Overview of Lower Limb
The lower limbs (extremities) are extensions from the trunk specialized to support body weight, for locomotion (the ability to move from one place to another), and to maintain balance.
The lower limbs have six major regions (Fig. 5.1):
Figure 5.1. Regions and bones of lower limb.
- The gluteal region (G. gloutos, buttocks) is the transitional region between the trunk and free lower limbs. It includes two parts of the lower limb: the rounded, prominent posterior region, the buttocks (L. nates, clunes), and the lateral, usually less prominent hip region (L. regio coxae), which overlies the hip joint and greater trochanter of the femur. The “width of the hips” in common terminology is a reference to one’s transverse dimensions at the level of the greater trochanters. The gluteal region is bounded superiorly by the iliac crest, medially by the intergluteal cleft (natal cleft), and inferiorly by the skin fold (groove) underlying the buttocks, the gluteal fold (L. sulcus glutealis). The gluteal muscles, overlying the pelvic girdle, constitute the bulk of this region.
- The femoral region (thigh) is the region of the free lower limb that lies between the gluteal, abdominal, and perineal regions proximally and the knee region distally. It includes most of the femur (thigh bone). The transition from trunk to free lower limb occurs abruptly in the inguinal region or groin. Here the boundary between the abdominal and perineal regions and the femoral region is demarcated by the inguinal ligament anteriorly and the ischiopubic ramus of the hip bone (part of the pelvic girdle or skeleton of the pelvis) medially. Posteriorly, the gluteal fold separates the gluteal and femoral regions (see Fig. 5.46A).
- The knee region (L. regio genus) includes the prominences (condyles) of the distal femur and proximal tibia, the head of the fibula, and the patella (knee cap, which lies anterior to the distal end of the femur), as well as the joints between these bony structures. The posterior region of the knee (L. poples) includes a well-defined, fat-filled hollow, transmitting neurovascular structures, called the popliteal fossa.
- The leg region (L. regio cruris) is the part that lies between the knee and the narrow, distal part of the leg. It includes most of the tibia (shin bone) and fibula (calf bone). The leg (L., crus) connects the knee and foot. Often laypersons refer incorrectly to the entire lower limb as “the leg.”
- The ankle (L. tarsus) or talocrural region (L. regio talocruralis) includes the medial and lateral prominences (malleoli) that flank the ankle (talocrural) joint.
- The foot (L. pes) or foot region (L. regio pedis) is the distal part of the lower limb containing the tarsus, metatarsus, and phalanges (toe bones). The toes are the digits of the foot. The great toe (L. hallux), like the thumb, has only two phalanges (digital bones); the other digits have three.
Development of Lower Limb
Development of the lower limb is illustrated, explained, and contrasted with that of the upper limb in Figure 5.2. Initially, the development of the lower limb is similar to that of the upper limb, although occurring about a week later. During the 5th week, lower limb buds bulge from the lateral aspect of the L2–S2 segments of the trunk (a broader base than for the upper limbs) (Fig. 5.2A). Both limbs initially extend from the trunk with their developing thumbs and great toes directed superiorly and the palms and soles directed anteriorly. Both limbs then undergo torsion around their long axes, but in opposite directions (Fig. 5.2B,C–D).
Figure 5.2. Development of lower limbs.
A–D.The upper and lower limbs develop from limb buds that arise from the lateral body wall during the 4th and 5th weeks, respectively. They then elongate, develop flexures, and rotate in opposite directions. Segmental innervation is maintained, the dermatomal pattern reflecting the elongation and spiraling of the limb. E and F.Future bones develop from cartilage models, demonstrated at the end of the 6th week (E) and beginning of the 7th week (F).
The medial rotation and permanent pronation of the lower limb explain how:
- the knee, unlike the joints superior to it, extends anteriorly and flexes posteriorly, as do the joints inferior to the knee (e.g., interphalangeal joints of the toes);
- the foot becomes oriented with the great toe on the medial side (Fig. 5.2D), whereas the hand (in the anatomical position) becomes oriented with the thumb on the lateral side; and
- the “barber-pole” pattern of the segmental innervation of the skin (dermatomes) of the lower limb develops (see “Cutaneous Innervation of Lower Limb”).
The torsion and twisting of the lower limb is still in progress at birth (note that babies’ feet tend to meet sole to sole when they are brought together, like clapping). Completion of the process coincides with the mastering of walking skills.
Bones of Lower Limb
The skeleton of the lower limb (inferior appendicular skeleton) may be divided into two functional components: the pelvic girdle and the bones of the free lower limb (Fig. 5.1). The pelvic girdle (bony pelvis) is a bony ring composed of the sacrum and right and left hip bones joined anteriorly at the pubic symphysis.
The pelvic girdle attaches the free lower limb to the axial skeleton, the sacrum being common to the axial skeleton and the pelvic girdle. The pelvic girdle also makes up the skeleton of the lower part of the trunk. Its protective and supportive functions serve the abdomen, pelvis, and perineum as well as the lower limbs. The bones of the free lower limb are contained within and specifically serve that part of the limb.
