The Femur

Gross Anatomy

Proximal Femur

• Roughly spherical (~2.5 cm radius of curvature)
• Largely covered by hyaline articular cartilage except at the fovea capitis (a pit on the medial surface for attachment of the ligamentum teres / ligament of the head)
• Articulates with the acetabulum to form a ball-and-socket (spheroidal) joint

• Connects the head to the shaft
• Neck-shaft angle averages 127° in adults (begins at ~141° in the fetus; decreases with age)
• Femoral neck is anteverted approximately 14° relative to the femoral condyles (range 1-40°)
• Largely intracapsular; the posterior neck is only partially covered by the joint capsule (basicervical and intertrochanteric regions are extracapsular)

Shaft (Diaphysis)

• Cylindrical; directed obliquely downward and medially, so the knee lies near the body's midline
• The posterior surface bears a prominent longitudinal ridge, the linea aspera, which serves as attachment for adductor and other thigh muscles
• The linea aspera splits distally into the medial and lateral supracondylar lines, which form the floor and sides of the popliteal fossa
• The pectineal line runs from the lesser trochanter to the linea aspera (adductor longus and brevis, pectineus insert here)

Distal Femur

• Expands into two large articular masses: the medial condyle (larger) and lateral condyle (smaller)
• The condyles articulate with the tibial plateaus and with the patella (via the patellar/trochlear surface anteriorly)
• Between the condyles posteriorly lies the intercondylar (intercondyloid) fossa:
  • Lateral wall = attachment of the anterior cruciate ligament (ACL)
  • Medial wall = attachment of the posterior cruciate ligament (PCL)
  • The roof is demarcated by the intercondylar line
• Non-articular bony prominences above the condyles = medial and lateral epicondyles (attachment sites of collateral ligaments and muscles)
• The adductor tubercle sits above the medial epicondyle; it is the termination of the medial supracondylar line and receives the adductor magnus tendon

Blood Supply

1. Medial circumflex femoral artery (MFCA) - the dominant supply; a branch of the profunda femoris (deep artery of thigh). It runs posteriorly around the neck and its deep branch penetrates retinacular vessels under the joint capsule to supply the femoral head
2. Lateral circumflex femoral artery (LCFA) - contributes mainly to the anterior neck via ascending branch
3. Artery of the ligamentum teres - a branch of the obturator artery; relatively minor in adults but contributes to the foveal area

Clinical point: Femoral neck fractures and hip dislocations can rupture the MFCA retinacular vessels, interrupting the main blood supply and causing avascular necrosis (AVN) of the femoral head. In children, the piriformis fossa nail entry point for intramedullary nailing risks damage to the posterosuperior retinacular vessels off the MFCA - this is why lateral trochanteric entry is preferred.

Ossification

• The distal femur is responsible for the highest percentage of longitudinal growth in the lower extremity (~70% of femoral length; ~37% of total lower limb length)
• Slipped capital femoral epiphysis (SCFE) occurs through the femoral head physis at the zone of hypertrophy

Key Angles and Measurements

Proximal Femoral Trabecular Architecture

• Compression trabeculae - arching from calcar to superolateral femoral head cortex
• Tension trabeculae - from the lateral cortex, arching medially toward the femoral head
• The Ward triangle is a relatively weak area between the two systems where trabecular density is low - a common site for stress fractures and osteoporotic fractures

Muscle Attachments Summary

Clinical Correlations

• Femoral neck fractures: Commonest in elderly osteoporotic women (minimal trauma); risk of AVN and non-union due to tenuous blood supply. Classification: subcapital, transcervical, basicervical
• Intertrochanteric fractures: More vascular region, lower AVN risk; treated with intramedullary nail or dynamic hip screw
• Subtrochanteric fractures: High biomechanical stress; treated with intramedullary nail
• Femoral shaft fractures: High energy in young (MVA), lower energy in elderly; intramedullary nail is gold standard treatment
• Distal femur fractures: Up to 6% of all femur fractures; bimodal distribution (high energy in young, low energy in elderly); thin cortices and wide medullary canal make fixation challenging