title: Why are bone and soft tissue measurements of the TT-TG distance on MRI different in patients with patellar instability?
abstract: OBJECTIVE
  To determine whether the tibial tuberosity-to-trochlear groove distance (TT-TG) and patellar tendon-to-trochlear groove distance (PT-TG) are equal, whether the bony and cartilaginous points coincide in the trochlea, and whether the insertion of the PT coincides with the most anterior point of the TT in patients with patellar instability.
  
  
  METHODS
  Fifty-three MRI scans of patients with patellar instability were examined. TT-TG and PT-TG were measured by three examiners in 31 knees. Additionally, the bone-cartilage distance in the trochlea [trochlear cartilage to trochlear bone (TC-TB)] and the distance between the mid-point of the PT insertion and the most anterior point of the TT (PT-TT) were measured by one examiner. The intraclass correlation coefficient was used to evaluate the reliability of the measurements between the three examiners. The relationships between the measurements were determined, the means of the measurements were calculated, and the correlations between PT-TG and TT-TG, PT-TT, and TC-TB were assessed.
  
  
  RESULTS
  The ICC was above 0.8. PT-TG was 3.7 mm greater than TT-TG. The TC and TB coincided in 73 % of cases, and the mean TC-TB was 0.3 mm. The PT was lateral to the TT in 94 % of the cases, and the mean PT-TT was 3.4 mm. The Pearson's correlation coefficients between PT-TG and TT-TG, PT-TT, and TC-TB were 0.946, 0.679, and 0.199, respectively.
  
  
  CONCLUSIONS
  TT-TG underestimated PT-TG, primarily due to the lateralization of the PT insertion relative to the most anterior point of the TT.
  
  
  CONCLUSIONS
  our study shows that in patients with patellar instability, there are differences in the absolute values of TT-TG and PT-TG, as previously reported for patients without patellar instability. Hence, normal cut-off values based on case-control studies of TT-TG cannot be equivalently used when measuring PT-TG to indicate TT medialization in patients with patellar instability. It is also important to note that the clinical outcomes cannot be directly compared between patients evaluated using TT-TG versus PT-TG measurements.
  
  
  METHODS
  III.
authors.affiliation: Department of Orthopaedics and Traumatology, Institute of Orthopaedics and Traumatology, University of São Paulo, Ovídio Pires de Campos St, 333, 2nd Floor, Cerqueira César, São Paulo, SP, 5403-010, Brazil.
authors.forename: Betina B
authors.initials: BB
authors.lastname: Hinckel
authors.affiliation: Department of Orthopaedics and Traumatology, Institute of Orthopaedics and Traumatology, University of São Paulo, Ovídio Pires de Campos St, 333, 2nd Floor, Cerqueira César, São Paulo, SP, 5403-010, Brazil. gobbiortopedia@gmail.com.
authors.forename: Riccardo G
authors.initials: RG
authors.lastname: Gobbi
authors.affiliation: Department of Musculoskeletal Radiology, University of São Paulo, São Paulo, SP, 55403-010, Brazil.
authors.forename: Eduardo N
authors.initials: EN
authors.lastname: Kihara Filho
authors.affiliation: Department of Orthopaedics and Traumatology, Institute of Orthopaedics and Traumatology, University of São Paulo, Ovídio Pires de Campos St, 333, 2nd Floor, Cerqueira César, São Paulo, SP, 5403-010, Brazil.
authors.forename: Marco K
authors.initials: MK
authors.lastname: Demange
authors.affiliation: Department of Orthopaedics and Traumatology, Institute of Orthopaedics and Traumatology, University of São Paulo, Ovídio Pires de Campos St, 333, 2nd Floor, Cerqueira César, São Paulo, SP, 5403-010, Brazil.
authors.forename: José Ricardo
authors.initials: JR
authors.lastname: Pécora
authors.affiliation: Department of Musculoskeletal Radiology, University of São Paulo, São Paulo, SP, 55403-010, Brazil.
authors.forename: Marcelo B
authors.initials: MB
authors.lastname: Rodrigues
authors.affiliation: Department of Orthopaedics and Traumatology, Institute of Orthopaedics and Traumatology, University of São Paulo, Ovídio Pires de Campos St, 333, 2nd Floor, Cerqueira César, São Paulo, SP, 5403-010, Brazil.
authors.forename: Gilberto Luis
authors.initials: GL
authors.lastname: Camanho
mesh_terms: D000328:Adult; D002358:Cartilage, Articular / Q000000981:diagnostic imaging* / Q000473:pathology; D005260:Female; D006801:Humans; D007593:Joint Instability / Q000000981:diagnostic imaging* / Q000473:pathology; D008279:Magnetic Resonance Imaging; D008297:Male; D008875:Middle Aged; D031222:Patellar Dislocation / Q000000981:diagnostic imaging* / Q000473:pathology; D017847:Patellar Ligament / Q000000981:diagnostic imaging* / Q000473:pathology; D012016:Reference Values; D015203:Reproducibility of Results; D013977:Tibia / Q000000981:diagnostic imaging* / Q000473:pathology; D055815:Young Adult
keywords: MRI; Patellar dislocation; Q angle; Risk factors; TT-TG