1 Comprehensive lower extremity anatomy

Synopsis

The reconstructive surgeon who embarks on surgery on the lower extremity must be well acquainted with its complex anatomy.

The essence of reconstructive surgery is using one’s thorough knowledge of the nuances of the anatomical structures to enable borrowing from one or more structures to reconstruct the loss of others.

This chapter aspires to provide a comprehensive review of the anatomical structures from the hip through to the plantar surface of the foot in order to provide the basis of mastery of lower-extremity reconstruction.

The components of the lower extremity vary widely in regard to the redundancy of tissue, density of distinct structures, and function as one travels from proximal to distal regions. It is for this reason that the anatomical depiction of the lower extremity is divided into discussion of the thigh, knee, lower leg, and foot. Each of these structures differs greatly from each other. However, many reconstructive tasks bridge and cross these regions.

Even though present in discrete sections, the anatomy of the lower extremity should be viewed as an interconnected and interrelated unit.

Embryology

The first recognizable sign of lower limb growth in the human embryo occurs during the third week of life. The initial limb development begins along the ventrolateral surface. Slow outgrowth of the apical epidermal ridge continues into the fifth week when a small flattened foot plate develops. The foot plate begins development straight with no dorsiflexion and the plantar surface pointed toward the head. Skeletogenesis occurs in the tibial/fibular region while the foot plate begins to rotate laterally (the right foot plate clockwise and the left counterclockwise). By the sixth week, digital rays appear and external notching occurs. Through the sixth to the eighth week chondrification progresses throughout the future bony structures and the lower limb elongates.

The transition from the embryonic to the fetal period occurs during the eighth week. At this point, the phalanges, metatarsals, tarsals, malleoli, tibia, fibula, and femur are all present. Between the third and ninth month the foot dorsiflexes and pronates, and the thigh internally rotates. During this time period, lower limb structures are recognizable. The soft tissues further develop and bones begin to ossify. The first signs of detectable ossification begin in the foot, specifically in the distal phalanges and metatarsal shaft. The intervening proximal and middle phalanges follow. Tarsal bones begin ossification in the fifth month beginning with the talus and the cuboid. Ossification of the remaining regions of the lower extremity continues throughout the rest of gestation.

The lower extremity vascular anatomy of the embryo differs from the adult primarily by the development and transient presence of the sciatic artery (also called the axis or ischiadic artery). This artery arises from the umbilical artery and is separate from the femoral artery as it courses inferiorly along the posterior surface of the thigh, knee, and leg and is a major contributor to lower extremity vascular supply in early fetal development. The femoral artery passes along the anterior side of the thigh and branches into the primitive posterior tibial and peroneal artery and an anterior tibial artery. As the fetus grows, the femoral artery increases in size and the sciatic artery becomes attenuated. In normal development the sciatic artery involutes during the eighth week of gestation and its remnants contribute to the formation of the inferior gluteal, deep femoral, popliteal, and peroneal vessels (Fig. 1.1). However, in 0.025–0.04% of the population a sciatic artery persists throughout fetal development and into adulthood. A persistent sciatic artery can lead to aneurysm formation, thrombosis, and embolization. Although it is a rare anomaly, failure to recognize a persistent sciatic artery can lead to inappropriate assumptions of vascular patterns to common lower extremity flaps.

Fig. 1.1 Vascular development of the lower extremity.

The gluteal region

Gluteal skeletal structure

The pelvis consists of the two paired hip bones and the midline sacrum, articulating together at the two sacroiliac joints. The hip bones are formed by the fusion of the ilium, pubis, and ischium. These three bony regions of the pelvis coalesce to form the acetabulum (Fig. 1.2). The complex bony structure of the pelvis serves to cradle the abdominal organs during upright ambulation, and the large thick bony prominences of the pelvis serve as muscle attachments for the muscles of the hip and thigh. Dense ligaments reinforce the pelvis and distribute the numerous opposing forces acting on it. The sacrospinous ligament runs from the sacrum to the ischial spine, bounding the greater sciatic foramen. The sacrotuberous ligament attaches the sacrum to the ischial tuberosity and encloses the lesser sciatic foramen. Running from the anterior superior iliac spine to the pubic tubercle is the inguinal ligament. Most of the muscles of the thigh originate from the bony pelvis (Fig. 1.3). The action of the multiple flexors, extensors, and internal and external rotators on the hip joint serves to stabilize and position the torso during the complex process of ambulation.

