Correlative Surgical Anatomy

ERIC I. FERKEL

RICHARD D. FERKEL

EVOLUTION

It is widely considered that the development of human bipedalism, ˜4 to 6 million years ago, is one of the most important adaptations humans have made. As tree-dwelling primates began to evolve into more terrestrial, highly adapted species, the lower limb, especially the foot, became uniquely adapted for locomotion. The descent from the trees represents a critical period in the evolution of human locomotion. The shift to an upright position and bipedalism required a dramatic restructuring of pelvic, lower limb, and foot morphology. The foot particularly had to adapt to deal with the increasingly direct pressure placed upon it as it came into contact with the ground while balancing propulsion and gait.¹

The human foot differs from that of other primates in three fundamental ways:

1. The human foot has both transverse and longitudinal arches.

2. The human foot is a rigid structure, with strong ligamentous support.

3. The human foot does not function as a grasping organ.

Because the foot brings humans into immediate and direct physical contact with their environment, it must withstand great stress, constant exposure, and the weight of the entire body. During the course of evolution, the human foot has developed into a superbly designed and exquisitely functional appendage.²

EMBRYOLOGY

The limb buds, consisting of mesoderm covered by ectoderm, first appear during the fourth week of embryonic development. The mesoderm gives rise to the future bones, muscles, tendons, and ligaments; the ectoderm goes on to form skin, nails, hair, and the sweat and sebaceous glands. The nerves and vascular elements grow into the limb buds from the trunk. The development of the lower limb parallels but lags slightly behind that of the upper limb. Structures appear in a proximal-to-distal sequence, from the thigh toward the foot. The future hallux, the preaxial or tibial border, and the postaxial or fibular border of the leg-foot axis are distinguishable shortly after the fifth week of embryonic life. Next the metatarsal condensations and future digits appear. By the end of the eighth week, as the embryonic phase ends and the fetal phase begins, the feet are fully formed, with the soles facing each other and the entire lower extremity in marked external rotation. This is the so-called praying position, where the feet remain until the seventh prenatal month. There is no angulation yet between the foot and the leg, and the ankle is in an equinus position.

During the ensuing fetal period, the leg-foot axis undergoes important rotational changes. There is progressive internal rotation of the thigh and leg, with subsequent dorsiflexion and pronation of the foot, bringing it close to the adult neutral position. The rotational changes during this critical period are due largely to the growth and interaction of the talus and calcaneus. Congenital foot disorders such as vertical talus and talipes equinovarus are thought to be related to abnormalities in talocalcaneal development during this phase of gestation.³

Studies of the embryology of the subtalar joint indicate that it is a newly acquired structure. The subtalar joint is the result of a series of modifications of the osseous elements in the course of developments. These changes typically occur during the embryonic period, before 8 weeks of gestation. The sinus tarsi has been found to appear late in embryogenesis, and its shape has been shown to allow for bipedal gait and needed to provide cushion and stability to the hindfoot.⁴

SKIN AND SUBCUTANEOUS FASCIA

The skin of the foot and ankle is similar in basic organization to skin elsewhere in the body. It consists of a layer of dense connective tissue, the dermis, covered by an outer layer of stratified squamous epithelium, known as the epidermis. The dermis is arranged in two layers: a deep or reticular layer and a superficial or papillary layer. The deeper reticular layer is responsible for the strength and toughness of the skin. It is also within this layer that sweat glands, sebaceous glands, smooth muscle, and hair follicles are found. The skin on the sole of the foot has no hair follicles and lacks sebaceous glands.

The arrangement of connective tissue bundles in the retinacular layer gives rise to various patterns of cleavage or tension lines within the skin, known as Langer lines.
An appreciation of the pattern of cleavage lines is important to the surgeon because incisions made along Langer lines are apt to heal with a fine, linear scar, whereas incisions that cross the cleavage lines are more likely to result in unsightly, irregular scars. On the medial aspect of the leg and across the anteromedial aspect of the ankle and dorsum of the big toe, Langer lines run parallel to the long axis of the foot. On the rest of the dorsal surface of the foot, the lines of cleavage are arranged obliquely at a 45° angle to the long axis, curving around the tip of the lateral malleolus. On the plantar aspect, the lines are arranged longitudinally with a gentle curvature, convex to the fibular side, and form a U-shaped pattern along the border of the heel (Fig. 5-1).

