CHAPTER 83 Leg

This chapter describes the shafts of the tibia and fibula, the soft tissues that surround them and the interosseous membrane between them. The superior (proximal) and inferior (distal) tibiofibular joints are described in Chapters 82 and 84 respectively.

SKIN AND SOFT TISSUE

SKIN

Vascular supply and lymphatic drainage

The cutaneous arterial supply is derived from branches of the popliteal, anterior tibial, posterior tibial and fibular vessels (see Fig. 79.5). Multiple fasciocutaneous perforators from each vessel pass along intermuscular septa to reach the skin; musculocutaneous perforators traverse muscles before reaching the skin. In some areas there is an additional direct cutaneous supply from vessels that accompany cutaneous nerves, e.g. the descending genicular artery (saphenous artery) and superficial sural arteries. Fasciocutaneous and direct cutaneous branches have a longitudinal orientation in the skin, whereas the musculocutaneous branches are more radially oriented. For further details consult Cormack & Lamberty (1994).

Cutaneous veins are tributaries of vessels that correspond to the named arteries. Cutaneous lymphatic vessels running on the medial side of the leg accompany the long saphenous vein, and drain to the superficial inguinal nodes, while those from the lateral and posterior sides of the leg accompany the short saphenous vein and pierce the deep fascia to drain into the popliteal nodes.

Innervation

The skin of the leg is supplied by branches of the saphenous, common fibular and tibial nerves (see below and Figs 79.17; 79.20).

SOFT TISSUE

Deep fascia

The deep fascia of the leg, the fascia cruris, is continuous with the fascia lata and is attached around the knee to the patellar margin, the patellar tendon, the tuberosity and condyles of the tibia, and the head of the fibula. Posteriorly, where it covers the popliteal fossa as the popliteal fascia, it is strengthened by transverse fibres and often perforated by the short saphenous vein and sural nerve. It receives lateral expansions from the tendon of biceps femoris and multiple medial expansions from the tendons of sartorius, gracilis, semitendinosus and semimembranosus. The deep fascia blends with the periosteum on the subcutaneous surface of the tibia and the subcutaneous surfaces of the fibular head and malleolus, and is continuous below with the extensor and flexor retinacula. It is thick and dense in the proximal and anterior part of the leg, where fibres of tibialis anterior and extensor digitorum longus are attached to its deep surface, and is thinner posteriorly where it covers gastrocnemius and soleus. On the lateral side it is continuous with the anterior and posterior crural intermuscular septa, which are attached to the anterior and posterior borders of the fibula respectively. A broad transverse intermuscular septum, the deep transverse fascia of the leg, passes between the superficial and deep muscles in the calf.

Deep transverse fascia

The deep transverse fascia of the leg is a fibrous stratum between the superficial and deep muscles of the calf. It extends transversely from the medial margin of the tibia to the posterior border of the fibula. Proximally, where it is thick and dense, it is attached to the soleal ridge of the tibia and to the fibula, inferomedial to the fibular attachment of soleus. Between these bony attachments it is continuous with the fascia covering popliteus which is, in effect, an expansion from the tendon of semimembranosus. At intermediate levels it is thin, but distally, where it covers the tendons behind the malleoli, it is thick and continuous with the flexor and superior fibular retinacula.

Interosseous membrane

The interosseous membrane connects the interosseous borders of the tibia and fibula (Fig. 83.1). It is interposed between the anterior and posterior groups of crural muscles; some members of each group are attached to the corresponding surface of the interosseous membrane. The anterior tibial artery passes forwards through a large oval opening near the proximal end of the membrane, and the perforating branch of the fibular artery pierces it distally. Its fibres are predominantly oblique and most descend laterally; those which descend medially include a bundle at the proximal border of the proximal opening. The membrane is continuous distally with the interosseous ligament of the distal tibiofibular joint. Tibialis anterior, extensor digitorum longus, extensor hallucis longus, fibularis tertius, the anterior tibial vessels and deep fibular nerve are all anterior to the membrane, and tibialis posterior and flexor hallucis longus are posterior.

