Femoral anteversion describes the degree of forward twist in the femur, or thigh bone, specifically the angle of the femoral neck relative to the knee joint axis. When this twist is outside the typical range, it can significantly alter joint mechanics, posture, and gait. Measuring this angle is a fundamental step in diagnosing conditions like in-toeing, where the feet turn inward, and assessing the risk for future hip or knee problems, such as patellofemoral instability or osteoarthritis. The measurement helps clinicians determine if a patient’s symptoms are related to this rotational alignment.
The Role of Clinical Assessment
The initial step in measuring femoral anteversion is a physical examination. This clinical assessment estimates the angle using the relationship between hip rotation and the bone’s anatomy. The most common technique is the Ryder’s or Craig’s Test, which relies on palpating the greater trochanter.
To perform the test, the patient lies face-down with the knee flexed to 90 degrees. The clinician rotates the hip inward and outward while feeling the greater trochanter. The goal is to find the point where the trochanter is most prominent laterally, indicating the femoral neck is parallel to the examination table. The angle of the lower leg relative to the vertical line is then measured with a goniometer.
The clinical test is simple and quick, offering an immediate assessment of the hip’s rotational profile. A hip with excessive anteversion shows markedly greater internal rotation compared to external rotation. While useful for screening, this method provides only an estimate and is often followed by imaging for a definitive measurement.
Radiographic and CT Measurement
To obtain a precise measurement of femoral anteversion, imaging techniques are necessary. Computed Tomography (CT) scanning is considered the most accurate method, often called the “gold standard” for measuring bony torsion. CT provides cross-sectional images that allow visualization of the two axes used to calculate the angle.
The measurement involves defining two key lines in the axial plane. The femoral neck axis runs through the center of the femoral head and neck. The transcondylar axis is a line tangential to the posterior surfaces of the femoral condyles at the knee joint. Femoral anteversion is the angle formed between the projection of these two axes.
Advanced CT techniques, such as axial-oblique reformations, enhance accuracy by aligning the cross-section parallel to the long axis of the femoral neck, minimizing positioning errors. While CT exposes the patient to radiation, Magnetic Resonance Imaging (MRI) is an alternative that measures the angle without radiation. CT remains the most common choice due to its superior bone detail and speed.
Interpreting Anteversion Values
In a typical adult, the femoral anteversion angle ranges between 8 and 15 degrees. This range represents the normal forward twist in the femur that allows for optimal hip joint mechanics and a straight gait pattern.
A measurement significantly higher than this range is classified as excessive anteversion, or antetorsion. This condition is often associated with in-toeing because the femur must internally rotate to align the hip joint optimally within the socket. In children, the angle is naturally higher (around 30 to 40 degrees at birth), but it gradually decreases with growth, stabilizing in the adult range by adolescence.
Conversely, an angle significantly lower than the normal range, or a negative value, is termed retroversion. This means the femoral neck is twisted posteriorly relative to the knee. Retroversion typically causes a compensatory out-toeing, as the leg externally rotates to compensate for the twist. The degree of deviation from the normal range guides the need for further intervention.
Treatment Decisions Based on Measurement
Measurement of femoral anteversion guides treatment decisions. For most children whose excessive anteversion causes in-toeing, the condition is monitored, as it typically self-corrects as the child grows and the angle naturally decreases. Non-surgical management, including observation and physical therapy, is the primary approach unless the rotational difference is severe or persists past skeletal maturity.
Surgical intervention is reserved for extreme cases where the angle causes significant functional impairment, pain, or joint instability. An angle greater than 50 degrees in an older child, or a persistent angle causing noticeable symptoms in an adult, may indicate a need for correction. The procedure is a derotational osteotomy, where the femur is cut, rotated, and fixed in a more normal position to restore rotational alignment and improve gait mechanics.