Abstract
The Q angle is defined as the angle formed by a line passing from the anterior-superior iliac spine and the centre of the patella and another line passing this point to the centre of the tibial tuberosity. Normal values reported in the literature for the Q angle widely range between 12.7° (± 0.72°) and 18.8° (± 4.7°). This variability depends on individual anatomical variations and method of measurement as well. In fact, several factors can affect the measurement of Q angle. The correct evaluation is carried out with a goniometer, in supine position and the knee in full extension. Q angle evaluation can be biased by standing position and quadriceps contraction, which can increase the Q angle; in contrast, knee flexion can reduce it. Furthermore, it has been demonstrated that the Q angle increases from external to internal rotation of the tibia, while it decreases from pronation to supination of the foot. Finally, patellar malalignment in the horizontal plane, such as subluxation or dislocation, causes a decrease in the Q angle, as the centre of the patella is laterally shifted. The accuracy of Q angle measurement can be also affected by an error in identifying the anatomical landmarks. An error in Q angle measurement below 5° requires an error in setting the anatomical landmarks of no greater than 2 mm. Several authors have shown that the Q angle is greater in females, as the proximal landmarks are more lateral and a greater valgus alignment is necessary to reestablish a correct mechanical axis of the limb. According to the side, there is no evidence that Q angle is symmetric.
The clinical significance of the Q angle is controversial. An increased Q angle was considered for a long time as the main cause of anterior knee pain and an important risk factor for patella subluxation or dislocation. Some authors showed a correlation between Q angle increase and symptomatic patellar chondromalacia. However, others showed no significant differences in Q angle values between symptomatic and asymptomatic patients. Presently, there is not sufficient clinical evidence that an increased Q angle predisposes to knee problems. Furthermore, it is impossible to assert that all the alterations of the extensor mechanism are exclusively due to an increase in the Q angle, as they can depend on other factors, such as: functional overloading of the knee, muscle and ligament insufficiency, bone and chondral morphological changes, malalignment or asymmetric length of the inferior limbs and foot alterations.
In conclusion, a homogeneous method of measurement and correct data interpretation are necessary to clarify the conundrum of Q angle. Moreover, it is important to understand that patellofemoral malalignment is not always the cause of knee pain and instability. This can reduce the risk of performing surgical procedures of extensor mechanism realignment that are technically perfect but potentially harmful.