Fig. 83.1 Posterior aspect of the interosseous membrane. Note the contrasting direction of the fibre bundles around the vascular openings.

BONE

TIBIA

The tibia lies medial to the fibula and is exceeded in length only by the femur (Figs 83.2, 83.3). Its shaft is triangular in section and has expanded ends; a strong medial malleolus projects distally from the smaller distal end. The anterior border of the shaft is sharp and curves medially towards the medial malleolus. Together with the medial and lateral borders it defines the three surfaces of the bone. The exact shape and orientation of these surfaces show individual and racial variations.

Fig. 83.2 A, Left tibia and fibula: anterior aspect. B, The muscle attachments. A: 1. Medial condyle. 2. Tibial tuberosity. 3. Anterior border of tibia. 4. Interosseous border of tibia. 5. Medial surface. 6. Medial malleolus. 7. Gerdy’s tubercle. 8. Lateral condyle. 9. Head of fibula. 10. Interosseous border of fibula. 11. Anterior border of fibula. 12. Medial crest. 13. Anterior surface. 14. Subcutaneous area. 15. Lateral malleolus. B: 1. Semimembranosus. 2. Medial patellar retinaculum. 3. Epiphysial line (growth plate). 4. Medial collateral ligament. 5. Gracilis. 6. Sartorius. 7. Semitendinosus. 8. Tibialis anterior. 9. Capsular attachment. 10. Iliotibial tract. 11. Capsular attachment. 12. Lateral collateral ligament. 13. Biceps femoris. 14. Patellar tendon. 15. Epiphysial line (growth plate). 16. Fibularis longus. 17. Extensor digitorum longus. 18. Tibialis posterior. 19. Fibularis brevis. 20. Extensor hallucis longus. 21. Extensor digitorum longus. 22. Fibularis tertius. 23. Epiphysial line (growth plate). 24. Epiphysial line (growth plate).

Fig. 83.3 A, Left tibia and fibula: posterior aspect. B, The muscle attachments. A: 1. Groove for tendon of popliteus. 2. Styloid process (apex) of fibula. 3. Head of fibula. 4. Neck of fibula. 5. Medial crest. 6. Interosseous border of tibia. 7. Posterior border. 8. Groove for fibular tendons. 9. Lateral malleolus. 10. Intercondylar eminence. 11. Groove for semimembranosus attachment. 12. Soleal line. 13. Nutrient foramen. 14. Vertical line. 15. Medial border of tibia. 16. Medial malleolus. 17. Groove for tibialis posterior tendon. B: 1. Gap in capsule for popliteus tendon. 2. Soleus. 3. Flexor hallucis longus. 4. Fibularis brevis. 5. Epiphysial line (growth plate). 6. Capsular attachment. 7. Semimembranosus. 8. Epiphysial lines (growth plates). 9. Popliteus. 10. Soleus. 11. Tibialis posterior. 12. Flexor digitorum longus. 13. Epiphysial line (growth plate). 14. Capsular attachment.

Proximal end

The expanded proximal end is a bearing surface for body weight, which is transmitted through the femur. It consists of medial and lateral condyles, an intercondylar area and the tibial tuberosity.

Condyles

The tibial condyles overhang the proximal part of the posterior surface of the shaft. Both condyles have articular facets on their superior surfaces that are separated by an irregular, non-articular intercondylar area. The condyles are visible and palpable at the sides of the patellar tendon, the lateral being more prominent. In the passively flexed knee the anterior margins of the condyles are palpable in fossae that flank the patellar tendon.

The fibular facet on the posteroinferior aspect of the lateral condyle faces distally and posterolaterally. The angle of inclination of the superior tibiofibular joint varies between individuals, and may be horizontal or oblique. Superomedial to it the condyle is grooved on its posterolateral aspect by the tendon of popliteus; a synovial recess intervenes between the tendon and bone. The anterolateral aspect of the condyle is separated from the lateral surface of the shaft by a sharp margin for the attachment of deep fascia. The distal attachment of the iliotibial tract makes a flat but definite marking, Gerdy’s tubercle, on its anterior aspect. This tubercle, which is triangular and facet-like, is usually palpable.

