Femoral Head

The femoral head is the spherical proximal end of the femur that articulates with the acetabulum of the pelvis to form the hip joint. As the ball component of this synovial ball-and-socket joint, the femoral head plays a pivotal role in weight bearing, mobility, and stability of the lower limb. Its smooth surface and congruent articulation allow for a wide range of movement, while internal architecture supports compressive forces transmitted through the hip.

Structure

The femoral head is nearly spherical and comprises approximately two-thirds of a sphere. It projects medially and slightly anteriorly from the femoral neck, forming an angle (the neck-shaft angle) that supports upright posture and bipedal locomotion. The surface is covered with hyaline cartilage except for a small central depression, the fovea capitis femoris.

Components

• Articular surface: Smooth and lined with hyaline cartilage, facilitating low-friction movement within the acetabulum.

• Fovea capitis: A pit located slightly posterior and inferior to the center of the head. It serves as the attachment site for the ligament of the head of the femur (ligamentum teres), which carries a small artery.

• Subchondral bone: The bone beneath the cartilage surface contains trabeculae that align with stress vectors to resist compressive forces.

Dimensions

• Diameter varies with sex and stature, averaging around 45 mm in adult males and 40 mm in adult females.

• The femoral head comprises approximately 2/3 of a full sphere.

Location

The femoral head is situated at the proximal end of the femur, medial to the greater and lesser trochanters. It lies within the acetabulum of the pelvis and faces medially, superiorly, and slightly anteriorly. Its orientation forms part of the angle of inclination and angle of anteversion, which are essential in maintaining proper biomechanics of the hip joint.

Function

• Articulation: Serves as the ball of the hip joint, articulating with the acetabulum to permit a wide range of movement including flexion, extension, abduction, adduction, rotation, and circumduction.

• Weight transmission: Transmits the body’s weight from the pelvis to the femoral shaft during standing and locomotion.

• Stabilization: Works with the acetabular labrum, joint capsule, and surrounding muscles and ligaments to stabilize the hip joint.

Articulations

The femoral head articulates exclusively with the acetabulum of the pelvis to form the acetabulofemoral joint, commonly known as the hip joint. This is a multiaxial synovial joint that allows extensive motion while providing strength and support.

Articulating Surface Joint Type Movements Allowed Acetabulum of pelvis Synovial (ball-and-socket) Flexion, extension, abduction, adduction, internal and external rotation, circumduction

Blood Supply

The blood supply to the femoral head is critical and has unique clinical importance due to the risk of avascular necrosis. It is supplied by three primary sources:

• Medial circumflex femoral artery: The main contributor in adults, particularly through its retinacular branches that ascend along the femoral neck.

• Lateral circumflex femoral artery: Minor contributor to the anterior part of the femoral head.

• Artery of the ligamentum teres (acetabular branch of the obturator artery): Important in children but often diminishes in adults.

These vessels penetrate the bone through foramina in the neck and ascend to the subchondral region. Disruption of these vessels due to trauma or dislocation can lead to ischemia and bone necrosis.

Nerve Supply

The innervation of the femoral head is derived from branches of the femoral nerve, obturator nerve, and sciatic nerve. These nerves primarily serve the joint capsule and periosteum rather than the articular cartilage, which is aneural.

Trabecular Architecture

The internal structure of the femoral head exhibits a highly organized system of trabeculae that reflect the direction of mechanical stress. These include:

• Principal compressive trabeculae: Extend from the superior cortex of the head to the medial cortex of the femoral shaft.

• Principal tensile trabeculae: Arise from the lateral cortex and cross the compressive system at right angles, resisting tensile forces.

• Ward’s triangle: A region of relative radiolucency between intersecting trabecular patterns, vulnerable in osteoporosis.

Ossification

The femoral head develops from a secondary ossification center that appears between 4 to 6 months of age postnatally. The epiphysis grows and eventually fuses with the femoral neck during adolescence, usually between ages 14 to 18 years. Proper ossification is critical for hip joint congruity and function.

Relations

• Superior: Acetabular roof (dome) of the pelvis

• Medial: Acetabular fossa and ligament of the head of the femur

• Anterior: Iliopsoas tendon, femoral neurovascular bundle (external)

• Posterior: Piriformis and short lateral rotators

Clinical Significance

• Avascular necrosis (AVN): Interruption of blood supply to the femoral head can lead to bone death, collapse, and joint dysfunction. Common causes include trauma, corticosteroid use, alcoholism, and sickle cell disease.

• Femoral head fracture: Often associated with posterior hip dislocation. These fractures are intra-articular and may impair blood supply.

• Osteoarthritis: Degenerative wear of the articular cartilage of the femoral head and acetabulum, causing pain and restricted mobility.

• Developmental dysplasia of the hip (DDH): Abnormal development may result in misalignment or subluxation of the femoral head from the acetabulum, requiring early detection and management.

• Slipped capital femoral epiphysis (SCFE): Occurs in adolescents when the femoral head slips posteriorly through the epiphyseal plate. It can lead to permanent deformity and early arthritis.

Palpation and Imaging

The femoral head is not palpable externally due to its deep position within the acetabulum. However, its location can be inferred clinically using surface landmarks such as the inguinal crease and greater trochanter. Imaging modalities used for evaluating the femoral head include:

• Plain radiographs: Useful for assessing joint space, alignment, and trabecular patterns.

• MRI: Superior for detecting early avascular necrosis, cartilage defects, or labral pathology.

• CT scans: Useful for complex fractures or preoperative planning.