title: Ultrasound of the knee during voluntary quadriceps contraction: a technique for detecting otherwise occult effusions.
abstract: OBJECTIVE
  To describe 1) a technique that can detect synovial effusions not seen on static ultrasound (US) examination and 2) the characteristics of patients with knee osteoarthritis (OA) for whom this technique proved useful.
  
  
  METHODS
  From reviewed records of 76 patients with knee OA (112 knees) that we had seen for US-guided injections over a defined period, we found 45 knees with no detectable effusion on static US, of which 18 (14 patients) showed fluid when scanned during voluntary quadriceps contraction. For all patients, we had recorded effusion features (physical examination, presence and size on US), and success of joint entry was determined by getting synovial fluid and/or seeing an air echo or inflow of injected material.
  
  
  RESULTS
  The 14 patients we studied were obese (mean +/- SEM body mass index 32.7 +/- 2.3 kg/m(2); 3 morbidly obese), with moderate to severe OA by radiography in most (Kellgren/Lawrence class 3 or 4 in 10 of 14 knees for which radiographs were available). The suprapatellar synovial space seen by US was small (mean +/- SEM depth 0.38 +/- 0.04 cm). Arthrocentesis obtained 0.5-16 ml of synovial fluid (mean +/- SEM 2.9 +/- 0.6 ml), which correlated with the depth of effusion as seen on US with the quadriceps in maximum contraction (Spearman's rho = 0.5597, P = 0.0157). In 4 knees where arthrocentesis failed to retrieve fluid, we observed at injection the inflow of material and a linear air echo.
  
  
  CONCLUSIONS
  US of the knee during voluntary quadriceps contraction can find effusions not detectable on static US. Such effusions provide targets for accurate aspiration and injection that would not be appreciated with static US.
authors.affiliation: Rheumatology Division, University of Michigan, Ann Arbor, MI 48109-0358, USA. rike@umich.edu
authors.forename: Robert W
authors.initials: RW
authors.lastname: Ike
authors.forename: Emily C
authors.initials: EC
authors.lastname: Somers
authors.forename: Erin L
authors.initials: EL
authors.lastname: Arnold
authors.forename: William J
authors.initials: WJ
authors.lastname: Arnold
mesh_terms: D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D005260:Female; D006801:Humans; D007270:Injections, Intra-Articular / Q000379:methods; D007719:Knee Joint / Q000000981:diagnostic imaging* / Q000473:pathology; D008297:Male; D008875:Middle Aged; D009119:Muscle Contraction; D020370:Osteoarthritis, Knee / Q000150:complications* / Q000000981:diagnostic imaging / Q000473:pathology; D019152:Paracentesis / Q000379:methods*; D052097:Quadriceps Muscle; D016059:Range of Motion, Articular; D013582:Synovial Fluid / Q000000981:diagnostic imaging*; D014463:Ultrasonography
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