The anterior condylar surfaces are continuous with a large triangular area whose apex is distal and formed by the tibial tuberosity. The lateral edge is a sharp ridge between the lateral condyle and lateral surface of the shaft. The condyles, their articular surfaces and the intercondylar area are described in Chapter 82.

Tibial tuberosity

The tibial tuberosity is the truncated apex of a triangular area where the anterior condylar surfaces merge. It projects only a little, and is divided into a distal rough and a proximal smooth region. The distal region is palpable and is separated from skin by the subcutaneous infrapatellar bursa. A line across the tibial tuberosity marks the distal limit of the proximal tibial growth plate (Fig. 83.2). The patellar tendon is attached to the smooth bone proximal to this, its superficial fibres reaching a rough area distal to the line. The deep infrapatellar bursa and fibroadipose tissue intervene between the bone and tendon proximal to its site of attachment. The latter may be marked distally by a somewhat oblique ridge, onto which the lateral fibres of the patellar tendon are inserted more distally than the medial fibres. (This knowledge is necessary to avoid damaging the tendon when sawing the tibia transversely just above the tibial tuberosity in a lateral to medial direction, e.g. in performing an osteotomy.) In habitual squatters a vertical groove on the anterior surface of the lateral condyle is occupied by the lateral edge of the patellar tendon in full flexion of the knee.

Shaft

The shaft is triangular in section and has (antero)medial, lateral and posterior surfaces separated by anterior, lateral (interosseous) and medial borders. It is narrowest at the junction of the middle and distal thirds, and expands gradually towards both ends. The anterior border descends from the tuberosity to the anterior margin of the medial malleolus and is subcutaneous throughout. Except in its distal fourth, where it is indistinct, it is a sharp crest. It is slightly sinuous, and turns medially in the distal fourth. The interosseous border begins distal and anterior to the fibular facet and descends to the anterior border of the fibular notch; it is indistinct proximally. The interosseous membrane is attached to most of its length, connecting tibia to fibula. The medial border descends from the anterior end of the groove on the medial condyle to the posterior margin of the medial malleolus. Its proximal and distal fourths are ill defined but its central region is sharp and distinct.

The anteromedial surface, between the anterior and medial borders, is broad, smooth and almost entirely subcutaneous. The lateral surface, between the anterior and interosseous borders, is also broad and smooth. It faces laterally in its proximal three-fourths and is transversely concave. Its distal fourth swerves to face anterolaterally, on account of the medial deviation of the anterior and distal interosseous borders. This part of the surface is somewhat convex. The posterior surface, between the interosseous and medial borders, is widest above, where it is crossed distally and medially by an oblique, rough soleal line. A faint vertical line descends from the centre of the soleal line for a short distance before becoming indistinct. A large vascular groove adjoins the end of the line and descends distally into a nutrient foramen. Deep fascia and, proximal to the medial malleolus, the medial end of the superior extensor retinaculum, are attached to the anterior border. Posterior fibres of the medial collateral ligament and slips of semimembranosus and the popliteal fascia are attached to the medial border proximal to the soleal line, and some fibres of soleus and the fascia covering the deep calf muscles are attached distal to the line. The distal medial border runs into the medial lip of a groove for the tendon of tibialis posterior. The interosseous membrane is attached to the lateral border, except at either end of this border. It is indistinct proximally where a large gap in the membrane transmits the anterior tibial vessels. Distally the border is continuous with the anterior margin of the fibular notch, to which the anterior tibiofibular ligament is attached.

