The articular type of the femoral head
The coxofemoral joint is a ball-and-socket joint; its articular surfaces are spherical. It therefore has 3 degrees of freedom (DoF), on the 3 axes of movement: X, Y, and Z.
The coxofemoral joint can therefore perform internal rotation, external rotation, adduction and abduction, as well as flexion and extension; the coupling of these movements corresponds to circumduction.
The anatomical shape of the femoral head
We can initially simplify the shape of the femoral head to a sphere covered with cartilage. To be exact, it is 2/3 of a sphere, with a vertical diameter slightly larger than its horizontal diameter. It has a radius of between 20 and 25 mm.
It features the fovea capitis on its postero-inferior quadrant.
Anatomical orientation of the femoral head
The femoral head is attached to the femur by the femoral neck. This femoral neck has a particular angulation on the frontal and horizontal planes; it is this angulation that determines the angle of inclination and the angle of declination, which can vary depending on the individual's morphology.
Angle of inclination of the head and femoral neck
The angle of inclination is formed between the axis of the femoral neck and the axis of the femoral shaft. The neck of the femur is oriented upward, forward, and inward. Its axis therefore forms with that of the femoral shaft an inclination angle, open downward and inward, of 125° to 140°.
Angle of declination of the head and femoral neck
The axis of the femoral neck and head—to repeat: upward, forward, and inward—forms, with the transverse axis of the lower end of the femur, an angle of declination open forward and inward of approximately 30°. Therefore, it is the axis of the femoral neck and the transverse axis of the femoral condyles that form this angle of declination.