Fig. 1.2 Bones and ligaments of pelvis.

Fig. 1.3 Bony attachments of buttock and thigh muscles.

Gluteal fascial anatomy

The fascial system of the gluteal region and the lower extremity contains various permutations of a nearly continuous superficial fascial and a deep fascial layer. The superficial system is located within the subcutaneous fat, usually as a dividing layer where superficial veins and nerves traverse to reach the overlying skin. The superficial layers are thinner than the deep layer, which becomes thinner with age to a point where its presence is described as variable. The deep fascial layer is thicker and frequently can be seen as a dual-layered fibrous band. It usually lies directly over the underlying limb musculature and its proper fascia.

The superficial fascia of the gluteal region is continuous with that over the low back and continues inferiorly into the proximal thigh. The deep fascia covering the gluteal muscles varies in thickness. Over the maximus it is thin, but over the anterior two-thirds of the medius it forms the thick, strong gluteal aponeurosis. This is attached to the lateral border of the iliac crest superiorly, and splits anteriorly to enclose tensor fasciae latae and posteriorly to enclose gluteus maximus.

Muscles of the buttocks

The gluteus maximus is the largest muscle in the body and lies most superficially in the gluteal region, originating from the posterior gluteal line of the ilium and the dorsal portion of the sacrum (Fig. 1.4). The superficial fibers coalesce into a thick tendinous expansion which contributes to the iliotibial band of the fascia lata, while the deep fibers insert on the gluteal tuberosity of the femur. The gluteus maximus acts as a hip extensor when the hip is in a flexed position. When the lower extremity is fixed in position – as in standing – the gluteus maximus dorsally rotates the pelvis and torso. The vascular supply is primarily derived from the inferior gluteal vessels, which supply the inferior two-thirds of the muscle. The superior gluteal vessels supply the superior portion and the first perforator branch of the femoral profunda contributes to the vascular supply of the muscle laterally. For ease of description, the Mathes–Nahai classification system is used when discussing the vascular supply to muscle in this chapter (Table 1.1). The vascular supply of gluteus maximus with two dominant pedicles (superior and inferior gluteal arteries) is considered to be type III.

Fig. 1.4 Muscles of hip and thigh: posterior views.

Table 1.1 Mathes–Nahai classification system for muscle vascular supply

Muscle vascular supply type	Description
I	Single vascular pedicle
II	Dominant vascular pedicle and one or more minor pedicles
III	Two dominant pedicles
IV	Segmental vascular pedicles
V	Single dominant vascular pedicle and secondary segmental pedicles

Innervation to the gluteus maximus is provided by the inferior gluteal nerve. Underneath the gluteus maximus lie three bursae: the trochanteric, the gluteofemoral, and occasionally the ischiofemoral bursae, which allow frictionless movement over its underlying structures. The gluteus medius, situated immediately deep to the gluteus maximus, arises from the outer surface of the iliac wing and inserts on the greater trochanter of the femur. It is innervated by the superior gluteal nerve and functions to abduct the hip and medially rotate the femur. Blood supply to this muscle is from the deep branch of the superior gluteal artery and from the trochanteric connection.

The gluteus minimus lies deep to the gluteus medius and arises from the outer surface of the ilium. Its fibers join the aponeurosis of the gluteus medius to insert on the greater trochanter, and the two muscles function together to abduct the hip. The gluteus minimus is innervated by the superior gluteal nerve and receives blood supply from the superior gluteal artery and trochanteric connection. Several small muscles arise from the medial pelvis and insert on the greater trochanter of the femur, functioning collectively to rotate the hip externally. These muscles include piriformis, superior and inferior gemellus, quadratus femoris, obturator internus, and obturator externus.