The anterior part of the medial collateral ligament is attached to an area approximately 5 cm long and 1 cm wide near the medial border of the proximal medial surface. The remaining medial surface is subcutaneous and crossed obliquely by the long saphenous vein. Tibialis anterior is attached to the proximal two-thirds of the lateral surface. The distal third, devoid of attachments, is crossed in mediolateral order by the tendons of tibialis anterior (lying just lateral to the anterior border), extensor hallucis longus, the anterior tibial vessels and deep fibular nerve, extensor digitorum longus and fibularis tertius.

On the posterior surface, popliteus is attached to a triangular area proximal to the soleal line, except near the fibular facet. The popliteal aponeurosis, soleus and its fascia, and the deep transverse fascia are all attached to the soleal line: the proximal end of the line does not reach the interosseous border, and is marked by a tubercle for the medial end of the tendinous arch of soleus. Lateral to the tubercle, the posterior tibial vessels and tibial nerve descend on tibialis posterior. Distal to the soleal line, a vertical line separates the attachments of flexor digitorum longus and tibialis posterior. Nothing is attached to the distal quarter of this surface, but the area is crossed medially by the tendon of tibialis posterior travelling to a groove on the posterior aspect of the medial malleolus. Flexor digitorum longus crosses obliquely behind tibialis posterior; the posterior tibial vessels and nerve and flexor hallucis longus contact only the lateral part of the distal posterior surface.

Distal end

The slightly expanded distal end of the tibia has anterior, medial, posterior, lateral and distal surfaces. It projects inferomedially as the medial malleolus. The distal end of the tibia, when compared to the proximal end, is laterally rotated (tibial torsion). The torsion begins to develop in utero and progresses throughout childhood and adolescence till skeletal maturity is attained. Tibial torsion is approximately 30° in Caucasian and Asian populations, but is significantly greater in people of African origin (Eckhoff et al 1994). Some of the femoral neck anteversion seen in the newborn may persist in adult females: this causes the femoral shaft and knee to be internally rotated, and the tibia may develop a compensatory external torsion to counteract the tendency of the feet to turn inwards.

The smooth anterior surface projects beyond the distal surface, from which it is separated by a narrow groove. The capsule of the ankle joint is attached to an anterior groove near the articular surface. The medial surface is smooth and continuous above and below with the medial surfaces of the shaft and medial malleolus respectively: it is subcutaneous and visible. The posterior surface is smooth except where it is crossed near its medial end by a nearly vertical but slightly oblique groove, which is usually conspicuous and extends to the posterior surface of the malleolus. The groove is adapted to the tendon of tibialis posterior, which usually separates the tendon of flexor digitorum longus from the bone. More laterally, the posterior tibial vessels and nerve and flexor hallucis longus contact this surface. The lateral surface is the triangular fibular notch; its anterior and posterior edges project and converge proximally to the interosseous border. The floor of the notch is roughened proximally by a substantial interosseous ligament but is smooth distally and sometimes covered by articular cartilage. The anterior and posterior tibiofibular ligaments are attached to the corresponding edges of the notch. The distal surface articulates with the talus and is wider in front, concave sagittally and slightly convex transversely, i.e. it is saddle-shaped. Medially it continues into the malleolar articular surface which may extend into the groove that separates it from the anterior surface of the shaft. Such extensions, medial or lateral or both, are squatting facets, and they articulate with reciprocal talar facets in extreme dorsiflexion. These features have been used in the field of forensic medicine to identify the racial origins of skeletal material.

Medial malleolus

The short thick medial malleolus has a smooth lateral surface with a crescentic facet that articulates with the medial surface of the talus. Its anterior aspect is rough and its posterior aspect features the continuation of the groove from the posterior surface of the tibial shaft for the tendon of tibialis posterior. The distal border is pointed anteriorly, posteriorly depressed, and gives attachment to the deltoid ligament. The tip of the medial malleolus does not project as far distally as the tip of the lateral malleolus; the latter also being the more posteriorly located of the two malleoli. The capsule of the ankle joint is attached to the anterior surface of the medial malleolus, and the flexor retinaculum to its prominent posterior border.