The skin on the plantar aspect of the foot is designed to act as a shock absorber during gait. There are several septa within the heel, each aiding in its own way to the mechanical stability of the heel as it undergoes torsional, shear, and compressive forces during gait. The epidermis/dermis is tightly bound to the underlying fascia by strong fibrous septa that limit movement between the two. The septa divide the subcutaneous fat into small chambers that act as protective shock absorbers in areas of increased pressure. Typically, the subcutaneous chambers are enlarged under the metatarsal heads and calcaneus. The skin on the anterior aspect of the ankle and dorsum of the foot is thin and only loosely connected to the underlying fascia. The skin in this area is supple and easily moved over the underlying structures.

FIGURE 5-1. Cleavage lines on the dorsal skin of the foot. On the dorsomedial aspect, the cleavage lines are parallel to the medial border of the foot. On the remaining surface, the lines are oblique, making about a 45° angle with the long axis of the foot. (Direction of cleavage lines adapted from Cox HT. The cleavage lines of the skin. Br J Surg 1941;29:234, illustration by Susan Brust.)

FIGURE 5-2. Topical lateral ankle anatomy. Note the position of the lateral malleolus.

LANDMARKS

Several bony landmarks and soft tissue structures are easily palpated on the ankle and provide constant orientation during arthroscopic surgery. The lateral and medial malleoli are readily identifiable bony landmarks. The lateral malleolus lies 1 cm distal and 2 cm posterior to the medial malleolus (Figs. 5-2 and 5-3). Passive dorsiflexion facilitates palpation of the anterior joint line, which lies on a line drawn 2 cm proximal to the tip of the lateral malleolus and 1 cm proximal to the tip of the medial malleolus (Fig. 5-4).

The posterior articular margin of the tibia is positioned about 5 cm distal to the anterior joint line, but is not palpable because it is located deep to a thick layer of fibroadipose tissue between the Achilles and flexor tendons (Fig. 5-5). Posterolaterally, however, between the lateral aspect of the
Achilles tendon and the peroneal tendons, only a thin layer of fibrous tissue lies between the skin and the posterior joint capsule. The bony landmarks to be palpated on the medial aspect of the foot are the sustentaculum tali, located 2.5 cm below the tip of the medial malleolus; the navicular tuberosity; and the medial aspect of the talar head, found at the midpoint of a line drawn between the medial malleolus and the navicular tuberosity. On the lateral border of the foot, the flared tuberosity at the base of the fifth metatarsal is easily found; 1 cm (approximately one fingerbreadth) proximal to the tuberosity is the calcaneocuboid articulation. The sinus tarsi can be felt by applying finger pressure to the soft tissue depression just anterior to the lateral malleolus. If the foot is then inverted, the lateral aspect of the talar dome can be felt by deep palpation of the sinus.

FIGURE 5-3. Topical medial ankle anatomy. Note the position of the medial malleolus.

FIGURE 5-4. Topical anterior ankle anatomy showing medial and lateral malleoli.

FIGURE 5-5. Topical posterior ankle anatomy showing medial and lateral malleoli.

Tendon landmarks of the foot and ankle are also easy to identify and palpate. On the anterior aspect of the ankle, the digital extensor tendons and anterior tibialis tendon can be felt. They run parallel to each other and under the extensor retinacula (Fig. 5-6). Pulsation of the dorsalis pedis artery can be felt between the tendons of the extensor hallucis longus and extensor digitorum longus on a line drawn from the midpoint of the bimalleolar axis to the proximal end of the first intermetatarsal space (see Fig. 5-6). On the lateral aspect of the ankle, the peroneal tendons pass immediately behind the lateral malleolus as they cross the ankle joint (Fig. 5-7). On the medial aspect of the ankle, a soft tissue depression is palpable between the posterior aspect of the medial malleolus and the Achilles tendon. Within this depression lie the tendons of the tibialis posterior, flexor digitorum longus, and flexor hallucis longus, as well as the posterior tibial artery and tibial nerve (Fig. 5-8). Pulsation in the posterior tibial artery is palpable behind the medial malleolus, 2.5 cm anterior to the medial border of the Achilles tendon.