Gluteal vasculature

The superior gluteal artery is the last branch of the posterior trunk of the internal iliac artery. It exits the pelvis through the greater sciatic foramen superior to the piriformis, dividing into two branches (Fig. 1.5). The deep branch runs deep to the gluteus medius, dividing into superior and inferior branches. The superior branch travels laterally to the anterior superior iliac spine and connects with the ascending branch of the lateral circumflex femoral and the deep circumflex iliac. The inferior branch supplies the gluteus medius and minimus, and later joins with the lateral circumflex femoral. The superficial branch of the superior gluteal artery pierces the gluteus maximus, connecting intramuscularly with branches of the inferior gluteal artery, and sending musculocutaneous perforators to the overlying skin. The inferior gluteal artery branches from the anterior trunk of the internal iliac and exits the pelvis through the greater sciatic foramen below the piriformis. It runs deep to the gluteus maximus, supplying it and its overlying skin with musculocutaneous perforators. The artery continues down the posterior thigh, connecting with the perforating branches to supply the skin.

Fig. 1.5 Arteries and nerves of thigh: deep dissection (posterior view).

Gluteal innervation

The posterior cutaneous nerve of the thigh exits the pelvis with the inferior gluteal vessels (Fig. 1.5). Several branches will curl around the inferior border of the gluteus maximus and run superiorly to innervate the overlying skin of the buttocks, while the posterior cutaneous nerve continues to descend down the posterior thigh deep to the fascia lata, sending segmental cutaneous branches to the skin of the posterior thigh. The gluteal muscles are innervated by the superior and inferior gluteal nerves. The superior gluteal nerve runs with the superior gluteal artery and divides into superior and inferior branches, which course respectively with the superior and inferior branches of the deep superior gluteal artery, supplying the gluteus medius and minimus. The inferior branch runs laterally to also innervate the tensor fascia latae. The inferior gluteal nerve runs with the inferior gluteal artery, exiting the pelvis below the piriformis and entering the gluteus maximus.

The thigh

The thigh serves a dual function of aid in truncal support and positioning of the knee in space; both processes are critical to ambulation. In the thigh, these functions are usually enacted by multiple muscles that perform in a redundant fashion. The muscles of the thigh are some of the largest in the body and their corresponding fasciocutaneous coverage is also large and relatively redundant. Thorough knowledge of the thigh anatomy is a key component that allows for exploitation of these redundancies. These attributes of the thigh allow it to be a generous source of harvest tissue for local, regional, and distant reconstructive surgical options.

Thigh skeletal structure

The thigh region is generally defined as the cylindrical proximal portion of the lower extremity that extends from the infragluteal crease and the inguinal ligament distally to the tibiofemoral joint. Most anterior thigh musculature originates from the pelvic bone and therefore the bony structure’s interrelation between the pelvis and the femur is a principal anatomic foundation.

The femoral head articulates with the pelvic acetabulum to form the only true ball-and-socket joint in the body, allowing for stability under load as well as smooth multiplanar movement. A dense ligamentous capsule surrounds the hip joint circumferentially, extending from around the pelvic acetabulum to encase the femoral head and insert on the femoral neck (Fig. 1.6). The ligaments of the hip joint are the iliofemoral, pubofemoral, ischiofemoral, transverse acetabular and the ligamentum teres. As the hip moves, so do the capsular ligaments. The ligaments are reinforced with capsular thickenings which wind and unwind, tightening around the hip, affecting stability, excursion, and joint capacity.

Fig. 1.6 Joints and ligaments of the hip.

The pelvic bone serves as the principal base for the thigh musculature. Eleven out of the 17 muscles of the thigh originate from the pelvis. Two bony prominences emanate from the femoral neck at about 90° from each other – the greater and lesser trochanters. These femoral prominences primarily serve as insertion sites for hip actors (Fig. 1.3). The greater trochanter receives insertions from the quadratus femoris, gluteus medius, obturator externus, piriformis, obturator internus, and gemellus muscles. The lesser trochanter is the insertion target for the iliopsoas muscle. The action of multiple flexors, extensors, and internal and external rotators on the hip joint serves to stabilize and position the torso during the complex process of ambulation.

