The fibula is one of two long bones in the lower leg. A long bone is defined as one whose body is longer than it is wide, with growth plates (epiphysis) at either end, having a hard outer surface of compact bone and a spongy inner surface known as cencellous bone containing bone marrow. Both ends of the bone are covered in hyaline cartilage to help protect the bone and aid in shock absorption. The fibula is much more slender than the tibia and is not directly involved in weight transmission. It has a proximal head, a narrow neck, a long shaft and a distal lateral malleolus.
Articulations & Ligaments
The fibula and tibia are the two long bones of the lower leg and run parallel to each other. They are similar in length but the fibula is much thinner than the tibia. This is suggestive of the weight bearing contributions of each bone. For example the thicker tibia has much greater role in weight bearing than the fibula.
The tibia and fibula articulate through two joints - the superior and inferior tibiofibular joints. The superior tibiofibular joint is a synovial plane joint with the transverse joint line spanning the lateral tibial condyle and the medial fibular head. The capsule is thickened anteriorly and posteriorly and joins with the anterior ligament, relating closely to the tendon of biceps femoris. The inferior tibiofibular joint is a syndesmosis joint, just above the ankle region which lies between the medial distal end of the fibula and the concave fibular notch region of the tibia.
The distal end of the joint has a 4 mm separation from the prolongation of the ankle joint. There is no fibrous capsule surrounding this joint but there is a flat band of ligament named the anterior tibiofibular ligament which descends laterally between the two leg bones.
The tibia and fibula also articulate via an interosseous membrane known as the middle tibiofibular ligament. This ligament extends through the fibula’s and tibia's interosseous crests and separates the muscles in the back of the leg from the muscles located in the front of the leg. It is made of an aponeurotic lamina, which is thin and made of oblique fibres.
Head of Fibula
The head of the fibula is irregular in shape and directed upward, forward, and medialward. A round facet on its proximomedial aspect articulates with the corresponding facet on the inferolateral surface of the lateral tibial condyle. The inclination of this facet can vary among individuals from almost horizontal to an angle of 45 degrees.
The styloid process is a palpable bony prominence located about 2 cm distal to the knee joint. The fibular collateral ligament attaches just in front of this and the tibiofibular capsular ligament attaches to the margins of the articular facet. A nerve (the common fibular nerve) passes over the head of the fibula and can be rolled against the bony fibula; this can be sensitive on palpation.
Shaft of Fibula
The shaft of the fibula has four borders and four surfaces each corresponding to a particular group of muscles.
The antero-lateral border begins above in front of the head, runs vertically downward to a little below the middle of the bone, and then curving somewhat lateralward, bifurcates so as to embrace a triangular subcutaneous surface immediately above the lateral malleolus. This border gives attachment to an intermuscular septum, which separates the extensor muscles on the anterior surface of the leg from the peroneus longus and peroneus brevis on the lateral surface.
The antero-medial border, or interosseous crest, is situated close to the medial side of the preceding, and runs nearly parallel with it in the upper third of its extent, but diverges from it in the lower two-thirds. It begins above just beneath the head of the bone (sometimes it is quite indistinct for about 2.5 cm. below the head), and ends at the apex of a rough triangular surface immediately above the articular facet of the lateral malleolus. It serves for the attachment of the interosseous membrane, which separates the extensor muscles in front from the flexor muscles behind.
The postero-lateral border is prominent; it begins above at the apex, and ends below in the posterior border of the lateral malleolus. It is directed lateralward above, backward in the middle of its course, backward, and a little medialward below, and gives attachment to an aponeurosis which separates the peronei on the lateral surface from the flexor muscles on the posterior surface.
The postero-medial border, sometimes called the oblique line, begins above at the medial side of the head, and ends by becoming continuous with the interosseous crest at the lower fourth of the bone. It is well-marked and prominent at the upper and middle parts of the bone. It gives attachment to an aponeurosis which separates the tibialis posterior from the soleus and flexor hallucis longus.
The anterior surface is the interval between the antero-lateral and antero-medial borders. It is extremely narrow and flat in the upper third of its extent; broader and grooved longitudinally in its lower third; it serves for the origin of three muscles:
The posterior surface is the space included between the postero-lateral and the postero-medial borders; it is continuous below with the triangular area above the articular surface of the lateral malleolus; it is directed backward above, backward and medialward at its middle, directly medialward below. Its upper third is rough, for the origin of the soleus; its lower part presents a triangular surface, connected to the tibia by a strong interosseous ligament; the intervening part of the surface is covered by the fibers of origin of the flexor hallucis longus. Near the middle of this surface is the nutrient foramen, which is directed downward.
The medial surface is the interval included between the antero-medial and the postero-medial borders. It is grooved for the origin of the tibialis posterior.
The lateral surface is the space between the antero-lateral and postero-lateral borders. It is broad, and often deeply grooved; it is directed lateralward in the upper two-thirds of its course, backward in the lower third, where it is continuous with the posterior border of the lateral malleolus. This surface gives origin to the peronei longus and brevis.
In front of the styloid process, the biceps femoris embraces the fibular collateral ligament. Extensor digitorum longus is attached to the head anteriorly, while fibularis longus attaches anterolaterally and soleus posteriorly. Extensor digitorum longus, extensor hallucis longus and fibularis tertius are attached to the extensor surface.
Muscle attachments to the posterior surface are complex. Between the crest and interosseous border is a concave posterior surface to which is attached the tibialis posterior. An intramuscular tendon may ridge the bone obliquely. Soleus attaches between the crest and posterior border on the proximal fourth of the posterior surface, its tendinous arch is attached to the surface proximally. Flexor hallucis longus attaches distally to soleus on the posterior surface, almost reaching the distal end of the shaft.