Normal Quadriceps Angle Size on X-ray, CT & MRI
The quadriceps angle (Q-angle) is the angle formed between the line of pull of the quadriceps muscle and the patellar tendon, measured at the center of the patella. It serves as a key indicator of patellofemoral alignment and is routinely assessed when evaluating anterior knee pain, patellar instability, and chondromalacia patellae. Accurate measurement guides both conservative and surgical management decisions.
Normal Reference Values
| Orientation | Measurement |
|---|---|
| Anteroposterior | 15-20¡ |
Clinical Significance
A normal Q-angle ranges from 15° to 20° in the anteroposterior plane. Values exceeding 20° suggest increased lateral patellar stress and are associated with patellofemoral pain syndrome, patellar subluxation, and chondromalacia. Females typically exhibit slightly larger Q-angles than males due to a wider pelvis, a physiological consideration when interpreting borderline measurements.
A Q-angle below 15° is less commonly discussed but may be seen in patients with genu varum or following tibial tubercle medialization procedures. Measurement pitfalls include patient positioning (the knee must be fully extended and the foot in neutral rotation), as hip rotation and foot pronation can artificially alter the angle. MRI and CT allow assessment in the axial plane for complementary patellar tilt and congruence measurements.
- Patellofemoral pain syndrome (lateral overload)
- Patellar instability / recurrent subluxation
- Chondromalacia patellae
- Post-surgical malalignment (tibial tubercle osteotomy)
- Genu valgum-related lateral patellar maltracking
Reference: Margheritini F. Orthopedic Sports Medicine, Principles and Practice. Springer. (2011).
Imaging Notes
On radiography, the Q-angle is approximated using anteroposterior views by identifying the anterior superior iliac spine (ASIS), patella center, and tibial tubercle; however, direct clinical goniometric measurement remains the standard. CT allows precise measurement of the tibial tubercle–trochlear groove (TT-TG) distance as a complementary index, with the knee in full extension. Ensure neutral limb rotation to avoid measurement error.
On MRI, axial sequences through the patellofemoral joint enable assessment of patellar tilt and trochlear morphology alongside Q-angle estimation. Sagittal and coronal sequences help identify associated soft-tissue abnormalities such as lateral retinacular tightening or medial patellofemoral ligament (MPFL) tears, which often coexist with Q-angle abnormalities.