title: Tibial tubercle-trochlear groove distance: defining normal in a pediatric population.
abstract: BACKGROUND
  The tibial tubercle-trochlear groove (TT-TG) distance is a useful tool in guiding surgical management for patients with recurrent lateral patellar instability. Current recommendations for tibial tubercle transfer are based on TT-TG distance thresholds derived from adult populations. Recurrent patellar instability, however, frequently affects children, but normal and pathological TT-TG values have not been established for pediatric patients. The objectives of this study were to (1) confirm that magnetic resonance imaging (MRI) measurements for TT-TG distance in a pediatric population are reliable and reproducible, (2) determine whether the TT-TG distance changes with age, (3) define normal TT-TG distances in a pediatric population, and (4) confirm that a subgroup of pediatric patients with patellar instability have higher TT-TG distances.
  
  
  METHODS
  Six hundred and eighteen MRIs were retrospectively collected for patients who were nine months to sixteen years old. Each MRI was measured twice in a blinded, randomized manner by each reviewer. Patient age, sex, knee laterality, magnet strength, underlying diagnosis, and pertinent previous surgical treatments were all recorded separately from the measurements. MRIs that were unreadable and those of patients who had previous extensor mechanism surgery, preexisting deformity, or destructive neoplasms were excluded.
  
  
  RESULTS
  There was excellent intraobserver and interobserver reliability of TT-TG distance measurements. TT-TG distance was associated with the natural logarithm of age (p < 0.001). A percentile-based growth chart was created to demonstrate this relationship. The median TT-TG distance for patients without patellar instability in this pediatric population was 8.5 mm (mean and 95% confidence interval, 8.6 ± 0.3 mm). Patients with patellar instability had higher TT-TG distances (median, 12.1 mm; p < 0.001). TT-TG distance measured nearly 2 mm less on MRIs performed with a 3-T magnet than on those acquired with a 1.5-T magnet (p < 0.001).
  
  
  CONCLUSIONS
  TT-TG distance changes with chronologic age in the pediatric population. As such, we developed a percentile-based growth chart in order to better depict normal TT-TG distances in the pediatric population. Like many issues in pediatric orthopaedics, an age-based approach for directing surgical treatment may be more appropriate for skeletally immature individuals with recurrent lateral patellar instability.
authors.affiliation: Departments of Orthopaedics and Rehabilitation (A.J.D., N.T.M., A.D., and G.T.) and Emergency Medicine (D.T.), University of New Mexico Health Sciences Center, 1 University of New Mexico, MSC10 5600, Albuquerque, NM 87131. E-mail address for A.J. Dickens:
authors.forename: Aaron J
authors.initials: AJ
authors.lastname: Dickens
authors.affiliation: Departments of Orthopaedics and Rehabilitation (A.J.D., N.T.M., A.D., and G.T.) and Emergency Medicine (D.T.), University of New Mexico Health Sciences Center, 1 University of New Mexico, MSC10 5600, Albuquerque, NM 87131. E-mail address for A.J. Dickens:
authors.forename: Nathan T
authors.initials: NT
authors.lastname: Morrell
authors.affiliation: Departments of Orthopaedics and Rehabilitation (A.J.D., N.T.M., A.D., and G.T.) and Emergency Medicine (D.T.), University of New Mexico Health Sciences Center, 1 University of New Mexico, MSC10 5600, Albuquerque, NM 87131. E-mail address for A.J. Dickens:
authors.forename: Andrew
authors.initials: A
authors.lastname: Doering
authors.affiliation: Departments of Orthopaedics and Rehabilitation (A.J.D., N.T.M., A.D., and G.T.) and Emergency Medicine (D.T.), University of New Mexico Health Sciences Center, 1 University of New Mexico, MSC10 5600, Albuquerque, NM 87131. E-mail address for A.J. Dickens:
authors.forename: Dan
authors.initials: D
authors.lastname: Tandberg
authors.affiliation: Departments of Orthopaedics and Rehabilitation (A.J.D., N.T.M., A.D., and G.T.) and Emergency Medicine (D.T.), University of New Mexico Health Sciences Center, 1 University of New Mexico, MSC10 5600, Albuquerque, NM 87131. E-mail address for A.J. Dickens:
authors.forename: Gehron
authors.initials: G
authors.lastname: Treme
mesh_terms: D000293:Adolescent; D002648:Child; D002675:Child, Preschool; D005260:Female; D006801:Humans; D007223:Infant; D007231:Infant, Newborn; D007593:Joint Instability / Q000473:pathology* / Q000601:surgery; D008279:Magnetic Resonance Imaging; D008297:Male; D010329:Patella / Q000033:anatomy & histology* / Q000473:pathology / Q000601:surgery; D031222:Patellar Dislocation / Q000473:pathology* / Q000601:surgery; D057071:Patellofemoral Joint / Q000033:anatomy & histology* / Q000473:pathology* / Q000601:surgery; D012016:Reference Values; D015203:Reproducibility of Results; D012189:Retrospective Studies; D013977:Tibia / Q000033:anatomy & histology* / Q000473:pathology / Q000601:surgery