Pelvic Girdle - Appendicular Skeleton

Appendicular Skeleton

The appendicular skeleton contains the upper and lower limb bones and the bones anchoring the limbs to the axial skeleton. The appendicular skeleton includes the pectoral girdle, upper limbs, pelvic girdle, and lower limbs.

Pelvic Girdle

Two hip bones, which articulate with each other and the sacrum, make up the pelvic girdle (FIGURE 7-32). The hip bones are also called the coxal or pelvic bones. The pelvic girdle attaches the lower limbs to the axial skeleton. Together, the sacrum, coccyx, and pelvic girdle form the pelvis.

The pelvis of females is usually wider in all diam-eters than that of males. The pelvic girdle supports, protects, and/or articulates with the trunk, lower limbs, urinary bladder, large intestine, and repro-ductive organs. The hip bones each have three parts (the ilium, ischium, and pubis) fused together into an acetabulum, which houses the rounded head of the femur (thigh bone).

Ilium

The largest portion of the hip bone is the ilium , form-ing the prominence of the hip. The margin of the prominence is called the iliac crest. This crest ends anteriorly in a blunt anterior superior iliac spine and ends posteriorly in a sharp posterior superior iliac spine. The anterior superior iliac spine is an import-ant landmark since it is easily felt through the skin, and is even visible when an individual is thin. Just inferior to the posterior inferior iliac spine there is a deep indentation in the ilium, which forms the greater sciatic notch. Through this notch, the thick and rope-like sciatic nerve passes to enter the thigh. The posterolateral surface of the ilium is broad and is called the gluteal surface. Three ridges cross this surface, which are known as the posterior, anterior, and inferior gluteal lines. The gluteal (buttock) mus-cles attach to these lines.

The ilium is made up of a body and a superior, wing- shaped portion called the ala. The body of the ilium joins the pubis anteriorly and joins the ischium inferiorly. The ilium joins the sacrum at the sacroiliac joint. A projection from the ilium provides attach-ments for ligaments and muscles. Ligaments from the iliac tuberosity stabilize the sacroiliac joint. Each sacroiliac joint allows the body’s weight to be transmitted from the spine to the pelvis. A promi-nent arcuate line runs inferiorly and anteriorly from the auricular surface. It is a ridge that helps cre-ate the pelvic brim, which is the superior margin of the true pelvis.

Ischium

The ischium is the lowest portion of the hip bone and is L-shaped. Its angle, the ischial tuberosity, points downward and posteriorly. Each ischial tuberosity is very strong and thick. It supports the body weight when sitting. There are two other important areas of the ischim: the ischial spine and the lesser sciatic notch . The ischial spine projects medially into the pelvic cavity. It is a point of attachment of the sacrospi-nous ligament that runs from the sacrum. Just inferior to this is the lesser sciatic notch, through which many nerves and blood vessels pass to supply the anogenital region. The inferior surface of the ischial body forms the ischial tuberosity, having a rough texture.

Pubis

The pubis is the anterior portion of the hip bone, and forms an angle known as the pubic arch or subpubic angle. This arch helps to differentiate between the pelvises of males and females. The pubis is basically V-shaped, with superior and inferior rami extending from its flattened, medial body. The pubic crest is formed by the thickened anterior border. The lateral end of the pubic crest is the pubic tubercle, which is one of the attachments for the inguinal ligament. The two pubic bones join at the symphysis pubis, the upper margin of which (the pelvic brim) separates the lower pelvis from the upper portion. A large open-ing, known as the obturator foramen, lies between the pubis and ischium. It is nearly closed by a fibrous membrane.

Lower Limbs

The lower limbs consist of the bones of the thigh, leg, and foot. In each lower limb, these bones include a femur, a patella, a tibia, a fibula, tarsals, metatarsals, and phalanges.

Femur

The femur or thighbone is the longest, strongest, and largest bone in the body, extending from the hip to the knee (FIGURE 7-33). Various processes from the femur provide attachments for muscles of the lower limbs and buttocks. A pit on the head of the femur called the fovea capitis marks the point of attach-ment of the ligamentum capitis. Just below the head is a neck (constriction) and large processes known as the lateral greater trochanter and the medial lesser trochanter, where the thigh and buttock muscles are attached. The trochanters are connected anteriorly by the intertrochanteric line and connected posteriorly by the prominent intertrochanteric crest.

