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HLT-RADI64-03 · Lesson 4 · 24 clock hours · theory

Skeletal Anatomy of the Lower Extremity

Objective. Students will identify all named bones and major bony landmarks of the toes, foot, ankle, lower leg, knee, femur, and pelvis on anatomical diagrams and radiographic images.

⚠ Pending professional review

This lesson, its worked examples and its practice questions has not yet been verified by a licensed clinician in the field or by a certification or approval body. It is a study draft and may contain errors. Do not use it to make decisions about any patient or medication. Always follow your instructor, your supervising professional, and current official references and regulations.

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Overview

A solid command of lower extremity skeletal anatomy is the foundation of every radiographic positioning decision you will make on the job. Before you can position a patient correctly, select the right projection, or communicate findings to a radiologist or ordering provider, you must be able to name every bone and landmark you see on the image. This lesson walks through the skeleton from the tips of the toes to the pelvis.

Toes and Foot

The foot contains 26 bones grouped into three regions: phalanges, metatarsals, and tarsals.

Each toe is built from small bones called phalanges (singular: phalanx). The great toe (first digit) has two phalanges — a proximal phalanx and a distal phalanx. Digits two through five each have three: proximal, middle, and distal phalanges. The joints between phalanges are called interphalangeal (IP) joints; in the great toe the single joint is simply the IP joint, while in the lesser toes they are the proximal interphalangeal (PIP) and distal interphalangeal (DIP) joints.

Proximal to the phalanges are the five metatarsals, numbered one through five from medial to lateral. They articulate distally with the proximal phalanges at the metatarsophalangeal (MTP) joints and proximally with the tarsals at the tarsometatarsal (TMT) joints, also called Lisfranc joints.

The seven tarsal bones form the rear and mid-foot: the calcaneus (heel bone, the largest tarsal), talus (which sits atop the calcaneus and articulates with the leg), navicular (on the medial side), cuboid (on the lateral side), and three cuneiforms — medial, intermediate (middle), and lateral. The calcaneus and talus together make up the hindfoot; the navicular, cuboid, and cuneiforms form the midfoot.

Ankle

The ankle joint (talocrural joint) is formed by three bones: the tibia, the fibula, and the talus. The distal tibia projects downward on the medial side as the medial malleolus. The distal fibula forms the lateral malleolus. Together these two malleoli form a mortise — a bracket-shaped socket — that cups the superior dome of the talus. The syndesmosis is the fibrous joint holding the distal tibia and fibula together just above the mortise; injury here is clinically significant and may be subtle on radiographs.

Lower Leg

The lower leg has two bones: the larger, weight-bearing tibia on the medial side, and the slender fibula on the lateral side. On the tibia, identify the tibial plateau (the broad proximal surface with medial and lateral condyles), the tibial tuberosity (a bony prominence on the anterior shaft where the patellar tendon inserts), the anterior crest (the sharp ridge felt under the skin), and the medial malleolus distally. The fibula has a head proximally, a slender shaft, and the lateral malleolus distally. The proximal tibiofibular joint and distal tibiofibular syndesmosis connect the two bones.

Knee

The knee is the largest joint in the body. It involves the distal femur, the proximal tibia, and the patella. On the distal femur, identify the medial condyle and lateral condyle, the medial epicondyle and lateral epicondyle above them, and the intercondylar notch (fossa) between the condyles posteriorly. The proximal tibia presents the medial tibial plateau and lateral tibial plateau separated by the intercondylar eminence (tibial spines). The patella is the largest sesamoid bone in the body; it sits within the quadriceps tendon and articulates with the patellar surface (trochlear groove) of the femur. On a standard AP knee, the joint space is the gap between the femoral condyles and the tibial plateau.

Femur and Proximal Femur

The femur is the longest and strongest bone in the body. Key landmarks include the head (a smooth hemisphere that fits into the acetabulum), the neck (the angled column connecting head to shaft), the greater trochanter (the large lateral prominence), the lesser trochanter (the posteromedial prominence), the intertrochanteric crest (posteriorly) and intertrochanteric line (anteriorly) connecting the two trochanters, the long shaft, and the medial and lateral condyles distally. Fractures of the femoral neck and intertrochanteric region are common in older adults and are among the most clinically important findings you will see in hip and pelvis radiography.

