Reliability of Tönnis classification in early hip arthritis: a useless reference for hip-preserving surgery

Introduction

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.

Materials and 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.