title: Re-evaluating the functional implications of the Q-angle and its relationship to in-vivo patellofemoral kinematics.
abstract: BACKGROUND
  The Q-angle is widely used clinically to evaluate individuals with anterior knee pain. Recent studies have questioned the utility of this measure and have suggested that a large Q-angle may not be associated with lateral patellofemoral translation, as often assumed. The objective of this study was to determine: 1) how accurately the Q-angle represents the line-of-action of the quadriceps and 2) if adding active quadriceps contraction or a bent knee position to the measurement of the Q-angle improves its reliability, accuracy, and association with patellofemoral kinematics.
  
  
  METHODS
  The study included individuals diagnosed with chronic idiopathic patellofemoral pain and control subjects (n=43 and n=30 knees). Three measures of the clinical Q-angle (straight- and bent-knee with relaxed quadriceps and straight-knee with maximum isometric quadriceps contraction) were obtained with a goniometer and compared to a fourth MR-based measure of Q-angle. Patellofemoral kinematics were derived from dynamic cine-phase contrast images, acquired while subjects extended/flexed their knee from approximately 0° and 45°.
  
  
  RESULTS
  The Q-angle did not represent the line-of-action of the quadriceps. The average difference between each clinical and the MR-based Q-angle ranged from 5° to 8°. These differences varied greatly across subjects (range: -28.5° to 3.9(o)). Adding an active quadriceps contraction or a bent knee position, did not improve the reliability of the Q-angle. An increased Q-angle correlated to medial patellar displacement and tilt (r=0.38-0.54, P<0.001) in the cohort with anterior knee pain.
  
  
  CONCLUSIONS
  Clinicians are cautioned against using the Q-angle to infer patellofemoral kinematics.
authors.affiliation: Functional and Applied Biomechanics, Department of Rehabilitation Medicine, NIH, Bethesda, MD, USA; Department of Bioengineering, University of Pennsylvania, Philadelphia, PA, USA. Electronic address: bfreed@seas.upenn.edu.
authors.forename: Benjamin R
authors.initials: BR
authors.lastname: Freedman
authors.affiliation: Physical Therapy and Rehabilitation Science, University of Maryland, College Park, MD, USA.
authors.forename: Timothy J
authors.initials: TJ
authors.lastname: Brindle
authors.affiliation: Functional and Applied Biomechanics, Department of Rehabilitation Medicine, NIH, Bethesda, MD, USA.
authors.forename: Frances T
authors.initials: FT
authors.lastname: Sheehan
mesh_terms: D000328:Adult; D000368:Aged; D000704:Analysis of Variance; D001696:Biomechanical Phenomena; D016022:Case-Control Studies; D002908:Chronic Disease; D015331:Cohort Studies; D005260:Female; D006801:Humans; D007537:Isometric Contraction; D007719:Knee Joint / Q000503:physiopathology*; D008279:Magnetic Resonance Imaging / Q000379:methods; D008297:Male; D008875:Middle Aged; D046788:Patellofemoral Pain Syndrome / Q000503:physiopathology*; D052097:Quadriceps Muscle / Q000502:physiology*; D016059:Range of Motion, Articular / Q000502:physiology; D015203:Reproducibility of Results; D055815:Young Adult
keywords: Kinematics; Knee; Magnetic resonance imaging; Patellofemoral joint; Patellofemoral pain syndrome