Arrangement of Lower Limb Bones
Body weight is transferred from the vertebral column through the sacro-iliac joints to the pelvic girdle and from the pelvic girdle through the hip joints to the femurs (L. femora) (Fig. 5.3A). To support the erect bipedal posture better, the femurs are oblique (directed inferomedially) within the thighs so that when standing the knees are adjacent and placed directly inferior to the trunk, returning the center of gravity to the vertical lines of the supporting legs and feet (Figs. 5.1, 5.3, and 5.4). Compare this oblique position of the femurs with that of quadrupeds, in whom the femurs are vertical and the knees are apart, with the trunk mass suspended between the limbs (Fig. 5.3B).
Figure 5.3. Pelvic girdle and related joints, demonstrating transfer of weight.
A.The weight of the upper body, transmitted centrally through the vertebral column (1), is divided and directed laterally by means of the bony arch formed by the sacrum and ilia (2). Thick portions of the ilia transfer the weight to the femurs (3). The pubic rami form “struts” or braces that help maintain the integrity of the arch (4). B.The arrangement of the lower limb bones of bipeds is compared to that of quadrupeds. The diagonal disposition of the femur recenters support directly inferior to the trunk (body mass) to make bipedal standing more efficient and to enable bipedal walking, in which the full weight is borne alternately by each limb. In quadrupeds, the trunk is suspended between essentially vertical limbs, requiring simultaneous support from each side.
Figure 5.4. Bones of lower limb.
A and B.Individual bones and bony formations are identified. The foot is in full plantarflexion. The hip joint is disarticulated (B) to demonstrate the acetabulum of the hip bone, which receives the head of the femur.
The femurs of human females are slightly more oblique than those of males, reflecting the greater width of their pelves. At the knees, the distal end of each femur articulates with the patella and tibia of the corresponding leg. Weight is transferred from the knee joint to the ankle joint by the tibia. The fibula does not articulate with the femur and does not bear or transfer weight, but it provides for muscle attachment and contributes to the formation of the ankle joint.
At the ankle, the weight borne by the tibia is transferred to the talus (Fig. 5.4). The talus is the keystone of a longitudinal arch formed by the tarsal and metatarsal bones of each foot that distributes the weight evenly between the heel and forefoot when standing, creating a flexible but stable bony platform to support the body.
The mature hip bone (L. os coxae) is the large, flat pelvic bone formed by the fusion of three primary bones—ilium, ischium, and pubis—at the end of the teenage years. Each of the three bones is formed from its own primary center of ossification; five secondary centers of ossification appear later.
At birth, the three primary bones are joined by hyaline cartilage; in children, they are incompletely ossified (Fig. 5.5). At puberty, the three bones are still separated by a Y-shaped triradiate cartilage centered in the acetabulum, although the two parts of the ischiopubic rami fuse by the 9th year (Fig. 5.5B). The bones begin to fuse between 15 and 17 years of age; fusion is complete between 20 and 25 years of age. Little or no trace of the lines of fusion of the primary bones is visible in older adults (Fig. 5.6). Although the bony components are rigidly fused, their names are still used in adults to describe the three parts of the hip bone.
Figure 5.5. Parts of hip bones.
A.An anteroposterior radiograph of an infant’s hips shows the three parts of the incompletely ossified hip bones (ilium, ischium, and pubis). B.The right hip bone of a 13-year-old demonstrating the Y-shaped triradiate cartilage.
Figure 5.6. Right hip bone of adult in anatomical position.
In this position, the anterior superior iliac spine (ASIS) and the anterior aspect of the pubis lie in the same coronal plane (blue). A.The large hip bone is constricted in the middle and expanded at its superior and inferior ends. B.The symphysial surface of the pubis articulates with the corresponding surface of the contralateral hip bone. The auricular surface of the ilium articulates with a corresponding surface of the sacrum to form the sacro-iliac joint.
Because much of the medial aspect of the hip bones/bony pelvis is primarily concerned with pelvic and perineal structures and functions (Chapter 3) or their union with the vertebral column (Chapter 4), it is described more thoroughly in those chapters. Aspects of the hip bones concerned with lower limb structures and functions, mainly involving their lateral aspects, are described in this chapter.
The ilium forms the largest part of the hip bone and contributes the superior part of the acetabulum (Fig. 5.5B). The ilium has thick medial portions (columns) for weight bearing and thin, wing-like, posterolateral portions, the alae (L. wings), that provide broad surfaces for the fleshy attachment of muscles (Fig. 5.3).
The body of the ilium joins the pubis and ischium to form the acetabulum. Anteriorly, the ilium has stout anterior superior and anterior inferior iliac spines that provide attachment for ligaments and tendons of lower limb muscles (Fig. 5.6).
