Radiographic Anatomy and Positioning of the Femur and Pelvic Girdle

Hip and Proximal Femur Anatomy

System Components: The lower limb anatomy involved in this study includes the pelvic girdle, the femur, the hip joint, the leg, and the foot.
Joints of the Pelvic Region: * Sacroiliac Joint: The articulation between the sacrum and the ilium. * Hip Joint: The articulation between the head of the femur and the acetabulum of the hip bone. * Symphysis Pubis: The cartilaginous joint between the two pubic bones.

Proximal Femur Structure and Angles

Anatomic Features of the Proximal Femur: * Head: The rounded proximal end that articulates with the acetabulum. * Fovea Capitis: A small depression or pit located at the center of the femoral head which serves as an attachment point for the ligamentum teres. * Neck: The constricted portion connecting the head to the body or shaft. * Greater Trochanter: A large, prominent, quadrangular process located superiorly and laterally to the femoral shaft. * Lesser Trochanter: A smaller conical projection located at the junction of the base of the neck and the shaft of the femur on the posterior-medial aspect. * Intertrochanteric Crest: A thick ridge on the posterior surface that connects the greater and lesser trochanters. * Body or Shaft: The long cylindrical portion of the bone extending distally from the trochanters.
Geometric Angles of the Proximal Femur: * Neck to Shaft Angle: The longitudinal axis of the neck forms an angle of approximately $125^\circ$ with the shaft. This angle varies slightly based on body habitus. * Longitudinal Angle: The femur sits at a longitudinal angle of approximately $10^\circ$ from the vertical. * Anterior (Antetorsion) Angle: The femoral head and neck are tilted anteriorly at an angle of approximately $15^\circ$ to $20^\circ$ relative to the body of the femur.

Anatomy of the Pelvis and Hip Bones

Composition of the Pelvis: The complete pelvis is composed of four bones: * Two Hip Bones (also known as ossa coxae or innominate bones). * One Sacrum. * One Coccyx.
Structure of the Hip Bone (Ossa Coxae): Each hip bone is formed by the fusion of three primary parts: 1. Ilium: The largest and most superior portion. 2. Ischium: The inferior and posterior portion. 3. Pubis: The lower and anterior portion.
The Acetabulum: A deep, cup-shaped socket formed by the union of the ilium, ischium, and pubis, which receives the head of the femur to form the hip joint.
Specific Anatomy of the Ilium: * Ala (Wing): The large, flared upper portion. * Iliac Crest: The superior border of the ala. * Anterior Superior Iliac Spine (ASIS): A prominent bony landmark located at the anterior end of the iliac crest. * Anterior Inferior Iliac Spine (AIIS): Located below the ASIS. * Posterior Superior Iliac Spine (PSIS): Located at the posterior end of the iliac crest. * Posterior Inferior Iliac Spine (PIIS): Located below the PSIS. * Body of the Ilium: The inferior portion that contributes to the acetabulum.
Specific Anatomy of the Ischium: * Upper Body: Contributes to the posterior section of the acetabulum. * Lower Body: Projects inferiorly from the upper body. * Ischial Spine: A sharp projection located posterior to the acetabulum. * Ischial Tuberosity: The heavy, rough projection that supports the weight of the body when seated. * Ischial Ramus: A thin process that extends anteriorly from the tuberosity to join the inferior ramus of the pubis. * Greater Sciatic Notch: A large notch above the ischial spine. * Lesser Sciatic Notch: A smaller notch below the ischial spine.
Specific Anatomy of the Pubis (Pubic Bone): * Body: Contributes to the anterior-inferior portion of the acetabulum. * Superior Ramus: Extends superiorly and laterally from the body. * Inferior Ramus: Extends posteriorly and inferiorly to join the ischial ramus. * Symphysis Pubis: The midline junction where the two pubic bodies meet. * Obturator Foramen: The large opening formed by the ischium and pubis bones.
Bony Landmarks for Palpation: * Iliac Crest. * Anterior Superior Iliac Spine (ASIS). * Greater Trochanter. * Ischial Tuberosity. * Symphysis Pubis.

Pelvic Cavities and the Birth Canal

Greater (False) Pelvis: The flared area above the pelvic brim. It supports abdominal organs but does not contribute to the birth canal.
Lesser (True) Pelvis: The area below the pelvic brim. It forms the birth canal in females.
The Birth Canal Structures: * Inlet (Superior Aperture): Defined by the pelvic brim. * Outlet (Inferior Aperture): Specifically defined by the span between the ischial tuberosities and the tip of the coccyx. * Cavity: The space between the inlet and the outlet. * Planes: The Plane of the Inlet and the Plane of the Outlet represent the orientation of these openings during the birth process.

Gender Differences in Pelvic Anatomy

General Shape: * Female: Typically wider, shallower, and lighter in structure. * Male: Typically narrower, deeper, and heavier.
Angle of the Pubic Arch: * Female: Forms an obtuse angle ( $> 90^\circ$ ). * Male: Forms an acute angle ( $< 90^\circ$ ).
Shape of the Pelvic Inlet: * Female: Often more oval or round. * Male: Often heart-shaped.

