The Skeletal System: Lower Extremity Anatomy and Osteology

Overview and Objectives of the Lower Extremity Skeletal System

• The skeletal system of the lower extremity is organized similarly to a bowl at the top (the pelvis) extending down into the thigh, the lower leg, and the foot.
• Key objectives for studying this system include:
  • Identifying all constituent parts of the pelvis.
  • Determining anatomical orientation (Anterior, Posterior, Medial, and Lateral sides).
  • Distinguishing between the male and the female pelvis based on functional and structural differences.
  • Understanding the orientation and parts of the femur (thigh bone) and the tibia (medial leg bone).
  • Listing the specific anatomical parts of the fibula (lateral leg bone).
  • Identifying the $7$ tarsal bones (ankle bones).
  • Correct naming and numbering of the metatarsals and phalanges.
  • Defining the hallux (the big toe).

The Pelvis: Os Coxae and Major Components

• The term "os coxae" refers to exactly one-half of the pelvis when divided down the sagittal midline.
• The pelvis (or each os coxae) is composed of three primary bones that fuse together:
  • Ilium: The most superior portion of the pelvis. (Note the spelling: "ilium" refers to the bone, whereas "ileum" refers to a section of the small intestine).
  • Ischium: The posterior-inferior portion of the pelvis. The speaker uses the mnemonic "You sit on your ish (ischium) which is your tish (tush)" to remember its posterior location.
  • Pubis (Pubic Bone): The most anterior portion of the pelvis.
• The Acetabulum:
  • This is the large, cup-like socket on the lateral side of the pelvis where the head of the femur articulates.
  • The name translates to "vinegar cup." The etymology stems from the word "acetic acid" (vinegar), as the socket resembles the small cups used to serve vinegar in ancient times.
• Pelvic Openings and Passageways:
  • Obturator Foramen: The large opening located between the pubis and the ischium. It serves as a passage for a neurovascular bundle, specifically the obturator nerve, artery, and vein.
  • Greater Sciatic Notch: A large indentation on the posterior aspect of the ilium, superior to the ischial spine.
  • Lesser Sciatic Notch: A smaller indentation located inferior to the ischial spine. The sciatic nerve travels through this region.
• Sacrum and Coccyx:
  • The sacrum is a large, triangular bone at the base of the spine, posterior to the pelvis.
  • It consists of $5$ fused vertebrae (numbered $1$ through $5$).
  • Sacral Ala: The wing-like lateral projections on the superior portion of the sacrum ("ala" means wing).
  • Sacral Promontory: The anterior-most portion of the superior sacrum.
  • Neuroforamina: Small holes in the sacrum through which sacral nerves exit.
  • Coccyx: Anatomically known as the tailbone, which follows the sacrum inferiorly.

Detailed Landmarks of the Pelvic Bones

• Landmarks of the Ilium:
  • Iliac Crest: The superior border of the ilium. This is the structure people usually feel when they "put their hands on their hips."
  • Iliac Fossa: A large, smooth, concave depression on the anterior surface of the ilium.
  • Anterior Superior Iliac Spine (ASIS): The upper-most point or process on the anterior side of the ilium crest.
  • Anterior Inferior Iliac Spine (AIIS): A bony process located just below the ASIS.
  • Posterior Superior Iliac Spine (PSIS): The upper-most point on the posterior side of the ilium.
  • Posterior Inferior Iliac Spine (PIIS): A bony process located just below the PSIS.
• Landmarks of the Ischium:
  • Ischial Spine: A sharp projection coming off the ischium, separating the greater and lesser sciatic notches.
  • Ischial Tuberosity: A rough, thickened area of the ischium. As a tuberosity is a rough spot for muscle attachment, this is specifically the site where the hamstring muscles of the posterior thigh attach.
• Landmarks of the Pubis:
  • Superior Ramus: The upper branch of the pubic bone extending toward the acetabulum.
  • Inferior Ramus: The lower branch extending toward the ischium.
  • Pubic Symphysis: The midline cartilaginous joint (specifically a symphysis joint containing fibrocartilage) that connects the left and right pubic bones.

