RT 211: Comprehensive Study Notes on the Lower Limb Bones and Joints
Overview of the Pelvic Girdle and Lower Limb Bones
The study of the lower limb, categorized under medical radiation technology, encompasses the skeletal structure and articulations of the pelvic girdle and the lower extremities. The pelvic girdle consists of three fused bones: the ilium, the ischium, and the pubis. These three elements unite to form the hip bone, also known as the os coxae or the innominate bone. The complete pelvis is a complex structure formed by the right and left hip bones, the sacrum, and the coccyx. This anatomical region is essential for supporting the trunk and facilitating locomotion.
The lower limb is further divided into specific regions: the thigh, the leg (or crus), and the foot. The thigh contains the femur, often referred to as the thigh bone, and the patella, or knee cap. The leg consists of the tibia, known as the shin bone, and the fibula, known as the calf bone. The foot is organized into the tarsus (instep), the metatarsus, and the digits (toes). The tarsus includes seven bones: the talus (astragalus), the calcaneus (os calcis or heel bone), the navicular (tarsal scaphoid), the cuboid, and three cuneiform bones (medial, intermediate, and lateral).
Detailed Anatomy of the Hip Bone (Os Coxae)
The hip bone is a large, irregular bone that serves as the connection point between the lower limb and the trunk. A primary feature of this bone is the acetabulum, a deep, circular, cup-shaped depression located on the lateral surface below the middle of the bone. Within the acetabulum is the acetabular notch, which is a gap on the inferior margin of the rim, and the acetabular fossa, a flat central portion. Another prominent feature is the obturator foramen, a large opening in the inferior part of the hip bone situated between the pubis and the ischium.
The ilium, or flank bone, forms the upper $2/5$ of the acetabulum and the entirety of the hip bone above it. It consists of a body and a wing (ala). The ala is the thin, flattened upper part, characterized by the iliac crest (the upper curved border) and the iliac fossa (the concave inner surface). The auricular surface is a rough, ear-shaped part posterior to the iliac fossa that articulates with the sacrum. The ilium features four distinct spines: the anterior superior iliac spine (ASIS), the anterior inferior iliac spine (AIIS), the posterior superior iliac spine (PSIS), and the posterior inferior iliac spine (PIIS).
The ischium forms the lower $2/5$ of the acetabulum and the part of the hip bone below and dorsal to it. Key landmarks include the ischial spine, a pointed process extending medially and dorsally, and the ischial tuberosity, a large rounded process on the posterior surface of the lower body. The pubis forms the anterior $1/5$ of the acetabulum. It includes a superior and inferior ramus, a pubic tubercle (a small process lateral to the symphysis), and a pubic crest. The pubic arch is the curved surface formed by the medial margins of the inferior rami of both pubic bones and the lower border of the symphysis pubis.
Characteristics and Sexual Dimorphism of the Pelvis
The whole pelvis acts as a hollow, curved cylinder (the term pelvis is derived from the Latin for "basin") at the inferior end of the vertebral column. It is divided into the true pelvis (minor pelvis), located between the inlet and outlet, and the false pelvis (major pelvis), which lies above the plane of the inlet. The linea terminalis (terminal line or arcuate line) is the ridge of bone beginning at the upper margin of the symphysis pubis that defines the pelvic rim or inlet. The superior aperture (inlet) is the heart-shaped opening into the true pelvis, while the inferior aperture (outlet) is the opening at the lower end composed of an incomplete bony ring and ligaments. The posterior margin of the hip bone features the greater sciatic notch and the lesser sciatic notch.
Significant differences exist between the male and female pelvis due to the requirements of childbirth. In females, the hips are more prominent, the iliac crests flare outward more and are less curved, and the anterior iliac spines are further apart. The female superior aperture is larger and more rounded, whereas the male inlet is heart-shaped. Additionally, in females, the acetabula and ischial tuberosities are further apart, the ischial spines are less medially directed, and the inferior aperture is larger. The female pubic arch is broader, measuring between , compared to the male pubic arch which is . The joint at the symphysis pubis is also shorter in females.
Osteology of the Thigh and Knee: Femur and Patella
The femur is the longest bone in the human body, extending from the hip to the knee. At its proximal end, it features a head with a small central pit called the fovea capitis femoris, a neck, and two large processes: the greater trochanter (lateral surface) and the lesser trochanter (medial surface). These trochanters are connected posteriorly by the intertrochanteric crest and anteriorly by the intertrochanteric line. The shaft or body of the femur contains the linea aspera, a double bony ridge running longitudinally down its posterior surface. The distal end of the femur features the medial and lateral condyles, the intercondylar fossa (a deep notch between the condyles), the patellar surface, the medial and lateral epicondyles, and the adductor tubercle on the medial epicondyle.
