Correlation Between Merchant's Standard Q Angle and TT-TG Distance

In response to the poor reliability of clinically measured quadriceps angle (Q angle), the standard Q angle (SQA) was developed as a standardized, repeatable measurement method.

To determine the correlation between SQA and tibial tubercle-trochlear groove (TT-TG) distance in awake and anesthetized patients with recurrent patellar instability.

Cross-sectional study; Level of evidence, 3.

We included 47 patients (94 knees; mean patient age, 23 ± 9 years) treated by 1 surgeon for recurrent patellar instability who had undergone computed tomography (CT) scans as part of their preoperative workup. SQA measurements were taken in the clinic and in the operating room after the induction of general anesthesia. Using these CT scans, we measured the TT-TG distance, Caton-Deschamps index (CDI), and lateral trochlear inclination (LTI).

The correlation coefficient (R² ) between the SQA and the TT-TG distance was 0.03 (β = 0.18) for awake patients, and 0.15 (β = 0.39) for anesthetized patients. When the CDI was <1.2, the correlation was stronger in anesthetized patients (R² = 0.32; β = 0.56) than in awake patients (R² = 0.06; β = 0.24). Similarly, when the LTI was >11°, the correlation was greater in anesthetized patients (R² = 0.27; β = 0.52) than in awake patients (R² = 0.02; β = 0.12). The correlation between the SQA and the TT-TG distance was strongest in anesthetized patients with both a CDI of <1.2 and an LTI of >11° (R² = 0.57; β = 0.76).

We observed a strong positive correlation between the SQA and the TT-TG distance in anesthetized patients with normal patellar height and a normal LTI. However, in awake patients and those with anatomic risk factors for recurrent patellar instability, correlations between the SQA and the TT-TG distance were weakly positive or nonsignificant. The SQA may provide useful information, but not in patients with anatomic findings that put them at increased risk for patellofemoral instability.