The vascular supply of the femoral bone arises from multiple sources. The femoral head and neck are encircled by an arterial-connecting arcade contributed by the medial and lateral circumflex arteries with lesser contribution by the superior and inferior gluteal vessels. These vessels piece the hip joint synovial capsule and form a subsynovial intra-articular ring. The femoral shaft has multiple nutrient foramina via which branches from the second perforating artery from the profunda femoris (Fig. 1.7) enter and deliver endosteal blood supply. These foramina usually run along the linea aspera (Latin for “rough line”) on the posterior surface of the femur in the middle one-third of the femoral shaft. Periosteal vascularity along the femur receives vascular input from either the perforating branches of the profunda or directly from the profunda femoris. The distal metaphyseal region of the femur also has multiple vascular foramina that are derived from the genicular arterial system.

Fig. 1.7 Osteology of the femur.

This genicular system, based on the superficial femoral artery, is the anatomical basis for the clinically relevant medial femoral periosteal bone flap. Using the descending genicular artery (a medial branch of the superficial femoral artery) as a pedicle, perisosteum, cortical and cancellous bone can be harvested from the medial femur as a vascularized flap. Usually used as a free flap (with or without an associated skin paddle off the saphenous artery), the medial femoral periosteal bone flap is utilized as an effective treatment for recalcitrant bony nonunions of the clavicle, humerus, tibia, and radius (Fig. 1.8).

Fig. 1.8 The medial femoral periosteal bone flap.

Thigh fascial composition

Like the abdomen, the thigh has a superficial and deep fascial system. The superficial fascia lies within the subcutaneous fat of the thigh encircling the entire structure. Superficial vessels and nerves pass along and pierce this fascia as they travel from their deeper tributaries and surface to the superficial skin targets. The thickness of the fascia varies throughout the thigh but thickens proximally in the inguinal region. At the inguinal ligament the superficial fascia fuses with the deep thigh fascia.

The deep fascia structure of the lower extremity is a tough, well-vascularized fibrous tissue that lies beneath the superficial fascia and all the subcutaneous fat. This fibrous structure encircles and constrains the thigh musculature in a near-complete sheath, often found to be dual layers. The deep fascia of the thigh has been referred to and described by several different names, making the terminology somewhat confusing. The deep fascia of the thigh is also called investing fascia of the thigh; the fascia lata (lata, Latin for wide or broad); or, even more confusingly, the “thick layer of the superficial fascia.” The deep fascia of the thigh attaches posteriorly to the sacrum and coccyx, anteriorly to the inguinal ligament, and laterally to the iliac crest. At the inguinal ligament, where the superficial fascia joins the deep fascia, lies the fossa ovalis. The fossa ovalis is an opening in both fascial layers through which the great saphenous vein, superficial branches of the femoral artery, and lymphatics pass through, transitioning between the deep and the superficial layers of the thigh (Fig. 1.9). This opening is covered by a membranous layer of the superficial fascia called the cribriform fascia (derived from the Latin cribrum or sieve). A thickening of the fibrous tissues is present on the lateral aspect of the deep fascia (fascia lata) which is called the iliotibial band or tract (Fig. 1.10). The iliotibial band is proximally attached to the tensor fascia latae muscle and together this myofascial unit aids in maintaining knee extension.

Fig. 1.9 Surface anatomy: superficial veins and nerves.

Fig. 1.10 Muscles of hip and thigh: lateral view.

Septa pass from the deep fascial sheath to the bones within, confining the functional muscle groups within osteofascial compartments. The fascia functions as additional areas of attachment to the muscles and as a partition of similarly functioning musculature which augments their efficiency. In addition to separating musculature, fascial planes also provide pathways for vessel perforators to travel from deeper vasculature to the overlying skin they supply.

