Clinical Significance of the Static and Dynamic Q-angle

Abstract

Q-angle represents the resultant force vector of the quadriceps and patellar tendons acting on the patella. An increased Q-angle has been considered a risk factor for many disorders and injuries. This literature review challenges the clinical value of static Q-angle and recommends a more dynamic movement evaluation for making clinical decisions. Although there are many articles about static Q-angle, few have assessed the value of dynamic Q-angle. We searched Scopus and PubMed (until September 2021) to identify and summarize English-language articles evaluating static and dynamic Q-angle, including articles for dynamic knee valgus (DKV) and frontal plane projection angle. We also used textbooks and articles from references to related articles. Although static Q-angle measurement is used systematically in clinical practice for critical clinical decisions, its interpretation and clinical translation present fundamental and intractable limitations. To date, it is acceptable that mechanisms that cause patellofemoral pain and athletic injuries have a stronger correlation with dynamic loading conditions. Dynamic Q-angle has the following three dynamic elements: frontal plane (hip adduction, knee abduction), transverse plane (hip internal rotation and tibia external rotation), and patella behavior. Measuring one out of three elements (frontal plane) illustrates only one-third of this concept. Static Q-angle lacks biomechanical meaning and utility for dynamic activities. Although DKV is accompanied by hip and tibia rotation, it remains a frontal plane measurement, which provides no information about the transverse plane and patella movement. However, given the acceptable reliability and the better differentiation capability, DKV assessment is recommended in clinical practice.

Keywords: dynamic knee valgus; dynamic q-angle; frontal plane projection angle; knee biomechanics; q-angle.

Figure 1. Static Q-angle.

Q-angle is defined as the angle formed between an imaginary line connecting the ASIS of the pelvis to the center of the patella and a proximal projection of the line running from the tibial tubercle to the patella’s center (red line). It appears that the measured static Q-angle does not reflect the true line of pull of the RF (blue line), which originates from the AIIS. Figure created by Harris Simeonidis. AIIS: anterior inferior iliac crest; ASIS: anterior superior iliac spine; RF: rectus femoris

Figure 2. Dynamic Q-angle.

Dynamic Q-angle is defined as the Q-angle through the knee joint’s flexion, with or without a dynamic activity. Its measurement requires either the same bony points as on static Q-angle or using DKV through the measurement of FPPA. The DKV is a combination of hip adduction and internal rotation and knee abduction, with the lower limb fixed on the ground. The FPPA is formed by lines connecting the ASIS, the midpoint of the femoral condyles (or the center of the patella), and the malleoli’s midpoint in the frontal plane. Although an ankle eversion normally accompanies tibial internal torsion, a subsequent greater internal rotation of the femur leads to a relative external rotation of the tibia in relation to the femur. This notion cannot be measured directly via FPPA because dynamic Q-angle has three elements: frontal plane (hip adduction, knee abduction), transverse plane (hip internal rotation, tibia external rotation), and patella behavior. Measuring one out of three elements (frontal plane) illustrates only one-third of the concept. Figure created by Harris Simeonidis. ASIS: anterior superior iliac spine; DKV: dynamic knee valgus; FPPA: frontal plane projection angle