title: Individualizing the Tibial Tubercle-Trochlear Groove Distance: Patellar Instability Ratios That Predict Recurrent Instability.
abstract: BACKGROUND
  Patients with patellar instability and a tibial tubercle-trochlear groove (TT-TG) distance ≥20 mm may be candidates for distal tubercle realignment surgery. Although this variable has proven valuable in predicting recurrent dislocations, it is not individualized to patient size, bony structure, or patellofemoral mechanics.
  
  
  OBJECTIVE
  To develop a patellar instability ratio (PIR) that predicts the risk of recurrent instability based on the TT-TG distance to patient-specific anatomy.
  
  
  METHODS
  Cohort study; Level of evidence, 3.
  
  
  METHODS
  On magnetic resonance imaging scans of 59 knees with patellar instability, the TT-TG distance, tibial tubercle-posterior cruciate ligament (TT-PCL) distance, sagittal patellar length (PL), sagittal trochlear length (TL), axial patellar width (PW), and axial trochlear width (TW) were calculated by 2 observers in a blinded and randomized fashion. Patients were divided into 2 groups: those with a single dislocation and those with multiple (≥2) dislocations. The ability of the TT-TG and TT-PCL distances as well as the 8 different ratios to predict recurrent instability was assessed by calculating odds ratios (ORs), sensitivity, specificity, positive predictive value (PPV), and negative predictive value (NPV) for each measure.
  
  
  RESULTS
  Twelve knees (20%) experienced a single dislocation, while 47 (80%) sustained multiple dislocations. A TT-TG distance ≥20 mm was predictive of recurrent instability, with a sensitivity, specificity, and PPV of 21%, 100%, and 100%, respectively. The OR for TT-TG ≥20 mm was 5.38 (P = .29), while the highest ORs for recurrent instability were noted for a TT-TG/PW ≥0.4 (OR, 7.37; P = .02) and a TT-TG/TW ≥0.5 (OR, 8.88; P = .04). The sensitivity, specificity, and PPV of a TT-TG/PW ≥0.4 were 62%, 83%, and 94%, respectively, while those of a TT-TG/TW ≥0.5 were 45%, 92%, and 95%, respectively.
  
  
  CONCLUSIONS
  Two novel PIRs (TT-TG/TW and TT-TG/PW) were identified and found to more effectively predict recurrent instability than the TT-TG distance alone. Each ratio takes into account patient-specific anatomy and can be measured in an accurate and reliable fashion by clinicians. These PIRs are a step toward overcoming some of the limitations of using the TT-TG distance in isolation. Further investigation into the clinical applications and utility of the TT-TG/TW is warranted.
authors.affiliation: Department of Orthopedic Surgery and Sports Medicine Center, Mayo Clinic, Rochester, Minnesota, USA.
authors.forename: Christopher L
authors.initials: CL
authors.lastname: Camp
authors.affiliation: Department of Orthopedic Surgery and Sports Medicine Center, Mayo Clinic, Rochester, Minnesota, USA.
authors.forename: Mark J
authors.initials: MJ
authors.lastname: Heidenreich
authors.affiliation: Department of Orthopedic Surgery and Sports Medicine Center, Mayo Clinic, Rochester, Minnesota, USA.
authors.forename: Diane L
authors.initials: DL
authors.lastname: Dahm
authors.affiliation: Department of Orthopedic Surgery and Sports Medicine Center, Mayo Clinic, Rochester, Minnesota, USA.
authors.forename: Michael J
authors.initials: MJ
authors.lastname: Stuart
authors.affiliation: Department of Orthopedic Surgery and Sports Medicine Center, Mayo Clinic, Rochester, Minnesota, USA.
authors.forename: Bruce A
authors.initials: BA
authors.lastname: Levy
authors.affiliation: Department of Orthopedic Surgery and Sports Medicine Center, Mayo Clinic, Rochester, Minnesota, USA krych.aaron@mayo.edu.
authors.forename: Aaron J
authors.initials: AJ
authors.lastname: Krych
mesh_terms: D000293:Adolescent; D000328:Adult; D015331:Cohort Studies; D004311:Double-Blind Method; D005260:Female; D006801:Humans; D007593:Joint Instability / Q000473:pathology* / Q000601:surgery; D008279:Magnetic Resonance Imaging / Q000379:methods; D008297:Male; D008875:Middle Aged; D015588:Observer Variation; D010329:Patella / Q000601:surgery; D031222:Patellar Dislocation / Q000473:pathology* / Q000601:surgery; D057071:Patellofemoral Joint / Q000473:pathology* / Q000601:surgery; D016119:Posterior Cruciate Ligament / Q000473:pathology; D012008:Recurrence; D013977:Tibia / Q000473:pathology / Q000601:surgery; D055815:Young Adult
keywords: patellar instability; patellar width; ratio; tibial tubercle–trochlear groove distance; trochlear width