The patella is the largest sesamoid bone in the body, which is a type of bone that develops within a tendon—specifically the tendon of the quadriceps muscle. It is a flat, rounded bone positioned anterior to the knee joint. Anatomical parts of the patella include the proximal margin (base), the pointed inferior end (apex), and the articular surface which allows it to glide against the femur. Another sesamoid bone mentioned is the fabella, which is frequently present posterior to and above the lateral condyle of the femur.
Anatomy of the Leg: Tibia and Fibula
The tibia, or shin bone, is a large, sturdy bone located medially in the leg. Its proximal end includes the medial and lateral condyles, the intercondylar eminence (a double-pointed process between articular surfaces), and the tibial tuberosity on the anterior surface. The distal end features the medial malleolus, which projects downward and reaches approximately below the level of the ankle joint. It also includes the inferior articular surface for the talus and the fibular notch on the lateral surface.
The fibula, or calf bone, is a long, slender bone situated on the lateral side of the tibia and parallel to it. It does not bear weight but serves as an attachment point for muscles. Its proximal end is the head, which terminates in a pointed upper end called the apex or styloid process. The distal extremity is the lateral malleolus, which forms the lateral prominence of the ankle. The tip of the lateral malleolus is positioned at a lower level than the medial malleolus.
The Foot: Tarsals, Metatarsals, and Phalanges
The foot structure is highly specialized for weight-bearing. The posterior tarsal bones include the talus and the calcaneus. The talus, also called the astragalus or ankle bone, resides between the tibia and the calcaneus; its upper convex surface is the trochlea. The calcaneus is the largest tarsal bone, forming the heel; it features the calcaneal tuberosity and the sustentaculum tali, a medial shelf of bone that supports the talus. The anterior tarsal bones consist of the navicular (which has a tuberosity), the cuboid, and the three cuneiform bones.
The metatarsus is composed of five bones numbered medial to lateral (first through fifth). Each metatarsal bone has a base (proximal), a body, and a head (distal). Notably, the fifth metatarsal features a prominent tuberosity at its base. The digits (toes) consist of phalanges. The great toe (hallux or first digit) has two phalanges: proximal and distal. The other four toes each have three phalanges: proximal, middle, and distal. Like the metatarsals, each phalanx has a base, body, and head. Sesamoid bones are often found in tendons on the plantar surfaces of the metatarsophalangeal joints, particularly the great toe.
Arthrology: Joints of the Lower Limb
The articulations of the lower limb are categorized by their movement and structure. The sacroiliac joints (2) connect the auricular surfaces of the sacrum and ilium and are classified as gliding joints. The symphysis pubis is a cartilaginous joint between the medial margins of the pubic bones. The hip joints (2) are ball-and-socket joints between the femoral head and the acetabulum, stabilized by the ligamentum teres (round ligament). The knee joints (2) are complex articulations involving the femoral condyles, the tibial condyles, and the patella. They are described as hinge, double condylar, and gliding joints. Internal structures of the knee include the menisci (semilunar cartilages) and the cruciate ligaments (anterior and posterior).
Other joints include the proximal tibiofibular joint (gliding) and the distal tibiofibular joint (cartilaginous). The ankle joint (talocrural joint) is a hinge joint formed by the tibia, fibula, and talus. Within the foot, there are intertarsal joints (gliding), tarsometatarsal joints (5 per foot, gliding), intermetatarsal joints (4 per foot), metatarsophalangeal joints (5 per foot, condylar), and interphalangeal joints (9 per foot, hinge), subdivided into proximal and distal articulations.
The Arches of the Foot and Clinical Considerations
The foot features arches that provide strength and flexibility. The longitudinal arch is visible on the medial border and is formed by tarsal and metatarsal bones. This is further divided into the internal (medial) and external (lateral) longitudinal arches. The transverse arch curves across the foot from the medial to the lateral side of the plantar surface.
Congenital anomalies of the pelvis and lower limb include congenital dislocation (where the femoral head is outside the acetabulum), the absence of patellae, and bipartite patella (where the patella is in two separate pieces). Other anomalies include the os trigonum (a small triangular bone posterior to the talus that failed to unite) and the accessory navicular bone (a separate posteromedial part of the navicular). Radiography of these regions focuses on landmarks such as the iliac crest, ASIS, ischial tuberosity, greater trochanter, tibial tuberosity, and the malleoli to ensure proper imaging of the bones and joints for diagnostic purposes.