Muscle attachments

The patellar tendon is attached to the proximal half of the tibial tuberosity. Semimembranosus is attached to the distal edge of the groove on the posterior surface of the medial condyle; a tubercle at the lateral end of the groove is the main attachment of the tendon of this muscle. Slips from the tendon of biceps femoris are attached to the lateral tibial condyle anteroproximal to the fibular facet. Proximal fibres of extensor digitorum longus and (occasionally) fibularis longus are attached distal to this area. Slips of semimembranosus are attached to the medial border of the shaft posteriorly, proximal to the soleal line. Some fibres of soleus attach to the posteromedial surface distal to the line. Semimembranosus is attached to the medial surface proximally, near the medial border, behind the attachment of the anterior part of the medial collateral ligament. Anterior to this area (in anteroposterior sequence), are the linear attachments of the tendons of sartorius, gracilis and semitendinosus: these rarely mark the bone. Tibialis anterior is attached to the proximal two-thirds of the lateral (extensor) surface. Popliteus is attached to the posterior surface in a triangular area proximal to the soleal line, except near the fibular facet. Soleus and its associated fascia are attached to the soleal line itself. Flexor digitorum longus and tibialis posterior are attached to the posterior surface distal to the soleal line, medial and lateral respectively to the vertical line (see above).

Vascular supply

The proximal end of the tibia is supplied by metaphysial vessels from the genicular arterial anastomosis. The nutrient foramen usually lies near the soleal line and transmits a branch of the posterior tibial artery; the nutrient vessel may also arise at the level of the popliteal bifurcation or as a branch from the anterior tibial artery. On entering the bone the nutrient artery divides into ascending and descending branches. The periosteal supply to the shaft arises from the anterior tibial artery and from muscular branches. The distal metaphysis is supplied by branches from the anastomosis around the ankle.

Innervation

The proximal and distal ends of the tibia are innervated by branches from the nerves that supply the knee joint and ankle joint respectively. The periosteum of the shaft is supplied by branches from the nerves that innervate the muscles attached to the tibia.

Ossification

The tibia ossifies from three centres, one in the shaft and one in each epiphysis. Ossification (Figs 83.2, 83.3, 83.4; see Fig. 82.6) begins in midshaft at about the seventh intrauterine week. The proximal epiphysial centre is usually present at birth: at approximately 10 years a thin anterior process from the centre descends to form the smooth part of the tibial tuberosity. A separate centre for the tuberosity may appear at about the twelfth year and soon fuses with the epiphysis. Distal strata of the epiphysial plate are composed of dense collagenous tissue in which the fibres are aligned with the patellar tendon. Exaggerated traction stresses may account for Osgood–Schlatter disease, where fragmentation of the epiphysis of the tuberosity occurs during adolescence and produces a painful swelling in the region of the tuberosity. Healing occurs once the growth plate fuses, leaving a bony protrusion. Prolonged periods of traction with the knee extended, both in children and adolescents, can lead to growth arrest of the anterior part of the proximal epiphysis, which results in bowing of the proximal tibia as the posterior tibia continues to grow. The proximal epiphysis fuses in the 16th year in females and the 18th in males. The distal epiphysial centre appears early in the first year and joins the shaft at about the 15th year in females and the 17th in males. The medial malleolus is an extension from the distal epiphysis and starts to ossify in the seventh year: it may have its own separate ossification centre. In 47% of females and 17% of males, an accessory ossification centre appears at the tip of the medial malleolus which fuses during the eighth year in females and the ninth in males: it should not be confused with an os subtibiale, which is a rare accessory bone found on the posterior aspect of the medial malleolus.

Fig. 83.4 Stages in the ossification of the tibia (not to scale).

FIBULA

The fibula (Figs 83.2, 83.3) is much more slender than the tibia and is not directly involved in transmission of weight. It has a proximal head, a narrow neck, a long shaft and a distal lateral malleolus. The shaft varies in form, being variably moulded by attached muscles: these variations may be confusing.

