Femoral anteversion (FA) refers to the angle of twist in the femur, or thigh bone, along its long axis. This anatomical measurement is defined by the angular relationship between the axis of the femoral neck and the transcondylar axis of the knee joint. The femoral neck connects the femoral head, which sits in the hip socket, to the main shaft of the femur. A certain degree of forward orientation is a normal component of human lower extremity alignment. This rotational relationship is influential in determining how the hip and knee joints align and function during walking and running.
Clinical Necessity of Measurement
Accurate measurement of femoral anteversion is necessary for diagnosing and managing a variety of musculoskeletal conditions that affect gait and joint health. The angle directly influences the rotational alignment of the entire leg, which can lead to functional problems if it falls outside the typical range. Clinicians often request FA measurement when a patient, particularly a child, presents with a persistent in-toeing gait, commonly known as being “pigeon-toed.”
Excessive anteversion can cause the knees and feet to turn inward to properly seat the femoral head within the hip socket during walking. Conversely, a lower-than-normal angle, known as femoral retroversion, can result in an out-toeing gait. Abnormal anteversion is associated with chronic knee pain, patellar instability, and may contribute to conditions like developmental dysplasia of the hip (DDH) or femoroacetabular impingement (FAI).
The measurement helps a healthcare provider determine the true source of a rotational issue, differentiating a problem originating in the femur from one in the tibia or foot. This diagnostic clarity is important for planning effective treatment, which can range from observation and physical therapy to surgical correction. In adults, an abnormal angle can alter the biomechanics of the hip, potentially increasing joint loading and acting as a risk factor for early-onset osteoarthritis.
Non-Imaging Clinical Assessment
Initial assessment of femoral anteversion often begins with a physical examination technique known as Craig’s Test, or the trochanteric prominence angle test. This method provides a clinical estimate of the angle by assessing the amount of hip rotation required to bring the femoral neck into a neutral position. The patient is positioned lying face down, or prone, with the knee of the leg being tested flexed to a precise 90 degrees.
The examiner then locates the greater trochanter, the large bony prominence on the upper, outer thigh, by palpating the area. The hip is slowly rotated inward and outward while the clinician monitors the position of the greater trochanter. The goal is to find the angle of rotation where the greater trochanter is at its point of maximal lateral prominence, meaning it is parallel to the examination table.
When the trochanter is positioned most laterally, the femoral neck is considered to be aligned in the transverse plane relative to the pelvis. At this point, the clinician measures the angle that the lower leg makes with a line drawn perpendicular to the examination table. This measured angle of internal rotation of the lower leg is then used as the clinical estimation of the femoral anteversion angle.
Because this test relies on palpation and estimation, it is considered a functional measurement rather than a precise quantitative one. It is a simple, non-invasive method that helps screen patients and provides immediate information about the hip’s rotational profile before committing to advanced imaging. The clinical utility of Craig’s Test is in its ability to quickly identify a significant rotational difference, but its reliability and validity can vary among different examiners.
Advanced Imaging Techniques
For a definitive and quantitative measurement of femoral anteversion, advanced imaging modalities like Computed Tomography (CT) or Magnetic Resonance Imaging (MRI) are used and are considered the gold standard. These scans provide cross-sectional images that allow for the precise definition of the bony axes required for angular calculation. The measurement is calculated as the angle formed between two specific reference lines drawn on axial images taken at different levels of the femur.
Defining the Axes
The first reference line is the femoral neck axis, defined on an axial image of the proximal femur. This line is drawn from the center of the femoral head through the center of the femoral neck, often approximated at the level of the lesser trochanter. The second reference line is the posterior condylar axis, established on an axial image taken at the distal end of the femur, at the knee joint. This distal axis is defined by connecting the most posterior points of the medial and lateral femoral condyles. The femoral anteversion angle is then mathematically determined by measuring the rotational angle between these two axes projected onto a single plane.
CT is historically the most common method, but it involves exposure to ionizing radiation, which is a concern, especially for children who may require repeat measurements. Newer techniques, such as low-dose biplanar radiography (LDBR), offer a precise, lower-radiation alternative that is particularly beneficial for pediatric patients. LDBR uses two perpendicular radiographic projections to create a three-dimensional model of the entire lower limb. Specialized software then uses the 3D reconstruction to automatically identify the bony landmarks and calculate the femoral anteversion angle with accuracy comparable to that of a CT scan.
Interpreting Anteversion Values
The interpretation of the measured femoral anteversion value depends heavily on the patient’s age. In adults, the normal range for femoral anteversion is generally considered to be between 8 and 15 degrees. Children are born with a much greater degree of anteversion, typically around 30 to 40 degrees, which gradually decreases as they grow and walk.
A measured value significantly greater than the normal range for that age group is classified as excessive anteversion. This increased internal twist of the femur is what causes the compensatory in-toeing gait pattern, as the patient internally rotates the hip to keep the femoral head centered in the hip socket. Conversely, a value below the normal range, or even a negative value, is termed femoral retroversion.
Retroversion indicates a posterior twist in the femur, which is commonly associated with an out-toeing gait. The interpretation of these values guides the treatment approach. Mild to moderate cases are often managed through observation, as the angle may self-correct during growth. If the excessive anteversion or retroversion is severe and causes significant functional disability or pain, a surgical procedure called a derotational osteotomy may be considered to physically rotate the femur back toward a more typical alignment.