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

Jacob D Mikula1, Alia J Mowery2, Micheal Raad1

  • 1Department of Orthopaedic Surgery, The Johns Hopkins University School of Medicine, Baltimore, Maryland, USA.

Orthopaedic Journal of Sports Medicine
|December 10, 2025
PubMed
Summary
This summary is machine-generated.

The standard Q angle (SQA) correlates with tibial tubercle-trochlear groove (TT-TG) distance in anesthetized patients but not in awake individuals with patellar instability risk factors.

Keywords:
Q anglepatella altapatellar instabilitypatellar trackingtibial tubercle–trochlear groove distancetibial tuberositytrochlear dysplasia

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Area of Science:

  • Orthopedic Surgery
  • Biomedical Engineering
  • Radiology

Background:

  • The standard Q angle (SQA) was developed to improve the reliability of clinical quadriceps angle (Q angle) measurements.
  • Assessing Q angle is crucial for diagnosing and managing knee conditions.

Purpose of the Study:

  • To investigate the correlation between the standard Q angle (SQA) and tibial tubercle-trochlear groove (TT-TG) distance.
  • To compare this correlation in awake versus anesthetized patients with recurrent patellar instability.

Main Methods:

  • A cross-sectional study included 47 patients (94 knees) with recurrent patellar instability.
  • Standard Q angle (SQA) and computed tomography (CT) scans were used to measure TT-TG distance, Caton-Deschamps index (CDI), and lateral trochlear inclination (LTI).
  • Measurements were taken both in the clinic (awake) and in the operating room (anesthetized).

Main Results:

  • A significant correlation between SQA and TT-TG distance was observed in anesthetized patients (R² = 0.15).
  • This correlation strengthened in anesthetized patients with normal patellar height (CDI < 1.2) and normal lateral trochlear inclination (LTI > 11°) (R² = 0.57).
  • Correlations were weak or nonsignificant in awake patients and those with anatomic risk factors.

Conclusions:

  • The SQA shows a strong positive correlation with TT-TG distance in anesthetized patients with normal knee anatomy.
  • The SQA's utility is limited in awake patients or those with predisposing anatomic factors for patellar instability.
  • Anesthesia may enhance the reliability of SQA measurements for assessing patellar instability.