Pelvic Joints and Movement Classifications

Sacroiliac Joint (2): * Classification: Synovial. * Mobility: Amphiarthrodial. * Movement: Limited.
Hip Joint (2): * Classification: Synovial. * Mobility: Diarthrodial. * Movement: Spheroidal (Ball and Socket).
Symphysis Pubis: * Classification: Cartilaginous. * Mobility: Amphiarthrodial. * Movement: Limited.
Union of Acetabulum (2): * Classification: Cartilaginous. * Mobility: Synarthrodial. * Movement: Immovable (in adults).

Hip Localization and Anatomic Positioning

Localization Methods for the Femoral Head and Neck: * Line Construction: Draw a line between the ASIS and the superior border of the symphysis pubis. Find the midpoint of this line. * Femoral Head (H): Located approximately $1\frac{1}{2}\text{ inch}$ ( $4\,cm$ ) distal and perpendicular to the midpoint of the ASIS-Symphysis line. * Femoral Neck (N): Located approximately $2\frac{1}{2}\text{ inches}$ ( $6-7\,cm$ ) distal and perpendicular to the midpoint of the ASIS-Symphysis line. * Alternative Landmark: The midpoint of the constructed line is roughly $1-2\text{ inches}$ ( $3-5\,cm$ ) above the head and $3-4\text{ inches}$ ( $8-10\,cm$ ) above the neck.
Comparison of Foot/Leg Rotation for Radiography: * Anatomic Position: Feet are straight up; the head and neck are tilted anteriorly ( $15^\circ$ to $20^\circ$ ). The lesser trochanters are clearly visible on the image. * External Rotation (Relaxed Position): Feet are turned outward. The lesser trochanters are shown in profile. * True AP Projection (Internal Rotation): The legs and feet are rotated internally $15^\circ$ to $20^\circ$ . This places the femoral neck parallel to the IR and hides the lesser trochanters from view (or makes them only slightly visible).

Radiographic Positioning of the Femur

AP Projection (Mid and Distal Femur): * Rotation: Internally rotate the leg $5^\circ$ . * Central Ray (CR): Perpendicular to the midpoint of the Image Receptor (IR). * Criteria: The knee joint must be included; no rotation of the knee; optimal exposure factors.
Lateral Projection (Mid and Distal Femur): * Rotation: True lateral position. * Central Ray: Perpendicular to the midpoint of the IR. * Criteria: Knee joint included; no rotation; optimal exposure factors.
Lateral Projection (Mid and Proximal Femur): * Criteria: Proximal femur must not be superimposed; true lateral representation.
Trauma Lateral (Lateromedial): Performed when the patient cannot move for a standard lateral.

Radiographic Positioning of the Pelvis and Hip

AP Pelvis Projection: * Rotation: Rotate limbs internally $15^\circ-20^\circ$ (nontrauma). * Central Ray: Centered midway between the level of the ASIS and the symphysis pubis. * Criteria: Entire pelvis and proximal femora included; no rotation of the pelvis; lesser trochanters should not be visible.
AP Bilateral Frog-Leg Projection (Cleaves Method): * Rotation: Abduct femora $40^\circ$ to $45^\circ$ from vertical ( $20^\circ$ to $30^\circ$ is recommended for less femoral neck distortion). * Central Ray: Centered $3\text{ inches}$ ( $7.5\,cm$ ) below the level of the ASIS. * Criteria: Pelvic girdle centered horizontally; lesser trochanters equal in size; greater trochanters superimposed over femoral neck.
AP Unilateral Hip: * Central Ray: Perpendicular to the mid-femoral neck. * Criteria: Proximal $1/3$ of femur included; hip joint space and acetabulum visualized; total existing orthopedic prosthesis must be demonstrated.
Axiolateral Projection (Danelius-Miller Method): * Rotation: Leg internally rotated (if non-trauma). * Central Ray: Perpendicular to the mid-femoral neck. * Cassette Placement: Parallel to the long axis of the femoral neck. * Criteria: Entire femoral head, neck, and acetabulum visualized; no grid lines; compensating filter recommended.

Questions & Discussion

Q: What is the depression at the center of the femoral head? * A: Fovea capitis.
Q: Which pelvic girdle bone is the largest? * A: Ilium.
Q: Which landmark is posterior to the acetabulum? * A: Ischial spine.
Q: Does the "false pelvis" form the birth canal? * A: False. The true (lesser) pelvis forms the birth canal.
Q: What is the joint classification of the symphysis pubis? * A: Amphiarthrodial.
Q: Which landmark should NOT be visible on a correctly positioned AP Pelvis? * A: Lesser trochanter.
Q: Where is the CR centered for an AP Pelvis? * A: Midway between the ASIS and symphysis pubis.
Q: How is the cassette aligned for the Danelius-Miller projection? * A: Parallel to the femoral neck.