title: The contribution of the tibial tubercle to patellar instability: analysis of tibial tubercle-trochlear groove (TT-TG) and tibial tubercle-posterior cruciate ligament (TT-PCL) distances.
abstract: OBJECTIVE
  The purpose of this study is to assess the reliability of measuring the tibial tubercle to posterior cruciate (TT-PCL) distance compared to the tibial tubercle to trochlear groove (TT-TG) distance on magnetic resonance imaging (MRI), establish baseline TT-PCL values in patellar instability patients, and determine the predictive value of an excessive TT-PCL distance (≥24 mm) for recurrent patellar instability compared to a TT-TG distance ≥20 mm.
  
  
  METHODS
  TT-TG and TT-PCL distances were calculated on MRI in a randomized and blinded fashion by two reviewers on 54 patients (59 knees) with patellar instability. Interobserver reliability was assessed using interclass correlation coefficients (ICC). TT-PCL distances were also assessed to establish mean values in patellar instability patients. The ability of excessive TT-PCL and TT-TG distances to predict recurrent instability was assessed by comparing odds ratios, sensitivities, and specificities.
  
  
  RESULTS
  Interobserver reliability was excellent for both TT-TG (ICC = 0.978) and TT-PCL (ICC = 0.932). The mean TT-PCL in these 59 knees was 21.7 mm (standard deviation 4.1 mm). Twelve (20 %) of 59 knees had a single dislocation, and 47 (80 %) exhibited 2 or more dislocations. The odds ratios, sensitivities, and specificities of a TT-TG distance ≥20 mm for identifying patients with recurrent dislocation were 5.38, 0.213, and 1.0, respectively, while those of a TT-PCL distance ≥24 mm were 1.46, 0.298, and 0.583, respectively. Of the 10 knees with a TT-TG distance ≥20 mm, all 10 (100 %) had recurrent instability, while 14 (73.7 %) of the 19 knees with a TT-PCL ≥24 mm experienced multiple dislocations (n.s.).
  
  
  CONCLUSIONS
  Both TT-PCL and TT-TG can be measured on MRI with excellent interobserver reliability. In this series, the mean TT-PCL value in patients with patellar instability was 21.8 mm, but the range was broad. A TT-PCL distance ≥24 mm was found to be less predictive of recurrent instability in this series. For patients experiencing multiple episodes of patellar instability in the setting of a normal TT-TG distance, obtaining the TT-PCL measurement may provide a more focused assessment of the tibial contribution to tubercle lateralization.
  
  
  METHODS
  III.
authors.affiliation: Department of Orthopedic Surgery and Sports Medicine, Mayo Clinic and Mayo Foundation, Mayo Clinic 200 First St., SW, Rochester, MN, 55905, USA.
authors.forename: Mark J
authors.initials: MJ
authors.lastname: Heidenreich
authors.affiliation: Department of Orthopedic Surgery and Sports Medicine, Mayo Clinic and Mayo Foundation, Mayo Clinic 200 First St., SW, Rochester, MN, 55905, USA.
authors.forename: Christopher L
authors.initials: CL
authors.lastname: Camp
authors.affiliation: Department of Orthopedic Surgery and Sports Medicine, Mayo Clinic and Mayo Foundation, Mayo Clinic 200 First St., SW, Rochester, MN, 55905, USA.
authors.forename: Diane L
authors.initials: DL
authors.lastname: Dahm
authors.affiliation: Department of Orthopedic Surgery and Sports Medicine, Mayo Clinic and Mayo Foundation, Mayo Clinic 200 First St., SW, Rochester, MN, 55905, USA.
authors.forename: Michael J
authors.initials: MJ
authors.lastname: Stuart
authors.affiliation: Department of Orthopedic Surgery and Sports Medicine, Mayo Clinic and Mayo Foundation, Mayo Clinic 200 First St., SW, Rochester, MN, 55905, USA.
authors.forename: Bruce A
authors.initials: BA
authors.lastname: Levy
authors.affiliation: Department of Orthopedic Surgery and Sports Medicine, Mayo Clinic and Mayo Foundation, Mayo Clinic 200 First St., SW, Rochester, MN, 55905, USA. krych.aaron@mayo.edu.
authors.forename: Aaron J
authors.initials: AJ
authors.lastname: Krych
mesh_terms: D000293:Adolescent; D000328:Adult; D005260:Female; D005269:Femur / Q000000981:diagnostic imaging* / Q000473:pathology; D006801:Humans; D007593:Joint Instability / Q000000981:diagnostic imaging* / Q000473:pathology; D008279:Magnetic Resonance Imaging; D008297:Male; D015588:Observer Variation; D031222:Patellar Dislocation / Q000000981:diagnostic imaging* / Q000473:pathology; D057071:Patellofemoral Joint / Q000000981:diagnostic imaging* / Q000473:pathology; D016119:Posterior Cruciate Ligament / Q000000981:diagnostic imaging* / Q000473:pathology; D011237:Predictive Value of Tests; D012008:Recurrence; D015203:Reproducibility of Results; D016037:Single-Blind Method; D013977:Tibia / Q000000981:diagnostic imaging* / Q000473:pathology; D055815:Young Adult
keywords: Patellar instability; Tibial tubercle to posterior cruciate ligament distance; Tibial tubercle to trochlear groove distance