Determination and significance of femoral neck anteversion

Femoral neck anteversion (FNA) describes the normal torsion or twist present in the femur. Femoral neck anteversion is defined as the angle between an imaginary transverse line that runs medially to laterally,through the knee Joint and an imaginary transverse line passing through the center of the femoral head and neck (Fig. 1)(.1,2) In adults without pathology, the femur is twisted so the head and neck of the femur are angled forward between 15 and 20 degrees from the frontal plane of the body.(1,2) In some instances, the FNA angle is directed forward or backward well beyond this angle. Some researchers(3-8) suggest that FNA angles outside this 15- to 20-degree average are a contributing factor in many different orthopedic problems in the lower extremity that are commonly seen by physical therapists. The purpose of this Update is to describe how FNA is related to hip rotation, how hip rotation range of motion can be used to predict abnormal FNA, and how asymmetries in hip rotation may be used to identify patients who may be at risk for developing various orthopedic problems in the hip and lower extremity. Femoral neck anteversion sometimes is called "medial femoral torsion."(2,3) It is thought to result from medial (internal) rotation of the limb bud in early intrauterine life.(3) In postnatal development, a reduction of the FNA angle usually occurs during growth.(2,3) In the newborn, the average FNA angle is 31 degrees.(2) The average FNA angle is 26 degrees by 5 years of age, 21 degrees by 9 years of age, and 15 degrees by 16 years of age.(2) The FNA angle, therefore, diminishes about 1.5 degrees a year until about 15 years of age.2 In the adult, average femoral head and neck torsion forward of the body's frontal plane is about 15 degrees, a little less in men (less than 15degrees) and a little more in women (18degrees) (Fig. 1).(2,4) Femoral neck anteversion angle is typically symmetrical from the left side to the right side.(5) In contrast to FNA, femoral neck retroversion is present when the head and neck of the femur are angled less than the average FNA angle (15degrees-20degrees) along the frontal plane of the body (Fig. 1).(6,7) In some cases, the femoral head and neck may even be angled backward from the frontal plane of the body.