20/09/2026
Biomechanics of the Femoral Neck–Shaft Angle
The image illustrates the angle of inclination of the femoral neck, which is the angle formed between the long axis of the femoral neck and the longitudinal axis of the femoral shaft. This angle is a major determinant of hip biomechanics, joint loading, muscle leverage, and lower-limb alignment. The image compares a more coxa valga position with a more coxa vara configuration.
In the illustrated normal range, the femoral neck–shaft angle is shown around 120°–135°. This angle positions the femoral head appropriately within the acetabulum and determines how forces are transmitted from the pelvis into the femoral shaft. The exact “normal” value varies with age, s*x, skeletal development, and measurement method, so the numbers should be considered reference ranges rather than absolute cutoffs.
🔵 Coxa Valga
Coxa valga refers to an increased femoral neck–shaft angle, typically greater than approximately 135° in adults. As the angle increases, the femoral neck becomes more vertically oriented. This changes the geometry of the hip and can alter the mechanical advantage of the surrounding muscles.
A more vertical femoral neck can reduce the horizontal offset between the hip joint centre and the femoral shaft. This may reduce the abductor muscles' moment arm, meaning the abductors may need to generate greater force to balance the external hip adduction moment during single-leg stance.
🟢 Coxa Vara
Coxa vara describes a decreased femoral neck–shaft angle, commonly below approximately 120° in adults. The femoral neck becomes more horizontally oriented, increasing the effective femoral offset and abductor moment arm in many configurations.
This can improve the mechanical leverage of the hip abductors, potentially reducing the muscle force required for pelvic stabilization. However, the altered geometry also changes the distribution of stresses within the femoral neck and proximal femur.
⚙️ Effect on Hip Joint Forces
The hip is subjected to substantial compressive and shear forces during weight bearing. Changes in the neck–shaft angle alter the direction and magnitude of these forces because they change the relationship between the ground reaction force, body weight, hip joint centre, and femoral shaft.
During single-leg stance, the hip abductors must counteract the external adduction moment created by body weight. Therefore:
Muscle force × abductor moment arm = external hip moment
Changing the femoral neck–shaft angle can modify the abductor moment arm and consequently influence the amount of muscle force required to maintain pelvic stability.
🦵 Effect on Lower-Limb Alignment
The neck–shaft angle also influences the orientation of the femur and can contribute to changes in hip range of motion, femoral alignment, knee mechanics, and overall lower-limb loading. Coxa vara and coxa valga are therefore not simply changes in the appearance of the hip—they represent changes in the geometry of the entire mechanical chain between the pelvis and lower limb.
🔑 Clinical Biomechanical Takeaway
Coxa valga → increased neck–shaft angle → more vertical femoral neck → potentially reduced abductor leverage → altered hip loading.
Coxa vara → decreased neck–shaft angle → more horizontal femoral neck → potentially increased abductor leverage → altered femoral-neck stresses.
The most important concept is that small changes in bone geometry can substantially influence joint mechanics. The femoral neck–shaft angle affects muscle moment arms, joint reaction forces, pelvic stability, and load transmission through the lower limb. However, neither coxa vara nor coxa valga should automatically be considered pathological—the clinical significance depends on the individual's anatomy, symptoms, function, and overall hip mechanics.