Head

The head of the fibula is irregular in shape and projects anteriorly, posteriorly, and laterally. A round facet on its proximomedial aspect articulates with a corresponding facet on the inferolateral surface of the lateral tibial condyle. It faces proximally and anteromedially and has an inclination which may vary among individuals from almost horizontal to an angle of up to 45°. A blunt apex (styloid process) projects proximally from the posterolateral aspect of the head and is often palpable approximately 2 cm distal to the knee joint. The fibular collateral ligament is attached in front of the apex, embraced by the main attachment of biceps femoris. The tibiofibular capsular ligament is attached to the margins of the articular facet. The common fibular nerve crosses posterolateral to the neck and can be rolled against bone at this location.

Shaft

The shaft has three borders and surfaces, each associated with a particular group of muscles. The anterior border ascends proximally from the apex of an elongated triangular area that is continuous with the lateral malleolar surface, to the anterior aspect of the fibular head. The posterior border, continuous with the medial margin of the posterior groove on the lateral malleolus, is usually distinct distally but often rounded in its proximal half. The interosseous border is medial to the anterior border and somewhat posterior. Over the proximal two-thirds of the fibular shaft the two borders approach each other, with the surface between the two being narrowed to 1 mm or less.

The lateral surface, between the anterior and posterior borders and associated with the fibular muscles, faces laterally in its proximal three-fourths. The distal quarter spirals posterolaterally to become continuous with the posterior groove of the lateral malleolus. The anteromedial (sometimes simply termed anterior, or medial) surface, between the anterior and interosseous borders, usually faces anteromedially but often directly anteriorly. It is associated with the extensor muscles. Though wide distally, it narrows in its proximal half and may become a mere ridge. The posterior surface, between the interosseous and posterior borders, is the largest and is associated with the flexor muscles. Its proximal two-thirds is divided by a longitudinal medial crest, separated from the interosseous border by a grooved surface that is directed medially. The remaining surface faces posteriorly in its proximal half; its distal half curves onto the medial aspect. Distally this area occupies the fibular notch of the tibia, which is roughened by the attachment of the principal interosseous tibiofibular ligament. The triangular area proximal to the lateral surface of the lateral malleolus is subcutaneous; muscles cover the rest of the shaft.

The anterior border is divided distally into two ridges that enclose a triangular subcutaneous surface. The anterior intermuscular septum is attached to its proximal three-fourths. The lateral end of the superior extensor retinaculum is attached distally on the anterior border of the triangular area and the lateral end of the superior fibular retinaculum is attached distally on the posterior margin of the triangular area. The interosseous border ends at the proximal limit of the rough area for the interosseous ligament. The interosseous membrane attached to this border does not reach the fibular head, which leaves a gap through which the anterior tibial vessels pass. The posterior border is proximally indistinct, and the posterior intermuscular septum is attached to all but its distal end. The medial crest is related to the fibular artery. A layer of deep fascia separating the tendon of tibialis posterior from flexor hallucis longus and flexor digitorum longus is attached to the medial crest.

Lateral malleolus

The distal end forms the lateral malleolus which projects distally and posteriorly (Figs 83.2, 83.3, 83.5). Its lateral aspect is subcutaneous while its posterior aspect has a broad groove with a prominent lateral border. Its anterior aspect is rough, round and continuous with the tibial inferior border. The medial surface has a triangular articular facet, vertically convex, its apex distal, which articulates with the lateral talar surface. Behind this facet is a rough malleolar fossa pitted by vascular foramina. The posterior tibiofibular ligament and, more distally, the posterior talofibular ligament, are attached in the fossa. The anterior talofibular ligament is attached to the anterior surface of the lateral malleolus; the calcaneofibular ligament is attached to the notch anterior to its apex. The tendons of fibularis brevis and longus groove its posterior aspect: the latter is superficial and covered by the superior fibular retinaculum.

