Lower Limbs

The Appendicular Skeleton: Pelvic Girdle and Lower Limbs

The pelvic girdle, which connects the lower limbs to the axial skeleton, comprises four bones: the left and right coxal bones (appendicular) and the fused vertebrae of the sacrum and coccyx (axial). Within the coxal bones, there are three primary regions: the ilium (superior), the pubis (anterior), and the ischium (posterior). A crucial feature is the coccyx's articulation with the sacrum, which forms part of the sacroiliac joint. The acetabulum, a large indentation on the coxae, accommodates the head of the femur, creating the largest ball-and-socket joint in the human skeleton.

Additionally, the pelvis's anatomy allows for gender identification. The pelvic inlet, marked by a brim from the sacral promontory to the pubic symphysis, differentiates the abdominal cavity (superior) from the pelvic cavity (inferior). More critical in medicine is the pelvic outlet, the narrowest part of the pelvis involved in childbirth, bounded by ischial tuberosities, the pubic symphysis, and the distal coccyx. Notably, the female pelvis is wider than the male pelvis, with a broader pubic arch angle and a sacrum and coccyx that point downward and outward, facilitating birth.

Transitioning to the lower limbs, the femur is the largest bone, crucial for transmitting upper body weight to the feet. Its spherical head articulates with the acetabulum of the coxae, making the hip joint strong but less mobile than the shoulder joint, thus reducing dislocation risks. Hip fractures, which often occur at the proximal femur, should be understood in this context.

The distal end of the femur connects with the patella through the patellar groove; the patella is a sesamoid bone aiding in force transmission across the knee. The tibia, or shin bone, is larger and bears weight on the medial side, articulating with the femur, fibula, and talus (ankle bone). The fibula, smaller, articulates with the tibia and talus but not with the femur directly.

In the foot, the talus articulates with both the tibia and fibula to form part of the ankle joint, while the calcaneus (heel bone) bears significant body weight. The foot also comprises other tarsal bones, the metatarsals, and phalanges. The digit structure of the foot mirrors that of the hand, but the arrangement is reversed: digit number one (big toe) is medial, while digit number five is lateral. Finally, the foot features longitudinal and lateral arches that distribute the upper body's weight and aid locomotion, preventing nerve and blood vessel compression in the soft tissue of the foot.