral malleolus projects downwards considerably below the tibia. Its lateral surface is subcutaneous and can be felt through the skin without difficulty ; its posterior aspect is marked by a broad groove with a prominent lateral border. Its anterior aspect is rough and rounded and is continuous below with the inferior border. The medial aspect (fig. 477) presents a triangular articular facet, with its apex pointing downwards, which articulates with the lateral aspect of the talus in the ankle-joint ; it is convex from above downwards. Behind the articular facet the bone is marked by a roughened depression, termed the malleolar fossa which readily admits the tip of a finger.

Figure 477
The shaft of the fibula (figs. 471, 473) possesses three borders and three surfaces, each of which can be associated with a particular group of muscles. The borders are anterior, posterior and interosseous, and they should be identified in the first instance. The lateral surface of the lateral malleolus is continuous above with an elongated triangular area on the shaft, which is subcutaneous. The rest of the shaft is covered with muscles and cannot be examined satisfactorily in the living subject. The anterior and posterior margins of this triangular area meet above, where they are continuous with the anterior border of the bone, which ascends to reach the anterior aspect of the head. The posterior border is continuous with the medial margin of the groove on the back of the lateral malleolus. Usually sharp and distinct in its lower part, it is often rounded in the upper half of its extent. The interosseous border lies to the medial side of the anterior border and as a rule is on a more posterior plane (fig. 476), but in the upper two-thirds of the bone these two borders are very close to each other, and the intervening surface may be r mm. or less in width.

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.

Figure 478
The anterior border of the fibula divides inferiorly into two ridges which enclose between them a subcutaneous triangular surface (fig. 477). The anterior intermuscular septum of the leg is attached to its upper three-fourths, and the lateral extremity of the superior extensor retinaculum (transverse ligament of the leg) to the lower part of the anterior border of the triangular area. The lower part of the posterior margin of the triangular area gives attachment to the lateral extremity of the superior peroneal retinaculum. The interosseous border terminates below at the upper extremity of the roughened area for the attachment of the tibiofibular interosseous ligament. It provides attachment for the interosseous membrane and, on account of the gap in the upper part of the membrane for the transmission of the anterior tibial vessels, does not reach so high as the head of the bone. The posterior border is not always recognizable at its upper end; below it becomes continuous with the medial border of the groove on the back of the lateral malleolus. Except at its lower end it gives attachment to the posterior intermuscular septum of the leg. The medial crest of the bone is intimately related to the peroneal artery, and the nutrient foramen of the fibula is situated either on the crest or in its immediate vicinity near the middle of the shaft. It gives attachment to a layer of the deep fascia of the leg which separates the tibialis posterior from the flexor hallucis longus and the flexor digitorum longus muscles.

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.

Figure 479
Below the origin of the soleus the remainder of this aspect gives origin to the flexor hallucis longus, which extends downwards almost to the lower end of the bone. The triangular area of the shaft above the lateral malleolus is covered only by the superficial fascia and the slain. Above the lateral malleolus a triangular area on the medial aspect of the shaft gives attachment to the interosseous tibiofibular ligament (fig. 477) and its anterior and posterior margins to the corresponding ligaments of the inferior tibiofibular joint.

Figure 480
The anterior aspect of the lateral malleolus gives attachment to the anterior talofibular ligament. The lower border is marked in front by a slight notch and behind by a small projection which constitutes the apex of the malleolus. It is to the notch that the calcaneofibular ligament is attached. The groove on the posterior aspect lodges the tendons of the peroneus brevis and peroneus longus ; the latter is the more superficial and is closely covered by the superior peroneal retinaculum. The malleolar fossa (fig. 477) is pitted by numerous small vascular foramina ; its upper part gives attachment to the inferior transverse tibioibular ligament; its lower part to the posterior talofibular ligament..

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.

Figure 481

 

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