Sex-Related Differences in the Pelvis

• The Pelvic Inlet and Outlet:
  • Pelvic Inlet: The superior opening of the true pelvis. The narrowest part of the inlet is the distance between the two ischial spines.
  • Pelvic Outlet: The inferior opening through which a baby passes during childbirth.
• Male Pelvis:
  • Tends to be narrower and heavier.
  • The pelvic inlet is often described as "heart-shaped." The speaker specifically compares it to a "Mickey Mouse" shape, where the sacrum and iliac fossa create the appearance of ears and a face.
  • The distance between the ischial spines is narrow.
• Female Pelvis:
  • Tends to be wider and shallower to facilitate childbirth.
  • The pelvic inlet is much more oval-shaped rather than heart-shaped.
  • The distance between the ischial spines is significantly greater compared to the male pelvis.

The Femur (Thigh Bone)

• The femur is the bone of the thigh. It is categorized into proximal, shaft (body), and distal regions.
• Proximal Femur:
  • Head: A ball-shaped structure that fits into the acetabulum of the pelvis to form the "ball and socket" hip joint. The head is positioned medially.
  • Fovea Capitis: A small indentation on the head of the femur that allows for the passage of nerves, arteries, and veins to nourish the bone.
  • Neck: The constricted region connecting the head to the shaft. This is a very common site for hip fractures.
  • Greater Trochanter: A large, lateral projection that can be felt on the outside of the upper thigh.
  • Lesser Trochanter: A smaller, medial projection located inferior to the neck.
• Shaft of the Femur:
  • Linea Aspera: A rough longitudinal ridge on the posterior surface of the femoral shaft. It serves as an attachment point for muscles, specifically the short head of the biceps femoris.
• Distal Femur:
  • Condyles: Rounded, doorknob-like surfaces that articulate with the tibia. There are Medial Condyles and Lateral Condyles.
  • Epicondyles: Bony bumps located just above ("epi-") the condyles. There is a Medial Epicondyle and a Lateral Epicondyle.
  • Intercondylar Fossa: A deep depression located between the two condyles on the posterior side.
  • Patellar Surface: Located on the anterior distal femur where the patella sits.

The Patella and Lower Leg Bones

• Patella:
  • Commonly known as the kneecap.
  • A triangular-shaped bone located only on the anterior side of the leg.
• Tibia:
  • The larger, medial bone of the lower leg.
  • Tibial Tuberosity: A rough spot on the anterior superior surface where the quadriceps tendon attaches.
  • Condylar Surfaces: The flat top of the tibia (sometimes called the tibial plateau) that articulates with the femoral condyles.
  • Medial Malleolus: The large bony bump on the medial side of the ankle (often called the "inner ankle bone").
• Fibula:
  • The thinner, lateral bone of the lower leg. (Mnemonic: "fi-bu-LA" for lateral).
  • It attaches to the tibia laterally, not to the femur.
  • Head: The proximal end.
  • Neck: Located just below the head. An important nerve, the common peroneal nerve (also known as the fibular nerve), travels around this neck from posterior to anterior.
  • Lateral Malleolus: The bony bump on the lateral side of the ankle (the "outer ankle bone").

The Bones of the Foot

• Tarsal Bones ($7$ in total):
  1. Talus: The superior-most tarsal bone that articulates with the tibia and fibula.
  2. Calcaneus: The heel bone, located inferior and posterior to the talus. It features a tuberosity for walking.
  3. Navicular: A boat-shaped bone located on the medial side, distal to the talus.
  4. Cuboid: A cube-shaped bone located on the lateral side, distal to the calcaneus.
  5. Medial Cuneiform (or $1^{st}$ cuneiform): Located on the medial side.
  6. Intermediate Cuneiform (or $2^{nd}$ cuneiform): Located between the medial and lateral cuneiforms.
  7. Lateral Cuneiform (or $3^{rd}$ cuneiform): Located most laterally among the cuneiforms.
• Metatarsals:
  • There are $5$ metatarsals, numbered $I$ to $V$ starting from the medial side (the big toe side) to the lateral side.
• Phalanges (Toe Bones):
  • Hallux: The anatomical name for the big toe. It consists of only $2$ phalanges: Proximal Phalanx and Distal Phalanx.
  • Digits $II, III, IV,$ and $V$: Each of these four toes consists of $3$ phalanges: Proximal Phalanx, Middle Phalanx, and Distal Phalanx.
  • To name a specific bone, use its number and position (e.g., "the $5^{th}$ distal phalanx").
  • There are a total of $14$ phalanges per foot. Knowing the naming convention for these (proximal, middle, distal, and numbering $1-5$) allows a student to identify roughly one-quarter of the bones in the entire body when combined with the hands.