There are three functional compartments of the thigh: anterior, posterior, and medial. The anterior compartment contains the knee extensor muscles; the posterior compartment is comprised of knee flexors; and the medial compartment retains the hip adductors (Fig. 1.11). The medial and lateral intermuscular septa travel from the fascia lata to attach to the femur on the linea aspera along its posterior aspect. These septa then divide the anterior compartment from the posterior compartments and partition the thigh musculature according to knee function. Only the anterior and posterior compartments have definite fascial boundaries. The medial (adductor) compartment is not a true anatomical compartment as it has no defined intermuscular septa separating it. The muscles of the adductor compartment – gracilis, pectineus, adductor longus, adductor brevis, and adductor magnus – form a functional compartment in the proximal aspect of the thigh.

Fig. 1.11 Thigh: serial cross-sections.

Thigh musculature

There are 17 muscles that have origins in, insertions to, or course through the region of the thigh. The iliacus and the psoas major originate from the lumbar vertebral column and insert on the lesser trochanter together. They are commonly considered as one functional unit – the iliopsoas – and are powerful hip flexors.

The three compartments of the thigh are a well-organized way to approach the thigh musculature (Table 1.2). The anterior thigh compartment contains the sartorius, articularis genu, and quadriceps femoris, comprised of the rectus femoris, vastus lateralis, intermedius, and medialis (Table 1.3). These muscles are the primary knee flexors. All of the anterior-compartment musculature is monoarticular functioning, with the exception of the sartorius and the rectus femoris, which crosses both the hip joint and the knee joint. These muscles are therefore biarticular muscles and flex both the hip and knee joints.

Table 1.2 Thigh musculature

Table 1.3 Anterior thigh compartment musculature

Muscle	Function
Sartorius	Flexes, abducts, and external rotation of thigh at hip
Sartorius	Flexes and internally rotates leg at knee
Rectus femoris	Hip flexion and knee extension
Vastus lateralis	Knee extension
Vastus medialis	Knee extension, patellar stabilization
Vastus intermedialis	Knee extension
Articularis genu	Retracts bursa as knee extends

The sartorius muscle is the most superficial muscle of the thigh. It obliquely crosses the anterior thigh from superolateral to medial inferior. The proximal end of the sartorius makes up one of the three limbs of the femoral triangle. The medial border of the sartorius makes up the lateral side; the medial border of the adductor longus makes up the medial side of the triangle; and the inguinal ligament makes up the superior limb (Fig. 1.12). The femoral triangle denotes the region where the femoral artery, nerve, and vein are transiently uncovered superficially by muscle in the proximal thigh. Here, all three structures radiate multiple branches in multiple directions to supply the vasculature and innervation to the groin region, lower abdomen, and proximal thigh. As the femoral artery, vein, and nerve descend distally in the thigh, they dive under the adductor longus and then proceed to the adductor canal. At the distal end of the sartorius, the muscle inserts on the proximal, medial surface of the tibia. The sartorius inserts in this region above the gracilis, which then inserts above the semitendinosus (Fig. 1.13). This three-muscle insertion point forms the pes anserinus (a Latin derivation for “duck’s foot”) which is so named because of the three-toed pattern of the duck’s foot. The sartorius’ superficial and femoral triangle proximity location makes it an ideal candidate for muscle transposition coverage over proximal thigh wounds with femoral vessel exposure. It has a type IV segmental blood supply but there exist more dominant segmental pedicles which allow for adequate vascular pivot points. In addition, partial sartorius muscle transfer can be effectively utilized to ensure vascular supply and close complex proximal thigh wounds.

Fig. 1.12 Arteries and nerves of thigh: anterior views.

Fig. 1.13 Pes anserinus.

