title: Reliability of Tönnis classification in early hip arthritis: a useless reference for hip-preserving surgery.
abstract: BACKGROUND
  The Tönnis classification is widely accepted for grading hip arthritis, but its usefulness as a reference in hip-preserving surgery is yet to be demonstrated. We aimed to evaluate reproducibility of the Tönnis classification in early stages of hip osteoarthritis, and thus determine whether it is a reliable reference for hip-preserving surgery.
  
  
  METHODS
  Three orthopaedic surgeons with different levels of experience examined 117 hip X-rays that were randomly mixed of two groups: a group of 31 candidates for hip-preserving surgery and a control group of 30 patients that were asymptomatic with respect to the hip joint. The surgeons were asked to rate an eventual osteoarthritis according to the Tönnis classification. After 2 months, the surgeons were asked to re-evaluate the X-rays in a random order. Intra- and interobserver reliabilities were calculated by comparing the observers' two estimations using Kappa statistics.
  
  
  RESULTS
  Kappa values for interobserver reliability were slight or fair (range 0.173-0.397). Kappa values for intraobserver reproducibility were fair (range 0.364-0.397). Variance in grading no and slight osteoarthritis was the most frequent cause for intra- and interobserver disagreements (76.3 and 73.01% of the non-concordant observations, respectively). The confidence interval analysis showed that the observers' experience did not affect reproducibility.
  
  
  CONCLUSIONS
  The Tönnis classification is a poor method to assess early stages of hip osteoarthritis. These findings suggest that its routine use in therapeutic decision-making for conservative hip surgery should be reconsidered.
authors.affiliation: Orthopaedic Department, Hip Unit, Hospital de la Santa Creu i Sant Pau, Universitat Autònoma de Barcelona, Sant Quintí, 89, 08026, Barcelona, Spain. mvalera@santpau.cat.
authors.forename: Marius
authors.initials: M
authors.lastname: Valera
authors.affiliation: Orthopaedic Department, Hip Unit, Hospital de la Santa Creu i Sant Pau, Universitat Autònoma de Barcelona, Sant Quintí, 89, 08026, Barcelona, Spain. naia1310@gmail.com.
authors.forename: Natalia
authors.initials: N
authors.lastname: Ibañez
authors.affiliation: Orthopaedic Department, Hip Unit, Hospital de la Santa Creu i Sant Pau, Universitat Autònoma de Barcelona, Sant Quintí, 89, 08026, Barcelona, Spain. rnavarro@santpau.cat.
authors.forename: Rogelio
authors.initials: R
authors.lastname: Sancho
authors.affiliation: Parc de Salut Mar, Hip Unit, Universitat Autònoma de Barcelona, Barcelona, Spain. marcteypons@gmail.com.
authors.forename: Marc
authors.initials: M
authors.lastname: Tey
mesh_terms: D000293:Adolescent; D000328:Adult; D019644:Arthroplasty, Replacement, Hip; D000066491:Clinical Decision-Making; D005260:Female; D006621:Hip Joint / Q000000981:diagnostic imaging*; D006801:Humans; D008297:Male; D008875:Middle Aged; D015588:Observer Variation; D015207:Osteoarthritis, Hip / Q000145:classification* / Q000000981:diagnostic imaging / Q000601:surgery; D011446:Prospective Studies; D011859:Radiography; D015203:Reproducibility of Results; D012720:Severity of Illness Index; D055815:Young Adult
keywords: Hip osteoarthritis; Hip-preserving surgery; Interobserver reliability; Tönnis classification