Proximally, the femur articulates with the hip bone, coursing medially over its length toward the knee. Because of this structure, the knee joints can remain closer to the body’s center of gravity, ­providing better balance. In females, the medical course of each femur is more pronounced, due to their pelvises being wider. This can cause a higher likelihood of knee problems in females who are athletic. The femur, after broadening distally, articulates with the tibia via the lateral and medial condyles, which are processes flanked superiorly by lateral and medial epicondyles or sites of muscle attachment. The superior region of the medial epicondyle has a bump known as the adductor tubercle. Between the condyles on the anterior femoral surface, the smooth patellar surface articulates with the patella. There is a deep, U-shaped intercondylar fossa between the condyles on the posterior aspect of the femur.

Patella

The kneecap (patella) is a triangular-shaped bone that articulates with the femur and is located in a ten-don passing over the knee. The patella has a convex, rough anterior surface and a broad base. The apex of the patella is connected to the tibia by the patellar lig-ament. The rough surface of the patella is where the anterior and superior surfaces of the quadriceps ten-don and the patellar ligament attach.

Tibia

The tibia or shinbone, is the larger of the two leg bones, located on the medial side. Its distal end expands to form a prominence on the inner ankle where liga-ments attach (FIGURE 7-34). A depression on its lateral side articulates with the fibula. The tibial tuberosity is the process where the patellar ligament attaches. The distal end of the tibia has an inner prominence (the medial malleolus) where ligaments attach.

Fibula

The fibula is a slender bone located on the lateral side of the tibia. It does not enter into the knee joint and does not bear any body weight. The head of the fibula articulates with the tibia. The fibula has slightly enlarged ends, a proximal head and a distal lateral malleolus.

Foot

The foot includes the ankle, instep, and toes. The ankle or tarsus is made up of seven bones called tarsals that are arranged so the talus bone moves freely where it joins the leg bones. The tarsal bones connect the tibia and fibula to the foot. The other tarsal bones are firmly bound in a mass supporting the talus. The largest tarsal bone is the calcaneus­(heel bone), which helps support body weight and provides muscle attachment for foot movement.

The Achilles tendon attaches to the calcaneus. The other tarsals are the lateral cuboid, the medial ­navicular, and the anterior medial, intermediate, and lateral cuneiform bones. The cuboid and cuneiform­ bones articulate with the metatarsal bones anteriorly.

Metatarsals

The instep (metatarsus) is made up of five bones called metatarsals numbered one through five, beginning with the medial side. These bones are small and long, except for the first metatarsal, which is thick and short, playing an important role in supporting the weight of the body. The metatarsals have a more par-allel arrangement than the metacarpals in the hands. The “ball” of the foot is formed by the enlarged head of the first metatarsal. It articulates distally with the proximal phalanges of the toes.

Phalanges

The 14 phalanges of the toes, similar to those of the fingers, are aligned with the metatarsals. Each toe has three phalanges except the great toe or hallux, which has only two—proximal and distal (­FIGURE 7 -35). The phalanges of the toes are smaller than the ­phalanges of the fingers and have less mobility, though they are similar in structure and arrangement.

Arches of the Foot

The two longitudinal arches of the foot allow for weight transfer to occur and are maintained by the ligaments and tendons that bind the calcaneus to the distal portions­ of the metatarsal bones. The degree of ­curvature from the medial to lateral foot borders is called the transverse arch. The foot bones ­interlock , which along with the strength of the ligaments and the pull of certain tendons maintains the arches during muscular activity. The arches stretch as needed when walking or running. The medial longitudinal arch curves significantly because of the position of the talus. The lateral longitudinal arch is lower, elevat-ing the lateral foot to redistribute some weight to the calcaneus and fifth metatarsal. The most important bone related to this arch is the cuboid. The two lon-gitudinal arches act like pillars for the transverse arch. The bones of the adult skeleton are listed in TABLE 7-1 and the terms describing skeletal structures are listed in TABLE 7-2.

1. Which bones are involved in the formation of the hip bone?

2. List the bones of the lower limbs.

3. Which bone is the strongest in the human body?

4. Which arches of the feet support the weight of the body?