The quadriceps muscles act as a unit through the patella and the patellar tendon to extend the lower leg at the knee. Knee extension is critical in human ambulation and upright posture maintenance. The redundancy in the function of these four muscles ensures this ability. However, removal of one or a portion of one of the quadriceps muscles can usually be tolerated with appropriate centralization techniques of the remaining musculature and with adequate physical therapy. This recovery potential is a primary factor in the ability to use the vascular type II rectus femoris muscle for regional and distant reconstructions. The dominant pedicle from the descending branch of the lateral circumflex artery allows for wide transposition of the rectus femoris muscle or musculocutaneous flap without sacrificing knee function after centralizing the remaining knee flexors.

The posterior compartment musculature is comprised of the knee flexion muscles, colloquially known as the hamstrings (termed referring to the posterior thigh culinary cut of the hog – ham) (Table 1.4). The three muscles of this compartment facilitate knee flexion. The biceps – as its name denotes – has two heads, each with a slightly different origin and action. All three of these muscles span the posterior knee, with the tendons of the semitendinosus and semimembranosus traveling medially and forming one border of the popliteal fossa and the biceps femoris tendon traveling laterally and forming the other border (Fig. 1.14).

Table 1.4 Posterior thigh compartment musculature

Muscle	Function
Biceps femoris	Long head: extends knee
Biceps femoris	Both heads: flexes and laterally rotates knee
Semitendinosus	Extends hip, flexes and medially rotates the knee
Semimembranosus	Extends hip, flexes and medially rotates the knee

Fig. 1.14 Muscles of the thigh.

The adductor compartment houses the adductor magnus, brevis, and longus, as well as the gracilis and pectineus (Figs 1.15 and 1.16 and Table 1.5). All five muscles cross the hip joint but only one – the gracilis – crosses the knee. The gracilis is also the most superficial muscle of the adductor compartment. It begins from its origin on the pubic bone as a flat, broad muscle that tapers down into a narrow tendon that inserts into the pes anserinus. Because of its superficial location, its muscle vascular supply type II, and an associated vascular branch that leads to the overlying skin, the gracilis is a frequent choice for a muscle or musculocutaneous flap. Due to their position and musculature structure, the remainder of the adductor muscles do not lend themselves easily for flap transposition.

Fig. 1.15 Muscles of thigh: anterior view.

Fig. 1.16 Muscles of the thigh, deep dissection: anterior view.

Table 1.5 Adductor compartment musculature

Muscle	Function
Adductor magnus	Hip adduction, internal hip rotation
Adductor longus	Hip adduction, internal hip rotation
Adductor brevis	Hip adduction
Gracilis	Hip adduction, knee flexion, and internal rotation
Pectineus	Hip flexion, adduction, and internal rotation

The tensor fascia latae is not classically designated in the previously described compartments but included in the discussion of the thigh because of its intimate relation to thigh musculature and it utility in thigh reconstructive options. The tensor fascia latae lies on the proximal lateral aspect of the thigh originating from the iliac crest. As it descends distally, it is enveloped by the two layers of the iliotibial band of the fascia lata (Figs 1.10 and 1.12). The tensor fascia latae muscle usually ends in the first third of the thigh but can extend down to the lateral femoral condyle. Its blood supply is from the ascending branch off the lateral circumflex femoral arterial system. This allows the muscle and fascia to be included with other flaps based off the same system if such a composite reconstruction is required. Also, with dissection to the branch to the source lateral circumflex femoral vessels, the tensor fascia latae with its long iliotibial band can be used to reconstruct other fascial defects in nearby regions such as the abdomen.

Thigh vasculature

The primary blood supply to the entire lower extremity is the femoral artery, passing beneath the inguinal ligament into the femoral canal (Fig. 1.17). It courses with the femoral vein through the femoral sheath, a short canal composed of continuation of the transversalis fascia anteriorly and the iliac fascia posteriorly. These fascial layers fuse with the vascular adventitia approximately 4 cm inferior to the inguinal ligament. The femoral artery crosses beneath the sartorius muscle and enters Hunter’s canal, a fibromuscular canal bounded laterally by the vastus medialis, inferiorly by the adductor longus and magnus, and anteromedially by the sartorius. The femoral artery exits Hunter’s canal through the adductor hiatus, entering the popliteal fossa, at which point its name changes to the popliteal artery.