Beginning at the anterior superior iliac spine (ASIS), the long curved and thickened superior border of the ala of the ilium, the iliac crest, extends posteriorly, terminating at the posterior superior iliac spine (PSIS). The crest serves as a protective “bumper” and is an important site of aponeurotic attachment for thin, sheet-like muscles and deep fascia. A prominence on the external lip of the crest, the tubercle of the iliac crest (iliac tubercle), lies 5–6 cm posterior to the ASIS. The posterior inferior iliac spine marks the superior end of the greater sciatic notch.
The lateral surface of the ala of the ilium has three rough curved lines—the posterior, anterior, and inferior gluteal lines—that demarcate the proximal attachments of the three large gluteal muscles (pl., glutei). Medially, each ala has a large, smooth depression, the iliac fossa (Fig. 5.6B), that provides proximal attachment for the iliacus muscle. The bone forming the superior part of this fossa may become thin and translucent, especially in older women with osteoporosis.
Posteriorly, the medial aspect of the ilium has a rough, ear-shaped articular area called the auricular surface (L. auricula, a little ear), and an even rougher iliac tuberosity superior to it for synovial and syndesmotic articulation with the reciprocal surfaces of the sacrum at the sacro-iliac joint.
The ischium forms the postero-inferior part of the hip bone. The superior part of the body of the ischium fuses with the pubis and ilium, forming the postero-inferior aspect of the acetabulum. The ramus of the ischium joins the inferior ramus of the pubis to form a bar of bone, the ischiopubic ramus (Fig. 5.6A), which constitutes the inferomedial boundary of the obturator foramen. The posterior border of the ischium forms the inferior margin of a deep indentation called the greater sciatic notch. The large, triangular ischial spine at the inferior margin of this notch provides ligamentous attachment. This sharp demarcation separates the greater sciatic notch from a more inferior, smaller, rounded, and smooth-surfaced indentation, the lesser sciatic notch. The lesser sciatic notch serves as a trochlea or pulley for a muscle that emerges from the bony pelvis. The rough bony projection at the junction of the inferior end of the body of the ischium and its ramus is the large ischial tuberosity. The body’s weight rests on this tuberosity when sitting, and it provides the proximal, tendinous attachment of posterior thigh muscles.
The pubis forms the anteromedial part of the hip bone, contributing the anterior part of the acetabulum, and provides proximal attachment for muscles of the medial thigh. The pubis is divided into a flattened medially placed body and superior and inferior rami that project laterally from the body (Fig. 5.6).
Medially, the symphysial surface of the body of the pubis articulates with the corresponding surface of the body of the contralateral pubis by means of the pubic symphysis (Fig. 5.3A). The anterosuperior border of the united bodies and symphysis forms the pubic crest, which provides attachment for abdominal muscles.
Small projections at the lateral ends of this crest, the pubic tubercles, are important landmarks of the inguinal regions (Fig. 5.6). The tubercles provide attachment for the main part of the inguinal ligament and thereby indirect muscle attachment. The posterior margin of the superior ramus of the pubis has a sharp raised edge, the pecten pubis, which forms part of the pelvic brim (see Chapter 3).
The obturator foramen is a large oval or irregularly triangular opening in the hip bone. It is bounded by the pubis and ischium and their rami. Except for a small passageway for the obturator nerve and vessels (the obturator canal), the obturator foramen is closed by the thin, strong obturator membrane. The presence of the foramen minimizes bony mass (weight) while its closure by the obturator membrane still provides extensive surface area on both sides for fleshy muscle attachment.
The acetabulum (L., shallow vinegar cup) is the large cup-shaped cavity or socket on the lateral aspect of the hip bone that articulates with the head of the femur to form the hip joint (Fig. 5.6A). All three primary bones forming the hip bone contribute to the formation of the acetabulum (Fig. 5.5).
The margin of the acetabulum is incomplete inferiorly at the acetabular notch, which makes the fossa resemble a cup with a piece of its lip missing (Fig. 5.6A). The rough depression in the floor of the acetabulum extending superiorly from the acetabular notch is the acetabular fossa. The acetabular notch and fossa also create a deficit in the smooth lunate surface of the acetabulum, the articular surface receiving the head of the femur.
Anatomical Position of Hip Bone
Surfaces, borders, and relationships of the hip bone are described assuming that the body is in the anatomical position. To place an isolated hip bone or bony pelvis in this position, situate it so that the:
- ASIS and the anterosuperior aspect of the pubis lie in the same coronal plane.
- Symphysial surface of the pubis is vertical, parallel to the median plane (Fig. 5.6).
In the anatomical position, the:
- Acetabulum faces inferolaterally, with the acetabular notch directed inferiorly.
- Obturator foramen lies inferomedial to the acetabulum.
- Internal aspect of the body of the pubis faces almost directly superiorly. (It essentially forms a floor on which the urinary bladder rests.)
- Superior pelvic aperture (pelvic inlet) is more vertical than horizontal; in the anteroposterior (AP) view, the tip of the coccyx appears near its center (Fig. 5.3).