Chapter 7: Lower Limb

Detailed Chapter Overview

Chapter 7 covers the radiography of the lower limb, a region that bears the full weight of the body and is subject to a vast range of traumatic and degenerative conditions. This chapter provides a meticulous guide to the anatomy and positioning of the foot, ankle, tibia, fibula, and knee. A deep understanding of the intricate anatomy, from the arches of the foot and the tarsal bones to the complex ligamentous structure of the knee, is paramount for producing diagnostic images. The chapter emphasizes the importance of precise positioning and CR angulation to correctly visualize joint spaces and anatomical relationships that are otherwise obscured by superimposition. It thoroughly details the anatomy of the ankle mortise and explains why a specific degree of internal rotation is necessary to view it as an open profile. The patella and its articulation with the femur are explored, along with specialized projections to assess patellofemoral tracking. Technical factors are carefully considered, highlighting the use of grids for larger body parts like the knee and the necessity of adjusting CR angles based on patient body habitus to accurately visualize the knee joint. The chapter provides detailed, step-by-step instructions for a multitude of projections, from routine AP, oblique, and lateral views to specialized weight-bearing methods for assessing joint stability and alignment. For each projection, exacting evaluation criteria are established, ensuring the radiographer can critically analyze their images for rotation, joint space visualization, and correct part-IR alignment.

In-Depth Study Guide

Anatomy of the Foot

The foot is comprised of 26 bones, categorized into phalanges, metatarsals, and tarsals, and is structured into two main arches: longitudinal and transverse.

Phalanges (Toes):

• There are 14 phalanges. The first digit (great toe) has two (proximal and distal), while digits 2-5 have three (proximal, middle, and distal).

Metatarsals:

• Five metatarsals are located between the phalanges and tarsals. The 5th metatarsal has a prominent, rough **tuberosity** at its base, which is a common fracture site.

Tarsals:

The seven tarsal bones of the posterior foot are larger and more irregular than the carpals of the wrist.

• Calcaneus (Os Calcis): The largest and strongest tarsal bone, commonly known as the heel bone. It has a posterior tuberosity for attachment of the Achilles tendon.
• Talus: The second largest tarsal bone, situated superior to the calcaneus. It is a key bone of the ankle joint, articulating with the tibia, fibula, calcaneus, and navicular.
• Navicular: A flattened, boat-shaped bone located between the talus and the three cuneiforms.
• Cuneiforms (3): Medial, intermediate, and lateral. They are located between the navicular and the bases of the first three metatarsals.
• Cuboid: Located on the lateral aspect of the foot, distal to the calcaneus and proximal to the 4th and 5th metatarsals.

Anatomy of the Ankle and Lower Leg

• Tibia: The larger, stronger, weight-bearing bone of the lower leg, located medially. Its distal end forms the prominent **medial malleolus**.
• Fibula: The slender bone of the lower leg, located laterally. It is not a weight-bearing bone. Its distal end forms the **lateral malleolus**.
• The Ankle Mortise: The true ankle joint is a synovial hinge-type joint formed by the articulations between the talus and the tibia and fibula. The deep socket formed by the inferior tibia and the medial and lateral malleoli is called the **mortise**. For the mortise to be seen as an open profile on a radiograph, the leg must be internally rotated 15-20 degrees.

Anatomy of the Knee

The knee is the largest and most complex joint in the body, involving the distal femur, proximal tibia, and the patella.

• Distal Femur: Features two large, rounded condyles: the **medial condyle** and the **lateral condyle**. The adductor tubercle is a small projection on the posterolateral aspect of the medial condyle.
• Proximal Tibia: The upper surface is the **tibial plateau**, which articulates with the femoral condyles. It has two small projections, the medial and lateral intercondylar tubercles, which together form the **intercondylar eminence**.
• Patella (Kneecap): The largest sesamoid bone in the body, embedded in the quadriceps tendon. It protects the anterior knee joint and acts as a fulcrum for the quadriceps muscle.
• Menisci: Two C-shaped fibrocartilage discs (medial and lateral) that sit on the tibial plateau, acting as shock absorbers.
• Ligaments: Provide stability to the knee. The two most important are the **Anterior Cruciate Ligament (ACL)** and **Posterior Cruciate Ligament (PCL)**, which prevent anterior/posterior sliding, and the **Medial (Tibial) Collateral Ligament (MCL)** and **Lateral (Fibular) Collateral Ligament (LCL)**, which prevent medial/lateral instability.

CR Angulation for the Knee:

The angle of the CR for an AP knee projection depends on the patient's body habitus, measured by the distance between the ASIS and the tabletop.

• Thin Pelvis (< 19 cm): 3-5 degrees caudad (toward the feet).
• Average Pelvis (19-24 cm): 0 degrees (perpendicular).
• Thick Pelvis (> 24 cm): 3-5 degrees cephalad (toward the head).

Routine and Special Projections

Foot

• AP (Dorsoplantar), Oblique, and Lateral: A 10-degree posterior angle is often used for the AP projection to open the intertarsal joints. The oblique requires a 30-40 degree medial rotation. The lateral must demonstrate the tibiotalar joint.
• Weight-Bearing AP and Lateral: Used to demonstrate the structural integrity of the longitudinal arches under the stress of body weight.

Calcaneus

• Axial (Plantodorsal) and Lateral: The axial projection requires a steep 40-degree cephalic angle to project the bulk of the calcaneus free of superimposition.

Ankle

• AP, Mortise, and Lateral: The AP view shows the joint but with some bony overlap. The **AP Mortise** view, achieved with a 15-20 degree internal rotation, opens the entire mortise joint space and is critical for assessing joint integrity.
• AP Stress Views: Performed to evaluate ligamentous tears. Inversion or eversion stress is applied to the ankle to see if the joint space widens unnaturally.

Knee

• AP and Lateral: The AP view requires correct CR angulation based on patient size. The lateral requires 20-30 degrees of flexion to relax muscles and show joint detail.
• AP Oblique (Medial and Lateral Rotation): A 45-degree rotation in each direction is used to visualize the tibial condyles and femoral condyles on the side "down" (closer to the IR).
• PA Axial (Rosenberg Method - "Tunnel View"): A weight-bearing view with the knees flexed 45 degrees and a 10-degree caudad CR angle. Excellent for demonstrating joint space narrowing associated with arthritis.
• Tangential (Sunrise/Skyline View): Used to visualize the patellofemoral joint and assess for subluxation or fractures of the patella.