Pelvis

The pelvis is a ring formed by the two hip bones (also called os coxae or innominate bones) and the sacrum posteriorly. Each hip bone has three developmental components fused in adulthood: the ilium (the broad wing superiorly), the ischium (the posteroinferior portion, including the ischial tuberosity — the bony seat we sit on), and the pubis (the anteromedial portion). The cup-shaped socket where the femoral head articulates is the acetabulum, formed by contributions from all three components. The two pubic bones meet at the midline at the pubic symphysis. Posteriorly, the ilium joins the sacrum at the sacroiliac (SI) joints. On an AP pelvis, the greater sciatic notch, obturator foramen, and anterior superior iliac spine (ASIS) are additional landmarks you must recognize.

Safety and Scope Considerations

Before positioning any lower extremity, communicate clearly with the patient about pain and mobility limitations. Never force a limb into a position that causes distress. Always immobilize with positioning sponges rather than asking patients or colleagues to hold a limb in the primary beam. Apply gonadal shielding when it will not obscure the anatomy of interest and per facility protocol. Collimate tightly to the area of interest to reduce scatter and patient dose. Confirm the correct patient and correct body part before exposure. Accurate anatomy identification is also a patient safety issue: misidentifying a bone or landmark can lead to repeat exposures, which increase dose, or to missed pathology, which harms the patient.

Key terms9

Phalanx (pl. phalanges)Any of the small bones that form the toes; the great toe has two (proximal and distal), and each lesser toe has three (proximal, middle, distal).
TarsalsThe seven bones of the hindfoot and midfoot: calcaneus, talus, navicular, cuboid, and medial, intermediate, and lateral cuneiforms.
Medial malleolusThe bony prominence at the distal end of the tibia that forms the medial wall of the ankle mortise.
Lateral malleolusThe bony prominence at the distal end of the fibula that forms the lateral wall of the ankle mortise.
Intercondylar eminenceThe bony projections (tibial spines) rising from the center of the proximal tibial plateau between the medial and lateral condyles; seen on AP knee images.
AcetabulumThe cup-shaped socket in the hip bone formed by the ilium, ischium, and pubis that articulates with the femoral head.
Greater trochanterThe large bony prominence projecting laterally from the proximal femur at the junction of the neck and shaft; a key landmark on AP pelvis and hip images.
PatellaThe largest sesamoid bone in the body, located within the quadriceps tendon; articulates with the trochlear groove (patellar surface) of the distal femur.
Pubic symphysisThe cartilaginous joint at the midline where the right and left pubic bones meet; a key midline landmark on AP pelvis radiographs.
In class: Label blank diagrams of each lower extremity segment from phalanges through pelvis, self-correct with the atlas, and record a personal error log to focus study. · Complete a radiographic image annotation activity: for each of eight provided radiographic images (foot AP, foot lateral, ankle AP, ankle lateral, knee AP, knee lateral, pelvis AP, hip AP), students write the anatomical name of every labeled arrow. · Participate in an instructor-led Socratic review using projected radiographs, identifying structures, naming adjacent articulations, and predicting which joint spaces are best demonstrated on each projection.
Materials: Merrill's Atlas, lower extremity skeletal models, skeletal diagram worksheets, projected radiographic images, whiteboard
Check: Score ≥ 80% on a 50-item image-based identification quiz covering toe-to-pelvis anatomy on radiographic images.

Practice check8

Answer each question, then check it. Nothing is recorded; this is your own practice.

Radiographic procedures / anatomy
1. On an AP radiograph of the foot, which bone is correctly identified as the largest of the tarsal bones?
Radiographic procedures / anatomy
2. Which structures form the mortise of the ankle joint?
Radiographic procedures / anatomy
3. A radiograph demonstrates a bony prominence on the anterior proximal shaft of the tibia where the patellar tendon inserts. What is this landmark called?
Radiographic procedures / anatomy
4. On an AP pelvis radiograph, the cup-shaped socket that articulates with the femoral head is identified. What is the correct name for this structure?
Radiographic procedures / anatomy
5. How many phalanges does the second (index) toe normally contain?
Radiographic procedures / anatomy
6. On a lateral knee radiograph, the large sesamoid bone visible within the anterior soft tissues overlying the distal femur is the:
Radiographic procedures / anatomy
7. Which of the following best describes the clinical importance of accurately identifying the intertrochanteric region of the proximal femur on a hip radiograph?
Patient care and safety
8. During an AP pelvis examination, which patient safety practice specifically related to anatomy identification should the radiographer perform before making an exposure?
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