NERVES

Subcutaneous Layer

Three sensory nerve systems and accompanying superficial veins can be found in the subcutaneous layer of the ankle: the superficial peroneal, saphenous, and sural. The cutaneous nerves and superficial veins of the ankle are important structures to identify and protect when establishing portals for arthroscopy.

Superficial Peroneal Nerve

The superficial peroneal nerve (SPN) is found subcutaneously and often can be palpated as a thin, tense cord along the lower leg and ankle with inversion and plantar flexion of the foot (Fig. 5-9A, B). It arises from the common peroneal nerve (L4, L5, S1, S2), which is derived from the sciatic nerve and passes down the leg between the peroneus longus muscle and the fibula. The SPN innervates the peroneus longus and brevis and provides sensory innervation to the dorsum of the foot, with the exception of the first web space between the great toe and the second toe. It is the only nerve in the human body that can be palpated and visualized by manipulation.

The SPN pierces the deep fascia of the leg, becoming subcutaneous, descending the leg between the peroneal group of muscles and the extensor digitorum longus, at various levels. In one study of 100 cadavers, the nerve pierced the fascia 12.5 cm above the tip of the lateral malleolus in 90% of patients, 15 cm above the time in 1%, 10 cm above the tip in 2%, 7.5 cm above the tip in 5%, and 5 cm above the tip in 1%. In another study of 118 cadaveric legs, the nerve piercing occurred 10.5 cm above the

tip of the lateral malleolus in 74.7% and at a higher level in 23.4%.⁵ It then divides into its terminal branches, the intermediate and medial dorsal cutaneous nerves, usually 6.4 cm above the tip of the lateral malleolus⁶ (Fig. 5-10). The intermediate dorsal cutaneous nerve passes over the inferior extensor retinaculum, crosses the common extensor tendons of the fourth and fifth digits, and then runs in the direction of the third metatarsal space before dividing into dorsal digital branches. The medial terminal branch of the SPN, the medial dorsal cutaneous nerve, passes over the anterior aspect of the ankle overlying the common extensor tendons. It runs parallel to the extensor hallucis longus tendon and divides distal to the inferior extensor retinaculum into three dorsal digital branches. The terminal cutaneous branches of the superficial nerve lie in close proximity to the dorsal venous plexus of the anterior aspect of the ankle.

FIGURE 5-6. Dorsal anatomy of the ankle and foot. Note the position of the deep peroneal nerve and dorsalis pedis artery and branches of the superficial peroneal nerve. (Illustration by Susan Brust.)

FIGURE 5-7. Lateral anatomy of the ankle and foot. The internervous plane lies between the superficial peroneal nerve and the sural nerve, along the posterior fibula and peroneal tendons, and extends along toward the base of the fourth metatarsal. (Illustration by Susan Brust.)

FIGURE 5-8. Medial anatomy of the ankle and foot. The posterior tibial nerve lies between the flexor digitorum longus and the flexor hallucis longus, divides into the medial and lateral plantar nerves, and gives off calcaneal branches. (Illustration by Susan Brust.)

FIGURE 5-9. The superficial peroneal nerve. (A) The superficial peroneal nerve can be identified by plantar flexing and inverting the foot. (Copyright Richard D. Ferkel.) (B) Usually only one of the two branches (arrows) can be identified subcutaneously. (Head is to the left and toes are to the right.)

This classic description of the course of the SPN³, ⁷, ⁸ has been expanded to include four anatomic variations⁹ (Fig. 5-11A-D). In a cadaveric study of 85 legs, the nerve was noted to lie solely in the lateral compartment in only 62 (73%) of the specimens. The other descriptions included passage through the fascia from the anterior compartment in 12 specimens (14%), branches in both anterior and lateral compartments in 10 specimens (12%) before dividing, and one case of the nerve descending on the superficial surface of the peroneus longus fascia and then dividing, never running deep to the muscle.