Fig. 83.5 Distal end of left fibula: medial aspect. 1. Attachment of posterior tibiofibular ligament. 2. Attachment of posterior talofibular ligament. 3. Area for attachment of interosseous tibiofibular ligament. 4. Articular facet for talus. 5. Malleolar fossa.

Muscle attachments

The main attachments of biceps femoris embrace the fibular collateral ligament in front of the apex (styloid process) of the fibular head. Extensor digitorum longus is attached to the head anteriorly, fibularis longus anterolaterally, and soleus posteriorly. Extensor digitorum longus, extensor hallucis longus and fibularis tertius are attached to the anteromedial (extensor) surface. Fibularis longus is attached to the whole width of the lateral (fibular) surface in its proximal third, but in its middle third only to its posterior part, behind fibularis brevis. The latter continues its attachment almost to the distal end of the shaft.

Muscle attachments to the posterior surface, which is divided longitudinally by the medial crest, are complex. Between the crest and interosseous border the posterior surface is concave. Tibialis posterior is attached throughout most (the proximal three-fourths) of this area; an intramuscular tendon may ridge the bone obliquely. Soleus is attached between the crest and the posterior border on the proximal fourth of the posterior surface; its tendinous arch is attached to the surface proximally. Flexor hallucis longus is attached distal to soleus on the posterior surface and almost reaches the distal end of the shaft.

Vascular supply

A little proximal to the midpoint of the posterior surface (14–19 cm from the styloid process), the fibular shaft is characterized by a nutrient foramen, directed distally, which receives a branch of the fibular artery. An appreciation of the detailed anatomy of the fibular artery in relation to the fibula is fundamental to the raising of osteofasciocutaneous free flaps incorporating segments of the bone. Free vascularized diaphysis grafts may also be taken on a fibular arterial pedicle. The proximal and distal ends receive metaphysial vessels from the arterial anastomoses at the knee and ankle respectively.

Innervation

The proximal and distal ends of the bone are supplied by branches of nerves that innervate the knee and superior tibiofibular joint and the ankle and inferior tibiofibular joints respectively. The periosteum of the shaft is supplied by branches from the nerves that innervate the muscles attached to the fibula.

Ossification

The fibula ossifies from three centres, one each for the shaft and the extremities (Fig. 83.6). The process begins in the shaft at about the eighth intrauterine week, in the distal end in the first year, and in the proximal end at about the third year in females and the fourth in males. The distal epiphysis unites with the shaft at about the 15th year in females and the 17th in males, whereas the proximal epiphysis does not unite until about the 17th year in females and the 19th in males‥ An os subfibulare is an occasional and separate entity and lies posterior to the tip of the fibula, whereas the distal fibular apophysis lies anteriorly. An os retinaculi is rarely encountered; if present, it overlies the bursa of the distal fibula within the fibular retinaculum.

Fig. 83.6 Radiograph of an ankle in plantar flexion in a child aged 10 years. A, Oblique anteroposterior view. Note that the fibular growth plate is approximately at the level of the ankle joint. 1. Inferior growth plate of fibula. 2. Inferior growth plate of tibia. 3. Ankle joint. 4. Talus. B, Lateral view. Note the trabecular pattern in the calcaneus. 1. Tibial growth plate. 2. Posterior growth plate of calcaneus.

MUSCLES

The muscles of the leg consist of an anterior group of extensor muscles, which produce dorsiflexion (extension) of the ankle; a posterior group of flexor muscles, which produce plantar flexion (flexion); and a lateral group of muscles, the fibulares, derived, embryologically, from the extensors. The greater bulk of the muscles in the calf is commensurate with the powerful propulsive role of the plantar flexors in walking and running.

ANTERIOR OR EXTENSOR COMPARTMENT

The anterior compartment contains muscles that dorsiflex the ankle when acting from above (Figs 83.7, 83.8, 83.9). When acting from below they pull the body forward on the fixed foot during walking. Two of the muscles, extensor digitorum longus and extensor hallucis longus, also extend the toes, and two muscles, tibialis anterior and fibularis tertius, have the additional actions of inversion and eversion respectively.