Normal Femoral Angle Size on Radiography and MRI
The femoral angle, also known as the distal femoral angle, describes the relationship between the long axis of the femoral shaft and the articular surface of the distal femur as measured on anteroposterior (AP) knee imaging. Accurate measurement of this angle is essential for evaluating lower-limb alignment, surgical planning for osteotomies, and assessing conditions such as genu valgum or varum. It serves as a foundational parameter in both orthopedic and radiologic assessment of knee biomechanics.
Normal Reference Values
| Orientation | Measurement |
|---|---|
| Anteroposterior | 75-85¡ |
Clinical Significance
A normal femoral angle falls between 75° and 85° on AP projection. Values outside this range indicate malalignment of the distal femur relative to the mechanical axis of the lower limb, which can significantly alter load distribution across the knee joint. Deviation toward a smaller angle typically reflects a valgus deformity, while a larger angle suggests a varus configuration.
Persistent malalignment is associated with accelerated cartilage degeneration, medial or lateral compartment osteoarthritis, patellar tracking abnormalities, and stress fracture risk. In the surgical context, preoperative measurement is critical for distal femoral osteotomy planning and total knee arthroplasty component positioning. It is important to note that patient rotation or beam angulation during radiography can introduce measurement error, so strict AP positioning is mandatory.
- Genu valgum — reduced femoral angle, lateral compartment overloading
- Genu varum — increased femoral angle, medial compartment overloading
- Lateral compartment osteoarthritis — associated with valgus malalignment
- Medial compartment osteoarthritis — associated with varus malalignment
- Post-traumatic deformity — malunited distal femur fractures altering the angle
Reference: Yochum TR, Rowe LJ. Essentials of Skeletal Radiology. (2005).
Imaging Notes
On AP radiography, the femoral angle is measured by drawing a line along the longitudinal axis of the femoral shaft and a second line along the distal femoral articular surface; the angle formed at their intersection is recorded. A true AP projection with the knee fully extended and the patella centered is essential to avoid rotational artifact. Weight-bearing long-leg AP radiographs provide the most clinically relevant alignment data when evaluating the full mechanical axis.
On MRI, coronal sequences (typically coronal PD or T1-weighted) allow analogous angle measurement with the added benefit of simultaneous cartilage and ligament assessment. MRI is particularly useful when correlating bony alignment with soft-tissue pathology such as meniscal or collateral ligament involvement. Consistent slice orientation through the true coronal plane of the femur is critical for reproducibility.