FIGURE 5-10. Anatomic dissection of a right ankle superficial peroneal nerve (SPN) and the intermediate dorsocutaneous nerve (IDCN) and medial dorsocutaneous nerve (MDCN). Note the proximity of the anterolateral portal to the superficial peroneal nerve and its branches.

FIGURE 5-11. Anatomic variations of the superficial peroneal nerve. (Redrawn from Adkison DP, Bosse MJ, Gaccione DR, et al. Anatomic variations in the course of the superficial peroneal nerve. J Bone Joint Surg 1991;73A:112, with permission.) (A) Type I: The superficial peroneal nerve lies only in the lateral muscle compartment until, as it courses inferiorly, it pierces the crural fascia 3 to 18 cm proximal to the lateral malleolus. The course of the deep peroneal nerve is shown in this figure for orientation. (B) Type II: The superficial peroneal nerve passes through the crural fascia in the anterior muscle compartment.

FIGURE 5-11. (Continued) (C) Type III: The superficial peroneal nerve branches near the fibular head, and the medial dorsal cutaneous nerve runs in the anterior compartment, while the intermediate dorsal cutaneous nerve runs in the lateral compartment. (D) Type IV: In this, the rarest of variations, the superficial peroneal nerve arises from the common peroneal nerve 1 cm from the posterior aspect of the fibular neck. The nerve never lies deep to the peroneal longus muscle and descends instead on the superficial surface of the peroneal longus muscle and pierces the crural fascia ˜11 cm proximal to the lateral malleolus. At this point, it then immediately divides into the medial and intermediate dorsal cutaneous nerve branches of the foot. (Copyright Richard D. Ferkel, 1995.)

Numerous communicating branches have been found linking the superficial peroneal and sural nerves. Drizenko et al. noted 35 separate communicating branches in 55 cadaveric lower limbs (58%). Half of the cases were seen proximally (perimalleolar or midtarsal), and half were seen distal in the metatarsal region.¹⁰ These communicating branches can be injured during ankle and foot arthroscopy leading to unexplained postoperative pre- and submalleolar pain.

Positioning of the foot and ankle during arthroscopic surgery can affect the location of the SPN in relation to its surrounding structures. de Leeuw et al. have shown the SPN consistently moved lateral when maneuvered from combined plantar flexion and inversion to neutral and dorsiflexion. These findings may affect anterolateral portal position.¹¹

It is important to be aware of the SPN anatomical variations, because injury to the SPN is the most common complication of ankle arthroscopy (see Chap. 22).¹²

Sural Nerve

The sural nerve (S1, S2) is created from a meeting of the medial sural nerve, a branch of the tibial nerve, and the anastomotic peroneal communicating nerve arising from the lateral sural nerve or the common peroneal nerve.⁵ The sural nerve provides sensory innervation to the lateral half of the foot. Posterolaterally, the sural nerve and short saphenous venous plexus lie in the subcutaneous tissue just posterior to the peroneal tendons behind the lateral malleolus (Figs. 5-12, 5-13 and 5-14). The sural nerve was found by Lawrence and Botte to be 14 mm posterior and 44 mm inferior to the lateral malleolus and lies anterior to the short saphenous vein.¹³ The sural nerve divides into its lateral and medial terminal branches at the base of the fifth metatarsal. When performing subtalar arthroscopy, the posterior subtalar portal has been found to put the sural nerve at most risk.¹⁴, ¹⁵ The short saphenous vein receives tributaries from the lateral calcaneal veins and from the dorsalis pedis vein as it reaches the lateral aspect of the foot. Extreme caution needs to be exercised when using the posterolateral portal of the ankle and subtalar joint to avoid injury to the sural nerve.

Saphenous Nerve

The saphenous nerve (L3, L4), the terminal branch of the femoral nerve, passes over the anterior aspect of the medial malleolus and is found posteromedial to the great saphenous vein. Both the nerve and vein cross over the anteromedial joint capsule. The saphenous nerve supplies the skin of the anteromedial aspect of the leg to the medial aspect of the foot. Feiwell and Frey reported that the average distances between the anteromedial portal and the saphenous nerve were found to be ˜7.4 mm.¹⁶ It is distributed distally to the medial side of the foot and may reach as far as the first metatarsophalangeal joint (Fig. 5-15).

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